Application

Sequence of Operation
Open Loop (Mode 3)
26
Siemens Industry, Inc.
Application Note, App 6752
140-1322
2015-07-07
The Venturi calibration table initially contains all zeros by default, that is, it contains no
calibration information. When the application detects a zero flow for the sixteenth entry
(the table entry with the highest flow), the application does operate, but runs with only
PID control. If PID only control is satisfactory for a given job, there is no need to
populate the Venturi tables.
Open Loop (Mode 3)
This application can operate in open loop mode. In open loop control, there is no
airflow sensor to provide an actual airflow measurement. Instead, operation is based
completely on the values in the Venturi table. For example, if the actuator’s setpoint is
600 cfm, the application will use the Venturi table to derive the voltage setting that
generates a flow of 600 cfm and then set the actuator to that voltage to generate the
desired flow. Assuming the table has accurate entries, the flow (if it were to be
measured) would be at setpoint.
Internally within the application, with open loop control, the flow is always assumed to
be at setpoint As a result there is virtually no difference between flow tracking and
setpoint tracking when the actuator being tracked is operating open loop.
Set
S OPEN LOOP or G OPEN LOOP to YES to indicate that the respective actuator
is to operate open loop. Setting an AVS enable point to
YES will suppress the AVS
failure indication that otherwise would occur when no airflow sensor is connected.
When operating open loop, there is no flow sensor for use in the calibration sequence.
Instead, the Venturi table values are directly input by the user. If the Venturi Valve has
end points for calibration such as 1200 cfm at end voltage, and an implied lower range
of 0 cfm at its other end of the voltage range, you should enter this information into
Point 16, as shown in Example 1 and 2 below.
The direction of actuation, Normally Open or Normally Closed is determined by
VENTRUI ACT.
Point
Flow
Volt
1
0
0.0
2
0
0.0
15
0
0.0
16
1200
10.0
Example 1 - Table with CFM End Limit – direct acting
It may not be necessary to enter 0 values since 0 values are initially in the table by
default when direct acting is selected.
Point
Flow
Volt
1
0
10.0
2
0
10.0
15
0
10.0
16
1200
0.0
Example 2 - Table with CFM End Limits – reverse acting