How to Achieve Precise Positioning With AC 3ph Motor, Encoder, and Brake Assembly?

Ive havent been a technician long and have been handed a project that requires an AC 3ph motor that will move forward/reverse to an exact position with the operator either typing a length in a HMI or a switch that i would install. With the switch i would have to put prox sensors at every position that this motor must stop along with indicator lamps to let the operator know that its reached the desired postion. This would require 7 prox and lamps. (prox every 2 ft for 14 ft total) I would like to make it more automated. The load on the motor is a chain driven "backstop" basically a plate that will have product slam into it so I will need a brake assembly to hold position. I have different ideas but seem to always hit a roadblock. Havent seen a motor with an encoder and brake asssembly since theyre placed in the same location. There is a high speed plc input card that is used for something else on this machine and was thinking about a prox reading the sprocket for certain counts for certain positions. Its just the forward and reverse programming that scares me and the math that may be involved (its a L32 compact logix). ANY ideas are apprecaited. Thanks 

Well, you do need some way of sensing the position, or how far the arm has moved relative to its rest or "zero" position. One way without using too many sensors is to use a multi-turn pot coupled directly to the motor's shaft that should be calibrated to give a readout as V/degree or mV/degree where degree is how much the shaft has turned relative to zero. Then you compare the potentiometer output against the desired reference voltage (calculated based on desired position) and either advance the motor up or down till the actual position is achieved. You will need to include some hysterisis into this else, the motor will "hunt" around the desired position once it is reached.