Encoder for axis tacho feed back

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30 Mar 2021 15:22 #204133 by scotth
Disconnect the motor from the drive. Use a 12 volt car battery to drive the motor in both directions. Monitor the tach with your scope and see what is happening. Brushes will wear and hang in the brush holder giving unstable readings also old coolant can give the same problem. Contact cleaner will normally remove any contaminates.
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31 Mar 2021 10:59 #204203 by johnmc1
Good Day All ,thanks for the replies

The problem with the negative direction oscillations applies to all three servo axis.

The motors are fanuc with no encoders, I am trying to simulate the tacho feedback with mesa card, for the servo amplifier.

Any information about tacho simulation would be most helpfull.

I will try a motor with a inbuilt tacho, but as it produces 32v/1000rpm,
will need to sort out a voltage divider to get the needed 6v/1000rpm.

Also having trouble setting up the pc ethernet connection to mesaflash
a 7i80hd card to replace the 5i24 to see if this will fix the problem.


At this point as well as changing to a new servo amp, encoder and all mesa cards except the 5i24 main card.

Have also checked the following,

Motor works perfectly both forward and reverse with 12 volt test

Encoder output cw and ccw checks out ok on the oscilliscope.

PCW wrote
I would disable the drives then verify that you
get correct +and - analog tach simulator outputs
when you rotate the shaft by hand. If you do, the
trouble is not related to the encoder feedback -->
analog simulated tach output path.


With the servo amp not enabled , and the motor turned by hand cw
the linuxcnc displays a positive movement output.
When in the motor is moved in opposite direction the linuxcnc displays a negative direction on the display.

Thanks for your interest and patience.
Cheers john

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31 Mar 2021 11:11 #204204 by johnmc1
In the previous message,I should have added, that the tacho input voltage to the servo, appears to be correct for both sign and voltage.

With the servo amp not enabled , and the motor turned by hand cw
the linuxcnc displays a positive movement output.
When in the motor is moved in opposite direction the linuxcnc displays a negative direction on the display.
Cheers john

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31 Mar 2021 15:31 #204234 by PCW
Maybe the Fanuc Tachometer simulator is unipolar?

You might try running the velocity through the abs component
before feeding it to the analog output hardware so the velocity signal
is always positive
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31 Mar 2021 19:20 #204271 by aleksamc
Have you read my post in the past?

What about polarity of your simulated output to servodrive?
I wrote you that Feadback from tacho always Positive.
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07 May 2021 12:08 #208205 by johnmc1
PCW replied
"Maybe the Fanuc Tachometer simulator is unipolar?

You might try running the velocity through the abs component
before feeding it to the analog output hardware so the velocity signal
is always positive"


I have not been able to run (or understand) the abs component , to get, only positive tacho feedback, and as I do not know where to start.
Any advice or direction will be most appreciated.

At present all I have managed is to add ,
loadrt abs count=1
addf abs.0



net x-pos-fb <= hm2_5i24.0.encoder.00.position
net x-vel-fb <= hm2_5i24.0.encoder.00.velocity
net x-pos-fb => joint.0.motor-pos-fb
net x-index-enable joint.0.index-enable <=> hm2_5i24.0.encoder.00.index-enable
net x-pos-rawcounts <= hm2_5i24.0.encoder.00.rawcounts


net x-enable hm2_5i24.0.7i83.0.4.analogena0
net x-vel-fb hm2_5i24.0.7i83.0.4.analogout0
setp hm2_5i24.0.7i83.0.4.analogout0-maxlim 277.7777
setp hm2_5i24.0.7i83.0.4.analogout0-minlim -277.7777
setp hm2_5i24.0.7i83.0.4.analogout0-scalemax 277.7777

cheers john

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07 May 2021 23:01 #208266 by PCW
loadrt abs count=1
addf abs.0 servo thread


net x-pos-fb <= hm2_5i24.0.encoder.00.position
net x-vel-fb <= hm2_5i24.0.encoder.00.velocity
net x-vel-fb => abs.0.in
net x-vel-fb-abs <= abs.0.out
net x-pos-fb => joint.0.motor-pos-fb
net x-index-enable joint.0.index-enable <=> hm2_5i24.0.encoder.00.index-enable
net x-pos-rawcounts <= hm2_5i24.0.encoder.00.rawcounts


net x-enable => hm2_5i24.0.7i83.0.4.analogena0
net x-vel-fb-abs => hm2_5i24.0.7i83.0.4.analogout0
setp hm2_5i24.0.7i83.0.4.analogout0-maxlim 277.7777
setp hm2_5i24.0.7i83.0.4.analogout0-minlim -277.7777
setp hm2_5i24.0.7i83.0.4.analogout0-scalemax 277.7777
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08 May 2021 10:40 #208298 by johnmc1
Many thanks PCW for your reply.

The Fanuc Tachometer simulator is not unipolar, when using the abs component.
A negative, axis motion command, will result in instant motor runaway.
A positive , axis motion command , will result with correct movement.

I intend to try another servo drive ,motor combination that is known to be correct .

cheers john

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