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2012-05-15T20:39:52Z

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{{DISPLAYTITLE: Moving until a certain position is reached}}
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There have been several questions recently about moving part of the robot until it has reached a certain position. I though I would post a RobotC example showing how to drive a motor forwards or backwards while monitoring an encoder. This same idea could be used for raising a lift or anything else where an encoder is used to monitor the rotation of a motor.

Anytime a loop is used waiting for a terminal condition to be found it's important to think about what will happen if something goes wrong. This code also shows how to use a timeout so that if an error occurs, the motor stalls for example, the test will stop after a period of time.

This code is a simple example and does not use multi tasking or the built in timer both of which could be used as part of this:<br />
{|
|-
|<syntaxhighlight lang="ROBOTC">
#pragma config(Sensor, dgtl1, encoder, sensorQuadEncoder)
#pragma config(Motor, port2, MyMotor, tmotorNormal, openLoop)
//*!!Code automatically generated by 'ROBOTC' configuration wizard !!*//

/*-----------------------------------------------------------------------------*/
/* */
/* Drive motor until an encoder has counted for a number of counts */
/* or a timeout period has passed. */
/* */
/* Assumes the encoder will count up when the motors are running forwards */
/* */
/*-----------------------------------------------------------------------------*/

#define TIMEOUT_CNT_PER_SEC 10
#define MOTOR_FWD_SPEED 64
#define MOTOR_REV_SPEED (-64)
#define MOTOR_STOP_SPEED 0

int
DriveByEncoder( int encoder_count, int timeout_in_seconds = 5 )
{
int timeout;

// Drive motor until encoder has moved a number counts or
// timeout_in_seconds seconds have passed

// Zero the encoder
SensorValue[ encoder ] = 0;

// Run the motor forwards or backwards
if( encoder_count > 0 )
motor[ MyMotor ] = MOTOR_FWD_SPEED;
else
motor[ MyMotor ] = MOTOR_REV_SPEED;

// run loop until encoder hits encoder_count counts or timeout reached

for( timeout=(timeout_in_seconds*TIMEOUT_CNT_PER_SEC); timeout > 0; timeout-- )
{
// check encoder
if( encoder_count > 0 )
{
// going forwards
if( SensorValue[ encoder ] >= encoder_count )
break;
}
else
{
// going backwards
if( SensorValue[ encoder ] <= encoder_count )
break;
}

// wait 1/10 second
wait1Msec( 100 );
}

// Stop the motor
motor[ MyMotor ] = MOTOR_STOP_SPEED;

// See if we sucessfully found the right encoder value
if( timeout <= 0 )
{
// there was an error - perhaps do something
// return error
return (-1);
}
else
// return success
return 0;
}

/*-----------------------------------------------------------------------------*/
/* */
/* Test the new function */
/* */
/*-----------------------------------------------------------------------------*/

task main()
{
// forwards until 200 counts
DriveByEncoder( 200 );

// short wait
wait10Msec( 200 );

// backwards for 3000 counts with a 10 second timeout
DriveByEncoder( -3000, 10 );

// demo done
while(true)
wait1Msec( 10 );
}
</syntaxhighlight>
|}<br />

{{tip-from-author|name=jpearman|link=http://www.vexforum.com/showpost.php?p=222061&postcount=9}}

Moving until position - Revision history

Bfeher: Created page with "{{DISPLAYTITLE: Moving until a certain position is reached}} Programming_Tips_Tricks|Programming Tips Trick..."