Although the tests are mainly using nxt sonar sensor, I suspect this is the issue may be typical for most sonar sensors.


To obtain distance data, sonar can indeed be very problematic when the object happens not to be flat; unlike distance feedback by, say, a laser sensor.

We collect some experimental data about mapping out the reading for a rectangular box. As expected, some interesting anomaly approaching the far corners. I tried to lookup online about technical information about the sonar sensor, but unfortunately all of them are about flat object only . Any technical information about how sonar works will be helpful.

This is not a great diagram, but hope to get the idea across.


the grey area is where the sonar's feedback varies far from expected (calculated values). Calculated distance = simple trig calculation from the bottom center.

Not to be rude, but the NXT sensor is actually a ultrasonic sensor, as in it sends ultrasonic sound waves out as opposed to the infrasonic usually used be actual sonar sensors.

Yes, Ultrasonic sensors such as these do have problems getting a distance on a curved or angled surface, as it bounces the sound waves off at an angle, away from the sensor. This is problematic, because the sensor gauges distance by how long it takes the sound it shot out to bounce back to it. This is a problem with practically all Ultrasonic sensors, especially the LEGO ones. IR Light reflectance works better, but only at small distances. As you said, Laser Rangers are by far the best, but are usually too exspensive for the common person (and kinda dangerous). The kind of sonar on submarines and such have many, many transmitters and receivers, so it is virtually surrounded by sound. Also, things such a cloth tend to absorb sound waves, causing NONE to be returned.

So, have I enlightened you at all?

cool. Thanks. This definitely helps. I did not know the difference between Infrasonic and ultra sonics. That's definitely something interesting to read on.

So, with the limitation of the technology available to the kids , I would assume they'll just have to create value map for the field using the ultrasonic sensor, or they will have to make sure the sensor is facing the flat surfaces only.

Take a look at the beam pattern section on this page: http://www.vexforum.com/wiki/index.php/ ... nge_Finder
It's for the VEX Ultrasonic Rangefinder, but the information still applies.