Rockwell Automation DeviceLogix System User Manual - Manual (Page 27)

Publication RA-UM003B-EN-P - February 2010 27
Navigate the Function Block Editor Interface Chapter 2
The PID equation controls the process by sending an output to the
actuator device. The greater the error between the setpoint and
process variable input, the greater the output will be. An additional
value (feedforward or bias) can be added to the control output as an
offset. The PID result (control variable) drives the process variable
toward the setpoint.
The PID function block monitors and controls the process loop for
analog process parameters such as pressure, temperature, flow rate,
and fluid level. Features of the PID function block include:
• PID equations expressed in Dependent Gains (ISA standard)
• Input scaling in engineering units
• Zero-crossing deadband
• Derivative term acts on PV
• Direct or reverse acting control
• Output alarms
• Output limiting with anti-reset windup
• Manual mode (with bumpless transfer)
• Feedforward or output biasing
The PID function block uses the following equation with dependent
gains:
Where:
K
c
is Control Gain
T
i
is Reset Term
T
d
is Rate Term
SP is Set Point
PV is Process Variable
E is (SP-PV) or (PV-SP)
CV is Output Control Variable
Δ
t is Loop Update Time