PI Controller

Operator Menu Location: Generic Hardware Operators -> Controller Blocks

Operator Bitmap:

Functional Description:
The PI Controller block implements the position form of a Proportional-Integral control algorithm . It computes a control signal M, as the sum of proportional, integral (reset), and derivative (rate) control modes:

Each mode is based on measurements (samples) of an error signal E. For direct-acting control, this error signal is defined as
E = PV - SP,
where PV is the process variable and SP is the set-point. For reverse-acting control, the error is defined as
E = SP - PV.

The direct/reverse acting attribute of a PI controller refers to the relative direction of movement of the process variable and controller output. Under a direct acting configuration, an increase in the process variable results in an increase in the controller output. Under a reverse acting configuration, an increase in the process variable results in a decrease in controller output. The proper setting depends upon the process itself. In most processes, the controller should be set to reverse acting. (Hence, the operator defaults to reverse acting.)
The proportional mode term, , is calculated as the product of the proportional gain, , and the error term E(t),

The integral mode term is obtained from

where is the integral or reset time expressed in minutes, t is the loop sampling time expressed in seconds, and S(t) is the sum of all past errors, computed from
S(_t) _= S_(t-(delta)_t) + E(t), S(0)=0

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

Initial Condition. The output value at startup, i.e., M(0)

Minimum Output. The minimum output value

Maximum Output. The maximum output value

Direct Acting. A boolean property that governs the relative direction of movement of the process variable and controller output. When the property is checked, an increase in the process variable results in an increase in the controller output. When it is not checked, an increase in the process variable results in a decrease in controller output. By default, this property is left unchecked (in most applications, a PI controller should be reverse acting.)

Comments:
•The reset time ( ) input is measured in minutes
•The minimum proportional gain ( ) input is 0.001 _(_if a smaller value is present, 0.001 will be used as the gain in the computation of the output signal.)

See Also:
PID Controller, PI Controller, PID Controller with External Feedback, Lead-Lag Controller