Application

Sequence of Operation
Airflow Control
18
Siemens Industry, Inc.
BACnet LCM-OAVS, Application 6754
140-1325
2015-07-07
When initiated, the firmware module sets DMPR STATUS from CAL to RECAL and
decrements/increments the damper position (in a reiterative sequence up to four cycles
in length) until sensed airflow matches setpoint.
NOTE:
It is important to realize that while the Damper Status module runs, the damper
position point (SUP DMP POS will change in value but the flow point (SUP VOL)
might not.
To change DMPR STATUS from RECAL back to CAL, set DMPR STATUS to CAL and
then release it. (A status of RECAL will not prevent the recalibration sequence from
running if needed.)
Venturi Operational Modes
The general exhaust Venturi Air Valve can operate in one of three modes:
 Mode 1 – Operates with both a PID loop and a Venturi table.
This mode provides the best control and is the most commonly used mode for
these applications. In this mode, the embedded Venturi table statements works
together with a PID feedback loop to operate the Venturi Air Valve so that the
measured air velocity is maintained at setpoint. The following several sections
describe this mode.
 Mode 2 - Operates with a PID loop, but no Venturi table.
In this mode, the controller operates with PID control based on a flow sensor input,
but the Venturi table is not used. See the PID Only Mode section, later in this
document, for specific information on this mode.
 Mode 3 - Operates with Venturi table, but no PID loop
In this mode, the controller operates open loop (without a flow sensor). There is no
PID control. Positioning of the actuator is based solely on a Venturi table consisting
of command voltages and their resultant corresponding airflows. See the
Open
Loop Mode
section, later in this document, for specific information on how this
mode differs from the normal Mode 1 above.
Feedback Loop Operation
When the flow is slightly below the setpoint, the LCM gradually opens the Venturi Air
Valve slowly, until the airflow reaches the setpoint, at which time the air valve’s position
remains constant. If the flow is far below the setpoint, the LCM opens the Venturi Air
Valve rapidly, and rapidly increases until the airflow reaches the setpoint, at which time
the air valve’s position remains constant. The feedback gain GEX P GAIN is adjusted
to tune the flow loop. The sample loop time for the flow loops is fixed at 2 seconds. I
and D gain are inherent in the system and do not need adjustment.
NOTES:
1. VOL DIF STPT must be set to 0 while the flow loops are being tuned.
2. The Venturi Air Valve command point, GEX DMPR AO3, indicates Venturi Air
Valve position. The Venturi Air Valve may be set up for normally open or normally
closed operation.
3. Adjusting P gain to a value greater than 0.1 may cause system instability.