Experimental Physics and
Industrial Control System

Hello to everyone at tech-talk,

I am currently working on implementing an EPICS driver for controlling axis which are stepper motors with absolute encoders.
My hardware controller is a Beckhoff PLC I am programming to interface with. I am planning to use mm as EGU.

I have read multiple documentation and I have started to implement a driver based on the Model 3 drivers, I have heavily inspired myself from the MCB4B Driver for ACS and the Phytron driver.

I am starting to get a better understanding on how it works but I still need clarifications on some points. Would you be so kind to help me on those matters?

 

First of all,

• I see that when creating an axis controller using the class asynMotorController, it creates a lot of parameters then linked to a motor record by their names (the string passed to the createParam() function), is that correct?
• How does each axis has its own set of parameters while connected to the same asynMotorController, is it because the asynMotorAxis class creates an AsynUser which connects to the controller?
  By doing this, it creates a separate set of parameters for the axis with the pasynUser_?

 

• I heavily rely on the record programming to carry out advanced functions, such as backlash compensation and closed loop motion on the encoder.
  Do you have experience in those matters? How will the system reach a specific encoder position? Will it use a linear term between encoder ticks and motor steps?
  But if the axis is not linear enough, there will be issues in reaching the position?
  If I understand correctly, usually motors use the FRAC and RTRY mechanism to reach a encoder position with a certain number of retry?
  In practice, how do most people operate with an encoder? Do they use  the motor in open loop and monitor the encoder, or do they set UEIP at 1 and use FRAC motion?

 

• My motors are able to do microstepping (decomposing a full step in 64 microsteps), should I implement this in the MRES field of the record, as not being 200 full steps but 12800 microsteps?
  I think so because I intend to give the motor positions in microsteps. Is there a low limit on the MRES value?

 

• On the system, we will be using absolute encoders, in that regard the encoder ratio is always 1 to transfer to encoder counts, correct?
  From my understanding, encoder ratio is mainly to interface with incremental encoders converting to raw A/B ticks in full encoder tick.

Because from what I gathered the motorEncoderPosition_ must be linked to the Field REP and is the hardware counter of the encoder.

• Is there a filter of some sorts on the encoder counts field or should I implement one directly on my Beckhoff controller and then use the delayAfterMove parameter to allow the encoder counts to stabilize?
• As I have absolute encoder on all axis, for an axis the parameter motorhasEncoder_ is always at 1 as well as the motorStatusHomed_ . However, to use and encoder, the record needs to have the record field UEIP at 1.

Is it defined by the user, or should initialize UEIP at 1, in my template defined in “motor.substitutions.irelec” adding the field UEIP to my .db file (which is derived from basic_asyn_motor.db with some field added)?

 

• From what I understand, the motor record is defined by the motorRecord.cc and motorRecord.dbd and Model 3 drivers mainly interface with the devMotorAsyn.c, is it correct?
  A change on a field of the record will be processed by motorRecord.cc and use the different functions I have redefined for my Driver (move, poll, etc…), is it correct?
  Does the devMotorAsyn.c defines the support entry table required to process the record commands? (with the function start_trans, build_trans and end_trans)
  Looking through the code, I understood that for Model 2 Driver and Support, one needs to defined the support entry table to process the record commands? (ex: MCB4B_build_trans function)

In Model 3, it seems the asynMotorController directly handles the modification of its parameters by calling the needed function of an axis. How does the record interface with the asynMotorController parameters?

I can’t find where devMotorAsyn.c writes to the asynMotorController parameters.

Bottom line of this topic is: Can I use the record advanced functions such as backlash compensation even with a Model 3 Driver?  

 

• The record always feed steps command to the controller if I understood the code correctly.
  However as I don’t have a home routine (due to absolute encoders), the open loop (step count) is reset at each hardware controller power reset.
  Thus, the user should never do move_absolute in openloop. To disable it, I would like the UEIP to be always at 1? Or something should be done to set the open loop position using setposition based on the absolute encoder at the start of the controller?

It could be the home procedure, just sending a setposition() to the stepper based on the encoder position? Or does the home position require motion?

 

• I noticed there is no process of error catching on the motor and error resetting, is that correct?
  Should I always reset an axis in Error on my Beckhoff hardware controller or should I include command on my EPICS driver to do so, thus adding record for those specific actions?
  Because for example, if my motor reaches a hardware limit switch while in motion it will put itself in error state, which then needs an error reset using the MC_Reset.

 

I need it’s a lot of questions, I hope it is clear enough.

In any case, thank you for your time and help.

 

Paul-Elie GUILLOT

Technology Manager in Automation and Robotics

Téléphone: +33 (0)4 76 44 98 93

 

 

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