Experimental Physics and
Industrial Control System

Mark Rivers <[email protected]> · Tue, 6 Jun 2000 18:53:31 -0500

Leonard J. Reder asked a question the other day about the PID record in EPICS.

I tried using this record some time ago and discovered some serious problems
with it.  I have documented these at:
http://cars.uchicago.edu/software/epidRecord.html#Problems

As a replacement I have written an Enhanced PID record (EPID).  Some of the
features of this record compared to the standard EPICS PID record are:

- The PID expression is computed as an absolute number, rather than a
  differential number to be added to the present output value. This simplifies
  database construction, and also permits the record itself to perform limit
  checking on the output. This is the most important enhancement, because the
  standard EPICS PID record simply cannot be used to directly control a real 
  analog output record in incremental mode if the user is operating within a
  proportional band.

- Separation of device support from the record. There is Soft Record device
  support which uses EPICS database links, and this is very similar to the PID
  record. However, the EPID record can also be used with other device support,
  for example to communicate with faster feedback software, or with hardware
  controllers. Device support is provided in the Message Passing Facility for
  fast feedback (> 10 kHz) using an Acromag IP330 ADC and a Systran DAC128V 
  DAC. 

- Addition of many fields (OUTL, DRVH, DRVL) to simplify construction of
  databases 

- Sanity checks and limits are placed on the magnitude of the integral term 
  (I) which are lacking in the PID record. 

- Monitors are posted for the CVAL field, which simplifies construction of
  user-interface tools, such as plotting. 

- The CVL field has been renamed INP. This field can now be modified (a feature
  of EPICS R3.12 and higher), so that a single EPID record can be used to
  control different processes at different times. 

- Changed the time units of the KI and KD terms from minutes to seconds 

I am using the EPID record in the following applications:

- Stabilization of the angle of the second crystal in a double-crystal 
  monochromator

- Furnace temperature control in a multi-anvil high-pressure apparatus

- Laser power control in a laser-heated diamond-anvil cell

Documentation and source code are available at:
http://cars.uchicago.edu/software/epidRecord.html

____________________________________________________________
Mark Rivers                             
Argonne National Laboratory             (630) 252-0422 (office)
Building 434A                           (630) 252-0405 (lab)
9700 South Cass Avenue                  (630) 252-1713 (beamline)
Argonne, IL 60439                       (630) 252-0443 (FAX)

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System