New MCV Software

On a recent project for a major water company, Max Wright Systems & Software were asked to produce software for an Allen Bradley SLC 500 PLC, which would replace four cell validation units to improve reliability and provide extensive logging and alarming facilities. The multiple cell validation systems had to be monitored via a MMI and provide a reliable analog signal to control dosing systems. The system was controlling the chlorine dosing of drinking water, so had to be accurate and reliable.

Multiple cell validation operates by comparing three or more analogue signals from cells and providing a mean average as an output. The high sensitivity of the cells meant that they had to be calibrated regulary, but despite regular calibration, the validation system had to detect when the calibration of a cell was necessary or if a cell had failed completely.

The PLC monitors each cell analogue individually, it checks to ensure the signal is within 4 and 20mA. If the signal is outside these limits, it is not included in the average calculation and alarm is raised via telemetry that the system is operating using only two cells. If all signals are healthy then they are checked against each other, if one of the cells deviates to far above or below the values being provided by the other cells, it is excluded from the average calculation. If the cell returns to a value closer to the others it is then included in the average equation. However, if the cell remains out of limits for over a set period, the cell is deemed to have failed and an alarm is raised via telemetry. If while operating using only two cells, they deviate from each other by a set amount for a set period the PLC shuts down the process completely until the cells have re-calibrated or repaired.

As well as monitoring and checking the cells, the MCV system provides Low Low, Low, High and High High alarms to telemetry. These signals are generated using the average signal. All the setpoints for the MCV can be adjusted by only access at chemist security level.

From time to time maintenance must be performed on different parts of the measurement cells. To allow the operator with the correct security access to maintain the operation of the plant, a maintenance switch is provided via the MMI for each cell. This switch can only be operated while the cell is healthy, which means it cannot be operated as a fault over-ride. While the switch is operated a constant value equal to the value that was provided by the cell when the switch was operated is used in the average equation. After a set time the switch reverts back to normal operation.

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