Page 167 - The Unofficial Guide to Lego Mindstorms Robots

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stop release;
OnFwd(OUT_A);
until (SENS OR_3 == 100);
armLock = 1;
OnRev(OUT_A);
until (SENSOR_3 != 100);
armLock = 0;
Off(OUT_A);
}

task release() {
until (armLock == 0);
stop grab;
OnRev(OUT_A);
until (SENSOR_3 == 1 00);
armLock = 1;
OnFwd(OUT_A);
until (SENSOR_3 != 100);
armLock = 0;
Off(OUT_A);
}

task heartbeatWatcher () {
while (true) {
if (missedBeats > 1) {
Pla ySound (SOUND_DOWN);
stop messageWatcher;
Off(OUT_C);
until (armLock == 0);
Off(OUT_A);
until (Message () == HEARTBEAT_MESSAGE);
PlaySound(SOUND_UP);
missedBeats = 0;
start messageWat cher;
}
missedBeats = missedBeats + 1;
Wait(HEARTBEAT_TIME);
}
}

Don't Break That Arm

The grab and release tasks, as I said, have the ability to interrupt each other. They use a variable, armLock, to avoid
potentially dangerous situations. For example, suppose that release is running. It drives the arm motor in reverse until the
arm is up and the limit touch sensor is pressed. Then the motor is run forward to release the touch sensor. Before the touch
sensor is released, however, suppose grab starts up. It stops release and runs the motor forward until the touch sensor is
pressed. It's already pressed, so grab starts running the motor in reverse, waiting for the touch sensor to be released. Since
the arm is already at the top of its travel, running the arm motor in reverse creates pressure on the gears and will either break
the arm apart or cause the gears to skip.