need help designing a non RC motor control circuit

[Donahue]

Let me start out by saying that I am not an electrical engineer, but I am learning quickly. I still need a little help however. I am trying to figure out a way to control a pair of DC gear motors in a particular way. The application is on a walk behind material handling machine that I am designing. The motors are identical, 600w 24v motors that have a 30:1 worm gear speed reducer on them. I plan on delivering the power to them with a 140 amp controller card we use at work. The cards have programmable speeds and acceleration rates among other things and I have most of the inner workings of those figured out. In this new application however, I would like each motor to drive a track much like a tank track. I will need to vary the amount of current going to each motor to change the speed so that I can effectively turn. Now here comes the challenging part: I need to do this from a mechanical source. The rear of the machine is going to have a tiller much like a pallet jack. As the tiller is rotated to one side, the power is going to be biased to one motor, causing the cart to turn. The throttle is applied via a wig wag switch operated by the thumbs. This wig wag controls both the speed and the forward/reverse operation of the control card. I believe I have found a way to use a potentiometer to achieve the effect I am after, but not at this high of current. In this experimental circuit, the motors are effectively wired in parallel with each other, both running at 24v and 70A each. But in this experimental circuit, there is a pot wired in the junction on one set of the leads. The wiper goes to the card, and the ends go to each motor. The position of the wiper would essentially favor which motor received the current, and at what ratio. This pot could be adjusted on the fly while driving said cart. Going all the way to one end would stop one motor and cause the other to receive all the current, allowing the cart to pivot on the inside track. I could then just hook up a mechanical extension of the pot knob to that rotating the tiller caused the pot to be adjusted. My first concern is if my concept even works. I do not have a good way to test this right now, though I may be able to find a couple of small DC motor to do a proof of concept. But if this works, I still need to find a solution that can handle 140 amps going through it.

any thoughts, comments? do i need to draw a schematic?

[JohnRobHolmes]

At that high of power, an electronic way of control will be much more reliable. While you could use relays to turn one motor on and off, getting proportional control through an analog method will take some large bits and a lot of waste heat.

A schematic would help.

[ThunderVP]

You said you have a set of industrial motor controller cards already so that is where you should look for some of your information. Most programmable controller cards can be controlled by an adjustable frequency clock pulse. Give both the same freq in, and they run the same speed. Adjust one up or down and the tow rig will turn. Remove the clock or the start control line and it will stop. To control the frequencies for each drive, a small PLC with analog and digital I/O would be best.
Simple DC motor controls used on industrial conveyors are speed adjusted with a simple pot. Getting two of these to work as you want may be possible but not as good in the long run. (although would be much cheaper)