Laboratory Final Design Project. PWM DC Motor Speed Control

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1 Laboratory Final Design Project PWM DC Motor Speed Control Bowen Wang, < > Siyang Xia, < > Renhao Xie, < > E E 331 Lab, Winter 2013

2 TABLE OF CONTENTS Purpose of project, features, ratings. 3 Block diagram Complete schematic... 5 Circuit simulation Test results Bill of materials and Cost estimate Summary/Conclusion

3 1.Purpose of project, features, ratings 1.1 Purpose: It is important for engineers to control a higher power load with a lower power circuit. However, if we perform a direct modulation, it will create a rather inefficient design with the control device (transistor). Thus a better way with a high power efficiency is to rapidly switch the control device on and off at a frequency which is high enough that the load effectively sees only the average over many cycles. And this is also the basic idea of this project. The purpose of this design project is to develop and prototype a high efficiency pulse width modulation (PWM) speed control for a small DC motor. 1.2 Features: VDD power supply (5V and 15V) Resistor: 1k, 100k, potentiometer (maximum 10k) Zener Diode: 1N4007 Capacitor: 100nF, 47nF NMOS: 2N7000 LM555 timer: LM555CN Comparator: LM339N Motor Jump wires Breadboards 1.3 Ratings Input Voltage: 5V for the chips, 15V for the motor Output voltage: triangle wave 0-3V for the saw tooth, square wave 0-5V for the motor 3

Block diagram Figure 2.1 Block Diagram of the PWM DC motor speed control There are four parts for the whole system: oscillator, PWM modulator, motor driver and the DC motor.

4 Block diagram Figure 2.1 Block Diagram of the PWM DC motor speed control There are four parts for the whole system: oscillator, PWM modulator, motor driver and the DC motor. The oscillator will create a triangle saw tooth waveform drive the motor control circuit. We build this by powering a 555- timer chip with 5 volt DC Source, which will create a linear waveform with a 10kHz frequency. And this signal will be sent to the next part: the pulse width modulation (PWM). In this part, we setup a control voltage input and compares it to the saw tooth wave, and get an output of a square- pulse wave. Then the square wave is sent to the motor driver that is comprised of a buffered transistor. The pulse is turning the switch on and off rapidly to keep the motor running while conserving power. In the end, we power the motor by a 15- volt DC source and the whole system works. 4

Complete schematic (Circuit Design) Figure 3.1 Schematic Diagram of the whole system Figure 3.1 shows a complete schematic diagram of the whole system.

5 Complete schematic (Circuit Design) Figure 3.1 Schematic Diagram of the whole system Figure 3.1 shows a complete schematic diagram of the whole system. The left part is oscillator, the up part is PWM, the right part is the motor driver and finally the big 3- D motor is in the middle. In order to build exact the same circuit, we buy some new components from the E E store beside the given lab kit. First for the oscillator, we setup the power- supply to 5V which is V1. And we use the 100k- ohm resistor for R1 and R3, and connect 2 100k- ohm resistors in series, which is 200k ohm for R3. Also we connect 100nF and 47nF in parallel, which is 147 nf for C1. And we buy the LM555CN chip for U1 and connect the circuit as shown above. Then for the comparator (PWM), we bought the LM339 chip from the E E store, and use the same power supply (V1) as for the oscillator. Also we use the 1k- ohm resistor for R4 and the potentiometer for X1. Then we connect the circuit as shown above. Then for the motor driver, we bought the diode 1N4007GP and the transistor 2N7000 from the E E store, and connect them as the circuit shown above. Finally for the motor, we bought it from the E E store and power it by a 15- volt power supply, and then connect it as the circuit shown above. 5

6 Circuit simulation Figure 4.1 Simulation of the oscillator and the PWM Parameter Design Specification Test Results Power Supply +5V for speed control 5.0V +15V for motor 15.0V Oscillator 10KHz +/- 5% frequency 9.7KHz, which is 3% lower than 10KHz, within the 5% range. Saw tooth asymmetrical ramp Yes, it s saw tooth asymmetrical ramp <10us fall time 160ns, which is far less than 10us Frequency does not Yes, it does not change PWM modulator change V output At least 10% to 90% modulation Correct Correct 6

7 Motor driver 0 to 500mA at +15.0V 420mA, within 0-500mA range Functions Speed increases w/input voltage Correct Table 4.1 Results of the measurement compare to the expected values Test results The following are the images of saw tooth, pulse with different duty cycles. From the measurement on the graph, we meet all the specifications, the oscillation frequency is within 10KHz +/- 5% range, the output of the oscillator must be a periodic saw tooth asymmetric ramp waveform. The fall time is far less than 10us. We chose to use a 555 Timer Chip because it can keep the circuit simple and it s cheap, maintaining ideal characteristics. When we first built the circuit according to our simulation, we found that in real circuit the output is not exactly like the simulation, some properties don t meet the requirements, this may be caused by the error of the resistors and capacitors, especially the capacitors, in real life the value of capacitor can be 40%+/- it s stated value. Then we adjust the values of our resistors and capacitors to achieve the correct frequency. The circuit meets all of these requirements. The saw tooth waveform is not completely ideally linear, but close enough. The output is shown below. The next step is to use an input control voltage to compare to the saw- tooth waveform in order to produce a square wave to help switch the motor on and off. The output should be in the range from 0-5 volts and able to vary the duty cycle at least between 10% and 90%. We designed this part of the circuit with a Voltage Comparator IC as it simple and cost effective. We used a potentiometer which act like a voltage divider to change the input control voltage without using another power source. We perfectly meet the requirements. When we added the motor driver to the circuit, we found there s current leaking, then we add a diode 2N4732 to avoid it, but the output is still wrong, because 2N4732 is a Zener diode, it will also permit it to flow in 7

8 the reverse direction when the voltage is above the breakdown voltage. So finally we use a 1N4007 rectifier to solve this. The output is nice and the motor works perfectly. Figure 5.1 nearly 0% duty cycle Figure 5.2 nearly 10% duty- cycle 8

9 Figure 5.3: 90% duty- cycle Figure 5.4: nearly 100% duty - cycle 9

10 We then measured the linearity of our pulse- width modulation by comparing the duty cycle to the input voltage. It is very close to linear with an R^2 value of The plot and data is shown below. Input Voltage(V) On- Period(us) Duty Cycle time(us) Table 5.6 Measurement of the duty cycle Duty Cycle vs Voltage y = x R² = Dulty Cycle Voltage Figure 5.7: Graph of the duty cycle measurement 10

11 Then we use a multimeter to measure the voltage and current across the system speed control and the motor, to calculate the power efficiency: the maximum output of the motor under full load is: 0.42A*15V = 6.3W, the power used by speed control is 2.5mA*5V = W. So the power efficiency = 6.3/( ) = 99.8% Bill of materials and Cost estimate Resistor * 4 = 0.2 * 5 = 1 Capacitor * 2 = 0.2 * 2 = 0.4 Transistor 2N7000 * 1 = 1 * 0.3 = 0.3 Diode 1N 4007 * 1 = 1 * 0.2 = 0.2 Potential meter * 1 = 1 * 1 = 1 LM555 timer * 1 = 0.4 * 1 = 0.4 LM339 comparator * 1 = 0.3 * 1 = 0.3 Total cost = $3.5 Summary and conclusion For this experiment, we build an oscillator to produce a saw tooth wave in the first part, convert those saw tooth wave into pulse wave in the second part, and use those pulse wave to control the speed of a motor. Basically, we meet all the specification of this experiment, the frequency is within 10kHz +- 5%, the linearity of duty cycle verses control voltage is very good, we can get the duty cycle from 10% to 90%. And there is a better way to improve our design procedure, after simulation, we built up the circuit. And the result from that circuit is not same to the result of simulation, and that makes us to change the value of resistance a little bit. That is because fro capacitor, there is some error on its actual value, so we need to consider those error properly. 11

Laboratory Design Project: PWM DC Motor Speed Control

EE-331 Devices and Circuits I Summer 2013 Due dates: Laboratory Design Project: PWM DC Motor Speed Control Instructor: Tai-Chang Chen 1. Operation of the circuit should be verified by your lab TA by Friday,