Characterizer

Operator Menu Location: Signal Conditioning

Operator Bitmap:
 

Functional Description:
The characterizer operator is a piecewise-linear interpolation mapping. Its output y is defined by

where x is the input, N is the number of points in the mapping (user-defined), and is an N-point user-defined data set to be interpolated.
Design Pad will issue an error on processing if the input pin is not connected.

The Characterizer operator is ideally suited for situations where an accurate mathematical model of equipment operation is not available, but measurement data is. For instance, consider a process loop with a proportional valve actuator with a marked non-linear response throughout its range of operation. Assume that experiments characterizing its performance yield the following test data:

In the table, the values in the left column represent the position the valve was commanded to reach (0-100% its range of operation). The data in the right column is the measured steady-state valve position in response to the corresponding command input. A pronounced non-linear valve response (such as the one shown in the preceding graph) may adversely affect overall system performance. PID tuning constants that yield good performance when the valve is operating in its linear range may yield inadequate performance when the valve is operating in its saturation region. You can address this problem by adding a Characterizer operator between the PID block and the Analog Output block. You could set up the Characterizer operator to invert the valve operating characteristics, thereby linearizing the process loop. The required Characterizer data set (the inverse of the valve performance data) is reproduced here. With this arrangement, the characterizer and valve positioner combination would respond to PID command signals linearly. For example, a PID output of 77.9% will be translated (by the characterizer) into a command input (to the valve) of 60.0%. When this 60.0% command input is applied to the valve, the resulting valve position will be 77.9%.

User-Defined Properties:
Object Name. A string label that identifies the operator

Number of Points. The number of points in the characterization mapping

X Increment. The default increment for x-coordinates in the mapping (applied by pressing the "Generate X and Y Coordinates using X and Y Increments" button)

Y Increment. The default increment for y-coordinates in the mapping (applied by pressing the "Generate X and Y Coordinates using X and Y Increments" button)

X and Y Coordinates. Data set to interpolate

Comments:
The data abscissas must be monotonically increasing: