Ultrasonic-based Localization

The ITL ultrasound positioning system tracks precisely the real-time location of devices or people in indoor environments. The basic idea of the system is that it uses the propagation delay of ultrasonic signal between transmitter and receiver to calculate the distance between them. Several techniques are implemented to achieve very high accuracy, including our innovation technique called Phase Accordance Method.

A demonstration video is available for download here.

Implementation

We use the combination of Time of Arrive (TOA) and Angle of Arrival (AOA) methods. In TOA method, the transmitter and receiver are synchronized using a common timing reference. The transmitter sends a message with timestamp. Then the receiver receives the message, compares with the local clock and the time different can be calculated.

AOA is a technique which is used to determine the direction of the incoming signal. In order to implement AOA method, two ultrasonic microphones are needed at the receiver. These two microphones are attached to the receiver board in parallel, separately at a certain length. The signal reaches each microphone at different moment.

Time of Arrival		Angle of Arrival

The figure on the right shows the implementation of AoA method. With known value L and the distances to two microphones, using simple trigonometric rule, the angle of the incoming signal can be calculated.

Phase Accordance Method

We have proposed an innovation method for rapidly detecting the incoming signal. In this method, we use an ultrasonic burst signal. Unlike the conventional burst pulse detection system which uses the envelope of the burst to indicate the start of package, instead our technique uses the burst which consists of two or more frequency sub-carriers and the carriers' phases accord at a single time marker called epoch. The superposition of the two frequency signals make a beat which shows a distinctive pattern on the envelope. We call the result waveform as the sync pattern. Sync Pattern The advantage of this method is the short detection time of the incoming signal. It only requires a burst of two milliseconds which in turn significantly limits the error occurred from the time used in detecting process.

Another benefit of this method is that the sync pattern synchronizes the receiver's local oscillator to the carrier's phase. Therefore, we can simply communicate data using phase shift keying or phase amplitude modulation.

System Overview

We design a simple system to demonstrate the performance of the proposed technique. The system consists of:

• Ultrasonic Transmitter Board
• Ultrasonic Receiver Board
• Signal Processing Board

These components are connected as shown in the figure below:

System Design

Here are some pictures of the hardware:

Transmitter Board	Receiver Board	Signal Processing Board

Experiment and Rusult

We have conducted the experiment at the range of three meters. First we observe the distance measurement. The experiment was repeated for 500 times to get the average value and the result is very satisfying. Mean value is 3002.4 mm Standard Deviation is 0.3 mm Then we also investigate on the angle measurement. We placed the device at the actual angle of 20 degree and again, the experiment was repeated for 500 times. Mean value is 20.48 degrees Standard deviation is 0.52 degree

The graph of the results are shown below:

Result from distance measurement		Result from angle measurement