Page 152 - The Unofficial Guide to Lego Mindstorms Robots

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grab
turnAround
return
release
turnAround
;

I won't describe every word in Minerva's program, but I do want to cover the more interesting ones. First, let's take a look at
s eek and return. The interesting thing about seek is that it uses one of the RCX's timers to figure out how long Minerva
moves forward befo re finding something to pick up. Timer 1 is used for this purpose; the sleep word uses timer 0. seek
fi rst sets timer 1 to 0 like this:

0 1 TIMER_SET

Then Minerva moves forward until she "sees" something to pick up. The current timer value is recorded in a variable,
returnTime, like this:

1 TIMER_GET returnTime !

When it's time for Minerva to drive back to her starting point, in ret urnTime, we just need to drive back for the stored
amount of time:

: return
forward
returnTime @ sleep
stop
;

A nother interesting word is calibrate, which takes ten readings of the light sensor and calculates an average value. This
value is used to determine if the light s ensor "sees" something that can be picked up or not. We begin by pushing the current
ru nning total on to the stack. To begin with, the total is zero:

: calibrate
0

T hen we just run in a loop from 0 to the constant value NUMBER_OF_SAMPLES. Each time through the loop, we read the
value of input 3 and ad d it to the running total on the stack. This is done every tenth of a second, by calling 1 sleep:

NUMBER_OF_SAMPLES 0 DO
2 sensorValue +
1 sleep
LOOP

Once this is done, calculating an average is a simple matter of division:

NUMBER_OF_SAMPLES /

Then the average value is stored in the threshold variable:

threshold !