E-Zine April 2012
Field instruments are the source of data used to control the process. When they supply data to the control system, they are often called "transmitters," and the data they transmit on the network is called a process variable. Process variables include pressure, temperature, level, and flow rates, but there are many other variables measured such as pH, opacity, voltage, amperage, and refractive index.
Another class of field instrument is the final control element. In many applications, a control valve positioner/actuator powered by compressed air is used to adjust the position of a control valve to regulate the flow of a process fluid, such as steam or cooling water. The positioner regulates the pressure of the air applied to the actuator to change the valve position using position feedback from the valve stem.
A less common type of final control element is a variable speed drive (VSD) that changes the speed of an electrical motor used to drive equipment, such as a pump or fan. By changing the drive speed of the equipment, the flow rate can be regulated.
Signal processing includes exponential or moving average smoothing (filtering), conversion to engineering units value, and alarm limit checking. In general, the exponential smoothing filter is more responsive to changes in actual process values, while the moving average filter is better for eliminating random noise from measurement values.
Depending upon the mechanism for sensing the raw measurement value, conversion to engineering units may use an equation or a table look-up to calculate the process value. For example, the equations for thermocouple conversion are complex and can require extensive floating point calculations. Most thermocouple lookups can be done using a relatively straightforward 20-30 point lookup table with a linear interpolation if the measurement is between point pairs.
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Excerpted from The Consumer Guide to Fieldbus Network Equipment for Process Control