Chapter 15 PID Function (Built-in function)
15 - 15
(i) Differential time
It sets the differential time of PID loop in question (Td). As larger the differential time, the
differential operation is getting stronger. The scope is real number at the unit of second.
(j) Limiting change of present value
It sets the limit of change in present value of PID loop in question. If PV suddenly changes due to
signal components such as sensor’s malfunction, noise or disturbance during control of PID, it
may cause sudden change of PID control output. To prevent the phenomenon, a user can set the
max. limit of change in present value that is allowed per PID operation cycle. If the change of
present value is limited accordingly, it may calculate the present value as much as the limit
although the present value is changed more than the limit once the limit of change in present
value is set. If using the PV change limit function, it may prevent against sudden change of
control output owing to noise or etc. If it is, however, set too small, it may reduce the response
speed to the PV change of an actual system, not to sudden change by noise or etc, so it is
necessary to set the value appropriately according to the environment of a system to control in
order that the PV toward the set value does not take a longer time. The available scope is
between -32,768 ~ 32,767. If setting the PV change limit as 0, the function is not available.
(k) Limiting change of MV (ΔMV function)
It limits the max. size that control output, which is output by PID operation is changed at a time.
The output MV in this operation cycle is not changed more than the max. change limit set in the
previous operation cycle. The function has an effect to prevent a drive from operating excessively
due to sudden change of output by preventing sudden change of output resulting from
instantaneous change of set value. If it is, however, set too small, it may cause taking a longer
time until PV reaches to its target, so it is necessary to adjust it appropriately. The available
scope is between -32,768 ~ 32,767. If setting it as 0, the function does not work.
(l) Max. MV
It sets the max. value of control output that may be output by the result of PID operation. The
available scope is between -32,768 ~ 32,767. if it exceeds the max. output designated by PID
operation result, it outputs the set max. output and alerts the max. output excess warning. For
the types and description of warnings, refer to Error/Warning Codes.
(m) Min. MV
It sets the min. value of control output that may be output by the result of PID operation. The
available scope is between -32,768 ~ 32,767. If it is smaller than the min. output value designated
by PID operation result, it outputs the set min. MV and alerts the min. output shortage warning.
For the types and description of warnings, refer to Error/Warning Codes.
(n) Manual MV
It sets the output when the operation mode is manual. The available scope is between -32,768 ~
32,767.
(o) DeadBand setting
It sets the deadband between set value and present value. Although it may be important to
reduce normal status reply of PV for its set value even when MV fluctuates heavily, depending on
control system, it may be more important to reduce the frequent change of MV although the
normal status reply is somewhat getting larger. DeadBand may be useful in the case.
Below figure shows an example of DeadBand setting.
To Next Page
Loading...