Page 370 - Understanding Automotive Electronics

2735 | CH 10 Page 357 Tuesday, March 10, 1998 1:27 PM

DIAGNOSTICS 10

To clarify some of the ideas embodied in an expert system, consider the
following example of the diagnosis of an automotive repair problem. This
particular problem involves failure of the car engine to start. It is presumed in
this example that the range of defects is very limited. Although this example is
not very practical, it does illustrate some of the principles involved in an expert
system.
A typical expert system The fundamental concept underlying this example is the idea of
formulates expertise in condition-action pairs that are in the form of IF-THEN rules. These rules
IF-THEN rules. embody knowledge that is presumed to have come from human experts (e.g.,
experienced mechanics or automotive engineers).
The expert system of this example consists of three components:
1. A rule base of IF-THEN rules
2. A database of facts
3. A controlling mechanism
Each rule of the rule base is of the form of “if condition A is true, then
action B should be taken or performed.” The IF portion contains conditions
that must be satisfied if the rule is to be applicable. The THEN portion states
the action to be performed whenever the rule is activated (fired).
The database contains all of the facts and information that are known to
be true about the problem being diagnosed. The rules from the rule base are
compared with the knowledge base to ascertain which are the applicable rules.
When a rule is fired, its actions normally modify the facts within the database.
The controlling mechanism of this expert system determines which
actions are to be taken and when they are to be performed. The operation
follows four basic steps:
1. Compare the rules to the database to determine which rules have the IF
portion satisfied and can be executed. This group is known as the conflict
set in AI parlance. A conflict set is a type of set, as in set theory.
2. If the conflict set contains more than one rule, resolve the conflict by
selecting the highest priority rule. If there are no rules in the conflict set,
stop the procedure.
3. Execute the selected rule by performing the actions specified in the
THEN portion, and then modify the database as required.
4. Return to step 1 and repeat the process until there are no rules in the con-
flict set.
In the present simplified example, it is presumed that the rule base for
diagnosing a problem starting a car is as given in Figure 10.15. Rules R2
through R7 draw conclusions about the suspected problem, and rule R1
identifies problem areas that should be investigated. It is implicitly assumed
that the actions specified in the THEN portion include “add this fact to the
database.” In addition, some of the specified actions have an associated

UNDERSTANDING AUTOMOTIVE ELECTRONICS 357