Page 151 - The Unofficial Guide to Lego Mindstorms Robots

140

: seek
forward
0 1 TIMER_SET
BEGIN 2 sensorValue threshold @ 3 - < UNTIL
2 sleep
1 TIMER_GET returnTime !
stop
;

: return
forward
returnTime @ sleep
stop
;

: retrieve
seek
grab
turnAround
return
release
turnAround
;

: Minerva
initialize
calibrate
5 0 DO
retrieve
LOOP
;

The pbFORTH implem entation of Minerva's program closely resembles the NQC version of the program presented in Chapter
5. By the time all the simpler words are defined, th e more complex words are simply combinations of the earlier ones. Keep
this i mind—I'll describe the words in Minerva's p rogram starting at the end and returning to the beginning.
n

The main program word, Mi nerva, pretty much describes itself. It initializes the RCX, which includes configuring input 3 as
a light sen sor inpu t. Then it calls calibrate to take an average reading of the light sensor. Finally, it loops five times,
calling retrieve to go pick up something and bring it back.

The retrieve word is also pretty self-explanatory. It calls seek to drive forward and look for an object to pick up. Then
grab is called to grab the object. Minerva turns around and heads back to her starting point with turnAround and
return. Then she drops the object, with release, and turns around again with turnAround. With the simpler words
defined correctly, retrieve is a highly readable piece of programming:

: retrieve
seek