Programmable Logic Controllers

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

2. Learning Outcome 1 • LO1: Understand the design and operational characteristics of a PLC system • 1.2 describe different types of input and output device

3. Lesson Outline • At the end of this session the students will be able to… • Specify the I/O units of a PLC. • Input devices • Output Devices

4. 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

5. 3. Output Devices • Relay • Directional Control Valves • Motors • Stepper Motors

6. 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.

7. 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.

8. 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.

9. Parameters used todescribe input devices • Errors can arise in a number of ways… • Non-linearity erroris 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 erroris 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

10. Parameters used todescribe input devices Measured Value Sensor Output Assumed relationship Hysteresis Error Decreasing Actual relationship Increasing Nonlinearity error True Value Value being measured Non-linearity Error Hysteresis Error

11. Parameters used todescribe input devices • Range: • the limits between which the inputs can vary

12. Parameters used todescribe input devices • 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

13. Parameters used todescribe input devices • 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.

14. Parameters used todescribe input devices Response Steady state reading Time

15. Parameters used todescribe input devices • 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

16. Parameters used todescribe input devices • 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.

17. Parameters used todescribe input devices • 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.

18. Parameters used todescribe input devices • Reliability: • the probability that it will operate to an agreed level of performance for a specified period subject to environmental conditions.

19. 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%)

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

21. 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

22. Mechanical Switches Supply Voltage Supply Voltage PLC Input Channel Work piece not present: 0 Work piece present: 1 The sense of the input is now changed Work piece not present: 1 Work piece present: 0

23. Mechanical Switches • Switches are available with normallyopen(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.

24. 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 ofthe number of poles, (that is the number of separate circuits that can be completed by the same switching action) and thenumber of throws(the number of individual contacts for each pole)

25. 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

26. 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 Lever pushed down by contact The output changes from a normally open, ‘0’ to a closed contact ‘1’

27. Mechanical Switches Roller Cam

28. Mechanical Switches

29. 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.

30. 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

31. Eddy Current TypeProximity Switch 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 Constant alternating current Range, typically 0.5 mm to 20 mm Metal Object X Eddy Current The voltage needed to maintain a constant current therefore changes. The voltage is thus a measure of the proximity of metal objects Back emf induced in coil, opposing the current producing the magnetic field

32. 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.

33. 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)

34. Reed Switch 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

35. Reed Switch

36. 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.

37. Capacitive ProximitySwitch

38. 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

39. Inductive ProximitySwitch

40. 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

41. Photoelectric Sensorsand Switches Transmissive Type Reflective Type Light-emitting diode Light-emitting diode Object Photo-detector Photo-detector

42. Photoelectric Sensorsand Switches

43. Photoelectric Sensorsand Switches • 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.

44. 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.

45. The BasicIncremental Encoder Light Detector 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 Fixed Disc Rotating Disc Single Aperture Apertures 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 !!

46. The BasicIncremental Encoder

47. 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.

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

49. Basic form of anAbsolute Encoder Bank of 4-detectors Light Apertures through which light can pass 1111 0000 0001 1110 0010 1101 0011 1100 The output from the 4-detectors depends on the position of the disc 1011 0100 0101 1010 1001 0110 1000 0111

50. 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