Field

The present invention relates to a sensor for load measurement.

Background

For conventional rail pad sensors, sensing materials (used for the sensors) are generally embedded in associated rail pads during manufacturing of the rail pads, which however causes the manufacturing process to be complicated and costly. Besides, sensing performance of the sensors is largely dependent on deformation of the rail pads. Since the dynamic characteristic of the rail pads tend to change with regards to encountered load amplitude and frequency, temperature, and etc., the sensitivity of the sensors is thus consequently also affected. This undesirably causes the linearity and accuracy of the sensors to deteriorate as a result.

One object of the present invention is therefore to address at least one of the problems of the prior art and/or to provide a choice that is useful in the art. Summary

According to a 1 ^st aspect, there is provided a sensor for load measurement, comprising: at least one sensing portion which includes a piezoelectric sensor and at least one sliding layer arranged to at least partially encapsulate the piezoelectric sensor; and at least one non-sensing portion arranged adjacent to the sensing portion, wherein in use, a force applied on the sensor causes the sensing portion to be actuated to generate a voltage corresponding to the applied force, and wherein the sliding layer is configured to enable lateral movement of the piezoelectric sensor when the force is applied to protect the piezoelectric sensor against lateral stretching, and the non-sensing portion is configured to transmit horizontal force component of the applied force to a support structure on which the sensor is to be arranged.

Preferably, the sensing portion may further include an insulation layer arranged to encapsulate the piezoelectric sensor to provide electrical insulation. Preferably, the insulation layer may be formed of polyethylene terephthalate or polyethylene naphthalate.

Preferably, the piezoelectric sensor may be further arranged with a pair of electrodes configured as an anode and a cathode.

Preferably, the piezoelectric sensor may be formed of polyvinylidene fluoride, polyvinylidene difluoride, or piezoelectric lead zirconate titanate. Preferably, the sensing portion may further comprise: a shielding layer arranged to encapsulate the sliding layer to provide electromagnetic shielding for the sensor.

Preferably, the sliding layer may be formed of a material having a low friction coefficient smaller than 0.1 , and may include polytetrafluoroethylene.

Preferably, the shielding layer may be arranged to be attached to the sliding layer using an adhesive. Preferably, the electrodes may be coupled to a charge amplifier to enable measurement of the force applied, based on the generated voltage.

Preferably, the sensing portion may be configured in the form of a strip. Preferably, the at least one sensing portion may include first and second sensing portions, which are arranged to be interposed by the non-sensing portion.

Preferably, the at least one sensing portion may also include a plurality of sensing portions, and the at least one non-sensing portion may include a plurality of non-sensing portions, in which each pair of sensing portions is arranged to be interposed by at least one non-sensing portion.

Preferably, the plurality of sensing portions and plurality of non-sensing portions may be cooperatively arranged to form a collective arrangement, which is configured to be symmetrical about both the longitudinal and transverse axes of the sensor.

Preferably, the anodes of the plurality of sensing portions may be electrically coupled to collectively provide a positive terminal and the cathodes of the sensing portions may then be electrically coupled to collectively provide a negative terminal, the positive and negative terminals arranged to be coupled to a charge amplifier to enable cumulative measurement of the force applied. Preferably, the electrodes of respective sensing portions may be arranged to be coupled to respective charge amplifiers to enable measurement of respective force applied to respective sensing portions.

According to a 2 ^nd aspect, there is provided a sensor device for load measurement, comprising: a base substrate; and a sensor of the 1 ^st aspect, wherein the sensor is attached to the base substrate.

Advantageously, the sensor device has a large frequency bandwidth and good linearity of output, which enables load measurement over a wide dynamic sensing range.

Preferably, the base substrate may be a rail pad.

Preferably, the device may further comprise a protection layer formed on the sensor, arranged in opposition to the base substrate.

Preferably, the device may be arranged to be interposed between a rail, and at least one associated tie sleeper for monitoring a load moving on the rail. It should be apparent that features relating to one aspect of the invention may also be applicable to the other aspects of the invention.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. Brief Description of the Drawings

Embodiments of the invention are disclosed hereinafter with reference to the accompanying drawings, in which:

FIG. 1 a depicts a sensor device for load measurement, and FIG. 1 b is a cross- sectional schematic view of the sensor device, according to an embodiment;

FIG. 2 depicts a cross-sectional schematic view of a sensing portion, which is incorporated in the sensor device of FIG. 1a;

FIGs. 3a and 3b depict two variations of the sensor device, according to different embodiments;

FIG. 4 is a table listing different materials used for manufacturing a prototype device of the sensor device of FIG. 1 a;

FIG. 5a and 5b are respectively a photograph and a schematic diagram of an experimental setup for evaluating the prototype device;

FIG. 6 is a graph show the prototype device's response to step load;

FIG. 7 is a graph show the prototype device's response to cyclic load;

FIG. 8 is a graph show a calibration curve of the prototype device, for a case where a Tokai rail pad is used as a base substrate of the device;

FIG. 9 is a graph show a calibration curve of the prototype device, for a case where a HDPE rail pad is used as a base substrate of the device;

FIG. 10 shows a photograph of the prototype device deployed in a field test;

FIG. 1 1 is a graph showing the prototype device's response to train load; and

FIG. 12 is a graph showing the prototype device's response to a bogie having a flat wheel. Detailed Description of Preferred Embodiments

1. Rail Pad Force Sensor

FIG. 1 a depicts a sensor device 100 for load measurement, and FIG. 1 b is a cross-sectional schematic view of the proposed sensor device 100, according to an embodiment. Broadly, the sensor device 100 includes a base substrate 102, and a sensor 103, wherein the sensor 103is arranged on (and attached to) the base substrate 102 (by means of a first adhesive layer 106a). The sensor 103 comprises at least one sensing portion 200 which includes a piezoelectric sensor 202 and at least one sliding layer 208 (i.e. see FIG. 2) arranged to at least partially encapsulate the piezoelectric sensor 202; and at least one non-sensing portion 104 arranged adjacent to the sensing portion 200, wherein in use, a force applied on the sensor 103 causes the sensing portion 200 to be actuated to generate a voltage corresponding to the applied force, and wherein the sliding layer 208 is configured to enable lateral movement of the piezoelectric sensor 202 when the force is applied to protect the piezoelectric sensor 202 against (any) lateral stretching, and the non-sensing portion 104 is configured to transmit horizontal force component of the applied force to a support structure on which the sensor 103 is to be arranged. As the name suggests, the piezoelectric sensor 202 is to be made of a piezoelectric material, and will be elaborated below. The non-sensing portion 104 is also known as a dummy portion, since there are no sensing functionalities configured. Further, a protection layer 108 can optionally be formed on top of the sensor 103, by being attached thereto via a second adhesive layer 106b. In one example, the base substrate 102 is a rail pad made of high-density polyethylene (HDPE), but is not to be construed as limiting since other suitable materials are also possible. Hence, the sensor device 100 can also be known as a "rail pad force sensor". In this case, the sensor device 100 is arranged to be interposed between a rail (for locomotives and trains), and at least one associated tie sleeper for monitoring a load moving on the rail, but other applications can be envisaged as well. It is to be appreciated that the sensing portion 200 is configured into a strip form in this embodiment, and in this case, the at least one sensing portion 200 includes a plurality of sensing portions 200, and the at least one non-sensing portion 104 includes a plurality of non-sensing portions 104, where each pair of sensing portions 200 is arranged to be interposed by at least one non-sensing portion 104. That is, where there are at least two sensing portions 200 arranged on the sensor device 100, then the two sensing portions 200 are configured to be interposed by at least one non-sensing portion 104. Also, the plurality of sensing portions 200 and plurality of non-sensing portions 104 are arranged on the base substrate 102 to form a collective arrangement/layout, which is configured to be symmetrical about both the longitudinal and transverse axes of the base substrate 102 (and of the proposed sensor 103 itself). Hence, different types of collective arrangement/layout of the plurality of sensing portions 200 and plurality of non-sensing portions 104 on the base substrate 102 are also possible (besides that 100 shown in FIG. 1 a), such as variant sensor devices 300 and 350, as shown in FIGs. 3a and 3b respectively. Now, the sensing portion 200 at its core (i.e. piezoelectric sensor 202) may be formed of polyvinylidene fluoride or polyvinylidene difluoride (PVDF), which is a specialty lightweight plastic material with piezoelectric properties, and has a number of advantages. Firstly, PVDF (which is widely used in many sensing applications, such as strain and strain-rate gauges, tactile sensors, and impact sensors) possesses high piezoelectric coefficients, large frequency bandwidth and a linear output over a wide dynamic range. This makes PVDF suitable for dynamic sensing applications. Secondly, PVDF is flexible and available in large area sheets (which are easily purchased off-the-shelf). Thus a PVDF sheet is easily formable into any desired shape for any desired application simply using a blade cutter. Thirdly, a PVDF sheet is relatively cheap (e.g. one sheet of size 203 mm by 280 mm costs about USD$200, which may be made into about seven sensing portions 200, in strip form, for a prototype device of the sensor device 100 intended for a field test). Furthermore, a PVDF sheet is fairly thin (e.g. available in thickness of 28 pm, 52 pm, or 1 10 pm). Hence, adding a thin sensing layer (by way of the PVDF sheet) to the rail pad provides negligible changes to the original track support structure. In view of these advantages, a PVDF film is therefore selected to be used to make the sensing portion 200 in this embodiment.

When attaching the PVDF film to the rail pad to create the sensor device 100, it is to be noted that the rail pad (especially those made of elastomeric materials) may experience a large in-plane stretch under a rail seat load, since the elastomeric materials tend to be nearly incompressible with a Poisson ratio close to 0.5. In view of this design constraint, the PVDF film is thus not directly bonded to the rail pad to avoid damage to the PVDF film due to possible excessive lateral stretching, when a load is imposed on the sensor device 100. If the friction coefficient between the PVDF film and the material in direct contact is large, the same problem may exist. Therefore, a sliding layer is preferably to be included into the structure of the sensing portion 200 to prevent the PVDF film potentially suffering from excessive lateral stretching (due to the Poisson effect). Having said that, it is however impractical (and also not safe) to have a full sliding surface arranged underneath the entire surface of the rail pad, because lateral forces (if present) can also occur even in straight rail track segments due to train movement caused by line or surface deviations or vehicle hunting. Lateral force has to be transmitted through the rail pad to the track substructure (i.e. tie sleepers). In view of the foregoing, the sensor device 100 is proposed to have an alternating-strip configuration as shown in FIG. 1a. 2. Alternating Strip Configuration

Referring to FIG. 1 a, for the sensor device 100, an arrangement with alternating strips of the sensing portions 200 and non-sensing portions 104 is formed on the entire area of the rail pad (i.e. the base substrate 102), as previously explained. For illustration purposes, three strips of the sensing portions 200 and four strips of non-sensing portions 104 are shown in FIGs. 1 a and 1 b, but is not to be construed as limited only to the arrangement of FIGs. 1 a and 1 b. In application on site, the pressure distribution on the rail pad may not be uniform, and so it is recommended to arrange the strips of the sensing portions 200 in a bi- symmetrical layout (i.e. symmetrical about the longitudinal and transverse axes of the rail pad) to eliminate any rotation-induced stress. By this configuration, the non-uniform pressure distribution can be captured by different strips of the sensing portion 200. More details about the sensing portion 200 are explained in "Section 3". The strips of the non-sensing portions 104 are configured with a same thickness and to have similar elastic properties as the strips of the sensing portion 200. The strips of the non-sensing portions 104 are bonded to the first and second adhesive layers 106a, 106b so as to resist any lateral forces imposed on the sensor device 100. In one example, the rail pad is an actual rail pad used in the track system and need not be specially made for the proposed sensor device 100. It is to be appreciated that the first adhesive layer 106a on the rail pad is to bond all the strips of the sensing portions 200 and non-sensing portions 104 to the rail pad. The second adhesive layer 106b then bonds all the strips of the sensing portions 200 and non-sensing portions 104 with the protection layer 108 to protect the sensor device 100, in which suitable materials with good mechanical strength and high/comparable friction of coefficient with rail pad materials are preferably used as the protection layer 108. It is also to be appreciated that when the sensor device 100 has been deployed, the protection layer 108 is in contact with a baseplate of the rail fastening system. As such, materials with a high friction coefficient (utilised as the protection layer 108) may therefore decrease potential occurrence of rail pad slippage. 3. Sensing Portion

FIG. 2 is a cross-sectional schematic view of the sensing portion 200, which comprises the piezoelectric sensor 202 (e.g. formed of the PVDF film) arranged with a pair of electrodes 204a, 204b configured to receive electrical charges generated by the piezoelectric sensor 202 (when actuated); an (optional) insulation layer 206 arranged to encapsulate the piezoelectric portion 202 and electrodes 204a, 204b to provide electrical insulation; and the sliding layer 208 arranged to encapsulate the insulation layer 206 (and the piezoelectric sensor 202) to beneficially enable lateral movement by the piezoelectric sensor 202 during application of the force, wherein the piezoelectric sensor 202 is actuated to generate the electrical charges when a force is applied to the sensing portion 200, in which a magnitude of the electrical charges generated corresponds to amount of the force applied. As mentioned, the sensing portion 200 may be configured and shaped to be in the form of a strip, but other suitable shapes are certainly not precluded. In an example, the sensing portion 200 may also further comprise a shielding layer 210 arranged to encapsulate the sliding layer 208 to provide electromagnetic shielding for the sensing portion 200. The sliding layer 208 is formed of a film or coating of a material having a low friction coefficient (i.e. preferably smaller than 0.1 ), such as polytetrafluoroethylene (PTFE). The shielding layer 210 is arranged to be attached to the sliding layer 208 using an adhesive 212. Since the sensing portion 200 has different multiple layers, it can also be called a multilayer device. Further details of the sensing portion 200 are set out below. Specifically, the piezoelectric sensor 202 is positioned at the center (of the sensing portion 200) with the electrodes 204a, 204b (e.g. formed as sputtered metalized electrodes or compliant silver ink) arranged on opposing faces of the piezoelectric sensor 202. As mentioned, the piezoelectric sensor 202 is encapsulated by the insulation layer 206, in which suitable materials with good electrical insulation properties as well as high mechanical strength are to be considered for use, e.g. polyethylene terephthalate (PET) and polyethylene naphthalate (PEN). Then, flexible printed circuits technology can be used to form two strips of conductor (not shown) on the inner surfaces of the insulation layer 206 to collect the electrical charges generated by the actuated piezoelectric sensor 202 and received by the electrodes 204a, 204b. Accordingly, the electrodes 204a, 204b are thus configured as an anode and a cathode, which can optionally be connected to a charge amplifier (not shown) for signal collection to enable measurement of the amount of force applied on the sensing portion 200, based on the magnitude of the generated electrical charges (i.e. as a voltage). It is to be appreciated that the piezoelectric sensor 202 is sealed in the insulation layer 206 to achieve good electrical insulation and water proofing. For the shielding layer 210, which is designed for EMI shielding, metal foil or fabric tapes can be used. Also, if the shielding layer 210 has its own adhesive layer, then it is not necessary to use the adhesive 212 for attachment. Further, the purpose of the sliding layer 208 is to protect the piezoelectric portion 202, the electrodes 204a, 204b, and the insulation layer 206 from any excessive lateral strain that may arise when the force is applied on the sensing portion 200. In instances where there are multiple of the sensing portions 200 (e.g. in FIGs. 1 a and 3a), to simplify collection of all the electrical charges generated, the multiple sensing portions 200 can also be electrically connected in parallel (e.g. via flexible printed circuits), thereby collectively providing only one set of positive and negative terminals, which in turn can be connected to one single charge amplifier. That is, all anodes of the multiple sensing portions 200 are electrically coupled together to form the positive terminal, while all cathodes of the multiple sensing portions 200 are electrically coupled together to form the negative terminal. So by such a connection, there is only one electrical output generated by the sensor device 100 corresponding to a total cumulative force acting on the sensor device 100. Alternatively, the respective sensing portions 200 are electrically connected to respective charge amplifiers, and in this manner, there will be multiple electrical outputs from the sensor device 100, corresponding to respective forces acting on the respective sensing portions 200 (at different portions of the sensor device 100). By such multiple connections, the load distribution and moments on the sensor device 100 are obtainable, if needed. An example illustration is given in FIG. 3a, in which the variant sensor device 300 is arranged to generate four electrical outputs corresponding to the respective forces acting on the respective four strips of the sensing portions 200. 4. Fabrication and Testing of a Prototype Device

For evaluation purposes, a first prototype device (not shown) of the sensor device 100 was fabricated and performance tested under both laboratory and site environments. The materials (along with associated dimensional parameters) used to form different portions of the first prototype device are listed in a table 400 shown in FIG. 4.

The rail pad used in the first prototype device for laboratory testing is manufactured by Sumitomo Riko Co. Ltd. (formerly known as Tokai Rubber Industries), which is made of elastic rubber with a vertical stiffness of about 180 kN/mm. The PVDF film used from Measurement Specialties™ is in the form of a big sheet with a size of 203 mm by 280 mm, and having sputtered Cu/Ni electrodes, which can be cut into any desired shapes using a blade cutter to fit the sizes of different rail pads. For other materials of the rail pad used, the thickness of the PVDF film can be changed as well. Based on the dimensional parameters of the materials listed in the table 400, a total thickness of the first prototype device (but excluding the rail pad) is only 0.65 mm approximately.

For the first prototype device, the three strips of the sensing portions 200 are connected in parallel and then connected to one charge amplifier with a large time constant. FIG. 5a and 5b respectively show a photograph 500 and schematics 550 of the experimental setup of a lab test for evaluating the first prototype device. Specifically, the first prototype device is arranged intermediate two steel plates 552, 554, which are to respectively simulate the rail and baseplate of a fastening system. A universal testing machine is used to apply a predetermined compression force (via a loading head 556) on the first prototype device.

FIGs. 6 and 7 show the graphical responses 600, 700 of the first prototype device due to step load and cyclic load respectively. FIG. 8 is a graph 800 of a calibration curve of the first prototype device, where it can be determined that the coefficient of determination (R ² ) for the linear fit is about 0.9992, demonstrating very good linearity of the first prototype device. To test performance of the sensor device 100 on site, a field test was conducted at a test track in a train depot located in Singapore. The rail pads used on the test track are rail pads made from high-density polyethylene (HDPE). The HDPE rail pad has a much higher stiffness (i.e. 560 kN/mm) compared to the rail pad from Tokai Rubber Industries. HDPE rail pads are used to avoid significant change of the original track stiffness, which may induce change in dynamic train- track interaction load. For purpose of the field test, ten second prototype devices with HDPE rail pads were fabricated based on the above-mentioned method for the first prototype device. The calibration curve of the second prototype device is shown in a graph 900 in FIG. 9. Particularly, the coefficient of determination is found to be about 0.9999, which shows that very good linearity of the sensor device 100 is also obtainable using the HDPE rail pad.

FIG. 10 is a photograph 1000 showing a second prototype device installed on a rail seat. The installation is simple, requiring only loosening some bolts and lifting the rail to allow replacement of the existing (non-instrumented) rail pads with the second prototype devices. FIG. 1 1 shows an example graph 1 100 of the second prototype devices' dynamic response to a train with six bogies (i.e. four axles per bogie, and 24 axles in total) moving at 70 km/h (each detected peak in the graph 1 100 corresponding to one wheel). FIG. 12 shows one of the second prototype devices' responses (i.e. at "Pad 2") in a graph 1200 to a bogie with a good wheel and a flat wheel. A flat wheel causes an impact force on the rail. From the response of "Pad 2", it is observed that when the first wheel (i.e. a good wheel) passed "Pad 2" at time around 4.25 s, there was no obvious impact force generated. But for the second wheel (i.e. a flat wheel), an impact force was captured at time about 4.5 s, indicating presence of a defect on the second wheel. Using the rail pad force time history captured by the ten second prototype devices, the wheel-rail contact force as well as the size of the flat wheel can be estimated using an appropriate algorithm. Hence, the field test results prove that the sensor device 100 does indeed perform well under site environment.

In summary, this disclosure describes a way to convert any conventional rail pads into load sensors by simply incorporating the sensor 103 (along with the necessary associated elements) to the rail pads. As it is simply an add-on to the rail pads, no special technologies are needed to integrate the rail pads with the proposed sensor 103. Further, materials used to make the sensor 103 are relatively low costs, and fabrication of the sensor 103 is also simple. Then by arranging the sliding layer 208 to encapsulate the piezoelectric sensor 202, the variation of the dynamic characteristics of the rail pad on the performance of the proposed sensor device 100 can be effectively minimized.

The proposed sensor device 100 is distinguished from existing (commercially available) rail pad sensors, which need to actually integrally form the sensors with rail pads during the rail pad production process. As demonstrated in testing, the proposed sensor device 100 shows good linearity and performs well under dynamic loading conditions. In the area of structural health monitoring for the railway industry, the sensor device 100 can be used for real time measurement of a load imposed on a rail seat, for conducting weigh-in-motion (WIM) under normal operational speed, as well as for real-time monitoring of surface defects on wheels of a running train. This advantageously facilitates train operators in diagnosing conditions of train wheels, such as detecting wheel surface defects such as skid flat, out-of-roundness, spalling and etc. The wheel- rail interaction force may also be determined by the sensor device 100. It is to be appreciated that the same idea may also be applied to convert elastomeric bearings utilised to support a floating slab track into load sensors, which can monitor forces exerted on the bearings for railway monitoring applications. The sensor device 100 can also be used to monitor forces on elastomeric bearings used in the support of other types of structures such as bridges, flyovers, viaducts and buildings.

To reiterate, the proposed sensor device 100 has the following key features and associated advantages. Firstly, the sensor device 100 is configured by converting existing rail pads into the sensor device 100 by integration with the sensor 103, which is typically of less than 1 mm thickness. It is to be appreciated that the sensor device 100 has practically the same stiffness as existing rail pads, thereby allowing direct replacement of existing rail pads with the sensor device 100 with minimal changes to the track support system. This enables ease of installation, and is also non-intrusive on existing rail support infrastructure. Secondly, the sensor device 100 has a large frequency bandwidth (e.g. 0.001 Hz to 10 Giga Hz for the PVDF material used) and good linearity of output, which enables load measurement over a wide dynamic sensing range (e.g. 100 nano- psi to 10 mega-psi for the PVDF material used) and provides ease of signal processing.

Thirdly, the sensor device 100 is beneficially arranged to use low-cost sensing materials and is made via simple fabrication procedures. This thus enables low cost production of the sensor device 100.

Fourthly, for the sensing portion 200, the sliding layer 208 (e.g. PTFE) is arranged to encapsulate both the insulation layer 206 and piezoelectric sensor 202 (e.g. the PVDF film) to protect the piezoelectric sensor 202 from large lateral stretching that may be encountered when a load is imposed on the sensor 103.

Lastly, in one embodiment, the sensor device 100 adopts an alternating-strip configuration in arranging the different strips of the sensing portions 200 and non-sensing portions 104. Specifically, the strips of the sensing portions 200 are configured to respond to vertical force components (of an applied force) acting on the base substrate 102 (i.e. the rail pad), whereas the strips of the non-sensing portions 104 are configured to transmit horizontal force components (of the applied force). In this manner, any potential horizontal force components exerted on the base substrate 102 can beneficially be transmitted to the track substructure through the strips of the non-sensing portions 104. Therefore, any cross-talk between vertical and horizontal force components acting on the sensor device 100 is thus minimized.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary, and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practising the claimed invention. For example, the piezoelectric sensor 202 may also be made of piezoelectric lead zirconate titanate, or any other suitable sensing materials instead.