PID Tuning

Technical Technical Discussion
1

We are tuning a velocity PID for a simple flywheel attached to a launcher. It is one Falcon 500 feeding into a gearbox ratio of 11:10.

Here are three different things we tried with Phoenix Tuner X:

  1. VoltageOutput with setpoint of 8v we get to 37 rps
  2. Using Ks, Kv, Ka, of 0, and a kP of 1, setpoint is 37, we get to 12 rps
  3. Using Ks of .7, Kv of .2, Ka of 0, kP values between .1 and 10, setpoint is 37, we get to 2 rps

We used Voltage Output to get 37 rps at around 8 volts. The mechanism started to move without stalling from friction at around 0.7 volts with was used for static friction.

Are we missing more input?
Is there a term that needs to be adjusted?

2

With kv of 0.2 and setpoint 37 your motor output should be 7.4 volts, can you check what output does the talon fx apply, it should be at least 7 volts.

3 
In a Velocity closed loop, the gains should be configured as follows:

Ks - output to overcome static friction (output)
Kv -  output per unit of requested velocity (output/rps)
Kp - output per unit of error in velocity (output/rps)

I had this very discussion yesterday or recently in a different thread. Basically, this page helped me a lot in my understanding.

But, it is the case that if you observed 8 volts at 37 rps, then a feed forward Kv of around .215 would be in the ballpark of 8v of output…which should bring you back to 37 rps (or so).

Did your mechanism really need .7 volts to overcome static friction? That seems like a lot.

4

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