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

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:

> 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.

>

Confused here Jon. The voltage from the tach would go from zero to
several volts in a fraction of a second also. How do you close the
loop with that? Or are you saying that any DAC used would be too
slow? Also, I don't see how a frequency to voltage converter would
be discontinous...at 0 hz you get 0V...same with a tach. At 0 rpm on
the motor I doubt your tach is putting out voltage, unless it is
violating some laws of physics/electronics.

> 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.
>

*shrug* delta sigma dacs are 24 bits (16 million to 1) and have a
response time of at least 30khz for the one I have in mind. Are you
sure the lines from your tach are not picking up noise greater than
7uV? I can't even measure 7uV very accurately on my lab bench with
my $$$ oscilloscope. Also, it may be feasable to forgoe using a DAC
and just pwm an RC buffered by an op-amp...depends on the clock speed
of the Xilinx part as to how many bits of resolution you can get vs
response time. Something on the order of 16-18 bits and 1khz should
be doable.

Curious what the brush amps that use encoder input for velocity
feedback are doing inside? Something similar?

Rick