Page 165 - The Unofficial Guide to Lego Mindstorms Robots

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To provide some feedback to the human operator, lightWatcher plays tones when it sends the grab or release commands.

The hearbeat task is very simple. It repeated ly sends the heartbeat command to the IR port.

Programming Minerva

Minerva's program, then, must listen for incoming messages on the IR port and respond to them. This is fairly simple, but it's
complica ted by two things:

1. Minerva should listen for th e heartbeat commands from the remote. If she doesn't hear them, she should stop what she's
doing until the IR link is established again.

2. The human operator should be abl e to change the direction of the arm as it is moving, without waiting for an entire grab or
release cycle to be completed. This feature, how ever, should not interfere with Minerva's ability to stop the arm when it moves
up as far as it can go.

Minerva's program is split into four primary task s:

message Watcher
The mes sagewatcher ta sk examines the IR port for incoming messages. When one arrives, it is examined and the
op
appr riat e action is taken.

grab and release
The grab and release tasks are kicked off by messageWatcher to control Minerva's arm. They are separate tasks so
that messageWatcher can continue to receive com mands from the remote while the arm is moving. You could, for
example, start driving forward while the arm was gr abbin g. grab and release have the ability to interrupt each other, so
you can change the direction of the arm as it's moving .

heartbeatWatcher
This task keeps count of missing heartbeats. When messageWatcher receives a heartbeat, it subtracts one from a tally of
missing heartbeats, kept in the missedBeats varia ble. The heartbeatWatcher adds one to this count; if it's ever more
than one, heartbeatWatcher assumes it h as lost con tact with the remote. It then stops the messageWatcher task and
shuts down the robot's motors. When the heartbeat i s heard again, heartbeatWatcher starts up messageWatcher
once again.

Here's the enti re program. I'll describe more of the details after the listing.