Re: Picking a gyro for field-centric swerve control
 

If you are going to use two gyros, I don't think you should pick and choose between which one you should look at - you should look at both of them!

Somebody a few posts back mentioned using high-pass and low-pass filtering, something I heartily recommend. Subjecting a coarse gyro to a low-pass filter and a fine gyro to a high-pass filter and combining the values should lead you to a value that is extremely accurate, assuming the coarse gyro can handle shocks (robot getting slammed around) appropriately.

Ideally, you'd want a sensor that does not accumulate drift (like a compass) in place of the coarse gyro, but it sounds like the EM situation on the robot is too intense to use magnetic compasses.

Re: Picking a gyro for field-centric swerve control
 

I'd like to caution against live-match calibration. The drivers already have a bit to worry about with the game itself and 2 minutes FLIES BY like there's no tomorrow. Having to line up and recalibrate a sensor on the fly seems simple enough, yet getting the calibration correct during a match will be near impossible without taking many many seconds of match time.

Re: Picking a gyro for field-centric swerve control
 

I feel the same way. It's a good option but I want our drivers focussed on one thing and one thing only - winning the game. Having to worry about our robot malfunctioning is hopefully the last thing on their minds.

Re: Picking a gyro for field-centric swerve control
 

We used the Honeywell HMC6352 compass (I2C), with a look-up compensation table, and two gyros (different sensitivities).

The general scheme for the look-up table was to put the robot on a rotating table outfitted with an encoder on it's shaft. The (encoder - compass) errors were written to the table. The table was indexed using the actual compass reading.

The compass ran at a max of 20 Hz, but the idea was to read it when the rotation rate was low, reset the gyros, and use the gyros (+ last compass) between compass readings.

On the table we were within +/- 3 degs at our max robot rotation rate. And at those rates the compass alone at 20 Hz was almost as good (eliminates a chunk of coding).

The compass needs to be several inches away from the nearest ferrous (or magnetic) component; and some mu metal shielding can be used if needed. Keep the mu metal away from the compass too!

The key is to find a mounting location where the compass readings change monotonically as the robot rotates, otherwise you have a multi-valued function that can't be easily compensated (if at all).

Knowing absolute field heading is of course only one part of this control system.

David

Re: Picking a gyro for field-centric swerve control
 

Enlighten me please, mu metal?

Re: Picking a gyro for field-centric swerve control
 

Mu Metal is used for shielding against magnetic fields.

When I was doing RF, we kept a bunch on hand.

Re: Picking a gyro for field-centric swerve control
 

Product of Carpenter Specialty Alloys [correction: looks like "Mu-Metal" and "Mumetal" are trademarks owned by Telcon Metals; Carpenter produces a higher-performance variant of it]. High magnetic-permeability metal typically in sheet form design to intercept magnetic fields. Loses its effectiveness when work-hardened. Usually has to be ordered by the ton (in my experience, your mileage may vary), so not really practical for teams to use unless they have some alternate means of obtaining it. Oh, and did I mention that it's very heavy...

Re: Picking a gyro for field-centric swerve control
 

I assume it would be used to keep the RF noise from the motors/wires away from the magnetic compass then? What is a good distance to keep it away from the compass? (Other than as far as possible)

Re: Picking a gyro for field-centric swerve control
 

Actually, it attenuates low frequency B-fields (magnetic fields) not the radio-frequency (high frequency) E-field (electrical fields). Similar concept, just different embodiments. Mu-metal can become magnetically saturated if the field strength is high enough, so it's not a fix-all. In the FRC application, the main B-field sources are the permanent magnets in the DC motors. Unlike RF noise, the wires will emit very little in the way of magnetic fields (consult Lenz's Law).

Re: Picking a gyro for field-centric swerve control
 

And here I thought you were talking about Limp Bizkit and Korn.

Re: Picking a gyro for field-centric swerve control
 

Here is what we used:

http://www.lessemf.com/mag-shld.html#276

To use it effectively you should read up on it a bit before using it.

David

Re: Picking a gyro for field-centric swerve control
 

+1 for finding a small-quantity supplier!