Railways

190

OBSTRUCTION DETECTOR USING ULTRASONIC SENSORS FOR UPGRADING THE SAFETY OF A LEVEL CROSSING Kazutoshi S A T 0 t Toshiyuki SHIMIZU 1

Hideki ARAI t Masahiro TAKADA

t

f;

f Railway Technical Research Institute JAPAN

1 Matsushita Communication Industrial co., Ltd. JAPAN ? $ Matsushita Electric Industrial co., Ltd. JAPAN INTRODUCTION Quite few train accidents, caused by the signal control system, have occurred since the Japanese National Railway (JNR) was put under private management in 1987. However, train accidents at a crossing have recently become important problems as the numbers of train density and vehicle traffic volume are increasing. A train accident at a crossing is very dangerous because it is not only a train-and-vehicle-collision,but may cause a secondary train-collision-accidentif the train runs off the rails. Therefore, it is an important issue to prevent a crossing-accident for a safe travel. A crossing-accident can be avoided by a grade separated crossing. But for this, it is required to make a huge

construction cost and to consider the environmental

with loop coils. Neither has not been generally introduced because of their weak points. Hence, the authors have developed a new obstruction detector, that can be useful for all lines including one in the snow-covered area, by studying ultrasonic-based obstruction detection at a crossing. In this paper, the authors will explain about the structure and the methods of the ultrasonic-based obstruction detector first, then about the safety logic of the detection.

THE SHIFT OF THE NUMBERS OF CROSSINGS AND THE CROSSING-ACCIDENTS There are three types crossings on Japan Railway (JR) lines. Type 1 crossing is installed with an alarm and a crossing gate. Type 3 is only with an alarm and Type 4

problems like the right to sunshine of the wayside residents. And all of these have made grade separation more difficult, even for the main lines.

is only with a display sign of Crossing. Neither alarm nor crossing gate is not installed for Type 4 crossing.

Several technical means have been taken for crossingaccident prevention, and the most promising one is to

Figure 1 shows the shift of the numbers of crossings and the crossing-accidents of all JR group from 1983 to 1993. The number of all crossings decreased from ap-

install an crossing obstruction detector. This equipment detects an obstruction at a crossing in early stages to re-

proximately 28900 in 1983 to approximately 23700 in 1993.

port it for a train crew. There are two types of the obstruction detector now, one with ray beams and another Number of crossings 40000

Crossing accidents

I

Crossing

35000

’

30000

1 I

800

700

mn

25000

an alarm and a crossing gate are installed with, has been increased in number. In contrast, Type 4 crossing has

20000 15000

become fewer. Taking measures to upgrade the crossing safety, the crossing-accidents have been reduced to half, fiom 738 in 1983 to 382 in 1993.

10000 5000 0

1983

This decrease is caused by the accelerated grade separation andor disuse of crossings. The disuse of crossings is especially due to discontinuing many loss-making local railroad lines about that time when the JNR was put under private management in 1987. In addition, one thing should be mentioned that Type 1 crossing, which

1985

1987

1989

1991

1993 year

Figure 1: The numbers of crossings and the crossingaccidents of all JR group from 1983 to 1993

Recently, however, it has become impossible to prevent a crossing-accident only by the equipment like an

International Conference on Developments in Mass Transit Systems, 20 - 23 April 1996, Cmference Publication No. 543 OlEE 1998

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191

Caused bv

crew. Each JR company has positively planed to intro-

etC

duce this equipment.

PROBLEMS OF THE EXISTING OBSTRUCTION DETECTOR

Type3

Y! Typc.4 16% Figure 2: the causes for crossing-accidents in 1993 alarm and a crossing gate. Figure 2 shows the causes for

Photoelectric method (light ray, infrared light ray, and laser) By this method, an obstruction is detected when the ray beam of the ray net on the railroad crossing is continuously cut off for a certain time length by an obstacle like a vehicle. The equipment consists of ray transmitter, which emits light beam by using semiconductor Imines cent element, and receiver, which receives the light ray to activate a detection relay. Figure 3 shows the outline

crossing-accidents in 1993.

of this equipment.

60% of all crossingaccidents was occurred when a vehicle ran across the crossing just before the trains coming. Besides, about 10% of it was lateral collision, which is unbelievable in normal cases. Drivers have an obligation to stop in front of a crossing to make sure their safety by Japanese Road Traffic Law, but there are many drivers who dlo not keep the law. Only the equipment like an alarm and a crossing gate is not enough for this kind of drivers. Therefore, each JR company has carried out regular campaigns to upgrade drivers manner at a crossing and accused bad drivers to the police. On the other hand, 27% of all accidents was occurred when a vehicle was made stopped on a railroad crossing by an engine stall failure. For this, it will be effective to install an obstruction detector, which detects an obsta-

This photoelectric obstruction detector has some problems such as that: a) cannot be used in the heavy-snow area, b) may malfunction by infrared light ray method because of the much beam attenuation volume toward snow and fog, c) does not work well for optical axis gap between a transmitter and a receiver by laser method, though it works well in snowy and foggy conditions, d) requires a regular maintenance to clean up lens. Currently, photoelectric obstruction detectors are not used in winter though they are already installed at crossings in the snow-covered area.

cle on a crossing in early stage and reports it to a train

l e v e l crossing ~”

F\;,- - m23

x

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Leve I c r o s s I ng

A crossing warnings 7%

Ray Transmitter

Crossing warnings Figure 3: Conventional method to detect the obstacles using ray beam

Figure 4: Conventional method to detect the obstacles using loop coil

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192

d) can not apply to a steel-plate-paved or a

Loop coil method

snow-melting-facilities-installed crossing.

By this method, an obstruction is detected when the inductance of loop coils was changed when a metallic object, like a vehicle, approaches to the loop coil under the railroad-crossing ground. This equipment consists of loop coils and a detector for signal processing. Figure 4 shows the equipment outline. The loop coil method obstruction detector is applicable to crossings in the snow covered and frequently fogged areas as well. At the same time, it has problems such as

is easy to be influenced by the temperature and the humidity because the signal change owing to vehicles is small, hence, requires a regular adjustment of the detection sensitivity, because the following is late and the correction range is small, though an automatic equilibristat corrects it, the cost for a concrete-paved railroad crossing (a connecting track), which prevents a loop coil from its shape change, is high,

I i

Ultrasonic wave is generally used for object detection and/or distance measurement since it is easy to get a sharp directivity and the energy reflectivity is 100% when ultrasonic wave vertically reflects from an object in air. Hence, we have developed a method for obstruction detection on railroad crossing by irradiating ultrasonic wave from an ultrasonic antenna, installed above the railroad crossing, to detect an obstacle by the reflective wave. This equipment consists of an ultrasonic antenna and a processor as shown in Figure 5. An ultrasonic antenna consists of a cylindrical ultrasonic vibrator and an antenna horn, and the frequency for this is 26 kHz, the transmission sound pressure level is 95 dB. This antenna is to be mounted on an arm at 5.0-6.0 meters above the road surface and to be adjusted to ver-

circle with a diameter of 1.2 meters, just below the an-

--

tenna. A vehicle will be detected when it enters this circle. The directivity features of the ultrasonic antenna is described in Figure 6.

j

Processing Lwie

_ _

Equipment outline

tically irradiate the ultrasonic wave toward the road surface. The detection range of this antenna is within a

Arm

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OBSTRUCTION DETECTOR USING ULTRASONIC SENSOR

Obstruction alarm

.Level crossing

Figure 5 : Structure of the obstruction detecting device I

A processor mainly consists of a signal sending and receiving unit that sends and receives ultrasonic signals, inputioutput unit that detects a vehicle or an obstruction for individual action conditions at a crossing, and a logical unit. Since it is required to have a fail-safe fimction for a processor, the double microcomputers method with a tight comparison at computer-bus level circuit in a logical unit is adopted to upgrade the safety level. It is applicable to various size crossings because signal sendigdreceiving unit of the processor can be linked with eight antennas for maximum. In addition, more signal sendingireceiving unit can be installed.

The detection logic and the safety

Figure 6: Directivity of the ultrasonic antenna

This equipment reports an obstruction detection to a train crew by activating a special signal if it detected an

193

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Figure 8: Relation between the speed of wind and the attenuation quantity of reflection wave level from the ground

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wave couldn't reach to any gate. A series of three Car I / Existence is to avoid the wind-caused malfunction to the for obstacles

G a t e f o r ground

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Figure 7: Decision of an obstruction using the ultrasonic reflection wave and gate signals obstruction for longer time than the appointed time length. The obstruction detection will be continued until

a train comes up to the crossing. Hence, the obstruction detection doesn't have to be done when a crossing alarm is not warning. For this equipment, it is important to distinguish the existence of an obstruction (a vehicle) though it is not necessary to detect a detailed vehicle height. According to the antenna height, we have set a vehicle detection gate and a road surface gate to know a vehicles existence by the return of the reflective wave within gates. Figure 7 shows the signal SendinpJreceiving mode of ultrasonic wave and the results of the obstruction detection. When the reflective wave is returned within a road surface gate, it is Mode 1 and the result is No Car. When the reflective wave is returned within a vehicle detection

best of its ability. Figure 8 shows the relation between the wind speed and the receiving level of the road-surface-reflective wave. The upper line of Figure 8 shows the wind speed change, the lower line shows the receiving level change of the road-surface-reflective wave, and the transverse axis shows the time lapse. For this measurement, ultrasonic wave was transmitted toward the road surface in every 100 ms from the antenna, installed at the 6.0 meters high, to measure the receiving level of the road-surfacereflective wave and the wind speed. The results show that the receiving level was not successively attenuated though the higher the wind speed becomes, the lower the receiving level will be. Therefore, the wind speed influence can be mostly eliminated by the Car Existence judgement when the reflective wave reaches to neither gate for three measurement in succession. Figure 9 shows the ultrasonic sendingreceiving signals, in case of the normal road-surface-reflective wave, and

gate, it is Mode 2 and the result is Car Existence. When the reflective wave is outside of both gates, ,it is Mode 3. A series ofthree Mode 3 will be resulted in Car Existence. Car Existence will be taken over until the reflective wave from the road surface is normally re-

"a" in Figure 9 is the rectification detection output of a receiving signal, "b" i s the vehicle detection gate, "c"

ceived. Car Existence for the case when reflective wave is returning to neither gate is to activate the equipment

is the road surface gate, and "d" is the digital output of a receiving signal. The first signal in Figure 5 shows the

to keep the safety level, even if the curved surface of a vehicle diffused the transmission wave and the reflective

sending wave and the next signal shows the road-

an example of measurement at a road surface gate and a vehicle detection gate.

surface-reflective wave.

The transmission time of

194

Transmission frequency 26kHz Transmission Reflection wave a Receiving signal (detective rectif b Gate for detecting obstacles c Gate for detecting ground d Receiving signal

(digital output)

- 2 2

J-3iiiP

L IOms-rt

Figure 9: Example of measuring the reflection wave from the ground ultrasonic wave is 2.0 ms. The reverberation of ultrasonic vibrator makes the sending wave time much longer.

Operational conditions of the equipment and future plan

Signals between the sending wave and the road-surface-

This equipment was introduced at a crossing near Hida-

reflective wave are multi-reflections caused by the

Furukawa station on Takayama Line,

antenna.

JR Tokai, in 1994 for the first time. Though Hida-Furukawa is well

Depending on the antenna height, the necessary time to open a vehicle detection gate or a road surface gate may differ. The width of each gate is fixed at 22.5 ms for a vehicle detection gate and 5 ms for a road surface gate. The time lapse between a vehicle detection gate and a road surface gate is to detect an obstruction at 0 3 me-

known as a heavy snow area, the equipment have normally worked without a mal-detection. The ultrasonic antenna could have been installed at 6.5 meters high for Takayama Line because it has not electrified yet. However, to use this equipment for electrified railroads, it is required to make the antenna height 8.5 meters to keep a

ters high from the road surface. It is clear from the re-

distance to the electrified railroad line. For this, an ul-

sult that the road-surface-reflective wave can be normally received at the centre of a road surface gate.

trasonic sensor with higher transmission sound pressure

The necessary time to sendlreceive signals by one ultrasonic antenna is fixed at 100 ms for this equipment. The multi-reflection and the antenna height are considered for this. Hence, even if more than eight antennas were

level has to be adopted to have a sufficient SignalNoise ratio.

installed, the obstruction detection needs just a second.

Therefore, we have developed a new sensor, shown in Figure 10. This sensor has the separated signal sendingireceiving functions. The maximum transmission sound pressure level is 124 dB, within the range

Considering the alarm behaviours at JRs crossings,

that an element stays still, as shown in Figure 11.

there is no problem for this detection time.

Authors have assumed that the loss of road-surfacereflective level, caused by the road surface shape and installation error of the ultrasonic antenna, is 10 dB, the loss caused by wind influence is 20 dB, and the loss

Signals are sentireceived, except for the crossing alarm, in every five minutes to adjust the onioff timings of both a road surface gate and a vehicle detection gate automatically. The onioff timings are adjusted by the average receiving time that is calculated by the time length from when you send the ultrasonic wave to when

cone

Piezo element

\

you receive the road-surface-reflective wave. This enables the equipment to be applicable to the height change by snow covering.

Figure IO: Piezoelectric unimorph transducer

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caused by snow is 20 dB by our study. So, the maximum loss of road-surface-reflective level is 50 dB in the nature environment. The directivity features of this sensor is described in Figure 12 and the features of distance/signal detection level to a flat level (as road surface), which are studied by the indoor tests, are shown in Figure 13. In this figure, 1.25 Vp-p and 2.5 Vp-p lines are measured in continuous mode, 144 Vp-p line in pulse mode.

Figure I 1: Relation between impressed voltage and sound pressure level of the transducer (indoor, Im)

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Figure 13 indicates that the aforementioned maximum loss can be fully covered when the signal detection level is set at -59.7 dBV, because the road-surface-reflective level is -9.7 dBV when the sensor is mounted at 8.5 meters high. Whether the signal detection level can be set at -59.7 dBV or not relies on the extemal receiving noise in the receiving circuit, or on the noise inside a set. The noise inside a set in the receiving circuit was measured to be -66.5 dBV, so that it was below 6 dB than signal detection level. There is no problem for this. The extemal receiving noise is planed to be studied in future field test.

CONCLUSION Obstruction detector using a new developed ultrasonic

50 -40-:10 20 -10 0 1R 21)30 4Q Stl (dr.9)

Figure 12: Derectivity of the ultrasonic sensor

sensor is mentioned in this paper. This detector has already been operated for non-electrified railroad lines. For electrified railroad lines, a new developed ultrasonic sensor is to be verified its performance by field tests. Field test will be started from October, 1997 in JR Kyushu and from December,

1997 in JR Hokkaido.

This detector has foresight to be in operation for also electrified railroad lines.

0

2

4

haterrr ti@

6

8

10

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Figure 13: Relation between signal detection level and the antenna height for each impressed voltages. (144 Vp-p line is measured in pulse mode that duty is 1 %)

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