Speed Measurement of Vehicles by Radio Wave - Absolute Measurement of Automobiles

Copyright © IFAC 12th Triennial World Congress, Sydney, Australia, 1993

SPEED MEASUREMENT OF VEHICLES BY RADIO WAVE - ABSOLUTE MEASUREMENT OF AUTOMOBILES K. Kobayashi and K. Watanabe College o/Engineering, Hosei University, 3-7-2, Kajino-cho, Koganei-shi, Tokyo 184, Japan

Abstract. This paper describes a new type of absolute speedometer of automobiles. It is known that the electric field intensity of the radio waves in the UHF/VHF range exhibits spatial periodic patterns due to the multi-pass fading phenomenon. We employ this pattern as the spatial scale for speed measurement by the spatial filtering method. We have examined if the pattern can be used as the spatial scale for speed measurement by laboratory experiments and we found it could. In this paper we consider how to realize an automobile speedometer based on the above idea. Further we went out from our laboratory to measure the electric field intensity at the various sites under various speeds. We found the method could measure the speed of automobile in the real situations. Keywords. vihicle; filtering; spatial filter; Sensors; absolute speedometer

1

INTRODUCTION

an antenna and tuner, which is not equipped with an AGC circuit and which is mounted in a vehicle being driven at a constant speed, is subject to distortion by periodic noise. This noise is called skip noise and is caused by a phenomenon known as multipass fading(Y.Okumura et al.,1986). Radio waves broadcasted from a station propagated to a receiving antenna along various paths and by way of one or more reflection. Thus, the radio waves received by an antenna exhibit an electric field intensity which is spatially periodic due to wave diffraction. This is a well known cause of noise in automotive communications. The spatial change in electric field intensity of the radio waves are not influenced by rain, mud or oil adhered to the antenna. Multi-pass fading occurs mainly because of reflections by buildings, mountains and geological features which vary in vertical height, and not because of reflections by automobiles which pass somewhat close to the antenna. Thus, spatial change in the radio wave electric field intensity can be effectively used for speed measurement by the correlation or spatial filtering method. The prior research verified the validity of the radio wave speedometer under restrictive circumstances such as on the roof of a building(K.Kajiro et al.,1991). The result of prior research provided that the speed is measurable. In this paper we investigate the electric field intensity in a real world environment and situations and examine if this pattern can be used for absolute speed measurement.For these investigation and examination, we developed a two channel electric field "level meter with wide dynamic range and quick response.

Accurate measurement of the absolute speed of automobiles leads to the safety driving, because the information of the absolute speed makes the control law of anti-skid breaking quite simple and accurate(London,1985). A current speedometer for measuring the speed of a vehicle is to multiply the measured wheel revolution rate to the wheel radius. This approach often includes abrupt unpredictable errors due to slip and skid of wheels and a biased error due to the steady state slip. These errors cause the difficulty in the implementation of an ABS system that can be easily realized if the absolute speed is known. A typical example of the absolute speed measurement method involves the use of optics(LIdogawa et al,1975). Optical reflection from the ground is used as a spatial pattern. (1) detects the optical reflection from the ground using two photo-sensors which are aligned in the direction of movement, (2) calculates the delay time between the two outputs using a correlation method, and (3) obtains the speed by dividing the distance between the two sensors by the delay time(T.Naito et al,1968). Optical spatial filtering methods are also very well known(M.Shirai,1982; A.Kobayashi et al.,1984). While the optical methods solve the problems of the wheel speed measurement, they do require heavy and complicated mechanisms to adjust the focus of the photo-sensors and also require cleaning arrangements for the optics which inevitably become soiled by rain, mud and oil. Methods employing radio wave Doppler effects are difficult to apply to vehicles which exhibit a change in attitude(Japan,1984). The radio waves from a FM broadcasting station received by

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2

2.1

The auto correlation of eq.(I) is given by Oth Bessel function, when electric field intensities come from all directions. The spatial auto correlation function of the electric field intensities is also given by the Bessel function thus it is periodic and its period is nearly equal .\j2 .

MULl-PASS FADING AND SPEED MEASUREMENT

!If ulti-pass fading

o OQ o O~

(2)

DOt!. 2.2

v

Speed measurement

The antennae set on the vehicle aligned with the direction of movement. Electric field intensity received by front and rear antennae are E(t), E(t - L/V), respectively. In order obtain the efficient signal from the spatial filter with two antennae, we let the span of the antennae

(3) by which the frequency I with maximum power of the output spatial filter (i.e., the difference of signals from F and R antennae) is equal to

1=

2V A

(4)

Id = -

which yields Fig.!.

Electric field intensity by the reflection and diffraction.

V

= Id A

(5)

2

Fig.I schematically illustrates how the radio waves propagated from a broadcasting station to the moving vehicles. The front antenna F and the rear antenna R are set on the vehicle and are aligned in the direction of movement. The spatial electric field intensity pattern is picked up by the two antennae. We apply the spatial filter technique to the signal from the antennae F and R to estimate the speed. We define variable and constants for the system as follows;

The speed is proportional to the frequency

3

(AI) The antennae on the vehicle can receive radio wave propagated from a broadcasting station; (A2) The radio waves from the station arrive, uniformly from all directions, at the antennae. (A3) The object moves within a certain speed range.

Variables of radio wave.

Among these assumptions, Assumption (AI) is a prerequisite for the system. The system does not work without radio waves being broadcasted. Assumption (A2) is not always necessary for the system. This assumption is made to make the theoretical development easier. Assumption (A3) is speed measurement methods. Under these assumptions we investigate: The change in the pattern of electric field intensity at various sites; To investigate this problem, we need a quick response electric field level meter which works accurate at the speed of IOOkm/hr of the automobile.

Variables of Electric field intensity. E(t) electric field intensity detected by antennae

fd

frequency change of electric field intensity PI auto correlation function electric field intensity

Variables of antenna. L span of antennae

Variable of moving vehicle. V speed of moving vehicle

General variables and constants. ,t time

4

shift time

Electric field intensity at the receiving antenna on moving vehicle is gi ven by the summation of ith path's electric field in tensi ty.

~

E() T = L

{ . li - VtcosBi Ei . exp -2J7r A

PROBLEM DESCRIPTION

Assume for the system in Fig.I that:

>- raclio wave len~th Ai the length of ,th path from broadcasting station ei angle of the ith incidence from broadcasting station Ei electric field intensity of ith path
T

!d.

4.1

.} + 14>i

1=0

(1)

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INVESTIGATION OF CHARACTERISTICS OF ELECTRIC FIELD INTENSITY"

Experiment system

Photo 1. shows the vehicle used for this experiment. The two antennae were set on the roof of the vehicle. The two antennae are those of >-/2 clipole type and they were aligned in the clirection of movement as shown Photo. I.

4.4

The height of the antenna was 1.8Sm from ground level. The span L of two antennae were O.71Sm. The radio wave used was channel 3( voice carrying) of N HK-TV broadcasting station(107.7SMHz). The level meter used to pick up the intensity of electric field was developed for the experiment and has the following characteristics.

In the urban 6ite

Fig.3 shows the pattern in the urban site Sinjuku, Tokyo(Photo.3 ), where many hi-rise buildings exist. The speed of the vehicle was 3Skm/h. The pattern was not clearly periodic, unlike the pattern observed on the highway. But very strong intensities were observed. This is because of the hi-rise buildings becoming giant reflectors.

(1) The meter picks up two electric field intensities by a multiplexer and one tuner. These two electric field intensities at the antenna site. (2) The response of the meter is very quick. Delay time of the meter is 1 mS. Therefore, the meter can pick up changes in the electric field intensity observed in the vehicles running at a speed of lookm/h. The electric field intensities measured were A/D converted with a sampling interval of 100 mS. The speed of the vehicles was estimated spatial filter method by using this data.

Fig. 2. Electric field intensity on the road in Joban highway.

Photo. 1

4.2

Experimental system

Pattern of the electric field inten$ity at vanolU $ite$

It is known that the pattern of the electric field intensity is strongly influenced by geographical features such as mountains, hills and/or valleys. It is even influenced by small buildings and bridges. We measured the pattern of the electric field in various and typical sites. Photo.2,3 and 4 ,Fig.2,3 and 4, respectively, show the sites and the patterns of the electric field intensities.

4.3

Photo. 3

Testing road in the urban site Shinjuku, Tokyo, in a hi-rise buildings environment.

On the highway

Fig.2 shows the pattern measured on the Joban highway(Photo.2 ) , when the vehicle was driven at a speed of 70kmjh. The pattern exhibited a periodic wave. This is because of the reason that, the structures around the highway were uniform and every structure was aligned along the highway.

1.2.5

:LS Time[sec]

3.7:5

Fig. 3. Electric field intensity on the road in the urban site.

Photo. 2

Testing road in Joban highway in Ibaragi.

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:5.0

4·5

In the open field

stably estimated by the proposed method for the vehicle tested at the various sites. From the above investigations, we could conclude that the spatial pattern of the electric field intensity of radio waves can be used as the spatial measure for automobile speed measurement.

Fig.4 shows the pattern measured in the open field(Photo.4) where no high buildings, no trees existed. The speed of the vehicle was 40km/h. The intensities were not as strong as those still observed in the urban site road test. The pattern was random but we could still observe the fading.

100 L:' z:

E 80

=..

.... 60 .. 40 c;

-0

~

-0

.5;;; 20

I

t:l

00 Photo. 4

Testing road in the open field.

20

40

60

80

100

Real speed [km/hr]

Fig. 5. Real speed vs. estimated speed ·~9.9

">

~

....

.!! U c;

REFERENCES

E c ::.

The Automobile Division of the Institute of Mechanical Engineers(1985). Anti-Lock Breaking Systems for Road Vehicles, Conference of Institution of Mechanical Engineers,London. I.Idogawa,H.Watai,R.Ohba and K.Takai (1975). Automobile Speedometer by Using Correlation, Trans. SICE Vol.l,No.4,pp.473-478 T.Naito,Y.Ohkami,A.Kobayashi(I968). Speed Measurement by the Spatial Filter, Journal of SICE VoI.7,No.ll,pp.761-772 M.Shirai Measurement Method of Speed of Moving Vehicles, Japan Patent Report,June,pp.127-131 A.Kobayashi,T.Yamaura,Y.Turutani(1984). Speed Measurement by Capacitive Spatial Filter, SICE 22thAnnual Meeting Japan Industry Report(1984). Sensor Technology and its Application, Industrial Research,pp.233-234 Y.Okumura and A.Shinsi(1986). Introduction to Automotive Telecommunication, IECE K.Watanabe,K.Kobayashi, Speed Measurement of Vesicles by Radio Wave, SAE TECHNICAL PAPER 910271 ,Detroit

:lE

e

u

-68.4

-61.9

i:i" ~

0

E

c .~ ::. c £

..s 23

3.75

~.o

Time[sec]

Fig. 4. Electric field intensity on the road in the open field.

4.6

Speed Estimation

We carried out experiments to estimate speed by the proposed spatial filtering method in the various sites as those described in (1). The speed range was from 10krn/h to 100km/h. Fig.5 shows the estimated speed vs. the real speed observed by the calibrated speedometer of the vehicle. The estimated speed was lineally proportional to the real speed and also no systematic error existed.

5

CONCLUSIONS

This paper describes a new speed measurement method which can be applied to moving objects without wheels. The method is based on the spatial filtering technique. We employ the electric field intensity pattern of radio waves, broadcasted from a station as the spatial measure. The electric field exhibits stochastically periodic changes in intensity. We investigated if the electric field intensity pattern could be used as the spatial measure for automobile speed measurement, using a spatial filter method. We o~ tained the following results. (1 )Stable patterns of the electric field intensity were observed in various sites (highway, downtown and open field). This fact yields the stable speed measured by the proposed method. (2)The speed ranging from 10krn/h to 100krn/h of the vehicle was accurately and

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