Friday, July 22, 2011

CODAC PLC Interface Good Practice

The interface of CODAC development is very important that variables of configuration are not overwritten in the core application. There is no variable read back, so if one of these variables is adapted in the PLC, the connected EPICS PV will be misaligned. A variable of configuration have not to be measured as a simple setting for an output of PLC. It is obviously identified in the requirements for the design conceptual of the PLC, that outputs of PLC are handled by the core application. Straight writing from CODAC to the outputs is prohibited.

The device is managed by the control loop, consequently the control value issued by the control loop must be transferred to CODAC as a variable of state, not as a variable of configuration. If the mode of “open loop” is realized, then the input of user must be implemented as a divide configuration variable.

Process Function Standard Structure
The organization conceptual of a control function has three major parts: control logic and the control loop, state management, and interlocks. - The part of state management is stand for by a Petri Net or a GRAFCET. The state management will be controlled by exchanges with the process inputs and the CODAC. State management never starts the process outputs straightly. The part of interlocks and control logic mostly manages the control function of digital process outputs. It is controlled by the process inputs and the state management. The standard is that the actuator command is written only at one place in the code, in sequence to create the code easier to read and maintain. A diagram of logic gathers:
• All process conditions needed to make sure that the actuator is technically ready.
• The conditions of state.

- The control loop controls the continuous process outputs. It is controlled by the state management and the continuous process inputs. It is very frequently necessary to modify the control loop configuration according to the operational state, e.g. change the inhibit the integral action, set point, open the loop, etc..

The control function implementation is also standardized. Every function of control will be executed in one control block created of the three parts explained above:
- Interlocks and control logic
- Control loop
- State management
The block of control can be an FC or an FB, base on if it is a unique case or multiple cases. The three parts can be realized in the same control block or in dedicated ones – regarding on the function complexity.


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