Basic Documentation

Open Loop Control: Another approach uses a
Venturi air valve as a simple metering device without
measuring airflow. In this scenario, the Venturi
operates as an open loop flow controller. This
concept is valid as long as the system keeps the
valve within the operating range of the spring and
cone mechanism.
In some installations, the Venturi assembly includes
a sensor, which measures position of the actuator
shaft. This can be useful. That position signal is
sometimes mislabeled as flow feedback, but it’s not
actually sensitive to airflow. In the open loop design,
events in the duct system that change the airflow do
not move the actuator, and are not reflected by the
position sensor. Operators and designers have
mistaken the speed and stability of the position
signal as a true indication of airflow, overlooking the
dynamic events that occur inside the valve, as the
cone and spring continually seek the changing
balance point.
Most ventilation system designers recognize the
value of an airflow measurement. Nearly all
specifications call for a flow feedback signal.
Position of the actuator should not be construed to
satisfy that need.
Linearity of Control Components
Control engineers characterize components in terms
of input/output responses. They consider dynamic
characteristics such as speed and overshoot, as well
as the steady-state responses. If the steady-state
response of a component (for example, airflow
versus damper position) can be described with a
straight line, the component is called linear.
In the days before digital control, linearity was
greatly beneficial to engineers piecing systems
together from signal processing components.
Product developers went to great lengths to linearize
responses because it simplified system design.
Today, non-linearity in a component is easier to
address in software.
The physics of airflow is full of non-linearity. For
example, the relationship between the position of the
cone in the valve, and the corresponding airflow at a
particular pressure depends on the complex
geometry of the cone and valve body, and is highly
non-linear. Similarly, the relationship between the
position of the actuator and the resulting position of
the cone depends on the non-linear spring in the
cone and the pressure forces in the duct.
Some manufacturers still linearize their valves by
adding a non-linear electronic circuit to distort the
relationship between the command voltage and the
position of the actuator. These days, that step is
usually unnecessary.
Airflow Control Accuracy
In a closed loop flow control application, accuracy
depends mainly on the airflow sensor; the
characteristics of the damper (blade or Venturi) have
little effect. In such a case, the expression accuracy
of the flow control damper doesn’t actually mean
anything.
Sensors are available for a wide range of accuracy
requirements. It’s important to select sensors
appropriate to the application. That means relating
the sensing range and accuracy to the ventilation
objectives. There is no single accuracy specification
that makes sense for all applications. It also means
considering the geometry and vulnerability of the
components in the air stream. In exhaust systems
it’s crucial to prevent fouling; a rugged geometry with
minimum obstruction of the air path is preferred. A
low-profile orifice ring with large pressure taps is
very reliable.
AIRFLOW
VENTURI AIR VALVE
SINGLE BLADE DAMPER
AIRFLOW
AIRFLOW
MEASUREMENT
SENSOR
Figure 3. Venturi Air Valve and Damper Air
Terminals with Integrated Airflow Sensors.
In an open loop application, (a Venturi air valve
without an airflow sensor) flow control depends
entirely on the reliability of the flow versus position
relationship of the valve. This depends, in turn, on
the precisely coordinated mechanical parameters
described earlier. Deviations over time in the
mechanical parameters degrade open-loop
Siemens Industry, Inc. Page 3 of 8
Document No. 149-985