Simple, practical sensor, a steal

Although the goods comes without any datasheet or similar However, in a padded envelope, can be found under the name of HC-SR04 readily various data sheets, radiation patterns, source code for the Arduino, etc. just type in the search engine.

I have the code of Robodino used and get instantly meaningful values. Yield identical results all four with me. To use reliable measurements for precise orientation of moving units, should look at the radiation patterns, conduct their own tests, using various mathematical methods to obtain "a clear picture", otherwise discourages the potential robot back before every crumb.

The beam angle is according to data sheet 15 degrees, about 20 cm comes out that even some out. By this he is going strong in the width up to almost 90 degrees less than 10 cm.
The reflection I testhalber performed with wallpapered walls, monitors, a piece of Velcro Velcro and a plastic ruler. Even the latter two materials supply side with a few millimeters reflecting surface (eg ruler edge) already a decent result.
The data sheet write something 0.3 cm accuracy. These ensure a certain averaging is necessary. The unaveraged readings fluctuate with me mostly to + - 1 cm, more rarely by + - 2 cm.

The module uses two signal pins of the Arduino (Ping and Echo). The source code makes it clear how it really works.

Conclusion: Works great. Value for money unbeatable. Information material for it are everywhere.