Making a VEX motor act like a servo?

[Huntzingercg]

I am working on an animatronics project for my engineering 2 class and need the VEX 2 wire motor 269 to turn a set  number of degrees. I have tried using time, like the motor time control module as well the regular motor module and adding in waits, and wait untils. My problem is that the time increments are not precise enough. The motor will turn farther on way than the other, and will eventually make a full circle. I could solve this problem if I could only figure out how to use degrees over time modules... Help!

[mklrobo]

  Hello! 
I have had any experience with the VEX motor. I will have to look up reviews
and datasheets to give any appraisals of your request. The programming of the turning and
timing seems to be an issue, and I could address this generically with C++ programming. as
if programming a beaglebone black. You would have to convert to your microcontroller language.
What microcontroller do you use?

[bdeuell]

I assume the "VEX 2 wire motor 269" is essentially a geared dc motor and does not have any feedback loop like a servo. Without adding anything to the system it will be very difficult to get accurate position control. My only suggestion would be a timer based control but it sounds like you already tried that.

Here are some ideas of how you could modify the system:

- Add two limit switches that would be activated when the motor reaches each end of the travel. you could use one limit switch with a timer control to prevent drift over time but it would not make the individual moves more accurate.

- use a hard mechanical stop that limits the motion, ideally with torque limiter between the motor and hard stops. The VEX "clutch" part is a torque limiter.

 - create a potentiometer feedback control loop, this is how "hobby" servos (and vex servos) work.

 - use a servo