Re: Position vs. Velocity
Posted by
Jon Elson
on 2000-11-29 22:17:22 UTC
Mariss Freimanis wrote:
But, with a digital encoder, there is neither continuous position resolution nor
continuous position readout! You have position quantized both by resolution
AND by time! So, you CAN NOT have instantaneous position. Whichever is
the lesser of the sampling rate or the count rate is your effective sampling
rate. Note that when the count rate is lower than the sampling rate, you
have a system that effectively has varible time quantizing, which adds some
additional problems of loop stability. But, anyway, you cannot have more
bandwidth than half this minimum count rate, by the Nyquist principle.
(Maybe I'm making a mistake here, but it seems right to me, for after
Midnight!) With a DC tach or other continuous time method of measuring
velocity, you do not suffer this bandwidth limitation.
You CAN control position by controlling velocity at a high bandwidth,
and correcting position at a lower bandwidth! The velocity loop keeps
the system 'on course' between position sampling cycles.
Jon
> --- In CAD_CAM_EDM_DRO@egroups.com, Jeff Barlow <jeff@b...> wrote:<snip>
>
> > Plus, I'm still hung up on the idea that instantaneous position is
> the
> Actually position is what you want to control.<snip>
But, with a digital encoder, there is neither continuous position resolution nor
continuous position readout! You have position quantized both by resolution
AND by time! So, you CAN NOT have instantaneous position. Whichever is
the lesser of the sampling rate or the count rate is your effective sampling
rate. Note that when the count rate is lower than the sampling rate, you
have a system that effectively has varible time quantizing, which adds some
additional problems of loop stability. But, anyway, you cannot have more
bandwidth than half this minimum count rate, by the Nyquist principle.
(Maybe I'm making a mistake here, but it seems right to me, for after
Midnight!) With a DC tach or other continuous time method of measuring
velocity, you do not suffer this bandwidth limitation.
You CAN control position by controlling velocity at a high bandwidth,
and correcting position at a lower bandwidth! The velocity loop keeps
the system 'on course' between position sampling cycles.
Jon