Outdoor, in urban or wild environments, you may be served by Global Positionning System, however, Indoor equivalent is quite rare. Some are focused on WIFI signal, others on RFID, trying (badly) to prove themselves as precise as GPS. Seeking a centimeter-accurate solution within closed space actually turned into assault course.
My first augmented environment prototype relied on stereoscopic vision using two webcams and an infrared marker. It was accurate but expensive (in both computing and cost). For more than a year I was working on an ultrasonic positioning system filling specific needs:
- Passive localization (once set up, receiver just receives, no dialog with emitter)
- Centimeter accuracy
- Cheap (and scalable)
- Fitting our everyday indoor spaces
The resulting system is similar to GPS :
- Scattering of several emitters of known position
- Every one broadcasting on different frequency
- Receiver calculates emitter remoteness
For a room, you just need 3 ultrasonic emitters and a radio channel. Ultrasonic signals are broadcasted one by one using distinct frequencies, while a radio signal is a sign of exact emission time.
Knowing the moment of emission, Reciever can work out a distance for any received ultrasonic signals. As 3 distances are known, receiver is able to figure out receiver’s position by trilateration.
Tools involved in this demonstration are simple:
- A Soundcard
- -> 5.1 surround sound, means 6 available channels
- 192 Khz sampling for microphone input
- A few Piezo Tweeters
- A radio emitter
- A radio receiver
- And a condenser microphone
In such circumstances, my microphone, for want of anything better (and much more expensive) was insufficient. A common microphone is not fitted for ultrasounds. Even my measurement microphone was limited to 20-22 Khz.
Acoustic pulse had to be played over multiple frequencies, but still being catch with high accuracy (travelling as fast as sound, a millisecond error brings you 0.34cm further). Bests results use frequency modulation, sound waves carrying the real synchronization pattern.
Repeated several times a second, each wave is recorded by receiver who analyses determined frequencies and estimates its position.
For this little experiment, 5.1 sound kit is plugged into the soundcard to amplify the signal. Four outputs are wired to three emitters (each).
Soundcard also broadcasts radio signal through a short range FM emitter.
The microphone is a condenser measurement microphone. It requires additional power supply and has a special plug.
To merge sound and radio, both microphone and radio receiver are linked up to the same soundcard input.