Proportional-only control

P-only control is just reset action, where loop output is directly proportional to the magnitude of the setpoint error (E_S) and the size of the proportional gain (K_P).

P-only loop output is linear, and is calculated as follows:

Output = (KP x ES) + bias (if action = direct) or

Output = - ((KP x ES) + bias) (if action = reverse)

where  ES = [PV - setpt]

If you are using proportional-only loop control, follow these guidelines:

• Output limits:  Define the Maximum Output and Minimum Output properties for the loop output, noting that the maximum value must be greater than the minimum.

• Proportional Gain:   Calculate and enter a proportionalConstant (K_P) property value starting with this formula:

  [output range (maxOutput - minOutput)] / throttling range

  where throttling range is the corresponding result in the process variable.

  For example, for a temperature loop where a 0-to-100% loop output results in a 20 degree swing in the process variable, a starting point KP is:

  [(100% - 0%)/ 20deg.] = [(100% / 20deg.] = 5

  When tuning the loop, you can try increasing this value (effectively using only a portion of the throttling range) to eliminate the amount of setpoint error. However, if you increase the K_P too much, this typically results in a constant oscillation of the process variable (above and below the setpoint).

• Bias: Assign the bias property an output-midpoint value (for example, 50.0). This allows for equal corrections for a process variable above or below setpoint.

• Integral and Derivitive Gain: Set the properties integralConstant and derivativeConstant to 0.0 (the defaults).