Rockwell Automation 1772-L8_LW_LWP_LX_LXP,D17726.5.8 User Manual - Manual (Page 234)

EAF
Process Control Instructions
Chapter 16
16-23
De-scaling is necessary to force the PID input range to 0 to 4095 when
using an input module that does not return this range. A thermocouple
input module is a good example. It returns a scaled temperature range,
typically -300
°
to +1200
°
Fahrenheit. When specifying the input range for
de-scaling, specify the maximum range that your process will actually see.
For example, if your thermocouple input range is -300
°
to +1000
°
C but
your process could only possibly experience 25
°
to +550
°
C, then use the
latter range in the de-scaling equation. This improves the resolution for the
PID instruction and results in better system control.
Figure 16.8
DeĆScaling Inputs
(
(
(
Processor
Scaling
0Ć4095
PID
Instr.
DeĆscaling
Equation
Operator
Interface
SP
PV
C0
0Ć4095
Analog
Signal
Analog
Output
25Ć550 o C Analog Signal
Heater
Thermocouple Thermocouple
Input
Smax
2
= 550 o C
Smin
2
= 25 o C
Smax
1
= 4095
Smin
1
= 0
10373-I
Example: If measuring a full scale temperature range of 25
°
to 550
°
C,
enter 25 for the minimum scaling value and 550 for the maximum
scaling value.
M
2
+ (M
1
* Smin
1
)
(Smax
2
* Smin
2
)
(Smax
1
* Smin
1
)
) Smin
2
where:
M = Measured value
Smin = Minimum Scaling Value
Smax = Maximum Scaling Value
DeĆScaling Inputs