User`s manual

Watlow Series SD • 68 • Chapter 12 Features
Proportional plus Integral (PI) Control
The droop caused by proportional control can be cor-
rected by adding integral (reset) control. When the sys-
tem settles down, the integral value is tuned to bring the
temperature or process value closer to the set point. Inte-
gral determines the speed of the correction, but this may
increase the overshoot at startup or when the set point is
changed. Too much integral action will make the system
unstable. Integral is cleared when the process value is
outside of the proportional band.
Integral is in effect if PID Units are set to SI, and is
measured in minutes per repeat. A low integral value
causes a fast integrating action.
Reset is in effect if PID Units are set to US, and is
measured in repeats per minute. A high reset value
causes a fast integrating action.
Adjust the integral with Integral Heat
[It;ht] or In-
tegral Cool [It;CL] (Operations parameters).
Adjust the reset with Reset Heat [rE;ht] or Reset
Cool [rE;CL] (Operations parameters).
Proportional plus Integral plus Derivative (PID)
Control
Use derivative (rate) control to minimize the over-
shoot in a PI-controlled system. Derivative (rate) adjusts
the output based on the rate of change in the tempera-
ture or process value. Too much derivative (rate) will
make the system sluggish.
Rate action is active only when the process value is
within twice the proportional value from the set point.
Adjust the derivative with Derivative Heat [dE;ht] or
Derivative Cool [dE;CL] (Operations parameters).
Adjust the rate with Rate Heat [rA;ht] or Rate Cool
[rA;CL] (Operations parameters).
Time
Temperature
Set Point
Reduced Overshoot
Proportional Band
Proportional Band x 2
Heating Slows
Dead Band
In a PID application the dead bands above and below the
set point can save an application’s energy and wear by
maintaining process temperature within acceptable rang-
es. Shifting the effective cooling set point and heating set
point keeps the two systems from fighting each other.
Proportional action ceases when the process value is
within the dead band. Integral action continues to bring
the process temperature to the set point. When the dead
band value is zero, the heating element activates when
the temperature drops below the set point, and the cool-
ing element switches on when the temperature exceeds
the set point.
Adjust the dead bands with Dead Band Heat [db;ht] and
Dead Band Cool [db;CL] (Operations parameters).
Time
Temperature
Heating Set Point
Heating Proportional Band
Cooling Dead Band
Cooling Set Point
Cooling Proportional Band
Power Limiting and Power Scaling
Power limiting and power scaling are two methods
of placing limitations on a control output. The functions
can be used independently or together. An output level
calculated from the PID algorithm first has the power
limit applied, then the resulting value is processed using
power scaling.
Output pow-
er calculated
using PID
Output pow-
er capped at
Power Limit
Output pow-
er scaled
Using both power limiting and power scaling would
not usually be necessary. Power limiting provides a basic
static cap on power, while power scaling provides a more
dynamic range of power limitation.
NOTE:
When output power must be limited, in most cases power scaling
will provide better autotune performance than power limiting.
NOTE:
In on-off control set Power Limit 1, 2 and 3 (
[PL`1], [PL`2] and
[PL`3]) and Output Power Scale High 1, 2 and 3 ([PSh1], [PSh2]
and
[PSh3]) to 100%. Set Output Power Scale Low 1, 2 and 3
(
[PSL1], [PSL2] and [PSL3]) to 0%.
The power limit sets the maximum power for a
heat or cool control output. Each control output has its
own power limit. For heating outputs it determines the
maximum level of heat power and for cool outputs it de-
termines the maximum level of cooling power. A power
limit of 100% in effect disables the power limit. If the
PID calculations yield a power level that is greater than
the power limit setting, then the output power level will
be the power limit setting. For example, with a power
limit setting of 70%, a PID-calculated power output of