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3rd Party Sensors with VEX: Sharp IR Rangefinder

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The VEX Robotics System includes a growing, powerful set of official sensors. That said, the electronics world is overflowing with other types of cool and useful sensors. The great news is, both the VEX PIC and the VEX Cortex microcontrollers are extremely versatile and can be used with many of these sensors. Both the VEX PIC and VEX Cortex can be used with Analog Sensors that operate between 0 and 5 volts (+/- .5 volts), and have Power, Ground, and Signal lines.

One such sensor is the Sharp GP2D12 IR Rangefinder.

The Sharp IR Rangefinder allows your robot to determine distance from the nearest object, much like the VEX Ultrasonic Rangefinder. Unlike the VEX Ultrasonic Rangefinder, which uses  sound waves to measure distance, the Sharp IR Rangefinder uses infrared light. On one side of the sensor an infrared LED shines a beam of light, which is reflected back by the closest object, and detected by the receiver on the other side of the sensor. The sensor then uses a method called triangulation to determine how far away the object was.

The basis for triangulation is that objects at different distances will reflect the infrared beam back to the receiver at different angles. The varying angles produce different voltage levels in the sensor, and in turn sensor values that can be used to calculate distance. See below:

The Sharp IR Rangefinder provides very reliable distance values ranging from 10 to 80 centimeters away. One big advantage with using the IR Rangefinder is that it’s not affected by soft for angled objects that cause the VEX Ultrasonic Rangefinder to fail.

One challenge with using the sensor is that the raw values provided do not directly correlate to useful distance values, and they’re also non-linear. This means that we must perform a calculation on the raw sensor data first. Documentation for the sensor tells us that we can calculate the distance using the following formula:

Voltage = 1 / ( Distance + 0.42 )

Which, when rearranged gives us:

Distance = (1/Voltage) – 0.42

One additional challenge is that we can’t directly access the voltage returned by the sensor in our program. What we can access is the sensor value, which is proportional to the voltage – we’ll just have to take a conversion factor into account.

Below is a sample program written for the Sharp IR Rangefinder. It contains the function “IRValue()” which will perform the necessary calculation and return the useful distance data, in centimeters. Note that the “sensorPotentiometer” type was used, since there is no “IR Rangefinder” sensor type in ROBOTC, and the Potentiometer type will return the full, unmodified analog data (unlike other sensor types like the Gyro).

#pragma config(Sensor, in5,    sharp,               sensorPotentiometer)
#pragma config(Motor,  port2,           rightMotor,    tmotorNormal, openLoop, reversed)
#pragma config(Motor,  port3,           leftMotor,     tmotorNormal, openLoop)
//*!!Code automatically generated by 'ROBOTC' configuration wizard               !!*//

//Function uses data from the IR Rangerinder to calculate and return distance
int IRValue(tSensors sensorPort, float conversionFactor = .0000469616, float k = .42)
{
return (1.0 / (SensorValue[sensorPort] * conversionFactor)) - k;
}

task main()
{
wait1Msec(2000);
clearLCDLine(0);
clearLCDLine(1);

//While true...
while(true)
{
//If the robot is greater than 25 cm away...
if(IRValue(sharp) > 25)
{
//Move Forward
motor[rightMotor] = 30;
motor[leftMotor] = 30;
}
else
{
//Stop
motor[rightMotor] = 0;
motor[leftMotor] = 0;
}

//Display IR Sensor Data to LCD
displayLCDString(0, 0, "IR Value:");
displayLCDNumber(0, 9, IRValue(sharp), 3);
displayNextLCDString("cm");
}
}

Our Sharp IR Rangefinder was previously configured wired for an older microcontroller called the Handy Board. Like the VEX Cortex and PIC, it had wires for Power, Ground, and Signal, so we were able to use simple jumper wires to adapt it for the Cortex.

Front of the robot with Sharp IR Rangefinder:

Top of the robot, with connection from Sharp IR Rangefinder to VEX Cortex Analog Port 5:

And finally, here’s a video of the code from above running on the robot:

For more information on interpreting and using the data provided by the Sharp IR Rangefinder, check out this tutorial.

Written by Jesse Flot

November 8th, 2011 at 11:13 am