Mechanical limit switches - acceptable practice?

[cfair]

Bmammen, your original thought is sound, but not allowed in FRC. My experience with machine tools is pretty dated and we made one-off tools rather than production tools, but we ALWAYS put a second limit switch beyond the first one, and the second switch was connected to a relay or contactor that killed the entire system (no software involved). This second switch is surprisingly expensive, because you need a couple of inches of extra travel to accommodate the switch plus deceleration of the carriage. This year we built a linear test fixture with exactly these provisions for testing software because:

In FRC, our experience is that the best method is to use encoders and PID, with soft limits, so that the actual limit switches should never be encountered except during initialization (homing). However, there are multiple conditions that will cause the system to lose zero during a match, which you are apparently aware of. It is therefore very important that software be able to deal with hitting a limit switch, and it is a continual battle to keep our software folks and their code doing the right thing. In particular, kids often think it's OK to use open loop control and just monitor the limits (particularly for joystick mode), or they find that they never actually hit the limits using programmed moves, so the software that responds to them "atrophies", and bad things happen when they inevitably hit them. The response once you're in a limit switch can be debated, but we generally allow motion in the appropriate direction to get off the switch, and what happens after that depends on your tolerance for risk, because it should never have hit it in the first place.

[MrForbes]

I learned long ago in my FRC career that it's a good idea to make mechanisms that don't get hurt, if the motor keeps running. Yes, it takes some effort to figure out how to do this, but it can usually be done.

one trick is to have motors that spin rollers. Another is to use pneumatics for stuff that has to move a limited distance. Another is to have a "clutch" of some sort, such as a V belt that can slip, instead of a toothed belt or chain.

It's not that I don't trust the programmers...it's just that things somehow always go wrong. Mr. Murphy is usually hanging around the shop.

[Mr V]

If you are worried about failure of a single limit switch then just wire in two so you have a back up. Use the NO contact and wire them separately and in your code have it stop if either of them put out a signal. Relatively simple and legal under last season's rules. Having one in the power path would not be legal under last year's rules and I'd bet that it won't be legal under next year's rules. Additionally the problem is finding a limit switch rated for the amperage that the motor is rated for along with the fact that it would kill that motor for the rest of the match.

[philso]

The commonly available limit switches are usually rated for something like 3-5Adc with (inductive) motor loads. The stall current of a BAG motor is 41Adc. It is quite possible that the arcing that would occur as the contacts open will cause them to weld closed, defeating your protection scheme.

[Al Skierkiewicz]

So far everyone is correct as to the rules for 2014 (Mark, Jon and Philso). I would like to add that is pretty easy to design the limit switch mounting such that it will not be damaged by over-travel in the mechanism it is designed to limit. Rather than hitting the switch lever head on, just design it so that the lever "follows" the mechanism that is being sensed. By doing that, no stress is placed on the switch or the operating lever.

[philso]

Originally Posted by Al Skierkiewicz

I would like to add that is pretty easy to design the limit switch mounting such that it will not be damaged by over-travel in the mechanism it is designed to limit. Rather than hitting the switch lever head on, just design it so that the lever "follows" the mechanism that is being sensed. By doing that, no stress is placed on the switch or the operating lever.

In other words, using the limit switch as the physical end-stop often leads to destruction of the limit switch.

[Ether]

Originally Posted by philso

In other words, using the limit switch as the physical end-stop often leads to destruction of the limit switch.

If the OP would be willing to post some pix of his limit switch installation I'm sure he would receive some helpful suggestions.

Some teams use a flexible extension on the switch lever (post#2). Others use a "follower" approach (post#20). Both can be made to work reliably.