Technology: Fibre-optic gyroscopes point pilots in the right direction

日期:2019-02-28 06:08:03 作者:谈湮职 阅读:

By JEFF HECHT, Boston A NEW turboprop airliner, built by Dornier of West Germany, will be the first to have gyroscopes based on fibre optics. These gyros sense rotation by measuring changes in light passing through a loop of optical fibre. By keeping track of these changes around each of three axes, the gyros determine the craft’s direction. Dornier’s DL328 aricraft, a turboprop built to take around 30 passengers, will make its first flight in 1991. The US military is also keen on using fibre-optic gyroscopes to guide aircraft and weapons in space, because they are small and rugged. Traditional mechanical gyroscopes are bulky, vulnerable to shock and slow to start running, and can be very expensive. An alternative is the ring laser gyroscope, which is standard equipment on Boeing 757s and 767s. But they are also costly and require mechanical accessories to avoid errors when they are spinning slowly. Fibre gyros have no moving parts, can be entirely solid-state, and can be small and lightweight – these are all important considerations for aircraft as well as spacecraft. Their first applications will be in altitude heading and reference systems (AHRS), which check only the direction of flight. These need accuracies of only 0.1 to 1 degree per hour. The inertial systems that guide rockets and spaceships over long distances require far greater accuracies of 0.01 to 0.001 degrees per hour. In the fibre gyro, light from an external source is split and travels in opposite directions around a rotating ring. The beam going in the direction of rotation will travel slightly farther than the one going opposite to the rotation. While any difference is small in a single loop of fibre, the change is measureable when hundreds of metres are wound into a compact coil. The gyro for the Dornier DL328 was developed by Honeywell’s research centre in Phoenix, Arizona. It contains 100 metres of fibre in a coil 75 millimetres across and 19 millimetres high. Full production will begin in 1992. Honeywell will not disclose how much the gyros cost, but an engineer said that the company hopes the price will be down to $1000 per axis by 1994. Honeywell will vie with Litton Guidance and Control Systems of Woodland Hills, California and other aerospace companies to make fibre gyros for more than 3000 new AHRS units for existing US Air Force and Navy planes. Eventually, up to 10 000 new systems may be needed. Some planes now have 25-year-old AHRS units with vacuum tubes, says David Klein, an executive at Litton. Litton has flight-tested AHRS gyros with 500 metres of fibre, but an engineer with the company, Eric Goldner, said production versions will have 200-metre coils. Litton also has tested fibre gyros for inertial navigation and missile guidance. Meanwhile, researchers at Massachusetts Institute of Technology reported last month a laser gyro that also uses an optical fibre. Ordinary laser gyros use a mirror system to split the light and guide the two beams around a circle. The wavelengths of the beams vary according to the rotation rate. In MIT’s system, the two ends of a 25-metre fibre are pumped by a gas laser, which generates laser light by a nonlinear process called ‘stimulated Brillouin scattering’. As in a ring laser gyro,