A controller seeks to maintain the measured process variable (PV) at set point (SP) in spite of unplanned and unmeasured disturbances. Since e(t) = SP - PV, this is equivalent to saying that a controller seeks to maintain controller error, e(t), equal to zero.
A controller repeats a measurement-computation-action procedure at every loop sample time. A sensor measures a temperature, pressure, concentration or other property of interest from our process. The sensor signal is transmitted to the controller. The pathway from sensor to controller might include: a transducer, an amplifier, a scaling element, quantization, a signal filter, a multiplexer, and other operations that can change the size, sign, and/or units of the measurement. After all electronic and digital operations, the result terminates at our controller as the "wire in" measured process variable (PV) signal...
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Doug Cooper