World of MindStorms

UPDATE: Download this program w/ setup, usage instructions and a demo here; get the driver (optional) here.

Programming my current project, Bionic NXTPod 3.0, I broke the actuators a few times by having the slide run into either the top or bottom. This was caused by my motor control: every time it went up or down, the encoders were reset, not considering whether or not the motor was exactly on target. As the errors built up, the actuators were torn down.

Looking for a solution, I saw HiTechnic‘s motor PID block – for NXT-G – which lets the motor move to an absolute encoder position, without resetting in between. Using this method, the error – usually not more than 2-3 degrees – is eliminated every time the motor moves to a new position, because the block compares the desired position to the actual position, not the assumed “0 point”. For example:

• The motor needs to go to 200 degrees and then back to 100
• The program starts, and the motor overshoots to 201 degrees
• Without the HiTechnic block: the motor would move back 100 degrees – because it thinks it’s at exactly 200 degrees – ending up at 101
• With the HiTechnic block: the motor would move back 101 degrees (201-100) ending up at 100

void PositionMotor(char motorToTurn, int absoluteDegrees){
  if(nMotorEncoder[motorToTurn] == absoluteDegrees){}
  else if(nMotorEncoder[motorToTurn] < absoluteDegrees){
    bMotorReflected[motorToTurn] = false;
    nMotorEncoderTarget[motorToTurn] = absoluteDegrees-nMotorEncoder[motorToTurn];
    motor[motorToTurn] = 75;
    while(nMotorRunState[motorToTurn] != runStateIdle){}
    motor[motorToTurn] = 0;
  }
  else{
    bMotorReflected[motorToTurn] = false;
    nMotorEncoderTarget[motorToTurn] = nMotorEncoder[motorToTurn]-absoluteDegrees;
    bMotorReflected[motorToTurn] = true;
    motor[motorToTurn] = 75;
    while(nMotorRunState[motorToTurn] != runStateIdle){}
    motor[motorToTurn] = 0;
  }
}