Quote:
Originally Posted by Joe Johnson
Sidebar:
Preamble:
In an unlimited rotation angle world, the natural thing to do is to put a quadrature encoder (like a US Robotics or a Grayhill or some such) in the swerve drive system somewhere and pretty much call it good.
But... ...I hate that idea. The main reason is that then you have to zero the count when the robot wakes up (or goes brain dead*). Yes, you can get around this with training and check lists and such but it is a long season, this decision is going to bite you in the tukhus before its over.
Here is what I would like. - Something that can deal with unlimited rotation (obviously)
- Something that is 1:1 with the actual steering angle and does not require the controller to be zeroed (other than a constant programmed into the robot once and only changed if there is work on the robot -- if ever).
- Something that has a resolution of under 1 degree -- preferably 1/10th a degree or better.
- Something with as little noise as possible (To get reasonable responses, we use a lot of D Gain in our PID Loop. Noise is really tough for Deriviative Feedback loops to deal with.)
- Something with at Digital Output if possible -- for two reasons. First, we had a heck of a time with noise on the 5V supply and on the GND from the RoboRIO, so it was hard to get a noise free signal out of a lot of our sensors (like pots for example). Second we ran out of analog inputs on the RoboRIO and had to add a coprocessor just to manage the extra analog signals. Not ideal. If we can put the steering position I2C or SPI, that may make our lives better -- though perhaps not because we are thinking of using the Talon SRX to do our PID loop, I don't think the Talon SRX likes getting its sensor input via anything other than Quadrature or Analog. So, I don't really know what I want in this department.
Anyway, I would like learn from folk with unlimited rotation swerve experience. What sensor scheme did you use and what are the pros (and cons) of those choices?
Do tell.
Dr. Joe J.
*don't get me started about FIRST's decision to allow the NI folks to allow the RoboRIO loose its mind at just under 7V -- this is something close to criminal. Really... this was a really dumb design direction given the customer base and the intended use of the RoboRIO. |
You could also use an indexing sensor such as a hall effect. The Encoder in WPILib supports indexing out of the box as does the Talon via CAN-Position Control.
My money is on the sensor Bryce listed because I'm cheap. And also lazy and don't like wiring extra sensors.