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How to make motors coast instead of brake 
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Joined: Thu Dec 29, 2011 8:34 pm
Posts: 14
Post How to make motors coast instead of brake
With a tip from this thread, I was able to get the motors to stop dynamic braking at speed=0 and instead coast. The motivation was that the hard braking was damaging the gearboxes in our motors.

The main idea is "don't use speed = 0." Setting the speed to 1 avoids the braking mode. But this only works if the current speed was > 0 (forward). If the current speed was < 0 (backward), setting speed to +1 caused braking to happen. In this case using -1 avoided braking mode. In other words, the value to use for "coast" depends on the current direction of the motor.

Therefore this requires access to the current speed setting of the motor. I created a "speed-setting" function for each motor that remembers the speed it was set to:
Code:
   setLeftSpeed( speed );
These functions are used in place of
Code:
motor[left] = speed;


The function also contains the logic to set the speed to +1 or -1 if a speed of zero is requested, thus causing the motor to coast instead of brake:
Code:
short leftSpeed  = 0; // global
bool coastMode = true;

void
setLeftMotor( short speed )
{
  // the motor controller will brake (short) the motor
  // if the speed value is zero.  But setting the motor to 1
  // only makes it coast if the previous motor value was > 0.
  // So we set it to -1 if the previous speed was < 0.
  if ( speed == 0 && coastMode )
  {
    if ( leftSpeed < 0 )
      speed = -1;
    else
      speed = +1;
  }

  leftSpeed  = speed; // remember it
  motor[left] = speed;

}
Below is an example of a simple "tophat drive" method that uses coasting:
Code:
// globals:
//
//
short rightSpeed = 0;
short leftSpeed  = 0;
void setLeftMotor( short speed );
void setRightMotor( short speed );

bool coastMode = true;

task main()
{

  short speed = 75;  // (could be changed using buttons or joystick.)

  initializeRobot();
  waitForStart();   // wait for start of tele-op phase.  REQUIRED BY FIRST!


  // let the driver press and hold RT to change from COAST to BRAKE mode.
  if ( (joystick.joy1_Buttons & BTN_RT) )
    coastMode =  false;  // brake
  else
    coastMode =  true;   // coast


  // simple tophat drive: forward, backward, spin left, spin right:
  switch( joystick.joy1_TopHat )
  {

  case 0:  // forward
    setLeftMotor ( speed );
    setRightMotor( speed );
    break;

  case 4:  // backward
    setLeftMotor ( -speed );
    setRightMotor( -speed );
    break;

  case 6:  // spin left
    setLeftMotor ( -speed );
    setRightMotor( +speed );
    break;

  case 2:  // spin right
    setLeftMotor ( +speed );
    setRightMotor( -speed );
    break;

  default:  // stop
    {
      setLeftMotor ( 0 );
      setRightMotor( 0 );
    }

  }  // switch( joystick.joy1_TopHat )

}



void
setLeftMotor( short speed )
{
  // the motor controller will brake (short) the motor
  // if the speed value is zero.  But setting the motor to 1
  // only makes it coast if the previous motor value was > 0.
  // So we set it to -1 if the previous speed was < 0.
  if ( speed == 0 && coastMode )
  {
    if ( leftSpeed < 0 )
      speed = -1;
    else
      speed = +1;
  }

  leftSpeed  = speed; // remember it
  motor[left] = speed;

}

void
setRightMotor( short speed )
{
  // (see note about coasting in setLeftMotor()).
  if ( speed == 0 && coastMode )
  {
    if ( rightSpeed < 0 )
      speed = -1;
    else
      speed = +1;
  }
 
  rightSpeed = speed; // remember it.
  motor[right] = speed;
}


Wed Feb 08, 2012 9:27 am
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Joined: Tue Sep 14, 2010 9:19 pm
Posts: 496
Post Re: How to make motors coast instead of brake
Nice job on figuring this out. There are some easier ways to do these things though.
If you want your motors to coast, you can always use
Code:
bFloatDuringInactiveMotorPWM = true;

There is also no need to store the current value of the motor in your program. Just reading motor[motorName] will give you the speed that it's been set at.
If you still want to do it by yourself, making all these functions for every motor is a hassle. Use the tMotor variable instead:
I'll try and follow your code as closely as possible.
Code:
void setMotorSpeed(tMotor myMotor, int speed){
  if(speed == 0 && coast){
    if(motor[myMotor] > 0){
      motor[myMotor] = 1;
    else if(motor[myMotor] < 0){
      motor[myMotor] = -1;
    }
  }
  else{
    motor[myMotor] = speed;
  }
}


A more concise way to make the same functino would be:
Code:
void setMotorSpeed(tMotor myMotor, int speed){
  if(speed == 0 && coast){
    motor[myMotor] = sgn(motor[myMotor]);
  }else{
    motor[myMotor] = speed;
  }
}

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Wed Feb 08, 2012 10:21 am
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Joined: Thu Dec 29, 2011 8:34 pm
Posts: 14
Post Re: How to make motors coast instead of brake
Thanks!! I imagined there was an easier way, but I haven't discovered extensive documentation.

I read post here that indicated that that bool flag was only meant for NXT motors, "not FTC". I just noticed that that post dates to 2008, so I guess it changed. (See the last post here: viewtopic.php?f=52&t=868&p=4728&hilit=motors+coast+nxt+motors+coast+brake&sid=ffebb1854d9c2c6faf8b2b03f80f62c4#p4728 )

Back in December I tried to read the motor value as you indicated. motor[left] looks like a pointer dereference, but the values I got back did not make any sense. So I figured it was some kind of special RobotC syntax. It's not? I wonder what I did wrong?

-norm


Wed Feb 08, 2012 5:22 pm
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Joined: Tue Sep 14, 2010 9:19 pm
Posts: 496
Post Re: How to make motors coast instead of brake
I did not know that you were referring to TETRIX motors in this post. Well, I guess that you should continue using the +-1 method. As for reading the motor value by calling motor[motorname], I know that this works on the VEX Cortex. I've not tested it with NXT/Tetrix yet, but I assume that it should work.

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Wed Feb 08, 2012 6:16 pm
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