How to make motors coast instead of brake

gnormhurst · Wed Feb 08, 2012 9:27 am

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; }

How to make motors coast instead of brake

Who is online