Ramp/Soak Generator; Introduction - Automation Direct DL05 User Manual

Chapter 8: PID Loop Operation

Ramp/Soak Generator

1

Introduction

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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
3
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
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at V+02 at any particular time.
Setpoint Sources:
5
Operator Input

Ramp/soak generator

Ladder Program
6 Another loop's output (cascade)
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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
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for these applications.
The terms "ramp" and "soak" have special meanings in the
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process control industry, and refer to desired setpoint (SP)
values in temperature control applications. In the figure to
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the right, the setpoint increases during the ramp segment.
It remains steady at one value during the soak segment.
Complex SP profiles can be generated by specifying a series
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of ramp/soak segments. The ramp segments are specified in
SP units per second. The soak time is also programmable in minutes.
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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
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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
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start/stop functionality of the ramp/soak generator. The ladder program can monitor the
status of the ramp soak profile (current ramp/segment number).
A
Ramp/soak table
B
Ramp/soak controls
C
D
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DL05 Micro PLC User Manual, 6th Edition, Rev. A
Setpoint V+02
Setpoint
Ramp/soak
Generator
+
Loop
Calculation
+
–
Process Variable
SP
Ramp
slope
Loop
Calculation
–
Process Variable
Control Output
Soak
Time
Control Output