How is the turn rate of the RMP computed?

RMP 210, 220, 440LE, 440SE, 440 Omni
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kmelotti
Posts: 3
Joined: Wed Oct 24, 2012 8:57 am

How is the turn rate of the RMP computed?

Post by kmelotti »

I'm a bit confused about how the RMP 440 determines its turn rate. We put the RMP 440 on blocks and ran it at a turn rate of 3 rad/s yet the differential_wheel_vel_rps read 2.5 rad/s. The wheel diameter and the track width were left at their default values. Why is there a difference between these two values? We also tried it in gravel with a differential_wheel_vel_rps of ~2.3 rad/s though it jumped around a lot more understandably. Unfortunately I currently cannot attach any of the log files to this post, as soon as I can I'll attach the logs. Thanks for any insight on this!

Kevin

Craig
Posts: 58
Joined: Thu Jan 26, 2012 3:02 pm

Re: How is the turn rate of the RMP computed?

Post by Craig »

This could be one of two things.

If you are using our standard python OCU:
The standard OCU apply's a deadband to the input in order to improve performance around 0. This results in the command being about 10% less than the maximum setpoint. If you change the code for the OCU to apply an exponential instead of a deadband this behavior can be removed. Its also possible to just set the maximum to 10% higher than the desired maximum.

If you are using your own OCU:
You should verify the maximum yaw rate set point as well as verify that you are indeed sending the platform the appropriate value for 1 in the yaw rate command (0x3F800000).

The RMP is a skid steer platform it cannot close the loop on yaw rate!!
So if there is any load on the wheels or any slip in the system the drain applied to the integrator will reduce the actual yaw rate to less than the command. The differential_wheel_vel_rps is exactly that: it is the measured differential speed of the wheels. In a skid steer platform that doesn't correlate to body yaw rate.
Craig Shaffer
Principal Engineer

STANLEY INNOVATION, INC
www.stanleyinnovation.com

kmelotti
Posts: 3
Joined: Wed Oct 24, 2012 8:57 am

Re: How is the turn rate of the RMP computed?

Post by kmelotti »

We made our own OCU which I verified the max yaw rate is at its max (4.5) and the value being sent for the yaw command is correct (we recorded the yaw_rate_target_rps as it was running).

So what is the turn rate based on? Because even on blocks (no load on the wheels) the differential_wheel_vel_rps is still short of what the target value is. I understand that while loaded there could be issues with the closed loop but shouldn't it be accurate while unloaded?

We conducted another test at a different turn rate while the RMP was on blocks (no load):
yaw_rate_target_rps,vel_target_mps,fram_tire_diameter_m,fram_wheel_track_width_m,differential_wheel_vel_rps
0.915527,0,0.915527,0.483616,0.739136

phussey
Posts: 79
Joined: Fri Apr 20, 2012 8:12 am

Re: How is the turn rate of the RMP computed?

Post by phussey »

The controller tuning for the 440 is was un-intentionally left soft in one of the earlier releases. Could you please respond with the build ID for the Segway processor and the serial number of the platform?

There will be a new release package posted on the forum next week sometime to address a few of the issues we have been tracking and to add support for the OMNI and balancing platforms.
PATRICK HUSSEY
Principal Engineer

STANLEY INNOVATION, INC
www.stanleyinnovation.com

phussey
Posts: 79
Joined: Fri Apr 20, 2012 8:12 am

Re: How is the turn rate of the RMP computed?

Post by phussey »

The new package is ready but there is an issue with loading a large file to the forum. The issue is being fixed and it should be up soon.
PATRICK HUSSEY
Principal Engineer

STANLEY INNOVATION, INC
www.stanleyinnovation.com

phussey
Posts: 79
Joined: Fri Apr 20, 2012 8:12 am

Re: How is the turn rate of the RMP computed?

Post by phussey »

PATRICK HUSSEY
Principal Engineer

STANLEY INNOVATION, INC
www.stanleyinnovation.com

kmelotti
Posts: 3
Joined: Wed Oct 24, 2012 8:57 am

Re: How is the turn rate of the RMP computed?

Post by kmelotti »

Thanks for the update!

phussey
Posts: 79
Joined: Fri Apr 20, 2012 8:12 am

Re: How is the turn rate of the RMP computed?

Post by phussey »

I hope you find it useful. I will be uploading a diff of the interface shortly so changes are clear for customers who have written custom software. I will post when it is available.
PATRICK HUSSEY
Principal Engineer

STANLEY INNOVATION, INC
www.stanleyinnovation.com

phussey
Posts: 79
Joined: Fri Apr 20, 2012 8:12 am

Re: How is the turn rate of the RMP computed?

Post by phussey »

The interface diff is now posted under the release announcement which should guide users with older firmware through updating their custom software to the new version of the interface.

http://rmp.segway.com/viewtopic.php?f=9&t=70#p171
PATRICK HUSSEY
Principal Engineer

STANLEY INNOVATION, INC
www.stanleyinnovation.com

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