Re: [CAD_CAM_EDM_DRO] To convert Quadrature Encoder signals to Tachogenerator signals

Yes , my aaplication is a DC servo motor for X-Axis of
a turning center .Actually the motor and Drive
(A20B-0007-0361) are both from Fanuc , The Drive have
3 different modes of operations:

1) Tacho 3V
2) Tacho 6 V
3) Pulse Encoder +F/V

In my machine the servo motor does not have the Tacho
and the drive was running in 3rd mode, the F/V logic
was implemented in orignal CNC control, which I am
replacing with Powerautomation GmbH CNC control.

Pete and Jon Elson are right about using the F to V
circuit, The main problem for me is how to convert the
sign of the analog voltage generated by the F/V, Jon,
you have recommended some "Linear inverter" and
"quadrature to step/direction converter " for this
purpose ,could you please give me some link to where I
can find these circuit schematics?The Pulse coder
installed in the motor is of 2500 cpr.

before this discussion ,I myself was planning to use
the drive in mode 1 or 2 and for this purpose to use
2 F/V ICs with a 4066 analog switch for the sign
conversion of the analog voltage, but I don't know
whether this circuit will be fast enough to meet the
servo's requirement.

Thanks and regards

Babar

--- Jon Elson <elson@...> wrote:

> Bot Great wrote:
>
> > I assume that you have two square waves (in
> >quadrature) coming from your encoder and you want
> an
> >analog signal proportional to the frequency of the
> >square waves which is proportional to the rotation
> >speed of the encoder shaft. What you are looking
> for
> >is a frequency to voltage converter. Analog Devices
> >has an IC, the AD650, which will do what you want.
> >
> I believe the OP needed a bipolar output, as he was
> going to
> be using this with a 4-quadrant axis servo. Also,
> to get the
> best performance, it needs to sense all transitions
> of the
> encoder, otherwise the axis could drift up to two
> encoder
> counts before movement is sensed. So, it would take
> a
> quadrature to step/direction converter (pretty
> well-known
> circuit), the F-to-V converter, and a linear
> inverter to flip
> the polarity of the velocity signal when the
> direction is
> negative.
>
> The whole problem with this approach is that to
> produce
> a smooth velocity signal at constant speed, there
> MUST,
> mathematically, be delay in the system. The servo
> drive
> was constructed to not expect ANY delay in the
> tachometer
> signal, as a DC tachometer is totally free of delay,
> except
> that which might be caused by a filter to remove
> brush noise.
> So, the servo drive will, absolutely, require
> re-tuning after
> an encoder to velocity converter is installed.
>
> The major machine tool builders use extremely high
> resolution
> encoders, often 100,000 counts/inch and above, to
> combat this
> problem. This greatly reduces the interval between
> encoder
> pulses even at low velocity, reducing the need for
> smoothing
> delay.
>
> Just to give some data points, I have a true
> velocity servo on
> my mill, with DC tachometers feeding a traditional
> 3-loop
> control system. (Position, velocity and torque) I
> tested it to
> see where the motion broke down into stepwise
> movement, and
> it came out about .01 IPM. You have no sense that
> it is
> actually moving except for the screen position
> display. This
> worked out to about 3 encoder counts/second. The
> tach output
> was calculated to be 7 micro-volts (no way I could
> measure such
> a small signal). But, if I took a screwdriver and
> shorted out the
> tachometer, the axis started to vibrate. So, the
> differential amps
> in the servo amplifier were accurately measuring the
> techometer
> feedback, and keeping the loop closed.
>
> Jon
>
>
>
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