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Precision Table-Top Portable Thermal Chamber with DoubleThermoelectric Module
ANDERSON W. SPENGLERUNICAMP
DEMICAv. Albert Einstein, 40013083-852 Campinas, SP
ELNATAN C. FERREIRAUNICAMP
DEMICAv. Albert Einstein, 40013083-852 Campinas, SP
JOSÉ A. SIQUEIRA DIASUNICAMP
DEMICAv. Albert Einstein, 40013083-852 Campinas, SP
Abstract:A portable thermal chamber with a volume of only2.5 l capable of reaching temperatures over the−5Cto 70C range was designed and constructed. In order to make the thermal chamber lightweight and portable, twoseries connected thermoelectric modules were used as actuators since their size and performance characteristicsallow the portability and precise temperature control. The PID control provided stability and errors better than nor-mally found in expensive commercial thermal chambers, with maximum temperature error of±0.2C with respectto the setpoint and with a fluctuation of±0.1C.
Key–Words:Thermal chamber, Thermoelectric modules, Stacked Thermoelectric modules, Analogue PID, Tem-perature control, H-Bridge power driver.
1 Introduction
Commercial thermal chambers have a large volume,which makes them inappropriate for portable bench-top or field applications. The technology used in com-mercial thermal chambers employ compressors andradiators to cool down the chamber, requiring a lot ofspace and making them difficult to control with lowsteady-state temperature errors. To overcome theseproblems, this paper presents a technique where ther-mal electric modules (TEM) were used, making theequipment small and easier to control, resulting in areduced size lightweight portable equipment that canbe used in the field or in bench-top applications whereprecise temperature control are needed.
2 Design of the thermal chamber
The chamber size, insulation and materials are deter-minant in estimating the amount of power that willbe required to cool or heat the chamber. The thermalchamber has the external dimensions of300 mm x240mm x120 mm and the internal space is240 mm x180mm x 60 mm (approximately 2.6 l). The insulation ismade of an elastomer foam with thermal conductivityof 0.037 W/(m.K) at 20C, and its thickness is only30 mm.
To estimate the required power to cool the cham-ber, the AZTEC software [1] was used, and for the
given dimensions and desired temperature, the pro-gram indicates that it is necessary10.58 W, consid-ering that an active internal load is dissipating 1W.
An interesting feature of this design is that twoTEMs are used in a series configuration, in orderto reduce the temperature difference between the ce-ramic plates and, therefore, allow for a higher heattransportation. The thermal chamber is built in twoparts, which are united by a central element with twoTeflon pieces in order to centralize the aluminum cen-tral parts. Fig. 1 shows the block diagram of the struc-ture, the position of TEMs, sensors and copper blocks.
Figure 1: Actuators assembling diagram.
Since a critical part of TEM is dissipating the
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transported heat and the generated heat in this trans-portation, it was necessary to use a heat sink to removethe heat from the hot side of the top TEM. Accordingto reference [2], the used heat sink will be15o C aboveroom temperature for a 125W load. The final assem-bling of the thermal chamber is done by fastening intothe inner piece the module composed by the TEMs,copper blocks, heat sink, sensors and insulator. Af-ter this, the internal fan (required to make the spacialdistribution of the heat more uniform and equalize theinternal temperature) and the heat sink are also fixedin the inner piece. A photograph of the constructedthermal chamber with the temperature control boardon its top is shown in Fig. 2.
Figure 2: Thermal chamber constructed.
3 Eletronics CircuitsThe block diagram of the thermal chamber circuits ispresented in Fig. 5. The electronics circuits are usedboth to read the temperature (with digital integratedsensors) and to control the internal temperature by ad-equately driving the TEMs.
The microcontroller (PIC16F872A) [3] com-municates with the interface module through atransceiver which has a galvanic insulation, theADM2483 [4]. The digital temperature sensors em-ployed were implemented with the LM95071 [5],which is a very fast and high resolution (12 bits) in-tegrated temperature transducer. To measure the rela-tive humidity inside de thermal chamber it was used aSHT15 [6]. Although it has the capability of mea-suring temperature, it was used only as a humiditysensor since is very slow and would jeopardize theperformance of the temperature control. The setpointvoltage of the PID controller for the bottom TEM isdetermined by the user, and is internally generatedwith a 12 bits D/A converter with SPI communication
Figure 3: Block diagram of the thermal chamber cir-cuits.
(TLV5616) [7].The analogs sensors used are two 2.2 kΩ
B57891M Series NTCs [8] and one AD590 [9]. TheNTCs are fixed in the middle block and in the heatsink, since high precision is not required in thesepoints. The AD590 is located inside the thermalchamber, where it is necessary a high accuracy in thetemperature measurement. The three sensors signalsare conditioned to present 32 mV/oC, which is thesame scale used in the D/A converter. In Fig. 4 theschematic diagram used in the signal conditioning ofthe NTCs is shown.
R81k
R7NTC 2k2
+5V
R910k
R1036k7
R1110k
R126k95
Vref R1310k
R1410k
R1510k
Vheatsink
Figure 4: NTC signal conditioning circuit.
The AD590 signal conditioning circuit is pre-sented in Fig. 5.
R1610k
R1710k
R1810k
R1932k
VrefAD590
-12VVchamber
Figure 5: Conditioning circuit for the AD590.
The setpoint of the PID controller of the bottom
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TEM is determined by the user, and it is defined by asignal that comes from the D/A converter. To optimizethe operation of the top TEM, its temperature setpointis automatically chosen by the circuitry, and given bythe following equation:
vsetpoint = 0.8vchamber + 0.2vheatsink (1)
wherevchamber is the voltage related to chamber tem-perature andvheatsink is the voltage related to theheatsink temperature. This equation was obtained ex-perimentally, measuring many TEM voltage configu-rations in order to obtain a higher cooling and heatingefficiency.
The complete PID circuit of the bottom TEM isshown in Fig. 6. The choice of using an analog PIDcontroller following the format proposed in [10] wasdue the low cost and good performance results whichcan be obtained with this circuit, so was unnecessaryuse a PID controller as describe in [11]. The PID pa-rameters (proportional, integral and derivative gains)are adjusted individually and separately, offering moreflexibility to the system. The summing amplifier canintroduce signal attenuations of each signal comingfrom the PID main block.
R2410k
R2510k
R2310k
Vchamber
Vset_chamber
R301k
R3250k
R2610k
R312k
R33100k R34
3k9
R281M
R293k
C2210nF
C231uF
C2150uF
R352k
Vcontrol
Figure 6: PID circuit used for the bottom TEM con-trol.
Due to the high current required by the CP14-127-045 TEMs [12], an H bridge composed of MOS-FET transistors driven by a PWM signal was used. Toobtain the PWM signal, a conventional approach wasadopted. A triangular wave with frequencyfPWM =
15 kHz is generated and compared to the PID outputsignal. The output of the comparator is the requiredPWM signal.
The PWM signal is sent to the H bridge circuitthrough an optoisolator HCPL4503 [13], connectedto two TC4428 [14] drivers, resulting in two comple-mentary PWM signals outputs. To guarantee a contin-uous voltage level, LC filters are used in the output ofthe H bridge.
D7
12V
C43100nF
C44100nF
D8
12V
R7647k
R7947k
R77100
R78100
D10
D9
Q2
IRFZ44N
Vcc
C513.3uF
L11mH
L21mH
TEM
C523.3uF
Q1
IRF5305
D13
12V
C47100nF
C48100nF
D14
12V
R81
47k
R8347k
R80100
R82100
D12
D11
Q4
IRFZ44N
Vcc
Q3
IRF5305
PWM1 PWM1
Figure 7: H bridge circuit.
4 Experimental resultsThe first test performed was to determine the mini-mum internal temperature that could be reached withthe thermal chamber, driving the TEMs with a con-stant voltage source. The measured results are pre-sented in Fig. 8. The test starts with a nitrogen fluxinside the chamber, with 13 V and 6 A on the top and5 V and 2.6 A on the bottom TEM. The relative hu-midity measured reaches−2%, but the sensor man-ufacturer [6] informs that negative values should beconsidered as zero. The nitrogen flow was turned off150 minutes after the beginning of the test and an in-crease of the humidity is observed.
0 100 200
0
20
Heatsink Temp. Middle Temp. Chamber Temp. Chamber Humidity
Time[min]
Tem
pera
ture
[o C]
0
10
20
30
40
Relative H
umidity [%
]
Figure 8: Measurement result to obtain the lowesttemperature in the chamber.
At the end of the measurement cycle, the follow-ing temperatures were observed:−6.4C inside thechamber,1C in the intermediary block, and27.8C atthe heat sink. Using these values of measured temper-atures and the graphs provided by [12], it was possibleto estimate the transported power by each of the TEMsmodules as about 20 watts for the top TEM (which iswell above the 11 watts calculated by the manufac-turer´s software) and around 30 watts for the bottomTEM, which is in agreement with the 28.8W predictedby the software.
The difference observed in the top TEM is justi-fied by the unavoidable heat leakage from the cham-
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ber and also due to the insulation between the heat andcold sides of the TEM.
Due to the system nonlinearity in the heating andcooling process the PID tuning parameters were ob-tained experimentally. The optimal voltage for thebottom and top TEMs were also determined experi-mentally, after several experimental tests.
The traditional PID tuning method employed[15], is based in keeping only the proportionalcontrolling part actuating, and then rising the propor-tional gain until the system starts to oscillate. Theproportional gain is then set at 80% of this value.Secondly, using the found proportional gain, theintegral and derivative gains are obtained in a similarway.
Table 1 - Gains obtained by the trial method.
Top TEMGain Kc Ti TcValue 20 17.625 2
Bottom TEMGain Kc Ti TcValue 49.9 9.8 0.664
A test with the chosen PID parameters with theset-point adjusted to0C is shown in Fig. 9, where thevariation of the temperature as a function of the timeis presented.
0 50 100 150
0
5
10
15
20
Tempe
rature[oC]
Time[min]Figure 9: Variation of temperature with time when thethermal chamber is set to0C.
The thermal chamber takes approximately 180minutes to reach−0.2C and no overshoot is found.An excellent temperature stability is observed, as seenin Fig. 10. The gray lines are the temperature sensormeasurements, which have a resolution of0.03125C.It is important to notice that the maximum deviation isonly around0, 16C. The black solid line shown is the
result of the calculated average using the 10 previousand 10 subsequent points at each instant of time.
20 25-0,30
-0,25
-0,20
-0,15
Taverage
= - 0,21oC
∆T=0,07oC
Tem
pera
ture
[o C]
Time[min]
Experimental Data Average Line
∆T=0,09oC
Figure 10: Temperature variation near0C.
The upper proposed temperature limit for thethermal chamber is70C. A test with the set-point setto this value is shown in Fig. 11. The response showsthat a small overshoot (2C ) is verified, but the sta-bility is soon reached. In the Fig. 11 is possible to seehow the middle temperature follows the internal tem-perature as planned. Oscillations are also visible inthe heatsink temperature, caused mainly by the roomair conditioning action.
x
x
x x
Figure 11: System temperature graph for70C set.
The temperature variation when the system is insteady state around70C is similar to that observedfor 0C. This variation is approximately0.16C, asshown in Fig. 12.
The Fig. 13 shows the transient response for var-ious temperature set-points, showing that the systemreaches its steady-state quickly when temperaturessteps of10C are applied.
Although the option of using two TEMs in serieshas been made to allow the thermal chamber to reachlowest temperatures, it was observed that this config-uration also improves the system´s response to exter-nal disturbances. This can be seen in Fig. 14, where
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26 28 30 32 34
70,10
70,15
70,20
70,25
Taverage
=70,18oC
∆T=0,07oCTem
pera
ture
[o C]
Time[min]
Experimental Data Average Line ∆T=0,09oC
Figure 12: Internal temperature variation at70C.
Figure 13: Temperature response for various set-points.
x
xx
x
x
Figure 14: System reaction to an external disturbance.
10 15 20 25
9,65
9,70
9,75
Taverage
=9,69oC
∆T=0,06oC
Tem
pera
ture
[o C]
Time[min]
Experimental Data Average Line
∆T=0,06oC
Figure 15: Internal temperature oscillation when anexternal disturbance is forced.
it is observed that an external disturbance in the heatsink temperature (provoked by turning off the exter-nal fan) affects only the intermediate temperature, anddoes not affect the internal temperature of the thermalchamber.
Fig. 15 shows the behavior of the internal temper-ature when an external disturbance is forced.
5 Conclusions
A low-cost and high performance table-top portablethermal chamber which covers the 0-70oC tempera-ture range was designed and fabricated, using a dou-ble TEM technique. Due to its reduced volume andweight and the fact that it can be powered by two 12V batteries, it can also be a valuable tool in field testsmake possible field measurements.
The operation range of the thermal chamber is−5C to 70C, and an entrance for an inert gas likenitrogen allows the realization of tests below 0oCwithout condensation. The temperature fluctuationin steady state is only±0.1C, and the relative tem-perature error to the set-point is±0.2C, which out-performs most of the available commercial thermalchambers which are ten times more expensive.
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