Application

Sequence of Operation
Airflow Control
17
Siemens Industry, Inc.
BACnet LCM-OAVS, Application 6754
140-1325
2015-07-07
When calibration is in progress, CAL AIR equals YES. After calibration, CAL AIR
returns to NO.
The application uses Autozero Modules connected to AUTOZERO DO8. This means
that the supply and general exhaust flow control devices do not close during calibration
of the transducers.
NOTE:
The LCM does not monitor Fume Hood flow changes for 3 seconds during AVS
calibration.
Airflow Control
The supply flow is controlled with a feedback loop to operate the floating control
damper motor so that the measured air velocity is maintained at setpoint. To enhance
stable flow control, an advanced algorithm is used to calculate a controllable setpoint
as the value approaches zero cfm (lps). This feedback loops works as follows:
When the flow is slightly below the setpoint, the LCM opens the floating control damper
motor slowly, more and more until the airflow reaches the setpoint, at which time the
air damper’s position remains constant. If the flow is far below the setpoint, the LCM
opens the floating control damper rapidly, more and more until the airflow reaches the
setpoint, at which time the air damper’s position remains constant. The feedback gain
SUP P GAIN is adjusted to tune the flow loop. The sample loop time for the flow loops
is fixed at 2.0 seconds. I and D gain are inherent in the system and do not need
adjustment.
The application contains logic that prevents the feedback loops from trying to move the
damper motors faster than they can actually go. This helps minimize airflow overshoots
and undershoots whenever there is a large airflow setpoint change. The greatest
amount of position change that the supply damper motor will be allowed to undergo
during each execution of the supply airflow feedback loop is ± (100% * flow loop
execution speed)/MTR1 TIMING. The flow loop execution speed is fixed in this
application at 2.0 seconds.
NOTES:
1. Open the door to the controlled space and set VOL DIF STPT to zero while the
flow loops are being tuned.
2. The damper motor command point for supply is SUP DMP CMD The supply
damper motor may be set up for normally open or normally closed operation
depending on the value of MTR SETUP. See the
Damper Configuration section of the
Start-up document
for more information on motor setup.
Damper Status
When an Autozero solenoid from an Offboard Air Module is wired to D08, the physical
damper position may vary from what the application’s position point is indicating. A
Damper Status firmware algorithm will detect and correct situations where airflow
velocity is greater than 200 fpm and the following is true:
 SUP DMP POS = 100% and SUP VOL < SUP STPT
OR
 SUP DMP POS = 0% and SUP VOL > SUP STPT