Page 810 - Mechatronics with Experiments

796 MECHATRONICS
Commercial H-bridge amplifiers accept two different types of PWM command signals
and there is a “H-bridge driver/controller” section of the circuit which translates the input
command signals to corresponding H-bridge drive signals to operate the transistor pairs:
1. PWM command signal (0–100% duty cycle, corresponding to 0–100% of maximum
current command) and direction signal. The direction signal (ON for clockwise
or positive, OFF for counterclockwise or negative) decides on the direction of the
current, the PWM signal’s duty cycle determines the magnitude of the commanded
current. Using these two signals, the H-bridge driver circuit generates two command
signals that drive one of the pair (two transistors) at a predefined PWM frequency.
2. PWM command signal only in (0–100% duty cycle), where 50% duty cycle is
considered as zero current command. A PWM duty cycle less than 50% is interpreted
as counterclockwise direction, and a PWM duty cycle above 50% is interpreted as
clockwise direction. Then, the H-bridge controller activates the two PWM channels
to control one of the two pair of power transistors.
The type of PWM command signal an H-bridge expects is a matter of interpreting the
command signals in a clear way so that the direction and magnitude of the signal is defined.
The direction information determines which one of the two pairs of transistors will be
activated. The magnitude information determines what percentage of the time (duty cycle)
the active transistor pair is kept ON and OFF.
If the H-bridge expects an analog voltage signal for current command, it again needs a
way to determine the magnitude and direction. For instance, 0–10 VDC analog input signal
defines the magnitude of desired current, and a direction input signal determines the sign
(direction) of the desired current. Likewise, an analog voltage command range of −10 V to
+10 V alone can be used to define both magnitude and direction of the commanded signal
as well.
Regardless of the type of command signal used, the H-bridge controller needs to be
able to determine the desired current direction and magnitude. Once that is known, then
the corresponding transistor pair is activated with a proper duty cycle of a fixed frequency
PWM signal.

Procedure

1. Assemble the circuit as shown in the circuit diagram (Figures 11.31 and 11.32).
2. Calculate the values of the PWM frequency and duty cycle, and implement the code
for these values.
3. Download and run the program. The motor speed should change based on the duty
cycle.
4. Vary the duty cycle value in the code gradually to force the motor to correspondingly
increase or decrease its speed. Notice that if we simply control the PWM signal based
on open loop logic, we are controlling the H-bridge amplifier in voltage mode. The
average voltage applied across the motor terminals will be simply proportional to the
PWM output signals from the microcontroller. We refer to this type of amplifier as a
“voltage amplifier.”
5. Change the direction and magnitude of the speed under software control.

Closed Loop Current Control If we can measure the current passing through the
motor windings, and read it back to the microcontroller, then we can use it to implement a
closed loop current control algorithm to decide on the PWM output. For instance,
PWM = K ⋅ (i cmd (t) − i mea (t)) (11.130)