King Fahd University of Petroleum and Minerals. Department of Electrical Engineering

Transcription

1 King Fahd University of Petroleum and Minerals Department of Electrical Engineering AN OPEN LOOP RATIONAL SPEED CONTROL OF COOLING FAN UNDER VARYING TEMPERATURE Done By: Al-Hajjaj, Muhammad Supervised by: Dr. M, Kassas & Mr. N, Tasaddug. Monday, May 15, 2006

2 2 ABSTRACT The aim of this project is to design, build, and simulate an open loop rational speed control of cooling fan under varying temperature. Based on the temperature value measured from a temperature sensor, the speed of the fan increases or decreases in accordance to the temperature value. The project consists of several stages. First, two signals should be taken from an electronic circuit, one for the speed voltage of the fan and the other for the temperature voltage of the thermistor resistance. Then, these signals are sent to software program through the terminal block. Finally, the reference voltage comes out from the computer to the electronic circuit, which controls the speed of the fan. The system has been designed and tested successfully where all components of the system operate as expected.

3 Acknowledgement 3 We gratefully acknowledge Dr. M, Kassas and Mr. N, Tasadduq for helping and guiding us until we accomplished the goal of this project. Thanks also extended to Dr. Abuelma'atti, Taher from Electrical Engineering Department and Dr. N, Maalej from Physics Department for their valuable advices and to Mr. Farrazi for hardware components

4 4 TABLE OF CONTENTS ABSTRACT 2 Acknowledgement 3 1. INTRODUCTION 7 2. HARDWARE STRUCTURE OF THE PROJECT Fan Speed Measurement Fan Speed Control Measuring Temperature with Thermistor SOFTWARE STRUCTURE OF THE PROJECT Introduction to LabVIEW LabVIEW Program Analysis CONCLUSION RECOMMENDATIONS 27 REFERENCES 28 Appendix A 29

5 5 LIST OF FIGURES No. TITLE Page 1.1 The structure of the system Fan speed-dc voltage Curve LM324 PWM circuit Triangle wave at pin Triangle wave at pin Pulse wave at pin Fan speed-reference voltage Curve Fan speed-fan voltage Curve Resistance-Temperature Curve of a Thermistor Voltage divider circuit VI front panel and block diagram VI block diagram, converting voltage to speed VI front panel, speed of the fan VI block diagram, temperature while loop VI front panel, temperature degrees VI block diagram, reference while loop VI front panel, reference voltage VI- front panel, reference voltage, temperature degrees, time Vs speed, speed 25

6 6 No LIST OF TABLES TITLE Fan speed with its terminals voltage reading. Thermistor resistance reading at different temperature degrees The voltage values of thermistor resistance Voltage values of thermistor resistance (Array #2) Voltage values of thermistor resistance (Array #1) Page

7 7 1. INTRODUCTION The objective of this project is to design, build, and simulate an open loop rational speed control of cooling fan under varying temperature. Based on temperature value measured using a temperature sensor, the speed of the fan increases or decreases accordingly. We then show that this open-loop control system is useful by inputting different temperatures. Through all of this, the control system should keep the RPM of the fan consistent with temperature. To accomplish our objective, we built two types of electronic circuits: (i) The Pulse Width Modulation (PWM) circuit to give pulses that can control the transistor to vary the input voltage, which will operate the fan with varying levels of speed, (ii) The Voltage Divider Rule (VDR) circuit to afford the voltage of the thermistor resistance. The measured voltages are transferred to a computer program (LabVIEW) through a special board called terminal block, which will output a voltage (reference) that is used for controlling the fan speed. This output voltage, as we will see, varies in accordance to the temperature degrees.

8 8 Figure 1.1: Block diagram of speed control of cooling fan This project consists of two main parts that were built and tested to achieve the project goal (see Figure.1.1). The first one is a hardware structure using electronic devices, which also consist of two main stages. The second one is a software structure using LabVIEW program. 2. HARDWARE STRUCTURE OF THE PROJECT Hardware part consists of three stages that were built and tested to get the reasonable data used later with LabVIEW program Fan Speed Measurement. Since the main purpose of the project is to design a closed loop rational speed control of cooling fan, we need to choose a fan and see the relationship between its speed and the voltage applied on it. (see Figure.2.1)

9 9 Figure 2.1 Fan speed vs. DC input voltage A simple connection was made in the laboratory for testing the fan. Ammeter was used for reading the current while the tachometer was used for reading the speed in revolutions per minute (rpm) Fan Speed Control Pulse Width Modulation (PWM) control method As known, the speed of a DC motor is directly proportional to the supply voltage, so if we reduce the supply voltage from 12 Volts to 6 Volts, the fan will run at nearly half the speed. (see Figure 2.1) The purpose of a fan speed controller is to control the speed according to the signal representing the required speed. Motors come in a variety of forms, and the speed controller's motor drive output will be unlikely dependent on these forms [1].

10 Generally, the speed controller works by varying the average voltage sent to the motor. It can do this by simply changing the voltage sent to the motor, but this is quite inefficient. A better way is to knob the motor's supply on and off very quickly. The DC voltage converted to a square-wave signal, alternating between fully on (12V) and zero, giving the motor a series of power "kicks [1,4] Circuit and its operation Figure 2.2 shows LM324 circuit, which we built on prototype board to generate PWM signal by comparing a triangular wave signal with a DC signal. The DC signal can range between the minimum and maximum voltages of the triangle wave. The electronic circuit always works with 6 khz.

11 11 Figure 2.2 LM324 PWM circuit. LM324, a 14-pin IC consists of four individual op-amps. The triangle signal is generated with two of them (U1A and U1B) (see Figure 2.3), and a third (U1C) is used as an amplifier which also inverts the wave signal and centralizes it in the 0-10V output range of the LM324 when on a 12V supply. (see Figure 2.4) Figure 2.3 Triangle wave at pin7

12 12 Figure 2.4 Triangle wave at pin8 The fourth op-amp (U1D) is used as a comparator to compare the triangle signal with the reference voltage set by the DC power supply, which generates the square wave. When the wave voltage goes above the voltage at the pot wiper, the comparator output goes high, turning on the transistor switch and power to the fan. (see Figure 2.5) Figure 2.5 Pulse waves at pin14 The frequency is controlled by C1, R1, and R2 (22k) & R3 (10k) according to the following formula: frequency = R2 / (4 x R3 x R1 x C1)

13 Experimental Result 13 Once the square signals executed at pin 14, the fan rotates. Then, we use tachometer for reading fan RPM and the voltmeter to measure fan terminal s voltage (Vt). The speed of the fan controlled by the reference signal, which generated by supply (Vdc). Table 2.1: fan speed with its terminals voltage reading Voltage Speed Fan voltage(vt) References voltage(vdc) rpm

14 As seen in table 2.1, we start with maximum references voltage (11.65 v), then 14 decrease it until the fan stopped. When a maximum voltage sent from the power supply, it will appear as 2300 rpm at the tachometer screen. However, at 0.8 volt, at DC power supply, we observe the speed to be 505 rpm at the tachometer screen. (see Figures 2.6& 2.7) Figure 2.6 Fan speed-reference voltages Curve Figure 2.7 Fan speed-fan voltages Curve

15 Problems and Solutions: Tachometer available in the lab reads the speed quickly, and we could not take the exact results, which will make some errors. We solved this problem by looking always to the maximum reading at every step. PWM circuit does not have DC power supply. This is our solution to handle the reference signal that goes to a comparator (U1D). When the fan rotates at around 0.8 volt, the noise starts increasing in an irritating manner. To solve this problem, we have increased the value of two capacitors across the +12V (C2 & C3). We lost one transistor through circuit testing because of back-emf from inductive loads (fan). So to overcome this problem, the diode has been used to protect the switching transistor from being damaged Measuring Temperature with Thermistor Thermistors are temperature sensitive resistors. All resistors vary with temperature, but thermistors are built of semiconductor material with a receptivity that is spicifically sensitive to temperature. There are two types of thermistors, negative temperature coefficient (NTC) thermistors, whose resistance decreases with increasing temperature, and positive temperature coefficient (PTC) thermistors, whose resistance increases with increasing temperature. NTC is much more commonly used than PTC, especially for temperature measurement applications [5].

16 In this part, resistances of thermistor are measured at different temperature 16 degrees. The voltage divider rule is used to produce a voltage that is sensed by a data acquisition system (LabVIEW program) Resistance/Temperature Characteristic of Thermistor Normally, thermistor companies make available the resistance/temperature curves or tables for their particular devices. The thermistor curve, on the other hand, could be approximated perfectly with the following Steinhardt-Hart equation [3]: 1/T = A + B*ln(R) + C*(ln(R)) 3 where T ( K) is the temperature in degrees Kelvin, equal to T ( C) , and R is the resistance of the thermistor in Ω. The coefficients A, B, and C are presented by the thermistor company, or determined from experimental measurements of resistance. Temperature vs. Resistance 20 Risistance (kω) Temperature ( C) Figure 2.8 Resistance-Temperature Curve of a thermistor

17 Table 2.2 Thermistor resistance reading at different temperature degrees Temp Resistance (kω) Table 2 & Figure 2.8 show the relationship between the resistance of thermistor and its body temperature. We basically used thermometer to measure the temperatures degrees in ( C) when water is heated by a heater. After that, we read resistance values of thermistor from ohmmeter screen Thermistor Measurement Circuit Figure 2.9 Voltage divider circuit

18 18 As the DAQ module Analog Input deals only with voltage, we will need to provide constant voltage source, V DC, and a reference resistor, R=10K, and set the thermistor in a simple voltage divider to convert the resistance R T to voltage. In this design, shown in Figure 10, the output voltage V 0 will be equal to: V 0 = (R T * V DC ) / (R T + 10k ) Table 2.3 the voltage values of thermistor resistance Resistance (kω) Vout SOFTWARE STRUCTURE OF THE PROJECT The program takes in inputs from the terminal block into the DAQ and sends out a voltage to drive the fan (see Figure 1.1). Our LabVIEW program is designed to show reference voltage in volts (DAQ output), fan speed in rpm (DAQ input 1), temperature degrees in ºC (DAQ input 2) and speed-time characteristic. Look up tables are used for

19 retrieving temperature values where reference voltage and mathematical operation are used to find the speed Introduction of LabVIEW LabVIEW (Laboratory Virtual Instrument Engineering Workbench) is a graphical programming language based on the concept of data flow programming. There are three important components involved in the testing and measurement application, namely data acquisition, data analysis and data visualization. LabVIEW features an easy to use graphical programming environment, which covers these vital components. [2] The LabVIEW graphical development environment provides powerful tools to create applications without writing any lines of text-based code. With LabVIEW, dragging and dropping per-built objects quickly and simply creates user interfaces for the application. Then, specifying system functionality by assembling block diagrams produces a natural design notation for scientists and engineers. [2] LabVIEW programs are called virtual instruments (VIs). Each VI has three components: a block diagram, a front panel and a connector pane. Controls and indicators on the front panel allow an operator to input data into or extract data from a running virtual instrument. In LabVIEW, we can build a user interface by using a set of tools and objects. The user interface known as the front panel. We then add code using graphical representations of functions to control the front panel objects. The block diagram contains this code. In some manner, the block diagram looks like a flowchart. [2] (see Figure 3.1) 19

20 20 Figure 3.1 VI-front panel and block diagram LabVIEW Program Analysis Data Acquisition Inputs include: DAQ input 1: The voltage comes out from the tachometer corresponding to the RPM of the fan DAQ input 2: The voltage across thermistor resistance. Figure 3.2 VI-block diagram, converting voltage to speed

21 As the DAQ measures only voltages, tachometer is used as an rpm-to-voltage 21 converter. Experimentally, we noticed that the maximum speed which tachometer can read is rpm at 5 volts. However, we got the rpm by multiplying whatever voltage entered to DQA input 1 by 2000, as shown in Figure 3.2. Indicator (speedometer) is used for viewing the speed on the front panel. (see Figure 3.3) Figure 3.3 VI- front panel, speed of the fan On the other hand, DAQ input 2 which is the voltage across thermistor resistance (V Temp. ) comes in while loop. Figure 3.4 shows subroutine program (while loop) that compares the voltage temperature (V Temp. ) and the actual temperature sensed by the thermistor which is stored in array # 2. Indicator (thermometer) is used for viewing the temperature degrees in ºC on the front panel. (see Figure 3.5)

22 22 Figure 3.4: VI-block diagram, temperature while loop Figure 3.5 VI-front panel, temperature degrees

23 Table 3.1 Voltage values of thermistor resistance with temperature degrees (Array #2) Vtemp (V) T [ C] Vtemp (V) T [ C] Vtemp (V) T [ C] DAQ output includes only one output voltage that goes to PWM circuit through the terminal block to control the fan speed. It comes out when the DAQ input 2 (V Temp. ) goes through the while loop that compares its value with reference voltage values which is stored in array #1. This subroutine program shown in Figure 3.6 works as:

24 24 Figure 3.6 VI-block diagram, reference while loop If the temperature is going less than or equal the desired temperature (20 ºC), less reference voltage is applied to get minimum fan speed (0 rpm). However, if the temperature is greater than or equals the desired temperature (50 ºC), high reference voltage is applied to get maximum fan speed (2200rpm), otherwise, the speed increases when the temperature increases from 20 ºC up to 50 ºC. An indicator is used for viewing the reference voltage (in volts) on the front panel. (see Figure 3.7) Figure 3.7 VI-front panel, reference voltage

25 25 Table 3.2 Voltage values of thermistor resistance (Array #1) Vtemp Vref Figure 3.8 VI-front panel, reference voltage, temperature degrees, time Vs speed, speed

26 4. CONCLUSION 26 A DC fan control system has been developed using National Instrument's LabVIEW software and Data Acquisition Board. A path generation technique has been utilized to compute a speed profile of the fan. Experimental results are promising. We were able to accomplish our goal of designing a system to control the rational speed of a computer fan in an open- loop. As the program is running, one can see that the error percentage of the actual rational speed (rpm) and the desired rpm is very low yet not zero. Since the fan sends pulses of voltage, the fan has a tendency to overshoot and undershoot the desired rpm. However, these differences are very low keeping the error percentage to be minimal. For example, when the fan is spinning at 2200 rpm and the desired speed is 2200 rpm, the error percentage is less than 5%. However, when the desired rpm changes to 505 rpm, the fan receives a very low voltage until the rational speed is 505 rpm. Unfortunately, the fan speed drops above the desired value of 505 rpm. However, the program identifies this problem and then sends a higher voltage until the fan is spinning at the desired rpm again. This loop goes on and on for the length of the program running time and the rational speed oscillates close to the desired value.

27 5. RECOMMENDATIONS 27 The variation of speed on the fan is based on varying the value of reference voltages, which comes from DAQ output to a hardware structure (PWM) circuit. So, designing a proper electronic circuit is critical to get the desired speed. The design of this electronic circuit can be build with LabVIEW. LabVIEW is used to control the speed of the fan through lookup tables. Good results could be achieved by designing a PID controller instead of arrays. Also, a special thermistor could be used to measuring the temperature of fan. Based on that, we can increase or decrease the fan seed.

28 28 REFERENCES 1. Fan noise solutions < ( 27 Feb ) 2. Getting started with LabVIEW. National Instruments Corporation < > ( 1 March 2006 ) 3. Measure Temperature using Thermistor. EmAnt Pte Ltd Co < > ( 10 April 2006 ) 4. Speed controllers < SpeedControl/ SpeedControllers.html> ( 5 March 2006) 5. Potter, David Measuring Temperature with Thermistors a Tutorial. 1-4.< urements.pdf >

29 29 Appendix ( LM 324 data sheet )

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