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

36 Chapter 3. Mechanical Construction
ized that the boards were configured to work at 8 V. This forced us to make
the decision of buying new batteries or new control circuits. Evaluating the
prices, we ended up buying new control circuits that are rated for 5 V.
MC33887 motor driver integrated circuit is an easy solution to connect
a brushed DC motor running from 5 to 28 V and drawing up to 5 A (peak).
The board incorporates all the components of the typical application, plus
motor-direction LEDs and a FET for reverse battery protection. A microcontroller
or other control circuit is necessary to turn the H-Bridge on and
off. The power connections are made on one end of the board, and the control
connections (5V logic) are made on the other end. The enable (EN) pin
does not have a pull-up resistor, so it be must pulled to +5 V in order to
wake the chip from sleep mode. The fault-status (FS, active low) output pin
may be left disconnected if it is not needed to monitor the fault conditions of
the motor driver; if it is connected, it must use an external pull-up resistor
to pull the line high. IN1 and IN2 control the direction of the motor, and
D2 can be PWMed to control the motor’s speed. D2 is the ”not disabled“
line: it disables the motor driver when it is driven low (another way to think
of it is that, it enables the motor driver when driven high). Whenever D1
or D2 disable the motor driver, the FS pin will be driven low. The feedback
(FB) pin outputs a voltage proportional to the H-Bridge high-side current,
providing approximately 0.59 volts per amp of output current.
Voltage Divider and Voltage Regulator Circuit
Batteries are never at a constant voltage. For our case 7.2 V battery will
be at around 8.4 V when fully charged, and can drop to 5 V when drained.
In order to power microcontroller (and especially sensors) which are sensitive
to the input voltage, and rated to 5 V, we need a voltage regulator
circuit to output always 5 V. The design of the circuit that will be used in
voltage regulation merged with the voltage divider circuit that will be used
for battery charge monitor shown in Figure 3.16.
To operate voltage divider circuit, the following equation is used to determine
the appropriate resistor values.
Vi
Vo = ∗R2
R1 +R2
Vi is the input voltage, R1 and R2 are the resistors, and Vo is the output
voltage.