Sure it's doable. How large of a 12 volt storage battery do you think it will need?
Paul, I was thinking that I could power everything with a 3cell li-po battery, but I need to determine the consumption of the 2 displays and the motor to see if I can reasonably power everything.
The pull force of the standard sled is divided into pulling the actual skid and moving the weight forwards. If you take away that second component, everyone will be setting new world records.
If you have a switch to adjust the force per distance ratio, then it's got to be totally visible to all spectators. You might accidentally switch it for one competitor and they'll get the win too easily. The accusations of cheating will never be settled. Family members may never speak to each other again.
I'd do the different ratios with different sleds. The "red sled" is the hard one. "Green sled" is easy and "blue sled" is in between. You may wish them to be reprogrammable but only with a USB cable and only with a password.
You think a NMEA17 is big enough? Why not do the calculations for force and speed and then you'll know?
Morgan, I agree that some of the force used is to move actual weight but I don't think it will change stopping weight all that much. The best anybody has pulled on my sled is mid. 20's and that was with a fully modified truck built to pull only. Will hooking up a fish scale to weight box (disconnected from axles) and pulling it give me a close representation of ho much force is needed to move a full box?
I cannot duplicate my sled. Material, time and other things make it not feasible. My cousin bought a sled that weighs the same as mine but it pulls easier so we're thinking about using his on trucks under a certain weight.
I plan to keep everything enclosed which is why I want to use dip switches to change ratios which I planned to put on outside of sled anyhow. A small rotary switch would be more visible (for ratio settings) would work too.
Without wishing to contradict anything that @Morgans has said I would like to suggest that you consider using a simple DC motor driving a threaded rod to move the load back and forth. You could use a simple detector to count the revolutions of the threaded rod. And limit switches (which you have already mentioned).
Robin, a DC motor may be easier but controlling the motion would be harder I think. As the weight gets closer to the front the trucks slow down and as a result the advance of the weight needs to slow as well. Maybe I need to look into your idea more.
Originally this started out as just putting an encoder on the weight box and use it to calculate distance the sled has traveled and put it on the display in real time. My thoughts behind using a stepper to move the weight is that when my sled hits 30' the axles lock up and because some guys don't let off quick enough they drag the sled and I don't want to break the chains. Using a motor would be easier because when weight tops out the motor just stops and no extra strain is put on the drive system. If my idea is going to be difficult I can scratch it and go back to my original plan of just using a mini controller to drive the displays.
Biggest concern with stepper is added weight of all the components to make it work.
Still open to ideas though. Thanks.