POSITION CONTENT EQUIPMENT REQUIRED CONTROL ASSIGNMENT SYSTEM The use of motor drive in an open system is investigated. loop positional control Qty Designation Description 1 OA150A Op Amp Unit AU150B Attenuator Unit PA150C Pre-Amp Unit SA150D Servo Amplifier PS150E Power Supply DCM150F DC Motor IP150H Input Potentiometer OP150K Output Potentiometer GT150X Reduction Gear Tacho Unit DC Voltmeter 30-0-30V (or voltmeter in GT150X) PRELIMINARY PROCEDURE mM Attach the OA150A, PS150E, IP150H and OP150K to the Baseplate by means of the magnetic fixings. m™ Connect the Power Supply to the mains supply line. DO NOT switch on yet. 56 5 oa (ae fi eine tot te ee SIMPLE * Modular Servo Scanned by CamScanner Scanned by |b bok ww Modular Servo SIMPLE POSITION OBJECTIVE CONTROL ASSIGNMENT 5 When you have completed this assignment you will: ™ M™ KNOWLEDGE LEVEL SYSTEM Have observed the operation of a basic open-loop position control system. Appreciate the need for closed-loop feedback in a position control system. Before starting this assignment you should: M™ Be familiar with the components of the DC Modular Servo and preferably have completed Assignment 1, Familiarisation. Understand the term ‘error channel’. mM Understand how two rotary potentiometers can be used to form an error channel. BELEK EUDUDU UU Mm a7 Scanned by CamScanner Scanned by Position INTRODUCTION Control System Assignment 5 Two rotary potentiometers can be used to generate an error signal to show the misalignment of the output cursor with that of the input cursor. Now if the output potentiometer is mounted on the shaft of a geared motor, we would have the basis of an automatic position control system. That is, we use the error signal to drive the motor in a direction such as to reduce the misalignment to zero. Before going on to carry out such an investigation, it would be as well to see what happens in a manual system. So set up as in fig 3.5.1, but remember students will to work together to that ideally two carry out the al uk kk investigation. have ©) Input Praeriicameter ® Op. Awp. Unit OAISOA Fig 3.5.1 58 (alla ec at a ft a Mat Simple nn el eu el ede bee Modular Servo Scanned by CamScanner Scanned by Modular Servo Glinple Poaltion Control System PRAGCTIGAL Manual Control Assignment 5 6.4 Posttion System Sel the feedback selector switch on the OA150A to the 100kQ resistor, Set the rotary potentiometers to zero and zero operational amplifier output, using the voltmeter. set the Initially one student should slowly turn the knob on the input potentiometer and set it at any value in the ranges 0 to -15V and 0 to 415V. Let the other student watch the voltmeter connected to the output Vo and by turning the shaft of the other potentlometer return the voltmeter reading to zero. rotary After setting the input cursor to various positions, each time first returning to zero, try seeing if the student operating the output potentiometer can manage to keep the needle of the voltmeter at zero if the position of the input is slowly and continuously varied. Finally, try suddenly changing the position of the input cursor, whilst attempting to keep the error at zero. Question 5.1 Write down the needle, zero point. your observations on what for the different inputs, as happened it neared to its You will, no doubt have noticed that it was quite easy to get the voltmeter to register zero for a slow movement but when the movement was fast (step input), the correcting action needed to bring the needle to zero could not occur immediately because of over-compensation or what is known as overshoot. Now if we analyse our experiment we can see that the position of the input potentiometer slider gives us a reference voltage and that the position of the output potentiometer slider is used to feed back an inverted signal that is summed and compared in the operational amplifier to form our error signal Vo, which will then give a reading on the voltmeter. In this system muscle power is the Controller with finger movement being used as the Actuator. There is even a stage of pre-amplification in which our eye observes the meter reading and signals the brain to send a message along the nerve fibres to the muscles in our arm and hand. = Scanned by CamScanner Scanned by CamScanner Modular Servo reference { transducer + 3 3 comparator (eyes) — pre-amplifier (input : : potentiometer) } oeneacacncsecnst 5 Assignment System Control Position | actuator controller (brain) (muscles) (fingers) transducer (output jg $< potentiometer) Fig 3.5.2 Because in fig 3.5.2 information is fed back for comparison with a reference signal, the system is a closed loop one. We are now going to show how the motor can be used make a simple automatic positional control system. As a to first step we shall simply cause the motor, through a gear box, to drive the shaft on the output position. What sort of automatic control system would As there is no electrical feedback signal to form signal, the system is an open loop one. an this error be? PTT 5.2 anapnpp? Question potentiometer to a required IEeeee) Simple 60 Scanned by CamScanner Scanned by Modular Servo Simple Position Control System Assignment 5 6 tT 8 YY Pi Feedback DCMI150F LE ren / owe deM | / i Reduction Gear Tacho Unit / O_)) w 1 Oo ov a 1 Jum @a Sow Fl input Potentiometers eet | El ouput Potentiometer PASAT ° OPIS Fig 3.5.3 62 Scanned by CamScanner Scanned by a Modular Servo Simple Position Control System PRACTICAL 5.2 Assignment 5 Attach the AU150B, SA150D and DCM150F to the Baseplate by means of the magnetic fixings. Fix the plug from the Servo Amplifier into the Power Supply. Fix the plug from the Motor Unit into the Servo Amplifier. Attach the GT150X to the baseplate by means of the magnetic fixing and position it so that it is connected to the motor shaft by means of the flexible coupling. Set up as in fig 3.5.3 A simple open-loop Control motor-driven Position System Use the push-on coupling to link a low-speed GT150X to the output potentiometer shaft. Starting with AU150B the potentiometer knob shaft of the at the fully counter-clockwise position gradually turn it till the motor just rotates and record: Scale position at which the motor just rotates (1) Direction in which the output rotary potentiometer moves (2) Return the output rotary potentiometer cursor to zero by turning the GT150X high-speed shaft. Decide on a position in the direction (2), you wish the potentiometer shaft to turn to and then turn the AU150B potentiometer knob to position (1). As the cursor nears the required angle, reduce this input signal so that the cursor comes to rest nearly at the required point. Repeat using increasingly large input signals instead of value (1). Question 5.3 What happens as you try to get rest at the required angle? the cursor to come to The system is limited in that the motor drive was unidirectional; let us see if we can control the direction of rotation of the motor by, say, reconnecting Terminal connection of the power supply. Question 5.4 1 on the AU150B to a -15V What result did you get on changing from a positive to a positive and negative supply and trying to drive the motor? 63 Scanned by CamScanner Scanned by Control System Assignment 5 As Terminal 3 is at +15V and Terminal 1 at -15V then positions 0 to 5 will give you decreasing positive voltages and 5 to 10 increasing negative voltages. Now let us look at the mimic on the Servo Amplifier, fig 3.5.4. Inputs 1 and 2 are connected to a pair of transistors which are part of a circuit arrangement known as a ‘long tailed pair’. The emitter currents of each transistor are added together and the total current is held at a fixed value by a constant current source which is represented on the mimic diagram by the resistor to -15 volts. When both inputs are at the same voltage, the current in each transistor will be equal and so the output voltage (measured at the collector of each transistor) are the same. This means that no current flows through the motor. SA150D Feedback +24V control voltage ree 9s monitor control ov +15V -15V OV current monitor -15V Servo Amplifier Fig 3.5.4 7 Position rpreprrrrrrprrr Simple 8 FPF HHASAAASAAAAKRSHA Modular Servo Now if the voltage at Input 1 is raised, then the current through transistor 1 increases (and so the voltage at its collector falls). But, since the sum of the emitter currents is fixed this means that the current in the other transistor must reduce (and so the voltage at its collector increases). 64 So we get a voltage Scanned by CamScanner Scanned by Simple Position Control Assignment System 5 a la eh Modular Servo amplifiers to provide a high current drive for the motor armature. Similar reasoning applies if we have the voltage at Input 1 lower than Input 2 except that the output voltage across the motor is reversed and so it rotates in the other direction. To summarise; the circuit arrangement provides two ways to drive the motor as shown in fig 3.5.5. * + * armature Vi, i vi Vin [ V2 - OV Fig 3.5.5 ( | Luu ( ,ueUDECDROXDDBY difference across the collectors which is then input to power Dual input in which speed in each direction is determined by which of the two inputs, Vi or Vo, is the greater. The motor is stationary when Vy = Vo. Any voltage which is common to both inputs will not result in any motor current and this is often an advantage, for instance, in industrial systems where there may be noise or interference affecting the signal wires to a power amplifier. In later assignments the necessary input signals are provided by a specialised Inputs 1 and 2. Pre-Amplifier Unit which is connected to “aT yO ‘ae » wh ib 18 “wb ib a tb a ib ip ib { S| bu Single input in which direction and speed are controlled by varying either V4 or Vo. 65 Scanned by CamScanner Scanned by Modular Servo Simple Position PRACTICAL ASPECTS Control Assignment System 5 Let us draw together the threads of understanding that we have gained from this assignment. We started by setting up a control system that produced an error which we visually had to interpret before making a manual correction to the system output. It was seen that the delay between the change in state i.e a step input and action to follow could cause over-compensation or ‘overshoot’. We then started to mechanise the system by attaching a geared motor to the output rotary potentiometer but found that the control was difficult because visual interpretation was needed to determine when the motor should be stopped. In the following assignment we will deal with three points. The use of a special amplifier, the Pre-Amplifier that can output forward or reverse drive signals depending on whether it receives a positive or negative input. Using the error signal as an input into the Pre-Amplifer, with its magnitude used to determine the amount of motor drive and its polarity as to the direction of rotation. However, the last item of overshoot can be a more difficult problem to deal with and later assignments will spend some considerable time on some of the methods available to deal with the problem. 66 Scanned by CamScanner Scanned by TYPICAL RESULTS AND ANSWERS ASSIGNMENT 5 ~F i y eo ee Modular Servo = Question 5.1 You will find that for a fixed position of the input dial the meter can be set to zero quite rapidly and accurately. For a steady rotation of the input it will be difficult to hold the Pn meter exactly at zero. os == er For sudden dial movements the meter can be brought to zero but only after a large initial error and possibly several overshoots. wo This will be an open-loop control system. =a | - ‘ Question 5.2 = . | =~ Question 5.3 ‘ Question 5.4 The output dial will usually overshoot the desired position but, if reactions are fast, it could fall short of it and then be ‘inched On’ to the desired position. The output dial can now be brought to a desired position by g rotating in either direction although this is difficult to do manually. = = i> i ——? —_ = — | 2 a) 2 2 2 @ @ 67 Scanned by CamScanner Scanned by CLOSED-LOOP POSITION SYSTEMS CONTROL ASSIGNMENT 6 The use of a servo motor in an automatic position control system is investigated. EQUIPMENT REQUIRED Qty Designation Description 1 OA150A Op Amp Unit AU150B Attenuator Unit PA150C Pre-Amp Unit SA150D Servo Amplifier PS150E Power Supply DCM150F DC Motor IP150H Input Potentiometer OP150K Output Potentiometer GT150X Reduction Gear Tacho Unit 3 CONTENT fF § RKCKAERKAAAAAA SS § 8 SF _§_8. Pe Pr_. RP PRPPS Modular Servo DC Voltmeter 30-0-30V (or voltmeter in GT150X) m Attach the AU150A, PA150C and PS150E to the Baseplate ™ Connect the Power Supply to the mains supply line. DO NOT switch on yet. Pf } aS by means of the magnetic fixings. 68 i PRELIMINARY PROCEDURE Scanned by CamScanner Scanned by Modular Servo CLOSED-LOOP OBJECTIVE POSITION SYSTEMS ASSIGNMENT 6 When you have completed this assignment you will: M™ M@ KNOWLEDGE LEVEL ConTROL Have established the operating characteristic of the Pre-Amplifier Have observed the action of a simple motor driven closed-loop position control system Before starting this assignment you should: mM Be familiar with the components of the DC Modular Servo and preferably have completed Assignment 1, Familiarisation. m@ Understand the operation of electronic amplifiers. mM Understand the operation of a simple position control system. 69 Scanned by CamScanner Scanned by sdular Garvo Closed-Loop Position Control Assignment Systems 6 Set up as in fig 3.6.1 and switch on power, @ Aterandor Unit AUISB @ Preamplifier Unld PAIS Switch In ‘normal position @ Power Supply ' J Fig 3.6.1 In this assignment we will look at the Pre-Amplifier, PA150C, which has two signal inputs and two outputs. If there is a positive voltage on either of its inputs then one of its outputs INTRODUCTION becomes positive; whilst if one of its outputs becomes negative then the other output becomes positive. Before we use the PA150C to control the motor, we will carry out a practical to find its characteristics. PRACTICAL 6.1 Characteristics the of Pre-Amplifier PA150C On the upper potentiometer of AU150B, using the voltmeter, set the output between terminal 2 and OV to +1V. To be able to obtain accurate readings we need a fine control and so we are going to use the lower potentiometer to give us fractions of the one volt that we have selected. Connect terminal 6 of the lower potentiometer to terminal 2 of the upper potentiometer. This means that the scale positions 1 to 10 give us input values in tenths of a volt. We are now in a position to measure output values of the PA150C for different values of inputs to see how the amplifier behaves. 70 Scanned by CamScanner Scanned by Madular Sorva Closed-Loop a ST Poston TSR Control RTS RTS Ee Ral ence Systems Assignment 6 = For each scale position on the lower potentiometer take readings with the valtmeter of each of the PA150C outputs win respect to OV as In tig 3.6.2. After you have done this repeat for the differant outputs with the voltmeter across both outputs. Record your results In your own copy of fig 3.6.2 Now connect tarminal 3 of the upper potentiometer to the -15V Supply and repeat your readings. Input signal Pre-amplitier output Vi volts Vo (3) Vo (4) Vo (4-3) Fig 3.6.2 Exercise 6.1 Plot as on graphs of in fig 3.6.3 how amplifier you the and have outputs. input fig volts 3.6.4. connected against The the the curves voltmeter output will volts depend across the 71 Scanned by CamScanner Scanned by CamScanner Systems Control Assignment A 15 pre-amplifier output V (4) \ > e| | | 2 > \ 3 2 \Lat 3 / HAA BAAR pre-amplifier output V 9(3) {10 ‘a. \Let I E o Q 40.4 40.30 402 «40.1 Qo 0.1 02 pre-amplifier input volts V j Fig 3.6.3 03 04 O05 -06 fe 205 nner +0.6 6 PP RHF HH Position annannaen Closed-Loop SSA SAADA. Modular Servo 72 Scanned by CamScanner Scanned by CamScanner Modular Servo Position Closed-Loop > _ |-08 Y 2 |-o6 $ | 2 \ 16 s -0.4 E -0.2 3 PS “12 NY 6 Assignment Systems Control 8 Qa. -8 a 0 voltage across pre-amplitier +4 +8 +12 outputs V,6(4-3) +0,2 > «+16 V N. 40.4 » +0.6 } +0.8 Fig 3.6.4 The ratio of the output voltage V, to the input voltage V; gives the gain K. Exercise 6.2 Question 6.1 State why you should measure straight part of the curve. Question 6.2 Explain Question 6.3 State the range of should be kept to. Question 6.4 What input outputs? Using the the straight Pre-Amplifier. curves. the reasons value part for of the the signals will give curves find the non-linear that a you nil the gain gain on portions think voltage the of the of the inputs across the If the answer to this is not zero it is because there is a zero offset in PA150C. This can be eliminated using the ‘zero set' control. 73 Scanned by CamScanner Scanned by Closed-Loop Position PRACTICAL 6.2 A simple Motor Driven Closed-loop Position Control System Control Assignment Systems 6 Retain the units of the first practical and also attach the OA150A, DCM150F and IP150H and OP150K Baseplate by means of the magnetic fixings. to the Connect the Servo Amplifier connector plug into the Power Supply. Connect the Amplifier. Motor Unit connector plug into the Servo We are now in a position to use the outputs of the PC150C to control the motor rotation and provide to it an error signal from the OA150A Set up as in fig 3.6.5. 74 AAN. RePe AARAAA RRPR SF ATAAAA PePprrrrrPrr Modular Servo Scanned by CamScanner Scanned by Closed-Loop Position Assignment Contro} Systems 6 ea et {ia » » » vb bebo 1 Ecce | Grace 1 ead) as bw ed Rema bw eet a a a aad wa ve ee TUT Ob 7% i ée¢¢ed Modular Seno Ons pad Avomdum eer OP1SeK Fig 3.6.5 75 Scanned by CamScanner Scanned by bs Modular Servo Closed-Loop Position Control This time Assignment Systems we shall utilise the error signal output 6 V, of the operational amplifier to drive the output potentiometer via the pre-amplifier and motor. The upper potentiometer on the AU150B can now be used as a gain control and should initially be set to zero before switching on the power. Adjust the PA150C ‘zero set' so that the motor does not rotate. Now set the IP150H to some arbitrary angle and increase the gain control setting. The output potentiometer cursor should rotate to an angle nearly equal to that of the input potentiometer cursor . If the output cursor stops before arriving at the set position, one is faced with the fact that the system is tolerant to an error and the motor will not respond till the error exceeds a certain value. In the next assignment we shall study this effect, which is know as 'deadband'. For the purpose of this practical, increase the gain so that this tolerance is overcome and you get the correct alignment. Exercise 6.3 Note the different results obtained in your copy of fig 3.6.6 for several settings of the input shaft angle. Output cursor position in degrees Required PRACTICAL ASPECTS Actual Misalignment Fig 3.6.6 In a position control system it is of great importance that there should be a smooth stop at any required position. If there is an overshoot then there must be a reverse movement to compensate. If movement is rapid where the system gain is high, it becomes very difficult to get smooth, accurate arrest. Indeed, the correcting movement might also produce an overshoot that will again need correction and in an extreme case a system could be moving to and fro across a required position in a state of instability. So far we have dealt with motors in the unloaded state. The Speed/Torque characteristics of a motor may not produce a suitable response under load. To overcome this problem gearboxes are very often used. However, they then introduce the further problem of free play in the gears or backlash. 76 Scanned by CamScanner Scanned by