CIM Motor Reversing

[cbale2000]

Originally Posted by GeeTwo

In particular with an arm, the load on the arm is almost always preferentially in one direction (probably what you are referring to as "back"). All other things being equal or even close, it will take more volt-seconds to raise a load than to lower it. If you need to get it to a certain spot, use an encoder, potentiometer, limit switch, or other sensor to tell when the process is complete.

To add to this, in a pinch, you can add a dead stop to the mechanism and then program it to slightly overshoot where you want it to go and always hit the dead stop, thus returning it to a consistent starting point. You do briefly stall the motor by doing this (though CIMs do have a bit of tolerance to this) and you have to have a mechanism beefy enough to take the repeated impacts, but if for whatever reason you are unable to add/program a sensor for the mechanism it will do the job. Just make sure you bring a spare motor or two. Of course this method pretty much only works with a 2-position mechanism.

That said, the other suggestions mentioned above are all far preferable to the option I just described. A physical dead stop for a mechanism should almost always be a last resort.