My bad the motors currently operates at a maxium of 30VDC and consumes 250mA. The motor takes voltage from 0-30VDC. The motor controls the claw and the more voltage the stronger the claw grips. So, I would like to increase the adjustable voltage range to 0-50VDC.

I was planning on using an op-amp to multiply output voltage from a 0-5vdc digital pot. I wanted to multiply that voltage by 10 giving me an output range of 0-50Vdc. I was than going to use that voltage to control the darlington transitor. However, I am having a hard time try to find a OP-amp that will operate at +50VDC (60vdc to be safe). I place an example of the circuit I wanted to create at the addess below. http://www.jernie.com/careers/EWB.pdf

If someone has a schematic or drawing of PWM design, MOSFET, SCR, or etc it would be a great help to me.
Thanks Everyone.