Answer to RobotC Q&A fast 393 vs 393

VEX V5 General Discussion Technical Discussion
1

This question was originally posted by mykiscool in the ROBOTC Technical Support Q&A:

I believe the difference between the different motor types is for when you are using an integrated motor encoder with the natural language programing. When you use a command like line track for rotations, or move straight for rotation the IME would have a different tick count for the rotation for each setting on a 393 motor as well as the 269 motor.

2

Hmm interesting. Then if someone isn’t using the IMEs then is there any reason they have to pick the correct motor? How about 269 vs 393? Is there a difference other than for IME counts?

3

To be honest, I don’t think so. I always set them correctly, but it didn’t make a difference outside of IMEs.

4

I’m looking forward to jpearman’s explanation of what goes on under the hood :smiley:

5

They are also used for RobotC’s built in PID functions (which are still in development). I am sure that each motor will have its own PID constants when/if RobotC finishes all of those functions.

It also helps for documenting purposes, and just for reference (two of the three main reasons that I have used the Motors and Sensors Setup).

~Jordan

6

It’s not very exciting.

You are all correct, the motor selection is used in some of the IME code, as well as having a different number of counts per rev, the 269 encoders also count the opposite way to the 393s. ROBOTC uses the motor type to reverse the IME counter on a 269, so does ConVEX, here is a snippet of code showing that.

/*-----------------------------------------------------------------------------*/
/** @brief      Get the current motor position                                 */
/** @param[in]  index The motor index                                          */
/** @returns    The motor position                                             */
/*-----------------------------------------------------------------------------*/

int32_t
vexMotorPositionGet( int16_t index )
{
    int32_t     position;

    if( (index < kVexMotor_1) || (index >= kVexMotorNum))
        return(0);

    if( vexMotors index ].motorPositionGet != NULL )
        {
        position = vexMotors index ].motorPositionGet( vexMotors index ].port );

        // 269 needs reversing
        if( vexMotors index ].type == kVexMotor269 )
            position = -position;

        if(!vexMotors index ].reversed )
            return( position );
        else
            return( -position );
        }
    else
        return(0);
}

Code built on top of the IME functionality, like my smart motor library, also use the motor type for setting internal parameters.