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•Full Article: The Impact Of Instruments, Sensors, And Switches On Your Automation System

By Grant Van Hemert,
Schneider Electric Water Wastewater Competency Center

Water and wastewater treatment is an automation intensive process. With the rapid acceptance of Ethernet and increased reporting requirements, the use of automation will only increase. Thus, the need for repeatable, accurate, and reliable control has never been greater.

The precision and reliability of your control equipment relies on the accuracy of data coming in from sensors, switches, and other analytical instruments. Even with the highest quality-control hardware and program design, your automation system can fall short of expectations if it doesn't have the proper control system interface or if the sensing equipment is not properly selected, installed, applied, configured, and calibrated.

Proper Control System Interface
Sensing devices come in two general categories: on/off-based, otherwise known as discrete, and value-based, or analog. Discrete devices typically are easy to install and require little maintenance; however, their data-gathering capabilities are limited to either a yes or no scenario. Thus, many devices may be needed to get all the required data (e.g. four float switches in a wet well for sequenced pump control). This extra equipment can significantly add to installation costs.

Analog devices provide real value, but often have calibration requirements that require increased maintenance time. Some analog devices may come with contacts that can be programmed to provide a greater-than or less-than indication. Sometimes these contacts are connected to the programmable logic controller (PLC) instead of the actual value.

From a control standpoint, using the analog device's contacts instead of its value-based signal has three drawbacks. First, two conductors for each contact typically are required, which can increase installation costs. Second, the actual reading is not entered into the PLC; thus a human machine interface (HMI) or supervisory control and data acquisition (SCADA) system must show that a sensor is above or below a certain range — but not by how much. This can have a limiting impact for record keeping or troubleshooting. Third, changing the set points must be done at the sensor. This can involve avigating through complex menus. The net effect of these three drawbacks is that one operator at the SCADA control interface may assume the set point is at one value, while another operator has adjusted it at the transducer, resulting in operation problems.

In addition, network connections such as asynchronous serial interface (ASi), open controller area network (CANOpen), and Ethernet are gaining in popularity for interfacing sensors to PLCs. These network connections can gather vast amounts of data from both analog and discrete sensors while lowering installed costs. Each of these connection methods has its own benefits and drawbacks which must be considered during design.

Click Here To Download:
•Full Article: The Impact Of Instruments, Sensors, And Switches On Your Automation System