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S12CPUV2 Reference Manual, Rev. 4.0

342

Freescale Semiconductor

9.2.2 Rule Evaluation (REV and REVW)

Rule evaluation is the central element of a fuzzy logic inference program.

This step processes a list of rules from the knowledge base using current

fuzzy input values from RAM to produce a list of fuzzy outputs in RAM.

These fuzzy outputs can be thought of as raw suggestions for what the

system output should be in response to the current input conditions. Before

the results can be applied, the fuzzy outputs must be further processed, or

defuzzified, to produce a single output value that represents the combined

effect of all of the fuzzy outputs.

The CPU12 offers two variations of rule evaluation instructions. The REV

instruction provides for unweighted rules (all rules are considered to be

equally important). The REVW instruction is similar but allows each rule to

have a separate weighting factor which is stored in a separate parallel data

structure in the knowledge base. In addition to the weights, the two rule

evaluation instructions also differ in the way rules are encoded into the

knowledge base.

An understanding of the structure and syntax of rules is needed to

understand how a microcontroller performs the rule evaluation task. An

example of a typical rule is:

If temperature is warm and pressure is high, then heat is

(should be) off.

At first glance, it seems that encoding this rule in a compact form

understandable to the microcontroller would be difficult, but it is actually

simple to reduce the rule to a small list of memory pointers. The antecedent

portion of the rule is a statement of input conditions and the consequent

portion of the rule is a statement of output actions.

The antecedent portion of a rule is made up of one or more (in this case two)

antecedents connected by a fuzzy and operator. Each antecedent

expression consists of the name of a system input, followed by is, followed

by a label name. The label must be defined by a membership function in the

knowledge base. Each antecedent expression corresponds to one of the

fuzzy inputs in RAM. Since and is the only operator allowed to connect

antecedent expressions, there is no need to include these in the encoded

rule. The antecedents can be encoded as a simple list of pointers to (or

addresses of) the fuzzy inputs to which they refer.

The consequent portion of a rule is made up of one or more (in this case

one) consequents. Each consequent expression consists of the name of a

system output, followed by is, followed by a label name. Each consequent

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