Chapter 10 Built-in PID Functions
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10.6.4 Operation and function of cascade (CAS)
PDCAS performs cascade PID control through sequential operation of two PID loops. Generally, cascade
PID control is used for temperature control through chemical process or fuel control; The two loops used
here are called master loop and slave loop. As an example of temperature control through fuel flow, in case
of a single loop PID control, the fuel valve is opened and closed to control the fuel flow and consequenctly
control the temperature of the heating furnace. This is, therefore, a system in which a single PID loop
indirectly controls temperature through fuel flow control. In this case, equipping the system with a fuel flow
meter enables cascade PID control that consists of flow control and temperature control: the slave loop
controls the flow using the valve and the master loop controls temperature using the flow. The master loop
transfers the desired flow to the slave loop, while the slave loop monitors the flow meter and adjusts the flow
using the valve so that fuel corresponding to the flow desired by the master loop is injected. The slave loop
operates on the flow Set Value set by the master loop, regardless of temperature.
In terms of the internal cascade operation, the master loop measures the temperature (PV_mst) in a more
delayed manner than the slave loop and transfers the flow value (MV_mst) computed for the user’s desired
temperature (MV_mst) to the slave loop. The slave loop sets the flow value (MV_mst) transferred from the
master loop as the Set Value (SV_slv) and measures the fuel injection amount (PV_slv) in a more frequent
manner than the master loop in order to control the valve opening and closing (MV_slv).
Cascade, therefore, functions to transfer MV (MV_mst) of the master loop to SV (SV _slv) of the slave loop
when two loops are in operation.
If the slave loop is converted to manual output mode, the master output is not used and the master loop is
also converted to manual output mode. The manual mode _PIDn_MAN bit is not turned ON in the master
loop. If the slave loop is converted to auto output mode again, the master loop is also turned to auto output
mode. If _PIDn_MV_BMPL is set to On, state data is exchanged between the two loops to ensure a smooth
conversion.
If the slave loop is in Anti Wind-up mode, the master loop operates in PIDPAUSE mode. When there is a
need to increase or descrease the slave Set Value (SV_mst) despite the occurrence of anti wind-up, this
function prevents the occurrence of 2
nd
wind-up for the whole cascade loop. This function always operates
according to the corresponding conditions and the _PIDn_PAUSE bit is not turned On.
Heater
Note
Auto-tuning (AT) of Cascade System
In case of AT of a cascade system, AT of the slave loop precedes AT of the master loop. For AT of
the slave loop, it is required to predict how much SV the slave loop receives from the master loop
and setting AT_SV to this value enables the slave loop to operate as an independent loop. AT
performance may differ according to the predicted value. Upon completion of AT of the slave loop,
AT of the master loop starts.
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