On 11/06/2017 11:37 PM, Gene Heskett wrote:
Greetings;
Some things in the man 9 pid page are too well hidden in the
mathematical "jargon".
So can we clarify a few things?
like:
pid.N.command-deriv float in
The derivative of the desired (commanded) value for the
control loop. If no signal is connected then the derivative will be
estimated numerically.
What for instance, would/should be connected here, and what would be the
desired result of such a connection? And in this context, what is a
derivative?
In a positioning servo, if you have a way of sensing
velocity (such as a tach + ADC, or derive velocity from
encoder timestamps) this can provide less noise than
deriving velocity by subtracting current position from
previous position.
A number of encoder systems (software encoder, Mesa and Pico
System UPC and PPMC) can provide this.
Coarse and noisy velocity gets multiplied by the D parameter
and adds a lot of noise to the velocity command output.
With nothing connected to this HAL pin, then actual velocity
is derived from subtracting position, and you get all the
noise of quantized position plus quantized sample time.
Like:
pid.N.error float out
The difference between command and feedback.
This, on my G0704 seems to be a constant zero at any gain setting of the
halscope. What is this trying to tell me? And yes, its suitably
addf'd. :)
Well, on a positioning servo, this definitely gets a
non-zero value, although it SHOULD be quite small. I have
following error fault set to .005", so I know it is less
than that, and I tune to try to get all unloaded moves down
to 200 uInch or so.
I have no idea what it would do on a speed servo, such as
when used for the spindle.
Jon
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