Is there a way to set a specific initial or start position with tetrix and nxt motors. We are working on an arm and wish to set the arm at a specific position - can this be accomplished with the tetrix encoders. I know we have servos that can do this, but we have tetrix and nxt motors attached currently.

Thanks

Yes, a tetrix motor with encoder can be used in place of a servo. You just need to do PID control on it so it will hold at the target position. NXT motors have built-in encoders so they can do it as well.

Thank you.
It is still unclear to me how to program this. Zeroing the encoder for a wheel is not too much of a problem, but I am not sure how to tell the robot where zero is for an arm. Seems like I might need some type sensor that would help let the robot know where it is so that we can tell it where to go - does this make sense?

If you have a default position that an arm starts in every time, then that can be your starting point. Otherwise, you could use a potentiometer.

Yes, if the starting position of the arm could be different every time when you start, you either need to use a potentiometer so you will know the absolute position or if you are using an encoder, you need to have a "calibration limit switch", basically a touch sensor that is placed at the "start position". When started, you program the arm to swing toward the direction of the "starting position". When the touch sensor is active, you zero the encoder. That's called zero calibration.

You could also use this: http://www.hitechnic.com/cgi-bin/commer ... ey=NAA1030
It can measure absolute position which persists through power cycles.

- Xander