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PID Loop Operation
Maintenance
8–65
PID Loop Operation
Proportional Gain A constant that determines the magnitude of the PID proportional term in response to
the current error.
PV Absolute Alarm A programmable alarm that compares the PV value to alarm threshold values.
PV Deviation Alarm A programmable alarm that compares the difference between the SP and PV values to
a deviation threshold value.
Ramp / Soak Profile A set of SP values called a profile, which is generated in real time upon each loop
calculation. The profile consists of a series of ramp and soak segment pairs, greatly
simplifying the task of programming the PLC to generate such SP sequences.
Rate Also called differentiator, the rate term responds to the changes in the error term.
Remote Setpoint The location where a loop reads its setpoint when it is configured as the minor loop in a
cascaded loop topology.
Reset Also called integrator, the reset term adds each sampled error to the previous,
maintaining a running total called the bias.
Reset Windup A condition created when the loop is unable to find equilibrium, and the persistent error
causes the integrator (reset) sum to grow excessively (windup). Reset windup causes
an extra recovery delay when the original loop fault is remedied.
Reverse-Acting Loop A loop in which the PV increases in response to a control output decrease. In other
words, the process has a negative gain.
Sampling time The time between PID calculations. The CPU method of process control is called a
sampling controller, because it samples the SP and PV only periodically.
Setpoint (SP) The desired value for the process variable. The setpoint (SP) is the input command to
the loop controller during closed loop operation.
Soak Deviation The soak deviation is a measure of the difference between the SP and PV during a soak
segment of the Ramp / Soak profile, when the Ramp / Soak generator is active.
Step Response The behavior of the process variable in response to a step change in the SP (in closed
loop operation), or a step change in the control output (in open loop operation)
Transfer To change from one loop operational mode to another (between Manual, Auto, or
Cascade). The word “transfer” probably refers to the transfer of control of the control
output or the SP, depending on the particular mode change.
Velocity Algorithm The control output is calculated to represent the rate of change (velocity) for the PV to
become equal to the SP.