Page 808 - Mechatronics with Experiments

794 MECHATRONICS
11.11 EXPERIMENT 11: DC MOTOR SPEED CONTROL
USING PWM

Objectives
To control the speed of a DC motor using a PWM signal from a PIC 18F452 microcontroller
in conjunction with an H-bridge amplifier circuit.

Components

Item Quantity Part No. Supplier
DC Motor 1 154915
Optoisolator 1 114083 www.jameco.com
Potentiometer (200 Ω) 1 181972
IRF510 (MOSFET) 2 06F8238 www.newark.com
IRF9520 (MOSFET) 2 07B1521 www.newark.com
1N4003 Diode 4 76970 www.jameco.com
PIC Demo Board/connectors 1 set DM163022 or www.microchipdirect.com
DV164006

Theory
Pulse Width Modulation A PWM signal has two variables that must be decided
by the control software:
1. PWM signal carrier (or base) frequency,
2. PWM signal duty cycle.
The PWM frequency is generally a constant frequency in the kHz range. The duty
cycle is the value decided in real-time to indicate the equivalent analog value of the signal.
This is accomplished by generating an ON-OFF signal of a high frequency, and then
varying the percentage of time that the signal is in the ON state versus the OFF state. This
is called varying the duty cycle. The average value of the signal is equivalent to an analog
signal, provided that the PWM switching frequency is much higher than the bandwidth
frequency of the electromechanical system. For example, assume that a voltage signal
between 2.5–3.0 V is required to drive a motor, while the supply voltage is a fixed 5 V.
The PWM signal will consist of a voltage that varies at a high frequency (i.e., 1 kHz)
between 0–5 V. If, in every time period (1 ms), the voltage level is kept high (5 V) for
50% of the period, and low (0 V) for the remaining 50%, then the average voltage seen at
the output will be 2.5 V. In this case, the duty cycle is 50%. Similarly, if a high voltage is
maintained for 60% of the period, and low for the remaining 40%, then the average voltage is
0.6 × 5V = 3V.
Care must be taken to ensure that PWM frequencies are kept reasonably high com-
pared to the bandwidth of the control system, since a low frequency PWM signal may
actually be seen as varying, and not continuous, voltages, especially for loads with a
smaller electrical time constant. Typical PWM frequencies are of the order of a few kHz.
The PWM signal format is used in two different contexts:
1. Power transistor circuits, where the transistors are operated in all ON and all OFF
alternating states, which is called the PWM mode. In this mode of operation, as
opposed to the linear mode of operation where the transistor is partially turned
ON/OFF, the efficiency of the power transistors is significantly improved. In high