Application

Sequence of Operation
Airflow Control
17
Siemens Industry, Inc.
Application Note, Application 6759
140-1335
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
Both supply flow and general-exhaust flow are controlled with a feedback loop to
operate the floating control damper motor (either supply or general-exhaust) 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 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 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. Likewise, the greatest amount of position change
that the general damper motor will be allowed to undergo during each execution of the
general airflow feedback loop is + (100% * flow loop execution speed) / MTR2 TIMING.
The flow loop execution speed is fixed in this application at 2.0 seconds.
NOTES:
1. Open the door to the controlled space or set VOL DIF STPT to zero while the flow
loops are being tuned.
2. The floating control damper motor command points are SUP DMP CMD and GEX
DMP CMD. Each floating control 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.
Floating Control Actuation Auto-correct
Floating Control Actuation Auto-correct
In addition to the existing options for floating control actuator full stroke actions, all
floating control actuators are provided with additional logic to fully drive open or closed
when commanded to 100% or 0%.