PULSE WIDTH MODULATION. DC Motor - Speed Control. DC Motor - Speed Control. What is a DC Motor? What to control in a DC motor?

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1 1 General Overview DC Motor 2 What is a DC Motor? A machine that converts electrical to mechanical energy. PULSE WIDTH MODULATION What to control in a DC motor? Speed Direction Start/Stop Positional? BENM 3223 : Microcontroller Technology 3 DC Motor - Speed Control Can be achieved by controlling voltage or current. The most simplest method of control is by inserting a resistor (high power resistor aka cement resistor) in series with the DC motor. This method is simple but not efficient due to high power loss (heat) at the resistor. 4 DC Motor - Speed Control Variable voltage approach By reducing/increasing the supply voltage of the DC motor will decrease/increase the DC motor speed. The most practical method to control voltage is via the PWM method.

2 5 DC Motor Direction The only way to control direction is change the polarity of the DC motor. The polarity of the DC motor will determine the direction of current flow hence will determine the direction of rotation of the DC motor. 6 DC Motor - Start The most easiest way to start a DC motor is by switching in the power supply. However, it would introduce high current spikes into the motor circuit due to the motor inertia, Inductive characteristics and also start with load condition. Consequences: fried controller, MOSFETs, stripboards, break down of insulation and also fried circuits! 7 DC Motor - Start By reducing the supply voltage during starting (ramp start). Introducing high resistance during starting. All this can be achieved by using a controller atmel, PIC, OOPic, etc etc. 8 DC Motor - Stop Simplest method is by switching off the power supply. However, if the motor has inertia, instantaneous stop is not possible.

3 DC Motor - Stop DC Motor - Positional 9 10 Sudden reversal of polarity Regenerative braking / Electronic Braking These solution would require the use of a controller. Is positional possible? Yes Servo motor / Stepper Motor. Servo motor position is based upon the pulse width of PWM signal going into the servo. Stepper motor position is possible by counting the number of pulse sequence given to the stepper. All this is possible by incorporating controller. H bridge DC Motor Drive Circuit H Bridge Components - RELAY 11 Shaped like the letter H Advantage ability to control direction, speed, start and stop. Disadvantage will be dependent upon the component used. 12 Mechanical switch Very robust Types of relay 5V, 9V, 12V.. Etc etc Relay types will be dependent upon supply voltage and the voltage at which pulse is given to the relay contacts. Speed control is not possible. Safety interface to relay must be protected, free wheeling diode must be installed to protect from the inductive kick.

4 H Bridge Components - MOSFET H Bridge Example Circuits 13 Semiconductor relay Capable of withstanding high current flow Eg: IRF, IRFZ series For optimized H bridge configuration, must have p and n channel MOSFETs. Disadvantage, gate is sensitive to high currents. 14 H Bridge Example Circuits H Bridge Example Circuits 15 16

5 H Bridge Example circuits H Bridge Example circuits H Bridge Example Circuits H Bridge Example circuits 19 20

6 H Bridge Example circuit H Bridge example circuit Vcc Relay Control Signal/ ULN2003 output Control Signal/ ULN2003 output Relay M Free Wheeling diode Free Wheeling diode Vcc 23 MOSFET Driver circuit Totem Pole Driver 24 MOSFET Driver circuit Totem Pole Driver with optoisolator Vg Vs Vg Vs R1 R microcontroller R1 R microcontroller ULN2003 Optoisolator

7 H-Bridge Circuit H-bridge Circuit Low Power Circuit High Power Circuit 27 28

8 Speed Control Issues Speed Control Issues 29 All relay driver Speed control is not possible All relay driver with MOSFET Speed control is possible Semiconductor based driver Speed control is possible 30 Speed control can be achieved by giving a PWM pulse train to the semiconductor swtich. This will determine the duration at which the motor receives voltage. If the switching is done fast enough, the average voltage received by the motor may be controlled. Therefore, the average voltage received by the motor will be dependent upon the duty ratio (time ratio at which the switch is on ) of the switch. Protection Issues Protection Issues Motor If the motor only turns in one direction, free wheeling diode is recommended to discharge any remaining voltage stored in the motor when the motor is not rotating. MOSFET A free wheeling diode is also recommended to discharge the internal capacitance in the MOSFET during turn off and turn on. Fuse It is also recommended to put a fuse to generally protect the circuit in the event of a current surge. Remember, fuse is cheap, MOSFETs and relays are expensive!. Connections All connections to the motor must be wired, DO NOT use the copper stripboard as the connectors to motors. Copper stripboard do not have current capacity.

9 33 Interfacing Issues MicroC via ULN 2003 vs MicroC via Optoisolators? Using ULN2003 is simple but it has a disadvantage of a common ground for both MicroC and Motor circuit. In the event of a current surge during start, stop, direction change, excess current will go to ground and could fry the microcontroller. Solution is by using an optoisolator (4n25) to separate the ground for both MicroC and Motor circuit. Cost wise, optoisolator would cost about RM1 each while a MicroC would cost a lot!!. 34 What is PWM Signal? A PWM signal is simply a pulse of varying length, in effect a rectangular wave. The maximum voltage outputted will be +5 VDC, and the minimum will be 0 VDC. The length of the pulse generated is some times characterized by a duty cycle. 35 PWM Signal 36 PWM Signal

10 37 Duty Cycle The duty cycle is the percentage of the signal that the output remains high. For instance, a constant +5V would be equivalent to a 100% duty cycle. A typical square wave output from a function generator has a 50% duty cycle. 0V would correspond to a 0% duty cycle. 38 Why PWM Signal? The speed of a DC motor can be varied by changing the average voltage applied to the input. A pulse width modulated signal is created by switching the output on and off at some duty cycle. 39 Applications One common application of PWM signals is motor control. By varying the duty cycle of a PWM signal sent to a motor, you can vary the effective power of the signal and thereby slow the motor down or speed the motor up depending on how long of a pulse you send to the motor. 40 Motor Control The signal generated by the PIC can not be directly connected to the motor, however, because the PIC is unable to handle the power required by the motor. It is therefore necessary to use a transistor to regulate the flow of current to the motor.

11 41 Switching Transistor A transistor is like an electric switch. When you send a logic high (+5V) to the transistor, it allows current to flow. When a logic low (0V) is sent, it restricts the flow of current. For digital signals, this means that the signal can be reproduced exactly, except the new signal is scaled up to a much larger current. 42 References Dr Izham Abu Bakar