FCL example explained

Fuzzy Control Langage FCL is defined by IEC 1331 part 7. It's a simple language to define a fuzzy inferece system. We'll take a look at an example, for a more detailed explanation, please read the spec.

Keep in mind that FCL is defined as a 'Control language', so the main concept is a 'control block' which has some input and output variables (it's not a 'programm' in the usual way).
We'll be using this example, first take a look at it.

Ok, let's try to annalize each line:

First you define each FUNCTION_BLOCK (there may be more than one in each file)
FUNCTION_BLOCK tipper
Then input and output variable/s are defined (variable type is only REAL, integer is not implemented yet)
VAR_INPUT service : REAL; food : REAL; END_VAR VAR_OUTPUT tip : REAL; END_VAR
How each input variable is fuzzified is defined in FUZZIFY block. In each block we define one or more TERMs (also called LinguisticTerms). Each term is composed by a name and a membership function. E.g.:
FUZZIFY service TERM poor := (0, 1) (4, 0) ; TERM good := (1, 0) (4,1) (6,1) (9,0); TERM excellent := (6, 0) (9, 1); END_FUZZIFY
In this lines we define how variable service will be fuzzified. Three terms are used, for instance term poor uses a piece-wise linear membership function defined by points x_0 = 0, y_0 = 1 and x_1 = 4, y_1 = 0

food variable fuzzify block is define likewise:
FUZZIFY food TERM rancid := (0, 1) (1, 1) (3,0) ; TERM delicious := (7,0) (9,1); END_FUZZIFY
Output variables are defuzzified to get a 'real' output number, this is defined in DEFUZZIFY block. Like FUZZIFY block, linguistic terms (or TERMs) are defined:
DEFUZZIFY tip TERM cheap := (0,0) (5,1) (10,0); TERM average := (10,0) (15,1) (20,0); TERM generous := (20,0) (25,1) (30,0);
Then we may define some other parameters:
METHOD : COG;
Use 'Center of gravity' as defuzzifier's method.
DEFAULT := 0;
Use '0' as default value (if no rule actuvates this variable).

We can define now the rules. This is done using a RULEBLOCK. First we define some parameters:

RULEBLOCK No1
    AND : MIN;

Use 'min' for 'and' (also implicit use 'max' for 'or' to fulfill DeMorgan's Law)

ACT : MIN;
Use 'min' activation method

ACCU : MAX;
Use 'maximum' as accumulation method.

And now define some rules (3 in this case)
RULE 1 : IF service IS poor OR food IS rancid THEN tip IS cheap; RULE 2 : IF service IS good THEN tip IS average; RULE 3 : IF service IS excellent AND food IS delicious THEN tip is generous; END_RULEBLOCK
Ok, that's it, you've got a fuzzy controller.