GB2234380A

GB2234380A - Axle sensors

Info

Publication number: GB2234380A
Application number: GB8917082A
Authority: GB
Inventors: Derrick Ashley
Original assignee: JH Fenner and Co Ltd
Current assignee: JH Fenner and Co Ltd
Priority date: 1989-07-26
Filing date: 1989-07-26
Publication date: 1991-01-30
Also published as: GB8917082D0

Abstract

An axle sensor comprises an elongate, channel shaped member (1), an elongate pressure transducer e.g. coaxial cable (2) located at the bottom of said channel shaped member (1) and filler means (3) for filling the channel shaped member (1) and encapsulating the elongate pressure transducer (2) therein. The base or bottom (4) of the channel shaped member (1) defines the upper or working surface of the axle sensor, thus ensuring that there is no bonded interface in the region of the sensor in contact with road vehicle wheels. <IMAGE>

Description

DESCRIPTION AXLE SENSORS The present invention relates to axle sensors for monitoring the passage of traffic along a road. Axle sensors are load sensitive devices which are embedded in a road surface and are capable of distinguishing between types of vehicles according to,the load imposed upon the road surface and the number of axles per vehicle.

Axle sensors are known which comprise an elongate piezoelectric element encapsulated in an elastomeric moulding which facilitates embedding in a road surface.

When the piezoelectric element is subjected to mechanical stress, as it is when a vehicle axle passes over the sensor, a potential difference is generated across the piezoelectric element. This potential difference is proportional to the stress and is, therefore, a direct indication of the axle load of the vehicle.

The signal output from the piezoelectric element can be processed in a variety of ways. For example, it can be used to operate a counter to provide an indication of the volume of traffic on the road and a break down of that traffic according to axle load and axle number.

Alternatively, it may form part of a road signalling system which controls traffic lights or other road signalling devices in accordance with a pre-arranged programme dependent upon the flow of vehicles.

In early axle sensors the piezoelectric element comprises a piezoelectric tape approximately 12mm in width. This is not entirely satisfactory as it requires a groove of relatively large width and depth to be cut in the road surface to accommodate the sensor. In order to circumvent this disadvantage it has been proposed that the piezoelectric tape be replaced by a coaxial cable.

This cable is approximately Dmm in diameter, thereby enabling the elastomeric moulding to be much reduced in width. The resultant axle sensor is of relatively narrow section and can, therefore, be accommodated in a relatively narrow groove in the road surface.

The elastomeric moulding comprises a channel approximately 5mm in depth in its upper surface into which the coaxial cable is laid. This groove is then filled, and the top of the moulding is covered, with polymeric material in order to encapsulate the coaxial cable and provide the axle sensor with a flat working surface. Finally, the sides of the elastomeric moulding may be machined to the required profile.

The groove in which the coaxial cable is laid is formed by a moulding bar which must be removed after casting. In order to facilitate removal of the moulding bar a mould release fluid must be applied in the vicinity of the groove. Unfortunately, this mould release fluid contaminates the groove and the landings immediately adjacent thereto, and much reduces the bond effectiveness of the elastomeric moulding and the polymeric filler. As a consequence, the polymeric filler, which also forms the working surface of the axle sensor, tends to detach itself from the elastomeric moulding under extreme working conditions. The tendancy for de-bonding to occur between the elastomeric moulding and the polymeric filler is further exacerbated by the relatively small bonded area which exists therebetween.

In this respect, it should be understood that the piezoelectric element must be embedded in the road surface at a position sufficiently close to the surface to be receptive to changes in pressure at the surface.

For this reason the groove in the elastomeric moulding must be relatively shallow in order to position the pizelectric element close to the upper surface of the sensor and because it is shallow the contact area with the polymeric filler is much reduced.

It is an object of the present invention to provide an axle sensor which obviates or at least substantially mitigates the problems associates with conventional axle sensors which have been referred to hereinabove.

According to the present invention there is provided an axle sensor comprising an elongate, channel shaped member, an elongate pressure transducer located at the bottom of said channel shaped member and filler means for filling the channel shaped member and encapsulating the elongate pressure transducer therein, wherein the base of the channel section defines the upper or working surface of the axle sensor.

Preferably, the channel shaped member and the filler material are comprised of a polymeric material such as polyurethane.

Advantageously, the channel shaped member is formed from a solid section in which a relatively deep groove is routed or machined. This avoids the use of mould release agents which could adversely affect bonding of the filler material thereto. The axle sensor of the present invention allows the pressure sensitive transducer to be located in a relatively deep channel which affords the maximum possible area of bonding between the channel shaped member and the filler material, whilst ensuring that the pressure transducer remains just beneath the working surface of the axle sensor. It also ensures that there is no bonded interface in the region of the sensor in contact with road vehicle wheels.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawing which shows a cross section through an axle sensor according to the present invention.

Referring to the accompanying drawing the axle sensor comprises an elongate main body 1 having a relatively deep groove or channel formed therein. A coaxial cable 2, capable of generating an electrical signal when mechanical stress is applied thereto, is accommodated within the groove and the groove is filled with a filler material 3 to encapsulate the coaxial cable 2 within the main body 1.

The main body 1 is moulded as a solid section in castable polyurethane having a curing temperature below 60 C (highter temperatures may have an adverse effect on the coaxial cable 2) and the groove is formed therein by routing or some other suitable machining process. It would, of course, be possible to form the groove during the initial moulding of the main body, but this is not considered to be as satisfactory as routing as traces of mould release agent may remain and these can prevent effective bonding of the filler material 3 to the walls of the channel. The filler material 3 is also comprised of polyurethane and after this has been introduced into the groove in the main body 1, the axle sensor as a whole is subjected to a curing operation so that a secure bond forms between the inner wall of the groove and the filler material 3.

The working surface of the axle sensor is defined by the base 4 of the main body. In other words, it is the base 4 which is uppermost and in contact with vehicle wheels when the axial sensor is embedded in a road surface. The groove in the main body 1 is cut to such a depth as to leave a base thickness of approximately -2mm.

However, if necessary, after the final curing operation has been carried out the working surface 4 can be machined to leave a covering of 2mm above the coaxial cable 9. Additionally, the sides of the main body 1 can be machined to give the required profile.

Claims

1. An axle sensor comprising an elongate, channel shaped member, an elongate pressure transducer located at the bottom of said channel shaped member and filler means for filling the channel shaped member and encapsulating the elongate pressure transducer therein, wherein the base of the channel shaped member defines the upper or working surface of the axle sensor.

2. An axle sensor according to claim 1, wherein the channel shaped member and the filler means are comprised of a polymeric material such as polyurethane.

3. An axle sensor according to claim 1 or 2, wherein the channel shaped member is formed from a solid section in which a relatively deep groove is routed or machined.

4. An axle sensor according to claim 1, 2 or 3, wherein the elongate pressure transducer comprises a coaxial cable, capable of generating an electrical signal when mechanical stress is applied thereto.

5. An axle sensor substantially as hereinbefore described with reference to the accompanying drawing.