DC servo Tacho generator

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01 Mar 2021 18:44 #200717 by Henk
Hi
I've been asked to retrofit a Matsuura VMC with linuxcnc.

The current system is a Fanuc 6m with Gettys motors and Fanuc A06b-6047 DC drives.

These drives need analog DC feedback for speed typically from a tach but in this setup Fanuc fitted incremental encoders on the motors. One of the Fanuc boards does a F-V conversion and sends the analog signal to the drive.

There are a couple of sollutions and the easiest is probably to use two 7i77's and use the analog out on the 2nd to provide the analog speed feedback.

I was just wondering what would happen if I short the velocity ref input on the drive and provide the analog signal for commanded velocity from lcnc to the tach input on the drive? The drive would want to keep the motor stationary but it should run when the tach input is non zero?

Would this work?

Probably a stupid idea....

Henk

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01 Mar 2021 19:44 #200720 by Todd Zuercher
You wouldn't need to buy two 7i77 cards, if you only need more analog outputs, you could simply add a 7i83 Smart Serial card. The 7i83 is basically the same 6 analog outputs as are used on the 7i77 by them selves without the rest of the 7i77's goodies. (the analog outputs on the 7i77 are also just an internal smart serial connection.) Now if you have a reason to need the additional 6 encoder inputs and 48 issolated IOs, then you might still want to buy the 2nd 7i77.

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02 Mar 2021 04:21 #200757 by Henk
Replied by Henk on topic DC servo Tacho generator
Probably have to add a 7i74 and a 7i70 as well...

Was just wondering what would happen...

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02 Mar 2021 14:45 #200788 by Todd Zuercher
Well like I said if you need more io and the 32in+16out would be enough inputs then two 7i77s ($398) would be cheaper than 7i77+7i74+7i83+7i84 (or 7i70) ($426). But it all comes down to exactly what do you need, If the 64 inputs or 32 outs isn't enough for you, then other combinations might be more economical.

There is also some debate over how well this might actually work. I had researched it some but never actually tried it, and the machine I was considering it for has since been cut up for scrap.

A better option may be to find an encoder to tach generator (if anyone still makes them.) Or set up the drives to run in torque mode, or simply replace the drives.

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03 Mar 2021 11:38 #200892 by johnmc1
Replied by johnmc1 on topic DC servo Tacho generator
Good Day ,
I am at present trying to fit linuxcnc to a matsuura also and am trying with problems, to use the mesa 7i83 analog output for the tacho input .
cheers john

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03 Mar 2021 23:55 #200962 by andypugh
Replied by andypugh on topic DC servo Tacho generator
Do the problems seem that they can be solved, or are you advising against this approach?

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04 Mar 2021 03:54 #200990 by johnmc1
Replied by johnmc1 on topic DC servo Tacho generator
andypugh wrote,
"Do the problems seem that they can be solved, or are you advising against this approach"
No
I believe the problem (spurious signals) can be solved, because it is working up to a point .
At present, I have started to connect up the scope and check the fanuc encoder output . to the linuxcnc .
I will update.
cheers john

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04 Mar 2021 09:28 #201007 by tommylight
Replied by tommylight on topic DC servo Tacho generator
What voltage does the tacho input require?
What voltage can the 7i83 output?
All the velocity servo drives i worked with have tacho outputs of 12-15V at 1000RPM, so at 3K or 4K that makes 45 to 60V+-, or 120V total, i doubt 7i83 can output that, but pretty sure drives have adjustments for that, not sure to what extent.

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04 Mar 2021 10:26 #201010 by johnmc1
Replied by johnmc1 on topic DC servo Tacho generator
The 7I83 is remote isolated six channel +-10V analog output card intended for analog drive interfacing or other +-10V analog output applications. The analog outputs,drive enables and field power (8-35v) are galvanically isolated from the communications link

From my hal file
################# x-TACHO-FB

#lead screw pitch, 10 mm
#encoder 100000 pulses / rev
#x-axis scale = 1000 pulses /mm
#scale = tacho feedback ratio of, 6volts/1000rpm, (max rpm required 1200)
# rev/sec = 16.7
# analog scale(scalemax?) * 10/6 = 278 (uses ratio of 10v +/-
# as i understand scalemax, assume x-vel-fb ( from encoder input mm/sec) = 27.8
# then 27.8/278 =1/10 scalemax output = 1/10 of 10v =1v at analogout0
#####


setp hm2_5i24.0.7i83.0.4.analogout0-maxlim 278
setp hm2_5i24.0.7i83.0.4.analogout0-minlim -278
setp hm2_5i24.0.7i83.0.4.analogout0-scalemax 278

net x-vel-fb hm2_5i24.0.7i83.0.4.analogout0

net mcc-x hm2_5i24.0.7i83.0.4.analogena0

###########

My problem max jog 500 mm/min -ve ,then following error.
max jog 2000plus mm/min +ve
cheers john:(

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