Watlow PPC-2000 - Manual (Page 135)

PPC-2000 User’s Guide Chapter 3: Operating with AnaWin 3
Doc.# 30002-00 Rev 2.3 Watlow Anafaze 111
Ratio Control Example
A chemical process requires a formula of two parts water to one
part Potassium Hydroxide (KOH) to produce diluted Potassium
Hydroxide. The desired flow of water is 10 gallons per second
(gps) and the KOH should flow at 5 gps. Separate feeder pipes
for each chemical feed in to a common pipe. The flow rate of
each feeder pipe is measured by flow transducers providing 0-
5Vdc signals to analog inputs on a PPC. Motorized valves
controlling the flow of each chemical are controlled by outputs
from the PPC. Figure 3.11 illustrates this application.
Figure 3.11 Sample Application Using Ratio
Control
In this application the first input on an Analog Input module is
set up to convert the linear voltage signals from the flow
sensors to gallons per second (gps). The water flow is controlled
by channel 1. The KOH flow is controlled by channel 2 and is
set up as the ratio channel. The KOH flow set point must be
half of the water flow value.
The process engineer doesn’t want the KOH set point to exceed
15 gps regardless of the water flow. Table 3.11 lists the channel
parameter settings for the ratio KOH channel.
Table 3.11 Ratio Channel Parameter Settings
Parameter Setting Description
Set Point Source PPC1:AI 1.1 The PV Source of the master channel.
Set Point Ratio 0.5
KOH should flow at one half of the water
flow rate.
Set Point Offset 0 gps
When water flow is 0 gps the KOH flow
should be 0 gps.
Min Set Point 0 gps
When the water flow is 0 gps the KOH flow
should be 0 gps.
Max Set Point 15 gps The KOH set point will not exceed 15 gps.
PPC
Mixture Out
Control Valves
Flow Sensors
Water Input KOH Input
Channel 1 Input
Channel 2 Input
Channel 1 Output
Channel 2 Output