Method and apparatus for analogue output current control

摘要

This invention relates to an apparatus and method for output current control in an Industrial Process Control System. The output module comprises two management units each of which calculates the current demand for the module independently from one another. One management unit controls an output current controller whilst the other unit monitors the current produced by the output current controller against an independently calculated demand.
The output module has multiple output modules. However rather than one module providing a backup for the other modules, or alternating between use of each module, in normal operation, each module provides a portion of the required output current, the total output current being equal to the sum of the currents output by each module. In the event of failure on one of the modules, the other module or modules switch to providing the total current required and the failed module is switched to a fail safe mode by using an isolation circuit to switch off the output current from that module.
The output module also provides improved fail safe protection by providing an improved isolation circuit and an improved method of continually testing that the isolation circuit will operate correctly in the event that it is required to switch a module to a fail safe mode.