Crab drive without a pot

[Bharat Nain]

Originally Posted by Ian Mackenzie

True, but normal potentiometers have the nasty disadvantage of having a hard stop. When we did swerve drive in 2004, we broke a lot of potentiometers (especially in testing) by turning them too far. In hindsight, it might have been a better idea to use encoders, set the wheel to a known position at the start of the match, and probably have a limit switch as a backup that would be triggered (e.g.) when the wheel was straight ahead - so if anything happened, you could have a reset button that would spin the wheel until it triggered the limit switch and then would reset the encoder count. (You could also do this on the fly - every time that switch happens to be triggered, have an interrupt that corrects the encoder count.)

Did you try using a 10-turn pot?

[Ian Mackenzie]

Originally Posted by Bharat Nain

Did you try using a 10-turn pot?

We had 5-turn pots, so 2.5 turns in either direction. 10 turns might have been better, but I still think we would have broken a few (and, of course, we would have had lower resolution - but I'm not sure that would have been significant).

[Al Skierkiewicz]

There are a couple of things that teams can do to use a standard 270 degree pot without breaking it. You can easily use a couple of nylon gears (from Small Parts) to couple the pot to your steering mechanism. You can mechanically limit the travel of your crab so that you stop turning before the pot is broken or you can spend some money and buy a continuous pot.
But dpick, as you can see, no one is telling you to do crab without a pot. There is no way for the robot or the driver to know that the crab system has actually gone where the driver has told it to go or for the software to get it back to driving forward.

[Tristan Lall]

Why not an absolute encoder? (Like this.)

[fledman]

Originally Posted by Tristan Lall

Why not an absolute encoder? (Like this.)

Is there any functional difference between a pot and an absolute encoder? (I realize that they are different internally.) Why would someone use one over the other (for low speed applications, eg arms, steering)? Is there a reason teams use encoders that give counts instead of absolute encoders on higher speed applications (wheels)?

[B.Johnston]

Yes, the most significant advantage of this encoder is its free rotating shaft.

Plus it costs about the same as a 10 turn precision pot without the potential for value shifting due to vibration or temperature.

One possible reason to use raw quadrature input is to "count" beyond 1 revolution.

The source of this encoder USDigital has several other devices which can emulate pots by actively providing an analog voltage "incremented or decremented" by a quadrature pulse train (edac2) or an analog voltage feedback of the rate of pulses per second(etach2).

Additionally these devices incorporate the ability to self calibrate via a precise index marker pulse.

By selecting your counts per revolution carefully you can tailor the precision of the output to the neccessary resolution.