Programmable Logic Controllers LO1: Understand the design and operational characteristics of a PLC...

Programmable Logic Controllers

LO1: Understand the design and operational characteristics of a PLC

system

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Learning Outcome 1

• LO1: Understand the design and operational characteristics of a PLC system– 1.2 describe different types of input and output device

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Lesson Outline

• At the end of this session the students will be able to…1. Specify the I/O units of a PLC.2. Input devices3. Output Devices

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2. Input devices – Mechanical switches– Proximity switches– Photoelectric sensors and switches– Encoders– Temperature sensors– Position / displacement sensors– Strain gauges– Pressure sensors– Liquid-level detectors– Fluid flow measurement– Smart sensors

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3. Output Devices

– Relay– Directional Control Valves– Motors– Stepper Motors

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Input Devices

• An input device (sensor) provides a usable output in response to a specified physical input.– A thermocouple for example is a sensor that

converts temperature difference into an electrical input

• The term transducer is generally used to refer to a device that converts a signal from one physical form to a different physical form. Thus sensors are often referred to as transducers.

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Input Devices

• Sensors that give discrete i.e. digital (on / off) outputs can easily be connected to the inputs of a PLC (it’s just a case of matching the appropriate voltage).

• Analogue sensors give an output proportional to the measureable variable. Such analogue signals have to be converted (signal conditioning) to digital signals before they can be put into a PLC input ports.

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Parameters used todescribe input devices

• Accuracy: this is the extent to which the value indicated by a measurement system or element might be wrong.– A temperature sensor might give an accuracy

of 1%.– Errors give an indication of accuracy.

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• Errors can arise in a number of ways…– Non-linearity error is an error that occurs as

a result of assuming a linear relationship between input and output over a working range. Few systems or elements have a truly linear relationship.

– Hysteresis error is the difference in outputs given from the same value quantity being measured according to whether that value has been reached by a continuously increasing or decreasing change

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True Value

Measured Value

Assumed relationship

Non-linearity Error

Actual relationship

Nonlinearity error

Value being measured

Sensor Output

Increasing

Decreasing Hysteresis Error

Hysteresis Error


• Range:– the limits between which the inputs can

vary

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• Response time:– the time that elapses after the input to a

system or element is abruptly increased from zero to a constant value up to the point at which the system or element gives an output corresponding to some specified percentage such as 95% of the value of the input

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• When looking at the specification of a device the Response Time might be quoted in terms of…– Rise time is the time taken for the output from the

device to rise to some specified percentage of steady state output. (Often taken for the output to rise from 10% of the steady state value to 90% or 95% of the new steady state value.

– Settling time is the time taken for the output from the device to settle to within some percentage, such as 2% of the steady state value.

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Time

Response

Steady state reading


• Sensitivity: – the extent to which the output of an

instrument or sensor changes when the quantity being measured changes by a given amount• A thermocouple might have a sensitivity of

20µV/oC i.e. the output will change by 20µV for every 1oC

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• Stability: – is the ability to give the same output

when used to measure a constant input over a period of time.• Drift is the term used to describe the

change in output that occurs over time.• Zero drift is the term used to describe the

change that occur when there is zero input.

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• Repeatability:– the ability of a measurement system to give

the same value for repeated measurements of the same value of a variable. Can be effected by changes in environmental conditions such as temperature and humidity.• Normally quoted as a error of the full range

i.e. 0.1%. Thus if a pressure range was 20KPa, then the error would be 20Pa.

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• Reliability: – the probability that it will operate to an

agreed level of performance for a specified period subject to environmental conditions.

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Example

• MX100AP pressure sensor– Supply voltage: 3 V (6V max)– Supply current: 6 mA– Full scale span: 60 mA– Range: 0 to 100 kPa– Sensitivity: 0.6 mV/kPa– Nonlinearity error: 0.05% of full range– Temperature hysteresis: 0.5% of full scale– Input resistance: 400 to 550Ω– Response time: 1ms (10% to 90%)

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The Input andOutput Interfaces

• The input and output interfaces are where the processor receives information from external devices.

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Mechanical Switches

• A mechanical switch generates an on/off signal or signals as a result of some mechanical input causing the switch to open or close.

• For example a switch might be used to indicate the presence of a work piece on a machining table– The work piece pressing switch and closing it.

• the absence of the work piece is indicated by the switch being open

• the presence by it being closed

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Mechanical Switches NDGTA

Supply Voltage PLC

Input Channel

Work piece not present: 0

Work piece present: 1

Supply Voltage

Work piece not present: 1

Work piece present: 0

The sense of the input is now changed

Mechanical Switches

• Switches are available with normally open (NO) or normally closed (NC) contacts or can be configured as either by choice of the relevant contacts.

• A NO switch has its contacts open in the absence of a mechanical input and the mechanical input is used to close the switch.

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Mechanical Switches

• A NC switch has its contact closed in the absence of a mechanical input and the mechanical input is used to open the switch.

• Mechanical switches are specified in terms of the number of poles, (that is the number of separate circuits that can be completed by the same switching action) and the number of throws (the number of individual contacts for each pole)

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Mechanical Switches

• Mechanical bounce– When a mechanical switch is opened or closed, the

contacts do not make or open cleanly. This is called mechanical bounce. This is due to an elastic member that bounces back and forth like an oscillating spring. This there is no clean signal for say 20 mS or so.

– A way of overcoming this is to include a delay in the software programme of 20 mS before any other signals are read.

– Other ways to over come this is to use latches. See Handout 1

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Mechanical Switches

• The term ‘limit switch’ applies to a switch that is used to detect the presence or passage of a moving part. This can be a cam, roller, or lever

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Lever pushed down by contact

The output changes from a normally open, ‘0’ to a closed contact ‘1’


Roller

Cam

Mechanical Switches

• Liquid-level switches are used to control the level of liquids in tanks. Essentially, these are vertical floats that move with the liquid level and this movement is used to operate switch contacts.

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Proximity Switches

• Proximity switches are used to detect the presence of an item without making contact with it.– Eddy-current type– Reed switch– Capacitive proximity switch– Inductive proximity switch

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Eddy Current TypeProximity Switch

Constant alternating current

Metal Object

Eddy Current

Back emf induced in coil, opposing the current producing the magnetic field

The voltage needed to maintain a constant current therefore changes. The voltage is thus a measure of the proximity of metal objects

X

The voltage can be used to activate an electronic switch (transistor) that has an output switched from low to high by the voltage change i.e. creating an on/off device

Range, typically 0.5 mm to 20 mm

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Proximity Switches

• Eddy-current Switch:– The target surface must be at least three times larger than

the probe diameter for normal, calibrated operation; otherwise, special calibration my be required.

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Proximity Switches

• Compared to other noncontact sensing technologies such as optical, laser, and capacitive, high-performance eddy-current sensors have some distinct advantages.– Tolerance of dirty environments– Not sensitive to material in the gap between the probe and target– Less expensive and much smaller than laser interferometers– Less expensive than capacitive sensors

• Eddy-Current sensors are not a good choice in these conditions:– Extremely high resolution (capacitive sensors are ideal)– Large gap between sensor and target is required (optical and

laser are better)

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Reed Switch NDGTA

Magnet Contacts

‘Springy’ strips

When a magnet or current-carrying coil is brought close to the switch, the strips become magnetised and attract each other. The magnet closes typically about 1 mm from the switch.

Widely used in burglar alarms

Reed Switch NDGTA

Capacitive ProximitySwitch

• The capacitance of a pair of plates depends upon the separation of the plates: the smaller the separation the higher the capacitance

• The sensor of the capacitive proximity switch is just one of the plates of the capacitor the other being the object (metal or non-metal) for which the proximity is to be detected.

• Thus proximity is detected with a change in capacitance• The change in capacitance can be used to activate an

electronic switch circuit and so create an on/off device• Range typically 4 mm to 60 mm.• E.g. may be used to detect the presence of a cake inside

a box on a production line.

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Capacitive ProximitySwitch

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Inductive ProximitySwitch

• An inductive proximity switch consists of a coil would around a ferrous metallic core.

• When one end of the core is place near a ferrous metal object, there is effectively a change in the amount of metallic core associated with the coil and so a change in its inductance.

• The change can be monitored using a resonant circuit, the presence of the ferrous metal object thus changing the current in that circuit.

• The change in current can be used to activate an electronic switch circuit and so create an on/off device

• Range typically 2 mm to 15 mm.• E.g. may be used to detect the presence of tops on bottles on

a passing conveyor

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Inductive ProximitySwitch

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Photoelectric Sensorsand Switches

• Two types of photoelectric switch device: transmissive and reflective– Transmissive: the object being detected

breaks a beam of light (usually infra-red radiation)

• Usually used in applications involved in counting of parts e.g. objects moving along a conveyor

– Reflective: the object being detected reflects a beam of light onto the detector

• Used to detect whether transparent containers contain liquids to the required level

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Light-emitting diode

Photo-detector

Light-emitting diode

Photo-detector

Object

Transmissive Type Reflective Type

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• The radiation emitter is generally a light-emitting diode (LED).

• The radiation detector might be a phototransistor or a pair of transistors called a Darlington pair (used to increase sensitivity).

• Depending on the circuit the output can be made to switch either high or low when light strikes the transistor

• A photodiode and photoconductive cell are further examples of photoelectric sensors.– With these sensors, light is converted to a current,

voltage or resistance change.

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Encoders

• The term encoder is used for a device that provides a digital output as a result of angular or linear displacement.

• An incremental encoder detects changes in angular or linear displacement from some datum position.

• An absolute encoder gives the actual angular or linear position.

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The BasicIncremental Encoder

Light

Single Aperture

Fixed Disc

Rotating Disc

Apertures

Detector

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When the disc rotates the light beam is alternatively transmitted and stopped.

The number of pulses is proportional to the angle through which the disc has rotated

The resolution is proportional to the number of slots in the disc.

E.g. 60 slots gives a resolution of 6 degrees

With off-set slots it is possible to have over 1-thousand slots for one revolution !!

The BasicIncremental Encoder

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Encoders

• The problem with the basic encoder is that there is just one track.

• With one track there is no way of determining the direction of rotation.

• Thus most encoder have two or three tracks with sensors.

• With two tracks, one track is ¼-of the cycle displaced from the other track.

• As a consequence the output from one track will lead or lag that from the other track depending upon the direction of rotation.

• A third track gives one pulse per revolution and so can be used to count the number of full revolutions.

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Absolute Encoder

• The absolute encoder differs from the incremental encoder in having a pattern of slots that uniquely defines each angular position.

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Basic form of anAbsolute Encoder

Apertures through which light can pass

0010

0011

0100

0101

0000

0001

011001111000

1001

1010

1011

1100

1101

1110

1111

The output from the 4-detectors depends on the position of the disc

LightBank of 4-detectors

Resolution of anAbsolute Encoder

• With 4-tracks the number of positions that can be detected is 24 = 16

• The resolution therefore is 360o/16=22.5o

• Typically encoders have 10 or 12 tracks. The number of bits in the binary number being equal to the number of tracks.

• Thus with 10-tracks the number of positions that can be detected is 210 =1024 i.e. 360o/1024 =0.35o

• In practice binary code is modified to what is termed the Grey Code

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Temperature Sensors

• A bimetallic strip:

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Brass

Iron contacts

Electrical circuit

The two metals have different coefficients of expansion

Thus when the temperature increases the strip curves – one metal expanding more than the other: the higher expansion metal being on the outside of the curve

Not very accurate: used commonly in domestic central heating thermostats

Temperature Sensors

• The Resistive Temperature Detector (RTD):

• The electrical resistance of metals of semi-conductors changes with temperature.– Metals commonly used are platinum; nickel or

nickel alloys– Such detectors can be used as one arm of a

Wheatstone bridge: the output of the bridge taken as a measure of temperature.

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The ResistiveTemperature Device (RTD)

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RTD

Output 12 v

S Q

R P

There is no output when the resistors in the bridge arms are such that P/Q = R/S

Any departure of a resistance from this balance value results in an output

One of the problems with a resistive thermometer is that the leads connecting to the bridge can be quite long and themselves have significant resistance which changes with temperature.

One way to over come this is to use a 3-wire circuit: changes in lead resistance thus affect two arms of the bridge and hence balance out

Thermistors

• Semiconductor devices such as thermistors show very large changes in resistance with temperature. However this change is generally non-linear.– NTC: negative temperature coefficient – i.e

the resistance decreases with increasing temperature

– PTC: Positive temperature coefficient – the resistance increases with increasing temperature

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Thermodiodes andthermotransistors

• Can be used as temperature sensors as the rate at which electrons and holes diffuse across semiconductor junctions is affected by the temperature.

• LM35 is an integrated package which gives an output of 10mV/oC when the supply voltage is +5V.

• A digital temperature switch can be produced with an analogue sensor by feeding the analogue output into a comparator amplifier which compares it with some set value producing an output that gives a logic 1 signal when the temperature value input is greater than the set point and otherwise gives a logic 0 signal

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Thermocouple

• A thermocouple consists of essentially two dissimilar wires forming a junction.

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Signal Processing

Metal A

Metal B

Hot Junction

Copper wire

Copper wire

Cold Junction

Thermocouple

• When the junction is heated so that it is at a higher temperature that the other junctions in the circuit, which remain at a constant temperature, an EMF is produced that is related to the hot junction temperature

• See handout for EMF values for thermocouples assuming that the cold junction is at 0oC

• The thermocouple voltage is small and needs amplification before it can be fed to an the analogue input of a PLC.

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Position DisplacementSensors

• The term position sensor is used for a sensor that gives a measure of distance between a reference point and the current location of the target.– Resistive linear and angular position sensors (Also

called linear and rotary potentiometers)

• A displacement sensor gives a measure of the distance between the present position of the target and the previous recorded position– Linear variable differential transformer (LVDT)– Capacitive displacement sensors

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Strain Gauges

• When a wire or strip of semiconductor is stretched, its resistance changes. The fractional change in resistance is proportional to the fractional change in length, that is, strain.

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ΔR = G x strainR

Changing plate separation

Changing area of overlap

Moving the dielectric

Where G is called the gauge factor

Pressure Sensors

• Pressure sensors can be designed to give outputs that are proportional to the difference in pressure between two inputs– If one port is left open to atmosphere, the gauge

measures pressure changes with respect to the atmosphere and the pressure measured is known as the gauge pressure.

– The pressure is absolute if measure with respect to a vacuum

– Two common types are the diaphragm and bellows type

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Other input devices

• Liquid-level sensors

• Fluid Flow measurement

• Smart Sensors

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Smart Sensors

• To use a sensor we generally need to add signal conditioning circuitry.

• This circuitry may be used amplify and covert analogue to digital, to get a sensor signal in the right form, take accounts of any non-linearities and calibrate it.

• Additionally we need to take account of drift – a gradual change in the properties of a sensor over time.

• Some sensors have all these elements taken care of in a single package: these are termed Smart Sensors

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Smart Sensors

• The term smart sensor is thus used in discussing a sensor that is integrated with the required buffering and conditioning circuitry in a single element and provides functions beyond that of just a traditional sensor.

• The circuitry with the element normally consists of data converters, a processor and firmware, and some form of non-volatile EEPROM.

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Smart Sensors

• Smart sensors can be produced on a single chip and can be programmed for specific requirements needed by the user.– i.e. it can be programmed to process raw data

, correcting for such things as non-linearities, before sending the data to a base station.

– Another example would be to programme the sensor to send a warning signal when it measures some critical value.

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Output Devices

• The output port of a PLC are relay, opto-isolator with transistor or triac depending upon the devices that are to be switched on or off.

• Generally, the digital signal from an output channel of a PLC is used to control an actuator, which in turn controls some process.

• An actuator is a term used for a device that transforms the electrical signal into some more powerful action which then results in control of the process

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Output Actuators• Using the internet research and make your notes on the following:• Relay• Latching relay• Contactor• Directional Control Values

– Solenoid operating value– Spool valve– Single acting cylinder– Double acting cylinder

• Motors– DC– AC– stepper

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Programmable Logic Controllers LO1: Understand the design and operational characteristics of a PLC...

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