Alternative to limit switches?

[sciguy125]

Originally Posted by Al Skierkiewicz

I agree with Ken here. There should be no reason to break a limit switch.

They are properly mounted, they usually break from battle damage. Either a direct result or something else gets misaligned and snags it. Although, I do admit that there are some that could be positioned better to prevent snags.

The problem is that they're usually the last thing to go on. Last year, they weren't on until our second match. By this time, they hadn't been tested yet. Needless to say, whoever made the decision to run the arm anyway made a pretty bad call. Although, the damage was a testament to how powerful our arm actually was.

[Sparks333]

Originally Posted by Al Skierkiewicz

As for reed switches, I do not recommend them for this application. They have a bad habit of disengaging with vibration and magnets get knocked off leaving you defenseless. Additionally, the contacts don't always meet properly and high resistance occurs. With very little current being supplied by the RC, there is a source of error there. Microswitches are designed to "wipe" the contacts when they make contact, minimizing this effect.

This is very true. The good news is that reed switches aren't used too much anymore (on account of being mechanical instead of solid-state) and the Hall-Effect sensors have all but replaced them (Could be hyperbolizing, but I see many more hall effect sensors than reed switches). Best of all, they both work about the same: bring 'em into a magnetic field, and they trip, except hall effect sensors output a digital 0-5vdc signal almost 100% of the time (depending on the sensor). So, if you have a real hankering for magnetic switches, halls are the way to go. The ones in the kit last year were decent, but they were designed to count the teeth on a gear of a ferrous metal. Halls in general just respond to magnets. One final note: Some halls (most? I actually don't know) respond to the flipping of a magnetic polarity (aka N, S, N, S, etc.). This is not what you want for limit switch application, but it works rather well for encoder operation (So you don't count a magnet twice). We used a pair of hall-effect sensors on our robot last year in place of encoders. Can't say we used them well, but the scope says they worked great.

Sparks

[mechanicalbrain]

Originally Posted by sciguy125

The thing is, encoders only tell you where the system is supposed to be, not where it is. A limit switch (or the like) will tell you when the system is at its limit and should stop. If you only have an encoder you know that you're supposed to be somewhere, but if you have a loose cable or chain, you'll actually be somewhere else.

Thats only if you design it that way. If you wanted a measurement on the extension of the arm you might use a linear encoder while if you wanted a measurement of arm angle you would mount an encoder to the arm. Your measurements would be accurate.

Personally I like a magnet sensors because the lack of actual contact. I especially like the ones that detect ferrous metals rather then other magnets because it leaves you room to be creative with how you position the sensors (like for example we used little metal strips that we could bend to different shapes). I don't exactly using pots for measuring the angle of an arm because of already said inaccuracy but for measuring objects with smaller radius it is extremely useful. By the way, have any FIRST teams used linear encoders?

[Bharat Nain]

Originally Posted by RbtGal1351

Hello,

Our team has been using limit switches for our past two years to see if our arm is fully extended, so if it is, then the software doesn't let the arm extend more. However, we've had some problems with the limit switches breaking, so is there an alternative to limit switches? Have any other teams tried something else? What about reed switches?

Thanks
~Stephanie
Control Systems Leader

We had problems with the limit switch breaking all year. We made it as simple as possible to replace it(not that easy). And then we put a limit switch override button in programming. That way in competition if it did not work the operator could use his/her judgement and decide how far up/down the arm should go. The button came in really handy, especially at palmetto where we took finalist.
-Bharat

[Gdeaver]

There are many different grades of limit switches. If you look in a Mcmaster-Carr or Grainger catalog you will find some real beefy limit switches.