Watlow Series 96 - Manual (Page 26)

5 . 7 ■ F e a t u r e s Wa t l o w S e r i e s 9 6
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
temperature 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.
View or change the Output 1 derivative or rate with
Derivative 1 [dE1`] or Rate 1 [rA1`] (PID 1 Menu).
Derivative appears if Units Type [Unit] (Global
Menu) is set to [``S1]. Rate appears if [Unit] is
set to [``US].
Figure 5.7a PID control.
Temperature
Set Point
Reduced Overshoot
Proportional Band
Proportional Band x 2
Heating Slows
Dual PID
In an application with one output assigned to heat-
ing and another assigned to cooling, each will have a
separate set of PID parameters and separate dead
bands. The heating parameters take effect when the
process temperature is lower than the set point and
the cooling parameters take effect when the process
temperature is higher than the set point.
PID 1 parameters apply to Output 1; PID 2 parame-
ters apply to Output 2. View or change PID 1 and
PID 2 parameters in the PID 1 and PID 2 menus
(Operations Page).
Figure 5.7b Dual PID.
Time
Temperature
Set Point
Heating Side Proportional Band
Heating Side Dead Band
Cooling Side Dead Band
Cooling Side Proportional Band
Dead Band
In a dual PID application the dead bands above and
below the set point can be used to save energy and
wear on an application by maintaining a process
temperature within an acceptable range. 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 value of the dead band is zero, the heating ele-
ment becomes active when the temperature drops
below the set point, and the cooling element switch-
es on when the temperature exceeds the set point.
The dead band for Output 1 can be viewed or
changed with Dead Band 1 [db`1] (PID 1 Menu).
The dead band for Output 2 can be viewed or
changed with Dead Band 2 [db`2] (PID 2 Menu). Figure 5.7c Cooling dead band.
Time
Temperature
Heating Set Point
Heating Proportional Band
Cooling Dead Band
Cooling Set Point
Cooling Proportional Band