Alternative to limit switches?

[sciencenerd]

Originally Posted by Andrew Blair

I'm wondering about the use of pots. I know that they can be a degree or two off occasionally, and all the teams I've seen using them have a limit switch backup. Does anyone use pots exclusively?

Last year our team used only pots to determine the angle of our arm. However, we found that because of the inherent error, if we wanted the middle joint to bend as close to 360º as was safe, we would often slightly overshoot, pushing together two of our arm segments, bending the metal slightly and putting a huge amount of strain on the chain/motors. This was obviously a big problem, so we ended up retrofitting a limit switch to our arm halfway through the regional. The moral of this story: don't try to cut weight/save time by skipping the limit switch, you'll always regret it in the end.

[Al Skierkiewicz]

I agree with Ken here. There should be no reason to break a limit switch. You must remember that the mechanism it is designed to protect has momentum and will not come to a stop immediately. The microswitches shipped in our kits have long actuating arms that can be bent and/or cut to any form that is useful for you application. Design mechanical stops accordingly, backup the backup.
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.
If you are at an event where I am present please ask me to look at problems on your robot. I love to see other robots and what other teams have done to solve problems.

[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.

[KenWittlief]

if you are not protecting your microswitches, then anything else you put on thats unprotected will get smashed as well.

You might consider a lever or spring that actuates the microswitch from a distance, so the switch doesnt need to be exposed to the parts of the bot that get bashed during a match.

An example is the power switch in most PCs. The switch is back by the power supply, but the on/off button is on the front panel, connected by a long push rod.

[Tristan Lall]

Originally Posted by KenWittlief

An example is the power switch in most PCs. The switch is back by the power supply, but the on/off button is on the front panel, connected by a long push rod.

Ken, I think you're trapped back in the days of the AT form factor (or before)! Modern PCs (i.e. nearly anything within the last eight years) use an electrical connection directly to the motherboard, with a momentary switch. Cross two pins (one is connected to 5VSB, if I remember correctly), and the motherboard instructs the power supply to turn fully on.

There is a second switch on the back of most power supplies, but I don't think that that's the one to which you were referring; it just interrupts the power completely (preventing the motherboard from receiving standby power).

[Cuog]

And did you know that if you touch those two wires together in that switch they will weld themselves together(I accidentally did that)

But instead of limit switches on the arm i like pots our 2006 robot is going to have almost all the analog in ports utilized if i can help it

[sciencenerd]

Originally Posted by Cuog

But instead of limit switches on the arm i like pots our 2006 robot is going to have almost all the analog in ports utilized if i can help it

Like I said in my post above, our team found out the hard way that pots have inherent inaccuracies that make it inadvisable to use them as the safety cutoff. They can be used to determine position when a couple of degrees don't matter, but when something like the motors of your arm/whatever the arm is capable of running into is at stake, don't trust a potentiometer to be 100% accurate.

[Gdeaver]

I'll post again on this subject. There are industrial limit switches in the 5 to 20$ range that are much more robust and reliable than the small micro limit switches. They come in several configurations of levers, rods, and rollers. Several brands also have die cast bodies and can take abuse.

[Billfred]

Originally Posted by Gdeaver

I'll post again on this subject. There are industrial limit switches in the 5 to 20$ range that are much more robust and reliable than the small micro limit switches. They come in several configurations of levers, rods, and rollers. Several brands also have die cast bodies and can take abuse.

Just for a point of reference, where do the Vex limit switches fall in the grand pecking order of limit switches?

[Cuog]

Thats why our arm has a physical stopper just in case, and the pot is set so that 0 is about 3 inches from the stopper,

Just for a point of reference, where do the Vex limit switches fall in the grand pecking order of limit switches?

somewhere in the big thing range i would certainly use them for a limit switch but as long as it isnt likely to get hit since the plastic on them is not the best.

[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?

[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

[Al Skierkiewicz]

Sparks,
That is true for most of the prepackaged sensors but the actual Hall effect device is much like an optical transistor. The magnetic field will cause an increase in current in the transistor or send it into saturation. Individual Hall transistors are available if you look for them. Interfacing is then up to you.