Modbus input for absolute encoder position?

Hey guys I'm preparing to update some drives and have decided to go with position control and the motors will have 20bit absolute encoders. The drives are capable of popping the absolute encoder position of the motors which it started can be used to send to the controller via modbus so that the controller can know the machine position at startup.

My question is to discover if this is something Linuxcnc can take advantage of? I have a 6i25, 7i76, and 7i77 to work with. Any thoughts?

Thanks!!


Chris

20 bit absolute encoders are typically not multiturn encoders so the absolute position is
not directly useful to LinuxCNC, If the drives have a simulated encoder interface with index
for tge controller, that will allow homing to index.

Here is the except from the drive manual. Unfortunately I don't understand it all to well..lol.

"11.3 Absolute encoders are used

11.3.1 Functional description

Using the servo motor with absolute encoders, the absolute value detection system can be built from the host device. With the absolute value detection system, it is not necessary to perform the home return operation each time the power is turned on. This function is based on MODBUS communication to read the absolute encoder number and position data, the upper device for processing control to achieve absolute encoder-related functions."

I also saw some blurbs in the manual about multi turn but I wasn't sure what it meant.

The manual file size is too late to attach, PDF format.

Chris

OK if the drive manual says it can avoid homing than it must have multiturn absolute encoders
I do suspect it would be a project to implement absolute encoder operation with non-reatime modbus initilaization
of position and then switching to quadrature for real time position feedback

Do the drives support any real time absolute position interfaces ( SSI, BISS etc )?

That is above my pay grade...lol. I shot you the manual via email. My plan was to use position control with the loop closed at the drive not linuxcnc. I was hopeful that with modbus the initial position could be sent for each servo and linuxcnc recognize the position to reconcile machine reference. Granted I'm very new to modbus and linuxcnc, so I'm just poking around trying to gather some knowledge to work with..

Chris

PCW wrote: OK if the drive manual says it can avoid homing than it must have multiturn absolute encoders
I do suspect it would be a project to implement absolute encoder operation with non-reatime modbus initilaization
of position and then switching to quadrature for real time position feedback

I don't think it would be that bad, actually.

1) A user-space Python module is loaded at startup, this pulls the data out of ttyUSB0 (or wherever the Modbus dongle ends up)
2) The three axis absolute positions are made available on HAL pins and added to the encoder module positions using a realtime sum2 module per axis.
3) The INI file specifies HOME_ABSOLUTE_ENCODER = 2 (New feature, 2.8 only) linuxcnc.org/docs/devel/html/config/ini-...t__lt_num_gt_section

I have a vaguely similar system on my lathe (with single-turn absolute resolvers) where the axis positions are saved to file on shutdown and then used to derive a full-turns count added to the resolver feedback.