Cycle time calculation-unit_4_sensors_and_actuators.pdf

7/27/2019 Cycle time calculation-unit_4_sensors_and_actuators.pdf

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Unit 4 Sensors and Actuators

Assigned Core Text Reading for this Unit:Groover, M. P. (2008),Automation, Production Systems, and Computer-Integrated Manufacturing, 3rd ed., Chapter 6.

4.1 Unit Introduction4.2 Unit Learning Objectives4.3 Sensors4.4 Actuators4.5 Analogue-to-Digital Converters4.6 Digital-to-Analogue Converters4.7 Input/Output Devices for Discrete Data4.8 Unit Review4.9 Self-Assessment Questions4.10 Self-Assessment Answers

Section 4.1 Unit Introduction

To achieve the goals of automation and process control, the computer mustcollect data from and transmit signals to the production process. This is done byusing hardware components that act as intermediaries between the controlsystem and the process itself. In the last unit process variables and parameterswere defined as being either continuous or discrete. The control computer tendsto use digital discrete (binary) data, however some of the data from the processmay be continuous and analogue. Therefore we must have some way to

accommodate this within the system, so that analogue data can be read in adigital format, and vice-versa. The main components that are required to supportthe interface between the controller and the process are:

BULLETLISTSensors for measuring continuous and discrete process variables

Actuators that drive continuous and discrete process parameters

Analogue to digital converters that convert continuous signals into binary

Digital to analogue converters that convert digital data into analogue signals

Input/output devices for discrete dataENDLIST

Figure 4.1 illustrates the relationship between some of the major components.This unit explores each of these main components in turn.


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Actuators

Computer

Controller

Transformation Process

Sensors

DAC ADC

Input DevicesOutput Devices

Continuous and Discrete

VariablesContinuous and Discrete

Parameters

Figure 4.1: Major components linking control to process

Section 4.2 Unit Learning Objectives

After completing this unit you will be able to:

BULLET LISTClassify sensors as analogue or discrete

Specify desirable traits of sensors

Define actuators and specify the different types available

Specify the operating conditions of DC, AC and stepper electrical motors

Outline other types of actuators, other than electrical

Outline the steps of the analogue-to-digital conversion process

Outline the steps of the digital-to-analogue conversion process

Show how discrete data is handled by computing systemsENDLIST

Section 4.3 Sensors

A sensor allows for the transformation of a signal, or other physical variable, fromone form to anothergenerally into a form that can be utilised more efficiently bythe system that deploys the sensor. In this sense, a sensor is what is termed as atransducer; that is, it translates a physical variable from a form that cannot beread by the process, to one which allows it to be interrogated successfully.


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Generally, within the manufacturing process, the sensor collects different types ofprocess data for feedback control.

KEYPOINTA sensor is a transducer that allows for the transformation of a signal, or other

physical variable, from one form to another.END KEYPOINT

Different sensors types are outlined in Table 4.1; they can generally be classifiedaccording to the category of stimulus or physical variable they are required tomeasure.

Table 4.1: Sensor categories by stimulusStimulus Example

Mechanical Positional variables, velocity, acceleration, force, torque, pressure, stress,strain, mass, density

Electrical Voltage, current, charge, resistance, conductivity, capacitance

Thermal Temperature, heat, heat flow, thermal conductivity, specific heatRadiation Type of radiation (e.g. gamma rays, x-rays, visible light), intensity, wavelengthMagnetic Magnetic field, flux, conductivity, permeabilityChemical Component identities, concentration, pH levels, presence of toxic ingredients,

pollutants

KEYPOINTSensors can be classified according to the category of stimulus or physicalvariable they are required to measure; these include stimulus that aremechanical, electrical, thermal, radiation, magnetic, and chemical in kind.END KEYPOINT

Sensors may also be classified as analogue or discrete. A sensor that isanalogue in operation produces a continuous analogue signal whose valuevaries in an analogous manner with the variable being measured. A sensor thatis discrete produces an output that can only have certain values. The twodiscrete sensor types are binary and digital. A binary device produces one of twovalues, for example on/off. A digital device produces a digital output signal aseither a set of parallel bits; or a series of pulses that can be quantified. In bothcases the digital signal represents the quantity to be measured.

Digital sensors are becoming more common owing to their compatibility tocomputing systems, and their relative ease-of-use. A new development in sensor

technology is the emergence of micro-sensors, tiny sensors only a few microns insize that are usually fabricated out of silicon. A list of common sensors andmeasuring devices used in automation, together with explanations or sources offurther information is given in Table 4.2.

Table 4.2: Common measuring devices and sensors used in automationDevice Explanation or further information:

Accelerometer http://en.wikipedia.org/wiki/Accelerometer
http://en.wikipedia.org/wiki/Accelerometerhttp://en.wikipedia.org/wiki/Accelerometerhttp://en.wikipedia.org/wiki/Accelerometer


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Ammeter http://en.wikipedia.org/wiki/AmmeterBimetallicswitch

Switch based on bimetallic strip:http://en.wikipedia.org/wiki/Bi-metallic_strip

Bimetallicthermometer

Thermometer based on bimetallic strip:http://en.wikipedia.org/wiki/Bi-metallic_strip

Dynamometer http://en.wikipedia.org/wiki/Dynamometer

Floattransducer Lever arm with float attached, used to measure liquid levels in vessel(analogue), or to active control switch (binary)Fluid flowsensor

http://en.wikipedia.org/wiki/Flow_sensor

Fluid flowswitch

http://en.wikipedia.org/wiki/Sail_switch

Linear variabledifferentialtransformer

http://en.wikipedia.org/wiki/Linear_variable_differential_transformer

Limit switch http://en.wikipedia.org/wiki/Limit_switchManometer http://en.wikipedia.org/wiki/Manometer Ohmmeter http://en.wikipedia.org/wiki/OhmmeterOptical encoder http://en.wikipedia.org/wiki/Rotary_encoder#Optical_Absolute_Encoders

Photoelectricsensor array http://en.wikipedia.org/wiki/Photoelectric_sensor

Photoelectricswitch

http://en.wikipedia.org/wiki/Photoelectric_sensor

Photometer http://en.wikipedia.org/wiki/PhotometerPiezoelectrictransducer

http://en.wikipedia.org/wiki/Piezoelectric_transducer

Potentiometer http://en.wikipedia.org/wiki/PotentiometerProximityswitch

http://en.wikipedia.org/wiki/Proximity_switch

Radiationpyrometer

http://en.wikipedia.org/wiki/Radiation_pyrometer

Resistance-

temperaturedetector

http://en.wikipedia.org/wiki/Resistance_thermometer

Strain gauge http://en.wikipedia.org/wiki/Strain_gageTachometer http://en.wikipedia.org/wiki/TachometerTactile sensor Measuring device that indicates physical contact between two objects; see

also:http://en.wikipedia.org/wiki/List_of_sensors Thermistor http://en.wikipedia.org/wiki/ThermistorThermocouple http://en.wikipedia.org/wiki/ThermocoupleUltrasonicrange sensor

http://en.wikipedia.org/wiki/Ultrasonic_sensor

KEYPOINTSensors and measuring devices come in a wide variety of forms, based uponmany different operative principles, from analogue and discrete, to thermal,electrical, mechanical, magnetic and radiation devices.END KEYPOINT

Sensors may also be active or passive. An active sensor responds to thestimulus without the need for any external powerin other words, it alreadypossesses the capability of powering itself and its hardware components,
http://en.wikipedia.org/wiki/Ammeterhttp://en.wikipedia.org/wiki/Ammeterhttp://en.wikipedia.org/wiki/Bi-metallic_striphttp://en.wikipedia.org/wiki/Bi-metallic_striphttp://en.wikipedia.org/wiki/Bi-metallic_striphttp://en.wikipedia.org/wiki/Bi-metallic_striphttp://en.wikipedia.org/wiki/Bi-metallic_striphttp://en.wikipedia.org/wiki/Dynamometerhttp://en.wikipedia.org/wiki/Dynamometerhttp://en.wikipedia.org/wiki/Flow_sensorhttp://en.wikipedia.org/wiki/Flow_sensorhttp://en.wikipedia.org/wiki/Sail_switchhttp://en.wikipedia.org/wiki/Sail_switchhttp://en.wikipedia.org/wiki/Linear_variable_differential_transformerhttp://en.wikipedia.org/wiki/Linear_variable_differential_transformerhttp://en.wikipedia.org/wiki/Limit_switchhttp://en.wikipedia.org/wiki/Limit_switchhttp://en.wikipedia.org/wiki/Manometerhttp://en.wikipedia.org/wiki/Manometerhttp://en.wikipedia.org/wiki/Ohmmeterhttp://en.wikipedia.org/wiki/Ohmmeterhttp://en.wikipedia.org/wiki/Rotary_encoder#Optical_Absolute_Encodershttp://en.wikipedia.org/wiki/Rotary_encoder#Optical_Absolute_Encodershttp://en.wikipedia.org/wiki/Photoelectric_sensorhttp://en.wikipedia.org/wiki/Photoelectric_sensorhttp://en.wikipedia.org/wiki/Photoelectric_sensorhttp://en.wikipedia.org/wiki/Photoelectric_sensorhttp://en.wikipedia.org/wiki/Photometerhttp://en.wikipedia.org/wiki/Photometerhttp://en.wikipedia.org/wiki/Piezoelectric_transducerhttp://en.wikipedia.org/wiki/Piezoelectric_transducerhttp://en.wikipedia.org/wiki/Potentiometerhttp://en.wikipedia.org/wiki/Potentiometerhttp://en.wikipedia.org/wiki/Proximity_switchhttp://en.wikipedia.org/wiki/Proximity_switchhttp://en.wikipedia.org/wiki/Radiation_pyrometerhttp://en.wikipedia.org/wiki/Radiation_pyrometerhttp://en.wikipedia.org/wiki/Resistance_thermometerhttp://en.wikipedia.org/wiki/Resistance_thermometerhttp://en.wikipedia.org/wiki/Strain_gagehttp://en.wikipedia.org/wiki/Strain_gagehttp://en.wikipedia.org/wiki/Tachometerhttp://en.wikipedia.org/wiki/Tachometerhttp://en.wikipedia.org/wiki/List_of_sensorshttp://en.wikipedia.org/wiki/List_of_sensorshttp://en.wikipedia.org/wiki/List_of_sensorshttp://en.wikipedia.org/wiki/Thermistorhttp://en.wikipedia.org/wiki/Thermistorhttp://en.wikipedia.org/wiki/Thermocouplehttp://en.wikipedia.org/wiki/Thermocouplehttp://en.wikipedia.org/wiki/Ultrasonic_sensorhttp://en.wikipedia.org/wiki/Ultrasonic_sensorhttp://en.wikipedia.org/wiki/Ultrasonic_sensorhttp://en.wikipedia.org/wiki/Thermocouplehttp://en.wikipedia.org/wiki/Thermistorhttp://en.wikipedia.org/wiki/List_of_sensorshttp://en.wikipedia.org/wiki/Tachometerhttp://en.wikipedia.org/wiki/Strain_gagehttp://en.wikipedia.org/wiki/Resistance_thermometerhttp://en.wikipedia.org/wiki/Radiation_pyrometerhttp://en.wikipedia.org/wiki/Proximity_switchhttp://en.wikipedia.org/wiki/Potentiometerhttp://en.wikipedia.org/wiki/Piezoelectric_transducerhttp://en.wikipedia.org/wiki/Photometerhttp://en.wikipedia.org/wiki/Photoelectric_sensorhttp://en.wikipedia.org/wiki/Photoelectric_sensorhttp://en.wikipedia.org/wiki/Rotary_encoder#Optical_Absolute_Encodershttp://en.wikipedia.org/wiki/Ohmmeterhttp://en.wikipedia.org/wiki/Manometerhttp://en.wikipedia.org/wiki/Limit_switchhttp://en.wikipedia.org/wiki/Linear_variable_differential_transformerhttp://en.wikipedia.org/wiki/Sail_switchhttp://en.wikipedia.org/wiki/Flow_sensorhttp://en.wikipedia.org/wiki/Dynamometerhttp://en.wikipedia.org/wiki/Bi-metallic_striphttp://en.wikipedia.org/wiki/Bi-metallic_striphttp://en.wikipedia.org/wiki/Ammeter


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typically by battery power; a passive sensor, meanwhile, requires an externalpower source in order to operatein other words, its hardware is powered bysources of power it accumulates from the variables which it is set to measure. Athermocouple is an example of an active device, while a thermistor operatesusing passive principles. Figure 4.2 illustrates four key sensors used in the

Mindstorms system from Lego.

Figure 4.2: Four sensors from Mindstorms modelling system (sound, touch, lightand ultrasonic respectively).

KEYPOINTSensors may also be active or passive.END KEYPOINT

PROFESSIONAL TRANSFERIBLE SKILLS [CRIT] [PROB] [WCOMM]LEARNING ACTIVITY 4.1Use your company or the internet to identify suppliers and their productcatalogues for five of the sensors listed in Table 4.2. Report on theirspecifications using the terminology used above i.e. analogue or discrete;mechanical or thermal, etc. and their transducer properties. Share your findingson the discussion board.END LEARNING ACTIVITY 4.1

The sensors transfer function is the relationship between the value of thephysical stimulus and the value of the signal produced by the sensor andcommunicated to the controller. It operates in an input/output relationship, withthe stimulus being the input, and the signal generated being the output. This canbe expressed as follows:

)(sfS=

where S is the output signal; s is the stimulus; and f(s) is the functionalrelationship between them. For sensors that are binary (i.e. have one of twopositions at any one time), such as limit switches, we can express them asfollows: S = 1 ifs > 0 and S = 0 ifs < 0.


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The ideal functional form for an analogue measuring device is a simpleproportional relationship, such as:

msCS +=

where C is the output value at a stimulus value of zero; and m is the constant ofproportionality between s and S. In essence, m can be thought of as thesensitivity of the sensor.

KEYPOINTThe sensors transfer function (S) is the relationship between the value of thephysical stimulus and the value of the signal produced by the sensor in response.END KEYPOINT

EXAMPLE 4.1The output voltage of a particular thermocouple sensor is registered to be 42.3

mV at temperature 105C. It had previously been set to emit a zero voltage at0C. Since an output/input relationship exists between the two temperatures,determine (1) the transfer function of the thermocouple, and (2) the temperaturecorresponding to a voltage output of 15.8 mV.

Answer(1) We know that :

msCS +=

where S is the output signal; s is the stimulus;C is the output value at a stimulusvalue of zero; and m is the constant of proportionality between s and S.

Therefore: 42.3 mV = 0 + m(105C) = m(105C)

Or m = 0.4028571429

=> S = 0.4028571429(s) (1)

(2) Using the relationship from (1) above, we replace S with the voltage outputgiven, i.e. 15.8 mV.

=> 15.8 mV = 0.4028571429(s)

=> 15.8 / 0.4028571429 = s

=> s = 39.22CEND EXAMPLE 4.1


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Calibration of the sensor before use is important. When the sensor is calibrated,its transfer function is determined, and the inverse of the transfer function issubsequently derived, so that the value of the stimulus (s) may be determined.The ease of calibration is another method for classifying sensors. Desirable traitsfor sensors include the following:

BULLETLISTHigh accuracyvery few systematic errors reported

High precisionrandom variability is kept to a minimum

Wide operating rangehigh accuracy and precision over a wide range of values

High speed of responseresponds quickly to changes in the physical variablebeing measured

Ease of calibrationquick and easy calibration expected

Minimum driftthe gradual loss in accuracy over time is minimised

High reliabilityfailures or malfunctions minimised

Low costcost of purchasing or creating device is not excessive.ENDLIST

KEYPOINTDesirable traits for sensors include: high accuracy, high precision, a wide

operating range, a high speed of response, an ease of calibration, a minimumdrift, a high reliability, and a low cost.END KEYPOINT

Section 4.4 Actuators

An actuator converts the controller command signal into a change in a physicalparameter. This change is usually a mechanical alteration, such as a change inposition, or a change in velocity. J ust like the sensor, an actuator is also atransducer, as it changes one type of physical quality into another. Manyactuators are fitted with amplifiers, to covert low level control signals into strongsignals sufficient to drive the actuator.

KEYPOINTAn actuator converts the controller command signal from the controller into achange in a physical parameter.END KEYPOINT


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Three types of actuator may be defined: electrical, hydraulic, and pneumatic.Electrical actuators include electric motors of all kinds, stepper motors andsolenoids; hydraulic actuators includes a wide variety of cylinder-devicescompressing hydraulic fluids, typically oils or water-oil solutions, to achieveoperation; while pneumatic actuators include a variety of piston-and-cylinder

devices that compress air or other gases to achieve changes in the physicalvariables. Figure 4.3 illustrates a number of different types of actuators.

Figure 4.3: Actuators servo motor, stepper motor, solenoid, hydraulic pistonand pneumatic piston respectively.

KEYPOINTThere are three types of actuator: electrical (motors and solenoids), hydraulic,and pneumatic.END KEYPOINT

Electric motors convert electrical power into mechanical power. The mostcommon type of electric motor consists of a rotor that rotates inside a stationaryhousing (the stator). The rotational movement is transferred to a shaft connectedto the rotor, and subsequently to a series of pulleys, gears, shafts, and spindlesarranged as necessary (See Figure 4.4). Electric current supplied to the stator,continuously changes the magnetic fields of the rotor, which in turn alters its

position so as to align its magnetic fields with those of the stator; this causes therotational effect of the rotor. The principle ways to select a motor are based ontype (i.e. servo motor or stepper motor), torque (i.e. force exerted over distance)and revolutions per minute (RPM). The current supplied to the motor can bealternating current (AC) and direct current (DC) motors. There is also a specialtype of motor called the stepper motor.

KEYPOINT


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Electric motors convert electrical power into mechanical power; the mostcommon type of electric motors are the ac or dc servo motor and stepper motor.END KEYPOINT

Figure 4.4: Servo Motor with motor, optical encoder and gearing

DC motors are powered by a constant current and voltage. DC motors are usedfor their convenience of power sourcing, and for their relatively high torque-speedrelationship. DC servo motors are a common type of DC motor used inmechanised and automated systems, where the term servo refers to a specificfeedback mechanism that is used to control the motors position and speed. Anoptical encoder is the most common type of feedback mechanism used (seeFigure 4.4).

KEYPOINTDC motors are powered by a constant current and voltage. The creation of themagnetic field is caused by using a rotary switching device called thecommutator, or in the brushless DC motor, solid-state circuitry.END KEYPOINT

LEARNING ACTIVITY 4.2


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Use the internet to learn more about the servomechanism principle at web-site:http://hades.mech.northwestern.edu/index.php/Main_PageEND LEARNING ACTIVITY 4.2

The most common source of electrical power in industry is alternating current or

AC. AC motors are predominantly used in industry. AC motors are powered bythe generation of a rotating magnetic field in the stator, where the speed ofrotation depends on the frequency of the input electrical power. The rotor isforced to turn at the same speed as the rotating magnetic field. There are twobroad categories of AC motor: induction motors, and synchronous motors.

KEYPOINTAC motors are powered by the generation of a rotating magnetic field in thestator, where the speed of rotation depends on the frequency of the inputelectrical power. The rotor is forced to turn at the same speed as the rotatingmagnetic field.

END KEYPOINT

Induction motors are extremely popular owing to their relatively simpleconstruction, and low cost of manufacture. Induction motors generally do notneed an external source of power, as a magnetic field is induced by the rotationof the rotor through the magnetic field of the stator. Synchronous motorsenergise the rotor with alternating current, which generates a magnetic field inthe gap between rotor and stator. This magnetic field creates a torque that turnsthe rotor at the same rotational speed as the magnetic forces in the stator. Theyalso incorporate a device called an exciter to initiate rotation of the rotor whenpower is first supplied to the motor.

All AC motors operate at a constant speed that depends on the frequency of theincoming electrical power; thus operations where fixed speeds are required areideal for AC motors. Issues can arise, however, where variations in speeds, andstarting and stopping, occur over frequent intervals. Adjustable-frequency drives,called inverters, address the issue by controlling the cycle rate of the AC powerto the motor.

LEARNING ACTIVITY 4.3Use the internet to find out the specifications for the motor used in the NXTMindstorms kits from Lego and illustrated in Figure 4.5. What are its torque, RPMand type characteristics? How does it measure rotational feedback?


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Figure 4.5: Mindstorms motor from Lego

END LEARNING ACTIVITY 4.3

A third motor type is the stepper motor, or step motor. This provides rotation inthe form of discrete angular displacements, called step angles, whereby each

angular step is created by a discrete electrical pulse. The total angular rotation iscontrolled by the number of pulses received by the motor, and rotational speed iscontrolled by the frequency of the pulses. The stepper is characterised by a multipronged rotor and set of poles in the stator and by electronic circuitry thatchanges the polarity to achieve a stepping effect as north and south are attractedto each other (See Figure 4.6).

Figure 4.6: Schematic of a stepper motor rotor and stator

KEYPOINTThe stepper motor provides rotation via step angles, which are created by adiscrete electrical pulse, the total number of which equates to the total angularrotation.END KEYPOINT

LEARNING ACTIVITY 4.4Learn more about induction motors at web-site:


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http://en.wikipedia.org/wiki/Stepper_motorAn excellent paper on the use and application of stepper motors is given by theSolarbotics.net web-site at:http://www.solarbotics.net/library/pdflib/pdf/motorbas.pdfEND LEARNING ACTIVITY 4.4

The step angle of stepper motors is related to the number of steps for the motoraccording to the relationship:

sn

360=

where is the step angle in degrees; and ns is the number of steps for thestepper motor, which must be an integer value. The total angle through which themotor rotates (Am) is given by:

pm nA = whereAm is the total angle through which the motor rotates in degrees; np is thenumber of pulses received by the motor; and is the step angle in degrees.Angular velocity is given by:

s

p

n

f

2=

where is angular velocity; fp is the pulse frequency; and ns is the number ofsteps for the stepper motor. The speed of rotation is given by:

s

p

n

fN

60=

where N is the rotational speed; fp is the pulse frequency; and ns is the number ofsteps for the stepper motor.

KEYPOINTFor the stepper motor the step angle and other angular measurements may becalculated; these metrics define the uses to which the stepper motor may be put.

END KEYPOINT

EXAMPLE 4.2

A stepper motor has a step angle = 3.6. (1) How many pulses are required forthe motor to rotate through ten complete revolutions? (2) What pulse frequency isrequired for the motor to rotate at a speed of 100 rev/min?

(1) We know that the step angle is given by:
http://en.wikipedia.org/wiki/Stepper_motorhttp://en.wikipedia.org/wiki/Stepper_motorhttp://www.solarbotics.net/library/pdflib/pdf/motorbas.pdfhttp://www.solarbotics.net/library/pdflib/pdf/motorbas.pdfhttp://www.solarbotics.net/library/pdflib/pdf/motorbas.pdfhttp://en.wikipedia.org/wiki/Stepper_motor


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sn

360=

where is the step angle in degrees; and ns is the number of steps for thestepper motor.

=> 3.6 = 360 / ns

=> 3.6 (ns) = 360

=> ns = 360 / 3.6 = 100 step angles

The total angle through which the motor rotates (Am) is given by:

pm nA =

whereAm is the total angle through which the motor rotates in degrees; np is thenumber of pulses received by the motor; and is the step angle in degrees.

Now, to rotate through ten complete revolutions: 10(360) = 3600 =Am

np = 3600 / 3.6 = 1000 pulses

(2) We know that:

s

p

n

fN

60=

where N is the rotational speed; fp is the pulse frequency; and ns is the number of

steps for the stepper motor.

Thus, from the information derived from (1) above, and where N = 100 rev/min:

100 = 60 fp / 100

=> 10,000 = 60 fp

=>fp = 10,000 / 60 = 166.667 = 167 HzEND EXAMPLE 4.3

Stepper motors are widely used in the following applications:

BULLETLISTOpen loop control systems

Low-to-medium torque and power scenarios


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Machine tools in production machines, industrial robots, x-y plotters, medical andscientific instruments, and computer peripherals.ENDLIST

A key characteristic for all motors is the torque-speed relationship. The

relationship between torque and speed for the DC servo motor, AC servo motorand the stepper motor is shown in Figure 4.6. Generally, the torque generated ishigher at lower speeds and reaches an operating point for particular loads. Theeffort of changing loads is to swing the load arm in Figure 4.6 around the point oforigin. As load increases (rotating the arm counter-clockwise) the speed reducesand torque increases.

Torque,T

Speed,

Load

Operating

Points

DC ServoAC Servo

Stepper

Figure 4.6: Torque-speed relationship for stepper motor

Other types of electrical actuators include the following:

BULLETLISTSolenoids these consist of a stationary wire coil inside of which is a moveableplunger. When an electric current is applied to the coil, the plunger is drawn intothe coil; when the current is switched off the plunger is returned to its previousposition by a spring. Actuator action type here is a linear, push-pull movement,but rotary solenoids are also available, usually over a limited angular range.

Electromechanical relaysan on/off electrical switch consisting of a stationarycoil and moveable arm that opens or closes an electrical contact by means of amagnetic field that is generated when current is passed through the coil.ENDLIST

KEYPOINTSolenoids and electromagnetic relays are other types of electrical actuators.END KEYPOINT


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LEARNING ACTIVITY 4.5Learn more about solenoids and electromagnetic relays from the followingresources. The basic principles and operating characteristics of solenoids areoutlined by DetroitCoil.com in two white papers:

http://www.detroitcoil.com/PAGES/What%20Is%20A%20Solenoid.pdfhttp://www.detroitcoil.com/PAGES/How%20A%20DC%20Solenoid%20Works1.pdfElectromagnetic relays are outlined in great detail in Wikipedia at:http://en.wikipedia.org/wiki/RelayEND LEARNING ACTIVITY 4.5

Hydraulic and pneumatic actuators are both operated by pressurised fluids orgases. Oil is used in hydraulic systems, and compressed air is used in pneumaticsystems. Both categories of device are similar in operation but different inconstruction, primarily owing to the differences between fluids and gases. Fluids

are non-compressible, whereas gas is compressible. In production automation,hydraulic systems are generally preferred when high forces and accurate controlare required. Pneumatic systems are generally used for low cost applications, orwhere high speed actuation is needed. Hydraulic systems generally demandintricate and precise device construction, with close tolerances on componentparts being essential; whereas pneumatic systems are generally not as fine inconstruction, with any problems with air leaks being prevented by the use ofgeneral-purpose components such as O-rings.

KEYPOINTHydraulic and pneumatic systems are types of actuators. Oil is used in hydraulic

systems, and compressed air is used in pneumatic systems.END KEYPOINT

Both hydraulic and pneumatic systems can be designed to provide linear orrotary motions. One of the most common hydraulic and pneumatic devices is thecylinder device, which provides a linear in/out motion. It consists of a cylindricaltube inside of which is housed a piston that moves in and out inside the cylinderhousing. The piston may or may not have a spring to allow it to return to its initialposition inside the cylinder; if it doesnt it uses the action of the fluid or gas toreturn to its initial position after it has performed a stroke. The former, spring-loaded piston is called a single-acting cylinder and piston system; the lattersystem is called a double-acting cylinder and piston (see Figure 4.7).
http://www.detroitcoil.com/PAGES/What%20Is%20A%20Solenoid.pdfhttp://www.detroitcoil.com/PAGES/What%20Is%20A%20Solenoid.pdfhttp://www.detroitcoil.com/PAGES/How%20A%20DC%20Solenoid%20Works1.pdfhttp://www.detroitcoil.com/PAGES/How%20A%20DC%20Solenoid%20Works1.pdfhttp://www.detroitcoil.com/PAGES/How%20A%20DC%20Solenoid%20Works1.pdfhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Relayhttp://www.detroitcoil.com/PAGES/How%20A%20DC%20Solenoid%20Works1.pdfhttp://www.detroitcoil.com/PAGES/How%20A%20DC%20Solenoid%20Works1.pdfhttp://www.detroitcoil.com/PAGES/What%20Is%20A%20Solenoid.pdf


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Figure 4.7: Single-acting and double-acting cylinder and piston types

KEYPOINTIn cylinder and piston systems there are two types: single-acting and double-actingEND KEYPOINT

The force and speed characteristics of pneumatic systems are difficult tocalculate owing to the fact that air is compressible. Hydraulic systems give nosuch problems since oil is incompressible. We are able to express therelationships between the speed and force of the piston inside the hydrauliccylinder with the fluid flow rate and pressure as follows:

A

Qv =

pAF =

Where v is the velocity of the piston; Q is the volumetric flow rate;A is the area ofthe cylinder cross section; F is the applied force; and p is the fluid pressure.

For the double-acting cylinder and piston system, the circumference and lengthof the piston rod changes the calculation for the return stroke of the system, asits presence in the chamber necessarily reduces the total cylinder area uponwhich the fluid can exert pressure. It ultimately results in a slightly greater pistonspeed and slightly less applied force on the reverse stroke than on the forwardstroke.

KEYPOINT


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The force and speed characteristics of pneumatic systems can be difficult tocalculate since air is compressible. Hydraulic systems give no such problemsince oil is incompressibleEND KEYPOINT

Fluid-powered rotary motors are also available to provide continuous rotationalmotion. The rotation speed of a hydraulic motor is directly proportional to the fluidflow rate, as defined in the equation:

KQ= Where is angular velocity; Q is the volumetric fluid flow rate; and K is aconstant of proportionality. Angular velocity can be converted to revolutions perminute (rpm) by multiplying by 60/2.

EXAMPLE 4.3

A double-acting hydraulic cylinder has an inside diameter = 75 mm. The pistonrod has a diameter = 14 mm. The hydraulic power source can generate up to 5.0MPa of pressure at a flow rate of 200,000 mm3/sec to drive the piston. (a) Whatare the maximum possible velocity of the piston and the maximum force that canbe applied in the forward stroke? (b) What are the maximum possible velocity ofthe piston and the maximum force that can be applied in the reverse stroke?

Solution:

Forward stroke areaA = 0.25(75)2 = 4418 mm2

Reverse stroke areaA = 4418 0.25(14)

2 = 4264 mm2

(a) Forward stroke v = 200,000 / 4418 =45.3 mm/sec

F = 5(4418) =22,090 N(b) Reverse stroke v = 200,000 / 4264 =46.9 mm/secF = 5(4264) =21,320 N

END EXAMPLE 4.3

Section 4.5 Analogue to Digital Converters

The key problem with analogue signals is their incompatibility with computingsystems, which operate in digital format. Process signals are generallycontinuous and analogue, so they need to be converted in some way into adigital format. Analogue to Digital (ADC) conversion provides this functionality.

KEYPOINTAnalogue signals must be converted into a digital format for processing bycomputers controlling the processEND KEYPOINT


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There are five general steps and/or devices deployed to convert signals fromanalogue into digital (see Figure 4.8):

NUMLISTSensor and transducerthis generates the original analogue signal.

Signal conditioningthis renders the signal into a suitable form for conversion; itcan include noise filtration steps, or the conversion from one signal form toanother (for example, from current into voltage).

Multiplexerthis is a time-sharing switching device which collects the incominganalogue signals and determines when their output should occur. In mostprocesses there are many analogue signals generated simultaneously; themultiplexer sorts out the priorities of the signals to be passed to the rest of theprocess.

Amplifieronce the signal is passed from the multiplexer, it is scaled-up orscaled-down by means of an amplifier. Amplification is important, as it ensuresthat the signal produced is compatible with the ADC.

Analogue-to-digital converter (ADC)the prepared signal is converted fromanalogue to digital.ENDLIST

Analog

Digital

Converter

Transformation Process

Sensors

& Transducer

Other Signals

ContinuousVariable

Signal

Conditioner

Multiplexer

Digital

Computer

Amplifer

Figure 4.8: Analogue to Digital conversion steps

KEYPOINT


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There are five general devices deployed in the process of analogue to digitalconversion; these are: the sensor and transducer; the signal conditioner; themultiplexer; the amplifier; and the analogue-to-digital converter.END KEYPOINT

The ADC component illustrated in Figure 4.8 operates in three phases: sampling;quantisation; and encoding. Sampling consists of transforming the continuoussignal into a series of discrete analogue signals at periodic intervals; quantisationconsists of assigning each resulting discrete analogue signal to one of a finitenumber of previously defined amplitude levels, each of which is a discrete valueof voltage ranging over the full scale of the ADC; while in encoding the result ofthe quantisation process is converted into digital code, represented as binarydigits. So for example the value of voltage at a particular moment in time of say4.234volts is converted in a digital number of say 10011.

KEYPOINT

The analogue-to-digital conversion process consists of sampling, quantisation,and encoding.END KEYPOINT

LEARNING ACTIVITY 4.6Learn more about the analogue to digital conversion principle at:http://en.wikipedia.org/wiki/Analog-to-digital_converterEND LEARNING ACTIVITY 4.6

The conditions that dictate the choice of ADC for a given application areillustrated in Table 4.3.

Table 4.3: Conditions that dictate ADC choiceCondition Description

Sampling rate The rate at which the continuous analogue signals are polled. Highersampling rates means a closer approximation to the original analoguewaveform is achieved. The maximum possible sampling rate for eachsignal is the maximum sampling rate of the ADC divided by themultiplexer number of channels.

Conversion time The time interval between the application of an incoming signal andthe determination of the digital value by the quantisation andencoding phases of the conversion procedure. The maximumpossible sampling rate is limited by conversion time. Conversion timedepends on the type of conversion procedure used, and the number

of bits used to define the converted digital value. As the number ofbits increases, so does the conversion time, however the resolution ofthe ADC improves.

Resolution The precision with which the analogue signal is evaluated. Precisionis determined by the number of quantisation levels, which in turn isdetermined by the bit capacity of the ADC and the computer.

Conversion method Various methods exist, the most common being the successiveapproximation method. In this method a series of trial voltages arecompared to the input signal whose value is unknown. The number oftrial voltages corresponds to the number of bits used to encode the
http://en.wikipedia.org/wiki/Analog-to-digital_converterhttp://en.wikipedia.org/wiki/Analog-to-digital_converterhttp://en.wikipedia.org/wiki/Analog-to-digital_converter


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signal.

KEYPOINTThe conditions that dictate the choice of ADC for a given application aresampling rate, conversion time, resolution, and the method of conversion.

END KEYPOINT

For resolution as outlined in Table 4.3, the number of quantisation levels isdefined as:

n

qN 2= where Nq is the number of quantisation levels; and n is the number of bits.Resolution can be defined in equation form as:

121 =

=n

qADC

L

N

L

R

where RADC is the resolution of the ADC; L is the full-scale range of the ADC; andNq is the number of quantisation levels.

Quantisation generates an error, because the digitised signal is only sampledfrom the original analogue signal. The maximum possible error occurs when thetrue value of the analogue signal is on the borderline between two adjacentquantisation levels, in which case the error is half the quantisation-level spacing;this gives us the following for quantisation error (Quanerr):

ADCRQuanerr

21=

where RADC is the resolution of the ADC.

EXAMPLE 4.4QUESTIONUsing an analogue-to-digital converter, a continuous voltage signal is to beconverted into its digital counterpart. The maximum voltage range is 25 V. TheADC has a 16-bit capacity, and full scale range of 60 V. Determine (1) number ofquantization levels, (2) resolution, (3) the spacing of each quantisation level, and

the quantisation error for this ADC.

ANSWER(1) Number of quantization levels:

n

qN 2=

where Nq is the number of quantisation levels; and n is the number of bits.

= 216 = 65,536


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END KEYPOINT

Decoding sees the digital value outputted by the computer being tied to a binaryregister that controls a reference voltage source. Each successive bit in theregister controls half a bit of its predecessor, so that the level of the output

voltage is determined by the status of the bits in the register. The output voltageis given by:

})2(...125.025.05.0{ 1321 nn

refO BBBBEE

++++=

where EO is the output voltage of the decoding step; Erefis the reference voltage;and B1, B2,, Bn is the status of successive bits in the register, 0 or 1; and n isthe number of bits in the binary register.

KEYPOINT

Decoding sees the digital output of the computer being structured into a binaryregister that controls a reference voltage source.END KEYPOINT

Data holding tries to approximate the envelope formed by the data series. Thecreation of an analogue signal from digital data requires extrapolation of existingdata points, finding commonalities and extending the digital points to form acontinuous envelope that approximates as closely as possible to digital outputunder investigation. Data-holding devices are classified according to the order ofthe extrapolation calculation used to determine the voltage output duringsampling intervals.

Most ADC and DAC conversions are carried out within the functionality of aprocess controller. Where a controller does not have this functionality (e.g. usinga general purpose computer), then easily configured ADC/DAC devices can bepurchased. For very special purpose systems, ADC and DAC requires the designof electronic and analogue circuits to process the various signals.

Section 4.7 Input/Output Devices for Discrete Data

Discrete data does not require conversion applications to be processed bycomputers. As described in the last unit, there are three types of discrete data:binary data; discrete data other than binary; and pulse data.

KEYPOINTDiscrete data consists of three typesbinary data, discrete data other thanbinary, and pulse datawhich uses input/output interfaces to connect to thecomputing system.END KEYPOINT


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The input/output interfaces for each of these data types are described in Table4.4. Note that there are two contact interfaces, input and output. These can bearranged as a single contact point (for example for binary data), or as an array ofcontact points (for example for discrete data other than binary).

A contact input interface allows binary data to be read from some externalsource, such as the process, by the computer. It consists of a series of simplecontacts that can be either closed or open to indicate the binary status ofconnected devices. The computer periodically scans the actual status of thecontacts to update the values stored in the memory. Limit switches, valves, andmotor pushbuttons are typical devices connected to the contact input interface.One example of an input/output device is the relay board illustrated in Figure 4.9.

Figure 4.9: Relay board for contact input/output

A contact output interface allows binary data to be read from the computer bysome external device. The contact positions are set either on or off, andmaintained at these positions until changed by the computer in response tochanging conditions in the process environment. Alarms, indicator lights,solenoids, and constant speed motors are typical devices connected to thecontact output interface.

Table 4.4: Input/output interfaces for discrete dataDigital data type Input interface with computer Output interface from computer

Binary Contact input Contact outputOther than binary Contact input array Contact output arrayPulse Pulse counters Pulse generators

KEYPOINT


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A contact input interface allows binary data to be read from some externalsource, such as the process, by the computer. A contact output interface allowsbinary data to be read from the computer by some external device.END KEYPOINT

Discrete data can also be transmitted as pulses by such devices as digitaltransducers and optical encoders, and used to control devices such as steppermotors. A pulse counter converts a series of pulses into a digital value, the mostcommon device being those used to convert electrical pulses into digital data bydeploying a series of sequential logic gates, called flip-flops, and a memorycapability for storing the results of the counting procedure.

KEYPOINTDiscrete data can also be transmitted as a series of pulses that is captured by apulse counter, which converts the pulses into a digital value.END KEYPOINT

Pulse counters are typically used for counting and measurement applications,such as counting the number of packages moving past a photoelectric sensor, orto indicate the rotational speed of a shaft. A pulse generator is a device used toproduced a series of electrical pulses whose total number and frequency arespecified by the control computer. Typical applications of the pulse generator isin positioning systems, whereby the number of pulses released drive the axis ofthe system and moves workheads into position.

KEYPOINTPulse counters are typically used for counting and measurement applications;

while pulse generators are used in positioning systems.END KEYPOINT

Section 4.8 Unit Review

BULLETLISTA sensor is a transducer that allows for the transformation of a signal, or otherphysical variable, from one form to another.

Sensors can be classified according to the category of stimulus or physicalvariable they are required to measure; these include stimulus that aremechanical, electrical, thermal, radiation, magnetic, and chemical in kind.

Sensors may also be classified as analogue or discrete. A sensor that isanalogue in operation produces a continuous analogue signal whose valuevaries in an analogous manner with the variable being measured. A sensor thatis discrete produces an output that can only have certain values.


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Sensors and measuring devices come in a wide variety of forms, based uponmany different operative principles, from analogue and discrete, to thermal,electrical, mechanical, magnetic and radiation devices.

Sensors may also be active or passive. An active sensor responds to the

stimulus without the need for any external power; a passive sensor requires anexternal power source in order to operate.

The sensors transfer function (S) is the relationship between the value of thephysical stimulus and the value of the signal produced by the sensor in response.

Desirable traits for sensors include: high accuracy, high precision, a wideoperating range, a high speed of response, an ease of calibration, a minimumdrift, a high reliability, and a low cost.

An actuator converts the controller command signal into a change in a physical

parameter. Actuators are transducers, and may be fitted with amplifiers tostrengthen initial control signals to drive the actuator.

There are three types of actuator: electrical, hydraulic, and pneumatic.

Electric motors convert electrical power into mechanical power; the mostcommon type of electric motor is the rotational motor.

DC motors are powered by a constant current and voltage. The creation of themagnetic field is caused by using a rotary switching device called thecommutator, or in the brushless DC motor, solid-state circuitry.

In a DC servomotor the magnitude of the rotor torque is a function of the currentpassing through the rotor, while the mechanical power delivered by the DCservomotor is the product of torque and velocity.

Typically the servomotor is connected to a piece of machinery, which representsthe load that is driven by the servomotor. The torque developed by the motor andthe torque required by the load must be balanced, and this amount of torque iscalled the operating point.

AC motors are powered by the generation of a rotating magnetic field in thestator, where the speed of rotation depends on the frequency of the inputelectrical power. The rotor is forced to turn at the same speed as the rotatingmagnetic field.

Induction motors generally do not need an external source of power as amagnetic field is induced by the rotation of the rotor through the magnetic field ofthe stator. Synchronous motors energise the rotor with alternating current, whichgenerates a magnetic field in the gap between rotor and stator.


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The stepper motor provides rotation via step angles, which are created by adiscrete electrical pulse, the total number of which equates to the total angularrotation.

For the stepper motor the step angle and other angular measurements may becalculated; these metrics define the uses to which the stepper motor may be put.

Electrical actuators include solenoids and electromagnetic relays.

Hydraulic and pneumatic pistons are types of actuators. Oil is used in hydraulicsystems, and compressed air is used in pneumatic systems.

In cylinder and piston systems there are two types: single-actingwhere a springis used to allow the piston to return to its initial position inside the cylinder; anddouble-actingwhere the action of the fluid or gas is used to return the piston to

its initial position after it has performed a stroke.

The force and speed characteristics of pneumatic systems are difficult tocalculate owing to the fact that air is compressible. Hydraulic systems give nosuch problem as oil is incompressible

Analogue signals from the process are incompatible with computing systems,which operate in digital format. They must be converted into a format that can bereadily read by the computers controlling the process, by using analogue todigital conversion methods.

There are five general devices deployed in the process of analogue to digitalconversion; these are: the sensor and transducer; the signal conditioner; themultiplexer; the amplifier; and the analogue-to-digital converter.

The analogue-to-digital conversion process consists of sampling, quantisation,and encoding.

The conditions that dictate the choice of ADC for a given application aresampling rate, conversion time, resolution, and the method of conversion.

Returning control signals to the process from the computer requires digital-to-analogue conversion to take place.

Digital-to-analogue conversion consists of two steps, decoding and data holding.

Decoding sees the digital output of the computer being structured into a binaryregister that controls a reference voltage source.


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Data holding tries to approximate the envelope formed by the digital data series.The most common data-holding device is the zero-order hold.

Discrete data does not need to be converted to be read by the computer system.It consists of three typesbinary data, discrete data other than binary, and pulse

datawhich uses input/output interfaces to connect to the computing system.

A contact input interface allows binary data to be read from some externalsource, such as the process, by the computer. A contact output interface allowsbinary data to be read from the computer by some external device.

Discrete data can also be transmitted as a series of pulses that is captured by apulse counter, which converts the pulses into a digital value.

Pulse counters are typically used for counting and measurement applications;while pulse generators are used in positioning systems.

ENDLIST

Section 4.9 Self-Assessment Questions

NUMLISTWhat classification can be applied to sensors?

What are the desirable traits of sensors?

What is an actuator? What are the available types?

What are the operating conditions of DC, AC and stepper electrical motors?

Outline other types of actuators, other than electrical.

What is the need for analogue-to-digital converters?

Outline the steps of the analogue-to-digital conversion process.

What is the need for digital-to-analogue converters?

Outline the steps of the digital-to-analogue conversion process.

Define how discrete data is handled by computing systems.ENDLIST

Section 4.10 Answers to Self-Assessment Questions


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NUMLISTSensors may be classified as analogue or discrete. A sensor that is analogue inoperation produces a continuous analogue signal whose value varies in ananalogous manner with the variable being measured. A sensor that is discreteproduces an output that can only have certain values.

Desirable traits for sensors include: high accuracy, high precision, a wideoperating range, a high speed of response, an ease of calibration, a minimumdrift, a high reliability, and a low cost.

An actuator converts the controller command signal into a change in a physicalparameter. Actuators are transducers, and may be fitted with amplifiers tostrengthen initial control signals to drive the actuator. There are three types ofactuator available: electrical, hydraulic, and pneumatic.

Electric motors convert electrical power into mechanical power. DC motors are

powered by a constant current and voltage. The creation of the magnetic field iscaused by using a rotary switching device called the commutator, or in thebrushless DC motor, solid-state circuitry. AC motors are powered by thegeneration of a rotating magnetic field in the stator, where the speed of rotationdepends on the frequency of the input electrical power. The rotor is forced to turnat the same speed as the rotating magnetic field. The stepper motor providesrotation via step angles, which are created by a discrete electrical pulse, the totalnumber of which equates to the total angular rotation.

Hydraulic and pneumatic systems are types of actuators, other than electrical.Both categories of device are similar in operation but different in construction,

primarily owing to the differences between fluids and gases.

Analogue signals from the process are incompatible with computing systems,which operate in digital format. They must be converted into a format that can bereadily read by the computers controlling the process, by using analogue todigital conversion methods.

The analogue-to-digital conversion process consists of sampling, quantisation,and encoding.

Digital signals from the control computer are incompatible with the process,which operates in analogue format. They must be converted into a format thatcan be readily read by the process, by using digital to analogue conversionmethods.

Digital-to-analogue conversion consists of two steps, decoding and data holding.

Discrete data does not need to be converted to be read by the computer system.It is handled by input/output interfaces to connect to the computing system, or by


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