Computer Solutions
as it can get for engineers. Rich Merritt, proprietor of M3 Marketing Communications, cites the case of an avionics company that abandoned Matlab in favour of C to develop resilient software. He says: “Matlab has three major issues: firstly it takes about ten times as much memory as an equivalent C program, secondly nobody understands what it is doing and thirdly it is almost impossible to test. “On one airplane, a staff of about 50 programmers
worked for two years and could not get it to work. So they called in a consultant and he rewrote the whole thing in C in six months - and it works. On another airplane, the thoroughly tested, simulated, and FAA-approved software went completely dead on the first flight. The test plane still had manual controls, so they could bring it home safely.
“When I worked for NASA, we had to test everything. Commercial jets continue to use ancient avionics and software developed in the 1980s, because they do not trust the new stuff or today’s crop of programmers. Let us just hope that avionics technology does not work its way over to automation and process control.” Brian Chapman, SCADA software engineer at Schneider
Electric, believes that centralised control is always more reliable than distributed control. “I do not believe that airliners or spaceships use distributed control. They use redundant central control. If anything goes wrong, the entire control system is replaced. As far as software validation is concerned, this is the best method. “All the communications between controllers is a huge source of unpredictable errors that you do not want to find out about 40,000 feet in the air. Distributed control does not really make sense. If a controller that actuates the rudder loses communications with the cockpit, just what kind of decision can it make? There is none. You lose control of that rudder and everyone dies. “Distributed control systems
Fig. 2. Engineering professionals need to integrate increasingly complex systems. Photo courtesy: Shanghai Fanuc
are not more reliable than distributed control systems; just the opposite. Distributed control systems are just cheaper. The hardware is cheaper. A PLC with 100 I/Os is more expensive than 4 PLCs with 25 I/Os each. Networking is cheaper. Add up the feet of wire and in most cases, there will be much less of it in a DCS. If one PLC dies, you only
Fig. 3. Engineers must ensure that individual component failure does not bring entire systems to a halt.
Photo courtesy: NASA. 56
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