Watlow Series SD PID Profiling Controller - Manual (Page 71)

Wa t l o w S e r i e s S D • 6 9 • C h a p t e r 1 2 F e a t u r e s
50% would result in an actual output power level of 50%.
But if the PID calculated power output is 100%, then the
power level will be 70%.
Power scaling establishes the maximum power output
and the minimum power output. The output power is
then linearly scaled within that range. The default val-
ues of Output Power Scale Low of 0% and Output Power
Scale High of 100% in effect disable power scaling.
Linear scaling allows the controller to do calculations
over the full range of power (0 to 100%) and adjust that
calculation within the actual output span. For instance,
if scale low is set to 15% and scale high is set to 80%, the
output power will always be between 15 and 80%. If the
PID calculation is 100%, the output power will be 80%,
which is the same result you would get from a power lim-
it of 80%. However, if the PID calculation for heat is 50%,
the output will be 50% of the allowable range, which
scales to an actual output of 47.5%.
Power limiting and power scaling affect the specified
output at all times, including in on-off control, manual
mode and autotuning.
100
90
80
70
60
50
40
30
20
10
0
P
ercent P
o
w
er Output
Time ➔
Power Limit 100%
Power Scale Low 0%
Power Scale High 100%
Power Limit 100%
Power Scale Low 15%
Power Scale High 80%
Power Limit 70%
Power Scale Low 0%
Power Scale High 100%
Power Limit 70%
Power Scale Low 15%
Power Scale High 80%
The Power Limit 1, 2 and 3 ( [PL`1], [PL`2] and
[PL`3]) and Output Power Scale Low 1, 2 and 3
(
[PSL1], [PSL2] and [PSL3]) and Output Power Scale
High 1, 2 and 3 ( [PSh1], [PSh2] and [PSh3]) appear in
the Setup Page. The calculated PID heat and cool power
values can be viewed with Power Heat
[Po;ht] and Pow-
er Cool [Po;CL] parameters in the Operations Page.
Non-linear Output Curve
A non-linear output curve may improve performance
when the response of the output device is non-linear. If
Output Non-linear Function is set to curve 1
[Cru1] or
curve 2 [Cru2], a PID calculation yields a lower actual
output level than the linear output provides. These out-
put curves are used in plastics extruder applications.
Curve 1 is for oil cooled extruders and curve 2 is for wa-
ter cooled extruders.
Change the linearity for each output with Out-
put Non-linear Function 1, 2 or 3 ( [nLF1], [ nLF2] or
[ nLF3]) in the Setup Page.
Actual Output P
o
w
e
r
0
20
40
60
80
100
PID Calculation
Linear
Curve 1
Curve 2
Independent Heat and Cool 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.
Adjust heat and cool PID parameters are Operations
parameters.
Time
Temperature
Set Point
Heating Side Proportional Band
Heating Side Dead Band
Cooling Side Dead Band
Cooling Side Proportional Band
Variable Time Base
Variable time base is the preferred method for con-
trolling a resistive load, providing a very short time base
for longer heater life. Unlike phase-angle firing, variable-
time-base switching does not limit the current and volt-
age applied to the heater.
With variable time base outputs, the PID algorithm
calculates an output between 0 and 100%, but the output
is distributed in groupings of three ac line cycles. For
each group of three ac line cycles, the controller decides
whether the power should be on or off. There is no fixed
cycle time since the decision is made for each group of
cycles. When used in conjunction with a zero cross (burst
fire) device, such as a solid-state power controller, switch-