Implementing Feedforward/Feedback Motion Profile with Talon SRX

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Originally Posted by sraque

You're welcome. Just to be clear, my paper does not cover the "buffered" motion profile capability of the Talon SRX. We did not get a chance to try that before releasing this paper, although Omar from CTRE did provide the essential parts of an example during the latter part of Beta testing. I know he plans to have a separate user manual for that feature soon.



In our case, we did log data to a file. Time-stamp, throttle value, and speed as measured by the Talon SRX. I wanted to see how everything behaved. I took the image for illustration when I was building the document. The easiest way for you to get that value is to drive at full speed and then refresh the web page. If you don't have the room to do all that at once, you can grab the logging VIs from our Marvin T code post or build your own.

There is one error in that document that I need to fix and repost. I said to take the cycles per rev from the spec sheet and multiply by 4. This is not correct. The help screen in labview for the configuration VI is confusing. It says:

"Allows setting the Counts Per Rotation of the Talon SRX's connected quadrature encoder. This is also referred to as the 'Number Encoder Lines', 'Counts Per Revolution', and 'Cycles Per Revolution'."

It turns out that the number you send to the config should be Cycles Per Revolution (128 for my Grayhill encoder). The Talon SRX multiplies it by 4. To me, this help text includes contradictory terms. Cycles do not equal counts or lines.

Now, you would rightly say "Dang didn't he notice a 4 to 1 error?" Well, unknown to me at the time, the encoder in our gearbox is not on the wheel shaft. It has a 4.14:1 ration with the wheel. Divide by 4 and multiply by 4.41 and you don't notice much :-) So in the end, my number ended up being 565 fed into the Config VI. Beware a shifting transmission! This ratio will probably change with gear shifts, but you can just change it on the fly in code depending on the status of the shifter.

The end performance of this approach is striking. Tuning the PID gains was really easy (just as the Poofs claimed in their presentation). Very tolerant of values (you see my round numbers..no fine tuning needed). This controls our wheels to within a tenth of a rotation or less every single time.

Hi Steve
How did you get that .97 throttle value for the right side? Having trouble understanding.