AMC servo amp and encoder feedback

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06 Aug 2012 13:32 #22906 by Zig
AMC analogue servo drive has velocity feedback in the form of quadrature channel A and B signals.

The feedback arrangements converts the quadrature signals into +/-10V analogue of velocity.

My problem is the frequency to voltage circuit generates 1 V/ 22KHz.

Maximum pulse rate I can expect is under 10KHz. Clearly an unsatisfactory state of things. I have no way of altering this encoder at present.

I have a number of options:

a) Interpose a microprocessor to modify the feedback rate

b) I think I may be able to modify the hal file to incorporate a quadrature encoder whose output will drive the velocity feedback path

c) modify the frequency to voltage circuit on board the servo amp to better match the available encoder signal.

Option c) seems like the most likely to provide a decent solution. The other two are a bit more difficult to achieve.

Any other solution out there?

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06 Aug 2012 14:06 #22909 by skunkworks
Can you give us a bit more info? I might not understand what you are saying. I know some versions of amc drives allow encoder for velocity feedback. I have never used this option on any of ours. I have used actual tack feedback for velocity. I am moving away from that and using mesa encoder velocity to drive an analog output for velocity feedback.

I would try these options..

1) don't use velocity feedback and put the amp in current or voltage mode and tune accordingly.

2) use emc to output a velocity signal for velocity feedback.

(what are you going to drive the amc amps with? mesa or such?

sam

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06 Aug 2012 20:42 #22919 by Zig
AMC will be driven by a combination of MESA5I20 and 7I33

This provides for the oportunity to use the encoder signals within hostmot to generate velocity and then rescale the quadrature encoder ( again within hostmot ) and output the feedback signal at a higher rate.

I have not done much in the way of HAL programing recently and a hardware mod to AMC is easier to effect.

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06 Aug 2012 23:33 #22922 by andypugh
Zig wrote:

AMC will be driven by a combination of MESA5I20 and 7I33


So where does your 10kHz limit come in then? The 5i20 can read or generate MHz signals.

I am a bit puzzled about what feeds what to what in your system. It sounds like the encoder goes to the amp, and the amp creates a +/-10V velocity signal.
Can't you just ignore that velocity signal? There is no reason LinuxCNC and the drive can't share the encoder.

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07 Aug 2012 00:33 #22924 by Zig
Andy,

I suspect the 22KHz /volt spec came from considerations of rotary shaft encoders sitting on the back of rotary motors which are subsequently geared down through gearbox/screw pitch combination.

In this instance the 10 KHz limitation is imposed by linear encoder. There is no way to gear the reader to give higher resolution.

So given the frequency to voltage slope of 1 V/ 22KHz at 7500Hz ( notional speed ) I can expect 7500/22000 x 1 = .341V as the full tacho feedback signal.

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07 Aug 2012 00:59 #22926 by skunkworks
what we are saying is - don't use the amc drives encoder velocity feedback. Set the drive to voltage or current mode and tune accordingly...

What model is the drive? sounds like a BE15A80 or similar.

sam

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07 Aug 2012 08:11 #22932 by Zig
Yes .. it is a BE15A8

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07 Aug 2012 08:57 #22937 by andypugh
Zig wrote:

In this instance the 10 KHz limitation is imposed by linear encoder. There is no way to gear the reader to give higher resolution.
So given the frequency to voltage slope of 1 V/ 22KHz at 7500Hz ( notional speed ) I can expect 7500/22000 x 1 = .341V as the full tacho feedback signal.


I still don't understand the problem. Can you explain the components of the system and the nature of the signals between them?

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07 Aug 2012 11:57 #22940 by skunkworks
here is the manual..

www.a-m-c.com/download/datasheet/be15a8.pdf

they talk about the the velocity conversion

Velocity Monitor. Analog output proportional to motor speed. In Encoder Velocity mode,
output is proportional to the encoder line frequency. Encoder Velocity scaling is 22 kHz/V.

Could you try and see if it is going to be an issue? you do have loop gain that would adjust for low velocity signal. How much I don't kow...

(still - you could just use the amp in voltage or current mode)

sam

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07 Aug 2012 12:02 #22941 by andypugh
skunkworks wrote:

Velocity Monitor. Analog output proportional to motor speed.


So, the amplifier works on encoder counts, and this voltage is just for velocity monitoring and can simply be ignored?

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