Motor Driver Board

To drive the motors my RC Tank, I needed to build two H-bridges, one for the left tread motor and one for the right tread motor. Rather than putting together an H-bridge with discrete components, I used two BA6886N modules. They're smaller, more powerful, cheaper, and much easier to use than building my own.

The BA6886N can use a supply voltage of up to 28V. It drive drive a motor up to 1A. It takes two inputs from the microcontroller: F_in and R_in. An analog input, V_ref (pin 3), can be used to set the voltage output to the motor.

I decided that, although I plan on using a regulated 5V to all of the digital logic in the RC Tank, I will connect the power the motor drivers directly from the batteries. I can connect the regulated 5V to V_ref. The driver will lower the voltage, which would reduce the burden on my voltage regulator. This means that I would be able to use a regulator more on the order of 150mA rather than >1A and a heat sink. It would also help isolate the line noise that is generated from the motors from the rest of the circuitry. I'm going to connect V_ref directly to V_M.

As I mentioned, I planned to tie V_ref to 5V. If I had tied V_ref to ground, the motor would move freely. If toggled between the two, I can use pulse-width modulation (PWM) to control the speed of the motor. I had two PWM modules in the microcontroller that I didn't plan on using yet. I added this to my design. Even if I didn't use PWM, I can still just set the pins active without any other change. It's a lot easier to change firmware than hardware.

Here is the truth table for the BA6886N from its data sheet:

I designed this schematic for my driver board:

%mytable%|Symbol|Part/Value|Notes ||------|---------:|------||C1 |0.10 µF | ||C2 |0.10 µF | ||C3 |0.10 µF | ||R1 |1 kΩ | ||R2 |1 kΩ | ||Q1 |BS170 |NMOS ||Q2 |BS170 |NMOS ||U1 |BA6886N |driver||U2 |BA6886N |driver|

Part Selection Notes

• I really want to say I carefully picked these, but they were all mostly chosen because they were laying on my desk at the time.
• MOTOR1 and MOTOR2 are connected opposite from each other. The dot on the schematic is pin1: the red wire going to the motor. Because the motors are facing different directions, they need to be connected in a way that rotating in opposite directions moves the tank in one direction.
• Changing the direction of one motor connection is a lot easier than trying to remember that F_in and R_in are basically reversed on one of the drivers. I'd never remember which one. I'd rather let F_in be F_in and R_in be R_in.
• C1 and C2 reduce the noise that the motors generate and help to reduce current spikes.
• C3 reduces the noise that the board will generate on the power lines.
• Did you ever notice that capacitor values are always in farads, microfarads, and picofarads? Is saying "zero point one microfarads" easier than saying "a hundred nanofarads?"
• If you don't know how to read a capacitor's markings, a 0.10 µF capacitor is marked as "104".
• R1 and R2 are functioning as simple pull-up resistors for V_ref on U1 and U2.
• Q1 and Q2 allow the microcontroller to toggle V_ref on and off. Note that by using a transistor, pulling Speed high activates the transistor, pulling V_ref low and stopping the motor. This means that Speed is active low, and could more accurately by labelled as Speed.

Here is a "map" of where everything is placed on the protoboard:

Finally, here is the total truth table for the entire board. I've lumped "brake" and "standby" into one item. They are considered the same thing pertaining to my project. I will only be using standby as the drivers use less power in this mode, but the motors themselves do not behave differently in either mode.