Portable Ultrasonic 3D Tag System
Based On a Quick Calibration Method∗
Yoshifumi Nishida
Digital Human Research Center, AIST
& CREST, JST
2-41-6, Aomi, Koto-ku, Tokyo,
135-0064, Japany.nishida@aist.go.jp

Akifumi Nishitani
Tokyo University of Science
2641, Yamazaki, Noda-shi, Chiba,278-8510, Japana-nishitani@aist.go.jp

Hiroshi Mizoguchi
Tokyo University of Science
2641, Yamazaki, Noda-shi, Chiba,278-8510, Japanhm@rs.noda.tus.ac.jp

Toshio Hori
Digital Human Research Center, AIST
& CREST, JST
2-41-6, Aomi, Koto-ku, Tokyo,
135-0064, Japant.hori@aist.go.jp

Abstract – The ability to quickly construct a human activity observation system is necessary in order to conduct fieldresearch on human activities. The authors have developed
an ultrasonic 3D tag system, which is a type of location sensing system for observing human activities. The present study
attempts to establish a systematic method for quickly constructing an ultrasonic 3D tag system in various environments. A calibration function that is available in various
environments is one of the basic functions of portable ultrasonic 3D tag systems. In the present paper, we propose several methods for calibrating the 3D positions of ultrasonicreceivers placed arbitrarily in a daily-use environment. The
proposed methods require a relatively small number of transmitters and are independent of room size. In addition, themethods proposed herein are compared and their respective
advantages are discussed. A portable system based on the
proposed methods is also described herein.

As a method for efficient and robust recognition of activities, the concept of object-based activity recognition has
been proposed[7]. Theoretically, the behavior of handling
objects in an environment such as an office or home can berecognized based on the motion of the objects. We have developed a three-dimensional (3D) ultrasonic location system
as a fundamental system for robustly tracking objects andhave verified that the observation of human activity basedon object tracking is possible. However, like other location
sensors, the developed ultrasonic location sensor has a disadvantage in that lack of portability makes it difficult to collectactivity data in the various environments in which actual human activities occur.
In the present paper, we propose quick calibration algorithms that enable portable ultrasonic location sensors so thatusers can quickly construct a human activity observation system for field research on human activities.

Keywords: Human Activity Observation, Ultrasonic Location Sensor, Calibration.

2 Related Work

1 Introduction
Recently, human-centered information processing services have been attracting increasing attention. The goal of
the present research is to establish a technique by which torecognize both human activity and state in a living space.
Therefore, it is necessary to observe human activity in realtime and with high-accuracy, without the presence of restraints that prevent natural human activities.
∗ 0-7803-8566-7/04/$20.00 
c 2004 IEEE.

Several types of ultrasonic location systems have been
proposed. The Bat Ultrasonic Location System [1, 2], developed by AT&T, and the MIT Cricket Indoor Location System [3] do not deal with a calibration method. The Novel
Broadband Ultrasonic Location System [4] overcomes ultrasonic noise generated suddenly through everyday actions ina living space. This system takes the surrounding environment into consideration but does not deal with calibration.
Although the DOLPHIN system [5] does not require calibration, this system is used only in narrow areas of approximately 2.5m × 2m, and the portability of the system is un-

1561

known. In addition, a calibration method using a robot [6]has been proposed, but the required calibration device is toolarge to use in a number of environments.
In contrast, the present study examines new calibration
methods that require a relatively small number of transmitters and are independent of room size.

Receivers

Calibration device(Transmitters)

3 Quick Calibration

Figure 2: Typical calibration method 1

3.1 Calibration and measurement
The calibration herein describes the estimation of the 3D
position of ultrasonic receivers required in order to measure
the 3D positions of transmitters. Figure 1 shows the relation between the calibration and the measurement. In themeasurement, transmitter positions are estimated from the
known positions of receivers by solving an equation relating the transmitters and the receivers. In the calibration, receiver positions are calculated based on the known relative
positions of transmitters. Both are essentially the same inthe sense that unknown positions can be calculated based on
known positions.

Receivers

Calibration device(Transmitters)

Figure 3: Typical calibration method 2
ReceiversPr

3.3 Proposal of a quick calibration method
Measurement

Pt

L

Calibration

In the present paper, we propose two calibration methods
in addition to two methods that can be used in conjunction
with the basic methods described below.

| Pri - Pt j | =L i,j
Transmitters

3.3.1
Figure 1: Calibration and measurement

3.2 Simple calibration methods and disadvantages thereofThis section describes two typical calibration methods bydirectly applying a known estimation method of 3D positions.

3.2.1

Simple method 1: Large calibration device

The calibration device consists of transmitters of which
the relative positions are known. This method involves theuse of a calibration device of which the size is almost equal
to the space containing the receivers, as shown in Fig. 2.
However, fabricating, repositioning and using this large device is difficult.

3.2.2

Simple method 2: Small calibration device located at a distance from the receivers

This method involves placing a calibration device far awayfrom receivers, the positions of which should be calibrated.
This method cannot be used in enclosed spaces, such as
spaces surrounded by walls and a ceiling.

Basic method 1: Three or more ultrasonic transmitters

The procedure for quick calibration method 1 is described
below.
1. Move the calibration device arbitrarily to multiple positions (A, B, and C in Fig. 4).
2. Calculate the positions of receivers in a local coordinate
system, with the local origin set at the position of the calibration system.
3. Select receivers for which the positions can be calculated
from more than two calibration system positions.
4. Select a global coordinate system from among the local
coordinate systems and calculate the positions of the calibration device in the global coordinate system using the receivers selected in Step 3. Then calculate transformation matrices (M1 and M2 in Fig. 4).
5. Calculate the receiver positions using the receiver positions calculated in Step 2 and the transformation matrices
calculated in Step 4.
Steps 2 and 4 are described in detail in the following.
Calculating the 3D positions of receivers in a local coordinate system based on distance data (Step 2) The receiver
position (x,y,z) is calculated by solving plane equations derived from the equations of intersecting spheres, as shownin Fig. 5, namely by the multilateration algorithm. This is
explained in detail below.

1562

equations asAPReceivers

where P
Place B

Place A(8)⎞x
= ⎝ y ⎠,z

(9)

Place C⎞2(x2 − x1 ) 2(y2 − y1 ) 2(z2 − z1 )⎜ 2(x3 − x1 ) 2(y3 − y1 ) 2(z3 − z1 ) ⎟
⎟
⎜
A= ⎜
⎟,
..
..
..
⎠
⎝
.
.
.
⎛

0

M1

Calibration device(Transmitters)

= B,
⎛

M2

Figure 4: Quick calibration method 1(10)2(xi − x1 ) 2(yi − y1 ) 2(zi − z1 )⎛ 2⎞
l1 − x21 − y12 − z12 − l22 + x22 + y22 + z22⎜ l12 − x21 − y12 − z12 − l32 + x23 + y32 + z32 ⎟
⎜
⎟
B= ⎜
⎟.
..
⎝
⎠
.
l12 − x21 − y12 − z12 − li2 + x2i + yi2 + zi2(11)
For the case of four or more transmitters that are not coplanar, the rank of A becomes 3. The position (x,y,z) can becalculated by the least squares method as
P = (AT A)−1 AT B.

(12)

Reference [7] presents a detailed description for the case inwhich the rank of A is 2.
Figure 5: Multilateration
Here, l1 , l2 , · · ·, li denote the distances from the ultrasonicreceiver at (x,y,z) to the ith ultrasonic transmitter at position(xi ,yi ,zi ). We obtain the spherical equations as2(y1 − y)22(y2 − y)22(x1 − x) +(x2 − x) +(x3 − x) +(xi − x)2 +2(y3 − y)..
.
(yi − y)22

+ (z1 − z) = l12 ,(1)+ (z2 − z) = l22 ,2+ (z3 − z) = l32 ,(2)+ (zi − z)2 = li2 .

(4)

2

(3)

We obtain equations for the intersecting planes between thespheres from (1), (2), (3) and (4) as
2 (x2 − x1 ) x + 2 (y2 − y1 ) y + 2 (z2 − z1 ) z=

l12 − x21 − y12 − z12 − l22 + x22 + y22 + z22 ,2 (x3 − x1 ) x + 2 (y3 − y1 ) y + 2 (z3 − z1 ) z(5)=l12 − x21 − y12 − z12 − l32 + x23 + y32 + z32 ,
..
.

(6)

=

2 (xi − x1 ) x + 2 (yi − y1 ) y + 2 (zi − z1 ) z
l12 − x21 − y12 − z12 − li2 + x2i + yi2 + zi2 .

(7)

Simultaneous linear equations are obtained from the above

Calculating the positions of the calibration device in the
global coordinate system (Step 4) Here, let us assume
that the calibration device has n transmitters, that m receivers
are to be calibrated, and that the user repositions the device
at four different positions (positions A, B, C, and D) for calibration.
In the local coordinate system having the origin
at A (position A), the calculated receiver positions
are denoted by (xrA 1 , yrA 1 , zrA 1 ), (xrA 2 , yrA 2 , zrA 2 ),· · ·, (xrA iA , yrA iA , zrA iA ) and the transmitter positions are (xtA 1 , ytA 1 , ztA 1 ), (xtA 2 , ytA 2 , ztA 2 ), · · ·,(xtA n , ytA n , ztA n ), where iA indicates the number ofreceivers calculated from position A. Generally, iA issmaller than m because of the directivity of the transmitters.
The corresponding transmitter positions in the global
coordinate system are denoted by (XtA 1 , YtA 1 , ZtA 1 ),(XtA 2 , YtA 2 , ZtA 2 ), · · ·, (XtA n , YtA n , ZtA n ).
Similarly, in the local coordinate system having theorigin at B (position B), the calculated receiver positions
are denoted by (xrB 1 , yrB 1 , zrB 1 ),(xrB 2 , yrB 2 , zrB 2 ),· · ·, (xrB iB , yrB iB , zrB iB ) and the transmitter positions are (xtB 1 , ytB 1 , ztB 1 ), (xtB 2 , ytB 2 , ztB 2 ), · · ·,(xtB n , ytB n , ztB n ).
In the global coordinate system,
the transmitter positions are denoted by (XtB 1 , YtB 1 , ZtB 1 ),(XtB 2 , YtB 2 , ZtB 2 ), · · ·, (XtB n , YtB n , ZtB n ).
There are receivers (pink spheres in Fig. 4) of which thepositions can be calculated from both positions A and B.
For example, if (xrA 10 , yrA 10 , zrA 10 ), (xrA 11 , yrA 11 , zrA 11 ),1563

· · ·, (xrA 18 , yrA 18 , zrA 18 ) are the positions in local coordinate system A, and xrB 9 , yrB 9 , zrB 9 ), (xrB 10 , yrB 10 , zrB 10 ),· · ·, (xrB 18 , yrB 18 , zrB 18 ) are the corresponding positions inlocal coordinate system B, then we can obtain the following
equation:
PA = MBA PB ,

(13)
(14)

where MBA is a 4 × 4 matrix, and⎛⎞xrA 10 xrA 11 · · · xrA 18⎜ yrA 10 yrA 11 · · · yrA 18 ⎟⎟PA = ⎜⎝ zrA 10 zrA 11 · · · zrA 18 ⎠ ,11 ··· 1⎛⎞
xrB 1 xrB 2 · · · xrB 9⎜ y rB 1 y rB 2 · · · y rB 9 ⎟⎟PB = ⎜⎝ z rB 1 z rB 2 · · · z rB 9 ⎠ .
11 ··· 1

1. Determine the positions of three ultrasonic receivers bymeasuring L1 , L2 and L3 among the three receivers selected
arbitrarily.
2. Calculate each of the positions of an arbitrarily positioned
transmitter from the three receiver positions calculated in
Step 2.
3. In a similar way, calculate the 3D positions of the receivers by selecting four or more such arbitrary transmitter
positions.
Known positions

(15)

L1
L2
L3

(16)
Calibration device(Transmitter)

The matrix MBA can be calculated by the least squares
method as follows:
T −1
MBA = PA PT
B (PB PB )cosα cosγ−sinα cosβ sinγ
sinβ sinγ0

sinα sinβ Ty
cosβ
Tz
0
1

,

(18)

where α is the angle of rotation about the z-axis, β is theangle of rotation about the new y-axis, γ is the angle of rotation about the new z-axis, and the matrix is translated by T x,
T y, and T z. We can calculate α, β, γ, T x, T y, and T z using
a non-linear optimization method, such as the Nelder-Meaddownhill simplex method.
The receiver positions PB in the local coordinate system
based on position B can be converted to the positions Pw
Bin the global coordinate system (or in the local coordinate
system based on position A) as follows:Pw
B = Mba · PB .

Figure 6: Quick calibration method 2(17)

The transformation matrix MBA in (17) has effects of not
only rotation and translation but also enlargement and reduction. This matrix distorts the coordinate system for the casein which the transformation of the coordinate system is performed using multiple matrices, as described later. We use
the matrix expressed bycosα cosβ cosγ−sinα sinγ −sinα cosγ−cosα cosβ sinγ cosα sinβ Tx
sinα cosβ cosγ−cosα sinγ
−sinβ cosλ0Receivers

3.3.3

Additional method 1: Coplanar constraint

The calibration error can be improved significantly basedon knowledge of configuration of the receivers to be calibrated. For example, by placing the ultrasonic receivers on
the ceiling or on a wall, we can take advantage of the factthat the receivers are located on the same plane. This section
describes a method for calibrating the positions of receivers
when the receivers are located on the same plane.
Figure 7 shows the coplanar constraint method. Pri (=(xi , yi , zi )) denotes the positions of receivers, Ptj denotes
the positions of transmitters, and n denotes the normal vector
to the plane.
Receivers

Pi,j
z

(19)

Similarly, we can also calculate MBA , MCB , and MDC .

ri,j

d i,j
y
x

Pt j

n
Pr i

Li,j

Transmitters

Figure 7: Coplanar constraint
Pw
C = MBA · MCB · PC ,Pw
D = MBA · MCB · MDC · PD .

(20)
(21)

The equation for the plane on which all of the receivers
are located can be written as

In this way, we can calculate all of the receiver positionsin the global coordinate system.

3.3.2

Basic method 2: Ultrasonic transmitter

The procedure for quick calibration method 2 is described
below.

ax + by + cz + d = 0.

(22)

We can calculate the optimal values of these variables(a, b, c, d) in the least squares sense using (23). Concretely,
these variables (a, b, c, d) can be calculated as a basic vectorof nullspace by performing singular value decomposition.

1564

⎞
x1 y1 z1 1 ⎛ a ⎞⎜ x2 y2 z2 1 ⎟ ⎜ ⎟
⎟ b⎟
⎜
= 0,
⎜ .. .. .. .. ⎟ ⎜⎝ . . . . ⎠⎝ c ⎠dxi yi zi 1⎛

(23)

where (xi , yi , zi ) are temporary solutions for receivers as
calculated by basic method 1 or basic method 2.
If perpendiculars are dropped from the transmitters, we
can calculate the intersection points of the plane and the perpendiculars denoted by Pi,j (= (xi,j , yi,j , zi,j )). We can thencalculate the distance di,j and the length ri,j .
The positions of the receivers are Pri , and the equationsof spheres centered at Pi,j with radii ri,j are(xi,1 − xi )2 + (yi,1 − yi )2 +(zi,1 − zi )2 = ri,1 , (24)(xi,2 − xi )2 + (yi,2 − yi )2 +(zi,2 − zi )2 = ri,2 , (25)..
.
(xi,j − xi )2 + (yi,j − yi )2 +(zi,j − zi )2 = ri,j . (26)The positions of receivers are located on the intersection
points of these spheres. The plane equations derived fromthese spheres are expressed as follows:than actual distance due to ultrasonic damping. This phenomenon causes an increase in the error. The damping is
caused by the directivities of the transmitter and the receiver.
This section describes a method that considers the directivities in order to reduce this effect.
The ultrasonic location system developed by our groupmeasures the distance from the time-of-flight of the ultrasonic pulse. The time at which the ultrasonic wave exceeds
the threshold is the arrival time of the ultrasonic pulse at thereceiver, as shown in Fig. 8. However, in the case of damping of the ultrasonic wave, due to the effects of directivity
and distance, the error of the measured distance increases asa result of the delay in the ultrasonic pulse arrival time expressed by ∆T.
Approach to the damping problem
There are two approaches to this problem; a hardware approach and a software approach. In the hardware approach,for example, the highest point of the ultrasonic wave can beassumed to be the ultrasonic pulse arrival time determined bysampling and analyzing the ultrasonic wave, which requires
expensive equipment. Therefore, in the present paper, we
describe the software approach.
Voltagewave shape

APri = B,
where Pri =

(27)
xi
yi
zi

,

⎛ 2(x

Threshold(28)∆T
Arrival time⎞

Figure 8: Measurement of ultrasonic pulse arrival timei,2 − xi,1 ) 2(yi,2 − yi,1 ) 2(zi,2 − zi,1 )
2(xi,3 − xi,1 ) 2(yi,3 − yi,1 ) 2(zi,3 − zi,1 ) ⎟⎜A = ⎝ 2(xi,4 − xi,1 ) 2(yi,4 − yi,1 ) 2(zi,4 − zi,1 ) ⎠ ,
..
..
..
.
.
.

(29)

⎛ 2

Time

⎞

2 − z 2 − r 2 + x2 + y 2 + z 2
ri,1 − x2i,1 − yi,1
i,1
i,2
i,2
i,2
i,2
2 − x2 − y 2 − z 2 − r 2 + x2 + y 2 + z 2
r
⎜ i,1
i,1
i,1
i,1
i,3
i,3
i,3
i,3 ⎟
⎜
2
2
2
2
2
2
2
2 ⎟
and B =⎝ ri,1 − xi,1 − yi,1 − zi,1 − ri,4 + xi,4 + yi,4 + zi,4 ⎠ .
..
.

Method for dealing with the damping problem
The distance between the receivers and transmitters is|Pri − Ptj | = Lij ,(34)

where Pri are the positions of the receivers, Ptj are the positionsof the transmitters, and Lij are the actual distances.
(30)
The distances Lm,ij measured ultrasonically are longer thanThe rank of A is always 2 because of the coplanar constraint. the actual distances Lij . The actual distances Lij are deWe calculate Pri using (22), and (31) ∼ (33).
scribed by
Pri = Pri0 + n · t ,Pri0 = A+ B,
n = (a, b, c)

(32)
(33)

where A+ indicates the Moore-Penrose inverse matrix of A.

3.3.4

Lij = Lm,ij − ∆Lij ,

(31)

(35)

where ∆Lij is the error of the distance.
The direction of the transmitting face is denoted by z andthe direction of the receiving face is denoted by n. Then

Additional method 2: Consideration of
directivities

Error due to ultrasonic damping
The authors discovered a phenomenon whereby the measured distance between receivers and transmitters is longer

1565

cos θij =(Pri − Ptj ) · z,|Pri − Ptj | · |z|(36)

cos φij =(Ptj − Pri ) · n.
|Ptj − Pri | · |n|(37)

ReceiverPr

4 Experiments

Receiver
Pr
n

L

L

4.1 System configuration

φ

θ
z
Pt
Transmitter

Pt
Transmitter

Figure 9: Angle and distanceIn this case all of the directions of transmitters are identical,
as are all of the directions of the receivers. Actually it iseasy to create the calibration device in which the transmitters
are attached in the same direction. In addition, setting thereceivers in the same direction is easy.
The experimentally obtained relationship between thetransmitting angle of a transmitter and the error and thatbetween the receiving angle of a receiver and the error are
shown in Figure 10 and Figure 11, respectively.

Figure 12 shows the system configuration for the proposed
3D ultrasonic tag system. The system consists of an ultrasonic receiving section, an ultrasonic transmitting section, atime-of-flight measurement section, a network section, and apersonal computer. The ultrasonic receiving section receives
ultrasonic pulses emitted from the ultrasonic transmitter andamplifies the received signal. The time-of-flight measurement section records the travel time of the signal from transmission to reception. The network section synchronizes thesystem and collects time-of-flight data from the ultrasonic receiving section. The positions of objects are calculated basedon more than three time-of-flight results.
PC
RS232(9.6k to 921.6kbps)
Collect data
SYNC signal
data

RS485
Measure time of flight (921.6kbps,1km)

PIC

up to 127 devices
60
50
40

up to 2032 devices

ID

Ultrasonic receiver & amplifierRadio Ultrasonicreceiver transmitter
40kHz

10

y = -61.323x + 58.205
0
0.2

0.4

-10

PIC

C&
YN

20

0

PIC

dS

30

PIC

Sen

Error ∆ L [mm]Radio transmitter& SYNC generator
VHF 314.9MHz
wireless RS232

1517[mm]1735[mm]
1940[mm]
2122[mm]2325[mm]2539[mm]

0.6

0.8

1

PIC

1.2

Battery

Ultrasonic 3D tag 28x20x13mm

cos θ

Figure 10: Relationship between transmitting angle and error
Figure 12: System configuration
40

1479[mm]
1687[mm]
1881[mm]2093[mm]2292[mm]
2508[mm]

Error ∆ L [mm]

35
30
25
20

4.2 Experimental results
4.2.1 Method for error evaluation

15
10
5

y = -75.622x + 73.457

0
-5 0

0.2

0.4

-10

0.6

0.8

1

e1

1.2

e2

True positions
Calculated positions

cos φFigure 11: Relationship between receiving angle and error

en

Here,
∆Lij = −61.323 cos θij − 75.622 cos φij + 131.662(38)was obtained experimentally.
In the equation
2|Ptj − Pri | − L2ij

E=

2

,

(39)

i,j

Figure 13: Method for calculating error
Figure 13 shows the method used to calculate error. The
distances between the calculated receiver positions and thetrue receiver positions are denoted by e1 , e2 , · · · , en . The
average error is defined by

the receiver positions Pri are calculated by minimizing E
using a non-linear optimization method (such as the NelderMead downhill simplex method).

1566

n

E=

1
ei .
n i=1

(40)

4.2.2

Accuracy evaluation

Calibration was performed in a room (4.0×4.0×2.5 m)
having 120 ultrasonic receivers embedded in the ceiling. Figure 14 shows the experimental results of proposed calibration
method 1 applied using the coplanar constraint. The authors
performed calibration at 16 points in the room. Seventy-six
receivers were calculated. In the figure, the red spheres indicate calculated receiver positions, whereas the black crosses
are the true receiver positions, and the blue spheres indicate
the positions of the calibration device. Figure 15 shows theexperimental results for proposed calibration method 1 considering directivities. Seventy-six receivers were calculated.
Figure 16 shows the experimental results of proposed calibration method 2 using the coplanar constraint. Seventy-six
receivers were calculated. Table 1 shows the average error
E, maximum error, and minimum error for these methods.
The transmitter position error was 40 to 320 mm in thecase of calibration based on basic method 1 considering directivities.

calculated receiver positions
true receiver positions
positions of the calibration device
1000[mm]

0

-1000
2000

[mm]
1000

y

0
0

x
z

1000

2000

[mm]

Figure 16: Experimental results for proposed method 2 withthe coplanar constraint
Table 1: Errors in the proposed methods
Averageerror

Maximum
error

Minimumerror

195 mm

399 mm

66 mm

coplanar constraint

186 mm

382 mm

26 mm

Method 1 considering
directivity

78 mm

341 mm

13 mm

74 mm

338 mm

11 mm

239 mm3000 mm

27 mm

coplanar constraint

112 mm

542 mm

2 mm

Method 2 considering
directivity

impossible

impossible

impossible

impossible

impossible

impossible

Method 1

calculated receiver positions
true receiver positions
positions of the calibration device

Method 1 with

[mm]
1500
1000
500
x

0

y

0
-1000
-2000
-3000

-1000

[mm]

0

1000

Method 1 considering

2000

[mm]

directivity
with coplanar constraint

Figure 14: Experimental results for proposed method 1 withthe coplanar constraint

Method 2
Method 2 withcalculated receiver positions
true receiver positions
positions of the calibration device

Method 2 considering

[mm]
1000

directivity
with coplanar constraint

500
z
x

0

y

0
-1000

[mm]

-1000

0

-2000

1000
2000

-3000

[mm]

Figure 15: Experimental results for proposed method 1 considering directivity

5 Advantages of the Proposed
Methods
The advantages and disadvantages of the proposed calibration methods are listed below.
Basic method 1: Three or more ultrasonic transmittersadvantage: Suitable for wide-area calibration.
disadvantage: Requires a calibration device.
Basic method 2: Ultrasonic transmitter
advantage: Easy to use because only a single transmitter isneeded.

disadvantage: Not suitable for wide-area calibration.
Additional method 1: Coplanar constraint
advantage: Improved accuracy.
disadvantage: Only applicable in the coplanar case.
Additional method 2: Consideration of directivities
advantage: Improved accuracy.
disadvantage: Calculation requires a great deal of time. The
directions of the receivers are assumed to be identical.

6 Realized Ultrasonic Portable 3D
Tag System
The proposed methods enable a portable ultrasonic 3D tag
system. Figure 17 shows a portable ultrasonic 3D tag system,
which consists of a case, tags, receivers, and a calibration
device. The portable system enables measurement of human1567

activities by quickly installing and calibrating the system onsite, at the location where the activities actually occur.

Ultrasonic sensors

Portable case
Calibration device
built in sections

In addition, a portable ultrasonic location sensor based on
the proposed methods was realized. The portable system allows human activities to be measured in the area where theactivities actually occur with quick installation and calibration.
Further studies will examine the refinement of the method
in order to measure the 3D positions with higher accuracy,
systematization of calibration methods, and the use of theportable 3D ultrasonic tag system in a living space.

Acknowledgment
This study was funded in part by a Grant-in-Aid as partof the Special Coordination Fund for Promoting Science andTechnology from the Ministry of Education, Culture, Sports,
Science and Technology of Japan.

References[1] Andy Ward, Alan Jones, and Andy Hopper, “A New Location Technique for the Active Office,” IEEE PersonalCommunications, Vol.4, No.5, pp. 42-47, October 1997.
[2] Mike Addlesee, Rupert Curwen, Steve Hodges, Joe
Newman, Pete Steggles, Andy Ward, and Andy Hopper, “Implementing a sentient computing system,” IEEE
Computer, Vol.34, No.8, pp. 50-56, August 2001.

Figure 17: Realized portable ultrasonic 3D tag system

7 Conclusion
In the present paper, we proposed new calibration methods
to enable a portable ultrasonic location sensor so that users
can quickly construct a system for observing human activity
in various environments where human activities actually occur. Two basic calibration methods and two additional methods for quick calibration were described: B1) a basic method
using three or more transmitters, B2) a basic method using asingle transmitter, A1) an additional method using the coplanar constraint, and A2) an additional method considering thedirectivities of ultrasonic transmitters and receivers. One of
the advantages of all of the proposed methods is the requirement for a relatively small number of transmitters. In addition, the proposed methods are not dependent on room size.
The performance of the proposed methods was tested in
an experimental room (4.0×4.0×2.7m) in which 120 ultrasonic receivers were attached to the ceiling. The experimental results showed that the proposed methods could be used
to calibrate the positions of receivers positioned over a spacelarger than that of the calibration device and that the errorof calibration could be improved significantly by considering coplanar constraints or ultrasonic directivity. In the caseof calibration using four transmitters with the coplanar constraint and considering directivities, the average of position
error was 76 mm, a 60% decrease compared to the case without the coplanar constraint or consideration of directivities.
The advantages and disadvantages of the proposed methods
were discussed herein.

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