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Microcontroller-based multi-sensor apparatus for temperature control and thermal conductivity measurement
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2003 Meas. Sci. Technol. 14 N45 (http://iopscience.iop.org/0957-0233/14/8/402) View the table of contents for this issue, or go to the journal homepage for more
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INSTITUTE OF PHYSICS PUBLISHING
MEASUREMENT SCIENCE AND TECHNOLOGY
Meas. Sci. Technol. 14 (2003) N45–N49
PII: S0957-0233(03)57109-0
DESIGN NOTE
Microcontroller-based multi-sensor apparatus for temperature control and thermal conductivity measurement R Mukaro1 , M Gasseller1 , C Kufazvinei1 , L Olumekor1,2 and B M Taele1,3 1
Physics Department, Bindura University of Science, P Bag 1020, Bindura, Zimbabwe Physics Department, University of Zimbabwe, PO Box MP 167, Mt Pleasant, Harare, Zimbabwe 3 Physics Department, National University of Lesotho, PO Roma 180, Lesotho 2
E-mail:
[email protected]
Received 4 December 2002, in final form 2 May 2003, accepted for publication 14 May 2003 Published 16 July 2003 Online at stacks.iop.org/MST/14/N45 Abstract A microcontroller-based multi-sensor temperature measurement and control system that uses a steady-state one-dimensional heat-flow technique for absolute determination of thermal conductivity of a rigid poor conductor using the guarded hot-plate method is described. The objective of this project was to utilize the latest powerful, yet inexpensive, technological developments, sensors, data acquisition and control system, computer and application software, for research and teaching by example. The system uses an ST6220 microcontroller and LM335 temperature sensors for temperature measurement and control. The instrument interfaces to a computer via the serial port using a Turbo C++ programme. LM335Z silicon semiconductor temperature sensors located at different axial locations in the heat source were calibrated and used to measure temperature in the range from room temperature (about 293 K) to 373 K. A zero and span circuit was used in conjunction with an eight-to-one-line data multiplexer to scale the LM335 output signals to fit the 0–5.0 V full-scale input of the microcontroller’s on-chip ADC and to sequentially measure temperature at the different locations. Temperature control is achieved by using software-generated pulse-width-modulated signals that control power to the heater. This article emphasizes the apparatus’s instrumentation, the computerized data acquisition design, operation and demonstration of the system as a purposeful measurement system that could be easily adopted for use in the undergraduate laboratory. Measurements on a 10 mm thick sample of polyurethane foam at different temperature gradients gave a thermal conductivity of 0.026 ± 0.004 W m−1 K −1 . Keywords: thermal conductivity, steady state, polyurethane, microcontroller, pulse-width modulation, temperature gradient
(Some figures in this article are in colour only in the electronic version)
0957-0233/03/080045+05$30.00 © 2003 IOP Publishing Ltd
Printed in the UK
N45
Design Note
1. Introduction
2. System description
Thermal conductivity of insulation materials is an essential parameter in the refrigeration industry. This information is important for the design and performance of refrigerators and cold rooms. Thermal conductivity is the factor that determines heat transport phenomena in materials. In foams, thermal conductivity depends on a variety of parameters and this poses some difficulties in theoretical predictions of the thermal conductivity of such materials (Piorkowska and Galeski 1986). It is an intrinsic property whose value depends on the chemical composition, porosity, structure and fabric of the material (Jumikis 1966). Hence, experimental methods for determining thermal conductivity are essential and several such methods have been reported. Each method requires a particular measuring system, so many different apparatuses for thermal conductivity measurements of different materials have been designed and constructed (e.g. Liang et al 1999, Moster et al 1989). The major disadvantage of some of these methods is that they require large samples, cylindrical or rodlike shapes or plates several centimetres thick, which in some cases can hardly be obtained with the desired internal morphology (Graebner 1993, Piorkowska and Galeski 1986). A drawback of the large sample thickness is the long time required to achieve steady state for steady-state methods. Some of the nonsteady methods require temperature measurements at several points inside the sample or complicated data processing (Log 1993). Several methods have been developed to measure thermal conductivity of different materials and each of these is suitable for a limited range of materials depending on the thermal properties. Absolute measurement of thermal conductivity requires the establishment of steady-state conditions. There is a distinction between steady-state and non-steady-state techniques. In general, for steady-state techniques, a measurement is performed when the material that is being analysed is in complete equilibrium. This makes the process of signal analysis easy. The disadvantage generally is that it takes a long time to reach equilibrium. Non-steady state requires that a measurement be performed during the process of heating up. The advantage is that measurements can be made relatively quickly. Steady-state methods for determining thermal conductivity have become more attractive in recent years so this method was employed in this design to determine the thermal conductivity of polyurethane foam. The measurement system described in this design note consists of a microcontroller-based data acquisition system which automatically acquires the relevant data from the experiment and at the same time controls the heating process. The introduction of the microcontroller and its predecessor the microprocessor has allowed a wide distribution of intelligence throughout most control processes at a relatively low price. The measuring instrument presented here is interfaced with an IBM compatible computer through an RS232 cable and the COM1 port. Through a Turbo C++ programme, the computer records the measured temperature gradient and then computes and displays the results (Mukaro and Carelse 1997).
The measurement system presented here was used to measure thermal conductivity of polyurethane foam used by a local refrigerator manufacturing company. It is a digital temperature control and measurement system that uses an ST6220 microcontroller to generate a desired temperature gradient and measure the thermal conductivity. The ST6220 is an eightbit microcontroller that requires very little support circuitry as most of the needed electronics are on the chip. The designed system offers eight single-ended analogue input channels and features an eight-bit A/D converter with typical conversion time of 70 µs. There are other recent, powerful and attractive microcontrollers available on the market, but this was the only microcontroller available to the authors. Figure 1 shows the circuit diagram of the temperature control and measurement system. The microcontroller, which is driven by an 8 MHz crystal oscillator, outputs a signal with varying duty cycle which depends on the difference between the set and the measured temperatures. This signal is used to control power dissipated in the heater. The system operates from +5 V derived from a 9 V battery using an MC78T05CT voltage regulator. Diode D1 protects the system against accidental power supply polarity reversal. The ADC is especially sensitive to the any power supply spikes that occur during conversion. These spikes or ripples arise in integrated circuits as a result of rapid logic switching so low inductance 0.1 µF tantalum capacitors were used for supply bypassing. The push button provides a user reset to the system while capacitor C1 is used for debouncing the switch (Mukaro and Carelse 1999). The MAX4558, a low-voltage CMOS analogue integrated circuit configured as an eight-to-one multiplexer switch, is used to sequentially connect the eight temperature sensors to just one analogue input, line PB1 . The multiplexer is used to minimize the size and cost of the system. After enabling this chip through PB5 a particular sensor is selected by placing the appropriate bits on lines PB2 –PB4 . Whilst the system makes a comparison of the set and the measured temperatures to control temperature every 5 s, a measurement of thermal conductivity is made when the system reaches equilibrium. In this case, if the average heat sink temperature does not change by 0.5 ◦ C it is assumed that steady state has been reached. The LM335 operates as a two-terminal Zener diode whose breakdown voltage is directly proportional to the absolute temperature at 10 mV K−1 . The temperature to be controlled is in the range just above room temperature to about 373 K. All the sensors were first calibrated for 2.73 V at 273 K. At minimum temperature the output was 2.90 V while at maximum it was about 3.90 V. To provide maximum resolution a zero and span circuit was used so that the input voltage utilizes the entire range of the ADC (Jacob 1988). With one sensor at the lowest temperature, the 500 k potentiometer was adjusted (zeroed) so that the input voltage, Vin , to the microcontroller’s ADC read 0 V. The sensor was then placed at the maximum temperature and the 100 k potentiometer adjusted (span) for Vin to read 5.0 V. This zero and span circuit, which consists of an inverting summer of gain 5.0, uses the ordinary 741 operational amplifier. When this is done the AD converter now uses eight bits to represent the signal in the range 0–5 V with a corresponding resolution of
N46
Design Note
µ
µ
µ
µ
µ
µ
µ µ
Figure 1. Circuit diagram for temperature measurement and control system.
about 0.3 ◦ C. A unity gain buffer then follows to isolate the circuit from the microcontroller’s analogue input. Two separate +12 V were used for powering the opamps and the heater. The software program running on the ST6220 contains the main program and several service subroutines. The main program waits for two interrupts, one which allows the user to supply the control temperature and the timer interrupt that initiates an AD conversion of the voltage corresponding to the temperature. The timer interrupt is generated within the microcontroller while the computer interrupt is generated when line PB0 , which is configured as a falling edge interruptable input, receives data (the start bit). The microcontroller software controlling power to the heater relies on a simple control that can be easily adjusted without resorting to more complex algorithms for the heater such as PID control (Molina et al 2002). The control is achieved using the pulse-widthmodulation (PWM) technique which is a serial signal in which the information is contained in the width of the pulse. In this application the period of the signal (250 ms) is not changed while the duty cycle is changed under program control and the system generates continuous PWM signals by simply comparing the set and the measured temperatures. If the measured temperature is less, a voltage which is proportional to this temperature difference drives the 2N3055H transistor
which is used to control power to the heater. A logic high inserted on line PA1 makes the base–emitter junction of the transistor forward biased, causing current to flow through the emitter. This current energizes the relay R L , closing the normally open contact, that allows a large current to flow through the heater. The heater remains on for a time determined by the duration of this high pulse. A logic low drives the transistor into cut-off, which turns off the heater. The heater consists of 21 gauge nichrome wire wound on a ceramic ring. If the measured temperature is higher than the set, the heater will stay off until system temperature drops to just below the set temperature. When the two temperatures are the same, the measured temperatures are immediately transmitted to the computer for thermal conductivity calculation and display. The data are transmitted serially at 600 baud, no parity and one stop bit and can also be saved in a data file. Unlike other micrcontrollers, the ST6220 does not have a serial hardware so serial transmission is done by software. The serial interface between the computer and the ST6220 microcontroller consists of the MAX232 line driver/receiver which converts the signals to the levels needed by both the microcontroller and the computer (Mukaro and Carelse 1999). During transmission, all microcontroller interrupts are disabled to prevent the baud rate from getting corrupted. N47
Design Note
Guard Ring
oil insulation
heat source sample
T1
T2
T3
T6
T4
T7
T5
ice
T8
heat sink (ice and water)
Figure 2. Schematic diagram of the set-up used to measure the thermal conductivity of polyurethane.
3. Principle of operation Polyurethane foam has a very low thermal conductivity so the sample used is a thin, flat disc of cross-sectional area 0.01 m2 and thickness 10 mm. This was done to generate a large measurable temperature gradient. This measurement system uses the absolute method of measurement where steady-state heat flux through the flat slab of polyurethane is measured using a guarded ring hot-plate apparatus. The heat source, in the form of a rectangular tank of dimensions 100 mm × 100 mm × 50 mm, is filled with engine oil and is completely surrounded by the guard ring, also containing oil. The guard ring is used to minimize radial heat transfer losses across the edges of the sample. It is controlled to have the temperature closely matching that of the heat sink. Figure 2 shows a schematic diagram of the set-up used for measuring the thermal conductivity of polyurethane. The sample is sandwiched between the heat source and the heat sink. Its upper and lower surfaces were smeared with grease or a thin veil of silicon compound to eliminate air pockets, thereby reducing thermal resistance at the interface. The heat sink is maintained at 273 K during measurement by using a mixture of ice and water. Five LM335 temperature sensors, embedded in the heat source, and the other three in the heat sink, provide measurements of temperature gradient at different axial locations. During a measurement and control routine, the temperature gradients across the sample for the different axial positions are measured and sent to the computer. The average temperature gradient is computed when steady state has been reached. Once the heating commences, an electrical stirrer is switched on to ensure uniform temperature distribution in the heat sink. Heat flow rate into the sample and the temperature gradient across the sample were measured and used to determine the thermal conductivity of the sample using the one-dimensional Fourier conduction equation: Q = −k A
dT dx
where Q is the rate of heat flow, k is thermal conductivity, A the cross-sectional area through which the heat flows and dT /dx the temperature gradient. dT is approximated by T , the finite temperature difference, and dx by x, the distance over which the temperature difference is measured. The heat N48
Thermal Conductivity, K
0.03
stirrer
heater
0.025 0.02 0.015 0.01 0.005 0 25
35
45 55 65 75 Temperature (degrees Celcius)
85
Figure 3. Variation of thermal conductivity of polyurethane foam with temperature gradient.
source temperature is the average of T1 –T5 while the heat sink temperature is the average of T6 –T8 . The difference between these two gives the average temperature gradient which is used in the computation of thermal conductivity. The microcontroller measures the electrical power dissipated in the heater, which is used to calculate thermal conductivity. The accurate determination of thermal conductivity requires a precise knowledge of the electric power dissipated in the heater and this power depends on the supply voltage, Vcc . Vcc is measured by scaling the supply whereby the 10 k pot is adjusted until Vre f is a quarter of Vcc . This calibrates the scale so that the supply voltage is simply four times Vre f . A logic 1 placed on line PB6 closes the two switches A and B on the HCF4066BE quad switch. This enables a measurement of Vre f to be made from the potential divider. A logic 0 turns off the switches, protecting the battery from depletion.
4. Experimental results and discussion The thermal conductivity of polyurethane foam was measured at different temperature gradients, and an average thermal conductivity of 0.026± 0.004 W m−1 K−1 was obtained. This for polyurethane ranges between 0.020 and 0.030 W m−1 K−1 (Oertel 1993); however, this value depends on the chemical composition, porosity, structure and fabric material of the foam (Jumikis 1966). The most critical component of the foam during its preparation is the blowing agent that is added to make the foam rise and attain a good cell structure and thermal characteristics. In particular, the thermal conductivity depends on the density. The higher the density the higher the thermal conductivity. The density depends on the amount of blowing agent added. If the amount of the blowing agent is large then the foam becomes fluffy and hence less dense, while if the amount is too small the foam is more compact and hence more dense. The foam specimen tested has a mixing ratio of 100:122 by mass of polyol to isocyanate. The variation of its thermal conductivity with temperature gradient is shown in figure 3. This shows that the thermal conductivity decreases with temperature gradient. One reason for this could be that although the system is well thermally insulated, as the temperature of the foam rises, control of heat losses become difficult and inevitably some fraction of the heat supplied is lost. A sampling frequency of 5 s was used and it takes about 45 minutes for the system to reach steady state. The accuracy of temperature control is about 1 ◦ C.
Design Note
5. Conclusion The objectives of this project, which were to use microcontrollers in process control and utilize the latest powerful, yet inexpensive, technological developments for research and teaching by example, were met. A microcontroller-based steadystate method for determining the thermal conductivity of a poor conductor has been developed. Measurements on a 10 mm thick sample of area 0.01 m2 yielded a thermal conductivity value of 0.026 ± 0.004 W m−1 K−1 . The results show that, within its limited range, the system developed here is a fairly accurate temperature measurement and control system that could be adopted as an experiment for the undergraduate laboratory.
Acknowledgments The main author of this design note is a Junior Associate Member of the Abdus Salam International Centre for Theoretical Physics, Italy. He is very grateful to the Swedish International Development Cooperation Agency (SIDA) for their generous grant that enabled him to travel to the Abdus Salam ICTP where part of this research was carried out. The authors are also grateful to the Research Board of Bindura University of Science for funding this project and the Localization and Training Board of the National University of Lesotho for the financial assistance.
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