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 the Phase Accordance Method.

A demonstration video is available for download here.

Implementation

We use the combination of Time of Arrival (TOA) and Angle of Arrival (AOA) methods. In TOA method, the transmitter and the receiver are synchronized using wireless communication. The transmitter sends both an RF packet and an ultrasonic signal to the receiver. Then the receiver compares the receiving times of the RF packet and the ultrasound, and treats the time difference as the propagation time of the ultrasound. The propagation time of the ultrasound is converted into the distance between the transmitter and the receiver.

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 figure below shows the ultrasonic signal which consists of two sub-carriers. The superposition of the two frequency signals makes a beat which shows a distinctive pattern on the envelope. We call the result waveform as the sync pattern.

Sync Pattern

Since the Phase Accordance Method looks at the phase of the signal instead of the amplitude, it is very tolerant of ultrasonic attenuation and environmental noises. It has been proved that the accuracy of our system in distance measurement is about 100 times better than the conventional ultrasonic-based methods.

In addition to the high accuracy, this method has the advantage of 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 localization system to demonstrate the performance of the proposed technique. The system consists of:

• Ultrasonic Transmitter
• Ultrasonic Receiver
• Signal Processing Board
  ... Memorizes digitized ultrasonic wave and executes the Phase Accordance Method
• Wireless Communication Modules (MICAz from Crossbow Technology inc.)
  ... Used to synchronize the transmitter and the receiver

These components are connected as shown in the figure below:

System Design

Here are some pictures of the hardware:

Transmitter	Receiver	Signal Processing Board

Experiment and Result

We have conducted the experiment at the range of 1.5m. First we observe the distance measurement. The experiment was repeated for 1200 times to get the average value and the result is very satisfying.

• Mean value is 1500.162mm
• Standard Deviation is 0.122mm

Then we also investigate on the angle measurement. We placed the device at the actual angle of 10 degree and again, the experiment was repeated for 1200 times.

• Mean value is 10.68 degrees
• Standard deviation is 0.043 degrees