FROM CONDUCTORS

This invention relates to a monitoring system for electrical vehicles drawing current from conductors, for example overhead conductors or powered rails, and to current collectors incorporating sensors particularly, but not exclusively, capable of use in such systems.

Electrified railway vehicles derive power from an overhead contact wire system (commonly known as an overhead contact line or OCL) or a powered rail. With the overhead system, typically a pantograph mechanism placed on the roof of the vehicle comprises a current collector that transfers current from the overhead wire to drive the vehicle. [An alternative arrangement is used for some trolley buses, which use a collector on a trolley pole. The present invention encompasses such arrangements and is intended to cover all systems in which a vehicle draws current from a conductor]. While this arrangement has been generally satisfactory, over the years the operational speed of railway vehicles has increased and the margins of acceptable current collection have been reduced. Moreover, cheaper and lighter overhead equipment on high voltage AC systems has been widely used. Dynamic impacts (which may damage the current collector on the overhead pantograph head) increase dramatically with the increase of speed and the use of lightweight overhead equipment. The following description therefore concentrates on high speed rail transport but the invention may find useful application on lower speed transport.

Any damage caused to the OCL has a negative effect on both track and train operators, with the producer of the damage being charged a penalty for preventing other company's scheduled operation of train or track.

With the increasing number of high speed trains in some European countries this problem is becoming more pronounced as any deviation from the correct contact force of the current collector produces excessive uplift of the OCL with risk of severe damage. At a

minimum, incorrect contact force produces excessive wear of the current contacts (collector and OCL) and possible environmental noise due to arcing.

Existing high speed train pantographs apply an upward force, however there is no direct measurement used presently for on-line monitoring of the contact force.

Neither is there any type of temperature measurement for detecting thermal overload conditions which can occur, for example, when the train is standing still at a station and the current collector draws power from the OCL for auxiliary systems (e.g. air conditioning) without the cooling effect from wind flow.

Several attempts have been made to provide on-line real-time monitoring of the electrical infrastructure. One approach is to measure the wear of the OCL with a specially equipped train with high speed video cameras installed on the top. By fast image processing the moving image recorded as the train passes under OCL, the cross section of the wire is calculated and the resultant wear is derived from its deviation from the ideal circular section. This method is quite successful but is intended for an inspection train only, due to its complexity and cost.

Cracks can develop in the current collector when the current collector suffers a large impact. A crack in the current collector may trap or pull down the overhead line. If this happens then the pantograph must be freed from the line and the line replaced before the vehicle can continue. Any damage to the overhead wire must be dealt with as soon as possible to maintain the reliability of the train service and limit costs. It is therefore important to locate actual damage on the overhead wire. In some cases, by the time the train operator has realised that damage has occurred, he is considerably further down the train track and is unable to tell with any accuracy where the damage to the line occurred.

Pantographs of modern, high-speed, electrified trains may use current collectors made of a carbon material to minimise the wear of the overhead wire. However, carbon is susceptible to crack damage.

Damage detectors for current collectors have been described. For example, GB 1374972 and GB 2107662 describe systems to measure damage to a current collector that comprise placing a tube in a cavity in the current collector. The tube bursts when a crack in the current collector reaches the cavity. Such systems can be used to provide an "auto drop" feature. When the tube bursts, pressure drops in the system. The loss is detected, and an automatic auto-drop device lowers the pantograph removing contact with the OCL and preventing any possible damage.

With such pneumatic systems there is no means of re-lifting the pantograph following a pressure loss, the detection response is relatively slow and they do not provide a means of predicting the life of the collector or providing information on the condition of the OCL.

Many sensing arrangements comprising optical fibres are known, for example EP-A- 0269307 describes embedding an optical fibre in a current collector. An optical signal (e.g., from a LED source) is transmitted through the optical fibre and is received by a detector at its opposite end. Any crack damage to the current collector that reaches the optical fibre affects the optical signal and is thus detected.

Systems have also been developed to detect wear of a current collector. For example, DE-U-8803377.5 and EP-A-0525595 describe systems in which a plurality of optical fibres are placed in the current collector at different distances from the wear surface. The wear level of the pantograph can be deduced when, for example, damage occurs to the optical fibre closest to the wear surface of the current collector but not to an optical fibre further away.

Fibre Bragg Gratings comprise an optical fibre with a grating arranged to provide a reflected component to light passing through the fibre and of appropriate wavelength to interfere with the spacing of the grating. Since the spacing will vary according to strain and/or temperature, the wavelength reflected will vary with these parameters.

Fibre Bragg Gratings can be designed to measure temperature or strain, and force can be derived from strain measurements. An example of a Fibre Bragg Grating that is temperature independent but which shows a good response to strain, is described in "Temperature-Independent Strain Sensor System Using a Tilted Fiber Bragg Grating Demodulator", IEEE Photonics Technology Letters, Vol. 10, No. 10, October 1998, Page 1461. Conversely, strain isolated Fibre Bragg Gratings can be used for temperature measurement.

Fibre Bragg Gratings sensors have the ability to measure directly at critical stress points and at high voltage while not being adversely affected by high temperatures and high electromagnetic fields. They are also capable of transmitting the optical signals over significant distances of fibre cable without degradation.

US 6587188 proposes the use of fibre Bragg Gratings as strain and temperature sensors in vehicle applications.

FR-A-2846415 discloses the use of Fibre Bragg Sensors in a pantograph or within a conductor to measure force between a pantograph (carrying a collector) and a conductor. However the sensors disclosed are oriented to respond to dimensional changes transversely to the collector [see Fig. 3 of FR-A-2846415] and in practice would not be effective in measuring the result of the major force component which is bowing of the conductor [resulting in dimensional changes in the z direction of Fig. 3 of FR-A- 2846415].

The applicant's co-pending application PCT/GB2004/004569 discloses the use of Fibre Bragg Grating (FBG) sensors placed within the collector itself and discloses preferred positions for such sensors, particularly when the collector comprises a carbon collector body and a carrier which may be metal.

The applicant's co-pending application PCT/GB2004/004737 discloses a composite electrical collector, for use in transferring electricity to or from a conductor and to make

sliding contact with the conductor, the collector comprising a metal mesh embedded in a tribologically acceptable matrix.

CN-A-1468891 discloses composite collector material containing carbon fibre and metal powder.

According to the invention there is provided a current collector comprising a fibre and/or mesh reinforced composite collector body, and a carrier, and comprising Fibre Bragg Grating strain sensors aligned to detect strain longitudinally of the carrier and mounted:-

• within and adjacent the base of the carrier remote the composite collector body and/or

• within the composite collector body

The present invention is not limited to any particular type of Fibre Bragg Grating, so long as it is capable of providing the desired indication of collector condition. However, an advantageous geometry for measuring strain comprises a strain grating and compensating temperature grating; preferably these are combined in a single unit. http ://www. vtt.fi/tuo/74/proi ects/conmo.htm describes a method of using multiple gratings to provide temperature compensation.

The present invention also provides an improved Fibre Bragg Grating sensor unit comprising a strain grating disposed to permit good mechanical coupling with an article, and a compensating temperature grating disposed adjacent the strain grating and mounted for uncoupled expansion or contraction relative to the article.

Further features of the invention will be apparent from the claims and the following illustrative description and drawings, in which:

Fig.l is a diagram showing a collector fitted with Fibre Bragg Grating sensors suitable for use in the piesent invention; Fig.2 shows an alternative collector fitted with Fibre Bragg Grating sensors suitable for use in the piesent invention;

Figs. 3 & 4 show a Fibre Bragg sensor in accordance with the present invention Fig. 5 shows a collector carrier in accordance with one aspect of the present invention.

Fig.1 shows a typical collector in accordance with the invention. A collector 2 is shown in side elevation (Fig. 2a) section on line A-A (Fig. 2b) and plan (Fig. 2c). The collector 2 comprises a composite collector body 10 and a carrier 11 which may be of metal, composite, or other material. A Fibre Bragg Grating temperature sensor 12 may be embedded within the composite collector body 10. Strain and temperature sensor units 13 are fixed to the carrier 11 and are aligned to detect strain longitudinally of the collector. Fibre optical cables 14 are connected to the sensors and pass to control circuitry [not part of this invention].

The number of sensors required and their positions will vary depending on the size of collector and the accuracy required from the measurement. The sensors will either be embedded and adhered into a channel machined in the carbon, as with the temperature sensor 12 above, or adhered into channels in the metal carrier 11 within and adjacent the base of the carrier and extending longitudinally of the carrier. The sensors 13 may be used to detect both strain and temperature.

Fig. 2 shows in analogous manner an alternative form of collector used in a series of trials of the invention. In this collector combined strain and temperature sensors 13 are aligned along the centre line of the collector rather than to one side. A combined strain and temperature sensor 26 is positioned to one side and at the front of the collector to serve as an impact sensor.

With the sensors fitted in position as shownthe following measurements will be possible:

• Temperature of the collector.

• Contact force with the overhead wire

• Detection of vertical force, particularly excessive vertical force

• Position of overhead on collector

• Force in the direction of travel

• Detection of impact force, particularly excessive impact force

This information will be used to inform the operator of possible damage or risk to the collector, pantograph or overhead contact line. The operator can then take action to prevent further damage occurring. The information may also be used to provide an automatic response to collector condition.

Although the regions of maximum strain in a collector are on the top of the collector body and base of the carrier, and the minimum strain is at the collector/carrier interface, to protect the combined strain and temperature sensors they are mounted within the collector body or within the carrier adjacent the base of the carrier where there will be a high straia Independent temperature sensors may be disposed in low strain regions to get independent temperature measurement.

The Fibre Bragg Sensors used may comprise a strain grating disposed to permit good mechanical coupling with an article, and a compensating temperature grating disposed adjacent the strain grating and mounted for uncoupled expansion or contraction relative to the article.

Such a sensor is illustrated in Figs. 3 and 4 in which sensor 30 comprises housing 31 having a cut out at one end and a foil 32 secured at said end. The housing may be of a relatively rigid material (e.g. aluminium) and the foil may be a polymer foil e.g. a polyimide such as Kapton® (Du Pont). Finger 34 of the housing 31 provides a degree of rigidity to the foil while still allowing deformation of the foil 32 as explained bdow.

The housing and foil secure a fibre optic 33 having two Fibre Bragg gratings, a first grating secured within the foil 32, and a second grating secured within the housing 31 but free for uncoupled expansion or contraction relative to the housing, being free for movement within chamber 35. The optical fibre is loosely secured at both ends of the chamber 35 and is loosely mounted within the chamber so that the grating does not experience any strains attributable to the housing. In use, the sensor is placed against an article whose strain is to be measured and pressed in place using adhesive so that foil 32 and the embedded Fibre Bragg grating make good mechanical coupling to the article and so experience the relevant strains. The housing 31 is cantilevered free of the surface of

the article so as to be free of strains from the article. Because the grating in chamber 35 is uncoupled it experiences little or no strain attributable to the article or the housing 31 and so can act as a temperature sensor and this can be used to compensate for temperature effects in the strain grating in the foil 32. Use of aluminium or other metals means that the sensor in chamber 35 experiences a temperature extremely close to that experienced by the Fibre Bragg grating embedded in foil 32.

Typical dimensions of such a sensor may be

Overall length of housing 31 40mm

Overall width of housing 31 4mm

Length of finger 34 13mm

Length of chamber 35 13mm

Thickness of foil 32 0.25mm

Thickness of housing 31 0.8mm

The sensor shown has a single optical fibre 33 with two gratings distinguished by wavelength, but it is possible to use independent fibres for the gratings. Rather than a finger 34 it is possible to have the foil 32 framed around its periphery leaving a window in the housing 31, however this may constrain the foil and result in the embedded sensor detecting a mix of strains between the article to which it is applied and the housing 31. Accordingly, a finger arrangement is preferred. Problems of mismatch between article and housing strains can be reduced by using the same material for the housing as the article to which it is to be applied.

Advantages to use of such sensor units include :-

• that the spacing between the temperature grating and strain grating is known and fixed so that each unit tends to behave in a uniform and predictable manner

• that a temperature and strain map of an article to which the sensor units are attached can be obtained

• that they are easy to apply in manufacture of articles.

As an example of ease of manufacture, Fig. 5 shows a carrier 40 having channels to receive the Fibre Bragg sensor units. The channels have slots 41 in the surface facing the composite collector body. This permits the sensors to be placed within the carrier and permits access to the sensors so that during manufacture pressure is more easily applied to the sensors to ensure good adhesion and mechanical coupling with the carrier. Similarly, if the composite collector body (or indeed a conventional carbon collector body) is removed easy access to the sensors is given so ensuring ease of replacement if necessary.

In addition to overhead collector strips, the Fibre Bragg Grating sensors may be utilised in collector shoes for traction and industrial applications. They can also be modified for use in carbon brushes for electrical machinery or in any other application where strain is measured in an environment where temperature change is a complicating factor.