sysId for other Characterizations

[narmstro2020]

So I guess one could argue that the most used feature of sysId is motor characterization with drivetrain probably being a very close second. I could be wrong.

Anyhoo. I know from discussions with others on either CD or github that you can use sysID for FOC characterization (torque/current) by passing in current for voltage.

What I want to know is this a general case for all systems that can be reduced to a model similar to that of the DC motor. I'm guessing so. I'm wanting to characterize a PID loop where velocity is an input and position of the commanded mechanism is the output.

[calcmogul]

SysId fits the model dx/dt = Ax + Bu + c where A = −kᵥ/kₐ, B = 1/kₐ, c = −kₛ/kₐ sgn(x). x and u can be whatever. The fit should work for a system with x = position and u = velocity. The ideal system would have A = 0 and B = 1 → kᵥ = 0, kₐ = 1, but friction exists, so they'll be a bit different in practice.

[narmstro2020]

And so I can then take the output of a sysIdRoutine run and dump it into sysId with measured voltage, velocity, and position as velocity, velocity, position.

When characterizing with PID in the drop down fields in the analyzer I would choose velocity to get P and D. I would guess that position wouldn't be helpful from the dropdown?

[calcmogul]

dump it into sysId with measured voltage, velocity, and position as velocity, velocity, position

Should be velocity, position, don't care (the position field isn't used).

Since your system is a single integrator, the "velocity PID" option is what you want.

[narmstro2020]

Slight addendum. The is a key difference between measured velocity and driven velocity. My u will be the velocity I'm using as input while dx/dt will be the output velocity. Unless I'm wrong about that.

[calcmogul]

dx/dt is the derivative of x with respect to time, so if u is velocity, A = 0, and B = 1, then dx/dt must be acceleration.

[narmstro2020]

You're right. I was mixing up the version of the FF equations where the input is on the left side (voltage) and everything else on the right with the one where dx/dt is on the left.