Omnidirectional Ultrasonic Location Sensor
Akifumi NISHITANI

Yoshifumi NISHIDA

Hiroshi MIZOGUCH

Tokyo Univ. of Science
2641, Yamazaki, Noda-shi,Chiba, 278-8510, Japan
Email: a-nishitani@aist.go.jp

Digital Human Research Center, AIST
2-41-6, Aomi, Koto-ku, Tokyo,
135-0064, Japan
Email: y.nishida@aist.go.jp

Tokyo Univ. of Science
2641, Yamazaki, Noda-shi,Chiba, 278-8510, Japan
Email: hm@rs.noda.tus.ac.jp

Abstract— Rapid construction of the human activity observation system at low cost is necessary with respect to research
into human activities and commercialization. The authors havedeveloped an ultrasonic 3D tag system that uses ultrasonictransmitters/receivers for observing human activities. The newly
developed system monitors the positions of objects to whichtransmitters are attached. The ultrasonic 3D tag system canbe installed in a wide variety of environments, but the time
and the installation cost required to install the numerous required receivers are prohibitive. Therefore, the present authors
developed an omnidirectional ultrasonic location sensor. Theomnidirectional ultrasonic location sensor is useful because thenumber of sensors to be attached can be reduced. The present
paper proposes an omnidirectional ultrasonic location sensor andreports its feasibility.

I. I NTRODUCTION
Recently, human-centered information processing services
have been attracting increasing attention. The goal of thepresent research is to establish a technique by which torecognize both human activity and state in a living space. Thisrequires human activity to be observed in real time, with highaccuracy, and without the presence of restraints that prevent
natural human activity.
As a method by which to realize efficient and robust
recognition of human activities, the concept of object-based
activity recognition has been proposed[1]. Theoretically, thebehavior of handling objects in an environment such as anoffice or home can be recognized based on the motion of theobjects. We have developed a three-dimensional (3D) ultrasonic location system as a fundamental system for robustly
tracking objects and have verified that the observation ofhuman activity based on object tracking is possible. However,like other location sensors, the developed ultrasonic location
sensor has a disadvantage in that it is necessary to installnumerous receivers in order to construct the ultrasonic 3D
tag system in a wide variety of environments. This requires a
great deal of time and the cost of installing a large number ofreceivers is high. Therefore it is difficult to collect activity datain the various environments in which actual human activities
occur. Thus, we believe that the omnidirectional ultrasoniclocation sensor will be useful for reducing the time associatedwith sensor attachment.
The Bat Ultrasonic Location System [2], the MIT Cricket
Indoor Location System [3], The Novel Broadband Ultrasonic
Location System [4], and The DOLPHIN system [5] have been
0-7803-9056-3/05/$20.00 © 2005 IEEE.

proposed as an ultrasonic location system. In contrast, thepresent paper puts an emphasis on quickly constructing thesystem at low cost. In this paper, we propose an omnidirectional ultrasonic location sensor that enables the 3D positionof ultrasonic transmitters to be estimated over all azimuths,
so that the time and cost of installing multiple ultrasoniclocation sensors is reduced. The user can easily construct ahuman activity observation system for field research on humanactivities.
II. O MNIDIRECTIONAL ULTRASONIC LOCATION SENSOR
Figure 1 shows an omnidirectional ultrasonic location sensor. The omnidirectional ultrasonic location sensor can not
only receive the ultrasonic waves in an omnidirectional manner, can estimate the 3D positions of transmitters over all
azimuths. An icosahedronn was then constructed by combining
several equilateral triangle units. Each unit has three receivers
in the vertex of each equilateral triangle. The three receivers oneach unit are used for estimating 3D positions of transmitters.

ReceiverFig. 1.

Icosahedron to which receivers were attached

III. S IMULATION OF MEASUREMENT ACCURACY AND
MEASURABLE AREA

The required measurement accuracy and the required measurable area differ depending on the purpose and locationof application. The development of simulation software thatcan calculate the measurement accuracy and measurable area
for an omnidirectional ultrasonic location sensor beforehand
would be useful. Measurement accuracy and measurable area
simulation is performed and the results of the simulation areevaluated.

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A. Simulation software of measurement accuracy and measurable area
In the simulation software, the measurement accuracy andthe measurable area in a space can be shown for various
numbers of receivers, the arrangement of the receivers, andsizes of the space. In the measurement accuracy simulation,
an error margin whereby the standard deviation is 1.5 mm
was added at random at each point. In the measurable area
simulation, the directivity of the ultrasonic receiver was set to
be 40 degrees.
B. Measurement accuracy simulation result

indicates the horizontal direction of the receivers arranged in
the equilateral triangle, and the z axis is indicates the verticaldirection of the receivers arranged in the equilateral triangle.
The orange spheres indicate the actual positions as obtainedby the motion capture, and blue spheres indicate the positionsof ultrasonic transmitters. The difference in distance between
positions of the motion capture and positions of the ultrasonictransmitter is error. Table II shows the average errors for eachcase. From Fig. 4, the measurement error is confirmed todecrease as the side lengths of the equilateral triangles become
long, as is the case for the simulation results as well.

The measurement error was obtained by simulation when
the side length of the equilateral triangle is changed. The
simulation was performed for three side lengths of 70 mm,150 mm, and 250 mm.
Figure 2 shows the simulation results for equilateral triangle
side lengths of 70 mm, 150 mm, and 250 mm. The black
spheres indicate the positions of the receivers. The difference
in distance from the actual position of the transmitter isassumed to be the error. Table I shows the average error.
When the side length becomes long, the measurement error
is confirmed to decrease.

Calculated transmitter positions
Motion capture positions
4500
[mm]

4000

4000

3500

3500

3500

3000

3000

3000

2500

2500

2500

2000

2000

2000

1500

1500zz0500

1000

1500[mm]

The sides 70mm

250

Fig. 3.

140

1500
z
1000

1000
x[mm]

700

4500
[mm]

4000

1000

Receivers

[mm]

4500
[mm]

0x

500

1000

1500 [mm]0The sides 150mm

x

500

1000

1500 [mm]The sides 250mm

Measurement error of actual measurement

[mm]

600
4000

200
4000

500

3500

4000
150

3500

400

3000

100

100

2500

2000
50z[mm]200z[mm]

2500

1500

1500

1000
500
0
2000

002000y[mm]0The sides 70mm

TABLE II

60

AVERAGE ERROR OF ACTUAL MEASUREMENT

40Side length
Average error

201000
2000

500

1000
1000x[mm]

2000
1500

1000
2000

80
3000

300
2000

100

3500

3000

2500z[mm]1201000
1000x[mm]0 y[mm]0The sides 150mm

2000

500
0
2000

150mm
48mm

250mm
53mm

1000
1000x[mm]0 y[mm]0

The sides 250mm

200Fig. 2.

70mm
121mm

Simulation data
Measured data

180

Measurement error of simulation

160
140

Error [mm]TABLE IAVERAGE ERROR OF SIMULATIONSide length
Average error

70mm
134mm

150mm
62mm

250mm
36mm

1201008060
40200C. Comparison of actual measurement accuracy with simulation measurement accuracy
The simulation of measurement accuracy is compared withthe actual measurement accuracy by motion capture to verify
the validity of the simulation software. The accuracy is measured by arranging the receivers as the top of each equilateraltriangle. Figure 3 shows the results of the actual measurement
for equilateral triangle side lengths of 70 mm, 150 mm, and250 mm. The receivers are placed on the origin. The x axis

0100200Sides length [mm]300Fig. 4.
Comparison of measurement errors of simulation and actual
measurement

D. Measurable area simulation results
In the omnidirectional ultrasonic location sensor system,
the transmitter is estimated by installing part of the omnidirectional ultrasonic location sensor, which consists of five685

equilateral triangle units, in the ceiling. Simulation of themeasurable area is performed using the length of the sideof the triangle of 100 mm. Figure 5 shows the results ofthe measurable area simulation for a height of 2 m for a
3 × 3 × 2.7 m room. The measurable area of one equilateraltriangle unit is indicated by the area shown in the same color
as the equilateral triangle unit.
Triangle attached 3 receivers
1500
[mm]

8 shows the measurable area for a height of 0 mm. The
experimental results indicate that the measurable area of thetriangular unit is an ellipse having a major axis of 3 m anda minor axis of 1.2 m. Figure 9 shows the case in which
the omnidirectional ultrasonic location sensor is installed in a
5 × 5 m room. The number of attached sensors is 36 in thecase of current ultrasonic sensors, but the number of times
that sensors must be attached is only three in the case of theomnidirectional ultrasonic location sensor. Thus the numberof times that sensors must be attached becomes 1/12 that ofprevious systems.

1000

Transmitter positions
Motion capture positions

500

0-500

3500

3500

3000

3000

-1000

-1500
y-1500

-1000

-500

0500

1000

2500

2000

2000

1500

1500

1000

1000

500

500

1500[mm]xFig. 5.

2500

Measurable area simulation

IV. E XPERIMENTS

0500 1000 1500 2000 2500 3000 [mm]The height 500mm

A. Prototype omnidirectional ultrasonic location sensor
Figure 6 shows a prototype omnidirectional ultrasonic location sensor. Three receivers are installed on each face of theicosahedron, the sides of which are 140 mm in length, and thedistance between receivers is 100 mm.

Fig. 7.

0500 1000 1500 2000 2500 3000 [mm]The height 1500mm

Measurement error

3m

100mm

Receivers

Fig. 6.

1.2m

Omnidirectional ultrasonic location sensor

B. Evaluation of measurement error and measurable area
The omnidirectional ultrasonic location sensor, which consists of five equilateral triangle units, is installed in the ceilingas a height of 3 m and the positions of the transmitter isestimated. One quarter of the whole area was measured. The
rest assumed similar because of symmetry. Figure 7 shows
the results of the measurable area of heights of 500 mm
and 1,500 mm. The orange spheres indicate the positions asobtained by the motion capture, and the blue spheres indicate
the positions of the ultrasonic transmitters. The average error
is 108 mm and the standard deviation is 52 mm. Figure

Fig. 8.

Measurable area

V. A PPLICATION
A system to estimate the position of a robot in a house is one
application of the omnidirectional ultrasonic location sensor.
Figure 10 shows a house in which a robot must approach an686

8Omnidirectional sensor
6Ultrasonic receiver

425m

Y[m] 0-2
ultrasonicultrasonic-4

ultrasonicultrasonic-6-8-8-6-4

Fig. 12.

Fig. 9. Comparison of the number of attached receivers and the number ofattached omnidirectionalsensors

individual in order to assist them. Thirteen omnidirectional
ultrasonic location sensors that consist of five equilateraltriangle units are installed on the ceiling of the house, asshown in Figure 11. The transmitters are attached to the robot,
which can then estimate its own position and approach theperson who has another transmitter. The position of the robot
was measured by 203 points. Figure 12 shows the resulting
robot trajectory. The total distance traveled by the robot was
53.425m, and the average error of measurement was 180[mm].

Fig. 10.

-2

02

4

68

X[m]

5m

Result of robot trajectorytransmitters over all azimuths and consequently reduce theinstallation cost for such systems. The proposed ultrasonic
3D tag system is low cost and easy to construct in various
environments where actual human activities occur. The measurement accuracy for various distances between the ultrasonicreceivers, which were arranged at the vertices of equilateraltriangles that were arranged to form an icosahedral sensor,
was simulated, and the simulation results were compared withthe actual measurement accuracy. The measurable area wassimulated. A prototype icosahedral omnidirectional ultrasoniclocation sensor, which combined three receivers arranged on
equilateral triangle units was made. The experimental results
showed that the average error was 108 mm for a distance
between sensors of 100 mm and the measurable area per thetriangular unit, was shown to be an ellipse having a major
axis of 3 m and a minor axis 1.2 m for the case in which
the omnidirectional ultrasonic location sensor was installed ata height of 3 m. The number of times that sensors must beattached for a 5 × 5 m room becomes 1/12 that of previoussystems.

Floor plan of a living space that must be navigated by a robot

ACKNOWLEDGMENT
The authors wish to thank the KANSAI Electric Poewerco., Inc. for cooperation of experiments in a house.
R EFERENCESFig. 11. Omnidirectional ultrasonic location sensors that consisting of fiveequilateral triangle units

VI. C ONCLUSION
The present paper described an omnidirectional ultrasoniclocation sensor, which can estimate the 3D positions of[1] Y. Nishida, H. Aizawa, T. Hori, N.H. Hoffman, T. Kanade, and M.
Kakikura, “3D Ultrasonic Tagging System for Observing Human Activity,” in Proceedings of IEEE International Conference on Intelligent Robots
and Systems (IROS2003), pp. 785-791, 2003
[2] A. Ward, A. Jones, and A. Hopper, “A New Location Technique for theActive Office,” IEEE Personal Communications, Vol.4, No.5, pp. 42-47,
October 1997
[3] N.B. Priyantha, A. Chakraborty, and H. Balakrishnan, “The Cricket
Location-Support system,” in Proceedings of the 6th International Conference on Mobile Computing and Networking (ACM MobiCom2000), pp.
32-43, August 2000
[4] M. Hazas, and A. Ward, “A Novel Broadband Ultrasonic Location
System,” in Proceedings of UbiComp 2002, pp. 264-280, September 2002
[5] Y. Fukuju, M. Minami, H. Morikawa, and T. Aoyama, “DOLPHIN: An
Autonomous Indoor Positioning System in Ubiquitous Computing Environment,” in Proceedings of IEEE Workshop on Software Technologies
for Future Embedded Systems(WSTFES2003), pp. 53-56, May 2003

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