User Program - While at the Beamline - EPICS and Motion Controls

The lower level of GM/CA control system is constituted by EPICS input-output controllers (IOC). Generally we have two VME-type IOC per beamline, one embedded IOC for bimorph mirrors power supplies, and Linux IOC providing communication with MAR CCD detector and frame acquisition. The electronics installed at the beamlines and controllable via EPICS includes: Turbo PMAC2 VME intelligent motion controllers by Delta Tau, SIS3801 Multichannel scaler, Joerger VS64 latching scaler, Systran DAC128 digital to analog converter, Acromag IP330 analog to digital converter, AVME9440 Digital I/O, XY542 combined DAC/ADC, Keithley-428 Current Amplifier, Canberra NIM modules (556 AIM, 9635 ADC, 2016 AMP/TCA) for fluorescent detector.

Our main EPICS development platform is Redhat Enterpise Linux and the VME IOC are operational under VxWorks. The EPICS kernel and drivers are compiled on Linux using the cross-compiler produced at the ORNL.

The core of the whole control system are Turbo PMAC2 motion controllers that we made availvable in EPICS. The advantages of these controllers are (see the picture below):

• Distributed controls allowing to place parts of the controller called Macro Stations close to motors (only two fiber links have to be layed instead of dozens of cables running from the control rack into endstation).
• Support for virtually any type of motor -- steper, servo, brushless servo, piezo, and so on. We only need one type of motion controller for all three beamlines.
• Support for many types of encoders, so that all motors including steppers can be encoded thus enhancing controls reliability.
• Support for coordinated motions: controller can synchronize motion of several motors on the fly.
• Option for 24-channel digital I/O that allows to synchronize motion with other beamline events
• Programmability allowing to write custom motion trajectories and custom servo cycles.

Using these controllers we implemented on-the-fly scans for beamline alignment. The scans are partially based on an open-source project formerly carried by Elena Kondrashkina and Sergey Stepanov while they were employed by BioCAT. The package consists of a number of progamas witten in C language with user-friendly GUI frontend developed in Java.

As illustrated by the scan plot, the actual motion time of the shutter is about 12.5 ms and the beam blocking occurs within 2.5 ms.

> JBluIce - EPICS source distribution