Thanks for all the help with the original post. After reading your ideas and input I have decided to revamp the design.

Bryan1 came up with a very very good idea. Why not just use a manual controller designed for the scooter motors and then regulate speed with a servo controlled by the PLC. Awesome idea; I don't know why I didn't think about it.

Here's another change I thought about. Instead of using the wheel on, wheel off steering method, I want to just steer the front wheels. I need some sort of servo type thing. I thought about some sort of piston device and also a stepper motor. What would be the best way to go? Any other way? (Remember the car could weigh up to 450 lbs when loaded).

Thanks

 

i've read your last two posts and have some questions:
Why do you need H-bridge?
How often you have to reverse?
How fast you have to reverse?

You could use single transistor to control speed of the motor - from
your basic stamp directly or by PWM circuit like this:
http://www.solorb.com/elect/solarcirc/pwm2/pwm3.jpg

by changing voltage of the pin 6 of the LM324 you control the duty
cycle of the PWM. This can be manual or automated...
MOSFETs are cheap, instead of H bridge just use switch to swap motor wires for running in reverse.

if you want steering mechanism then why not use servo?
i know, the cart is some 500lbs...but...an 18-wheeler truck weighs
way more but truck driver doesn't have to be able to lift it - just strong enough to steer.

 

its called powersteering. a hydraulic piston helps the guy steer. otherwise it is very very difficult to drive such a vehicle.

i could definitely use a geared servo. anyone have any reccomendation for one with enough troque/power to do what im talking about

by the way, disregard the H-Bridge part. I've found another way.