How to make a Sharp IR sensor linear

Sharp IR proximity sensors are cheap and easy to use, but do not produce linear output. NOTE: this is a different issue than noise or jitter in the sensor data. That can be fixed with capacitors to ensure stable power – read this for explanation.

Here is one simple conversion to linear for the commonly used GP2YOA1YK sensor, accuracy is about +/-5%, but its drawback is that pow() is a slow function on the Arduino:

// Linearize Sharp GP2YOA1YK
float distance = 12343.85 * pow(sensorValue, -1.15) // centimeters


Here is a library that uses a lookup table – a much faster method because no calculations are done on the fly, it’s all done beforehand (you can see the list if you look inside DistanceGP2Y0A21YK_LUTs.h (LUT = Look Up Table) – http://code.google.com/p/gp2y0a21yk-library/.

Here is an equation tweaked for the longer range Sharp GP2Y0A02YK:

// Linearize Sharp GP2Y0A02YK
float distance = 30431 * pow(sensorValue, -1.169) // centimeters
// *or: *
float distance = 21208 * pow(sensorValue, -1.251) // inches


Here is a similar conversion function for the Sharp GPD212: http://www.arduino.cc/playground/Main/ReadGp2d12Range

If you measure values yourself, you can make an array of values and this is a very efficient way to look up those values (an example is included for the Sharp 2Y0A02): http://arduino.cc/playground/Main/MultiMap

Here is another solution that gives you a universal function that can be calibrated for different sensors (more info):
typedef const struct
{
const signed short a;
const signed short b;
const signed short k;
}
ir_distance_sensor;

// Parameters for GP2Y0A21YK sensor
const ir_distance_sensor GP2Y0A21YK = { 5461, -17, 2 };

// Converting the values of the IR distance sensor to centimeters
// Returns -1 if the conversion did not succeed
signed short ir_distance_calculate_cm(ir_distance_sensor sensor,
unsigned short adc_value)
{
if (adc_value + sensor.b < 0)
{
return -1;
}

return sensor.a / (adc_value + sensor.b) - sensor.k;
}
 

Here is another accurate function that also accounts for how curve changes when <10cm, if you have other sensors detecting that (normally objects that close give unusable values with this sensor): http://clockfort.com/code/CSHSharpIRSensor/CSHSharpIRSensor.cpp