Dave’s Revision Notes Programmable Systems Gary Plimer 2004
Programmable Systems Outcome 1Outcome 1 - Control SystemsThe purpose of this unit is to introduce the operation of control systems.When students have completed this unit they should be able to: Recognise and identify common control systems. Describe the operation of open and closed loop control systems. Make use of systems diagrams and systems technology.
Programmable Systems Outcome 1Systems DiagramsMost industrial product design is solved by the systems approach. Thisapproach involves studying the desired function of the product, and thenbreaking this function down into a series of subsystems.When applied to control systems, a Systems Diagram is a useful way ofvisually representing the desired function of the system. The systemsdiagram is a form of block diagram than contains all the subsystems within adashed box, called the systems boundary. The systems boundary indicatesthe extent of the control system. CONTROL OUTPUT INPUT OUTPUT DRIVER
Programmable Systems Outcome 1 CONTROL OUTPUTINPUT OUTPUT DRIVER The "real world" input and output conditions of the system are shown as arrows entering, and leaving, the systems diagram.
Programmable Systems Outcome 1A simplified systems diagram of a washingmachine is shown below. SET WASHING CYCLE CONTROL CLEAN COLD HEATER MOTOR PUMP HOT DIRTY WATER WATER DIRTY DRUM CLEAN CLOTHES CLOTHES
Programmable Systems Outcome 1Open Loop ControlAt the simplest level a control system can process an input condition toproduce a specified output.A good example of this type of system is a hand-held electric hairdryer. The heating element and fanmotor are switched on when the appropriateswitches are held down.SWITCH OUTPUT CONTROL FAN & HEATER HELD DRIVER SWITCHED ON DOWN
Programmable Systems Outcome 1SWITCH OUTPUT CONTROL FAN & HEATER HELD DRIVER SWITCHED ON DOWN This is an example of Open Loop Control, where an input is processed to produce an output. With the hairdryer example the heater and fan motor are held on until the switches are released. The air being blown out of the hairdryer is not temperature monitored or adjusted - the air is simply just blown out at whatever temperature the heater is capable of achieving.
Programmable Systems Outcome 1 DETECT SWITCH CONTROL OUTPUT LAMP LIGHT DRIVER LEVEL ON/OFFAnother Open Loop System isshown above. VccBy splitting the circuit shownopposite into INPUT, PROCESS& OUTPUT, describe theoperation of each subsystem 0V
Programmable Systems Outcome 1OPEN LOOP An open loop control system represents the simplest and cheapest form of control. However, although open loop control has many application, the basic weakness in this type of control lies in the lack of capability to adjust to suit the changing output requirements.
Programmable Systems Outcome 1CLOSED LOOP Closed loop control systems are capable of making decisions and adjusting their performance to suit changing output conditions. A personal cassette player is capable of detecting the end of the tape and switching the motor off, hence protecting the tape from snapping (or the motor burning out). END OF TAPE CONTROL OUTPUT DRIVER
Programmable Systems Outcome 1 CLOSED LOOP FEEDBACK SENSING CONTROL OUTPUT INPUT OUTPUT DRIVERAll closed loop control systems include a feedback sensing subsystem withinthe systems diagram. The control subsystem will process the feedback signal bymaking a decision on whether the state of the output should change.
Programmable Systems Outcome 1Pupil Assignment 1 Explain what is meant by the term systems diagram. Describe the differences between an open loop and closed loop control system. Describe the purpose of the feedback sensing subsystem in a closed loop control system.
Programmable Systems Outcome 1Pupil Assignment 2 The figures above show two types of electric fire. The second electric fire is a more modern device fitted with a thermostat. a) Name the type of control system used in each type of electric fire b) Draw a system diagram for each type of electric fire. c) Name a type of electronic sensor that may used for measuring temperature.
Programmable Systems Outcome 1 Pupil Assignment 3 LIMIT SWITCH SWITCH TRANSDUCER INPUT/OUTPUT INVERTER LATCH UNIT DRIVER UNITThe systems diagram for a simple liftcontrol system is shown below. MOTOR/ LIFT PLATFORM GEARBOX PLATFORM POSITION SENSORSTART CONTROL OUTPUT RAISE LIFT LIFT DRIVER PLATFORM Continued
Programmable Systems Outcome 1Pupil Assignment 3A limit switch at the top of the lift detects when the lift platform has reached thetop of the run. The block diagram below shows the system diagram sub-systemsbroken down into smaller blocks. LIMIT SWITCH START SWITCH TRANSDUCER INVERTER LATCH LIFT UNIT DRIVER MOTOR GEARBOX RAISE LIFT PLATFORM Continued
Programmable Systems Outcome 1 Pupil Assignment 3 +VSimulate the circuit shown.a) Clearly explain S Q how the system operates. R Q 0V RELAY (b) Explain the purpose of the relay, transistor and diode within the relay driver circuit.
Programmable Systems Outcome 1Pupil Assignment 4Build the circuit shown, using modular electronics system boards.a) Draw a system diagram of the control system.b) Draw a circuit diagram of the control system.c) Explain the operation (function and effect) of the DPDT relay. LIMIT SWITCH 1 INPUT/OUTPUT TRANSDUCER INPUT/OUTPUT INVERTER LATCH UNIT DRIVER UNIT MOTOR/ LIFT PLATFORM GEARBOX +V LIMIT OV SWITCH 2
Programmable Systems Outcome 2Outcome 2 - Analogue Control SystemsThe purpose of this unit is to introduce the operation of analogue control systems.When students have completed this unit they should be able to: Correctly identify common applications of analogue control. Represent analogue control systems using control diagrams and circuit diagrams. Select the appropriate configuration of op-amp in the design of analogue closed loop control systems. select appropriate output drive subsystems for control applications.
Programmable Systems Outcome 2 Analogue Closed Loop Control Systems V1 Vo V2Many control systems involve processing analogue signals such as heat, lightand movement. Therefore analogue closed loop control systems requireanalogue processing devices such as the operational amplifier (op-amp).One of the most common control applications involves using the op-amp as acomparator. In its simplest form a comparator just compares two voltagesignals, V1 and V2. If V1 is higher than V2 the output, Vo, is low, if V1 is lowerthan V2 the output is high
Programmable Systems Outcome 2 Pupil Assignment 1The block diagram below simulates an automatic heating system. Whenthe bulb heats up the temperature rise is detected by the bead thermistor,which sends a feedback signal back to the comparator. BEAD THERMISTOR INPUT/OUTPUT TRANSDUCER BULB UNIT COMPARATOR UNIT DRIVER Continued
Programmable Systems Outcome 2 Pupil Assignment 1 TEMPERATURE FEEDBACK SIGNAL +VCCR RT The circuit for the heating system is shown oppositeRV RVR OV -VCC Continued
Programmable Systems Outcome 2 Pupil Assignment 1 The variable resistor, RV1 (connecting to the non-inverting input of the op- amp) is used to set the reference level (or threshold). This sets the desired temperature of the bulb. Simulate the circuit shown. Calibrate the system to the following performance criteria: When the bulb heats above the reference level the thermistor should sense the temperature and send a signal to the comparator which will switch the bulb off. When the bulb cools below the reference level the bulb should switch on again. The system should operate continuouslya) Describe how the circuit operates.b) Explain clearly how you calibrated the system.
Programmable Systems Outcome 2 Control Diagrams TEMPERATURE SENSOR SET CONSTANT CONTROL OUTPUT TEMPERATURE OUTPUT DRIVER LEVEL TEMPERATUREThe circuit constructed in the last activity, can be shown as a ControlDiagram, (see above).However, when we use an Op-Amp, the circuit shown on the next slide is used.
Programmable Systems Outcome 2 Control Diagrams TEMPERATURE SENSOR SET OPERATIONAL OUTPUT CONSTANT TEMPERATURE AMPLIFIER DRIVER OUTPUT LEVEL TEMPERATURE The error-detectionThe control diagram symbol is also used tobreaks the generalised indicate that the controlcontrol subsystem of the involves two signals. Thesystems diagram into feedback signal is connectedmore specific blocks. to the negative symbol to indicate the use of negative feedback
Programmable Systems Outcome 2 Negative Feedback The purpose of closed loop control is to ensure that the output is maintained, as closely as possible, to the desired output level. In the case of a central heating system, a graph of the temperature in a room might appear as in the graph below. ACTUAL TEMPERATURE As can be seen from the graph, the control system isTEMPERATURE constantly trying to pull the SET TEMPERATURE temperature of the room back towards the set temperature level by reducing the error. This type of control uses negative feedback to reduce the error. TIME
Programmable Systems Outcome 2 Positive FeedbackThe opposite effect can be created by reinforcing the error, as cansometimes happen with public address systems when the microphone isheld too close to the speakers. A sound is picked up by themicrophone, amplified, and then output through the speaker. Theamplified sound is then picked up, re-amplified and so on. The net resultis a high pitch sound, which can be represented by the graph below ACTUAL SOUND SOUND SIGNAL SIGNAL REQUIRED SOUND SIGNAL TIME
Programmable Systems Outcome 2Pupil Assignment 2 Explain the following terms when applied to control systems: open loop, closed loop, negative feedback, positive feedback, error detector
Programmable Systems Outcome 2Positional ControlA good example of positional control is the model servo motor commonlyfound in radio-control cars and aeroplanes.The servo contains a motor, gearbox, feedbackpotentiometer and electronic control circuit on asmall printed circuit board.The feedback potentiometer is directly connectedto the output shaft. Therefore, when the motorspins, the output shaft is turned slowly by thegearbox, which in turn moves the potentiometer.Naturally movement of the output shaft isrestricted to 180º as the potentiometer cannotspin continuously.
Programmable Systems Outcome 2Positional Control The control diagram for the servo is shown below. FEEDBACK POTENTIOMETER SET OPERATIONAL OUTPUT CONSTANT POSITION AMPLIFIER DRIVER POSITIONNote how the feedback signal from the feedback potentiometer is fed into thenegative symbol of the control diagram, indicating negative feedback.
Programmable Systems Outcome 2 Positional ControlPush-Pull Follower Analogue DriverAn important element in producing +Vccpositional control, such as that requiredin the servo, is a dual rail push-pull NPNfollower analogue driver circuit, thatallows the motor to spin in both OP-AMP LOAD SIGNALdirections. The op-amp cannot sourcesufficient current to drive most output PNP 0Vcomponents, and so the dual rail push-pull follower is required to drive the -Vccoutput components.The dual rail push-pull follower is basedaround a two transistor circuit as shownopposite.
Programmable Systems Outcome 2Positional ControlWhen the signal from the op-amp is positive the NPN transistor will switchon, and current will flow through the load from the positive supplyrail, +Vcc, to the ground rail, 0V. +Vcc I I I OP-AMP +V LOAD SIGNAL I 0V -Vcc
Programmable Systems Outcome 2Positional ControlWhen the signal from the op-amp is negative the PNP transistor will switch onand current will flow through the load from the ground rail, 0V, to the negativesupply rail -Vcc. +Vcc I OP-AMP -V LOAD SIGNAL I I 0V I -Vcc
Programmable Systems Outcome 2Pupil Assignment 4A proportional control system isused to regulate the flow rate ofcoal onto a conveyer belt. The POWER SUPPLYsystem should sense the weight ofcoal on the conveyer belt and RACK AND CONTROLautomatically adjust the gate height PINION UNITto ensure that a constant flow ofcoal is supplied. VARIABLE HEIGHT GATE P CONVEYOR FRAME PIVOTED HERE CONSTANT SPEED DRIVE MOTOR LOAD SENSOR
Programmable Systems Outcome 2Pupil Assignment 4• Draw a systems & control diagram of the flow rate control system• Explain the term proportional control‘• Name the configuration of op-amp used in proportional control systems POWER SUPPLY• Name a suitable output driver circuit RACK AND CONTROL which could be used PINION UNIT with the control system VARIABLE HEIGHT GATE• Draw a circuit diagram of the driver P circuit. CONVEYOR FRAME PIVOTED HERE CONSTANT SPEED DRIVE MOTOR LOAD SENSOR
Programmable Systems Outcome 2Pupil Assignment 5The figure shows the layout of an audio mixing desk. TACHOGENERATOR AUDIO MIXING The tape is fed through the audio HEAD mixing head by being pulled on to a recording reel driven by a dc motor. As the tape builds up on the recording reel, the tape speed through the audio mixing head will increase. To prevent this from happening, the dc RECORDING REEL DRIVEN BY DC MOTOR motor is fitted to a closed loop control system. A tachogenerator connected to a pulley senses the tape feed rate and sends an error signal to the control system.
Programmable Systems Outcome 2 Pupil Assignment 5• Name the type of closed loop TACHOGENERATOR control used in this AUDIO MIXING HEAD application• Name the amplifier used in this type of closed loop control• Draw a control diagram of RECORDING REEL DRIVEN BY DC MOTOR the system• Draw a circuit diagram of the control system and explain the function of each part of the circuit.
Programmable Systems Outcome 2Pupil Assignment 6The figure illustrates a system for controlling the wing mirrors on a car byadjusting remote dials on the dash. DIALSA control diagram of the system forrotational movement in the X-axis (onemirror) is shown in the figure below.Similar systems are used for the Y-axisand for the other mirror. FEEDBACK SENSOR REMOTE OPERATIONAL OUTPUT WING MIRROR MOTOR DIAL AMPLIFIER DRIVER POSITION
Programmable Systems Outcome 2Pupil Assignment 6 FEEDBACK SENSOR REMOTE OPERATIONAL OUTPUT WING MIRROR MOTOR DIAL AMPLIFIER DRIVER POSITION• With reference to the control diagram, explain clearly how the system operates• Name the type of control used in this system• Name the configuration of op-amp required• State two reasons why the op-amp cannot be used to drive the motor directly• Name a suitable output driver which could be used with this system.
Programmable Systems Outcome 2Sequential Control Systems Sequential control is used to control systems whose outputs are required to follow a fixed cycle of events (i.e. a sequence). An interesting example of sequential control was used in music boxes. Actuators on the drum are arranged to produce notes, as the rum rotates the notes are played in the correct sequence to produce a tune. A more modern example of sequential control is a robot arm used to weld cars together on a large production line.
Programmable Systems Outcome 2Common Input Transducers used in Computer Control Potentiometers are used to provide feedback on position. This might be a simple potentiometer or a combined system such as a servo motor. Tachogenerators are used to provide feedback on speed. Light Dependent Resistors are used to provide feedback on lux level changes. Thermistors are used to provide feedback on temperature changes.However, there are other methods available to us, Slotted Discs, BinaryCoded Discs, Grey Scale Discs.
Programmable Systems Outcome 2Binary Coded Disc Slotted Disc A problem with Binary discs is that more than 1 bit can change at a time which may cause errors. A gray disc Gray Code changes only 1 bit at each stage.
Programmable Systems Outcome 2We need to be aware of the types of signal produced from these encodersare and how a computer could use them to control its output.Consider each of the encoders or input transducers, statefor each the type of signal produced and how this couldbe used by the microcontroller.