The Catapult

Figure 4-4 shows the completed catapult robot, programmed using the NXC code in this excerpt.

Figure 4.4 The completed catapult

NXC

For the NXC program, start with declaring the two touch sensors:

// enable NXC
#include "NXCDefs.h"
// main
task main() {
	// define sensors at port 1 and 2 to be touch sensors
	SetSensorType(IN_1,IN_TYPE_SWITCH);
	SetSensorType(IN_2,IN_TYPE_SWITCH);
}

The infinite loop looks very similar to the one in RobotC, so proceed to the lifting of the crank mechanism:

 …
task main() {
	…
	SetSensorType(IN_2,IN_TYPE_SWITCH);
	// endless loop
	while(1) {
		// lift crank mechanism
		RotateMotor(OUT_A, 75, 30);
	}
}

Running the gear wheel until the first touch sensor triggers a "pressed" event loads the catapult:

…
task main() {
	…
	while(1) {
		…
		RotateMotor(OUT_A, 75, 30);
		// load catapult
		// run motor until the first
		// touch sensor gets pressed
		OnFwd (OUT_C, 100);
		while(Sensor(IN_1) == 0) {
			// do nothing
		}
		Off(OUT_C);
	}
}

Waiting for the user to press the second touch sensor is done as follows:

…
task main() {
	…
	while(1) {
		…
		Off(OUT_C);
		// wait for the user to press the second touch sensor
		while(Sensor(IN_2) == 0) {
			// do nothing
		}
	}
}

Now drop the crank:

…
task main() {
	…
	while(1) {
		…
		while(Sensor(IN_2) == 0) {
			// do nothing
		}
		// drop crank mechanism
		RotateMotor(OUT_A, -75, 30);
	}
}

And wait:

…
task main() {
		…
	while(1) {
		…
		// drop crank mechanism
		RotateMotor(OUT_A, -75, 30);
		// wait for two seconds
		Wait(2000);
	}
}
Here's the complete NXC program for the catapult:
// enable NXC
#include "NXCDefs.h"
// main
task main() {
	// define sensors at ports 1 and 2 to be touch sensors
	SetSensorType(IN_1,IN_TYPE_SWITCH);
	SetSensorType(IN_2,IN_TYPE_SWITCH);
	// endless loop
	while(1) {
		// lift crank mechanism
		RotateMotor(OUT_A, 75, 30);
		// load catapult
		// run motor until the first
		// touch sensor gets pressed
		OnFwd (OUT_C, 100);
		while(Sensor(IN_1) == 0) {
			// do nothing
		}
		Off(OUT_C);
		// wait for the user to press the second touch sensor
		while(Sensor(IN_2) == 0) {
			// do nothing
		}
		// drop crank mechanism
		RotateMotor(OUT_A, -75, 30);
		// wait two seconds
		Wait(2000);
	}
}