View Single Post
  #18   Spotlight this post!  
Unread 08-02-2017, 19:36
Tom Line's Avatar
Tom Line Tom Line is offline
Raptors can't turn doorknobs.
FRC #1718 (The Fighting Pi)
Team Role: Mentor
 
Join Date: Jan 2007
Rookie Year: 1999
Location: Armada, Michigan
Posts: 2,569
Tom Line has a reputation beyond reputeTom Line has a reputation beyond reputeTom Line has a reputation beyond reputeTom Line has a reputation beyond reputeTom Line has a reputation beyond reputeTom Line has a reputation beyond reputeTom Line has a reputation beyond reputeTom Line has a reputation beyond reputeTom Line has a reputation beyond reputeTom Line has a reputation beyond reputeTom Line has a reputation beyond repute
Re: Recommended gyros that can keep up with fast rotation?

Quote:
Originally Posted by slibert View Post
Well, I'm biased a bit, but wanted to share my thoughts:

Pigeon specs are similar to navX-MXP/navX-Micro for 6-axis performance from a yaw drift rate, make sense as they use the same chipset which performs the 6-axis fusion onboard. The navX-MXP/navX-Micro firmware for calibration has improved over the two years, but it's likely safe to assume the Pigeon orientation angle accuracy after calibration is similar (sounds like it, according to published specs).

Pigeon update rate is 100hz, navX-MXP/navX-Micro are 200hz, so if you want faster update rate the latter is superior. 200Hz is great if you want to drive rapid PID loops that manage the orientation of multiple wheels on the robot.

We will have to see if there's any CAN bus contention limits the Pigeon IMU from consistently achieving update rate at 100hz, CTRE should be able to say on that. At 1mbps w/the PDP, 4 talons and a PCM all chatting on the CAN bus, and 100hz Pigeon updates there may be some contention starting to arise - that remains to be seen. However, in the configuration to connect directly a Pigeon to a Talon this would likely not be a concern.

From a software perspective, the Pigeon docs indicate it provides yaw, pitch and roll - however the navX-MXP/navX-Micro also (perhaps because it's not limited to the 8-byte CAN bus data payload sizes) provide synchronized quaternions and a sensor timestamp as well. Timestamped Quaternions (and the fact that navX-MXP/navX-Micro have multiple simultaneous communcation interfaces) are being used effectively in the Sensor Fusion Framework (SF2) to correct for video processing latency, and create timestamped orientation histories. As a contributor to SF2, I can say that more features along this line are coming. Software is key to enabling more autonomous features.

For newer teams the number of examples, the training materials and the large community that support navX-MXP/navX-Micro and help each other too are awesome.

So in addition to orientation accuracy specs which I believe are very similar, there are several system and software level capabilities to also consider.
There we go . That's great info. Thank you.

I believe the Talon srx update rate is at a default of 10 ms. So a matching update rate of the Pigeon at 10 ms means you'll be receiving 1 new sample per loop. I'm curious though how much the law of diminishing returns comes into effect when trying to run things at an update rate of 10 ms versus 5 ms. It's well beyond the scope of what I know how to do to measure the effect of that. You probably have more experience with that than I do since we rarely worry about update rates (except on shooters....).

I suspect based on the maximum number of devices allowed on the Can Bus that the update rate will not become bottlenecked, but like you said we should know pretty shortly. We haven't seen a problem on ours with 13 srx's.

Last edited by Tom Line : 08-02-2017 at 19:41.
Reply With Quote