IEEE-ISIE 2004: International symposium on industrial electronics, Ajaccio, France, Volume 1
Accurate temperature control of a ultrasonic cell using a microcontrolor. A. Malaoui¹, K. Quotb¹, M. Ankrim², M. Benhayoun². ¹ Laboratoire de Physique des Interactions Ioniques & Moléculaires, Service électronique, Centre saint Jérôme, Université de Provence Aix Marseille, France, e-mail :
[email protected],
[email protected] ² Laboratoire d'Electronique et Instrumentations, Dpt de Physique, Faculté des Sciences, Semlalia, Université Cadi Ayyad, B.P 2390 Marrakech, Maroc, e-mail :
[email protected],
[email protected]
Abstract— In this paper we present the description of the realization of an electronic system and ultrasonic cell. The electronic system allows the control of cell temperature. This device is a compact system, less expensive and constructed around a microcontrolor ST6225. The ultrasonic cell was designed and realized to measure with high precision the time of flight in the agroalimentary liquids between –10°C and +150°C, it is associated to a device system realized by our team. The combinations of the automatic algorithms were used to obtain a precision on the temperature measurement of 0.1 °C. The display and The processing of the results were carried out using the microcontrolor and an acquisition card connected to the computer. For that we developed a data-processing application using the LABVIEW. We study the performances of our system following several factors and constraints. We present simulations of the temperature change and we compare the results with those of commercial devices of temperature. We also present acoustic velocity measurements of the agroalimentary liquids using the our device. Keywords— Microcontrolor, Temperature Control, Acoustic Velocity.
I. INTRODUCTION The ultrasonic technique is one of the most recent methods in the last years. It is used by Wells in 1977 in the medicine, by Papadakis in 1976 for the test of metals, in the quality control of the agroalimentary products [1], pharmaceutical product, electronic component and oceanography [2]. The measured value of the acoustic parameters of these products, such, speed, attenuation, impedance and frequency spectra, depend on several physical parameters. The variation of these parameters with the temperature is not easily controllable with precision by traditional instruments. The controlling temperature devices, the most used are expensive, slow and calling algorithms managed by a microcomputer. This presents many difficulties for characterization of the produced samples on the industrial sites. The objective of this project is to use the last powerful developments of integrated electronic systems. However, inexpensive, for a better control in temperature. For that, we designed and developed an electronic device at base of the microcontrolor, in order to control the temperature of an ultrasonic cell which we especially designed and realized for this objective [2]. The basic principe is to combine a process of heating, which uses the joule effect with resistance and a process of
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cooling which using Peltier plates (Thermo Electric Cooler) or TEC, thus the time of stability and the relative error of the required temperature are tiny compared to the traditional methods of cooling. The two mechanisms are managed by a microcontrolor (µC) of the family ST62xx de STMicroelectronics [3]. We modeled the temperature control system by an experimental automatic model. Data-processing programs with assembler language were developed and charged in the microcontrolor. The group allows calculate the suitable control law and generate the electric signals with variable cyclic ratio, to command the electronic power. We use PID and ON-OF algorithms, to have a precision on the temperature measurement of about 0,1 °C in [–10°C, +150°C]. A data-processing platform was developed using LABVIEW, to treat and display the results on computer, using an acquisition card DAQ NI 5102. II. THE SYSTEM DESCRIPTION. The device realized is composed of three principal parts, designed and realized: the ultrasonic cell, the electronic part, the algorithms and the software allowing to take measurements. A. Ultrasonic Cell (fig 1) It’s a cell of parallelepiped form in brass (170x50x50 mm³), is composed of cylindrical cavity with ray r equalize at 17.5 (mm). The cavity contains a liquid has to test by the ultrasonic technique. The ultrasonic transducers for the generation and the detection of the acoustic waves are ceramics who form stoppers of cavity, The are in titanate-zirconate of lead whose resonance frequency is 5 MHz.
Fig.1 Ultrasonic cell.
IEEE-ISIE 2004: International symposium on industrial electronics, Ajaccio, France, Volume 1
B. Electronics realized (Fig 2) The central element of the electronic realization is the microcontrolor ST6225. It has 4 kilos bytes of programs memory and 64 bytes of data memory. The Inputs/outputs constitute three ports (A, B and C). They can be configured either in inputs or at outputs. The microcontrolor integer the converters A/N which are of type "Approximations successive". These converters have multiplexed entries. The conversion duration is a 70 µs with a quartz of 8 MHz. The converter resolution under a tension of 5 V, is 19.5 mV/bit. The temperature sensor used is the AD590, it delivered a output current proportional to the absolute temperature. It has a sensitivity of 1µA/°K between -55°C and 150°C. We used the port C for the analog/numeric conversion of the resulting signals from the measured and consign temperatures. One of the pin is used as input of the consign y(t), the error e(t) is recorded by µC in the pins PC5 and PC7 according to whether the sign of the tension. The input pin PA7 indicate to the µC that the display in the port B is y(t) or e(t) respectively for a logical level 1 or 0. The Peltier parallelepiped elements are connected electrically in series and thermically in parallel. They are stuck directly on external surface of the cell and are fed by a stabilized source power (fig. 3). It uses an electronic assembly which realize the linearity between the control order generated by the µC and the temperature creates by the cold side of the plate. On this figure the comparator A1control the transistor ballast T3 according to the difference tension raised on its inputs, to await the equilibrium condition [ e+ = e - ].
The tension applied to the negative input represents the image of the feeding current of Peltier module. This current is measured with a resistance of 0,1 ohm placed in series with the Peltier module. The low value of R is explained by a weak thermal dissipation in this resistance. The tension taken on this resistance is thus equal to 0,2 volts for 2 (A). To obtain the equilibrium condition of the assembly (under 5 volts in the inverses input of A1), a stage of amplification A2 was inserted in the assembly. The reference voltage standard regulate linearly the current between 0.5 and 3 (A) thanks to a simple device, thus we have for 5 volts a current of 2 (A). in figure 2, the J1 output of sensor AD590 is converted into tension thanks to the amplifier OP27 which is stable and reliable. The tension s(t) in its output S1 is proportional to the temperature. The temperature consign is produced by the P1 potentiometer, it constitutes the input y(t) of the µC. We use the operational amplifier U1 like subtracter of the value of the consign and the output temperature. Its output S2 represents the error of the chain in closed loop. The amplifier U3 provided y(t), the U2 amplifiers and U4 provide e(t) according to that is positive or negative. The sign of e(t) is indicated to the µC by PA6. In order to simulate the precision of the desired temperature and the various time-constants, we developed the thermal equations adapted to our system [4]. The control system of temperature considered contains two different processes, that of heating by joule effect made up of eight wound heating resistance (model WH50) of 6.8 ohms and that of cooling by Peltier plates (model CP0.8-31-06). We summarize on figure 4, the mechanism of µC operations. It generates electric signals with variable cyclic ratio η through PA0 and PA1 outputs. These signals tackle the electronics power so as to respect the linearity of the two processes.
Fig.3 Diagram of Peltier electronic power
III. STUDY OF SYSTEM A. Algorithm of control
Fig.2 electronic diagram of the temperature control assembly
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The objective of this realization is to have a precision on the temperature measurement of 0.1 °C in the range [- 10 °C, +150 °C], and the system must function under several constraints, we must solve two essential problems: 1- To obtain a precision of 0.1°C by a converter of 8 bits, the range of temperature is reduced 160°C (-10, +150 °C) to
IEEE-ISIE 2004: International symposium on industrial electronics, Ajaccio, France, Volume 1
25.6 °C. we replace part of the µC program, which is responsible to the comparison for the consign and output temperature, by the amplifiers U1 and U2. Because the control law of the regulator depends only with e(t) and these former states (equation 6), the µC will convert the error e(t) instead of the consign y(t) and the output temperature s(t). That enables us to increase the range of the temperature and release memory capacity. 2- Because the two processes (heating and cooling) have two different time-constants τ 1 and τ 2. it is very difficult at the same time to operate the temperature control with two regulators PID. To solve this problem we developed the algorithm of figure 5, which uses the PID of only one process and not both at the same time. The algorithm work according to the position of three critical temperatures; the ambient temperature (Tamb), the initial temperature (Tini) with which begins the control and the output temperature (Ts). Our algorithm combines the PID of such a process and the algorithm ON-OF of the other process. The basic idea is: if the control requires the PID of only one process, the regulation work, so not, the other process function to the state ”ON” by µC until the equality of the Ts and Tamb, then the PID of the other process work.
Fig. 4 Mechanism of the µC operation.
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B. Energy study P is the power dissipated by Joule effect in a metal plate of resistance R. This plate have a surface S in contact with air and the another surface S’ in contact with material. Φ p is the power dissipated by surface S in an environment carried with temperature Te, h is the heat exchange coefficient of metal with air(w/m².°c), To is the initial temperature and C is the total heat capacity. P-Φp = P- h.S.(T-Te)= C.dT/dt
(1)
The solution of this differential equation is: T= T1+ (To-T1).exp(-t/A)
(2)
With T1 = Te + P/h.S and A= C/h.S. The transfer of heat in our system is characterized by the power Φ exchanged between the face a and the face b (see fig.1) This power is given by [5]: Φ = K. λ. (Ta-Tb)
(3)
λ is the thermal conductivity of metal (W.m-1.K-1), face a is the internal face in contact with the liquid, face b is the external face contacts with the Peltier plates.
Fig. 5 Algorithm of temperature control developed with the µC
IEEE-ISIE 2004: International symposium on industrial electronics, Ajaccio, France, Volume 1
K is the factor of form :
K = 2.π .
L a
(4)
− 2 .9 b ln[ (0.637 − 1.781e b )] r
IV.
r is the ray of the cylindrical cell interns in (mm). a, b and L are dimensions of the external cell in (mm). C. Automatic study The combination of the operation of the two processes is difficult, our study amounts determining the control low of order which determine the equation of recurrence which connects regulator input and output. For that We wish to minimize the variation e(t) of measured temperature S(t) and that of the consign y(t) (fig. 6), so as to ensure that the total chain has the desired performances, such as stability, the precision and the speed [6]. Controllers ON-OF and PID represent the majority of the controllers used in the industrial control processes. Discrete regulators P.I.D are elements whose input and output are sampled. The equation of such a regulator in a continuous system is given by: t
de(t) U(t)= K pe(t)+ 1 ∫e(t)dt +Td Ti 0 dt
To be able to apply the automatic laws to this system, we adopt algorithm 2, describe in the following paragraph. The determination of the transfer function and the constants of the regulators are done by the method of Strejć [9].
(5)
RESULTS AND DISCUSSIONS
The exploitation of the preceding equations, allows us the simulation of travel time of heat, in the ultrasonic cell according to the power provided. to see (fig. 7). For that we vary the rate of power loss in the ambient air, supposing that the brass cell is filled of pure water. In the figure 7, it appears that the speed of our control system, depends on the output electric puissance provided to heating resistance and to the Peltier plates, also depends on the conditions of energy losses by the ambient air and depends on the liquid to be analyzed by the ultrasonic technique. In order to compare the reliability of our system with other existing devices, we present in the figure 8, controlled temperature measurements of our ultrasonic cell and those of a thermostat (Ministat Huber, DIN 12879-KI). We also present in figure 9, the response simulation of the system controlled using Tolbox simulink of Matlab with C2 = 0. The curve 9-a presents a going beyond for Co = 0.99785 and C1= 0.04, but it to tend towards quickly to the consign. The curve 9-b presents a the response for C1= 0.02 and Co = 0.9978, it shows a slow response time. On the other hand the curve 9-c presents the response with C1 = 0.025 and Co = 0.9976, that we regard as(5) an optimal answer for our corrector.
With Kp, Ti and Td are the constants of the regulator, are given in experiments [7]. In a discrete system the integral and the derivative can be approached by the trapeze method [8]. The control law of this regulator is:
U(k)=U(k-1)+C1.e(k)-C0.e(k-1)+C2.e(k-2) With C1 = (1 +
(6)
∆ Td T T + ) K , C0 = (1 + 2 d ) K , C 2 = d K ∆ ∆ Ti ∆
∆ is the period of sampling, are choice depends on smallest time constant on the process. The system is nonlinear by nature: indeed a tension of negative input will not make cool resistance and a great power in resistance causes a saturation of heating. We can define a linear model around a operation point [10]. It should be noted that we nap owe a physical system composed of two processes (heating and cooling) of different time-constants
τ1
and τ 2 .
Fig. 6 Synoptic diagram of the regulation
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Fig. 7 Curve of our system speed. Fig. 8 Experimental curve of temperature measurement by ministat
IEEE-ISIE 2004: International symposium on industrial electronics, Ajaccio, France, Volume 1
V. CONCLUSION In this work we studied, developed and realized an electronic assembly for the temperature control of an ultrasonic cell. The system developed is composed of three principal elements: an electronic card developed at base of the microcontrolor of the ST62xx family, an ultrasonic cell to measure acoustic speed and an power electronics which command the processes of heating and cooling. These two processes use Joule effect with a heating resistances to increase the temperature and Peltier effect to decrease it. In a first stage, we gave a description of the various system electronic components, we study the thermal and automatic equations. Then we proposed and discussed an control algorithm which combines regulator PID and ON-OF. We also presented, compared and discussed results them measured and simulated with our system and another commercial device of regulation. Finally we presented ultrasonic velocity measurements of pure water with 0.1 °C. VI. [1]
Fig. 9 Curves of simulation of heating with regulator.
We also notice that starting from these curves that the least variation of the values of the constants C0, C1 and C2 changes quickly the corrector. Also that the time of stability by the new regulation is decreased by three times compared to the existing regulator. In order to see used control of the temperature in ultrasonic measurements, we present in figure 10, acoustic speed in pure water measured with the precision of temperature of 0.1 °C. The results obtained are in concord with those obtained by Del Grosso [ 10 ].
Fig. 10 Curve of acoustic velocity measurements.
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K. Quotb and al. ; “An accurate method for the evaluation of ultrasonic velocity according to the temperature in a viscus medium”, in Proceedings of the 11 th International Symposium on Acoustic Remote Sensing and Associated Techniques of the Atmosphere and Oceans, 24-28 June 2002,Rome , Italy. [2] M. Benhayoun, A. Malaoui, M. Ankrim, K. Quotb, “Realization and first tests of an impedance measuring device in the [1mHz – 1MHz] Frequency interval”, PCN journal, V 12, accepted 19 March 2003. [3] A. Malaoui, M. Benhayoun, M. Ankrim, K. Quotb ; « Développement d’un outil informatique à base de programmation G, pour la caractérisation de l’eau de la méditerranée par spectroscopie acoustique», in Proceedings of the International congrée of spectroscopie, Marrakech, 23 avril 2003. [4] J. Marc Delaplace, J. Luc Grégoriadès ; Le ST62xx Mise en œuvre progressive d’un microcontrôleur, DUNOD 1993. [5] Bernard Eylument, Manuel de thermique théorie et pratique 2° édition revue et augmentée [6] F.Kreith, Transmission de la chaleur et thermodynamique, Masson 1967 [7] Direction de la recherche et ingénierie de la formation, cours 16A Régulation PID. [8] VODA A. A., Landu I.D. A, “Method of the Auto-calibration of PID Controllers”. Automatica, Vol. 31, N° 1,pp. 41-53 1995. [9] Yves Granjon, Automatique, Systèmes linéaires, non linéaires, à temps continus, à temps discret, représentation d’état, Edition Dunod. [10] P. Borne ; G. Dauphin, Analyse et Régulation des Processus Industriels T1, Edition Technip. [11] V. A. Del Grosso and C. W. Mader; “Speed of sound in pure water”; J. Acoust. Soc. Am; Vol 52 Number 5 (Part2) PP 1442-1446, (1972).
