Biomedical Engineering Reference
In-Depth Information
FIGURE 3-18
PWM of a DC motor.
Because of the high switching rate required, typically in excess of 1 kHz PWM,
controllers use not physical switches or relays but power transistors, as shown in the
partial schematic.
To maintain a constant speed or a specific motion profile that is torque dependent,
closed-loop or feedback control is generally used. This involves sensing the motor speed
using one of the sensors discussed in Chapter 2 and using that to control the duty cycle of
the PWM signal to maintain the required speed. This principle is illustrated in Figure 3-19.
Simple PWM controllers cannot reverse the direction of rotation of a motor unless a
dual rail supply is used. This is often inconvenient, particularly if the motor is large and
requires a high current, so a more common alternative is to use a switching configuration
that can reverse the polarity of the motor supply. This configuration, known as an H-bridge
because of its shape, is shown in Figure 3-20.
The H-bridge uses four power transistors that are independently controlled to ensure
that the appropriate pulse width is applied to the motor with the correct polarity. If tran-
sistors Q
1
and Q
3
are on simultaneously, then current will flow through the motor in the
direction shown in Figure 3-20. However, if Q
2
and Q
4
are on, then the current flow is
reversed and the motor will turn in the opposite direction. Care needs to be taken to ensure
that Q
1
and Q
2
or Q
3
and Q
4
are never conducting simultaneously, or they will provide a
direct path to Earth with catastrophic consequences.
FIGURE 3-19
PWM motor speed
control.
