VEX Cortex Motor Speeds up under load

Question

I am trying to get my robot to drive straight and am having trouble. I find that when running the motors with no load they run fine. If I put a load on one motor it accelerates. The other performs as expected, it tries to maintain speed. I am running 393 motors with encoders and PID selected. I am running robot C.

See the following video: https://youtu.be/u3P0Wectwco

program is as follows;

#pragma config(I2C_Usage, I2C1, i2cSensors)
#pragma config(Sensor, dgtl12, killB,          sensorTouch)
#pragma config(Sensor, I2C_1,  ,               sensorQuadEncoderOnI2CPort,    , AutoAssign )
#pragma config(Sensor, I2C_2,  ,               sensorQuadEncoderOnI2CPort,    , AutoAssign )
#pragma config(Sensor, I2C_3,  ,               sensorQuadEncoderOnI2CPort,    , AutoAssign )
#pragma config(Motor,  port2,           rmotor,        tmotorVex393_MC29, PIDControl, reversed, driveRight, encoderPort, I2C_1)
#pragma config(Motor,  port3,           lmotor,        tmotorVex393_MC29, PIDControl, driveLeft, encoderPort, I2C_2)
#pragma config(Motor,  port4,           topmotor,      tmotorVex393_MC29, openLoop, encoderPort, I2C_3)
#pragma config(Motor,  port5,           pmotor,        tmotorVex393_MC29, openLoop)
//*!!Code automatically generated by 'ROBOTC' configuration wizard               !!*//

void StopAll(){
    motor[rmotor] = 0;
    motor[lmotor] = 0;
    motor[topmotor] = 0;
    motor[pmotor] = 0;
}

//Stops the program at the push of a button
task eStop(){
    while (SensorValue(killB) == 0){
        wait1Msec(10);
    }
    StopAll();
    stopAllTasks();
}


task main()
{
    startTask(eStop);

    nMotorEncoder[rmotor] = 0;
    nMotorEncoder[lmotor] = 0;

    motor[rmotor] = 15;
    motor[lmotor] = 15;
    wait1Msec(20000);
    motor[rmotor] = 0;
    motor[lmotor] = 0;
    StopAll();


}

Thank you,

Mark

Answer 1

The problem was with the motor encoder. Upon disassembly, I discovered that the encoder was installed improperly. For whatever reason, the encoder gear was installed without an axle. Under no load, it worked. When under load apparently it would shift enough to cause it to miss counts causing the encoder to under sense speed, thus speeding up to motor.