AU2014100668A4

AU2014100668A4 - Transducer

Info

Publication number: AU2014100668A4
Application number: AU2014100668A
Authority: AU
Inventors: Philip Wayne Loveday
Original assignee: Council for Scientific and Industrial Research CSIR
Current assignee: Council for Scientific and Industrial Research CSIR
Priority date: 2014-06-03
Filing date: 2014-06-18
Publication date: 2014-07-24
Anticipated expiration: 2022-06-18
Also published as: RU151141U1; CN204027692U; BR202014018935Y1; AU2014100668B4; JP3193898U; BR202014018935U2; ZA201404072B

Abstract

THIS invention relates to a transducer for use in a system for monitoring the condition of elongate structural elements, and more particularly but not exclusively to a transducer for use in a system for monitoring and detecting cracks and breaks in railway rails. The transducer including a cylindrical metal housing and a piezoelectric actuator located inside the housing. The piezoelectric actuator includes a plurality of piezoelectric elements stacked on top of one another, the piezoelectric elements being sandwiched between a backing ring and a front mass, with the piezoelectric rings and the backing ring located on a locating pin extending from the front mass. The transducer is characterised in that the front mass has an operatively outer surface which is angularly offset relative to a radial plane of the cylindrical housing. Figure 1. 41 41 10 41 Fig. I Fig. 2

Description

-1 TRANSDUCER BACKGROUND TO THE INVENTION THIS invention relates to a transducer for use in a system for monitoring the condition of elongate structural elements, and more particularly but not exclusively to a transducer for use in a system for monitoring and detecting cracks and breaks in railway rails. There are several methods and systems which have been proposed for use in monitoring the integrity of elongate structural elements, and in particular railway rails. These methods and systems are aimed at detecting cracks in the rails before they develop into complete breaks, and also to detect breaks in a railway network where they have already occurred. If a crack or break in the rail is not detected beforehand, it could result in the derailment of the railway vehicle travelling on the track. It will be appreciated that such derailments will result in financial loss and can also result in injury and loss of life. It should be noted that while reference is made to railways, these systems are, or can potentially be, equally applicable to other applications where long lengths of structural steel are utilised, such as for example mine shafts and bridges.

-2 One method of detecting cracks and breaks in the rails of railway tracks is disclosed in South African patent 99/6936, the contents of which is incorporated herein by reference. The method includes the step of providing a number of autonomous acoustic transmitter units, and a number of acoustic receiver units located between the transmitter units. The various units are spaced apart from one another by predetermined distances. The transmitter units introduce a series of acoustic pulses with specific frequency composition into the rails and the receiver units detect and analyse the pulses to monitor any unwanted condition concerning the rail. This method requires the use of transmitters and the use of receivers in order to monitor the condition of the rail. Development of transducers for this method of detecting and monitoring cracks and breaks in railway rails is discussed in "Development of piezoelectric transducers for a railway integrity monitoring system", Philip W. Loveday, Smart Structures and Materials 2000: Smart Systems for Bridges, Structures, and Highways, Proceedings of SPIE Vol. 3988, 2000, Newport Beach, pp. 330-338. The system makes use of piezoelectric transducers which are mounted (clamped) under the crown of the rail on the outside of the track. The method of clamping the piezoelectric transducers is described in PCT patent application WO 2004/098974, the content of which is incorporated herein by reference. The piezoelectric transducers are spaced along the length of the railway network and they periodically transmit ultrasonic waves through the rails. The waves propagate through the track from one transducer towards a downstream transducer which acts as a receiving station. Typically, the transducers are spaced about 1 km apart. If the ultrasonic signal is not detected at the receiver station, the receiver station activates an alarm indicating that the rail either has a crack or is broken. A disadvantage associated with the above system is that the piezoelectric transducers are attached (clamped) under the crown of the rail on the outside of the track. The piezoelectric transducers are large and cannot be -3 attached under the crown on the inside of the track because they would interfere with the train wheels. The piezoelectric transducers have to be removed from the rail during routine track maintenance because a 'tamping' machine used to re-pack the ballast under the sleepers has wheels that engage the outside of the crown. The removal and re-attachment (which requires re-tightening of the clamps two weeks after re-attachment) of the piezoelectric transducers increases the maintenance cost of the system and results in periods of time when the system is inoperable. In addition, the existing system is not suited for distance in excess of 1 km, as the transmitted signal is not strong enough, and because the transducer is also not accurately matched to the particular structural element to which it will be attached from a propagation and operating frequency point of view. A further problem often encountered with transducer design is that connector pins of the transducer are not properly isolated from the rail to which they are secured, resulting in shorts and false signals. In addition, transducers used in the applications described above are prone to vibration, which may result in excessive vibration of the internal wiring and shims, and which may again result in electrical shorts and/or damage to the wires and shims. It is therefore an object of the invention to provide a piezoelectric transducer that will address the disadvantages described above. It is also an object of the invention to provide a piezoelectric transducer which will be a useful alternative to existing piezoelectric transducers.

-4 SUMMARY OF THE INVENTION According to the invention there is provided a transducer, suitable for use in a system for monitoring the condition of elongate structural elements, the transducer including: a cylindrical metal housing; a piezoelectric actuator located inside the housing, the piezoelectric actuator including: a plurality of piezoelectric elements stacked on top of one another, the piezoelectric elements being sandwiched between a backing ring and a front mass, with the piezoelectric rings and the backing ring located on a locating pin extending from the front mass; characterised in that the front mass has an operatively outer surface which is angularly offset relative to a radial plane of the cylindrical housing. The transducer may include four annular piezoelectric elements that form a piezoelectric stack located on the locating pin. The transducer may include a first isolation disc that is located between the piezoelectric stack and the backing ring, and a second isolation disc that is located between the piezoelectric stack and the front mass. The transducer may include an isolation cap that encapsulates the backing ring, and an isolating sleeve that is located in an annular space formed between an inner surface of the cylindrical metal housing and the outer surface of the piezoelectric stack. There is also provided for the cylindrical metal housing to be filled with a potting compound for improving the resistance of the transducer to shock and vibration.

-5 BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention is described by way of a non limiting example, and with reference to the accompanying drawings in which: Figure 1 is a perspective view of the transducer in accordance with one embodiment of the invention; Figure 2 is a side view of the transducer of Figure 1; and Figure 3 is a cross-sectional side view of the transducer of Figurel. DETAILED DESCRIPTION OF INVENTION Referring to the drawings, in which like numerals indicate like features, a non-limiting example of a transducer in accordance with the invention is generally indicated by reference numeral 10. The transducer 10 comprises of a cylindrical housing 20 that houses a piezoelectric actuator 30. The cylindrical housing 20 is typically made from metal, resulting in a robust and structurally sound support structure. The housing 20 has an open upper end 21 and an open lower end 22. A rebate 23 is provided on the inside surface of the housing, and extends from the upper end 21 of the housing 20 about two thirds of the length of the housing towards the lower end 22. A piezoelectric actuator 30 is located inside the housing 20. The piezoelectric actuator 30 includes a piezoelectric stack, which in turn comprises four piezoelectric discs 31. An important aspect of the invention is that four 2mm thick discs are used, and not for example two 4mm discs.

-6 A 400Vpk (peak voltage) drive will be required to actuate the four 2mm discs, whereas an 800Vpk (peak voltage) drive will be required to actuate a two disc configuration. The piezoelectric discs 31 forms a piezoelectric stack, and isolation discs 32 are provided at both ends of the stack so as to electrically isolate the connector pins (which connects the piezoelectric discs 31 to the signal cable 70) from the rail (not shown) to which the transducer 10 is secured. A backing ring 33 is located on a side of the piezoelectric stack opposite the front mass 40 of the transducer 10. The piezoelectric discs 31, the isolation discs 32 and the backing ring 33 are located on, and carried by, a locating pin 34, with the locating pin 34 in turn being secured to the front mass 40. More particularly, an end of the locating pin 34 engages, more particularly threadingly engages, a complementary aperture 44 provided in the front mass 40. The front mass 40 is in the form of a substantially solid metal cylinder that engages the upper end 21 of the cylindrical housing 20. An outer surface 41 of the front mass is angularly offset relative to a radial plane of the cylindrical housing 20. In other words, the outer surface 41 is not perpendicular to the wall of the cylindrical housing 20, but is orientated at an angle relative to the wall of the cylindrical housing 20. This is an important aspect of the invention, as it allows the transducer 10 to be securable to a flange of the rail (not shown) on the inside of the rail. The lower end 22 of the transducer 10 is therefore slightly spaced apart from the web (not shown) of the rail, which makes it easier to install the transducer, and which also provides more space for the signalling cable 70 protruding from the transducer 10. A circumferential lip formation 42 extends from the front mass 40, and in use locates on the upper end 21 of the cylindrical housing 20. A circumferential groove 43 is furthermore provided in the side of the front mass 40, and in use improves the seal formed when the front mass 40 engages the cylindrical housing 20, and more particularly when the groove is filled with potting media.

-7 An electrical isolation cap 50 covers the backing ring 33, and an electrical isolation sleeve 60 is provided between the piezoelectric stack and the housing 20. The cap 50 and sleeve 60 further improves the electrical isolation between the piezoelectric stack and the rail. The sleeve 60 locates on the rebate 23 provided in the inner surface of the housing 20. An end cap 80 engages the lower end 22 of the housing 20 so as to define an enclosed volume that encapsulates the piezoelectric stack. A central bore is provided through the end cap 80 for receiving the signal cable 70 therethrough. The cable 70 is sealed relative to the end cap 80 by way of a suitable cable gland. A conductor 71 of the cable 70 is connected to the piezoelectric stack. The entire internal volume 90 of the transducer is filled with a potting media, which prevents wire and shim vibration inside the transducer. In use the transducer 10 can be secured to a rail, and more particular to a lower surface of the web on the inside of the rail. The transducer 10 can be permanently installed due to its small size, and the angled head results in simple and ergonomic installation. The selection and arrangement of the piezoelectric discs ensures that the transducer remains powerful despite its small size. The transducer is furthermore well isolated, and also protected against vibration. The front mass and the end cap in addition forms a good seal, resulting in an environmentally sealed unit. All the above aspects of the new design render the transducer particularly suitable for use in a system for monitoring the condition of a rail, and the inventor believes that the novel and inventive design will be a major improvement over prior art transducers. It will be appreciated that the above is only one embodiment of the invention and that there may be many variations without departing from the spirit and/or the scope of the invention.

-8 It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

Claims

1. A transducer, suitable for use in a system for monitoring the condition of elongate structural elements, the transducer including: a cylindrical metal housing; a piezoelectric actuator located inside the housing, the piezoelectric actuator including: a plurality of piezoelectric elements stacked on top of one another, the piezoelectric elements being sandwiched between a backing ring and a front mass, with the piezoelectric rings and the backing ring located on a locating pin extending from the front mass; characterised in that the front mass has an operatively outer surface which is angularly offset relative to a radial plane of the cylindrical housing.

2. The transducer of claim 1 including four annular piezoelectric elements that form a piezoelectric stack located on the locating pin.

3. The transducer of claim 1 including a first isolation disc that is located between the piezoelectric stack and the backing ring, and a second isolation disc that is located between the piezoelectric stack and the front mass.

4. The transducer of claim 1 including an isolation cap that encapsulates the backing ring, and an isolating sleeve that is located in an annular space formed between an inner surface of the cylindrical metal housing and the outer surface of the piezoelectric stack.

5. The transducer of claim 1 in which the cylindrical metal housing is filled with a potting compound for improving the resistance of the transducer to shock and vibration.