Watlow Series SD PID Profiling Controller - Manual (Page 69)

Wa t l o w S e r i e s S D • 6 7 • C h a p t e r 1 2 F e a t u r e s
mode and it is off while in the auto mode. For the profil-
ing controller, the letter P appears in the left side of the
lower display followed by the manual % power set point
value. For example,
[P~75] indicates the profiling con-
troller is in Manual Mode at 75% power output. You can
easily switch between modes if the Auto-Manual Mode
[A-M] parameter is selected to appear in the Opera-
tions Page.
To transfer to manual mode from auto mode, press
the Advance Key ‰ until [A-M] appears in the lower
display. The upper display will display
[Auto] for auto
mode. Use the Up ¿ or Down ¯ keys to select [Man] .
The manual set point value will be recalled from the last
manual operation.
To transfer to auto mode from manual mode, press the
Advance Key ‰ until [A-M] appears in the lower dis-
play. The upper display will display
[MAn] for manual
mode. Use the Up ¿ or Down ¯ keys to select [Auto] .
The automatic set point value will be recalled from the
last automatic operation.
Changes take effect after three seconds or immedi-
ately upon pressing either the Advance Key ‰ or the
Infinity Key ˆ.
On-Off Control
On-off control switches the output either full on or
full off, depending on the input, set point and hysteresis
values. The hysteresis value indicates the amount the
process value must deviate from the set point to turn
on the output. Increasing the value decreases the num-
ber of times the output will cycle. Decreasing hysteresis
improves controllability. With hysteresis set to 0, the
process value would stay closer to the set point, but the
output would switch on and off more frequently, and may
result in the output “chattering.” On-off control can be se-
lected with Heat Control Method
[ht;m] or Cool Control
Method [CL;m] (Operations parameters).
NOTE:
Input Error Failure Mode
[FAIL] does not function in on-off con-
trol mode. The output goes off.
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%.
Set Point
Time
Temperature
The heating action switches off when the process
temperature rises above the set point.
The heating action
switches on at startup.
Hysteresis
Process Temperature
Hysteresis
Time
Temperature
The cooling action
switches
on at startup.
Process Temperature
The cooling action switches on when
the process temperature rises above
the set point plus the hysteresis.
Set Point
The heating action switches on when the process temperature
drops below the set point minus the hysteresis.
The cooling action switches off when the process
temperature drops below the set point.
Proportional Control
Some processes need to maintain a temperature or
process value closer to the set point than on-off control
can provide. Proportional control provides closer control
by adjusting the output when the temperature or process
value is within a proportional band. When the value is in
the band, the controller adjusts the output based on how
close the process value is to the set point.
The closer the process value is to the set point, the
lower the output power. This is similar to backing off
on the gas pedal of a car as you approach a stop sign. It
keeps the temperature or process value from swinging as
widely as it would with simple on-off control. However,
when the system settles down, the temperature or pro-
cess value tends to “droop” short of the set point.
With proportional control the output power level
equals (set point minus process value) divided by the pro-
portional band value.
Adjust the proportional band with Proportional Band
Heat [Pb;ht] or Proportional Band Cool [Pb;CL] (Opera-
tions parameters).
Time
Temperature
Set Point
Proportional Band Droop
Overshoot