4. Feedback in mechanical engineering
This goes back a long way, right back to ancient times, where float valves
were used to regulate the speed of Greek and Roman water clocks. A float valve
uses some air filled object floating on the surface to operate a valve which
lets water in (or out) to restore the desired level. Float valves are still
used today in toilet flush systems, where a ballcock is used to open or close the water inlet. This (as
you should know by now) is an example of negative feedback, because it keeps
the water level steady. Another current use of float valves is in carburettors,
though these are dying out in favour of fuel injection systems.
These
ancients were pretty smart guys though, not limited to simple water clocks. The
maestro seems to be one Ctesibius or Ktesibios (around 270 bc), who invented a number of mechanical devices,
including a water organ with rows of sound pipes and a keyboard; and a very
fancy water clock with a number of
whistling birds and ringing bells, no less. Unfortunately, none of his
work survives, but he was certainly a pioneer of feedback mechanisms.
A little later on brings us to the windmill,
which was enhanced
in 1745 by a blacksmith, one Edmund Lee who added a fantail to keep the face of
the windmill pointing into the wind. This of course is yet another example of
negative feedback. These mechanical negative feedback mechanisms are also
examples of simple control systems, but purely mechanical rather than
electronic, and it is the
electronic form which dominates these days.
The next big step forward in mechanical control was
the use of centrifugal governors. It seems their first use was in 1787,
when Thomas Mead regulated the speed of rotation of a windmill by using a
centrifugal pendulum.
But the best known early use was by James Watt in 1788 to regulate the
speed of his steam engine. This was a big step forward in mechanical engine
design, and one of the keystones of the industrial revolution.
It works
by having a pair of weighted balls suspended from arms attached to a spindle,
which is connected to the output shaft of the engine. The arms are linked to a
valve that regulates the steam input. As the engine speeds up beyond the
desired rate, the spindle rotates faster, and the flyballs are driven outwards
and upwards by centrifugal force. This movement partly closes the valve,
reducing the amount of steam delivered to the engine. Likewise if the engine
slows too much, the valve lets in more steam to speed it up again. A beautiful
example of negative feedback, keeping the speed stable.
Governors are
also used in gas and steam turbines and internal-combustion engines, where the
governor regulates the flow of fuel. In hydro-electric turbine generators, the
governor alters the water flow by opening and closing gates and valves, in
aircraft engines it varies the pitch of the propeller blades attached to the
engine.
An ideal governor
controls speed exactly, with no variation. This requires something that
responds quickly and accurately. But things don’t always work do smoothly;
there is often a time lag while the governor responds, and this can cause
instability, with the system varying alternately between too fast and too slow.
People can also
act as governors. When you drive a car, you exert pressure on the accelerator
to keep the car at the desired speed (well, you should). If the car starts to
go too fast, you ease off the pressure, and if it starts to go too slow, you
increase it. A learner driver can have problems with this control, and this can
lead to the classic lurching motion, which is a form of the instability
mentioned. As the car speeds up, the learner’s foot is thrown back a little too
much, and the car slows quickly, which throws the learner forward and the
pressure is increased too much. This is a governor that is over responding.
Responding too much, too little or too late is a classic problem with any
feedback control system, and there are many examples of problems this can cause
in life, from central heating systems to the management of the country’s
economy.
Another early
example of mechanical feedback was in The Great Eastern, which
was one of the largest steamships of its time. It possessed a steam powered
rudder with feedback mechanism designed in 1866.
By the late 20th
century, mechanical feedback was giving way all round to microprocessor based
electronic control systems. Systems using vacuum pressure to adjust the timing
of car engines were being replaced by engine management systems. As is often
the case with developing technology, the components for electronic engine
management had actually been in place for quite a long time before they found
their way into the average production car.
There are a
couple of reasons for this, and these reasons are often not well understood by
pundits trying to predict when new developments will become widespread in use.
Firstly, there is the simple reason that it takes a long time for a new
technology to be developed from initial prototypes to large scale mass
production. There is a huge difference between making one or two, or even a few
dozen working devices, and making hundreds of thousands or millions. Production
engineers are quite a different breed from research and development engineers,
and there is often a certain amount of healthy friction between them. The R
& D (research and development) guys tend to look down on the production
teams as merely putting into practice what the geniuses in R & D have
produced. The production teams are somewhat scornful of the R & D guys,
regarding them as living in an ivory tower, with no concept of real world
problems. And there is also a big difference between getting something to work
in the development department to making sure that it works ‘in the field’ where
variations in components and environment can play havoc with the intentions of
the designers. As indeed can the infinitely subtle ways in which those
confounded people, the users, can abuse and misuse the product – a process
which engineers often refer to as ‘finger trouble’. Just as teachers sometimes think
that schools would be fine if it was not for the students, engineers sometimes
cannot help feeling that the product would be fine without people using it.
But there is a
second reason why it often takes a long time for a new development to make it into
the marketplace. A new technology may have a cost/benefit advantage over the
old technology, but the trouble is, the old technology keeps improving as well.
Or at least, it keeps improving up to a point where it finally runs out of
steam. So it can take much longer than expected for the new development to
overtake the old one. In fact, sometimes it never does. It took longer than
many people expected for semiconductor memories to overtake the now obsolete
‘core’ memories, back in the 1960-70’s, because people kept finding better and
cheaper ways to make the old core memories. Primarily, this involved using
female labour in the Far East ,
whose nimble fingers could make the small devices better and faster than
previously expected. But later on, Intel spent a large amount of time and money
developing ‘bubble’ memories, based on magnetic bubbles. These never really
caught on, because semiconductor memories and floppy/hard disc technology was
developing at such a pace.
Machine
Tools.
I have already
described how robots use feedback, but there is another kind of mechanical
system that uses feedback, and that is the Machine Tool. Machine Tools are in a
sense, the ancestors of robots. Whereas robots can be (but are not always)
mobile, machine tools are always firmly fixed to the floor. It is a question of
bringing Mohammed to the mountain, if the mountain cannot be moved. Parts to be
manufactured are ferried to one or more machine tools to be punched, drilled,
milled, bent, cropped or otherwise manipulated. Simple machine tools (such as a
lathe) can be, and still are, operated manually. But large complex tools are
now generally controlled by programs, like computers.
The program
controls the sequence of operations to be performed, and as in robots, feedback
can be used to detect the results of the operation so it can be controlled more
precisely. For example, feedback can be used to take into account the wear and
tear on the machine’s own tools. So as the tool wears, the machine can be made
to apply more pressure, or to move further.
Early machine
tools were controlled by programs on a variety of media – punched paper tape or
cards, magnetic tape – but now generally they are driven by microprocessors
with semiconductor memory. Most small ‘computers’ now are in fact
microprocessors which are built into other equipment – cars, washing machines,
Televisons, DVD players, as well as machine tools. My car has, I believe, about
6 microprocessors hidden away somewhere, one of which just controls the ‘crash
detection’ system. Microprocessors like these are called ‘embedded’, because
they are included in the product, but hidden away from the user. Which is just
as well, otherwise there would be even more finger trouble.
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