Field This invention relates to monitoring linear displacement of an object over time. The invention has particular, but not exclusive, relevance to monitoring vertical movement of a railway track.

Background.

As trains move along sections of railway track there is often some vertical movement of the track. The track (including the sleepers) may move as a result of the weight of the train and due to the underlying ballast settling. If voids in the ballast have formed below the train, the vertical movement of the track is liable to be more extreme. If the vertical movement becomes too great, the tracks can become damaged. In general, vertical movement in excess of approximately 100mm is considered to be too large.

Damage caused by vertical movement of railway track is particularly problematic near points and crossovers, as such vertical movement may damage the electrical and mechanical devices at these locations. Modern point machines must be electrically controlled and be able to apply sufficient force to bend the point blades into position. If the vertical track movement at the point becomes too great, the shaft (or other component) of the point machine that imparts force to the point blades can become misaligned with the point blades, causing significant damage to itself as well as other components of the point machine.

Simple devices called void meters are widely used to monitor track movement. A void meter is left resting on the ballast below a rail, and records the maximum displacement of the rail over the time that the void meter has been in place by the rail pressing a ring down a metal shaft or moving a lever. Although easy to position, such void meters cannot be monitored in real time from a remote location - someone has to access the track in person to take readings. Being able to monitor vertical movement of railway tracks in real time allows remedial action to be taken before any serious damage occurs, thereby saving time and money and reducing potential danger to railway users. Another problem of a conventional void meter is that as the void meter rests on the ballast, the void meter is not able to take account of any movement in the ballast as a train passes.

A more sophisticated optical system for monitoring track movement exists. This optical system uses software to measure track movement from video data. However, this system is physically bulky and too expensive to be provided at every required location in a typical rail network. Further, such a system is easily removable by unauthorised personnel, so may be the target of thieves.

Many of the deficiencies of the void meters described above are solved by an earlier device developed in part by the inventor of the present application. This device uses a large spring that presses down on a sleeper and is mechanically coupled to a magnet. As a train passes and the sleeper moves, the spring base keeps in contact with the sleeper and the magnet moves along the length of a vertical column of reed switches, producing an electrical output (voltage or current). This output can be constantly monitored from a remote location. However, the reed switches are fragile mechanical devices that are liable to break under stress, particularly in vibrating environments like railways. This earlier device can also be bulky because the spring is large and therefore a large, flexible dust shield is required to protect the spring and reed switches.

Summary of the Invention

Aspects of the present invention, as described in the claims, provide compact and durable solutions to the problem of point void monitoring. Other features and advantages of

embodiments of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings

Brief Description of the Drawings

Various embodiment of the present invention will now be described, by way of example, with reference to the accompanying figures in which: Fig. 1 schematically shows a side view of a void meter according to the invention mounted to a support post and resting on a railway sleeper, the void meter being in a retracted state;

Fig. 2 schematically shows a view into the void meter of Figure 1 with a top section of casing removed;

Fig. 3 shows a perspective view of the void meter of Fig. 1;

Fig. 4 schematically shows the components of the printed circuit board shown in Fig. 2;

Fig. 5 schematically shows a perspective view of the void meter of Fig. 1 with the casing removed; Fig. 6 is a side view of the void meter illustrated in Figure 1 in an extended state;

Fig. 7A is a side view of an alternative void meter having a flexible bellows cover in a retracted state; and

Fig. 7B is a side view of the alternative void meter illustrated in Fig. 7A in an extended state.

Detailed Description

As shown in Fig. 1, in an embodiment, a void meter 1 according to the invention is attached to a vertical post 3 by a mounting member 5. The vertical post 3 is driven below the surface of the ground (not shown) such that the vertical post 3 remains substantially fixed relative to the ground. In particular, the vertical post 3 remains fixed relative to the ground regardless of any movement of the ballast below the rail track.

The void meter 1 has a shaft 7 mounted in a casing 9. The shaft 7 is arranged generally vertical when the void meter 1 is mounted to the vertical post 3. In this embodiment, the shaft is made of steel. A lower end 11 of the shaft 7 rests on top of a sleeper 13. As such, the lower end 11 is a contact point to the sleeper 13. A shaft cover 15, fixed relative to the casing 9, protects the top end of the shaft 7 from damage and dust when the shaft 7 moves up from its lowest position. When a train passes over the track, any vertical movement of the sleeper 13 due to the underlying ballast having moved and become less tightly packed causes the shaft 7 to move vertically upwards or downwards through the casing 9. As will be described in more detail hereafter, components internal to the casing 9 measure the vertical movement of the shaft 4 and output an electrical signal conveying information indicative of the vertical movement for remote monitoring.

The arrangement shown in Fig. 1 is advantageous over conventional void meters because, after installation, the void meter 1 is left in place and monitored remotely. Further, the void meter 1 measures the movement of the sleeper regardless of the condition of the ballast. The void meter 1 may additionally have a smaller form factor than prior void meters.

The contents of the void meter 1 will now be described with reference to Figures 2 to 5. As shown in Fig. 2, the inside of the casing 9 has two compartments: a main compartment 21, housing the majority of the mechanical components; and printed circuit board compartment 23, housing a printed circuit board (PCB) 25 on which a Hall effect chip 27 is mounted. In this embodiment, the Hall effect chip is an EM-3242 one-chip monolithic rotation angle sensor, available from Asahi Kasei Microelectronics, which outputs an electrical signal indicative of the rotational orientation of a magnet (not shown in Figure 2). The PCB 25 and Hall effect chip 27 are remote from the mechanical components and can be fully encapsulated to prevent damage. The shaft 7 is mounted within an upper shaft bearing 29a and a lower shaft bearing 29b (see

Figure 5), which are fixed to the casing 9 and allow linear movement of the shaft 7 relative to the casing 9. Typically, the shaft bearings 29a and 29b are made of acetal or other low friction plastic. The shaft 7 is coupled to pulley wheel 31 by flexible cable 33. In this embodiment, the flexible cable 33 is attached to the upper end of the shaft 7; in particular an end bulb of the flexible cable 33 having an increased cross-section rests in a hollow formed at the top of the shaft 7. The flexible cable 33 passes through an angled slot, emerging on the side of the shaft 7. The flexible cable 33 then runs along part of the length of the shaft 7 and passes below and around the pulley wheel 31 within a groove formed in the circumferential surface of the pulley wheel 31. The other end of the flexible cable 33 is attached to the pulley wheel 31 at a notch 35 in the circumference (see Figure 3) and held in place by an end bulb 37 of the flexible cable 33 similar to that at the other end of the flexible cable 33.

The pulley wheel 31 is mounted to a horizontal axle, which is supported for rotational movement by two axle bearings 39a, 39b, which are mounted to the casing 9. Typically, the axle bearings 39a and 39b are also made of acetal or other low friction plastic. A torsion spring 41 is arranged around the axle and is attached to the casing 9 at casing spring leg 43 and to the pulley wheel 31 at wheel spring leg 45, where it is pulled over stud 47. In such an arrangement, the torsion spring 41 urges the pulley wheel 31 in a rotational direction that biases the shaft 7, when the void meter 1 is installed, into contact with the sleeper 13. Accordingly, as the sleeper 13 moves, the resultant linear movement of the shaft 7 causes rotational movement of the pulley wheel 31.

The torsion spring 41 allows 270° of rotation from its relaxed state without the torsion spring 41 becoming distorted. In this embodiment, however, pulley wheel rotation is limited to 180° such that the movement does not exceed the limits of the torsion spring 41. In particular, the range of downward movement of the shaft 7 is limited by a top shaft cap 49 impinging upon the upper shaft bearing 29a, and the range of upward movement is limited by the underside of the main casing 9 . This has the effect that a certain amount of tension (i.e. equivalent to 90° of pulley wheel rotation) is applied to the cable 33 even when the shaft 7 is at its lowest position.

Also mounted to the pulley wheel axle is a magnet housing 51, which holds a magnet 53 (see Figure 5) with its north/south axis mounted perpendicular to the axis of the axle. The magnet housing 51 is positioned adjacent the PCB compartment 23 so that the magnet 53 is a short distance from the Hall effect chip 27. The Hall effect chip 27 only measures rotation of a magnet about a certain axis. As such, the axis of the Hall effect chip 27 is aligned with the axis of the axle. As the pulley wheel 31 rotates in response to vertical motion of the shaft 7, the alignment of the magnet 53 rotates in the plane perpendicular to the axle. The Hall effect chip 27 outputs an analog voltage proportional to the angle by processing the outputs of multiple Hall Elements formed on the silicon chip. The output voltage varies linearly with rotation angle from 0°-360°. However, if the magnet 53 were to rotate more than 360° the output would return to its initial value. By restricting rotation of the pulley wheel 31 to 180° between full extension and full retraction of the shaft 7, any such ambiguity is avoided.

In this embodiment, the Hall effect chip 27 has a resolution of 10 bits over a 360° rotation. Therefore, restricting the rotation to 180° means that for a range of movement of the shaft 7 of 100mm, the resolution is approximately 0.2mm. However, for practical reasons, the accuracy may be limited to 1mm resolution. Furthermore, in this embodiment the cycle time of the Hall effect chip 27 is 40μ8, so very fast measurements are achievable. Again though, operating software may be configured to limit measurements to every 10ms to reduce power consumption. As shown in Fig. 4, electronic components provided on the PCB 25 are made up of four main sections: 1: A voltage regulator 55 section, which provides a 2-stage drop from 24 Volts to 3 Volts.

2: A microcontroller 57 that has four functions: to turn the Hall Effect chip 27 on and off; to measure the output voltage of the Hall effect chip 27; to create an analog voltage using a pulse width modulation (PWM) feature; and to store calibration information.

3: The Hall Effect chip 27.

4: A 4 - 20 mA output section 59 includes a simple operational amplifier to convert the voltage produced by the smoothed PWM feature of the microcontroller 55 into a current output.

An automatic calibration routine is provided, and the void meter 1 will also update the calibration if needed. In particular, the lowest output seen by the microcontroller 57 is stored in EEPROM. This corresponds to the shaft 7 being fully extended. This lowest output is then taken as the base level from which the final output is calculated. If, for example, the flexible cable 33 stretches over time, the stored level would be updated if/when the shaft 7 is next fully extended. The ability to update automatically is particularly valuable as what is needed is a measurement of the relative movement of the track when a train passes over.

The signal output by the output section is transmitted, through wires or converted into a digital signal and transmitted through wires, wirelessly or via a combination of wires and wirelessly, to a remote control section along with signals from other void meters for central monitoring.

Fig. 5 shows more detail of the internal mechanical components of the void meter 1, particularly of the arrangement of the magnet 53. In this view, the north/south axis of the magnet 53 is arranged horizontally and perpendicularly to the pulley wheel axle. Fig. 6 shows an embodiment with a fixed shaft cover 15, as previously shown with reference to Fig. 1. This embodiment has the advantage that the cover 15 may be made of steel, which is sufficiently robust to protect the shaft 7. When first installed, the lower end 11 of the shaft 7 rests on the sleeper 13 and the shaft 7 remains stationary in a nearly fully retracted state (see Fig. 1). However, when a train passes over the track, the sleeper 13 moves upward and downward. When the sleeper 13 moves downward, the shaft 7 is able to extend and does so because the torsion spring 41 acts to rotate the pulley wheel 31 on its axle, pulling the flexible cable 33 around the circumference of the pulley wheel 31 and the shaft 7 downward. In this way, the lower end 11 of the shaft 7 always remains in contact with the sleeper 13. Thus, the north/south axis of the magnet 53 rotates, which the Hall Effect chip 27 measures as a varying output voltage. As described above with reference to Fig. 4, output section 59 converts the output voltage to a 4-20mA current output signal. From calibration of the system, the output signal is known to equate to a certain amount of vertical movement of the sleeper 13. The output signal can be monitored at a remote location and, if the motion is deemed to be higher than a certain safe threshold limit, use of the section of railway where the void meter is installed can be restricted and/or a track maintenance team can be dispatched to assess and/or rectify any damage.

The void meter 1 can monitor movement between a position corresponding to the void meter 1 being in a fully retracted state, as shown in Fig. 1, and a fully extended state, as shown in Fig. 6.

In comparison to the conventional void meters and the previous optical systems, the void meter according to the present invention has the advantage that it can be installed and remotely monitored for long periods of time. This is not only safer for railway maintenance workers, who will not have to access the track as often, but also more convenient as such void meters can be installed in many locations on a railway network, monitoring large sections of track continuously in real-time. When a vertical movement above a threshold value is detected, immediate action can then be taken.

The void meter 1 also provides a significant advantage over the earlier large spring device designed in part by the same inventor and discussed above. Firstly, the overall design is substantially smaller, which means it is less likely to cause a danger should it become disconnected. The Hall effect chip 27 is also less liable to break than the column of reed switches described above, as, not including moving parts, it is less likely to wear and if the sleeper were to suddenly jerk upwards, the Hall chip 27 would not lie in the sleepers path.

Finally, not only is the large spring an expensive component, but over time, the contact position on the sleeper may drift so that the spring becomes misaligned. The shaft in the preferred embodiment does not suffer from the same problem.

Further Embodiments

In contrast to Fig. 6, Figs. 7A and 7B show an alternative design for the cover to protect the shaft 7 as it passes up through the void meter casing 9. This design includes a flexible bellows cover 59 arranged above the shaft 7. In the retracted state of Fig. 7A (i.e. when the shaft 7 is prevented from moving downward by the sleeper 13), the cover 61 expands to accommodate a large length of the shaft 7. When the shaft 7 is fully extended, as in Fig. 7B, the bellows cover 61 is compressed down into a substantially smaller form factor. This arrangement has the advantage that it reduces the overall height of the void meter. There is a higher probability of a taller device being knocked or damaged in a railway environment. Moreover, if a taller void meter becomes disconnected from its support post, there is a higher likelihood that the void meter might lie over tracks, posing a significant threat to public safety. Without resting on a sleeper (as would be the case if the void meter were lying on the ground), a void meter with a flexible bellows shaft cover 61 would be in the fully extended state of Fig. 7B. The alternative embodiment of void meter has a substantially shorter longest dimension and therefore is less likely to pose a safety hazard.

Although the pulley wheel and cable arrangement of the preferred embodiment offers an elegant solution to the problem of track monitoring, one skilled in the art will recognise that other designs are also possible. Generally, the movement of a contact point in response to the displacement of the object being monitored is converted into a corresponding rotational movement and that rotational movement is measured to generate a signal indicative of the displacement of the object. For example, a rack and pinion system may be used instead to convert the linear motion of a vertical shaft to rotational motion of the magnet. Teeth on the shaft (the rack) can be designed to mesh with teeth on a gear wheel or pinion. All other components are substantially the same as that of the preferred embodiment. The rack and pinion however do require fine machining in order that they mesh appropriately.

The cable/pulley system and rack and pinion arrangements are similar in that they both convert vertical movement of a shaft, corresponding to vertical displacement of a sleeper, into a rotational movement that causes a magnet to rotate on an axle. However, it will be recognised that there are other ways of converting the linear displacement of a sleeper into rotational motion of a magnet that can be measured by the Hall effect chip 27. For example, in accordance with a further embodiment of the invention, the vertical motion of a sleeper is converted into rotational motion of the axis of the magnet by use of an arm that is pivotally connected to the body of a void meter. One end of the arm is attached at a hinge to a flat-bottomed foot, which remains in contact with a sleeper due to the arm being sprung downward. The opposite end of the arm, on the opposite side of the pivotal connection, includes a magnet arranged to rotate near a PCB with a Hall chip, substantially the same as that of the preferred embodiment. As the sleeper moves up and down, the foot will slide along the sleeper, causing the arm to pivot and the magnet to rotate. Thus, the vertical motion of the sleeper can be converted to a simple voltage by the Hall Effect chip. This design is simple and easy to manufacture compared to the prior art void meters, but has the disadvantage that the arm needs to be long to be able to measure the required linear range of movement, leading to a bulky device which may be damaged easily.

Although the embodiments described above use a Hall effect chip that is operable to detect rotational movement of a magnet, it will be appreciated that other forms of rotational position sensor can be employed. For example, various inductive rotary encoders are available that could be used in place of the Hall effect chip and magnet. A similar result could also be achieved by fixing a potentiometer to the axle of the pulley wheel.

As many apparently different embodiments of the present invention can be made without departing from the scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof.