Page 78 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

Pulse-width modulation
The H-bridge controls the on and off function as well as the direc-
tion of DC motors. The function of the H-bridge can be enhanced by
using PWM to control the speed of the motor. The PWM signal is
illustrated in Fig. 4.18. When the PWM signal is high, the motor is
on; when low, the motor is off. Since the signal turns the motor on
and off very quickly, the voltage delivered to the motor becomes an
average of the time on versus the time period of the cycle (T-on/T-
period). The greater the on time, the higher the average voltage.
The average voltage (VDC steady-state) is always less than the
voltage delivered (Vcc). PWM essentially controls the motor
speed.
Motors are inductive loads. When current is switched on and off, a
transient voltage is generated in the (motor) windings that can
damage the solid-state components used in the H-bridge. This
transient voltage can be controlled by using a snubber diode
bridged across each transistor, as illustrated in Fig. 4.19.
The snubber diode dissipates the transient voltage by creating a
voltage path directly to ground for the transient voltage. This effec-
tively protects the semiconductor the diode is bridged over. The
snubber diodes should be rated to handle the normal current the 57
motor typically draws.

Period

T-on
ON
Vcc
VAverage
OFF
GND

ON Vcc
VAverage
OFF GND
4.18 Pulse-width modulation for H-bridge

Team LRN Movement and drive systems