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, 08/16 5pm. You are responsible for scheduling an appointment with the TA. 2. Design documentation must be submitted to the TA by Monday, 08/19 5pm. Grading of the project: 1. Demonstration of the circuit operation in the lab (100 points) 2. Complete design documentation (100 points) 3. Extra credits (30 points) DESIGN PROJECT DESCRIPTION: The objective of this design project is to develop and prototype a high efficiency pulse width modulation (PWM) speed control for a small DC motor. A block diagram of the overall system is shown below: Background: Figure 1. Block diagram of the PWM DC motor speed control Controlling a higher power load with a lower power control circuit is an extremely common electrical engineering task. Some type of switching device, such as a transistor, is required to control the current flow through the load, but if direct modulation is performed, this creates a rather inefficient design with the control device, the transistor, often using more electrical power than the load itself. A much more power efficient control method 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. Since the transistor is either full on or full off, either its current is zero, or its voltage is small, and therefore the power dissipation of the control device can be significantly reduced. This is a common operating principle of many power electronics circuits. 1

Figure 2. Ramp generation of PWM digital output The most common method for providing this type of switch-mode power control is through pulse width modulation or PWM. In PWM, the drive to the control device is a train of pulses whose frequency is kept constant, but whose pulse width is adjusted. As shown in the lower part of Fig. 2, is held constant, but the high-time is varied to adjust the effective voltage that is given to the control device and also the current through the load. The duty cycle is the fraction of a cycle that the control device is turned on, D.C., usually expressed as a percent. If the voltage present in the ON state is, then the effective output from the PWM is a voltage equal to times the duty cycle:. The great advantage of this type of control is that it provides a highly linear transfer function, since pulse widths can be controlled quite precisely in most electronic circuits. As can be seen, the scaling factor between output voltage and the pulse width is, which can be made a constant for the circuit. The heart of a PWM system is the modulator which controls the PWM pulse width as a function of some external control voltage input,. As the input control voltage is varied over a range of 0 to, it is desired to change the output pulse width from to, corresponding to duty cycles of 0% to 100%. In practice, it is difficult to go fully from 0% to 100%, but most PWM systems come within a few percent of either extreme limit, still giving a large range over which the control is linear and predictable, for example, 2% to 98%. A periodic ramped voltage waveform is usually the most convenient and simplest means to make the conversion between an input control voltage and the output PWM pulse width. Such a periodic voltage ramp, or saw-tooth, waveform is shown in Fig. 2. If the saw-tooth waveform goes from 0 to some fixed limit of, then the PWM output pulse is simply when the input control voltage is greater that the voltage of the saw-tooth waveform,, as shown. This operation of comparing two voltages and outputting a binary 0 or 1 level is precisely what a voltage comparator does, and the critical piece of the PWM modulator is this voltage comparator. The vast majority of all PWM systems use an internal voltage ramp or saw-tooth waveform in this manner. The waveforms shown in Fig. 2 are ideal to illustrate the concepts, but the closer 2

that the actual waveforms of a circuit can mimic these, the better it will perform as a PWM modulator. The design of the PWM DC motor speed control thus consists of three subsystems: an oscillator to create the reference voltage saw-tooth waveform, the PWM modulator to compare this sawtooth voltage waveform against some externally applied control voltage and create the output digital PWM pulse train, and finally a control device or motor driver to rapidly turn the load on and off according to this pulse train. The most important performance measures for a PWM controller are usually the control linearity and the power efficiency. While the load for this design problem is a DC motor, this type of system could equally well be used to control the power supplied to lights, heaters, electromagnets, electrochemical cells, transformers, or any other type of heavy electrical load. Required Specifications: The required specifications for the design are as follows. Refer to the test results sheet to see how many points each of these specifications are worth. Power Supply: The oscillator, PWM modulator, and motor driver must all operate from a single VDC power supply, while the motor will have a single DC supply voltage of VDC. All parts of the system will share the same ground, V. Oscillator: The main oscillator must be created as part of the design. The oscillation frequency must be 10 khz ± 5%. The output of the oscillator must be a periodic asymmetric ramp waveform, as shown as in Fig. 2. This is commonly known as a saw-tooth waveform. The fall time must be less than or 10 μs. The period of the oscillator should not change when the input control voltage is varied. Input Control Voltage: An analog input DC control voltage is used to set the motor speed. The motor speed should increase smoothly and linearly with the DC voltage on the input. The range of the input control voltage is not specified, but this should be a convenient subset of the available power supply voltages. When the input control voltage terminal is left unconnected, the motor should not turn. PWM Modulator: The digital output from the PWM modulator must be in the range from 0.0 V to +5.0 V (V GND to V DD ). The PWM modulator must be able to vary the output duty cycle over a range of at least 10% to 90%. Motor Driver: The motor driver should be capable of turning the motor on and off with a motor supply voltage of Volts, and with a maximum current of up to ma. The DC motor, when spinning with no attached load, will not draw this much current, but the motor driver should be able to supply this much current. Function: With the input control voltage terminal left unconnected, the motor should not turn. When a DC input voltage is applied to the input control voltage terminal and slowly increased over its range, the motor should spin up and its no-load speed should increase along with the value of the DC input control voltage. The DC motor should be able to achieve its full no-load speed of approximately 7000 rpm at the top end of the input control voltage. This corresponds to approximately a 90% duty cycle from the PWM output, or an equivalent DC voltage of 13.5 V applied to the motor. Linearity: No specification on the linearity is given, but better linearity will result in more points awarded for the design. Since the motor speed cannot be easily measured in the laboratory, the linearity of the PWM output duty cycle versus the value of the input control 3

voltage will be used to measure the linearity of the circuit. Take some measurements and make a plot of PWM duty cycle versus input control voltage, and then find the best fit straight line through these points (a linear regression of the data). Power Efficiency: The efficiency of the system is the power used by the motor divided by the total power used by the motor and its speed control. The maximum output of the motor under full load is about 5.0 Watts, so use this figure in determining the efficiency of the speed control system. For example, if the speed control were to use 0.5 W, then the efficiency would be η = 5.0 W / 5.5 W = 90.9%. Be sure to include the power used by the motor driver as part of the speed control. Parts Cost: The cost of the parts used to construct the motor speed control should be as low as possible. Use the EE Stores cost for each part in the bill of materials. Do not include the cost of the motor, circuit board, or any connectors. Some hints to get started: 1. The oscillator (saw tooth) generator can be achieved using a commercial timer chip. The most common timer is a 555 timer. EXTRA CERDIT (20 points): The design of the saw tooth oscillator is not using a commercial timer chip. A good starting point for the oscillator is the CD4011B circuit examined in Laboratory Experiment 5, Procedure 6. It is simple, robust, and self-starting. It also has the advantage of internally using a ramp-like waveform to set its timing. The capacitive charge and discharge voltage waveforms are exponential decays, but only the first 2/3 of that waveform are used before the circuit switches, and this early part of the exponential is often sufficiently close to a linear voltage ramp that it can be used for PWM applications. 2. The PWM modulator is essentially a voltage comparator, and one solution is to simply use a voltage comparator IC for this purpose. EXTRA CERDIT (10 points): The alternative strategies which can be used to create the same function as the voltage comparator, but with a smaller parts count and a smaller parts cost. One such approach is to consider the basic CMOS inverter, whose voltage transfer characteristic can be considered very similar to a voltage comparator where the input voltage is compared to. The trick would be to restructure the inverter so that it can compare two input voltages against each other. 3. The motor driver can be as simple as a single transistor, but it will have to be chosen properly to handle the current and voltage range of the motor drive branch. Its input will also have to be matched to the PWM output pulse voltages. Characteristics of the DC motor: The DC motor used for this design is a Jameco part number 231732, which is a manufactured by Sinotech Shanghai as part number PC-130SF-09480-R. The data sheet is available on the class web site, and the motor has the following ratings: Voltage: 6.0 24.0 VDC, 12.0 V is nominal, Current: 50 375 ma, no load to stall point, Torque: 0.0 55.0 g-cm, maximum efficiency occurs at 12.3 g-cm, Speed: 0 6000 rpm at 12.0 V. 4

The above ratings are for continuous duty, but the motor can be used beyond these ratings for intermittent periods. At 24 Volts, the motor will happily spin up to over 12,000 rpm with no load. Some no-load test results on the motor are shown below in Figs. 3 and 4. Figure 3. DC motor no load current versus applied voltage Figure 4. DC no load speed versus applied voltage 5

As shown, the no load speed is quite linear with the applied voltage, while the no load current remains more constant at 30 40 ma over the whole speed range. As load is applied to the motor and its output torque increases, for a fixed voltage, the speed will drop and the current will increase. Providing a speed control that adjusts to the load would require monitoring the shaft rotation speed and creating a feedback control loop to automatically adjust the drive to the motor to compensate. For this design, the motor will be operating under no load conditions so that the effective or average drive voltage to the motor will provide adequate speed control. The PWM DC motor speed control therefore only needs to adjust the effective value of the voltage that is applied to the motor. No current sensing or speed sensing for closed loop control is required. 6

EE-331 Devices and Circuits I Winter 2013 Laboratory Design Project: PWM DC Motor Speed Control Instructor: Tai-Chang Chen VERIFICATION OF DESIGN TEST RESULTS: Group Members: Date Tested: TA: Parameter Design Specification Test Results Points Power Supply +5.0 V for speed control /5 +15.0 V for motor /5 Oscillator 10 khz ± 5% frequency /5 Saw tooth asymmetrical ramp /10 < 10 µs fall time /10 Frequency does not change /5 PWM Modulator 0.0 5.0 V output /10 At least 10% to 90% modulation /10 Motor driver 0 to 500 ma at +15.0 V /10 Functions No rotation with no input /5 Speed increases w/ input voltage /5 linearity As linear as possible /10 Power efficiency As high as possible /5 Parts Cost As low as possible /5 Test results /100 7