Application

Sequence of Operation
Airflow Control
26
Siemens Industry, Inc.
Application Note, App 2926
140-1308
2015-07-07
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 controller opens the Venturi Air Valve
slowly, more and more 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 controller
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 gains
SUP P GAIN and GEX P GAIN are adjusted to tune the flow loops. The sample loop
time for the flow loops is fixed at 0.2 seconds. I and D gain are inherent in the system
and do not need adjustment.
NOTES:
1. Open the door to the controlled space and set VOL DIF STPT must be set to 0
while the flow loops are being tuned.
2. The Venturi Air Valve command points, SUP DMPR AO2 and GEX DMPR AO3,
indicate Venturi Air Valve position. Each Venturi Air Valve may be set up for normally
open or normally closed operation.
3. Adjusting P gains (supply and/or exhaust) to values greater than 0.1 may cause
system instability.
Table Statement and Feedback Loop Interaction
Supply Air Velocity Control
A table statement and the supply air velocity feedback control loop work together to
control the supply air velocity. (The table statement values are generated automatically
during calibration of the supply Venturi Air Valve.) The table statement and the control
loop work together as follows:
When the supply air velocity is too low (less than 350 fpm), the feedback control loop
suspends operation and the table statement performs all supply air velocity control.
The application uses SUP AIR VOL/SUPDUCT AREA as the value for supply air
velocity.
When the supply air velocity is greater than 350 fpm, but SUP AIR VOL is less than
SUP FLO STPT × LO LIMIT, the table statement and the air velocity feedback loop
work together to control the supply air velocity. Once SUP AIR VOL becomes greater
than SUP FLO STPT, the air velocity feedback loop suspends operation and the table
statement takes over all supply air velocity control.
When SUP AIR VOL is greater than SUP FLO STPT × HI LIMIT, the table statement
and the air velocity feedback loop work together to control the supply air velocity. Once
SUP AIR VOL becomes less than SUP FLO STPT, the air velocity feedback loop
suspends operation and the table statement takes over all supply air velocity control.
NOTE:
If HI and LO LIMIT are set equal, then the feedback loop will always be active as long
as the supply air velocity is greater than 350 cfm.