7i80 -- tracking a 23bit shaft encoder

15 Mar 2017 14:38 - 15 Mar 2017 14:48 #89664 by jCandlish
Does there exist a quadrature divider for the 7i80 firmware that can downsample an encoder input?

What is the practical bandwidth limit with perfect signal integrity at the 7i80 input header.

Above the most conservative shaft speeds I don't get any counts.


Do I understand correctly that the 10Mhz limit of the connected 7i47 implies 40Mhz quadrature counts at the 7i80 which for a 23 bit encoder limits the shaft speed to < ~ 290rpm?

40000000/(2^23) ~= 4.8 r/s ~= 290rpm
Last Edit: 15 Mar 2017 14:48 by jCandlish. Reason: added
15 Mar 2017 16:30 - 15 Mar 2017 16:30 #89676 by PCW
A 23 bit encoder is not likely a quadrature encoder and even if it is, its outputs are probably limited to 10 MHz or so maximum by line driver/cable characteristics.

Most high resolution encoders use absolute (serial) position data transfer, Either SSI, BISS, Fanuc I, Fanuc II, Yaskawa or Misubishi formats for just this reason (too high quadrature data rates), plus the absolute number can be used for commutation
Last Edit: 15 Mar 2017 16:30 by PCW.
15 Mar 2017 18:24 - 15 Mar 2017 18:26 #89680 by jCandlish
The servo has a Tamagawa 23-bit absolute serial encoder. The amplifier presents the shaft position in relative quadrature.

Yes. The amplifier is also bandwidth limited.

I am trying to get the documentation from the amplifier manufacturer for a divisor parameter for the shaft position output.

So far ... no joy.
Last Edit: 15 Mar 2017 18:26 by jCandlish. Reason: absolute -- relative
15 Mar 2017 23:11 #89700 by PCW
It should be fairly easy to find the existing setting of quadrature counts / turn by
disabling the drive and turning the shaft 1 turn. It seems unlikely that the
simulated encoder output is more than 100000 counts per turn or so...
16 Mar 2017 06:07 - 16 Mar 2017 06:07 #89729 by jCandlish
The simulated output is 1:1, or 2^23 per turn. 8388608 counts!

I have not been able to find a divisor parameter.

The amplifier does have two encoder inputs however. The 2nd input is for a spindle mounted encoder. So it may be possible to inform the amplifier of the rotor/stator angle with this 2nd encoder derived and simulated from the FPGA by reading the Tamagawa serial input.

If there was a hostmot2 Tamagawa serial component.
Last Edit: 16 Mar 2017 06:07 by jCandlish.
16 Mar 2017 13:47 #89740 by PCW
That's bizarre, and really makes no sense for a servo amp
what is the motors maximum speed?

Is there no absolute interface output from the drive?
16 Mar 2017 14:16 - 16 Mar 2017 14:21 #89741 by jCandlish
It is bizarre indeed.

The servo amp is a universal type (synchronous/asynchronous) to 15000rpm. TTL, Serial and resolver type encoders are supported by daughter board plug-in.

The servo's max rated speed is 2000rpm.

There are EtherCat and Mechatrolink interfaces that are potentially able to access the absolute position of the servo from the amplifier. A path I chose not to pursue at the onset of this project, but now I am looking into EtherCat for LinuxCNC with rt-preempt.

I was hoping the rabbit hole didn't go so deep.

Last Edit: 16 Mar 2017 14:21 by jCandlish.
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