Micromachined Silicon Sensors
Measuring up to pressures of the Aerospace industry.
The Industry Takes Off
In the last thirty years, both civil and military sectors of the
aerospace industry have seen unprecedented growth with
developments to meet the increasing demand for effective
and safe air based transportation and defences.
This has in turn created a significant increase in global competition
and the use of many advanced and innovative technologies from a
wide spectrum of engineering disciplines, at all levels of the industry
supply chain.
Well known civil examples include the introduction of supersonic and
large passenger/freight aircraft - the Concorde and Boeing's 747 Series.
Also, manufacturers such as Airbus Industrie emerged and went on to
develop a range of fly-by-wire aircraft and following an industry
de-regulation in the seventies, a vast amount of new airline operators
entered the market. As the aerospace transportation infrastructure
developed even further, so did the market for regional jets to cover
the higher frequency, shorter haul/"hop" flights and an increasing
number of corporate owners started to buy business jets to run their
own flight missions. Although of a small seat capacity, many modern
business jets can now fly a non-stop distance of 5000 miles and beyond.
The next decade will see further developments in all these sectors
including, for example, the introduction of the 500 plus seater and
"Super Jumbo".
Military forces use a diverse range of aircraft to fulfil defence related
functions. These include attack, bomber, cargo, reconnaissance and
training amongst others. This sector has seen the introduction of stealth
aircraft technology and an acceleration in the race to produce the world's
most agile fighter aircraft. This has led to Europe moving on from the
Tornado programme to the Eurofighter Typhoon project which has
developed through the nineties, led by project partners in the UK,
Germany, Italy and Spain.
The Typhoon is designed to be aerodynamically unstable with active
computer control of the various flight control surfaces. This enables
the aircraft to perform advanced combat manoeuvres with the speed,
ease and precision which secure its place as a world class fighter.
First deliveries of this advanced Fighter are scheduled for later this year.
To keep pace with developments, US based manufacturers Boeing
and Lockheed Martin are submitting plans for the Joint Strike Fighter
(JSF). This aircraft will compete head on with the Typhoon early
in the present decade.
Worldwide, the industry is developing and manufacturing an extremely
broad range and high volume of gas turbine engines, fixed wing and
rotary wing aircraft to meet the demands in growth. The industry is
continually driven by prime factors such as performance, reliability,
efficiency and safety.
Micromachined Silicon Sensors
It was during the early seventies that the concept of an "integrated"
micromachined silicon sensor proved its ability to deliver a substantially
better performance for aerospace pressure measurement than other
traditional technologies available at that time. Due to its inherent
operating characteristics and solid state construction, it offered high
accuracy, response and overload capability coupled with a virtual
insensitivity to severe environmental effects such as vibration, shock
and acceleration.
Exploiting the piezo-resistive properties of a monolithic silicon diaphragm,
with atomically diffused strain gauges arranged in a wheatstone bridge
configuration, a continuous, high millivoltage output could be achieved
proportional to applied pressures ranging from tens of millibars upwards.
The compact nature of this sensor technology also enabled a modular
approach to packaging design which could be readily modified for
many different purposes including built-in signal conditioning where
required. Also, by the use of low thermal coefficient materials such
as a glass mount, together with fully encapsulated passively or actively
temperature compensated electronics, its performance could be
maintained even across wide extremes of temperature.
For the aerospace industry, effectively this technology offered all the
benefits of improved performance, reliability, efficiency and safety,
which were then of course, soon applied within the industry led by
specialist manufacturers such as Druck.
Ground and Flight Test Transducers
Originally, micromachined Druck silicon pressure sensors were used
within a scanning valve system for the high speed monitoring of
various pressures in the early seventies, testing jet engines such
as the Rolls Royce RB211. Ground and flight applications present
the broadest range of measurement requirements to a pressure
transducer manufacturer and range from engine test stand duty for
fuel, oil and air measurement through to surface profiling transducers
which monitor airflow on wings and fuselage surfaces. High stability
sensors are also used with digital outputs for air data recording and
airframe test sensors are often of small size and lightweight to meet
overall critical weight restrictions.
Engine Mount Applications
Measurement of critical engine performance parameters is vital in
modern military and commercial engines and pressure transducer
applications interface with engine fuel controls, cockpit instruments
and can be direct mounted for oil, pressure and filter condition
monitoring. Integrated electronic packaging and the advancement
of EMC/EMI protection circuitry provide good environmental
performance in exposed locations where higher temperatures and
increased vibration levels exist. A pressure transducer is also used
to monitor engine transients to prevent engine stall and so dynamic
response is essential.
Airframe Transducers
From a small fixed wing jet to a rotary wing military attack helicopter,
a wide variety of environmental conditions are encountered by airframe
mounted pressure sensors.
In particular, the introduction of fly-by-wire and closed loop control of
hydraulic systems has dramatically increased the use of airframe
pressure transducers. Transducer selection may include high level
millivolt outputs with passive-resistive components resulting in a
good signal-to-noise ratio and high EMC/EMI immunity or amplified
transducers which provide a variety of output signals including voltage,
current, frequency and digital. These devices also use advanced
hybrid electronics and ASIC technology which offer performance
and reliability to match their equivalent passive devices. From avionics
cooling to tyre pressure, de-icing systems to brake pressure, cabin
pressure and oxygen monitoring, pressure sensors play a vital role
within all major aircraft control systems.
Air Data Measurements
Today's sophisticated range of cockpit instrumentation and air data
computers (ADCs) require pressure input signals of either analogue
or digital form to measure parameters such as Altitude, Rate of Climb,
Airspeed and Mach. With the introduction of RVSM (Reduced Vertical
Separation Minima) legislation, altitude in particular needs to be
measured even more accurately. A new type of sensor technology,
which exploits the mechanical properties of a three dimensionally
micromachined resonant silicon element, has been developed
particularly for these relatively low absolute pressure measurements.
Eurofighter Typhoon Sensors
The new Eurofighter Typhoon aircraft is a good example of leading
edge pressure sensor technology working in partnership with one
of the world's largest aerospace consortiums. Druck are classed as
a major supplier for the Typhoon and with 620 aircraft already on
order and a realistic market potential of over 1000, project partner
BAe Systems are ready for deliveries to commence next year.
In addition to pressure transducers used on the Typhoon hydraulics
and fuel systems, Druck have now been selected by BAe Systems to
supply the aircraft ECS (Environmental Control System) making
a total of twenty-six Druck sensors on board every Typhoon aircraft
built. This additional contract further endorses the use of
micromachined silicon technology to provide a reliable high
performance solution for critical measurement applications in
severe aerospace environments. For example, on each aircraft,
Druck PDCR 300 series transducers are used within the hydraulic
control system. Because the aircraft is aerodynamically unstable
with computer controlled airframe surfaces, the Druck transducers
play an extremely dynamic and flight critical role monitoring the
various hydraulic pressures and communicating direct with the
aircraft computer by using a compatible digital interfaced, developed
specifically for the Typhoon.
Druck have been involved with the development work since 1990,
working closely with the four nation project partners including
BAe to meet the extensive and stringent specifications required
for the programme.
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June 2001
