Simatic 505 SoftShop
9 - PID Loops
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Ramp/Soak
The ramp/soak feature allows you to define a variation for the process
variable by specifying the time characteristics of the loop setpoint (Figure
9.4). The capability of varying the loop setpoint can be useful in a
number of processes such as heat treating and batch cooking.
Time
Setpoint
Figure 9.4 Example Ramp/Soak Cycle
You can use simple ramp operations to improve some process startup
procedures. For example, the controllers do a bumpless transfer from
manual to automatic mode. This transfer holds the process at the initial
state when the mode change occurs. A two-step ramp/soak profile can
then move the setpoint to a predefined value following the mode change,
with minimal disturbance to the process.
Defining Ramp/Soak Steps:
Ramp/Soak is programmed as a set of time periods, or steps. A step can
be one of three types: a ramp, a soak, or an end
.
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The ramp step changes the loop setpoint linearly from its current
value to a new value, at a specified rate of change.
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The
soak step holds the setpoint constant for a specified period
of time. You can guarantee a soak period by entering a deadband
value. This form of soaking ensures that the process variable is
within a specified deadband around the setpoint for a specified
period of time.
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The end step terminates a ramp/soak profile. When the program
reaches an end step, the loop remains in automatic mode and
holds the setpoint constant.
You can program a status bit for each step of the ramp/soak. This bit is
set to 1 when the loop is executing this step. It is reset when the loop
leaves the step. This allows for easy tracking in the RLL program.
Controlling the Ramp/Soak Operation
Ramp/Soak operation can be controlled by two methods: allowing the
profile to be executed automatically, or by writing values to the variables
that control ramp/soak.
Automatic
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Whenever the loop changes from manual to automatic
mode, the loop begins to execute the ramp/soak profile at the initial step
(Step 1). The loop continues to execute the profile until an end step is
encountered in the profile. At this point, the loop remains in automatic
mode, and the setpoint is held at the last value in the profile.
Using Ramp/Soak Number
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Each loop ramp/soak profile has a
corresponding 16-bit variable, LRSN that contains the current step. You
can monitor LRSN with a SF program and also write a step number to it
with a SF program. The ramp/soak profile changes to the step that is