Special Motor Drive Systemp yBy:

Dr. Rosmiwati Mohd-Mokhtar

Stepper motors are special motors that are used when motionand position have to be precisely controlled.

It is a digital electro-mechanical device where each stepcommand pulse (electrical pulse input) results in movement ofthe shaft by a discrete angle called step angle of the motor.

By varying the pulse rate, the motor can be made to advancevery slowly, one step at a time, or to rotate stepwise (clockwise& counter clockwise) at speeds as high as 4000 rpm& counter clockwise) at speeds as high as 4000 rpm.

The behavior of a stepper motor depends greatly upon thepower supply that drives it.power supply that drives it.

Application: Precise positioning device in machine tools, X-Yplotters, typewriters, tape-decks, valves, robotics and printers.

Simple stepper motor in which each step moves the rotor by 60° is shown below.

Stepper motor can be divided into 3 categories:

Permanent magnet stepper motor that advances 30° per step

Stepper motor can be divided into 3 categories:1. Variable reluctance – single-stack & multi-stack2. Permanent magnet3. Hybrid y

Hybrid motor having a 4-pole stator and two 5-pole armatures mounted on the same shaft. The salient poles on the first armature are all N poles, while those on the second armature are all S poles.

Each step produces an advance of 18°

Advantages:1. Compatible with digital systems and do not require D/A conversion at the

input.2. Simple open-loop control is good enough to control position and speed.

Wide range of step angles is available (1 8 to 90) The range of torque is from3. Wide range of step angles is available (1.8 to 90). The range of torque is from1 Nm (tiny wristwatch motor) to 50 Nm (machine tools application).

4. Bidirectional control is available.5. Max torque occurs at low pulse rate. Thus, stepper motor can accelerate its

load easily.6. Low speeds are possible without a reduction gear.7. Moment of inertia is usually low.8. Starting current is low.9. Multiple stepper motors driven from the same source can maintain perfect

synchronisationsynchronisation.

Disadvantages:1. Efficiency is low.2 Proper matching between load motor and its drive is required2. Proper matching between load, motor and its drive is required.3. Resonance can be a problem with variable reluctance motors.

Torque developed by a stepper motor depends upon current.The torque that the motor can exert while moving from oneposition to the next is called pull-over torque.

As the stepping rate is increased, rotor has less time to drivethe load from one position to the nextthe load from one position to the next.

Beyond certain pulsing rate, rotor cannot follow the commandand begins to miss pulses.g p

In order to maintain synchronism, therotor must settle down before advancingt th t itito the next position.

If the value of load torque and pulsing rateare such that the point of operation lies toare such that the point of operation lies tothe left of curve 1, then the motor canstart & also synchronise without missing apulse.pulse.

Say for torque 2.2 Nm, the maximum possible stepping rate,without losing count, is 500 steps per sec. Beyond that, motor willl h ilose synchronism.

If the motor drives a device having some inertia, the permissiblestart stop rate drops to about 400 steps per sec for the same loadstart-stop rate drops to about 400 steps per sec for the same loadtorque (curve 2).

Area between curve 1 and 3 or curve 2 and 3 represents variousl h f i ll bl i h l itorque values, the range of stepping rate allowable without losing

step provided that it has already been started and synchronisedbefore.

Area is known as slew range and motor is said to operate inslewing mode.

Slew range is useful for speed control but not suitable for positioncontrol.

To start/stop the motor, it has to be graduallyaccelerated/decelerated. This process is known as ramping.

A stepper motor is usually driven from a low voltage dc source.

In order to maximize torque capability of a step motor driveIn order to maximize torque capability of a step motor, drivecircuit should be such that the current builds up and decay asfast as possible.

This is important especially when high stepping rates arerequired.

The drive circuits are designed to incorporate this requirement.

h l b l d hStepper motors use either a unipolar or bipolar winding on thestator.

Unipolar winding consists of two coils per pole (4-pole stator).

U i l th t th t i i di l fl i thUnipolar means that the current in a winding always flow in thesame direction.

Th i i li h l fThe transient response is slightly faster.

The pull-out curve corresponds toThe pull out curve corresponds tothe slewing characteristics; the pull-in curve corresponds to the start-without-error characteristic.

a) Coil arrangement in a 4-pole unipolar winding. b) Schematic diagram of coils,switches, and power supply in a unipolar drive. c) Current pulses in a wave driveusing a unipolar winding.

When switch s1 is closed, phase A winding is connected to the dcsource Vd and the phase current builds up.

When it is opened, the phase current decays in the freewheeling pathconsist of phase A, DF and RF.

The external resistor RE reduces the electrical constant therebyThe external resistor RE reduces the electrical constant, therebyspeeding up the current build-up.

Value of external resistor RE is chosen to fix the value of the electricalti t t d th th lt V i h t d thtime constant and then the source voltage Vd is chosen to produce therated current IR in the phase winding.

RRIV pERd RRIV

Rp – phase winding resistance

During the ON period of the switch, phase current also flowsthrough the external resistor RE causing energy dissipated.

Also, energy stored in the phase winding inductance during ONgy p g gperiod is all dissipated in free-wheeling circuit resistance whenthe switch is turned OFF.

Thus, unipolar circuit is highly inefficient, and only suitable forlow power stepper motors.

By using chopper principle, the unipolar drive circuit is capableon providing fast current build-up and decay.

In a 4-pole stator, the bipolar winding consists of two coils sets.

The coils can be excited sequentially in three different ways:1. Wave drive2. Normal drive2. Normal drive3. Half-step drive

Wave drive – one set of coils is excited at a time.a e d e o e set o co s s e c ted at a t e

Normal drive – both sets of coils are excited at a time.

Half-step drive – obtained by combining the wave drive and the normal drive.

The pull-out curve corresponds to the slewing characteristics; the pull-

in curve corresponds to the start-without-error characteristic.

Schematic diagram showing how the stator coils A1, A2 and B1, B2 are connected to

the common dc source by means of switches Q1 to Q8. The dc source is shownswitches Q1 to Q8. The dc source is shown twice to simplify the connection diagram.

Step 1 2 3 4 1

Q1 Q2 On OnQ1 Q2 On - - - On

Q3 Q4 - - On - -

Q5 Q6 - On - - -

Q7 Q8 - - - On -

Bipolar circuit Bipolar circuit –– Normal DriveNormal Drive

Step 1 2 3 4 1

Q1 Q2 On On OnQ1 Q2 On - - On On

Q3 Q4 - On On - -

Q5 Q6 On On - - On

Q7 Q8 - - On On -

Step 1 2 3 4 5 6 7 8 1

Q1 Q2 On On - - - - - On On

Q3 Q4 - - - On On On - - -

Q5 Q6 - On On On - - - - -

Q7 Q8 - - - - - On On On -

The phase winding carries a positive current when semiconductorswitches s1 and s2 conduct and it carries negative current when s3 ands4 conduct.

The external resistance reduces the electrical time constant allowingrapid build-up of phase current.

The phase is de-energised by turning off s1 and s2. Winding currentnow flows through the path consisting of D3, source Vd and D4.

Major proportion of energy stored in phase winding inductance is fedMajor proportion of energy stored in phase winding inductance is fedback to the source and phase current decays rapidly to zero.

Also inefficient but better thani lunipolar.

It becomes efficient when chopperprinciple is employed to the circuit.p p p y

To avoid using the external resistor, the bilevel drives is employedwhere it enables to obtain fast rise and fall times of current.

a) Circuit of a bilevel drive when current in coil is zero b) Equivalent circuit when current ina) Circuit of a bilevel drive when current in coil is zero. b) Equivalent circuit when current in coil is increasing. c) Rate of increase of current and time to reach 10 A. d) Equivalent circuit

when current in coil is constant. E) Equivalent circuit when current in coil is decreasing.

The pull out curve corresponds to the slewingcharacteristic.

The pull-in curve corresponds to the start-without-error (start-stop) characteristic.

Power developed,55.9nP

Time constant,RLT 0

Torque when device have moment of inertia,t

nJ

559

t55.9

A stepper motor has a unipolar winding. Itoperates in the start-stop mode at a pulserate of 150 per second.

) What is the maximum torque it can develop?a) What is the maximum torque it can develop?b) How much mechanical power does it

develop?develop?c) How much mechanical energy [J] does it

produce in 3 seconds?pd) What is the max slew speed in rpm?e) Calculate the time constant of when it is

rated at 12 V.

a) At a pulse rate of 150 per second, the pull-incurve corresponds to the start-stop mode, max =30 N30 mNm.

b) From specification, step angle = 7.5.

Speed (rpm), rpm 5.18760360

5.7150 n1 sec 150 pulse60 60/1*15060 sec 60/1*150

= 9000 pulse

360 (1 rotation) 9000 pulse7.5 7.5/360*9000

55.9

nPm

7.5 7.5/360 9000

W59.055.9

10305.18755.9

3

nPm

c) The mechanical energy produce in 3 seconds

d) From the pull out graph, max slew speed

J 77.1359.0 tPW m

p g p p= 350 pps

I i i 5.7In rotation per minute,

e) Time constant

rpm5.43760360

5.7350 n

e) Time constant,

sec10938108.46 43

LT sec1093.8

4.520 R

T