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8--74
PID Loop Operation
Maintenance
and Troubleshooting
PID Loop Operation
Glossary of PID Loop Terminology
Automatic Mode
An operational mode of a loop, in which it makes PID calculations and update the loop’s
control output.
Bias Freeze
A method of preserving the bias value (operating point) for a control output, by inhibiting
the integrator when the output goes out-of-range. The benefit is a faster loop recovery.
Bias Term
In the position form of the PID equation, it is the sum of the integrator and the initial
control output value.
Bumpless Transfer
A method of changing the operation mode of a loop while avoiding the usual sudden
change in control output level. This consequence is avoided by artificially making the SP
and PV equal, or the bias term and control output equal at the moment of mode change.
Cascaded Loops
A cascaded loop receives its setpoint from the output of another loop. Cascaded loops
have a major/minor relationship, and work together to ultimately control one PV.
Cascade Mode
An operational mode of a loop, in which it receives its SP from another loop’s output.
Continuous Control
Control of a process done by delivering a smooth (analog) signal as the control output.
Direct-Acting Loop
A loop in which the PV increases in response to a control output increase. In other
words, the process has a positive gain.
Error
The difference in value between the SP and PV,
Error Deadband
An optional feature which makes the loop insensitive to errors when they are small. You
can specify the size of the deadband.
Error Squared
An optional feature which multiplies the error by itself, but retains the original algebraic
sign. It reduces the effect of small errors, while magnifying the effect of large errors.
Feedforward
A method of optimizing the control response of a loop when a change in setpoint or
disturbance offset is known and has a quantifiable effect on the bias term.
Control Output
The numerical result of a PID equation which is sent by the loop with the intention of
nulling out the current error.
Derivative Gain
A constant that determines the magnitude of the PID derivative term in response to the
current error.
Integral Gain
A constant that determines the magnitude of the PID integral term in response to the
current error.
Major Loop
In cascade control, it is the loop that generates a setpoint for the cascaded loop.
Manual Mode
An operational mode of a loop, in which the PID calculations are stopped. The operator
must manually control the loop by writing to the control output value directly.
Minor Loop
In cascade control, the minor loop is the subordinate loop that receives its SP from the
major loop.
On / Off Control
A simple method of controlling a process, through on/off application of energy into the
system. The mass of the process averages the on/off effect for a relatively smooth PV. A
simple ladder program can convert the DL350’s continuous loop output to on/off control.
PID Loop
A mathematical method of closed-loop control involving the sum of three terms based
on proportional, integral, and derivative error values. The three terms have independent
gain constants, allowing one to optimize (tune) the loop for a particular physical system.
Position Algorithm
The control output is calculated so it responds to the displacement (position) of the PV
from the SP (error term)
Process
A manufacturing procedure which adds value to raw materials. Process control
particularly refers to inducing chemical changes to the material in process.
Process Variable (PV)
A quantitative measurement of a physical property of the material in process, which
affects final product quality and is important to monitor and control.
DL350 User Manual, 2nd Edition
Error=SP -- PV