Monday, January 17, 2011

PLC and Sensor Constraint in Water Irrigation Control

We have encountered several sensor and PLC constraints. We have learned that many PLCs are requiring a significant part of a minute just to run through various checks of equipment on the PLC side, to read values from sensors, and to communicate with supervisory control and data acquisition systems. Furthermore, the manufacturers are often unable to tell us how much time is required. Because the simulations should duplicate the actual computation, it is the problematic. We have also found that some brands will take 60 sensor values per minute and provide an average, but those 60 sensor values are all read within the last 1 – 5 seconds of a minute.

We now insist on redundancy of key items for applications. Specifically, we state that all of the key sensors be duplicated, plus the sensors must be wired into different power supplies and A/D converter modules in the PLCs. We assume that it is not a question of ‘if’ a sensor will fail, but a question of ‘when’. Although there are various techniques to use software to check for problems with single sensor, we have found out that this adds a tremendous complexity to programming of the PLC that is unnecessary if redundancy exists.

There are the classic problems with accuracy, resolution and calibration on the sensor side. But a new challenge is presented when one uses an AFDM or similar device to measure flow rates at the head of a canal for control purposes. We have examined numerous filtering techniques, but in the end we have concluded that we need at least 10 minutes of continuous readings before we can use an average value for a control decision. This complicated headwork’s control on some canals. In contrast, using a flume of replogle to measure flows at the head of a canal has the advantages of little or no random noise in the signal, inexpensive redundancy of the water level sensor, and very importantly for control device.

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