A while back, I made myself a promise that as soon as enough USD had accumulated in my PayPal account I would upgrade the drive motors. So, using the $2000 odd seed funding from the Hackaday prize and some advertising income from one of my other websites, I made the plunge and ordered those crazy expensive Ampflow motors shown in a previous log. The motors are used by large combat robots for TV shows and don't have optical encoders and positioning control, which, I think, will be an advantage as I have already proven that I can balance the torques on the left and right drive motors using current sensors. The motors themselves (A28-400-F48) are brushed, high torque, high current DC devices with expensive neodymium magnets and a staggering 9.1 Hp at 200 amps for help with manoeuvring over rough ground. They are fan cooled, but will they get hot? Will the VEX Victor BB controller box get hot? Will the motor mounting bolts shear? ….. It's all very risky, but we all got to take risks when the odds are ok and we can afford, both financially and emotionally, to lose.

Firstly, the motors need to have adapter plates made and adapter collars for the shafts to fit the drive gearboxes. These are currently being manufactured so will be shown in the next log.

Next, the motors and drivers boxes are set up on a bench and tested using the proprietary VEX software with a Prolific USB to serial cable. Once I'd got a genuine cable from a reputable supplier, the software ran very easily and I was able to make the motors run in no time. 

I was a bit concerned about how to control the motors using an MCU, but Alex at VEX's technical support soon put me on the right track. The signal that the Victor BB requires is not a straightforward PWM, but rather an 'asynchronous' PWM where the LOW part of the waveform must be 20ms long. The duration of the HIGH's gives the motor speed and a HIGH of duration 1.0 ms gives full speed forwards, 2.0 ms gives full speed backwards and 1.48 gives neutral ….. Simples!

The VEX Victor BB driver box is wired to an Arduino Uno for testing and the wiring is as below:

It would be possible to code the Arduino with two delay() functions:

digitalWrite(13,LOW);
delayMicroseconds(20000);
digitalWrite(13,HIGH);
delayMicroseconds(1400);

But that's no use in my TC275 code as delay cannot be used as it will screw up all the other motor timings. Instead, asynchronous motor timing is done with the micros() function. The code was run with an LED and serial print console at very low speed to check that the asynchronous ratios were correct. Serial must be removed, as shown, when running the code at high speed.

// Asynchronous timer
float speedTimerA = 0.001;
float speedTimerB = 0.001;
int LEDState = HIGH;
unsigned long previousMicrosTimerA = 0; 
unsigned long previousMicrosTimerB = 0; 
unsigned long intervalTimerA = 0;
unsigned long intervalTimerB = 0;
int count =0;

void setup() 
{
  Serial.begin(9600);
  pinMode(13, OUTPUT);   // LED
  pinMode(3, OUTPUT);    // motor
  delay(5000);
}
void loop()
{
  motor();
  //printOut();                     // Remove this when using at high speed
}
///////////////////////////////////////////////////
void motor()
{
  unsigned long currentMicros = micros();
/////////////////////////////////////////////////////
  speedTimerA = 650;                                     // Between 1000 and 500. Neutral is 675.
  intervalTimerA = 1000000/speedTimerA;
  speedTimerB = 50;                                      // B MUST be slower than A. Speed of 50 Hz == 20 ms.
  intervalTimerB = 1000000/speedTimerB; 
  intervalTimerB = intervalTimerB + intervalTimerA;

  count++;
  if ((currentMicros - previousMicrosTimerA) >= intervalTimerA)
  {
    previousMicrosTimerA = currentMicros;
    if(LEDState == HIGH)
    {
       count =0;             // Count was used to help prove the maths worked ok.
    }
    LEDState = LOW;
    if ((currentMicros - previousMicrosTimerB) >= intervalTimerB)
    {
      LEDState = HIGH;
      count =0;
      previousMicrosTimerB = currentMicros;
    }
    digitalWrite(13,LEDState);
    digitalWrite(3,LEDState);
  }
} // motor
void printOut()
{
 unsigned long currentMicros = micros();
 Serial.print("  Count: ");Serial.print(count); 
 Serial.print("  LEDState: ");Serial.print(LEDState); 
 Serial.print("  TimerA: "); Serial.print(currentMicros - previousMicrosTimerA);
 Serial.print("  TimerB: "); Serial.println(currentMicros - previousMicrosTimerB);
 delay(1000);
}