The Experimental Physics and Industrial Control System (EPICS) is a software environment used to develop and implement distributed control systems to operate devices such as particle accelerators, telescopes and other large experiments.
EPICS also provides SCADA capabilities. The tool is designed to help develop systems which often feature large numbers of networked computers providing control and feedback.
EPICS uses client/server and publish/subscribe techniques to communicate between the various computers. One set of computers (the servers or input/output controllers), collect experiment and control data in real-time using the measurement instruments attached to it.
This information is given to another set of computers (the clients) using the Channel Access (CA) network protocol. CA is a high bandwidth networking protocol, which is well suited to soft real-time applications such as scientific experiments.
EPICS interfaces to the real world with IOCs (Input Output Controllers) . These are either stock-standard PCs or VME standard embedded system processors that manage a variety of “plug and play” modules (GPIB, RS-232, IP Carrier etc.) which interface to control system instruments (oscilloscopes, network analyzers) and devices (motors, thermocouples, switches, etc.). The IOC holds and runs a database of ‘records’ which represent either devices or aspects of the devices to be controlled. IOC software used for hard-real-time normally use RTEMS or VxWorks, though work has been ongoing in porting to other systems. Soft real-time IOC software sometimes runs on Linux or MS-Windows based machines.
Other computers on the network can interact with the IOC via the concept of channels. Take, for example a particle accelerator with shutters between sectors. There would typically be several channels corresponding to a shutter: an output channel to activate shutter motion, an input channel to see the status of the shutter (e.g. shut, open, moving, etc.), and probably some additional analog input channels representing temperatures and pressures on each side of the shutter. Channel names are typically in the form EQUIPMENT:SIGNALNAME (e.g. ACCELERATOR_RING:TEMP_PROBE_4, although they can be much less verbose to save time).
Most operations are driven directly from a standalone GUI package such as EDM (editor/display manager) or MEDM (Motif/EDM). These allow creation of GUI screens with dials, gauges, text boxes, simple animations, etc.
However it is not just GUI software which can interact with EPICS: any software which can speak the CA protocol can get and put values of records. For example on the EPICS website there are several extension packages which allow CA support in things like MATLAB, LabVIEW, Perl, Python, Tcl, ActiveX, etc. Hence it is easy to do things like make scripts which can activate EPICS controlled equipment.
There are different types of records available in EPICS. Here are some common types. Note that in addition to the other records not mentioned here, it is possible to create your own record type to interact with a device.
Each record has various fields in it, which are used for various tasks. AI and AO Analog Input and Output records can obviously store an analog value, and are typically used for things like set-points, temperatures, pressure, flow rates, etc. A limited amount of conversion to and from raw device data is available natively in the record (typically scaling and offsetting, but not advanced conversion like two’s complement or logarithmic). BI and BO Binary Input and Output records are generally used for commands and statuses to and from equipment. Calc and Calcout These two records can access other records and perform a calculation based on their values. (E.g. calculate the efficiency of a motor by a function of the current and voltage input and output, and converting to a percentage for the operator to read). Stepper Motor Control of a stepper motor. Allows settings of things like accelerations and velocities, as well as position.
Records in EPICS must have a designated scan time , otherwise they are automatically set as passive . A passive record will never process (unless its PROC field is written to). Mostly records are set to process on a periodic scan (such every 0.1 second).
Alternately records may be set to process only on events.