Submitted version of the thesis - Airlab, the Artificial Intelligence ...

3.7. Hardware Architecture 35
The simplest case is a switch to connect a motor to a power source, such
as in small appliances or power tools. The switch may be manually operated
or may be a relay or conductor connected to some form of sensor to automatically
start and stop the motor. The switch may have several positions
to select different connections of the motor. This may allow reduced-voltage
startingof themotor, reversingcontrol or selection of multiplespeeds. Overload
and over-current protection may be omitted in very small motor controllers,
which rely on the supplying circuit to have over-current protection.
Figure 3.15: Structure of an H bridge (highlighted in red)
The DC motors cannot be controlled directly from the output pins of
the microcontroller. We need the circuit so called ’motor controller’, ’motor
driver’ or an ’H-Bridge’. The term H-Bridge is derived from the typical
graphical representation of such a circuit. An H-Bridge (Figure 3.15) is
built with four switches (solid-state or mechanical). When the switches S1
and S4 are closed (and S2 and S3 are open) a positive voltage will be applied
across the motor. By opening S1 and S4 switches and closing S2 and
S3switches, this voltage is reversed, allowing reverse operation of the motor.
To drive motors we used a PWM signal and vary the duty cycle to act
as a throttle: 100% duty cycle = full speed, 0% duty cycle = coast, 50%
duty cycle = half speed etc. After some testing we optimized the percentage
of the duty cycle in order achieve a better performance. This optimization
will be mentioned later in Control Chapter.
For the motor control, we started by using the H-Bridge motor control
circuits provided by our sponsor. The initial tests have been performed by
implementing the correct PWM waves using these boards. Later, we real-