Moving a rail cart with two motors

Hi all,
I have a rail cart moving for 20m on two rails that are 6 meters wide.



I was thinking to use two stepper motor with closed loop like this

In this way is easy to move them at the same time of the same quantity to avoid that the cart warps and get an alarm if one motor fails. At start the cart goes home and, with two proximity sensors, one per rail, it can perform a fine setup, moving one motor per time, to stay perfectly aligned.
I'm wondering if you have suggestions or comments about this solution.


Might be a silly question, but do you need 2 motors?

Hi markd833,
not a silly question.
I can connect the motors only directly on the wheels and with only one motor, since the cart is not very strong, it moves before on the side with the motor and then on the other side moving crooked.

Finding powerful enough steppers might be hard , maybe normal geared DC motors and encoders or similar to keep them in step
( although an axle is better )

Will the two driven wheels have IDENTICAL circumferences? If not your cart will still get distorted.
I guess wheel to rail friction also is involved.

Hi hammy, you are right but I'm worried to keep the DC motor in step

Good question, I will check if the wheels do different turns on same distance and, in case, I can take in count on the software. About friction I hope the wheels will not slip. Thanks Paul

Hence the encoders ….

True, but I suppose that with stepper I'm sure that they always do the same turns, with DC I have to check the encoders, do a difference and, in case, modify the pwm on one motor to reset the error, so with stepper I don't have error with DC I always reset the error.

Steppers can miss steps if heavily loaded , and if the cart then stops out of alignment , you have no reference to re align it .
I’d would look at using an axle across the cart - easiest/robust /proven solution

Have a look at the “cameras on wires “ used high up in football stadia for possible inspiration.

The shape of you cart is odd , longer thin ones would be naturally more stable - some I’ve seen like yours have additional rails down the centre - a 6m span requires a substantial build.
Have you tried powering just one side of yours to see what happens ? Wheel shape etc also important .

Bit of an odd ball project really , and I think you are electronically trying to overcome a bad mechanical design

Its only a 20m run on 6m gauge, this is not an issue. Gradual slippage over time will be an issue though... (perhaps switching to toothed belt drive would fix this)

Normally a stepper would be a poor choice for traction purposes due to the massive inefficiency of steppers, but this might be the exception as you require lock-step positioning. Presumably this is some sort of gantry?

Do you have a good idea of the required torque - you will need to size the motors.

Thanks everyone for the tips, I will look for “cameras on wires “.
They, at the moment, move this cart by hand but they would like to add a motor that's why was not designed for that. This cart has several lamps on it to dry a material on the floor.
The rail is of steel so the friction is not much and all the cart has a weight og 50 kg.
The speed is about 0.5 m/s.

Another silly question, but could you use a winch at one (each?) end to pull the cart along. If your attachment point was on the centre line of the cart then would that avoid the cart warping?

Unfortunately I can't.

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