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Ultrasonic Radar System for Minimally Privacy-Violative Activity Observation


We are currently developing an ultrasonic system for tracking people and objects as a way of observing activity in living spaces while keeping invasion of privacy to a minimum. Our aim is to create a system that improves quality of life (QOL). By tracking people and objects, our system would allow the observation of activities of daily living (ADL). Data from these observations could then be used to construct behavior models, which could in turn facilitate the provision of evidence-based care. We suppose an ultrasonic radar based system that can comply with the following: 1) unconstraint measurement of location for the human, 2) minimally privacy-violation, which are compounding this human activities observing system.

Ultrasonic Radar System

Ultrasonic radar detects human location by determining time-of-flight among human head and ultrasonic sensors. Figure 1 and figure 2 show the ultrasonic radar system for observing human daily activities. The system is composed of 117 transmitters, 117 receivers, transmitter-controllers, receiver-controllers, network device and a host computer. The transmitters and receivers are embedded in a ceiling, and other devices are set on a attic.

Figure 1: Ultrasonic Radar System
Figure 2: System Configuration

Principle of measurement in ultrasonic radar system

In the ultrasonic radar system developed by the authors, it is assumed that the human head is an object moving at a relatively high vertical position in a living area, and emitting ultrasonic sounds and receiving them back as they are reflected from the head can detect the position of the head. This section explains the principle used to measure and calculate, that is, to locate, the position of a human head with unconstraint.

If the positions of the i-th transmitter, j-th receiver and head are


respectively, and the propagation distance is

as shown in Figure 3, then the following equation of a spheroid can be obtained.




are known, then the head position

can be calculated from the three equations of a spheroid.

Figure 3: Principle of measurement

Resolution and measuring error

The resolution in the x, y, and z directions is illustrated in figure, which shows the probability density distribution for 1000 locations of head calculated by the system. The resolution in x and y directions is about 34mm, while that in the z direction is about 10mm. The average of measuring error is 43mm.

Figure 4: Probability distributions for human head detection

Tracking result

The upper part of the figure shows the measured trajectory of the human head when the test subject moves as shown in the lower part of the figure. The figure shows that the system can detect the positions of the head at a frequency of 1Hz.

Figure 5: Tracking position of a human head

Experiment for discrimination based on the height difference among objects

The system can distinguish the detected objects by their height difference. For example, there is the difference between the head and a desk. The figure shows a result of experiment for discrimination. In the figure, red squares show the highest position of an object and green squares that of another. The figure indicate that the system can distinguish the detected objects and trace.

Figure 6: Result of experimememt for discrimination


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