Step Seminar

8/9/2019 Step Seminar

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STEPSEMINAR ON

ELECTRIC MOTORS,

ENCODERS &

RESOLVERS

SUBMITTED TO -: SUBMITTED BY -:

Mr. RAMANDEEP SINGH #1 JAGDEEP SINGH#2 GURDEEP SINGH

#3 VARINDER SINGH

#4 MUNISH GARG


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Electric motor.

An electric motoruses electrical energy to produce mechanical energy, very typically through the

interaction ofmagnetic fields and current-carrying conductors. The reverse process, producing electrical

energy from mechanical energy, is accomplished by analternator, generatorordynamo. Many types of

electric motors can be run as generators, and vice versa. For example a starter/generator for agas

turbine orTraction motors used on vehicles often perform both tasks.

Electric motors are found in applications as diverse as industrial fans, blowers and pumps, machine tools,

household appliances,power tools, and disk drives. They may be powered by direct current (e.g.,

a battery powered portable device or motor vehicle), or byalternating current from a central electrical

distribution grid. The smallest motors may be found in electric wristwatches. Medium-size motors of highlystandardized dimensions and characteristics provide convenient mechanical power for industrial uses. The

very largest electric motors are used for propulsion of large ships, and for such purposes as pipeline

compressors, with ratings in the millions ofwatts. Electric motors may be classified by the source of electric

power, by their internal construction, by their application, or by the type of motion they give.

The physical principle of production of mechanical force by the interactions of an electric current and a

magnetic field was known as early as 1821. Electric motors of increasing efficiency were constructed

throughout the 19th century, but commercial exploitation of electric motors on a large scale required

efficientelectrical generators and electrical distribution networks.

Some devices, such as magneticsolenoids and loudspeakers, although they generate some mechanical

power, are not generally referred to as electric motors, and are usually

termed actuators[1] and transducers,[2] respectively.


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HISTORY& DEVELOPMENT :-

THE PRINCIPLE :-

The conversion of electrical energy into mechanical energy byelectromagnetic means was demonstrated

by the British scientist Michael Faraday in 1821. A free-hanging wire was dipped into a pool ofmercury, on

which a permanent magnet was placed. When a current was passed through the wire, the wire rotated

around the magnet, showing that the current gave rise to a circular magnetic field around the wire.[4] This

motor is often demonstrated in school physics classes, butbrine (salt water) is sometimes used in place of

the toxic mercury. This is the simplest form of a class of devices calledhomopolar motors. A later

refinement is the Barlow's Wheel. These were demonstration devices only, unsuited to practical

applications due to their primitive construction.[citation needed]

In 1827, Hungariannyos Jedlik started experimenting with electromagnetic rotating devices he called

"lightning-magnetic self-rotors". He used them for instructive purposes in universities, and in 1828demonstrated the first device which contained the three main components of practicaldirect

current motors: thestator, rotorand commutator. Both the stationary and the revolving parts were

electromagnetic, employing no permanent magnets. Again, the devices had no practical application.[


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The first electric motors

The first commutator-type direct current electric motor capable of turning machinery was invented by the

British scientist William Sturgeon in 1832.[12] Following Sturgeon's work, a commutator-type direct-current

electric motor made with the intention of commercial use was built by Americans Emily and Thomas

Davenport and patented in 1837. Their motors ran at up to 600 revolutions per minute, and poweredmachine tools and a printing press.[13] Due to the high cost of the zinc electrodes required by primary

battery power, the motors were commercially unsuccessful and the Davenports went bankrupt. Several

inventors followed Sturgeon in the development of DC motors but all encountered the same cost issues

with primary battery power. No electricity distribution had been developed at the time. Like Sturgeon's

motor, there was no practical commercial market for these motors.[citation needed]

In 1855 Jedlik built a device using similar principles to those used in his electromagnetic self-rotors that

was capable of useful work.[6][8] He built a model electric motor-propelled vehicle that same year.[14] There is

no evidence that this experimentation was communicated to the wider scientific world at that time, or that it

influenced the development of electric motors in the following decades.[citation needed]

The modern DC motor was invented by accident in 1873, when Znobe Gramme connected

the dynamo he had invented to a second similar unit, driving it as a motor. The Gramme machine was the

first electric motor that was successful in the industry.[citation needed]

In 1886 Frank Julian Sprague invented the first practical DC motor, a non-sparking motor capable of

constant speed under variable loads. Other Sprague electric inventions aboutthis time greatly improved

grid electric distribution (prior work done while employed byThomas Edison), allowed power from electric

motors to be returned to the electric grid, provided for electric distribution to trolleys via overhead wires and

the trolley pole, and provided controls systems for electric operations. This allowed Sprague to use electricmotors to invent the first electric trolley system in 1887-88 in Richmond VA, the electric elevator and control

system in 1892, and the electric subway with independently powered centrally controlled cars, which was

first installed in 1892 in Chicago by the South Side Elevated Railway where it became popularly known as

the "L". Sprague's motor and related inventions led to an explosion of interest and use in electric motors for

industry, while almost simultaneously another great inventor was developing its primary competitor, which

would become much more widespread.

In 1888 Nikola Tesla invented the first practicableAC motorand with it the polyphase power transmission

system. Tesla continued his work on the AC motor in the years to follow at the Westinghouse

company.[citation needed]

The development of electric motors of acceptable efficiency was delayed for several decades by failure to

recognize the extreme importance of a relatively small air gap between rotor and stator. Early motors, for

some rotor positions, had comparatively huge air gaps which constituted a very high reluctance magnetic

circuit. They produced far-lower torque than an equivalent amount of power would produce with efficient

designs. The cause of the lack of understanding seems to be that early designs were based on familiarity


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of distant attraction between a magnet and a piece of ferromagnetic material, or between two

electromagnets. Efficient designs, as this article describes, are based on a rotor with a comparatively small

air gap, and flux patterns that create torque.[15]

Note that the armature bars are at some distance (unknown) from the field pole pieces when power is fed

to one of the field magnets; the air gap is likely to be considerable. The text tells of the inefficiency of the

design. (Electricity was created, as a practical matter, by consuming zinc in wet primary cells!)

In his workshops Froment had an electromotive engine of one-horse power. But, though an interesting

application of the transformation of energy, these machines will never be practically applied on the large

scale in manufactures, for the expense of the acids and the zinc which they use very far exceeds that of the

coal in steam-engines of the same force. [...] motors worked by electricity, independently of any question as

to the cost of construction, or of the cost of the acids, are at least sixty times as dear to work as steam-

engines.

Although Gramme's design was comparatively much more efficient, apparently the Froment motor was still

considered illustrative, years later. It is of some interest that the St. Louis motor, long used in classrooms to

illustrate motor principles, is extremely inefficient for the same reason, as well as appearing nothing like a

modern motor. Photo of a traditional form of the motor: [3] Note the prominent bar magnets, and the huge

air gap at the ends opposite the rotor. Even modern versions still have big air gaps if the rotor poles are not

aligned.

Application of electric motors revolutionized industry. Industrial processes were no longer limited by power

transmission using shaft, belts, compressed air or hydraulic pressure. Instead every machine could be

equipped with its own electric motor, providing easy control at the point of use, and improving power

transmission efficiency. Electric motors applied in agriculture eliminated human and animal muscle powerfrom such tasks as handling grain or pumping water. Household uses of electric motors reduced heavy

labor in the home and made higher standards of convenience, comfort and safety possible. Today, electric

motors consume more than half of all electric energy produced.


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Categorization of electric motors

The classic division of electric motors has been that ofAlternating Current (AC) types vs Direct

Current (DC) types. This is more a de facto convention, rather than a rigid distinction. For example, many

classic DC motors run on AC power, these motors being referred to as universal motors.

Rated output power is also used to categorise motors, those of less than 746 Watts, for example, are often

referred to as fractional horsepower motors (FHP) in reference to the old imperial measurement.

The ongoing trend toward electronic control further muddles the distinction, as modern drivers have moved

the commutator out of the motor shell. For this new breed of motor, driver circuits are relied upon to

generate sinusoidal AC drive currents, or some approximation thereof. The two best examples are:

the brushless DC motorand the stepping motor, both being poly-phase AC motors requiring external

electronic control, although historically, stepping motors (such as for maritime and naval gyrocompass

repeaters) were driven from DC switched by contacts.

Considering all rotating (or linear) electric motors require synchronism between a moving magnetic field

and a moving current sheet for average torque production, there is a clearer distinction between

an asynchronous motorand synchronous types. An asynchronous motor requires slip between the moving

magnetic field and a winding set to induce current in the winding set by mutual inductance; the most

ubiquitous example being the common AC induction motorwhich must slip to generate torque. In the

synchronous types, induction (or slip) is not a requisite for magnetic field or current production (e.g.

permanent magnet motors, synchronous brush-less wound-rotor doubly-fed electric machine).

[


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Comparison of motor types

Comparison of motor types[16]

Type Advantages Disadvantages Typical Application Typical Drive

AC Induction

(Shaded Pole)

Least expensiveLong life

high power

Rotation slips fromfrequency

Low starting torque

FansUni/Poly-phase

AC

AC Induction

(split-phase

capacitor)

High power

high starting torque

Rotation slips from

frequency

Appliances

Stationary Power Tools

Uni/Poly-phase

AC

Universal motor

High starting torque,

compact, high speed

Maintenance

(brushes)

Medium lifespan

Drill, blender, vacuum

cleaner, insulation blowers

Uni-phase AC or

Direct DC

AC

Synchronous

Rotation in-sync with

freq

long-life (alternator)

More expensive

Industrial motors

Clocks

Audio turntables

tape drives

Uni/Poly-phase

AC

Stepper DC Precision positioning

High holding torque

High initial cost

Requires a controller

Positioning in printers and

floppy drives

DC

Brushless DC

Long lifespan

low maintenance

High efficiency

High initial cost

Requires a controller

Hard drives

CD/DVD players

electric vehicles

DC

Brushed DCLow initial cost

Simple speed control

Maintenance

(brushes)

Medium lifespan

Treadmill exercisers

automotive motors (seats,

blowers, windows)

Direct DC

orPWM

Pancake DC Compact designSimple speed control

Medium costMedium lifespan

Office EquipFans/Pumps

Direct DCorPWM


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Servo motor

A servomechanism,or servo is an automatic device that uses error-sensing feedback to correct the

performance of a mechanism. The term correctly applies only to systems where the feedback or error-

correction signals help control mechanical position or other parameters. For example, an automotive power

window control is not a servomechanism, as there is no automatic feedback which controls positiontheoperator does this by observation. By contrast the car's cruise control uses closed loop feedback, which

classifies it as a servomechanism.

Synchronous electric motor

A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets

at the same rate as the alternating current and resulting magnetic field which drives it. Another way of

saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor,

which must slip to produce torque. A synchronous motor is like an induction motor except the rotor isexcited by a DC field. Slip rings and brushes are used to conduct current to rotor. The rotor poles connect

to each other and move at the same speed hence the name synchronous motor.

Induction motor

An induction motor (IM) is a type of asynchronous AC motor where power is supplied to the rotating device

by means of electromagnetic induction. Another commonly used name is squirrel cage motor because the

rotor bars with short circuit rings resemble a squirrel cage (hamster wheel). An electric motor converts

electrical power to mechanical power in its rotor (rotating part). There are several ways to supply power to

the rotor. In a DC motor this power is supplied to the armature directly from a DC source, while in an

induction motor this power is induced in the rotating device. An induction motor is sometimes called a

rotating transformer because the stator (stationary part) is essentially the primary side of the transformer

and the rotor (rotating part) is the secondary side. Induction motors are widely used, especially polyphase

induction motors, which are often used in industrial drives.

Electrostatic motor (capacitor motor)

An electrostatic motor or capacitor motor is a type of electric motor based on the attraction and repulsion of

electric charge. Usually, electrostatic motors are the dual of conventional coil-based motors. They typically

require a high voltage power supply, although very small motors employ lower voltages. Conventional

electric motors instead employ magnetic attraction and repulsion, and require high current at low voltages.In the 1750s, the first electrostatic motors were developed by Benjamin Franklin and Andrew Gordon.

Today the electrostatic motor finds frequent use in micro-mechanical (MEMS) systems where their drive

voltages are below 100 volts, and where moving, charged plates are far easier to fabricate than coils and

iron cores. Also, the molecular machinery which runs living cells is often based on linear and rotary

electrostatic motors.


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Brushless DC motors

Some of the problems of the brushed DC motor are eliminated in the brushless design. In this motor, the

mechanical "rotating switch" or commutator/brushgear assembly is replaced by an external electronic

switch synchronised to the rotor's position. Brushless motors are typically 85-90% efficient or more (higher

efficiency for a brushless electric motor of up to 96.5% were reported by researchers at the TokaiUniversity in Japan in 2009),[17] whereas DC motors with brushgear are typically 75-80% efficient.

Coreless or ironless DC motors

Nothing in the design of any of the motors described above requires that the iron (steel) portions of the

rotor actually rotate; torque is exerted only on the windings of the electromagnets. Taking advantage of this

fact is the coreless or ironless DC motor, a specialized form of a brush or brushless DC motor.

Optimized for rapid acceleration, these motors have a rotor that is constructed without any iron core. The

rotor can take the form of a winding-filled cylinder, or a self-supporting structure comprising only the

magnet wire and the bonding material. The rotor can fit inside the statormagnets; a magnetically soft

stationary cylinder inside the rotor provides a return path for the stator magnetic flux.

Because the rotor is much lighter in weight (mass) than a conventional rotor formed from copperwindings

on steel laminations, the rotor can accelerate much more rapidly, often achieving a mechanicaltime

constant under 1 ms. This is especially true if the windings use aluminum rather than the heavier copper.

But because there is no metal mass in the rotor to act as a heat sink, even small coreless motors must

often be cooled by forced air.

Related limited-travel actuators have no core and a bonded coil placed between the poles of high-flux thin

permanent magnets. These are the fast head positioners for rigid-disk ("hard disk") drives.

Printed Armature orPancake DC Motors

A rather unique motor design the pancake/printed armature motor has the windings shaped as a disc

running between arrays of high-flux magnets, arranged in a circle, facing the rotor and forming an axial air

gap. This design is commonly known the pancake motor because of its extremely flatprofile, although the

technology has had many brand names since its inception, such as ServoDisc.

The printed armature (originally formed on a printed circuit board) in a printed armature motor is made from

punched copper sheets that are laminated together using advanced composites to form a thin rigid disc.

The printed armature has a unique construction, in the brushed motor world, in that it does not have a

separate ring commutator. The brushes run directly on the armature surface making the whole design very

compact.

An alternative manufacturing method is to use wound copper wire laid flat with a central conventional

commutator, in a flower and petal shape. The windings are typically stabilized by being impregnated with

electrical epoxy potting systems. These are filled epoxies that have moderate mixed viscosity and a long


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gel time. They are highlighted by low shrinkage and low exotherm, and are typically UL 1446 recognized as

a potting compound for use up to 180C (Class H) (UL File No. E 210549).

The unique advantage of ironless DC motors is that there is no cogging (vibration caused by attraction

between the iron and the magnets) and parasitic eddy currents cannot form in the rotor as it is totally

ironless. This can greatly improve efficiency, but variable-speed controllers must use a higher switching

rate (>40 kHz) ordirect current because of the decreased electromagnetic induction.

These motors were originally invented to drive the capstan(s) ofmagnetic tape drives, in the burgeoning

computer industry. Pancake motors are still widely used in high-performance servo-controlled systems,

humanoid robotic systems, industrial automation and medical devices. Due to the variety of constructions

now available the technology is used in applications from high temperature military to low cost pump and

basic servo applications.

Universal motors

A series-wound motor is referred to as a universal motorwhen it has been designed to operate on either

AC or DC power. The ability to operate on AC is because the current in both the field and the armature

(and hence the resultant magnetic fields) will alternate (reverse polarity) in synchronism, and hence the

resulting mechanical force will occur in a constant direction.

Operating at normal power line frequencies, universal motors are often found in a range rarely larger than

one kilowatt (about 1.3 horsepower). Universal motors also form the basis of the traditional railway traction

motorin electric railways. In this application, the use of AC to power a motor originally designed to run on

DC would lead to efficiency losses due to eddy current heating of their magnetic components, particularlythe motor field pole-pieces that, for DC, would have used solid (un-laminated) iron. Although the heating

effects are reduced by using laminated pole-pieces, as used for the cores of transformers and by the use of

laminations of high permeabilityelectrical steel, one solution available at start of the 20th Century was for

the motors to be operated from very low frequency AC supplies, with 25 and 16.7 hertz (Hz) operation

being common. Because they used universal motors, locomotives using this design were also commonly

capable of operating from a third rail powered by DC.

An advantage of the universal motor is that AC supplies may be used on motors which have some

characteristics more common in DC motors, specifically high starting torque and very compact design if

high running speeds are used. The negative aspect is the maintenance and short life problems caused by

the commutator. As a result, such motors are usually used in AC devices such as food mixers and power

tools which are used only intermittently, and often have high starting-torque demands. Continuous speed

control of a universal motor running on AC is easily obtained by use of athyristorcircuit, while (imprecise)

stepped speed control can be accomplished using multiple taps on the field coil. Household blenders that

advertise many speeds frequently combine a field coil with several taps and adiode that can be inserted in

series with the motor (causing the motor to run on half-wave rectified AC).


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Universal motors generally run at high speeds, making them useful for appliances such

as blenders, vacuum cleaners, and hair dryers where high RPM operation is desirable. They are also

commonly used in portable power tools, such as drills, sanders (both disc and orbital), circularandjig

saws, where the motor's characteristics work well. Many vacuum cleaner andweed trimmermotors exceed

10,000 RPM, while Dremel and other similar miniature grinders will often exceed 30,000 RPM.

Universal motors also lend themselves to electronic speed control and, as such, are an ideal choice

fordomestic washing machines. The motor can be used to agitate the drum (both forwards and in reverse)

by switching the field winding with respect to the armature. The motor can also be run up to the high

speeds required for the spin cycle.

Motor damage may occur due to overspeeding (running at an RPM in excess of design limits) if the unit is

operated with no significant load. On larger motors, sudden loss of load is to be avoided, and the possibility

of such an occurrence is incorporated into the motor's protection and control schemes. In some smaller

applications, a fan blade attached to the shaft often acts as an artificial load to limit the motor speed to a

safe value, as well as a means to circulate cooling airflow over the armature and field windings.

AC motors

In 1882, Nikola Tesla discovered the rotating magnetic field, and pioneered the use of a rotary field of force

to operate machines. He exploited the principle to design a unique two-phase induction motor in 1883. In

1885, Galileo Ferraris independently researched the concept. In 1888, Ferraris published his research in a

paper to the Royal Academy of Sciences in Turin.

Tesla had suggested that the commutators from a machine could be removed and the device could operate

on a rotary field of force. Professor Poeschel, his teacher, stated that would be akin to building a perpetual

motion machine.[18] Tesla would later attain U.S. Patent 0,416,194, Electric Motor(December 1889), which

resembles the motor seen in many of Tesla's photos. This classic alternating current electro-magnetic

motor was an induction motor.

Michail Osipovich Dolivo-Dobrovolsky later invented a three-phase "cage-rotor" in 1890. This type of motor

is now used for the vast majority of commercial applications.

Components

A typical AC motor consists of two parts:

An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field,

and;

An inside rotor attached to the output shaft that is given a torque by the rotating field.


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Torque motors

A torque motor (also known as a limited torque motor) is a specialized form ofinduction motorwhich is

capable of operating indefinitely while stalled, that is, with the rotorblocked from turning, without incurring

damage. In this mode of operation, the motor will apply a steady torque to the load (hence the name).

A common application of a torque motor would be the supply- and take-up reel motors in a tape drive. In

this application, driven from a low voltage, the characteristics of these motors allow a relatively constant

light tension to be applied to the tape whether or not the capstan is feeding tape past the tape heads.

Driven from a higher voltage, (and so delivering a higher torque), the torque motors can also achieve fast-

forward and rewind operation without requiring any additional mechanics such asgears orclutches. In the

computer gaming world, torque motors are used in force feedback steering wheels.

Another common application is the control of the throttle of an internal combustion engine in conjunction

with an electronicgovernor. In this usage, the motor works against a return spring to move the throttle in

accordance with the output of the governor. The latter monitors engine speed by counting electrical pulsesfrom the ignition system or from a magnetic pickup[19] and, depending on the speed, makes small

adjustments to the amount ofcurrent applied to the motor. If the engine starts to slow down relative to the

desired speed, the current will be increased, the motor will develop more torque, pulling against the return

spring and opening the throttle. Should the engine run too fast, the governor will reduce the current being

applied to the motor, causing the return spring to pull back and close the throttle.

Slip ring

The slip ring is a component of the wound rotor motor as an induction machine (best evidenced by the

construction of the common automotive alternator), where the rotor comprises a set of coils that are

electrically terminated in slip rings. These are metal rings rigidly mounted on the rotor, and combined with

brushes (as used with commutators), provide continuous unswitched connection to the rotor windings.

In the case of the wound-rotor induction motor, external impedances can be connected to the brushes. The

stator is excited similarly to the standard squirrel cage motor. By changing the impedance connected to the

rotor circuit, the speed/current and speed/torque curves can be altered.

(Slip rings are most-commonly used in automotive alternators as well as in synchro angular data-

transmission devices, among other applications.)

The slip ring motor is used primarily to start a high inertia load or a load that requires a very high starting

torque across the full speed range. By correctly selecting the resistors used in the secondary resistance or

slip ring starter, the motor is able to produce maximum torque at a relatively low supply current from zero

speed to full speed. This type of motor also offers controllable speed.


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Motor speed can be changed because the torque curve of the motor is effectively modified by the amount

of resistance connected to the rotor circuit. Increasing the value of resistance will move the speedof

maximum torque down. If the resistance connected to the rotor is increased beyond the point where the

maximum torque occurs at zero speed, the torque will be further reduced.

When used with a load that has a torque curve that increases with speed, the motor will operate at the

speed where the torque developed by the motor is equal to the load torque. Reducing the load will cause

the motor to speed up, and increasing the load will cause the motor to slow down until the load and motor

torque are equal. Operated in this manner, the slip losses are dissipated in the secondary resistors and can

be very significant. The speed regulation and net efficiency is also very poor.

Stepper motors

Closely related in design to three-phase AC synchronous motors are stepper motors, where an internal

rotor containing permanent magnets or a magnetically soft rotor with salient poles is controlled by a set of

external magnets that are switched electronically. A stepper motor may also be thought of as a cross

between a DC electric motor and a rotary solenoid. As each coil is energized in turn, the rotor aligns itself

with the magnetic field produced by the energized field winding. Unlike a synchronous motor, in its

application, the stepper motor may not rotate continuously; instead, it "steps" starts and then quickly

stops again from one position to the next as field windings are energized and de-energized in sequence.

Depending on the sequence, the rotor may turn forwards or backwards, and it may change direction, stop,

speed up or slow down arbitrarily at any time.

Simple stepper motor drivers entirely energize or entirely de-energize the field windings, leading the rotor to

"cog" to a limited number of positions; more sophisticated drivers can proportionally control the power to

the field windings, allowing the rotors to position between the cog points and thereby rotate extremely

smoothly. This mode of operation is often called microstepping. Computer controlled stepper motors are

one of the most versatile forms of positioning systems, particularly when part of a digitalservo-

controlled system.

Stepper motors can be rotated to a specific angle in discrete steps with ease, and hence stepper motors

are used for read/write head positioning in computerfloppy diskette drives. They were used for the same

purpose in pre-gigabyte era computer disk drives, where the precision and speed they offered was

adequate for the correct positioning of the read/write head of a hard disk drive. As drive density increased,the precision and speed limitations of stepper motors made them obsolete for hard drivesthe precision

limitation made them unusable, and the speed limitation made them uncompetitivethus newer hard disk

drives use voice coil-based head actuator systems. (The term "voice coil" in this connection is historic; it

refers to the structure in a typical (cone type) loudspeaker. This structure was used for a while to position

the heads. Modern drives have a pivoted coil mount; the coil swings back and forth, something like a blade


15/46

of a rotating fan. Nevertheless, like a voice coil, modern actuator coil conductors (the magnet wire) move

perpendicular to the magnetic lines of force.)

Stepper motors were and still are often used in computer printers, optical scanners, and digital

photocopiers to move the optical scanning element, the print head carriage (of dot matrix and inkjet

printers), and the platen. Likewise, many computer plotters (which since the early 1990s have been

replaced with large-format inkjet and laser printers) used rotary stepper motors for pen and platen

movement; the typical alternatives here were either linear stepper motors or servomotors with complex

closed-loop control systems.

So-called quartz analog wristwatches contain the smallest commonplace stepping motors; they have one

coil, draw very little power, and have a permanent-magnet rotor. The same kind of motor drives battery-

powered quartz clocks. Some of these watches, such as chronographs, contain more than one stepping

motor.

Stepper motors were upscaled to be used in electric vehicles under the term SRM (Switched Reluctance

Motor).

Linear motors

A linear motor is essentially an electric motor that has been "unrolled" so that, instead of producing

a torque (rotation), it produces a straight-line force along its length by setting up a traveling electromagnetic

field.

Linear motors are most commonly induction motors or stepper motors. You can find a linear motor in

a maglev (Transrapid) train, where the train "flies" over the ground, and in many roller-coasters where the

rapid motion of the motorless railcar is controlled by the rail. On a smaller scale, at least one letter-size

(8.5" x 11") computer graphics X-Y pen plotter made by Hewlett-Packard (in the late 1970s to mid 1980's)

used two linear stepper motors to move the pen along the two orthogonal axes.

Feeding and windings

Doubly-fed electric motor

Doubly-fed electric motors have two independent multiphase windings that actively participate in theenergy

conversion process with at least one of the winding sets electronically controlled for variable speed

operation. Two is the most active multiphase winding sets possible without duplicating singly-fed or doubly-

fed categories in the same package. As a result, doubly-fed electric motors are machines with an effective

constant torque speed range that is twice synchronous speed for a given frequency of excitation. This is

twice the constant torque speed range as singly-fed electric machines, which have only one active winding

set.


16/46

A doubly-fed motor allows for a smaller electronic converter but the cost of the rotor winding and slip rings

may offset the saving in the power electronics components. Difficulties with controlling speed near

synchronous speed limit applications.[20]

Singly-fed electric motor

Singly-fed electric motors incorporate a single multiphase winding set that is connected to a power supply.

Singly-fed electric machines may be either induction or synchronous. The active winding set can be

electronically controlled. Induction machines develop starting torqueat zero speed and can operate as

standalone machines. Synchronous machines must have auxiliary means for startup, such as a starting

induction squirrel-cage winding or an electronic controller. Singly-fed electric machines have an effective

constant torque speed range up to synchronous speed for a given excitation frequency.

The induction (asynchronous) motors (i.e., squirrel cage rotor or wound rotor), synchronous motors (i.e.,

field-excited, permanent magnet or brushless DC motors, reluctance motors, etc.), which are discussed onthis page, are examples of singly-fed motors. By far, singly-fed motors are the predominantly installed type

of motors.

Nanotube nanomotor

Researchers at University of California, Berkeley, recently developed rotational bearings based upon

multiwall carbon nanotubes. By attaching a gold plate (with dimensions of the order of 100 nm) to the outer

shell of a suspended multiwall carbon nanotube (like nested carbon cylinders), they are able to

electrostatically rotate the outer shell relative to the inner core. These bearings are very robust; devices

have been oscillated thousands of times with no indication of wear. These nanoelectromechanical systems

(NEMS) are the next step in miniaturization and may find their way into commercial applications in the

future.


17/46

Servo Motors

A servo motor consists of several main parts, the motor and gearbox, a positionsensor, an error amplifier and motor driver and a circuit to decode the requested

position. Figure 1 contains a block diagram of a typical servo motor unit.

The radio control receiver system (or other controller) generates a pulse of varyinglength approximately every 20 milliseconds. The pulse is normally between 1 and 2milliseconds long. The length of the pulse is used by the servo to determine theposition it should rotate to.

Figure 1. Servo MotorBlock Diagram

Starting from the control pulse we will work though each part of the diagram andexplain how it all fits together. Once we have gone through how the servo works wewill investigate how the control pulses can be generated with a microcontroller.

Pulse width to voltage converter

The control pulse is feed to a pulse width to voltage converter. This circuit charges acapacitor at a constant rate while the pulse is high. When the pulse goes low thecharge on the capacitor is fed to the output via a suitable buffer amplifier. Thisessentially produces a voltage related to the length of the applied pulse.

The circuit is tuned to produce a useful voltage over a 1ms to 2ms period. The outputvoltage is buffered and so does not decay sign ificantly between control pulses so thelength of time between pulses is not critical.

PositionSensor


18/46

The current rotational position of the servo motor output shaft is read by a sensor.This is normally a potentiometer (variable resistor) which produces a voltage that isrelated to the absolute angle of the output shaft.

The position sensor then feeds its current value into the Error Amplifier whichcompares the current position with the commanded position from the pulse width to

voltage converter.

Error Amplifier

The error amplifier is an operational amplifier with negative feedback. It will alwaystry to minimise the difference between the inverting (negative) and non -inverting(positive) inputs by driving its output is the correct direction.

The output of the error amplifier is either a negative or positive voltage representingthe difference between its inputs. The greater the difference the greater the voltage.

The error amplifier output is used to drive the motor; If it is positive the motor will turnin one direction, if negative the other. This allows the error amplifier to reduce thedifference between its inputs (thus closing the negative feedback loop) and so makethe servo go to the commanded position.

The servo normally contains a single integra ted circuit and a hand full of discreetcomponents to implement the entire control system.

Controlling a Servo Motor with a Microcontroller

From the above we can determine that we need to generate a pulse approximatelyevery 20ms although the actual time between pulses is not critical. The pulse width

however must be accurate to ensure that we can accurately set the position of theservo.

PWM modules

Many microcontrollers are equipped with PWM generators and most people initiallyconsider using these to generate the control signals. Unfortunately they are not reallysuitable.

The problem is that we need a relatively accurate short pulse then a long delay; andgenerally you only have one PWM generator share between several servos whichwould require switching components outside the microcontroller and complicate the

hardware.

The PWM generator is designed to generate an accurate pulse between 0% and100% duty cycle, but we need something in the order of 5% to 10% duty cycle(1ms/20ms to 2ms/20ms). If a typical PWM generator is 8 or 10 bits say, then wecan only use a small fraction of the bits to generate the pulse width we need and sowe loose a lot of accuracy.

Timers


19/46

A more beneficial approach can be implemented with simple timers and softwareinterrupts. The key is realising that we can run a timer at a faster rate and do a singleservo at a time, followed by the next and the next etc. Each of the outputs is driven inturn for its required time and then turned off. Once all outputs have been driven, thecycle repeats.

This approach is demonstrated in the PIC servo controller project.

The timer is configured so that we have plenty of accuracy over the 1 to 2millisecond pulse time. Each servo pin is driven high in turn and the timer configuredto interrupt the processor when the pulse should be finished. The interrupt routinethen drives the output low.

For simplicity, the output pins can be arranged on a single port and the value zero(0x00) written to the port to turn off all pins at once so that the interrupt routine doesnot need to know which servo output is currently active.

After the pulse has ended, the microprocessor sets up the next pulse and begins theprocess again.


20/46

SH

EPPER MOTOR

I rame P Q R S e topelectromagnet

T U ) iV

turnedon, attracting tS enearest toot

S

ofagear-shaped ironrotor.W

ith theteethaligned toelectromagnet

U, they X ill

Yeslightlyoffset fromelectromagnet

.I rame 2 Q R he topelectromagnet

T U) is turnedoff, and theright electromagnet

T

) isenergi a ed, pulling thenearest teethslightly to theright.Rhis

results inarotationofb

.c in thisexample.

Irame 3

Q

Rhe

Yottomelectromagnet

T

b

) isenergi a ed; anotherb

.c

rotationoccurs. I rame 4 Q R he left electromagnet Td

) isenabled, rotatingagainbyb

.c.

W hen the topelectromagnet

T U ) isagainenabled, the teeth in thesprocket X ill haverotatedbyone toothposition; since thereare` e

teeth, it X ill

takeU

f f

steps tomakea full rotation in thisexample.

Becauseofpowerrequirements, inductionof thewindings, and temperaturemanagement, motorscannot be

powereddirectlybymost digital controllers. Somecircuitry that canhandlemorepowerg

amotorcontrollersuchas


21/46

an H-bridge must be inserted between digital controller and motor's windings. The above image shows the basic

circuit of a motor controller that can also sense motor current. The circuitry to control one winding of a motor i s

shown; a stepper motor would use a circuit that could control four windings, and a normal DC motor would need

circuitry to control two windings. All of this c ircuitry is typically incorporated in an integrated H-bridge chip.

A stepper motor(orstep motor) is a brushless, synchronous electric motorthat can divide a full rotationinto a large number of steps. The motor's position can be controlledprecisely without any feedback

mechanism (see Open-loop controller), as long as the motor is carefully sized to the application. Stepper

motors are similar to switched reluctance motors (which are very large stepping motors with a reduced pole

count, and generally are closed-loop commutated.)

Fundamentals of Operation

Stepper motors operate differently from DC brush motors, which rotate when voltage is applied to their

terminals. Stepper motors, on the other hand, effectively have multiple "toothed" electromagnets arranged

around a central gear-shaped piece of iron. The electromagnets are energized by an external control

circuit, such as a microcontroller. To make the motor shaft turn, first one electromagnet is given power,

which makes the gear's teeth magnetically attracted to the electromagnet's teeth. When the gear's teeth

are thus aligned to the first electromagnet, they are slightly offset from the next electromagnet. So when the

next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next

one, and from there the process is repeated. Each of those slight rotations is called a "step," with an integer

number of steps making a full rotation. In that way, the motor can be turned by a precise angle.

Stepper motor characteristics

1. Stepper motors are constant power devices.2. As motor speed increases, torque decreases. (most motors exhibit maximum torque when

stationary, however the torque of a motor when stationary 'holding torque' defines the ability of the

motor to maintain a desired position while under external load)

3. The torque curve may be extended by using current limiting drivers and increasing the driving

voltage (sometimes referred to as a 'chopper' circuit, there are several off the shelf driver chips

capable of doing this in a simple manner).

4. Steppers exhibit more vibration than other motor types, as the discrete step tends to snap the rotor

from one position to another, (this is important as at certain speeds the motor can actually change

direction).

5. This vibration can become very bad at some speeds and can cause the motor to lose torque (or

lose direction).

6. The effect can be mitigated by accelerating quickly through the problem speeds range, physically

damping (frictional damping) the system, or using a micro-stepping driver.

7. Motors with a greater number of phases also exhibit smoother operation than those with fewer

phases (this can also be achieved through the use of a micro stepping drive)


22/46

Open-loop versus closed-loop commutation

Steppers are generally commutated open loop, i.e. the driver has no feedback on where the rotor actually

is. Stepper motor systems must thus generally be over engineered, especially if the load inertia is high, or

there is widely varying load, so that there is no possibility that the motor will lose steps. This has often

caused the system designer to consider the trade-offs between a closely sized but

expensive servomechanism system and an oversized but relatively cheap stepper.

A new development in stepper control is to incorporate a rotor position feedback (e.g.

an encoderorresolver), so that the commutation can be made optimal for torque generation according to

actual rotor position. This turns the stepper motor into a high pole count brushless servo motor, with

exceptional low speed torque and position resolution. An advance on this technique is to normally run the

motor in open loop mode, and only enter closed loop mode if the rotor position error becomes too large

this will allow the system to avoid hunting or oscillating, a common servo problem.

TypesThere are three main types of stepper motors:[1]

1. Permanent Magnet Stepper (can be subdivided in to 'tin-can' and 'hybrid', tin-can being a cheaper

product, and hybrid with higher quality bearings, smaller step angle, higher power density)

2. Hybrid Synchronous Stepper

3. Variable Reluctance Stepper

Permanent magnet motors use a permanent magnet (PM) in the rotor and operate on the attraction or

repulsion between the rotor PM and the stator electromagnets. Variable reluctance (VR) motors have a

plain iron rotor and operate based on the principle that minimum reluctance occurs with minimum gap,

hence the rotor points are attracted toward the stator magnet poles. Hybrid stepper motors are named

because they use a combination of PM and VR techniques to achieve maximum power in a small package

size.

Two-phase stepper motors

There are two basic winding arrangements for the electromagnetic coils in a two phase stepper motor:

bipolar and unipolar.

Unipolar motors

A unipolar stepper motor has two windings per phase, one for each direction of magnetic field. Since in this

arrangement a magnetic pole can be reversed without switching the direction of current, the commutation

circuit can be made very simple (eg. a single transistor) for each winding. Typically, given a phase, one end

of each winding is made common: giving three leads per phase and six leads for a typical two phase motor.

Often, these two phase commons are internally joined, so the motor has only five leads.


23/46

A microcontrollerorsteppermotorcontrollercanbeused toactivate thedrive transistors in theright order,

and thiseaseofoperationmakesunipolarmotorspopularwithhobbyists; theyareprobably thecheapest

way toget preciseangularmovements.

hnipolarsteppermotorcoils

i portheexperimenter, oneway todistinguishcommonwire fromacoil-endwire isbymeasuring the

resistance.qesistancebetweencommonwireandcoil-endwire isalwayshalfofwhat it isbetweencoil-

endandcoil-endwires.r

his isdue to the fact that there isactually twice the lengthofcoil between the

endsandonlyhalf fromcenteri

commonwire) to theend.) A quickway todetermine if thesteppermotor isworking is toshort circuit every twopairsand try turning theshaft, wheneverahigherthannormal

resistance is felt, it indicates that thecircuit to theparticularwinding isclosedand that thephase is

working.

Bi larm r

Bipolarmotorshaveasinglewindingperphase.r

hecurrent inawindingneeds tobereversed inorderto

reverseamagneticpole, so thedrivingcircuit must bemorecomplicated, typicallywithan s -

bridgearrangement i howeverthereareseveral off theshelfdriverchipsavailable tomake thisasimple

affair).r

hereare two leadsperphase, nonearecommon.

Static frictioneffectsusingans-bridgehavebeenobservedwithcertaindrive topologies[citt tiu v v w w x w x ].

Becausewindingsarebetterutiliy ed, theyaremorepowerful thanaunipolarmotorof thesameweight.

his isdue to thephysical spaceoccupiedby thewindings. A unipolarmotorhas twice theamount ofwire

in thesamespace, but onlyhalfusedat anypoint in time, hence is

% efficientorapproximately

% of

the torqueoutput available).

houghbipolar ismorecomplicated todrive, theabundanceofdriverchip

means this ismuch lessdifficult toachieve.

An -leadstepper iswound likeaunipolarstepper, but the leadsarenot joined tocommon internally to the

motor.

hiskindofmotorcanbewired inseveral configurations:

nipolar.

Bipolarwithserieswindings.

hisgiveshigher inductancebut lowercurrent perwinding.

Bipolarwithparallel windings.

hisrequireshighercurrent but canperformbetteras thewinding

inductance isreduced.

Bipolarwithasinglewindingperphase.

hismethodwill run themotorononlyhalf theavailable

windings, whichwill reduce theavailable lowspeed torquebut require lesscurrent.


24/46

Higher-phase count stepper motors

Multi-phase stepper motors with many phases tend to have much lower levels of vibration, although the

cost of manufacture is higher. These motors tend to be called 'hybrid' and have more expensive machined

parts, but also higher quality bearings. Though they are more expensive, they do have a higher power

density and with the appropriate drive electronics are actually better suited to the application[citation needed],

however price is always an important factor. Look in your average printer and it will be a hybrid type design,

and these printers are manufactured down to a cost.

Stepper motor drive circuits

Stepper motor performance is strongly dependent on the drive circuit. Torque curves may be extended to

greater speeds if the stator poles can be reversed more quickly, the limiting factor being the winding

inductance. To overcome the inductance and switch the windings quickly, one must increase the drive

voltage. This leads further to the necessity of limiting the current that these high voltages may otherwise

induce.

L/Rdrive circuits

L/R drive circuits are also referred to as constant voltage drives because a constant positive or negative

voltage is applied to each winding to set the step positions. However, it is winding current, not voltage that

applies torque to the stepper motor shaft. The current I in each winding is related to the applied voltage V

by the winding inductance L and the winding resistanceR. The resistance R determines the maximum

current according to Ohm's law I V/R. The inductance L determines the maximum rate of change of the

current in the winding according to the formula for an InductordI/dt

V/L. Thus when controlled by an L/R

drive, the maximum speed of a stepper motor is limited by its inductance since at some speed, the voltage

U will be changing faster than the current I can keep up. In simple terms the rate of change of current is L X

R (e.g. a 10mH inductance with 2 ohms resistance will take 20 ms to reach approx 2/3rds of maximum

torque or around 0.1 sec to reach 99% of max torque). If you need to use the torque at high speeds you

need a large drive voltage with a low resistance/inductance. With an L/R drive it is possible to control a low

voltage resistive motor with a higher voltage drive simply by adding an external resistor in series with each

winding. This will waste power in the resistors, and generate heat. It is therefore considered a low

performing option, albeit simple and cheap.

Chopperdrive circuits

Chopper drive circuits are also referred to as constant current drives because they generate a somewhat

constant current in each winding rather than applying a constant voltage. On each new step, a very high

voltage is applied to the winding initially. This causes the current in the winding to rise quickly since dI/dt

V/L where V is very large. The current in each winding is monitored by the controller, usually by measuring

the voltage across a small sense resistor in series with each winding. When the current exceeds a

specified current limit, the voltage is turned off or "chopped", typically using power transistors. When the

winding current drops below the specified limit, the voltage is turned on again. In this way, the current is


25/46

heldrelativelyconstant foraparticularstepposition.

hisrequiresadditional electronics tosensewinding

currents, andcontrol theswitching, but it allowssteppermotors tobedrivenwithhighertorqueat higher

speeds than/

drives. Integratedelectronics forthispurposearewidelyavailable.

Phasecurrent waveforms

A steppermotor isapolyphaseACsynchronousmotor see

heorybelow), and it is ideallydrivenby

sinusoidal current. A full stepwaveform isagrossapproximationofasinusoid, and is thereasonwhy the

motorexhibitssomuchvibration. Variousdrive techniqueshavebeendeveloped tobetterapproximatea

sinusoidal drivewaveform: thesearehalfsteppingandmicrostepping.

Different drivemodesshowingcoil current ona

-phaseunipolarsteppermotor

Full stepdri e twophaseson)

his is theusual method forfull stepdriving themotor. Bothphasesarealwayson.

hemotorwill have full

rated torque.

Wavedrive

In thisdrivemethodonlyasinglephase isactivatedat a time. It has thesamenumberofstepsas the full

stepdrive, but themotorwill havesignificantly less thanrated torque. It israrelyused.

Hal stepping

henhalfstepping, thedrivealternatesbetween twophasesonandasinglephaseon.

his increases the

angularresolution, but themotoralsohas less torqueapprox

%)at thehalfstepposition

whereonlya

singlephase ison).

hismaybemitigatedby increasing thecurrent in theactivewindingtocompensate.

headvantageofhalfstepping is that thedriveelectronicsneednot change tosupport it.

Mi rostepping


26/46

What is commonly referred to as microstepping is actual "sine cosine microstepping" in which the winding

current approximates a sinusoidal AC waveform. Sine cosine microstepping is the most common form, but

other waveforms are used [1]. Regardless of the waveform used, as the microsteps become smaller, motor

operation becomes more smooth, thereby greatly reducing resonance in any parts the motor may be

connected to, as well as the motor itself. While microstepping appears to make running at very highresolution possible, this resolution is rarely achievable in practice regardless of the controller, due to

mechanical stiction and other sources of error between the specified and actual positions. In professional

equipment gearheads are the preferred way to increase angular resolution.

Step size repeatability is an important step motor feature and a fundamental reason for their use in

positioning. Example: many modern hybrid step motors are rated such that the travel of every Full step

(example 1.8 Degrees per Full step or 200 Full steps per revolution) will be within 3% or 5% of the travel of

every other Full step; as long as the motor is operated within its specified operating ranges. Several

manufacturers show that their motors can easily maintain the 3% or 5% equality of step travel size as step

size is reduced from Full stepping down to 1/10th stepping. Then, as the microstepping divisor numbergrows, step size repeatability degrades. At large step size reductions it is possible to issue many microstep

commands before any motion occurs at all and then the motion can be a "jump" to a new position.

Theory

A step motor can be viewed as a synchronous AC motor with the number of poles (on both rotor and stator)

increased, taking care that they have no common denominator. Additionally, soft magnetic material with

many teeth on the rotor and stator cheaply multiplies the number of poles (reluctance motor). Modern

steppers are of hybrid design, having both permanent magnets and soft iron cores.

To achieve full rated torque, the coils in a stepper motor must reach their full ratedcurrent during each

step. Winding inductance and reverse EMF generated by a moving rotor tend to resist changes in drive

current, so that as the motor speeds up, less and less time is spent at full current thus reducing motor

torque. As speeds further increase, the current will not reach the rated value, and eventually the motor will

cease to produce torque.

Pull-in torque

This is the measure of the torque produced by a stepper motor when it is operated without an acceleration

state. At low speeds the stepper motor can synchronise itself with an applied step frequency, and this pull-

in torque must overcome friction and inertia. It is important to make sure that the load on the motor isfrictional rather than inertial as the friction reduces any unwanted oscillations.

Pull-out torque

The stepper motor pull-out torque is measured by accelerating the motor to the desired speed and then

increasing the torque loading until the motor stalls or "pulls out of synchronism" with the step frequency.

This measurement is taken across a wide range of speeds and the results are used to generate the stepper

motor's dynamic performance curve. As noted below this curve is affected by drive voltage, drive current


27/46

and current switching techniques. It is normally recommended to use a safety factor of between 50% and

100% when comparing your desired torque output to the published pull-out torque performance curve of a

step motor.

Detent torque

Synchronous electric motors using permanent magnets have a remnant position holding torque

(called detent torque or cogging, and sometimes included in the specifications) when not driven electrically.

Soft iron reluctance cores do not exhibit this behavior.

Stepper motor ratings and specifications

Stepper motors nameplates typically give only the winding current and occasionally the voltage and

winding resistance. The rated voltage will produce the rated winding current at DC: but this is mostly a

meaningless rating, as all modern drivers are current limiting and the drive voltages greatly exceed the

motor rated voltage.

A stepper's low speed torque will vary directly with current. How quickly the torque falls off at faster speeds

depends on the winding inductance and the drive circuitry it is attached to, especially the driving voltage.

Steppers should be sized according to published torque curve, which is specified by the manufacturer at

particular drive voltages and/or using their own drive circuitry. It is not guaranteed that you will achieve the

same performance given different drive circuitry, so the pair should be chosen with great care.

Applications

Computer-controlled stepper motors are one of the most versatile forms ofpositioning systems. They are

typically digitally controlled as part of an open loop system, and are simpler and more rugged than closed

loopservo systems.

Industrial applications are in high speedpick and place equipment and multi-axis machine CNC machines

often directly driving lead screws orballscrews. In the field of lasers and optics they are frequently used in

precision positioning equipment such as linear actuators, linear stages, rotation stages, goniometers,

and mirror mounts. Other uses are in packaging machinery, and positioning of valve pilot stages for fluid

control systems.

Commercially, stepper motors are used in floppy disk drives, flatbed scanners, computer

printers, plotters, slot machines, and many more devices.

Some people looking for generators for homemadeWind Turbines found success in using stepper motors

for generating power


28/46

E O ERS

A

raycodeabsoluterotaryencoderwith tracks. At the topcanbeseen thehousing, interrupterdisk, and light

source; at thebottomcanbeseen thesensingelement andsupport components.

A rotaryencoder, alsocalledashaftencoder, isanelectro-mechan icaldevice that converts

theangularpositionofashaft oraxle toananalogordigital code, making it anangletransducer.otary

encodersareused inmanyapplications that requirepreciseshaft unlimitedrotationincluding industrial

controls, robotics, special purposephotographic lenses[ ], computer input devices suchas

optomechanicalmiceandtrackballs), androtatingradarplatforms.

hereare twomain types: absoluteand

incrementalrelative).

Contents

[hi j

1 Absolk t l rotm ry encoder

o 1.1Construction

o 1.2Mechanical absolute encoders

o 1.3 Optical absolute encoders


29/46

o 1.4 Standard binary encoding

o 1.5 Gray encoding

o 1.6 Single-track Gray encoding

o 1.7 Encoder output formats

2 Incremental rotary encoder

3 Incremental versus absolute encoderterminology

o 3.1 Traditional absolute encoders

o 3.2 Traditionalincremental encoders

o 3.3Battery backed incremental encoders

4 Sine wave encoder

5 Use in industry

o 5.1 Encoders used on servomotors

6 Encodertechnologies

7 See also

8 Externallinks

9References

Absoluterotaryencoder

Absoluterotaryencodern

o

D

Construction

Absolutedigital typeproducesauniquedigital code foreachdistinct angleof theshaft.

heycome in twobasic types: optical andmechanical.

Mechanical absoluteencoders

A metal disccontainingaset ofconcentricringsofopenings is fixed toan insulatingdisc, which isrigidly

fixed to theshaft. A rowofslidingcontacts is fixed toastationaryobject so that eachcontact wipesagainst

themetal discat adifferent distance from theshaft. As thediscrotateswith theshaft, someof thecontacts


30/46

touchmetal, whileothers fall in thegapswhere themetal hasbeencut out.

hemetal sheet isconnected

toasourceofelectriccurrent, andeachcontact isconnected toaseparateelectrical sensor.

hemetal

pattern isdesignedso that eachpossiblepositionof theaxlecreatesauniquebinarycode inwhichsome

of thecontactsareconnected to thecurrent source i.e. switchedon)andothersarenot i.e. switchedoff).

Optical absoluteencoders

heoptical encoder'sdisc ismadeofglassorplasticwith transparent andopaqueareas. A light source

andphotodetectorarrayreads theoptical pattern that results from thedisc'spositionat anyone time.

hiscodecanbereadbyacontrollingdevice, suchasamicroprocessor, todetermine theangleof the

shaft.

heabsoluteanalog typeproducesauniquedual analogcode that canbe translated intoanabsolute

angleof theshaftbyusingaspecial algorithm).

Standardbinaryencoding

otaryencoder forangle-measuringdevicesmarked in

-bit binary. he innerringcorresponds toContact in the

table. Blacksectorsarez

on".{

erodegrees ison theright-handside, withangle increasingcounterclockwise.

Anexampleofabinarycode, inanextremelysimplifiedencoderwithonly threecontacts, isshownbelow.

Standard Binary Encoding

Sector Contact 1 Contact 2 Contact 3 Angle

1 off off off 0 to 45

2 off off ON 45 to 90


31/46

3 off ON off 90 to 135

4 off ON ON 135 to 180

5 ON off off 180 to 225

6 ON off ON 225 to 270

7 ON ON off 270 to 315

8 ON ON ON 315 to 360

In general, where there are n contacts, the number of distinct positions of the shaft is 2n. In this

example, n is 3, so there are 2 or 8 positions.

In the above example, the contacts produce a standard binary count as the disc rotates. However, this has

the drawback that if the disc stops between two adjacent sectors, or the contacts are not perfectly aligned,

it can be impossible to determine the angle of the shaft. To illustrate this problem, consider what happens

when the shaft angle changes from 179.9 to 180.1 (from sector 4 to sector 5). At some instant, according

to the above table, the contact pattern changes from off-on-on to on-off-off. However, this is not what

happens in reality. In a practical device, the contacts are never perfectly aligned, so each switches at a

different moment. If contact 1 switches first, followed by contact 3 and then contact 2, for example, theactual sequence of codes is:

off-on-on (starting position)

on-on-on (first, contact 1 switches on)

on-on-off (next, contact 3 switches off)

on-off-off (finally, contact 2 switches off)

Now look at the sectors corresponding to these codes in the table. In order, they are

4, 8, 7 and then 5. So, from the sequence of codes produced, the shaft appears to

have jumped from sector 4 to sector 8, then gone backwards to sector 7, thenbackwards again to sector 5, which is where we expected to find it. In many

situations, this behaviour is undesirable and could cause the system to fail. For

example, if the encoder were used in a robot arm, the controller would think that the

arm was in the wrong position, and try to correct the error by turning it through 180,

perhaps causing damage to the arm.

Gray encoding


32/46

|

otaryencoder forangle-measuringdevicesmarked in}-bit binary-reflected

~raycode

B|

~C).

he innerringcorresponds toContact

in the table. Blacksectorsare"on".

erodegrees ison theright-handside, withangle increasinganticlockwise.

oavoid theaboveproblem, rayencoding isused.

his isasystemofbinary

counting inwhichadjacent codesdiffer inonlyoneposition. orthe three-contact

examplegivenabove, theray-codedversionwouldbeas follows.

Gray Coding

Sector Contact 1 Contact 2 Contact 3 Angle

1 off off off 0 to 45

2 off off ON 45 to 90

3 off ON ON 90 to 135

4 off ON off 135 to 180

5 ON ON off 180 to 225

6 ON ON ON 225 to 270

7 ON off ON 270 to 315


33/46

8 ON off off 315 to 360

In thisexample, the transition fromsector

tosector, likeall othertransitions,

involvesonlyoneof thecontactschanging itsstate fromon toofforviceversa.

his

means that thesequenceof incorrect codesshown in theprevious illustration

cannot happen.

Single-track Grayencoding

If thedesignermovesacontact toadifferent angularposition but at thesame

distance from thecentershaft), then thecorresponding"ringpattern"needs tobe

rotated thesameangle togive thesameoutput. If themost significant bitthe inner

ring in igure ) isrotatedenough, it exactlymatches thenext ringout. Sinceboth

ringsare then identical, the innerringcanbeomitted, and thesensorforthat ring

moved to theremaining, identical ring

but offset at that angle from theothersensoron that ring).

hose twosensorsonasingleringmakeaquadratureencoder.

ormanyyears,

orsten Sillkeandothermathematiciansbelieved that it was

impossible toencodepositiononasingle trackso that consecutivepositionsdiffered

at onlyasinglesensor, except for the two-sensor, one-trackquadratureencoder.

owever, in

. B. Speddingregisteredapatent

Patent

)showing it

waspossiblewithseveral examples. SeeSingle-track raycode fordetails.

Encoderoutputformats

Incommercial absoluteencoders thereareseveral formats fortransmissionofabsoluteencoderdata, includingparallel binary, SSI, "BiSS", ISI, Profibus, CA

DeviceNet, CANopen, EndatandHiperface, dependingon themanufacturerof the

device

Incremental rotaryencoder

Encoder

D


34/46

An incremental rotary encoder, also known as a quadrature encoder or a relative

rotary encoder, has two outputs called quadrature outputs. They can be either

mechanical or optical. In the optical type there are two gray coded tracks, while the

mechanical type has two contacts that are actuated by cams on the rotating shaft.

The mechanical type requires debouncing and is typically used as digitalpotentiometers on equipment including consumer devices. Most modern home and

car stereos use mechanical rotary encoders for volume. Due to the fact the

mechanical switches requiredebouncing, the mechanical type are limited in the

rotational speeds they can handle. The incremental rotary encoder is the most

widely used of all rotary encoders due to its low cost: only two sensors are required.

The fact that incremental encoders use only two sensors does not compromise their

accuracy. One can find in the market incremental encoders with up to 10,000 counts

per revolution, or more.

There can be an optional third output: reference, which happens once every turn.

This is used when there is the need of an absolute reference, such as positioning

systems.

The optical type is used when higher RPMs are encountered or a higher degree of

precision is required.

Incremental encoders are used to track motion and can be used to determine

position and velocity. This can be either linear or rotary motion. Because the

direction can be determined, very accurate measurements can be made.

They employ two outputs called A & B which are called quadrature outputs as they

are 90 degrees out of phase.

The state diagram:

Gray coding for

clockwise rotation

Phase A B

1 0 0

2 0 1

Gray coding for

counter-clockwise rotation

Phase A B

1 1 0

2 1 1


35/46


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Traditional absolute encoders

Traditional absolute encoders have multiple code rings with various binary

weightings which provide a data word representing the absolute position of the

encoder within one revolution. This type of encoder is often referred to as a parallel

absolute encoder. The distinguishing feature of the absolute encoder is that itreports the absolute position of the encoder to the electronics immediately upon

power-up with no need for indexing. [2]

Traditional incremental encoders

A traditional incremental encoder works differently by providing an A and a B pulse

output which provide no usable count information in their own right. Rather, the

counting is done in the external electronics. The point where the counting begins

depends on the counter in the external electronics and not on the position of the

encoder. To provide useful position information, the encoder position must be

referenced to the device to which it is attached, generally using an index pulse. The

distinguishing feature of the incremental encoder is that it reports an incremental

change in position of the encoder to the counting electronics.[2] Encoder

manufacturerPrecizika Metrology

Battery backed incremental encoders

Some encoder manufacturers, such as Fanuc, have taken a different approach to

this terminology. These manufacturers use absolute as their terminology for

incremental encoders with a battery backed up memory to store count information

and provide an absolute count immediately upon power up.[2]

Sine wave encoder

A variation on the Incremental encoder is the Sinewave Encoder. Instead of

producing two quadrature square waves, the outputs are quadrature sine waves (a

Sine and a Cosine). By performing the arctangent function, arbitrary levels of

resolution can be achieved.

Use in industry

Encoders used on servomotorsRotary encoders are often used to track the position of the motor shaft on

permanent magnet brushless motors, which are commonly used

on CNC machines, robots, and other industrial equipment. Incremental (Quadrature)

encoders are used on Induction Motor type servomotors, but absolute encoders are

used in Permanent Magnet Brushless Motors, where applicable. In these

applications, the feedback device (encoder) plays a vital role in ensuring that the


37/46

equipment operatesproperly.

heencodersynchroni es therelativerotormagnet

andstatorwindingpositions to thecurrent providedby thedrive. aximumtorque

results if thecurrent isapplied to thewindingswhen therotormagnetsare ina

particularpositionrangerelative to thestatorwindings.

hemotorwill perform

poorlyornot at all if this timing isnot adjustedcorrectly. Improperencoderalignmenton themotorcanactuallycause it torunbackwardssometimesresulting ina

hazardousrunawaycondition. Correct alignment isabsolutelyessential toproper

operationof thesemotors.[ ]

Encodertechnologies

Hall-effect quadratureencoder, sensinggear teethon thedriveshaft ofarobot vehicle.

Encodersmaybe implementedusingavarietyof technologies:

Conductive tracks. A seriesofcopperpadsetchedontoa PCB isused to

encode the information. Contact brushessense theconductiveareas.

his form

ofencoder isnowrarelyseen.

-ptical.

hisusesa light shiningontoaphotodiode throughslits inametal or

glassdisc.eflectiveversionsalsoexist.

his isoneof themost common

technologies.

agnetic. Stripsofmagnetisedmaterial areplacedon therotatingdiscandare

sensedbyaHall-effectsensorormagnetoresistivesensor. Hall effect sensors

arealsoused tosensegearteethdirectly, without theneed foraseparate

encoderdisc.


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Linear encoder

linear encoder is a sensor, transducer or readhead paired with a scale that encodes position.

The sensor reads the scale in order to convert the encoded position into an analog ordigital

signal, which can then be decoded into position by a digital readout (DRO) or motion

controller. The encoder can be eitherincrementalorabsolute. Motion can be determined bychange in position over time. Linear encoder technologies include optical, magnetic,

inductive, capacitive and eddy current. Optical technologies include shadow, self imaging

and interferometric. Linear encoders are used in metrology instruments and high precision

machining tools ranging from digital calipers to coordinate measuring machines.

Contents

[hide]

y 1 Physical principle

o 1.1 Opticalo 1.2 Magnetic

o 1.3 Inductiveo 1.4 Capacitive

o 1.5 Eddy currenty 2 Applications

o 2.1 Measuremento 2.2 Motion systems

y 3 Output formato 3.1 Analogue

3.1.1 Incremental signals 3.1.2 Reference mark

o 3.2 Digitalo 3.3 Limit switches

y 4 Physical arrangement / protectiony 5 Encoder termsy 6 Booky 7 See also

Physical principle

Linear encoders are transducers that employ many differentphysical properties in order to

encode position.

Optical

Optical linear encoders [1] dominate the high resolution market and may employ shuttering /Moir, diffraction orholographic principles. Typical incremental scale periods vary from

hundreds down to a few microns and following interpolation can provide resolutions as fine


39/46

as a nanometre. Light sources used include infrared LEDs, visible LEDs, miniature light-bulbs and laser diodes.

Magnetic

Magnetic linear encoders [2] employ either active (magnetized) or passive (variable

reluctance) scales and position may be sensed using sense-coils, Hall Effect ormagnetoresistive readheads. With coarser scale periods than optical encoders (typically a few

hundred microns to several millimeters) resolutions in the order of a micron are the norm.

Inductive

A popular application of the inductive measuring principle is the Inductosyn [3]. In effect it is

a resolverunwound into a linear system.

The Spherosyn encoder[4] is based on the principle of electromagnetic induction and uses

coils to sense nickel-chrome ball-bearings mounted within a tube.

Capacitive

Capacitive linear encoders work by sensing the capacitance between a reader and scale.Typical applications are digital calipers.

Eddy current

US Patent 3820110, "Eddy current type digital encoder and position reference", gives an

example of this type of encoder, which uses a scale coded with high and low permeability,

non-magnetic materials, which is detected and decoded by monitoring changes in inductance

of an AC circuit that includes an inductive coil sensor. Maxon [5] makes an example (rotaryencoder) product (the MILE encoder).

Applications

There are two main areas of application for linear encoders:-

Measurement

Measurement application include coordinate-measuring machines (CMM), laser scanners,

calipers, gear measurement [6], tension testers [7] and Digital read outs (DROs).

Motion systems

Servo controlled motion systems employ linear encoder so as to provide accurate, high-speed

movement. Typical applications include robotics, machine tools,pick-and-place PCB

assembly equipment; semiconductors handling and test equipment, wire bonders, printers and

digital presses.[8]

Output format


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Linear encoders either have analogue or digital outputs.

Analogue

Incremental signals

The industry standard, analogue output forlinear encoders is sine and cosine quadraturesignals.

The sine and cosine outputs.

These are usually transmitted differentially so as to improve noise immunity. An early

industry standard was 12 A peak-peak current signals but more recently this has been

replaced with 1V peakto peak voltage signals.Compared to digitaltransmission, the

analogue signals' lower bandwidth helps to minimise emc emissions.Quadrature sine/cosine signals can be monitored easily by using an oscilloscope in XY mode

to display a circularLissajous Figure. Highest accuracy signals are obtained ifthe Lissajous

Figure is circular (no gain or phase error) and perfectly centred.Modern encoder systems

employ circuitry to trim these error mechanisms automatically. The overall accuracy ofthe

linear encoderis a combination ofthe scale accuracy and errors introduced by the readhead.

Scale contributions to the error budgetinclude linearity and slope (scaling factor error).

Readhead error mechanisms are usually described ascyclic errororsub-divisionalerror

(SDE) as they repeat every scale period. The largest contributorto readhead inaccuracy is

signal offset, followed by signalimbalance (ellipticity) and phaseerror (the quadraturesignals not being exactly 90 apart). Overall signal si e does not affect encoder accuracy,

however, signal-to-noise and jitter performance may degrade with smaller signals. Automaticsignal compensation mechanisms can includeautomatic offset compensation (AOC),

automatic balance compensation (ABC) and automatic gain control (AGC). Phase is moredifficultto compensate dynamically and is usually applied as one time compensation during

installation or calibration. Other forms ofinaccuracy include signal distortion (frequentlyharmonic distortion ofthe sine/cosine signals).

Reference mark


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Mostincremental, linear encoders can produce an index or reference mark pulse providing adatum position along the scale for use at power-up or following a loss of power. This index

signal must be able to identify position within one, unique period ofthe scale. The referencemark may comprise a single feature on the scale, an autocorrelator pattern (typically aBarker

code) or a chirp pattern.Distance coded reference marks (DCRM) are placed onto the scale in a unique pattern

allowing a minimal movement (typically movingpasttwo reference marks) to define thereadhead's position.Multiple, equally spaced reference marks may also be placed onto the

scale such that following installation, the desired marker can be selected - usually via amagnet or optically.

Digital

The A and B quadrature channels.

Many linear encoders interpolate the analogue sine/cosine signals in orderto sub-divide the

scale period, providing a higher measurementresolution. The output ofthe interpolation

process are quadrature squarewaves - the distance between edges ofthe two channels being

the resolution ofthe encoder. The reference mark orindex pulse will also processed digitally

and will be a pulse, usually one to four units-of-resolution wide.

The major advantage of encoders with built-in interpolation and digital signaltransmission is

improved noise immunity. However, the high frequency, fast edge speed signals may produce

more emc emissions.

Incremental encoders with built-in digital processing make it possible to transmit position toany subsequent electronics in a position counterin included in the logic.As well as traditionalincremental, linear encoders, linear encoders may beabsolutein nature.

With suitably encoded scales (multitrack, vernier or pseudorandom) an encoder candetermine its position without movement or needing to find a reference position. Such

absolute encoders also communicate using serial communication protocols.Many oftheseprotocols are proprietry - Fanuc, Mitsubishi, EnDat, DriveCliq, Panasonic,

Yaskawa - but open standards such as BiSS [9] are now appearing, which avoid tying users toa particular supplier.

Limit switches

Many linear encoders include built-in limit switches - either optical or magnetic. Two limit

switches are frequently included such that on power-up the controller can determine ifthe

encoderis at an end-of-travel and in which direction to drive the axis.

Physical arrange

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