Principle Of Operation; Electronic Circuit; Magnetic Actuator; Pilot Stage - Emerson Fisher 846 Instruction Manual

Instruction Manual
Form 5739
February 2002
Section 4
Principle of Operation4-4-
The following paragraphs describe the functional
parts of the Type 846 and Model 3311. Figure 4-1
shows the block diagram.

Electronic Circuit

During operation, the input current signal is received
by the transducer's electronic circuit and compared
to the output pressure from the booster stage. A
solid-state pressure sensor is part of the electronic
circuit and monitors the booster stage output. The
silicon-based sensor uses strain gauge thin film
technology.
The sensor's pressure signal is fed to a simple
internal control circuit. By using this patented
technique, the transducer's performance is set by
the sensor/circuit combination. Changes in output
load (leaks), variations in supply pressure, or even
component wear are sensed and corrected by the
sensor/circuit combination. Electronic feedback
allows crisp dynamic performance and readily
compensates for output changes induced by
vibration.
Because the transducer is electronic
in nature, it is not well-modeled in the
loop as a simple resistor in series with
an inductor. Also it is better thought
of as a 50-ohm resistor in series with a
6.0 V voltage drop, with negligible
inductance.
This is important when calculating the loop load.
When the transducer is used in series with a
microprocessor-based transmitter, the noninductive
nature of the transducer allows digital signals to
successfully pass through undistorted.

Magnetic Actuator

The electronic circuit controls the level of current
flowing through the actuator coil, which is located in
the pilot/actuator assembly. A change to the level of
coil current is made by the electronic circuit when it
senses a discrepancy between the pressure
measured by the sensor and the pressure required
by the input signal.
Note

Principle of Operation

The actuator performs the task of converting
electrical energy (current) to motion. It uses a
patented, coaxial moving magnet design optimized
for efficient operation and is highly damped at its
mechanical resonance. A silicone rubber diaphragm
protects its working magnetic gaps from
contamination.

Pilot Stage

The patented pilot stage contains two opposed fixed
nozzles: the supply nozzle and the receiver nozzle.
It also contains the deflector, which is the moving
element.See figures 4-2 and 4-3. The supply nozzle
is connected to the supply air and provides a
high-velocity air stream. The receiver nozzle
captures the air stream and converts it back to
pressure. The receiver nozzle pressure is the output
pressure of the pilot stage.
To vary the pilot output pressure, the high-velocity
stream is diverted away from the receiver nozzle by
the deflector, which is a cylindrical, aerodynamic
body located between the two nozzles.
In response to a change in actuator coil current, the
deflector is repositioned between the nozzles. There
is a linear relationship between the coil current and
the pilot stage output pressure. For direct action
units, the power-off, or fail-safe, position of the top
of the deflector is near the center of the stream and
results in nearly zero pilot output pressure. As the
coil is energized, the deflector is drawn out of the
stream.
For reverse action units, the power-off, or fail-safe,
position of the deflector is completely out of the
stream. The result is maximum pilot output pressure.
As the coil is energized, the deflector moves into the
stream, resulting in a decreased pilot output
pressure.
The deflector material is tungsten carbide, and the
nozzles are 316 stainless steel. The nozzles have a
large bore of 0.41 mm (0.016 inches), which
provides good resistance to plugging.
4–1