Issue
93 April 1998
Picaro:
A Stamp-like Interpreted Controller
by
Tom Napier
For
years, Tom’s been itching to control the instruction
sets of processors. Using a PIC, some memory, and
an interpreter, he bypasses the processor hurdle
and writes his own language. He shows you how to
do it, too.
The
system performed very well during informal testing.
As expected, the algorithm learns occupancy patterns
within a maximum of three weeks. When changes to the
schedule are slight, the system the following week.
When
presented with a markedly different pattern, the software
waits a couple weeks, as designed, before adopting the
new pattern into its permanent record. Because the algorithm
is still verifying the validity of a pattern on the
second week, it’s too late to act on it even if
it passes the test, so the third week is when the anticipator
responds to a new pattern.
Although
the algorithm performs well, it has one weak spot. When
the patterns contain a large number of alternately occupied
and unoccupied periods, as shown in Figure 7, the algorithm
is threatened by what I call the "comb problem."
The problem is that the shifting process used in the
matching algorithm can yield a high score for patterns
that look similar (like two combs) but that are almost
opposite in timing from each other.
Needless
to say, this mode of operation is not likely to occur
because of the rarity of a comb-like pattern arising
in normal circumstances. And in practice, even if the
comb problem were to appear, it may be irrelevant because
the anticipator looks ahead for some time.
In
other words, if the area were seeing very frequent occupancy
changes, the anticipator ends up expecting arrival pretty
much all the time. Fortunately, this is the behavior
you want out of the system anyway, so the comb problem
is fairly benign. However, I feel that it’s important
to point out this theoretical weakness.
