The two screws on the top cover access holes to
a mounting plate assembly. The body and
two potentiometers positioned on the upper
mounting plate are machined from stainless steel.
circuit board assembly.
A filter cover assembly protects the sensing
element while allowing the passage of atmospheric
air over two sensors. It consists of a temperature
sensor and a humidity sensor. Electrical con-
One of the more important parameters sensed
nection to the sensing element is made through
in a gas turbine propulsion plant is speed. Speed
a fixed plug secured to the mounting plate.
is measured on reduction gears and gas turbines.
Changes in temperature of the air flowing over
The reduction gear speed is monitored to inform
the sensor assembly vary the electrical resistance
the propulsion plant operators of the propeller
of the temperature sensor. As temperature
shaft speed. Gas turbine speed is measured to
decreases, the resistance of the temperature
ensure the turbine does not overspeed. Also, gas
increases; at 41°F, resistance is about 2.4 kilohms
(kΩ). The change in resistance modifies a dc signal
turbine speed is used as an input to the control
systems of the engine. Two types of speed
in the ice-detector signal conditioner.
sensors are used to measure the required speeds.
Changes in the moisture content of the air
flowing over the sensor assembly vary the
reduction gears. The other type is the magnetic
electrical resistance of the humidity sensor. The
speed pickup, used to sense gas turbine speeds.
humidity sensor is a salt-treated wafer. As it
In some applications, it is used to sense reduction
absorbs moisture from the surrounding air, its
resistance decreases as the humidity increases; at
70 percent relative humidity, resistance is about
435 kΩ . The change in resistance affects the feed-
back to an amplifier in the ice-detector signal
Main reduction gears (MRGs) use tachometer
generators to provide electrical signals for remote
indication of propulsion shaft speed. The output
Ice-Detector Signal Conditioners
of the tachometer generator is voltage pulses
proportional to the speed of the propeller shaft.
The ice-detector signal conditioner (fig. 5-3 1)
The tachometer signal is amplified. Then it is sent
is contained in a metal box. The box is attached
to the propulsion electronics for use in data and
to the underside of the base/enclosure. It
bell logging and display of shaft speed. You can
contains two printed-circuit board assemblies, a
find more information on tachometer generator
transformer, and a relay and capacitor. The box
operation by referring to the related technical
is closed by a cover bearing identification labels.
manual found on your ship.
Magnetic Speed Pickup Transducer
The magnetic speed pickup transducer is
usually referred to as a magnetic speed pickup and
is the most common type of speed sensor found
in gas turbine propulsion plants. It is used for
sensing gas turbine speed on the LM2500 and the
Allison 501-K17. It is also used to measure
reduction gear speed for controllable pitch
propeller (CPP) systems on the FFG-7 class ships.
The MRGs of the DD-963 class ships use magnetic
speed pickups for sensing clutch engagement
Magnetic speed pickups sense speed by using
a toothed gear that cuts the magnetic field of the
pickup. The output of the sensor is a square-wave
ac voltage. This voltage is converted to a
proportional dc voltage in a signal conditioner.
The output of the signal conditioner is used in
Figure 5-31.--Ice-detector signal conditioner.