Verify encoder position against index (when available) (ros ticket #4821)

[ahendrix]

There are only a couple encoders in the PR2 that have an index signal. However, with these encoders, it is easy to constantly monitor for encoder drift / error by checking difference in encoder position when index pins goes high.

Unfortunately, the actuator data stored on MCB does not specify whether the encoder index is present. There a couple of ways to get this functionality.

1. Up-rev actuator data to include information about encoder index. Support both old and new actuator version. Provide script to update MCB from old version to new version. Check for out-of-date actuator data in pr2-system check.
2. Use URDF to define rosparam that indicates index is available. Have ethercat_hardware check for this rosparam when starting.
3. Use UDRD to load controller that monitors index position and publishes to diagnostics.

Of the options, #1 is best, it is probably what we would do if there wasn't any versioning issue with the actuator data on the MCB.

trac data:

• Owner: berger
• Reporter: dking
• Reported at: Wed Oct 20 17:44:08 2010
• URL: https://code.ros.org/trac/wg-ros-pkg/ticket/4821

[ahendrix]

[dking] Eric, I've already discussed this with Kevin, we would like to know if you have any opinions on this.

[ahendrix]

[berger] It's interesting, but I'm not sure it gets us a huge win because it's not on many of the encoders.

If we want to detect and respond to encoder errors like what the lab automation team saw last week, I would prioritize making detected encoder errors visible instead of adding a second type of detection.

The existing encoder error detection was clearly pointing to the problem in the incident last week - the issue was that the users weren't aware of it.

[ahendrix]

[dking] Yeah, I guess doing matching on the encoder index is not worth the effort since there are only 2 encoders with indexes in the PR2. There is already a change that creates a diagnostic warning when encoder errors occur.