Watlow ez-zone pm - Example 7: Wet Bulb; Dry Bulb

Watlow EZ-ZONE
®
PM Integrated Controller • 221 • Chapter 10 Applications
Cascade control uses two control loops (outer - loop 1 and inner - loop 2) to control the process.
The outer loop (Analog Input 1) monitors the
process or part temperature, which is then com-
pared to the Set Point. The result of the compari-
son, the error signal, is acted on by the PID
settings and the Range and Scale high/low set-
tings. Ultimately, the outer loop produces a
remote set point for the inner loop. The inner
loop input (Analog input 2) monitors the energy
source (heating and cooling), which is compared
to the remote set point generated by the outer
loop. The result of the comparison, the error
signal, is acted on by the PID settings in the
cascade inner loop (2), which generates an out-
put power level between -100% to +100%. If the
power level is positive the heat will be on; if the
power level is negative the cool will come on.
Power from the energy sources are supplied by the outputs of choice always referenced to Control
loop 2.
When cascade control is disabled (Source E
is true), the Math function output will equal
Control Loop 1, Set Point.
Note:
If an input sensor on the outer loop fails
when using deviation cascade the inner loop
will continue to drive the output.
Example 7: Wet Bulb / Dry Bulb
Requirements:
Two analog inputs and at least one output are
required to adjust the controlled part of the
processes.
Overview:
Wet Bulb/Dry Bulb is a conguration where a
dry bulb connected to Analog Input 1 measures
temperature on Analog Input 1. A wet bulb
sensor that is maintained with moisture has air
moved over the sensor. As moisture evaporates
from the wet bulb, the temperature drops. A
wet bulb input on Analog Input 2, in combina-
tion with the dry bulb temperature, senses
relative humidity. The controller calculates the
temperature difference between the two sen-
sors to determine percent relative humidity.
The humidify and dehumidify outputs are disabled when Analog Input 1 temperature falls below
32 F/0 C, or goes above 212 F/100 C. When function is set for Wet Bulb/Dry Bulb, the PV Function
output equals calculated humidity. Control loop 1 will control Analog Input 1 to Set Point 1. Con-
trol loop 2 will control Analog Input 2 to Set Point 2.
-100% = Range Low
+100% = Range High
Outer Loop 1
Process
Control Loop 1
Control Loop 2
Inner Loop 2
Energy
Cascade
Filter
Scale Low
Scale High
Range High
Range Low
Function
Remote Set
Point
Source E
Output
Heat Power
Cool Power
Set Point
Math Function
DIO
or
Function Key
0 to 100%
0 to 100%
Analog Input 1
Analog Input 2
Process Sensor
Energy Source
Set Point 1
Heat Algorithm
Cool Algorithm
Heat
Cool
Output n
Output n
Heater
Oil In
Limit
Limit Thermocouple
Lube Oil Tank
Oil Out
Output 1
Input 2
Input 1
Inner Loop
Thermocouple
Outer Loop
Thermocouple
Wet Bulb/Dry Bulb
Set Point 2
Set Point 1
Function
Filter
Source A
Source A
Source B
Source A
Heat Power
Heat Power
Cool Power
Cool Power
0 to 100%
0 to 100%
0 to 100%
0 to 100%
Output
Heat
Heat
Cool
Cool
Output n
Output n
Output n
Output n
Analog Input 1
Analog Input 2
Control Loop 2
Control Loop 1
PV Function
Temperature
Humidity