PID Servo Loop rates

Hi all,

I see that Linux CNC defaults to 1KHz PID rate. 
What do you lose when something like 250Hz is used. 

For the first time in decades, I have been experimenting with this 250Hz on one of my own developments and I found that tuning the loop was a breeze and I don't seem to lose anything. Historically I've been in the 1KHz to 4KHz range. I'm kinda liking this. 

​​​​​​Craig 

If your PC has poor latency, a faster loop could be detrimental to your pid servo loop tuning. I can see how slowing your loop down so that the resolution of latency noise is a smaller relative to the thread speed could give better tuning results.

Hi Todd, 

Yes I can see that which is why I don't use the PC platform. 
I use microcontrollers for motion control and I can achieve ridiculously fast rates but I'm starting with a clean sheet and for fun, I have been playing with this 250Hz rate and pretty impressed. 
Granted, my applications typically involve bigger servos and associated transmission systems and so response time is not super critical but hey, I have never experienced such easy PID tuning. 

Craig 

You lose bandwidth by going to a lower servo PID rate. If that doesn't
matter because the system you're controlling is slow, there is no price
in going for a lower control speed.

With a sampling rate of 250Hz, you can actually measure up to 125Hz. In reality
the limit is usually between 1/5 and 1/3 that frequency before difference between
a sampled and a time continuous system become apparent.

So with 250Hz you're approximately limited to 50Hz of control
bandwidth. To me that seems terribly low. On my own lathe I have a
resonance at approximately 300Hz. Since I have the servo thread running
at 4500Hz (because that's the maximum rate the encoders I use support)
I was able to eliminate that resonance.

On the other hand, what do you lose when going for a higher sampling
rate? The only thing I can come up with is processor time, but that
seems hardly an issue on a modern PC. And I'm doing this on an RT_PREEMPT
kernel, so it's not even Xenomai or RTAI.

The required PID rate also depends on what you are controlling.
 If you are controlling torque, you need a higher PID rate
than if you are controlling velocity (with the drive controlling the
lower level, higher frequency loops)

A typical position mode servo drive may have 3 nested PID (or similar)
loops, from fastest to slowest:

torque command to PWM out
velocity command to torque out
position command to velocity out