Re: [CAD_CAM_EDM_DRO] encoder to tach converter idea (was killer deal...)

skykotech wrote:

>the amc 25a8 brush amps I posted about will work fine for my motors with tachs, but it got me thinking. Why not use a small cpld and dac to make a simple encoder frequecy to tach voltage converter? Should be simple and under $10 in parts and would open our options to many of these surplus amps for velocity control. Something to think about anyway...
>
>
>

The reason is that the DC tach is what is called a continuous-time
device. This is good in control
systems, as there is no change in bandwidth at different speeds. An
encoder is discontinuous-time,
in that there is no information provided between encoder counts. This
is not a big deal at high
speed, but it becomes a big deal at low speeds, and standstill. The
slower it is moving, the slower
the updates are produces. A system that depends on constant feedback,
like a servo positioning
system, needs constant velocity information, if it is to respond
vigorously to perturbations, such
as cutting teeth biting into workpieces. It is almost impossible to
truly close the loop when the
sampling rate goes from zero to tens of KHz in a fraction of a second.

I should point out that my analog servo system on my Bridgeport is able
to prevent stick-slip
friction from domination motion down to .01 IPM. That works ou to an
encoder count rate of
about 3 Hz. At that velocity, the DC tach is producing 7 uV of output.
No, I can't measure it,
I can only extrapolate it from the V/KRPM rating of the tach. You'd
need SOME fancy DAC
to handle that kind of dynamic range (in this case, roughly 7 uV to 14
V, or 2 million to 1).
And, you can't depend on filtering, as the servo drifts, using a low-res
DAC, it would occasionally
have to put out a big pulse to restore the servo balance. Otherwise,
the servo would just drift
around open loop.

Jon