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ove hoot For temperature control without overshoot rs PID Temperatur What’s a PID controller? PID stands for “proportional integral differential” and relates to a process which seeks to continuously correct the error between a measured variable and a desired setting by calculating an appropriate correction process. In practice, it can largely avoid the large overshoots and undershoots that occur in simple temperature control systems. By LEONID LERNER 58  Silicon Chip siliconchip.com.au re Controller K EEPING TIGHT TEMPERATURE control is essential in many processes. For instance, good temperature control of a PC board etching bath is essential for best results. Too low a temperature and the process will be very slow, while too high a temperature will cause the etch resistant film to degrade and the solution to steam appreciably. Another, arguably more important, process where precise temperature control is vital is in a good home brew! Getting good temperature control is not as easy as it may seem. Consider the setup in the photo at left and represented below in Fig.1. When the hot plate is turned on, heat passes from the hotplate to the solution through the walls of the container. When the temperature of the solution reaches the desired value, the controller (the Digital Thermometer/ Thermostat featured in the August Fig. 1: model diagram of vessel on a hotplate showing equivalence to an electrical circuit consisting of a series connection of two RC circuits. THERMOMETER C3 VESSEL R2 R3 I R2 C1 2002 issue of S ILICON CHIP) switches the hot plate off but the temperature will continue to rise. This is because there is significant thermal resistance in the surface contact between the container and the hot plate, so that the temperature of the hot plate when it is turned off is much higher than the set temperature. The actual amount by which it is higher than the set temperature depends on the relationship of the set temperature to the maximum temperature attainable by the hot plate. For instance, a 2000W hot plate was found to have heated to 150°C when the temperature of the solution reached the optimal temperature of a persulphate bath – 65°C. Heat therefore continues to pass through the walls of the container HOT PLATE Temperature difference Thermal resistance C1 R1 OUT R3 R1 = Power flow Voltage difference Resistance = Current Temperature difference x Heat Capacity Voltage difference x Capacitance = Power flow x Time = Current x Time siliconchip.com.au C3 and the temperature continues to rise, until the hot plate and the solution are in thermal equilibrium. The result is temperature overshoot. Furthermore, after the peak temperature has been reached the system starts to cool down towards the desired temperature and the hot plate turns on again at the set temperature. The overshoot repeats, although this time it is smaller. The end result is that with on/off control, the temperature of the system in Fig.1 oscillates periodically. For etching PC boards, the result is the inconvenience of a lot of steam being generated but for many processes such as distillation, overshoot is simply not permissible. Our aim here is to understand the operation of the thermal delay of the system in Fig.1 and counteract it to achieve good temperature control. To do this we need to design a controller which has a more complicated response than simply on/off. But first we shall develop a model to make it easier to understand what is happening. It turns out that the thermal setup on the left side of Fig.1 is well modelled by an electrical circuit consisting of resistors and capacitors, as shown on the right side of Fig.1. This is more than just a convenient picture; it is based on actual mathematical July 2007  59 OVERSHOOT (DEGREES) DEGREES SECONDS SET TEMPERATURE / MAXIMUM TEMPERATURE Fig.2: temperature plot of a typical “switched” hot-plate which has an initial large overshoot, followed by a series of smaller over-temperature peaks. correspondence. If we make the equivalence temperature power <-----> <-----> voltage current then the equations in Fig.1 show that Newton’s law of cooling corresponds exactly to Ohm’s law, while the Law of Heat Capacities corresponds exactly to Coulomb’s Law for the capacitor. Therefore, we can view the thermal system as a good approximation to a passive resistor-capacitor network. CONTROLLER Fig. 3: temperature overshoot for a typical vessel on a hot-plate as a function of set temperature for equal time constants (blue line) and their ratio equal to 5 (red line). The heat capacities of the hot plate and of the vessel correspond to capacitors C1 and C2, the thermal resistance between the hot plate and the vessel corresponds to resistor R2, and the heat loss of both the hot plate and vessel to the surroundings is modelled by resistors R1 and R3, respectively. The ambient temperature in the thermal system then corresponds to earth potential in the electrical circuit. However, it is easy for confusion SYSTEM Tset Tout G(s) C(s) Tout = Tset x C(s) x G(s) (a) – OPEN LOOP CONTROL CONTROLLER Tset SYSTEM – Tout C(s) G(s) Tout = Tset x C(s) x G(s) 1 + C(s) x G(s) (b) – FEEDBACK CONTROL Fig. 4: block diagram showing the response to a change in set temperature (Tset) of a controller-system combination connected in (a) open loop and (b) with a portion of the output (Tout) fed back to the input. 60  Silicon Chip to arise due to the fact that in the correspondence, thermal power is equivalent to electric current and not to electric power. Thus expressions such as V2/R and 1/2 CV2 which correspond to power and energy in electric networks have no simple interpretation in the equivalent thermal circuit. Theory of PID control If we use a computer to switch the electric circuit of the right side of Fig.1 on or off, depending on whether a preset temperature has been reached, the response is shown in the graph of Fig.2. It is characterised by an initial large overshoot, followed by a series of smaller oscillatory overshoots, which tend to an oscillation of constant amplitude above and below the preset temperature. In fact, the initial overshoot and oscillation amplitude are easily calculated in terms of the time constants of the RC circuit and the set voltage, as shown in Fig.3. The overshoot depends on two ratios; the ratio (τ1:τ2) of the time constants of the hot plate and vessel RC circuits, and the ratio of the set temperature to the maximum steady state temperature, when the hot plate is on continuously. From Fig.3 we can establish a simple rule of thumb to determine whether overshoot is likely to be a problem in a given situation. First of all, if the maximum temperature achievable by the heating (or cooling) element, when it is on continuously, is much greater siliconchip.com.au CONTROLLER OUTPUT TIME Fig. 5: the user interface of the PID controller showing a typical impulse response. in magnitude than the desired temperature, then substantial overshoot is a possibility. For it definitely to occur, the time constants of the heating element and the heated vessel must also be within an order of magnitude of each other. So if τ1 is much greater than τ2 for example, the vessel reacts much faster than the hot plate and is able to follow its temperature much more closely, meaning there is no substantial overshoot. Similarly, if the set voltage is near the maximum voltage, no large overshoot is possible since we are operating close to the maximum temperature anyway. Both these criteria are satisfied for the usual situation of a vessel heated on a hot plate. The equivalent maximum temperature for typical hot plates is of the order of 1000°C (which is the value used in Fig.3) and so is much greater than typical set temperatures. To cope with the overshoot problem we adopt a simple strategy. The response of the equivalent RC circuit to a step in the set voltage, shown in Fig.2, is a calculable function of the circuit. If we feed this circuit from a controller with an exactly inverse response, then the response of the whole system will be flat – ie, it will behave like a resistor. The situation is shown in Fig.4(a). The response of the RC network in Fig.1 we write as G(s), while the response of the controller we write as C(s). Then if we choose a controller so G(s)C(s) = R, the combined circuit behaves as a resistor, so that the output voltage is related to input control siliconchip.com.au Fig. 6: drive energy provided by the PID controller to the thermal system as a function of time for a critical response. Starting with 100%, the drive has a trough to avoid overshoot, and then equilibrates to a steady-state value. current by Ohm’s law: Vout = Icontrol x R For a resistor, on/off control gives no overshoot. The controller with the required response – R/G(s) – does not have to be built physically. We can calculate this response on a microprocessor and pass the digital values it generates to a DAC which provides the control currents. The system is still driven by the computer directly, not in simple on/off fashion but with an R/G(s) response. However we have to investigate our ability to realise the R/G(s) controller response using a microcomputerDAC combination. It turns out this is a problem because G(s) for a two time-constant system requires infinite control currents to achieve an inverse response. For finite voltages, we cannot achieve G(s)C(s) equivalent to a resistor. The best we can do is approximate a 2-pole RC network with a single time constant τd, which we can choose so that Tset is reached in the minimum possible time with no oscillation. This corresponds to what is called the “critical response”. Obviously, if we set τd = 0 we get the inverse response, which as we stated above is impossible. Hence we have to determine the minimum value of τd corresponding to our maximum current. Our controller drive will initially be 100% so as to attain the preset temperature as quickly as possible and will then drop quickly to avoid overshoot, before levelling off to its steady-state value. Fig.6 shows the result. If we push the time constant of the response below a critical value, a point will be reached where negative drive is required. Since this is impossible, overshoot will result. Hence the critical value of τd corresponds to that where the curve of Fig.6 just touches the horizontal axis. What we are required to do to complete this program is to measure the system response G(s). This is most simply done by pulsing the circuit for a set period, usually of the order of a minute and measuring the response. For typical systems, the temperature hardly rises during the pulsing (this is why it is called an impulse response) and what we see is a large overshoot after the power had been turned off, followed by decay to ambient temperature. Typical results obtained with the present project are shown in the diagram of Fig.5. In fact, this curve is characterised by just three parameters. The first is the maximum steady-state temperature. We cannot measure it directly because this would mean overheating the hot plate, in which case other thermal processes, such as convection and radiation will come into play. Our thermal to electrical correspondence is based only on conduction, so the actual maximum temperature of the hot plate is substantially less than we estimate from the curve in Fig.5. However this does not matter, provided we operate the system at temperatures below about 500°C or so, when these other processes are unimportant. The other two parameters are the July 2007  61 62  Silicon Chip siliconchip.com.au 4.3k IC4 OP37 3 4 7 2 +9V 4.3k 47 100nF 6 1 F B 3 2 C E 6 –9V Q1 MJE2955 –9V 4 IC3 OP37 7 100nF 100k B PD6 PB0 PB6 PB7 PB5 V– C E R V+ LM334 Vdd C 10 GND IN GND 22pF BTA10-600B G OUT 2 1 1 F 1 F 10 F X1 4MHz 220 7805 5 4 15 3 A1 A2 XTAL1 XTAL2 PB3 PD1/TxD 2 100nF PD0/RxD 20 IC1 AT90S2313 RESET PB1 MJE2955 11 12 18 6 1 1 13 19 150 3 +9V R 82nF V– V+ 4 82k 10k CS1 LM334 17 5 4 x 100 470 F +5V PID TEMPERATURE CONTROLLER 470 F GND OUT Fig. 7: circuit diagram of the PID controller. The inset shows the modifications required to the thermometer circuit published in SILICON CHIP in August 2002. 2007 SC  CON3 DIN SKT –9V THERM SET 1 3 THERM EARTH THERM SIGNAL 5 +9V 2 IN REG1 7805 CON2 ISP CONN 4 FROM DIGITAL THERMOMETER 2 +9V 470 F +9V 1k 0.5W G 390  0.5W 14 13 5 4 A1 A2 39  0.5W 1 F 10nF 250V X2 TRIAC1 BTA10-600B 1 F A 240V AC OUTPUT SOCKET (FEMALE) A 240V AC INPUT PLUG (MALE) 2 (ADDED DIN SKT) 4 5 1 3 –9V +9V IN HI 7 2 4.7k (LCD MODULE) 11 DP1 S2b 1 S3b 9 8 7 6 N 10A SB TO PC NO NC N E CASE E 5 4 3 2 1 CON1 DB9F CAUTION! ALL WIRING WITHIN RED SECTION OPERATES AT 240V AC CON3b 4 6 15 IC2 MAX232 6 SOCKET & CONNECTIONS ADDED TO AUGUST 2002 THERMOMETER 22pF  OPTO 1 MOC3061 11 12 3 1 2 16 time constants τ1 and τ2, which in the usual case when R2 is small, can be understood as the heating constant of the hot plate, which is usually smaller than the third parameter, the cooling constant of the hot plate-vessel combination. All three parameters are extracted by the software from the curve in Fig.5. Another problem with the open loop controller configuration (Fig.4(a)) we have been considering, is that it relies on our ability to measure model parameters exactly, allowing for no variations in time. PID control The fact that we are not able to measure system parameters exactly, as well as slight variations in these parameters during the experiment (for instance a breeze arising), means we have to introduce negative feedback into the system to reduce errors. This changes the system from open-loop to closed-loop as shown in Fig.4(b). The output temperature is sampled and fed back into the controller input. In effect, the system functions as a feedback amplifier. And just as in that case, the feedback changes the response of the system. In order to achieve an RC type of response now, it turns out that the controller can be of the proportionalintegral-differential variety (PID). This is a particularly simple type of control where the control current is based on the sum of three terms: a term proportional to the input voltage to the controller, a term proportional to the integral of this voltage, and a term proportional to the differential of this voltage. The constants of proportionality are the tricky parts requiring calculation and are determined by the requirement that we obtain our desired RC response with minimum τd. These values, as we have seen, are available directly from the impulse response. Thus, for a new system, we operate the controller by carrying out an impulse response (this can take up to a few hours depending on the system) and registering its parameters as well as the ambient temperature. These can also be entered manually, if desired. For instance, the value of τd can be decreased from critical if a faster response is desired and some overshoot can be tolerated. Now we enter the set temperatures and siliconchip.com.au RECEIVE COMMAND FROM PC COMMAND = PULSE? YES RECEIVE DUTY FROM PC PULSE FOR 1 SEC NO MEASURE TEMP SEND TO PC COMMAND = RUN? YES RECEIVE PID DATA FROM PC CALCULATE DUTY NO COMMAND = Tambient? NO YES ABORT received from PC? YES MEASURE AMBIENT TEMP SEND TO PC NO SET button pressed? YES SET held > 2 sec? YES MEASURE SET TEMP NO Simplified Flow Chart of AT90S2313 Code Fig. 8: this flow chart shows how the microcontroller interprets a range of commands from the PC. durations we wish to cycle our system through (the thermal regimes of the system) and run the controller. Circuit operation The hardware part of the project is fairly straightforward – see Fig.7. It is designed to be used in conjunction with the Digital Thermometer/ Thermostat project referred to earlier and published in the August 2002 issue. The heart of the PID Controller circuit is an AT90S2313 microcontroller IC1 which, in addition to an extensive ALU (arithmetic logic unit), features 1kb of program flash memory, 128 bytes each of SRAM and EEPROM memory, a UART and a fast analog comparator. The analog comparator is used in conjunction with the LM334 constant current source (CS1) and an 82nF capacitor to form a simple tracking ADC (analog-to-digital converter) which the microcontroller uses to measure temperature. The voltage signal representing temperature is passed to the ADC by op amp IC3, configured as a noninverting amplifier with a gain of 20. Its high-impedance non-inverting input is fed directly from the digital thermometer via a shielded 4-core cable and DIN socket CON3. Thus IC3 provides minimum loading to the thermometer circuitry. This is important since the digital thermometer outputs voltages in the range 0-200mV and its temperature precision corresponds to a voltage of 100mV. Since the ADC has an input voltage range of 0-4V, maximum precision and minimum non-linearity due to input offset requires a gain of 20 for IC3. The OP37 op amp was chosen here for its low-noise/input offset characteristics. The AT90S2313 (IC1) drives the load (ie, a 240VAC heating element) from its PB3 output (pin 15) via a MOC3061 optically coupled driver (OPTO1) and an insulated tab Triac (TRIAC1). OPTO1 July 2007  63 64  Silicon Chip siliconchip.com.au TP3 A K 1N4004 – + S1 POWER ADJ –2.49V D6 1N914 VR6 10k D5 1N914 D4 1N914 VR1 10k LM335, LM336 3.3k ADJ ADJ TP1 – + –9V TP4 D2 1N4004 –16V 470 F 25VW 470 F 25VW +16V IC1 7 OUT GND GND OUT REG2 7909 IN IN 1k 10k TP2 10 F 25VW 10 F 25VW VR5 500 6 10 F 25VW VR4 500 0.1 F 10k –9V REG1 7809 –9V 4 3 LM627 2 SENSOR1: K TYPE THERMOCOUPLE + 1.1k 430 750k 100k VR2 10k D1 1N4004 5.6k ADJ SENSOR2 LM335 VR3 10k 100k NC NO VR7 1k S3a VR8 500 22k TP5 –9V +9V C 2 S2: POS1 –55° – 199.9°C POS2 –55° – 1200°C 2 RANGE 1 S2a –2.49V 5.6k 27 470 5.6k K-TYPE THERMOCOUPLE THERMOMETER/THERMOSTAT – + – + D3 1N914 +2.49V 5 4 –9V 4 IC2 OP77 7 –9V  6 3 1 E B 7 + 1 – 2 –2.49V +2.49V A –16V D8 1N914 K D7 1N914 +16V 11 DP1 ZD2 15V 1W B B S2b 2 1 150  0.5W 2.2k 10k 10k 2.2k 150  0.5W C 6 5 8 RFL D G 2N7000 INLO COM S 9 RFH A K 10 ROH ZD1 15V 1W GND IN TO RELAY1 COIL -1V G NO NC OUT 10k TO RELAY2 COIL GND OUT 7809 Q3 2N7000 IN S D BUZZER* * ONLY ONE BUZZER USED BUZZER* 7909 12 DISP– 4 DP2 C S3b Q2 BC327 Q1 BC337 LED C E E C LCD MODULE INHI BC327, BC337 ADDED 5-PIN DIN SKT 4.7k K A LED1 RED/GRN  2.2k +9V A 10 F 25VW 10M S3: PUSH TO SET ALARM TEMP 2 3 Fig.9: the red wiring (ie, to the DIN socket) shows the modifications required to the Thermometer/ Thermostat project (August 2002) so that it can be used with the PID Controller. SC 2007 12V AC IN REF2 LM336 -2.5 REF1 LM336 -2.5 +2.49V 3.3k provides zero voltage switching of the Triac for minimum electromagnetic interference. Resistor R1 is used to limit the current to the MOC3061 LED, consistent with reliable triggering. Communication with an optional PC is provided via the UART serial interface of the AT90S2313 using outputs PB0 & PB1 and the MAX232 level converter, IC2. The latter changes the unipolar 0-5V signals of the AT90S2313 to the ±10V of the RS232 specification. The serial interface is used by the PC to send control codes to the microcontroller and receive digitised values of the temperature. The PID Controller circuit is powered from the ±9V regulators on the Thermometer PC board and is interfaced to the LCD module in the same circuit. These connections are made via a 4-core shielded cable and 5-pin DIN socket CON3. Negative current arrangement A problem would have arisen if we had attempted to power the microcontroller and MAX232 directly from the Digital Thermometer, since these require a single +5V supply. Then the supply for the digital circuitry would share a common ground return path with the analog temperature signal and since the latter must be precise to 100mV, spikes due to load switching and UART transmission would be superimposed onto the signal due to the voltage drop across the connecting cable. To prevent this, the negative currents from the AT90S2313 and MAX232 are returned to the -9V supply, instead of the Digital Therm­ometer ground, via PNP power transistor Q2. Its emitter is held at ground potential (0V) by voltage follower op amp IC4. Thus, the earth line from the Digital Thermometer only carries signal current. The controller can be run in standalone mode without serial connection to a PC and to support this, triggering of the controller by the temperature set button of the Digital Thermometer is provided. The sensing is performed at PD6 (pin 11) of the AT90S2313, which detects a train of pulses generated by the digital thermometer when the set button is pressed. These pulses are normally generated by the decimal point driver of the thermometer’s LCD panel. The original Digital Thermometer was wired in such a way that the siliconchip.com.au decimal point is disabled when the set button is pressed and this is used by the PID Controller circuit. Note that in the original therm­ometer project, the “temperature set” button was wired as a 2-contact switch, which will therefore still function if the two wires are interchanged. In the PID project, all three contacts of the switch are used, hence you must ensure the wiring is as per the circuit of Fig.7. Fig.9 shows the complete circuit of the Digital Thermometer with the necessary modifications to connect it to the PID Controller. These changes are shown in red. PID software Most of the project complexity is in the software but unless you want to examine the source code, you only need to acquaint yourself with the user-interface screen shown in Fig.5. Although the controller can be used to control the load in stand-alone mode, a PC is required initially to set the load parameters. Once the PC is connected via the serial cable to the programmed microcontroller and the latter connected to the powered Digital Thermometer, the PC application can be launched, whereupon the screen of Fig.5 appears. The first step is to make the software connection to the controller box. This is done by selecting the appropriate COM port (1-4) and pressing any command button. For first time use, the test button is best. If no error message appears, a connection has been made. The controller has several modes of operation, as shown by the simplified (not all-encompassing) flow chart in Fig.8. The test and pulse modes can be started immediately. The test mode pulses any load connected to the controller at a 50% duty cycle and sends temperature values to the PC, which graphs them. The pulse mode is similar except a duty cycle percentage must be entered in the appropriate edit box. The other modes require a thermal regime to be entered. This is done by entering up to four different temperatures and durations in the appropriate edit boxes. Following this, the ambient temperature needs to be recorded. You must either enter a known temperature and press the enter button on the screen, or press the measure button and let the controller measure it as Parts List 1 PC board, code 04107071, 85.5 x 79mm 1 diecast metal case, 115 x 55 x 90mm (eg, Jaycar HB 5042) 1B9F 9-pin DIN socket (CON1) 1 6-pin PC-mount ISP connector (CON2) 1 5-pin DIN socket (CON3) 1 240V fused male IEC socket 1 240V female IEC socket 16 10mm M3 screws 8 M3 nuts & washers 4 M3 x 10mm threaded standoffs 1 M3 x 10mm csk screw, nut and washer 1 M4 x 10mm screw, nut & washer 10 Nylon cable ties Semiconductors 1 AT90S2313 IC (IC1) programmed with pid.hex 1 MAX232 RS232 interface (IC2) 2 OP37 op amps (IC3,IC4) 1 MOC3061 optocoupler (OPTO1) 1 BTA10-600B insulated tab Triac (TRIAC1) (do not substitute) 1 7805 5V regulator (REG1) 1 LM334 current source IC (CS1) 1 MJE2955 PNP transistor (Q1) 1 4MHz crystal (XTAL1) Capacitors 3 470mF 16V electrolytic 1 10mF 16V electrolytic 4 1mF 16V electrolytic 3 100nF MKT polyester 1 82nF MKT polyester 2 22pF ceramic 1 10nF 250V AC Class X2 Resistors (0.25W, 1%) 1 100kW 1 82kW 2 4.3kW 1 150W 4 100W 1 47W Resistors (0.5W, 1%) 1 1kW 1 390W 1 10kW 1 220W 1 39W Miscellaneous Mains-rated cable (brown, blue & green/yellow, medium-duty hookup wire the current temperature of the probe. The mode of temperature control is then chosen. If overshoot is not expected to be a problem (for instance the system is effectively a single-pole system), on/off control can be chosen since it is faster than PID. Otherwise PID control is chosen and PID parameters need to be entered. July 2007  65 IC3 OP37 4.3k 82nF 100k C AB 1k 0.5W IC4 OP37 CABLE TIE T IE 220 E BL CA 100 10k CON2 150 These parameters (as well as the 5 1 2 3 CABLE TIES SHOULD BE USED TO SECURE ALL necessity for PID control) can be CONNECTING WIRES TO EACH OTHER (IE established by running the system CON1 LOW VOLTAGE TO LOW VOLTAGE, MAINS TO MAINS, ETC) in impulse response mode – that 6 9 is, entering the pulse duration CON3 (the default is 60 seconds) in the CA 1 3 BL corresponding box and pressing the E TIE impulse response button. The load 5 4 will be turned on for the chosen 2 duration and subsequently left CABLE TIE alone, while the temperature is being recorded. 10 F 4.3k A system with manifest 2-pole 100 82k + 100nF (ie, two time-constants) response 1 F 100 will show almost no increase in 100 CS1 temperature during the on period and subsequently a large overshoot 100nF Q1 IC1 AT90S2313 470 F will be recorded, following by a cooling down period. X1 470 F Data acquisition can be ended REG1 MAINS OUTPUT: SIDE 2 x 22pF FEMALE IEC OF once a response resembling that + CONNECTOR BOX in Fig.5 has been obtained. The 1 F IC2 MAX232 1 F 470 F essential element here is to allow + + + 47 the load to cool at least 20% below TRIAC1 100nF 1  F 1  F its peak temperature before ending data acquisition. CS A If at any stage during the run, OPTO1 39  0.5W 17070140 the temperature or time limits of E the waveform displayed on the 390  0.5W MOC3061 screen are exceeded, or if different 10nF 250V X2 N scales are desired for any of the two axes, new initial and/or final time/ E TI LE temperature values can be entered AB C into the appropriate four edit boxes CAUTION! (FUSE) adjacent to the graph axes and the THESE COMPONENTS redraw button pressed. AND TRACKS ARE AT 240V At this stage the four PID MAINS POTENTIAL. A parameters can be calculated N DO NOT CONNECT TO POWER WHEN immediately, by pressing the MAINS INPUT: OUTSIDE CASE OR E FUSED MALE IEC Calculate button. The values WITHOUT CASE LID. COMMON CONNECTOR appearing in the corresponding boxes EARTH POINT (CASE) are the three main characteristics of Fig.1, discussed in the PID Control NOTE: INSULATE ALL TERMINALS ON THE IEC SOCKETS WITH HEATSHRINK SLEEVING Theory section: ie, the maximum Fig.10: the PC board parts layout and external board wiring. Note that the case effective temperature of the element must be earthed to the mains earth and the PC board also earthed at this point. and the time constants τ1 and τ2. If the response shows no peak or a weak peak, error messages will appear. In this case, you In the PC mode, the PC calculates the required duty cycle must either re-acquire the response (after changing the pulse and sends it to the controller, which pulses the load and sends duration appropriately) or run the load in on/off mode. You the current temperature value back to the PC to be displayed. can alter the PID parameter values if you wish, at this stage. In stand-alone mode, the controller runs its own algorithm This might be necessary to improve control, since the real to calculate the duty cycle but still sends temperature values setup can deviate from the model of Fig.1. to the PC. In this mode, the PC can be disconnected at any Thus if previous runs with the same system have shown time with no effect on system operation. Both modes can be that the final temperature is always 1% lower than the set terminated using the “Abort” button. value, the maximum effective temperature of the element The third mode of operation does not require the can be decreased by 1%. Once the parameters have been participation of a PC at all. In this case, the Digital calculated, the “Set Parameters” button needs to be Thermometer and PID Controller are used to run the thermal pressed, so they and the thermal regime are recorded in regime already stored in the controller EEPROM. This mode the microcontroller’s EE­PROM. is initiated by pressing the temperature set button on the The load can now be run in the desired regime. Three digital thermometer for at least 0.5s. choices are available, two operating under PC control and If this button is held down for longer than two seconds, one stand-alone. an additional feature is turned on, whereby rather than LE TIE C LE AB LE AB T IE C T IE 66  Silicon Chip siliconchip.com.au SECURE LOW-VOLTAGE WIRING AT BOTH ENDS WITH CABLE TIES NOTE: INSULATE ALL TERMINALS ON IEC SOCKETS Here’s how it fits together in the diecast case as per the diagram at left. Not shown here are the cable ties used to prevent wires moving and shorting. Note: use medium-duty hook-up wire for the connections to CON1 & CON3 (not rainbow cable as shown here) and keep the connections as short as possible (see safety panel). using the set temperature stored in memory as the first set temperature value, the controller reads the temperature set on the digital thermometer and uses it instead. This mode is terminated, either when the time stored in memory elapses or the thermometer is turned off. Construction All the components of the PID Temperature Controller are mounted on a PC board measuring 85.5 x 79mm and coded 04107071. This is housed in an aluminium diecast case measuring 115 x 55 x 90mm (eg, Jaycar HB-5042). The complete wiring diagram is shown in Fig.10. An accompanying photo shows the wiring layout of the prototype which is slightly different to that shown in Fig.10. The diecast case needs to be drilled to accept an IEC male mains socket which contains an integral 250V 10A slowblow fuse, an IEC female mains socket, and 5-pin DIN and RS232 sockets. The required cut-outs for the IDE sockets (male and female) can be made either by drilling around the periphery with a small drill and filing out, or by using a mill if one is available. Two other holes also need to be drilled: one 3mm hole to affix the isolated-tab Triac and a 4mm hole to affix the mains earth lugs. Assembly of the PC board is quite straightforward but we suggest the following procedure. Install the PC pins (at the low-voltage wiring points), the sockets and connector siliconchip.com.au CON2 first, then the passive components such as resistors and capacitors, followed by the link and 4MHz crystal. Next are the 3-terminal regulator, transistor Q2 and the op amps, current source (LM336) and the optocoupler. Ensure that the 1mF and 10mF tantalum capacitors are connected with the correct polarity. The Triac should be soldered to the PC board, keeping its leads as short as possible, while still allowing them to be cranked slightly, so that its insulated tab can be secured to the case. Be sure to use an insulated tab Triac, as specified in the parts list. Note that the entire mains section of the track needs to be tinned with a layer of solder about 1mm thick, to reduce PC board heating when high-power loads are being controlled. Use a 40W soldering iron or higher for this. Once the diecast case has all holes drilled and machined, the IEC male and female power sockets can be installed, followed by the 5-pin DIN socket and 9-pin D socket. Don’t get the IEC sockets mixed up – the male socket mounts at the end of the case. Before installing the board in the case, it will be necessary to solder the two brown mains wires to it, near the 1kW 0.5W resistor – see Fig.10. Do not use PC stakes to terminate these leads – solder them direct to the PC board. In addition, you will need to connect the green/yellow earth wire to the board at bottom right. Be sure to use 240VAC cable for all the wiring to the IEC sockets and use heatshrink sleeving to insulate all the terminals. The Triac tab can now be smeared with some heatsink compound and the assembled PC board mounted in the case, using the four corner mounting holes pre-drilled in the box. Secure the Triac tab to the case, using a screw, nut and lockwasher, then connect the wiring to the two IEC sockets and install the cable ties. The earth leads are connected to solder lugs which are then bolted to the diecast case using an M4 x 10mm screw, nut and lockwasher. Make sure the mains wiring is as short as possible and is kept well away from the low voltage parts of the circuit. Once the controller is assembled, the Digital Therm­ ometer must be equipped with a DIN socket so that a connection can be made to the controller. This is done by drilling a hole in the back panel, fitting a DIN socket and wiring it as shown in Fig.8. Our photos show the Dick Smith Electronics version of the Digital Thermometer, which was different in a number of aspects to the original project featured in August 2002. The DSE version July 2007  67 These photos show the modifications to the Digital Thermometer – the DIN socket on the rear panel allows interfacing to the PID Controller. This is the DSE kit version which is slightly different to the original August 2002 project. had additional 5V regulators for its LCD module and slightly different interfacing to the LCD. However, the DIN socket connections are still the same. Operation Connect the controller to the Digital Thermometer using a 5-wire DIN cable and to the PC with a 9-way RS232 cable (do not apply mains power to the controller at this stage). Turn on the thermometer and check for the presence of +9V & -9V on pins 7 & 4 of either of the two op amps. Check for +5V at pin 20 of the AT90S2313 and confirm that the voltage difference between the controller box and the digital thermometer earth is no more than 1mV or so. When the unit has passed the above tests, connect an AVR programmer to the programming header and program the microcontroller’s FLASH and EEPROM. The software will be available on our website at www.siliconchip.com.au Remove the programming connector and close the box. Connect a suitable resistive load and mains power to the controller and launch the PC application. Choose a suitable COM port number and press “Test”. If an error message indicates “no connection”, change the COM port number (in the range 1-4) and try again. If everything is working, the load should be pulsed on/ off with a 50% mark-to-space ratio and a graph of the probe temperature-versus-time should appear. You can touch the temperature probe with your fingers and check that the temperature rises and then falls. Calibration It is likely that at this stage you will notice some difference between the Digital Thermometer reading and the temperature displayed on the screen. This is mainly due to the particular ADC component values in your circuit. Choosing the scaling and offset appropriate to your components can reduce these errors. This is done by pressing the “Calibrate” button and changing the scalings displayed in the dialog which appears. You should only do this after you have taken comparative readings of the temperature at two different points and calculated the required changes in the scaling and offset. 68  Silicon Chip Some residual random variation between the Digital Thermometer and screen temperature readings might still be observed after this but it should not exceed a few tenths of a degree. The reason is that the LCD voltmeter in the thermometer averages temperature values over an interval of about one second, whereas the controller reads instantaneous values. We should also note that the accuracy with which the desired temperature is maintained depends, amongst other things, on the accuracy with which the ambient temperature is measured. If the latter varies during the run, this variation will be reflected in the temperature set by the controller. This is most easily seen by referring to Fig.1. A change in ambient temperature is equivalent to a change in earth potential, which is reflected in all voltages which are measured with respect to it. To counteract errors introduced this way, the run should be aborted, a new ambient SC temperature entered and then operation resumed. Check These Important Safety Points (1) Use medium-duty hookup wire for the connections between the PC board and connectors CON1 & CON3. These leads must be kept as short as possible and secured at both ends using Nylon cable ties. That way, if a lead comes adrift, it cannot move and contact any mains-operated components on the PC board or the terminals of the IEC sockets. DO NOT use rainbow cable (as shown in the prototype) – it breaks too easily. (2) Use mains-rated cable for all connections to the IEC sockets and insulate the terminals using heatshrink tubing. Alternatively, use insulated spade lugs (use a ratchet-driven crimping tool to properly secure the spade lugs to the leads). (3) Secure the high-voltage wiring between the PC board and the IEC sockets with cable ties. Again, the idea is to make it impossible for any leads to move and contact other parts of the circuit if they come adrift. (4) Part of the circuitry on the PC board operates at mains potential (as do the terminals of the IEC sockets). Do not touch any part of this circuitry while this device is plugged into the mains. DO NOT attempt to build this device unless you know what you are doing and are familiar with high-voltage wiring. siliconchip.com.au