Microcontroller Based Electric Expansion Valve Controller for Air Conditioning System

Microcontroller Based Electric Expansion Valve Controller for Air Conditioning System Thae Su Aye, and Zaw Myo Lwin Abstract In the air conditioning system, the electric expansion valve (EEV) is one of the most important parts of the system. It regulates how much liquid refrigerant enters the evaporator and tries to maintain a preset temperature difference between the inlet and outlet opening of the evaporator. In this paper, the author would like to implement how to control the EEV by comparing the temperature difference between the actual temperature and predetermined situation temperature to the reference setting point. Here, the author does not use the real EEV used in the aircon, but only uses the stepper motor to control the EEV. Stepper motor operates the precise refrigerant control because it does not rotate continuously but stops at the position predetermined by the control algorithm. In this paper, proportional control algorithm is applied. As the stepper motor driving system, PIC based control system is designed including the assembly software technology and dirlington control circuit is used. For the implementation of the stepper motor driving system, PIC16F877A is used and to detect the surrounding temperature LM35 temperature sensor is used. The outputs of the PIC are commands to drive the stepper motor, inputs of the dirlington control circuit. this paper mainly focuses on how the stepper motor controls the EEV for the air conditioning system and finally shows the test and result of the outputs of the PIC16F877A. temperature converted from the evaporating pressure sensed by the pressure sensor using pressure-temperature table. In this paper, pressure sensor is not used and this situation temperature is predetermined as 27 C in the program. The difference between these two temperatures is called superheat For sensing superheat, there are two basic methods: pressuretemperature method and two temperature method. The pressure-temperature method is applied in this paper. By comparing this actual superheat and reference setting superheat point, the EEV is controlled. EEV can be divided into four types. They are pulse, analog, heat motor and step motor. In the step motor valve used in this paper, a stepper motor is used to open or close the valve port. A stepper motor converts electronic pulses into mechanical movement. Each electronic pulse causes the shaft to rotate a certain number of degrees or step angle. The movement created by each pulse is precise and repeatable, which is why stepper motors are so effective for precision positioning applications. It does not rotate continuously but rotates a step by step for each signal sent by the controller. The number of step signals is recorded by the controller and the controller return the valve to any previous position at any time. Keywords Air Conditioning System, Electric Expansion Valve (EEV), Proportional Control Algorithm, Stepper motor, PIC16F877A. A I. INTRODUCTION IR conditioning or refrigeration system is widely used all over the world. This system is a closed loop system where the process of absorbing and rejecting heat is performed by the alternate compression, condensation and evaporation of a working fluid. There are two difference types of temperature in the system: actual temperature and situation temperature. Actual temperature is external pipe wall temperature sensed by the temperature sensor and situation temperature is the Manuscript received November 15, 2007. This work was supported in part by the Ministry of Science and Technology, Union of Myanmar. Authors are with the Mandalay Technological University, Mandalay, Myanmar (phone: 095-2-88704(Electronic Engineering Department), fax: 095-2-88702(Office, MTU), e-mail: thae.su.aye14@gmail.com and zawmyolwinn@gmail.com). Fig. 1 Basic Refrigerant Cycle 387

II. SYSTEM HARDWARE COMPONENTS The overall system configuration is briefly represented in this section and the hardware used in this research and the physical integration of the components are also described. A. PIC Microcontroller The PIC 16F877 (Microchip Technology, Inc., www.microchip.com) 8-bit microcontroller was chosen to obtain the analog data from the LM35 temperature sensor and process this data and output the command to drive the unipolar stepper motor connected to the expansion valve. This microcontroller has a 25 MHz processor (the current compiler runs the processor at 20 MHz), 33 input/output (I/O) pins, (8k*14words) of Enhanced FLASH program memory, (386*8bytes) of RAM, (256*8bytes) of data EEPROM. The PIC does not have an operating system and simply runs the program in its memory when it is turned on. This PIC microcontroller has several hardware features that are very useful and simplify the interfacing of sensors and motors with the microcontroller. B. Temperature Sensor and its Interfacing Although there are many types of sensor LM35 IC sensor is used to send the outlet of the evaporator in this project. The followings are its feature and it is calibrated directly in degree Celsius. 1. Linear +10mv/c scale factor 2. Rated for full -55 to +150 C range 3. Less than 60 A current drain 4. Low impedance output, 0.1 for 1mA load 5. Operate from 4 to 30 volts This is very simple sensor to interface.non-inverting buffer which gained one is used to interface for LM35 and it is shown in Fig. 2. example, two-degree increments, half-degree increments, etc.). They are used in the devices where precise positioning of the motor is necessary. In this project, the precise value of step is requires, so this type of stepper motor is used. To control a unipolar stepper, a Darlington Transistor Array is used. The stepping sequence is as shown below. Wires 5 and 6 are wired to the supply voltage. Step wire 1 wire 2 wire 3 wire 4 1 high low high low 2 low high high low 3 low high low high 4 high low low high Fig. 3 Motor Driving Circuit Fig. 2 LM35 interfacing Circuit C. Stepper Motor and its Control Circuit The unipolar stepper motor with six wires is used to close and open the expansion valve. A stepper motor is a motor controlled by a series of electromagnetic coils. The center shaft has a series of magnets mounted on it, and the coils surrounding the shaft are alternately given current or not, creating magnetic fields which repulse or attract the magnets on the shaft, causing the motor to rotate. This design allows for very precise control of the motor: by proper pulsing, it can be turned in very accurate steps of set degree increments (for III. PROPORTIONAL CONTROL ALGORITHM There are three types of control mode for refrigerant system used in air conditioning system. They are (1).Two-position control, (2).floating control and (3).proportional control. In the proportional control mode, actual temperature will approach to its set point. But it may not reach this point because of the various factors. The difference is called offset. If offset were constant, this difference could be programmed into the controller. In real world, offset changes over time with load conditions, so some means of predicting them must be used. To implement control algorithm, the followings must be considered. 1 let X be reference superheat set point 2 let Y be actual superheat (the difference between actual temperature and situation temperature) 3 if X=Y, the valve is stopped 4 if X >Y, the valve is opened one step 5 If X< Y, the valve is closed one step Go to 3. 388

By implementing this procedure, the valve will reach its required position by counting step by step. The number of steps required to rotate according to the sensory information is calculated by the followings. the situation temperature and the reference superheat set point as well as the number of steps to rotate the motor. Error e=h- h 0 where h 0 = reference superheat set point (8 C) h = actual superheat In refrigeration system, full scale range of superheat is considered between 5 to 15 C. In this paper, 10 C is applied. % change in superheat = 100 (h-h 0 /full scale range) = 100 (h- h 0 /10) = 10(h-h 0 ) The stepper motor having 1.8 degree/step resolutions is used, therefore, 200 steps is required to rotate one revolution. And change of valve position can be calculated according to the following. % change in valve position= 100 (v-v 0 /valve position in full scale range) = 100 (v-v 0 /200steps/rev) = 0.5 (v-v 0 ) Where v = desired valve position in step v 0 = valve position corresponding to h 0 in step Proportional Gain P = 0.5(v-v 0 )/ 10(h-h 0 ) = 0.05a/b Where a = v-v 0 (in step) b = h-h 0 (in C) For linear system, P = 1 0.05a/b = 1 a = 20b if actual superheat (h 0 ) = 10 C, b = 10 8 = 2 a = 20 2 = 40 steps the motor rotates 40 steps that may be clockwise or counter clockwise direction and EEV will open. If actual superheat (h 0 ) = 5 C, b = 5 8 = -3 a = 20 3 = 60 steps the motor rotates 60 steps that may be clockwise or counter clockwise direction and EEV will close. IV. SOFTWARE CONSIDERATION In order to determine how to rotate the step motor expansion valve, the author considers the actual temperature, Fig. 4 Flow Chart for EEV Controller In this system, expansion valve is open or close by comparing superheat set point (8 C) and temperature difference that is the difference of actual temperature and situation temperature (27 C). temperature difference is computed and compares this value with the set point. According to this comparison value, the valve selects the correct mode direction. This process is expressed in the following flow chart shown in Fig. 4. In this paper, although the stepper motor has 200 steps/rev and 1.8 deg/step resolutions is used, its sequences rotate four patterns: 0101, 1001, 1010 and 0110 continuously. 389

V. SOFTWARE IMPLEMENTATION For implementation of the program, the author uses MPASM for window from Microchip and MPLAB software in order to minimize the cost of the system and it is simply easy to use. The main program has been done using assembly language. By using MPASM assembler, the main program is assembled to get the hex file shown in Fig. 5. When we get this file, it is programmed to the PIC by using PIC STARTPLUS programmer and test circuit. VII. EXPERIMENTAL RESULT The experimental results for each output ports of PIC and the control system testing circuit is shown in fig.6 to fig.10. All of these outputs are square wave and these are fed to the base of driver darlington transistor pairs to drive the motor according to the required sequences. For the assembly software of the control system, the process is very simple and the procedure can be mentioned as the following steps. A. Initialization All port A pins are declared as inputs and port C pins are also declared as outputs. Constant 8 C and 27 C are set in sup1 register and sattemp register respectively. B. Start Program When the program starts, analog input from the output of the temperature sensor is converted by bit in ADC, calculate the actual superheat and compare reference superheat, calculate the number of steps to rotate, check motor position and determine the motor direction. Fig. 5 Hex File of application VI. CIRCUIT EXPLANATION OF EEV CONTROLLER In the circuit shown in Fig. 2, temperature sensor senses the actual temperature. The sensor output is the input to the PIC16F877A at port RA0. Depending on the input state, the output conditions that control the motor through the TIP120 darlingon transistor circuit are provided by assembly software at port C, RC0_ RC3. As for PIC microcontroller, this part is supplied with 5V DC. In this circuit, PIC is used with a simple clock condition with 4MHz crystal. C. Determine Output According to the sensory data and calculation result, the outputs of the PIC will be 0101, 1001, 1010 and 0110 alternately in clockwise or counter clockwise direction. The out put of the microcontroller that is fed to the darlington array is shown in the following figures. Fig. 7 Output waveform of RC0 Fig. 6 PIC-based circuit diagram for EEV controller Fig. 8 Output waveform of RC1 port The darlington connection type TIP120 NPN transistor pairs are used for the drive of the stepper motor coils. The diode putting between the collector and the power is for the protection of the transistors. There are four TIP120 NPN transistors to drive motor and this depends on the output bits of PIC16F877A. 390

[8] HIS Stepper Motor Theory n.d. Retrieved October 31, 2005, from http://eceserv0.ece.wisc.edu/~morrow/ece315/hsi_stepper_motor_th eory. Fig. 9 Output waveform of RC2 port Fig. 10 Output waveform of RC3 port VIII. CONCLUSION The electrically control expansion valve is more efficient than other types of valve. And the motor is translated into a linear movement to operate the valve which can be accurately positioned to open or closed the valve. Since it is easier to modify the electronic system than to try and adapt the valve, EEVs readily accept the new refrigerant and able to get the best performance. The quality of control depends on the electronic system rather than the design of valve. the program embedded in the PIC can be changed as the user require without needing to change the design of valve. ACKNOWLEDGEMENT Firstly, the author would like to pay her respectfully thanks to her beloved parents, teachers and her admire supervisor. Special thanks are due to her partner for his encouragements. The author also expresses her greatly thanks to all persons who will concern to support in preparing this paper. REFERENCES [1] Penfold,R.A. An Introduction to PIC microcontroller [2] Microchip Technology, 2001, PIC16F877A Datasheet www.microchip.com [3] Microchip Technology, 1998, MPASM user s guide www.microchip.com [4] Anonymous, April 2001, Electric Valve for Refrigerant Control by Sporlan Valve Company, www.sporlan.com [5] Dossat, R.J. 1997, Principle of Refrigerant System Fourth Edition, by Simon & Schuxter/A Viacom Company [6] Inove, S. October 2005, Stepper Motor Controller PIC Circuit Gallery, www.hobby-elec.org\step7.htm [7] Robert N. Bateson, 1989 Introduction to Control System Technology, Third Edition 391