Page 373 - Understanding Automotive Electronics

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10 DIAGNOSTICS

task to have requests for tasks explained in terms of both these rules and the
experience base that has led to the development of the expert system.
The general science of expert systems is so broad that it cannot be covered
in this book. The interested reader can contact any good engineering library for
further material in this exciting area. In addition, the Society of Automotive
Engineers has many publications covering the application of expert systems to
automotive diagnosis.
From time to time, automotive maintenance problems will occur that are
outside the scope of the expertise incorporated in the expert system. In these
cases, an automotive diagnostic system needs to be supplemented by direct
contact of the mechanic with human experts. The GM-CAMS system, for
example, has incorporated this feature into its customer support center.
Vehicle off-board diagnostic systems (whether they are expert systems or
not) continue to be developed and refined as experience is gained with the
various systems, as the diagnostic database expands, and as additional software
is written. The evolution of such diagnostic systems is heading in the direction
of fully automated, rapid, and efficient diagnoses of problems in cars equipped
with modern digital control systems.
OCCUPANT PROTECTION SYSTEMS
Occupant protection during a crash has evolved dramatically since about
the 1970s. Beginning with lap seat belts, and motivated partly by government
regulation and partly by market demand, occupant protection has evolved to
passive restraints and airbags. We will discuss only the latter since airbag
deployment systems can be implemented electronically, whereas other schemes
are largely mechanical.
Conceptually, occupant protection by an airbag is quite straightforward.
The airbag system has a means of detecting when a crash occurs that is
essentially based on deceleration along the longitudinal car axis. A collision that
is serious enough to injure car occupants involves deceleration in the range of
tens of gs (i.e., multiples of 10 of the acceleration of gravity), whereas normal
driving involves acceleration/deceleration on the order of 1 g.
Once a crash has been detected, a flexible bag is rapidly inflated with a gas
that is released from a container by electrically igniting a chemical compound.
Ideally, the airbag inflates in sufficient time to act as a cushion for the driver (or
passenger) as he or she is thrown forward during the crash.
On the other hand, practical implementation of the airbag has proven to
be technically challenging. Considering the timing involved in airbag
deployment it is somewhat surprising that they work as well as they do. At car
speeds that can cause injury to the occupants, the time interval for a crash into
a rigid barrier from the moment the front bumper contacts the barrier until the
final part of the car ceases forward motion is substantially less than a second.
Table 10.4 lists required airbag deployment times for a variety of test crash
conditions.

360 UNDERSTANDING AUTOMOTIVE ELECTRONICS