PID Loop Operation
Maintenance
8–57
PID Loop Operation
Ramp/Soak Generator
Our discussion of basic loop operation noted the setpoint for a loop will be generated
in various ways, depending on the loop operating mode and programming
preferences. In the figure below, the ramp / soak generator is one of the ways the SP
may be generated. It is the responsibility of your ladder program to ensure only one
source attempts to write the SP value at V+02 at any particular time.
Process Variable
Loop
Calculation
S
+
–
Control OutputSetpoint V+02
Setpoint Sources:
Operator Input
Ramp/soak generator
Ladder Program
Another loop’s output (cascade)
If the SP for your process rarely changes or can tolerate step changes, you probably
will not need to use the ramp/soak generator. However, some processes require
precisely-controlled SP value changes. The ramp / soak generator can greatly
reduce the amount of programming required for these applications.
The terms “ramp” and “soak” have special
meanings in the process control industry,
and refer to desired setpoint (SP) values in
temperature control applications. In the
figure to the right, the setpoint increases
during the ramp segment. It remains
steady at one value during the soak
segment.
Time
SP
Ramp
Soak
slope
Complex SP profiles can be generated by specifying a series of ramp/soak
segments. The ramp segments are specified in SP units per second time. The soak
time is also programmable in minutes.
It is instructive to view the ramp/soak generator as a dedicated function to generate
SP values, as shown below. It has two categories of inputs which determine the SP
values generated. The ramp/soak table must be programmed in advance,
containing the values that will define the ramp/soak profile. The loop reads from the
table during each PID calculation as necessary. The ramp/soak controls are bits in a
special loop table word that control the real-time start/stop functionality of the
ramp/soak generator. The ladder program can monitor the status of the ramp soak
profile (current ramp/segment number).
Process Variable
Loop
Calculation
S
+
–
Setpoint Control Output
Ramp/soak
Generator
Ramp/soak table
Ramp/soak controls
Introduction
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