quote:
Electrical buzz in the air
Electronics Weekly 07/21/04
author: Steve Bush
Aeroplanes have had electronics in them for almost as long as aircraft have had engines. First in radio sets and then in radar and navigation devices. In parallel, as aircraft grew, hydraulic amplification and control supplemented, and then replaced, mechanical linkages between pilots and the various flaps, elevators and rudders used for manoeuvring.
Fly-by-wire saw computers finally break all mechanical links between the pilot and control surfaces, although hydraulics still provided the muscle required in the wings.
Now, as planes have grown huge and every possible kilogram must be saved for passengers, hydraulics are getting the push and electric motors are taking their place.
The concept, called the 'all-electric aircraft', is still some way off as airborne electric actuator technology is too young for the notoriously conservative aircraft industry. In the mean time, the 'more-electric' aircraft is with us.
In a more-electric aircraft, gone are engine-driven hydraulic pumps and hundreds of metres of fluid-filled high-pressure hydraulic tubing, to be replaced with an electrical power bus and small electric pumps producing hydraulic power where it is needed [Electro-Hydraulic Actuators].
The Airbus A380 is to be the first more-electric airliner, following in the footsteps of the latest military jets which lost hydraulic power mains through their vulnerability to puncturing.
Airbus was also the first company to put fly-by-wire into a commercial airliner. Boeing's planned 7E7 Dreamliner is also more-electric. More electrical power means bigger generators, and efforts to increase their efficiency.
Aircraft power has traditionally been produced as 115V 400Hz three phase, using variable speed gearing to keep the frequency constant as engine revs vary.
With the latest airliners needing 150kVA from each generator, inefficiencies of variable gearboxes can no longer be tolerated and more efficient fixed-ratio gearing is being introduced, resulting in an output anywhere between 300 and 800Hz.
"'Frequency wild' is an expression you are going to be hearing more of," says Adrian Nicol, managing director of Cambridgeshire-based aircraft electronics manufacturer Io. Nicol has a DTI SMART award to develop power supplies to work across this range to produce 28V local busses.
Electric power to replace hydraulics is not the only significant new electrical load. Keeping passengers happy with video screens, laptop power supplies and, for the truly pampered, back-massaging seats, could cost upwards of 400W per seat in the future - 160kW for 400 passengers.
As they are ultimately moving control surfaces, the motors operating local hydraulic pumps need to operate with considerable finesse and deliver variable flows. There are two options here. Fixed-speed motor attached to variable displacement pumps, or variable-speed motors on fixed-displacement pumps.
As the second option is mechanically simpler, and a fixed-speed motor is a tricky thing on a frequency wild AC bus, semiconductor-based variable speed motors are favourite.
While there are many traditional ways to control the speed of a three-phase AC motor, much effort is going into perfecting matrix converters which can control the speed and direction of a three phase motor, direct from a three-phase any-frequency supply, with no heavy inductive or bulky capacitive energy storage.
The drawback is, that a matrix converter needs nine fast bi-directional semiconductor switches - one between every combination of input and output wire.
This is hardly a drawback if you are a maker of fast bi-directional switches, but power device makers are striving hard to develop something suitable and matrix switch modules are becoming available.
Looking into the future, all-electric aeroplanes seem inevitable - although it seems unlikely that aircraft will go quite as far as all-electric ships and have a central generator and electric motors producing primary thrust any time soon. So long as the industry's confidence is not dented by a disaster which can be traced back to some all-electric feature, there will be a fair market for high-reliability electronics in the aircraft industry
