LEGO MINDSTORMS Robots, Manual, Page: 142 / 226

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grabber arm command it sent to avoid unnecessarily sending the same command twice.
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 repeatedly 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 complicated by two things:
1. Minerva should listen for the 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 able 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, however, 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 tasks:
message Watcher
The messagewatcher task examines the IR port for incoming messages. When one arrives, it is examined and the appropriate 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 commands from the
remote while the arm is moving. You could, for example, start driving forward while the arm was grabbing.
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
variable. The
heartbeatWatcher
adds one to this count; if it's ever more than one,
heartbeatWatcher
assumes it has lost contact with the remote. It then stops the
messageWatcher
task and shuts
down the robot's motors. When the heartbeat is heard again,
heartbeatWatcher
starts up
messageWatcher
once again.
Here's the entire program. I'll describe more of the details after the listing.
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#define FORWARD_MESSAGE 16
#define SPIN_MESSAGE 17
#define STOP_MESSAGE 18
#define GRAB_MESSAGE 19
#define RELEASE_MESSAGE 20
#define HEARTBEAT_MESSAGE 21
#define HEARTBEAT_TIME 20
int message;
int armLock;
int missedBeats;
task main() {
SetSensor (SENSOR_3, SENSOR_LIGHT);
armLock = 0;
missedBeats = 0;
start messageWatcher;
start heartbeatWatcher;