[KenWittlief]

I like the idea of using limit switches in addition to the pot sensor. This is standard practice for many motion control systems. A limit switch is pretty bulletproof, so it not only covers pot/sensor failures, it covers your whole control system.

if the switch is closed you know your arm is at the end of its travel, no matter what the rest of your control system is telling you - in fact many systems have two at each end - an end of travel switch, and a hard cutoff (kill) switch

We used a pot at the base of our arm and we did have one fail for no apparent reason.

OK to get back to your original question then - one simple way to check your sensors is to have a jog function - you tell the arm to move a bit, and watch the sensor. If the sensor reading doenst change, then its failed.

You can do this on powerup, or under driver command (used in the pits) or under a periodic self test, but when a bot only runs for 2 minutes putting periodic tests into it SW isnt very useful.

the timeout test is another option - if you tell the arm to move read the pot first, then if it doenst change within a few SW cycles its probabally gone south (in my mind Im gone to Carolina, cant you see the sunshine, can you just feel the.. HEY WAKE UP! WERE IN THE MIDDLE OF THE FINALS - Oops, sorry, 58, 58, 58, 62, 65, 73). <- this is not very realistic - once a sensor goes south it almost never comes back.

BTW - when you loose your sensor input in a closed loop PID control system, its called 'going open loop' - its usually very funny to watch when it happens - kinda like putting a blindfold on a boxer during a prize fight, and watching him flail about in the ring trying to hit his opponent when he cant see what hes swinging at.

sometimes engineers go open-loop too, which is also amusing to watch :^)