Distance Measuring Options
If you are building an autonomous robot then you need to have some sort of obstacle avoiding sensors. I have attached a Parallax PING ultrasonic sensor to the front of AVA using a servo (so that I can scan 180 degrees). Ultrasonic sensors are generally pretty accurate but since they use reflected sound to calculate distance, they don't perform well if the obstacle is sound absorbing. Ultrasonic sensors can also miss thin objects or objects that reflect the sound away from the sensor. However, the range of ultrasonic sensors is much better than IR. For the PING, the available sensing range is 2 cm to 3 m.
To address the ultrasonic issues, I also mounted a Sharp IR Distance Sensor (GP2Y0A02YK0F) above the PING. The IR sensors don't perform well outside but indoors there accuracy is good enough as long as you stay within the quoted detection limits. IR sensors are generally cheaper than ultrasonic, their beams are more directional (narrower) and reflectivity of the surface is more important than the sound absorbing properties of potential obstacles.
Putting the two sensors together is complementary and allows the short comings of both sensors to be addressed (to an extent).
Sharp GP2Y0A02YK0F IR Distance Sensor (20-150 cm)
Sharp manufactures a range of IR Distance Sensors. For the front sensor I selected the GP2Y0A02YK0F, which has a usable detection range of 20 to 150 cm's.
The Sharp GP2Y0A02YK0F measures distances in the 20–150 cm range using a reflected beam of infrared light. By using triangulation to calculate the distance measured, this sensor can provide consistent readings that are less influenced by surface reflectivity, operating time, or environmental temperature. The Sharp GP2Y0A02YK0F outputs an analog voltage corresponding to the distance to the reflecting object.
If you have a look at the GP2Y0A02YK0F datasheet, you will see that the analog voltage output does not have a linear relationship to distance. You can also see that the values go crazy below about 20 cm.
Noah over at the Arduino Mega Blog has reversed engineered this plot to work out the relationship between distance and the output voltage.
distance = 10650.08 * sensorValue ^ (-0.935) - 10 cm
Sharp GP2Y0A02YK0F IR Distance Sensor Arduino Library
To connect to an Arduino and get a distance you could just use Noah's formula above, but sometimes it is easier to wrap the complexity up in a library. I did a search and didn't find an existing library, so I decided to do one myself. Partly because I haven't done one before.
I did find a library for the GP2Y0A21YK IR Distance sensor (10 - 80 cm), but the characteristics must be different to the GP2Y0A02YK0F as the distances provided by the library are way off. Noah's formula on the other hand, provides very good correlation with the distances measured by the PING. For consistency, I based my library on what jeroendoggen did for his.
You can download the Sharp GP2Y0A02YK0F IR Distance Sensor (20-150 cm) Arduino Library files, and then follow these instructions to use it:
- Create a directory called GP2Y0A02YK0F within the libraries sub directory where your Arduino sketches are saved.
- Copy GP2Y0A02YK0F.h, GP2Y0A02YK0F.cpp and keywords.txt into the GP2Y0A02YK0F directory.
- Within the GP2Y0A02YK0F directory, create a sub directory called examples.
- Copy DisplayCM.ino into the examples sub directory.
- Restart the Arduino IDE to see the new library.
Sharp GP2Y0A02YK0F Mounted on AVA
The following photo shows the Sharp IR sensor mounted above the front PING on AVA. I have fitted a sensor shield to the Arduino Mega which makes it very easy to connect the various sensors to the micro controller.