Chapter 10 Built-in PID Functions
10 - 6
10.4 PID Instructions
10.4.1 PID loop states
A PID loop has 5 states: PIDSTOP, AT (Auto-tuning), PIDRUN, PIDCAS, and PIDPAUSE.
(1) PIDSTOP is a state in which the output (MV) is represented by MV_min, the internal states are
initialized, and user settings are maintained. Under this condition, it is impossible to enter into PIDPAUSE.
(2) AT can be entered into by, in PIDSTOP only, setting the PIDxx_AT_EN bit to On and then executing
the PIDRUN instruction. Once the AT operation is completed, the system automatically enters into
PIDRUN. Tasks in AT include monitoring the system’s response to a series of inputs and determining the
PID coefficients (K_p, T_i, T_d) and operation cycle (T_s). Upon completion of AT, those values are
updated and the previous coefficients are lost.
(3) PIDRUN is a state in which the PID loop executes a normal control operation. MV by PID operation is
output and the changed settings are all applied since each scan operation is executed independently. If the
contact in front of the PIDRUN instruction is set to On or if the PIDRUN instruction exists on the ladder
program and PIDxx_REM_RUN is set to On, then it is possible to enter into PIDRUN.
(4) PIDCAS is a state in which twoo loops (master and slave loops) execute a control operation. Setting
the two loops in the same way as with PIDRUN and then using the PIDCAS instruction enables to enter
into PIDCAS, and the internal connection necessary for the interworking between the two loops is
automatically generated allowing data exchange between the loops. Loops operated in cascade are
displayed in the state flag PIDxx_STATE, under which state the remote operation PIDxx_REM_RUM bit
does not operate.
(5) PIDPAUSE is a state in which output, internal states and user settings are all maintained and the
control operation is paused. Setting PIDxx_PAUSE bit to On or using the PIDPAUSE instruction enables to
enter into PIDPAUSE. But, this is only possible when the previous state is PIDRUN.
PIDRUN
PIDPAUSE
PIDSTOP
PIDCAS
AT