Mechatronic system used for maintaining constant temperature with the LM 335 sensors.

Spanu, Alina

1. INTRODUCTION

The systems used for maintaining constant temperature have a wide range of applications regarding the material researches, such as their magnetic properties, in spectroscopy, electrical experiments, superconductor studies, biological sample studies.

As a main requirement, we have to pay attention to the constant temperature level inside a closed volume as well as to the stability and its control with great accuracy. There are technical studies that assume proportional cooling control to ensure energy--saving performance to reduce heat generation to the environment. The Cryo--Compact Circulators with small overall dimensions provide a heating capacity about 2 KW and the maximum ambient temperature +40[degrees]C.(*** Julabo.de, 2009).

The research we have done is focused on the designing a mechatronic system, which may control the temperature level using the temperature sensor LM 335 (Gassmann, 2001). We were interested in computing the airflow for energy saving during an established period, as well as in studying the influence of airflow dynamic parameters on the propeller system. The theoretical results were compared with the experimental ones.

2. THREE DIMENSIONAL MODEL

In order to study the technical methods for maintaining a constant temperature with great accuracy and stability we have designed a mechatronic system, which has the block diagram in Fig. 1. As it is shown it comprises a PC, a serial bus RS 232, the PIC 18 F microcontroller, three temperature sensors drawn as red rectangle, the air cooling system drawn as dark blue rectangle and the optical device for process control.

The three dimensional model of the experimental system is shown in Fig. 2. In order to measure the temperature values of the environment, the temperature sensors are placed as follows: one of them is near the air cooling system, another one near the heating device and the last one at the opposite side of the cooling system (*** Microchip.com, 2009).

We have paid attention to the LM 335 temperature sensor with its main technical performances. The Zener diode provides electrical tension losses about +10mV/[degrees]K in direct dependency on the temperature. The linear output signal as function of electric tension has a wide range between 25[degrees]C and 100[degrees]C with an error about 1o C.

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3. MATHEMATICAL MODELLING AND SIMULATION FOR THE AIR COOLING SYSTEM

The main goal of the mathematical modelling and simulation was the computation of the variation of the temperature values inside the closed volume taking into account the air-cooling flow over an imposed period of time. Finally, we have to compare the experimental results with the theoretical ones. We have made the mathematical model considering the flow running over a sonic and subsonic range, established as a function of the pressure values before and after the section of the propeller system. The air mass flow computed in the exit section of the air system is given in (1) (Demian & Banu, 1984), where: [d.sub.m] / dt is the air mass flow; [chi] = 1.4 is the

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

adiabatic coefficient; A--the section for the air flow; [DELTA]P, [P.sub.1] are the variation of the pressure and the pressure in the computation section of the air flow; [[rho].sub.2] - air density.

The mechanical torque as a pressure function is given by:

M = [[eta.sup.*] [V.sub.D]/[2.sup.*] [pi] * ([P.sub.2] - [P.sub.2]) (2)

where: M--the torque of the electrical D.C. brushless motor; [eta]--the efficiency; [V.sub.D]--the volume closed by two adjacent blades of the propeller; [P.sub.1], [P.sub.2]--the air pressure values before and after the computation section.

Finally, we could compute the temperature variation during a given period, considering the sections between the air-cooling system and the heating source by using the following equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

where: [T.sub.1]--the temperature of the input section; [A.sub.i], [N.sub.i]--the input area and its flow function; [epsilon]--the pressure ratio for the input nozzle; [A.sub.o], [N.sub.o]--the output area and flow function for the bearing; [A.sub.p], [N.sub.p]--the output area and flow function for the space between the propeller and the frame; [A.sub.b], [N.sub.b]--the output area and flow function for the bearing.

4. NUMERICAL APPLICATION.

In order to study the theoretical aspects regarding the way of providing a constant temperature using the mechatronic system we have developed a three dimensional model of the air-cooling device. The theoretical aspects were focused on the dynamic parameters of the air flow and its influence on the propeller system. Fig. 3 shows this 3D model. The mathematical model above was applied for the tested system using the following numerical values: [T.sub.1] = 27.2[degrees]C; [[rho].sub.2] = 1.2047Kg/[m.sup.3]; [chi] = 1.4; [P.sub.1] = 1.013e+05N/ [m.sup.2]; [P.sub.2] = 1.0385e+05 N/[m.sup.2];M = 0.4 Nm; [eta] = 0.8; [epsilon] = 3.5;

Solving the numerical equations defined above we have computed the results presented in Fig. 4 and Fig. 5. All these results emphasize the influence of dynamic parameters over the air-cooling process. Based on the results we have done FEM model and analysis of the propeller system, so that we infer the great efforts of the blend are found beside the shaft of the D.C. electrical brushless motor. (Spanu et al., 2006) The results are presented in Fig. 6. We infer the influences of the internal efforts as well as the displacements are as minimum as possible. The future research will be developed considering more complex sections for the airflow passing and more complex mathematical model for describing the external temperature influence.

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5. CONCLUSIONS.

The paper was focused on theoretical and experimental study of the constant temperature maintaining process taking into account the dynamic parameters. In order to do this with high accuracy we have designed a mechatronic system including three temperature sensors LM 335 and the brushless D.C. motor. We have solved the mathematical model describing the process. In order to improve the accuracy, we have made a FEM analysis of the propeller system, so that the internal efforts and displacements have minimum values.

6. REFERENCES.

Demian, Tr. & Banu, V. (1984). Micromotoare pneumatice liniare sirotative (Linear and rotational pneumatics micromotors), Editura Tehnica, Bucuresti

Gassmann, O. & Meixner, H. (2001). Sensors in Intelligent Buildings, Wiley-VCH Verlag GmbH, Weinheim

Spanu, A.; Hadar, A. & Dragoi, G. (2006). Modelarea tridimensionala a paletei de antrenare a suhstantelor folosite pentru generarea spumei umede din lichide (Three-dimensional modelling of the propeller hlend used for providing the wet foam of liquids),.Revista Materiale Plastice (Plastics Material Review), Nr. 3/2006, pp 199203, ISSN 0025/5289

***(2009) http://www.microchip.com., PIC Microcontroller Oscilator Design Guide, Accessed on: 2009-06-10

***(2009) http://www.julabo.de, Accessed on: 2009-06-10