Fullscreen HTML5Med Res (??MB)HTML4

THE WAY I SEE IT By NEVILLE WILLIAMS Fly by wire: is it safe from electromagnetic interference? Future aircraft, ships, trains and road vehicles are likely to be guided and controlled by electronic rather than mechanical means. But while electronic systems may be reliable in themselves, it has yet to be shown that they can be fully protected from electromagnetic interference. We know what can happen if the brakes fail or the steering linkage comes adrift in a present-day tourist bus. It can easily end up in the creek at the bottom of the hill! A similar fate could befall its future electronically controlled counterpart if the data circuits from the driver's cab to the road wheels were to be corrupted by a chance signal from a high-power mobile transmitter in the adjacent traffic lane. There is certainly no lack of precedent for accidental transmitter or other electromagnetic breakthrough into all sorts of electronic equipment, from radio receivers to radio telescopes. Ask any design engineer or service technician who's been around for a while; it's something that they have always had to live with! The question is not so much why interference should be a potential problem with electronic control systems but why it should not be, based on a mass of past experience! The vital difference is that whereas to date, electromagnetic breakthrough has merely been embarrassing, frustrating or mildly expensive, it is much more serious 36 SILICON CHIP when the lives of passengers are directly involved. Fly-by-wire concept In aircraft, electronic control or the so-called "fly by wire" concept has been around for some time but I first encountered it in 1984, when researching an article on Australia's then-new FA-18 Hornet attack fighter. Up until then, I must confess, the term fly-by-wire seemed to relate more to the woven steel cables strung from the cockpits to the control surfaces of ancient biplanes the cables that helmeted pilots religiously checked before take-off. In the present context, and certainly as applied to the FA-18, the word "wire" has to do with control circuits. These take over from the usual mechanical or hydraulic links, conveying encoded instructions from the cockpit to the flight computers and thence to actuators coupled to the various control mechanisms throughout the plane. The computers process and execute instructions from the pilot while optimising - or configuring - the plane for take-off, landing, or J:!Ormal flight, taking into account the constantly changing fuel/ weapons load. Being connected also to strain gauges and other status indicators, they can intervene to protect the airframe from excessive forces, unless deliberately overridden by the pilot to cope with emergencies. Like a super "George" (auto pilot) the flight control computers can be programmed to relieve the pilot (or crew) of routine tasks, freeing them for activities which call for intuitive human reaction - a major requirement in a combat aircraft as complex as the FA-18. Indeed, in more radical military designs, where aerodynamic stability has been deliberately sacrificed for sheer get-up-and-go performance, the planes are reputed to be virtually unflyable without constant computer monitoring and intervention. The bottom line is that the desired end result can be achieved much more efficiently with a dedicated all-electronic control system, than by attaching separate electronic modules to otherwise conventional mechanical and hydraulic linkages. The allelectronic fly-by-wire approach is lighter, more accessible for automated system checking, and capable of higher redundancy to cope with possible airframe damage, especially in combat aircraft. Passenger, cargo planes Modern airliners like the new A-320 Airbus are also being designed around fly-by-wire technology but with emphasis on operational The Switch to UHF TV On the NSW South Coast My perception of the Australian radio/TV broadcast scene was recently altered by a couple of weeks spent holidaying on the NSW mid-north coast. The winter days were pleasantly mild but whatever else the area may have going for it, a surfeit of broadcast entertainment is certainly not one of them. Back in Sydney, most residents have access to five local TV stations, eight AM-stereo radio stations and about as twice as many FM stations, if one includes the community broadcasters. At most times, if one wants to look or listen, there's a reasonable chance of finding something to fit the mood. But in the unit where I was staying, despite a newly installed antenna/booster system, only two worthwhile TV signals were available - from the ABC and a regional commercial station. As for radio, the score was the noise-free ABC regional transmitter on the FM band and on AM, Radio National from the ABC, plus the local commercial broadcaster and an assortment of other signals that were only just listenable. In that short list, the role of the economy, reduced crew requirements and detailed monitoring of the flight envelope, both to assist pilots and to counteract possible errors of human judgment. According to news reports, one of the demonstration routines for the new A-320 Airbus has been a low-level fly-over at reduced speed and in such an attitude as to be inadvisable, if not hazardous, under ordinary manual control. It therefore came as quite a shock when a brand new A-320 recently crashed in full view of spectators. It was apparently performing just such a manoeuvre an event that gave rise to speculation that the flight computer system may have "crashed" a few critical seconds before the plane it was controlling! The item concerned the plight of a small group of viewers in the southern highlands of NSW, who would be affected by the pending re-deployment of TV stations in the adjoining lllawarra area of NSW from their present VHF channels to UHF - to make room for new FM radio services. Preliminary tests had indicated that the UHF signals might not reach this and other small communities that were currently able to receive the VHF transmissions. Naturally, the residents were upset. Admittedly, the DOTC is planning to explore some of these problems before the VHF transmitters are actually switched off but how isolated small communities will fare in the all-UHF era is the subject of assurances rather than guarantees. What I found disturbing in the admittedly off-the-cuff TV interview was the facile dismissal of the viewers'objections: (1 ). They were fringe viewers who'd been lucky enough to receive useable signals from the VHF transmitters. They might not be so lucky with the UHF service. (2). The government has made other provisions for fringe viewers. All they need to do is to install a dish and pick up services direct from the satellite. Not surprisingly, the viewers concerned were unimpressed by the idea of having to spend $2500-odd to receive fewer TV services than at present - the ABC and SBS only. While the DOTC has a job to do, it is to be hoped that they will be no less keen to advise and assist the disadvantaged as they may be to "pull the plug" on the VHF services by the due date. I support in principle more intelligent long-term . use of the VHF/UHF bands. But the need for a full complement of FM radio broadcasters is not so urgent that TV fringe viewers need to be summarily disadvantaged. The matter may possibly have been clarified before you get to read this but in the meantime, I was intrigued by further speculation that radar blips or other powerful radio signals may have corrupted the plane's control logic at a critical moment. Such an eventuality was named as the most likely reason for the puzzling crash of a West German fly-by-wire Tornado fighter in 1984, killing both crew members. They certainly appeared to have ignored warnings to keep well clear of ultra-powerful radio transmitters near Munich carrying transmissions for the "Voice of America" and "Radio Free Europe". It is true that both military and commercial aircraft carry communications and radar transmit- ters, some delivering quite high RF output. It can also be assumed that precautions would have been taken at the design stage against possible breakthrough into the flight control system from these sources. It does not follow, however, that filtering, shielding and isolation from identified potential inter£erence will guarantee ,immunity at all times, from other external sources of a different frequency, spectral content and power level whether unintentional or deliberately hostile. Just consider the following: • Interconnecting leads can act as receiving antennas, especially at frequencies to which they happen to be electronically resonant. • The growing use of plastics rather than metal in aircraft con- ABC is crucial and as if to emphasise the point, chairman David Hill chose that same period to announce plans to provide additional program services in the area. His positive note was in sharp contrast to the attitude of a staff member of DOTC (Department of Transport & Communications) who had been quoted in a TV news item a few days previously. OCT0BER 1988 37 THE WAY I SEEIT-CTD struction may cause on-board wiring to be less effectively shielded. • While much more compact and efficient, solid-state equipment is relatively vulnerable to high level electromagnetic radiation - suggested as a reason why some Russian combat aircraft are reputed to use valves for critical functions. Problem that won't go away After many years in the business, I have no illusions about the persistent and intrusive nature of unwanted interference affecting electrical signals. As already suggested, it has always been with us in one form or another. I find it difficult to believe that it will suddenly and obligingly disappear from the present scene. In the 1920s and 1930s, when half the fun of radio was in logging weak and distant stations, we had to battle daily with atmospheric and man-made "snap, crackle and pop"! Oh yes and with heterodyne whistles from other listeners' regenerative detectors. In the 1940s and 1950s, with a rising emphasis on car radios and mobile 2-way equipment, we had to deal with vibrator hash, silence the ignition of the host vehicle, and put up with the invisible cocoon of interference that surrounded many other vehicles on the road. For good measure, rural vibrator-powered receivers involved a parallel set of problems. The preoccupation with television in the 1960s and 1970s exposed the pattern of harmonics being radiated by most old-style amateur transmitters, while the CB radio boom, which followed, emphasised the vulnerability of contemporary consumer equipment generally to nearby radio frequency emissions. In Britain, for example, official complaints about interference from CB radio transceivers averaged several thousand per month during 1981/82! More recently, computer and other digital equipment has added a further dimension to the conflict between wanted signals, "dirty" power lines, and the complex clutter of extraneous electromagnetic radiations which is poised to corrupt the flow of encoded information. This leads to further observations: • Digital equipment can itself be a frequent source of interference. • Some CD players radiate enough RF to noticeably desensitise the associated FM tuner in a hifi system. • Sydney airport radar caused major hassles for engineers when setting up the UHF TV relay channels in Kings Cross. • A Sydney sound studio recently encountered serious hash problems from newly installed switchmode power supplies. • People fitted with heart pacemakers are warned against certain environments which might affect the electronics. • Lightning strikes or near misses are still a fact of life, as pointed out in "Serviceman's Log" (August 1988, page 31). It's all a matter of EMC In European technical parlance, if not elsewhere, discussion of such problems is commonly lumped under the heading "EMC". This is short for electromagnetic compatibility, or the compatibility of particular technologies and/or items of equipment with the overall electromagnetic environment. It involves both sides of the equation: (1). The ability of electrical or electronic equipment to operate efficiently without needlessly polluting the already cluttered electromagnetic environment. (Strictly speaking, even a clinically clean transmitter complicates the environment by emitting yet another signal). (2). The ability of electronic equipment to function normally, unaffected by extraneous electromagnetic energy, natural or man-made - a critical consideration in the present context. Despite countless discussions and conferences over the years, at national and international levels, it is doubtful whether governments and industry have handled electromagnetic compatibility any more deftly than other major environmental issues. Historically, we've tended to put up with RF interference problems - output and input - until they've become utterly intolerable and even then, settled for corrective measures that didn't involve dipping too deeply into the too hard basket! Rather than tackle spurious emissions head-on, it's been easier to maintain the S/N ratio by progressively upping the power of transmitters around the world, thereby complicating the electromagnetic environment in another way. As mentioned in the August issue, it took decades for the Australian Government (and others) to give their inspectors effective control over RF sources other than regulation transmitters. In Europe, the EEC group is at odds over standards, with full legal implementation still two or three years away. Wise after/before the event? If there's one lesson to be learned from contemporary environmental debate, it's that we tend to barge ahead on the basis of present need (or fad) only to later concede that more consideration should have Any new technology certainly hos to be a lot better than what was aboard the Tornado fighter that crashed in Germany in 1984. 38 SILICON CHIP 'Problems? ... and you don't have our I20page catalogue . .. Modern fighter aircraft use fly-by-wire technology but is it sufficiently developed for civilian passenger aircraft? been given to the consequences of so doing. It's easy enough to be perceptive after the event, but ever so much harder to correct the situation when the damage has been done. Now - not later - is the time to debate this whole fly-drive-ride by wire push. Does it represent real progress or the use of high technology for the s.ake of commercial exploitation? It may seem utterly presumptuous for a mere technical columnist in a magazine to query the judgment or the motives of the world aircraft industry or, in longer term, the manufacturers of ships, trains and road vehicles. But in effect, the question boils down to this: • Have electronic engineers associated with those industries really overcome, once and for all, the perennial problem of electromagnetic interference? • Can they guarantee that their electronic control circuits are totally proof against penetration and corruption by all conceivable extraneous sources of electro-magnetic radiation? • Assuming that reality falls somewhat short of perfection, can they be confident that a functional all-electronic control system can be at least as reliable as the most proven conventional methods? Any new technology certainly has to be a lot better than what was aboard the ill-fated Tornado fighter in 1984. The mere fact that the crew had been warned to keep away from the Munich transmitters indicates that the control equipment was known to be vulnerable to high-level electromagnetic radiation. In terms of EMC - electromagnetic compatibility - how much better is the equipment aboi:trd the FA-18, the A-320 and other comparable aircraft in 1988? Drive-by-wire cars? If it is appropriate to speculate about aircraft which are technologically up front in terms of design, production standards and maintenance, what is the likely position in relation to road vehicles and, in particular, to the ordinary family car? How compatible will it/they be in an increasingly cluttered electronic environment, especially after encounters with some of our local "she'll-be-rightmate" mechanics? In considering this subject, it certainly isn't reassuring to read reports in overseas journals of obscure malfunctions in vehicle cruise and engine management systems, which appeared to be due to electromagnetic interference. Those would be annoying, frustrating, perhaps expensive but a similar malfunction directly affecting control of the vehicle could be disastrous. Best we ask the questions now, rather than wait until we're behind the wheel! A promising way of "hardening" communications and control equipment (reducing its vulnerability both to the electromagnetic environment and to deliberate zapping) would be to replace the "wires" with optical fibres. Maybe we should shift the emphasis to flying-riding-driving by glass! ~ At last . .. a TRADE catalogue for the consumer ARISTA ... your one-stop problem solver. Video plugs and sockets ... Video extension speakers . . . Video flyleads ... Video RF interference filters .. . Video splitters ... Indoor antennas ... Video switching units . . . Down convertors . . . Video speaker controllers . . . Video camera lights . .. Video tape rewinders ... Video cine adaptors . . . Video head cleaners . . . Video splicing kits . . . Video storage cases ... Video dust covers . .. Video leads .. . Searl plug leads ... Video dubbing kits .. . Video headphones . . . Video shotgun and wireless microphone systems . . . Pre-amplifiers with video inputs .. . Video camera stands . . . Just about anything you want. ... Try us ... NOW! Get your catalogue FREE from your local ARISTA dealer or send $2.50 P&H and your return address to: ARISTI\. ELECTRONICS PTY LTD PO BOX 191, LIDCOMBE, NSW 2141 OCTOBER 1988 39