Are potentiometers reliable position sensors? (and some Axon file questions...)

[mstacho]

Sorry if the topic is a bit all over the place, but it's one of those odds-and-ends things I can't really fit anywhere else :-)

I'm working on my robot hand.  I am currently using two potentiometers for my two-link finger in order to sense the rotation angle.  Now, I know my motors have encoders that I can buy, so if pots stop working then I'll just get them.

Unfortunately, one pot works great, the other doesn't.  And interestingly enough, they're both the same pot, and at one time they BOTH stopped working.

Now: Is this just because I (probably) soldered them a bit weirdly, or is it because potentiometers in general are less robust than encoders?  Should I just go for broke and get the encoders anyway?

One HUGE advantage to pots is the ability to have absolute positioning.  The encoders are all relative.  i wonder: can the Axon be programmed to store a small file in its flash memory that stores the encoder readings once the axon is shut off?

MIKE

[waltr]

In my experience Pots can be trouble and have reliability issues. Also not all pots are created equal. Some, manufacture/type, can be very reliable.
What kind of pots are you using? Link to specs?

Maybe try to find a better quality pot as the absolute position output is a good feature for your application.

[mstacho]

I'm using the "whatever potentiometers I found in the lab" :-P So I'm actually not sure what the specs are, other than they are 10KOhm pots (I think.  They say "1 KB" on them...)

What I'll probably end up doing is using the encoders anyway, since I'm also looking at some size constraints.  The easiest way I see of storing the position data is just sending it to the PC, although I guess if the hand moves when it's off it'll be lost anyway.  Maybe there's some way of having a calibration action that the hand goes through at the beginning of its run?

MIKE

[Conscripted]

Hobby servo's us a pot for position don't they? I can't imagine there would be so many to choose from if the technology was all crap.

Conscripted

[nickc]

Encoders come in absolute or relative and there are also resolvers which are absolute.  At work, we have several robots with relative encoders and none store data.  With relative encoders, you need to 'home' the joint. Normally this is done with a limit switch at one end of travel.  Drive the joint till the limit trips then back off a small number of counts and zero the encoder. You want to back off the limit so that you are not constantly tripping it.  If you put a limit switch at both ends of travel and keep the programmed range of motion just inside the limits, you can use the limit switches as fault detectors.

[Soeren]

Hi,

Now: Is this just because I (probably) soldered them a bit weirdly, or is it because potentiometers in general are less robust than encoders?  Should I just go for broke and get the encoders anyway?

While I cannot comment on your soldering (since I haven't seen it), I'd think that your problems stems from using common carbon track potentiometers, which can have different issues in your application (besides a short life and a general low reliability).

If you switch to conductive plastic potentiometers, you shouldn't have any issues, but they're quite expensive compared to carbon track types.
An intermediate quality is ceramic potentiometers (can even be found with PTFE coating) and with potentiometers, both price and longevity usually follows the price.

One HUGE advantage to pots is the ability to have absolute positioning.  The encoders are all relative.

... Except for the absolute position encoders of course

i wonder: can the Axon be programmed to store a small file in its flash memory that stores the encoder readings once the axon is shut off?

It has got 4kB of EEPROM and that's where you would usually store such info.

However... A better solution is to start by fully contracting (or extending) all controllable joints to whatever limit switches you may use for a known start up, as storing a position that might be moved by external force when shut down will lead to errors (or worse).
Only ever store and rely on info that you are sure cannot be altered by other means.

Whether you should be using potentiometers or encoders will depend on the size restraints and your budget - If the budget's large enough, you can have whatever size and form you like.

Here's another possible solution from France, that might be worth taking a look at.