WO1987006349A1

WO1987006349A1 - Apparatus for measuring speed

Info

Publication number: WO1987006349A1
Application number: PCT/SE1987/000188
Authority: WO
Inventors: Bo Gunnar Mattsson
Original assignee: Telefonaktiebolaget Lm Ericsson
Priority date: 1986-04-16
Filing date: 1987-04-14
Publication date: 1987-10-22
Also published as: SE8601740D0; GB2196741A; SE8601740L; JPS63503011A; GB2196741B; GB8726412D0; SE452916B; DE3790190T1

Abstract

Sensor device for measuring the speed of wheeled vehicles. Two elongate sensor elements (26, 27; 28, 29) are placed substantially transverse a roadway and are connected to equipment (14) for measuring the time difference between the signals generated when the hoses are passed over by vehicle wheels. The sensor elements which are used are mutually joined along their entire length by a stiff web formed such as to define spacing between the sensor devices which is constant and independent of the passage of the vehicles. This spacing is in the order of magnitude 10-100 mm, i.e. very much less than the width of the roadway.

Description

APPARATUS FOR MEASURING SPEED

TECHNICAL FIELD

The invention relates to an apparatus for measuring the speed of vehicles in such as traffic checks, and particularly to a sensor device for placing on a roadway.

BACKGROUND ART

The common mehtods for measuring the speed of a vehicle at speed checkpoint is radar measurement and measuring the time required for passage between two points. Radar checks require expensive equipment and specially trained person¬ nel for interpreting the measurement results. In spite of this, the uncertainty is great, particularly in dense traffic, and it has often been necessary to rely upon simultaneously photography for interpreting the measurement results. Neither do these steps always allow such reliable measurement that it is accepted by the courts.

When measuring the time for a vehicle to pass between two points, it is usual to use two air- or liquid-filled hoses, the hoses being placed across the road at a spacing of some meters. When a vehicle wheel passes over a hose there is obtained a pressure impulse which is detected by suitable measuring equipment.

The intention with the method is to measure the time that passes between the instant when a given wheel passes the first hose and the instant when the same wheel passes the second hose. Due to the great distance between the detector hoses, the uncertainty in detection will be great, particularly in dense traffic in several lanes or on a two-way roadway. The probability is then great that several vehicles pass the measuring device approximately at the same time. It may then happen that wheels on different vehicles and not one and the^v same wheel on a given vehicle actuate the sensor hoses. In addition, it is difficult to set up a measuring station when the measurement of the measuring distance between both hoses must be made with great precision, and it is important that the hoses are mutually parallel and at right angles to the roadway. An attempt to solve this problem is shown in the German Auslegeschrift 1 673 386 where bars are used to keep the distance between the hoses constant. It is further known from EPO 120 999 to use fibre-optic sensors to discover objects passing the sensor. Such a sensor is based on a light beam passing through an optical fibre being modified as to amplitude or polarization when the sensor is subjected to a force which bends the fibre.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a sensor device which can be quickly laid out with a minimum input of personnel, and which enables speed measurement with great accuracy and great security against measurement errors due to interference between different vehicles. A sensor device for measuring vehicle speeds in accordance with the invention includes two elongate sensor devices extending transverse at least one lane on which the vehicle is travelling. The characterizing features of the invention are disclosed in the accompanying claims.

BRIEF DESCRIPTION OF DRAWING

The invention will now be described in detail with reference to the accompa¬ nying figures, where Figure 1 is a sketch of a roadway provided with speed control equipment

Figures 2A and 2B illustrate two embodiments of the invention, Figure 3 illustrates in principle the sequence when a vehicle wheel passes the sensor device, Figure 4 is a time chart depicting actuation of the sensor devices.

In figure 1, the reference 10 denotes a roadway on which a vehicle 11 is travelling. Right across the roadway there is a cable containing two sensors 12,13 connected to measuring equipment 14. When the vehicle passes over the sensors 12,13 with a pair of wheels, e.g. 15, the sensors are actuated in rapid succession by the wheel pressure. Since the spacing between the sensors is small (10-100 mm), the probability is very great that it is one and the same wheel which actuates first one sensor and then the other. Let it be assumed that the vehicle speed is 20 m/s (about 70 km/h). and the distance between the sensors is 20 mm, then the interval between the passage of the wheels over both sensors is in the order of magnitude 1 ms. which gives extremely good discrimination between different vehicles. The impulses from the sensors are evaluated in measuring equipment 14 which, in a manner known per se, measures the distance between the impulses (tl and t2 in Figure 4) and converts the time difference t to a speed v using the distance s between the sensors. The equipment 14 is suitably based on a microcomputer in order to enable the correction of the measuring result in a simple way, taking into account sensor type, temperature variations etc.

Two different embodiments of a device in accordance with the invention are illustrated in figures 2A and 2B. The device according to Figure 2A utilizes flexible, electrically insulated hoses 21,22 filled with a conductive liquid as sensor elements. At both ends, the liquid is in contact with suitable, unillustra- ted conductive terminating plugs. Between the hoses 21 and 22 there is a web 23 which fixes the spacing between them along their entire length. The web 23 is formed such that the spacing is kept constant even when a vehicle wheel passes over it. It may be desirable to reinforce the web 23 with a metal plate, netting or the like.

A return conductor 24,25 is suitably arranged for each sensor hose 21,22. For example they may be arranged in the web 23 so that the entire measuring equipment 14 can be placed on the same side of the roadway. A quiescent current passes through the hoses 21,22 and the associated return conductors 24,25. When a vehicle passes the sensor device, the first hose 21 will be compressed by a wheel. The cross-sectional area of the conductive liquid is heavily reduced and the current is decreased heavily or ceases entirely. For a given threshold value of the current, a pulse circuit in the measuring equipment is activated and a counter or other form of time circuit is started. When the wheel passes the hose 22 a second pulse is generated and the timing circuit is stopped. The timing circuit is designed in such a way that once it is started it cannot be actuated for example by the second wheel on the same axle that may pass the same hose a short time afterwards. The measured time is converted to a speed with the aid of analogue or digital circuits, known per se.

Figure 2B illustrates a corresponding device where the sensor elements (12,13) are optical fibres 26,28 bedded into a flexible non-transparent casing 30 in a manner known from cablemaking technology. In this case as well, return conductor loops 27,29 are suitably placed in the casing 30 so that both transmitting and receiving equipment can be placed on the same side of the roadway. The casings 30 are joined together by a stiff web 23 in the same way as illustrated in Figure 2A so that the sensor elements are kept on a constant distance from each other.

In operation, the measuring equipment 14 sends light, which passes through the optical fibres 26,27 and 28,29 respectively, forming two closed loops across the roadway from the equipment 14 and back again to detecting means in the measuring equipment.

When a vehicle passes the sensor devices (see Figure 3) the casing 30 is deformed, which is illustrated by the reference 32. This in turn results in that the embedded optical fibres are subject to bending and thus to a mechanical stress. In the litterature pertaining to the art it is known, e.g. from the US patent 3982123, that an optical fibre subjected to bending increases its attenuation. Similarly, it is known e.g. from EP 120999 that an optical fibre changes its polarisation state when it is bent. One of these phenomena can be used to detect when a vehicle wheel passes the sensor devices 12 and 13, and the technique required for this detection is already described in the mentioned patent specifications. The detected changes in state actuate a measuring circuit in the same way as described in connection with Figure 2A. It is also possible to permit the sensor devices to be parts of one and the same optical fibre, e.g. so that the fibre 26 is the outgoing conductor and the fibre 28 is the return conductor, in which case the fibres 27 and 29 are unnecessary.

By utilizing sensing principles which give very rapid response to the pressure increase caused by a passing wheel, and rapid electronics in the sensor circuits it is possible to obtain the rapid measurement required by the short measuring distance and this gives good discrimination between different objects, even in dense traffic. The rapid indication can, if so desired, also be utilized for controlling cameras for identification, and the control of caution signs for indicating acutal speed and the like.

Claims

C L A I M S

1. A sensor device for measuring the speed of wheeled vehicles, including at least two elongate sensor elements intended to be placed transverse a roadway with the devices substantially parallel, also including equipment for measuring the time difference between the signals generated by the sensor elements when they are passed over by vehicle wheels, characterized in that the sensor elements (12,13) are mutually joined by a stiff web (23) formed such as to define the spacing between the sensor devices, this spacing being constant and independent of a vehicle passage, said passage being much less than the width of the roadway (alternatively in the order of magnitude 10-100 mm).

2. Sensor device as claimed in claim 1 characterized in that said spacing is between 10 and 100 millimeters.

3. Sensor device as claimed in claim 1, characterized in that the web (23) is reinforced.

4. Sensor device as claimed in claim 1, characterized in that the sensor elements comprise elastic, non-conductive hoses (21,22) filled with a conductive liquid.

5. Sensor device as claimed in claim 1, characterized in that the sensor elements comprise optical fibres (26,27,28,29) enclosed in a flexible casing.

6. Sensor device as claimed in claim 3, characterized by at least one return conductor (24,25) coupled in series with the conductive liquid in the hoses.

7. Sensor device as claimed in claim 4, characterized in that the sensor device contains an optical fibre loop with the input and output of the fibre at one end of the sensor element.

8. Sensor device as claimed in claim 4, characterized in that the sensor elements (e.g. 26,28) comprise parts of one and the same optical fibre.