Quote:
Originally Posted by Joe Johnson
TI think that the performance of the velocity control would be much enhanced by a fast PID such as the 1000hz loop time that I have heard that the Jaguars use in PID mode.
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From an analytic perspective, I don't necessarily believe that 1000hz operation will yield significantly better response than, say, a 100-200hz velocity loop running on the cRIO. The important thing is that your loop rate is higher (at least 10x higher is the rule of thumb my thesis advisor used) than 1/the time constant of the system you are controlling. When brushed DC motors are connected to a load, their time constant may be on the order of 100s of milliseconds or more (depends on the load's inertia and damping). In other words, a 4 CIM drivetrain with traction wheels geared for 10 fps free speed won't hit (63.2% of) 10 fps instantaneously when you floor the accelerator. Additionally, as you pointed out, getting good velocity feedback at 1khz rates presents a challenge in its own right (I think magnetic encoders are the best way to go here).
Your mileage may vary, and I'd welcome anyone to do empirical studies that show whether 1000hz gives them better dynamic velocity tracking performance than 200hz (for Jags) or 120hz (for Vics).*
* 200hz and 120hz are the maximum input rates for Jags and Vics, respectively.