Throttle-by-Wire

[Phil Hunter]

The analogy between cruise control and TBW is a very poor
one from a "systems" perspective, and illustrates my concern
that many of you are "in over your head" and don't even know
it.

Cruise control is basically a governor, the error is the diff
between the set speed and actual speed, what's being contolled
weighs a lot and reacts slowly in terms of human response times,
the control response time is purposely slow also, there is a
speed threshold below which is doesn't run, it's optional, and
it's designed to fail, in time, generally in a non-op state.

Conversely, TBW is always on, needs to be as close to 100%
reliable as possible, has a faster than human response time
otherwise only locomotive engineers would buy it, the mass
being controlled ( throttle butterfly ) is tiny, has multiple
control loops such as the difference between where the blades
actually are and where they are supposed to be as well as the
diff between where they are and where the driver wants them to
be, possibly cruise, traction and/or turbo anti-lag control
loops as well.

In my limited experience, control loops can appear to be stable,
but then lose stability due to a transient or step function input.
They can fail to either limit, as in going to idle or WOT, or they
can oscillate back and forth, i.e., WOT, idle, WOT, idle...

Simple scenario: twisty mountain road, nighttime, foggy, slick.
Few drivers in the world would be using cruise control. The TBW
controller sees full voltage from the throttle potentiometer,
did the driver floor it, or did the ground wire fall off from
hitting a pothole? The lives of the occupants probably depends
on how it reacts, 'cuz if you can turn the ignition key to just
the right spot where the engine stops but the steering wheel isn't
locked in such a situation, you're much better than 99 and 44/100ths
percent of the drivers on the road.

Another simple scenario: the butterfly feedback pot says the blade
angle is less than the TBW controller thinks it should be, is it
because the driver commanded more throttle, is the shaft binding,
did the voltage droop due to the AC compressor engaging, or did
the positive wire fall off? Each situation requires a different
response from the controller.

Hopefully you folks have got the idea, there's plenty of low to
moderate risk projects to get us into trouble, those who really
know how to do TBW probably wouldn't, those who think they know
may win a Darwin award or jail time for manslaughter before they
learn.

rgds,
philh
(digest)


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