User`s manual

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.
Integral 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) ad-
justs the output based on the rate of change in the tem-
perature 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).
Dead Band
In a PID application the dead bands above and be-
low the set point can save an application's energy and
wear by maintaining process temperature within ac-
ceptable ranges. Shifting the effective cooling set point
and heating set point keeps the two systems from fight-
ing 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 parame-
ters).
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 us-
ing power scaling.
➔➔
Using both power limiting and power scaling would
not usually be necessary. Power limiting provides a ba-
sic 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 max-
imum level of heat power and for cool outputs it deter-
mines the maximum level of cooling power. A power lim-
it 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 50%
Output
Power
scaled
Output
power
capped at
Power
Limit
Output
power cal-
culated
using PID
Time
Temperature
Heating Set Point
Heating Proportional Band
Cooling Dead Band
Cooling Set Point
Cooling Proportional Band
Time
Temperature
Set Point
Reduced Overshoot
Proportional Band
Proportional Band x 2
Heating Slows
Watlow Series SD ■ 62 ■ Chapter 10 Features