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. ____________________________________________ June 2001

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