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How to make motors coast instead of brake
http://www.robotc.net/forums/viewtopic.php?f=52&t=4217
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Author:  gnormhurst [ Wed Feb 08, 2012 9:27 am ]
Post subject:  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;
}

Author:  magicode [ Wed Feb 08, 2012 10:21 am ]
Post subject:  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;
  }
}

Author:  gnormhurst [ Wed Feb 08, 2012 5:22 pm ]
Post subject:  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

Author:  magicode [ Wed Feb 08, 2012 6:16 pm ]
Post subject:  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.

Author:  ronb [ Sat Nov 01, 2014 2:54 am ]
Post subject:  Re: How to make motors coast instead of brake

Thanks for these suggestions. I was thinking of using SGN and then saw you did this. How elegant.
I think even a more concise way of writing this is to alter the formal parameter and thus only have one line that sets the motor speed. like this:

Code:
void setMotorSpeed(tMotor myMotor, int speed){
  if(speed == 0 && coast) {
    speed = sgn(motor[myMotor]);
  }
  motor[myMotor] = speed;
}


That being said I think you may not always want a global variable to set coast mode. I am thinking a motor specific variable would provide ultimate functionality.

Code:
static bool coast[NUM_MOTORS]; // Use of static guarantees initialization to false by default
coast[Left_Motor] = true; // Set coast mode for selected motors
coast[Right_Motor] = true;
...

void setMotorSpeed(tMotor myMotor, int speed){
  if(speed == 0 && coast[myMotor]) {
    speed = sgn(motor[myMotor]);
  }
  motor[myMotor] = speed;
}

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