A bidirectional pressure sensor

Abstract

A Bidirectional pressure sensor comprising an elastomeric membrane 12 having a first and second surface, and one or more sets of strain gauges 14 attached to the membrane, each set of gauges include i

Description

The present invention relates to a pressure sensor for measuring negative or positive pressure. BACKGROUND TO THE INVENTION 5 Traditional pressure sensors often struggle with accurately measuring both positive and negative (vacuum) pressures, especially in environments where precise detection of differential pressure is crucial. This limitation impacts a wide range of applications, from automotive and aerospace engineering to marine and environmental sciences, where the accurate characterisation of 10 pressure profiles can significantly influence performance, safety and efficiency. Environmental factors such as temperature variations and electrical noises have historically compromised the accuracy and reliability of pressure sensors. These external influences can lead to erroneous pressure readings, which in critical applications could result in catastrophic failures. 15 Furthermore, traditional pressure sensors are often limited by their size, rigidity, or the need for direct contact with the medium being measured, which can restrict their use in complex or sensitive environments. It is an object of the present invention to reduce or substantially obviate the aforementioned problems. 20 STATEMENT OF INVENTION According to a first aspect of the present invention, there is provided a pressure sensor comprising: an elastomeric membrane having a first surface and a second surface; and 25 one or more sets of strain gauges attached to the elastomeric membrane, the or each set of strain gauges including: a first strain gauge attached to the first surface of the elastomeric membrane, and a second strain gauge attached to the second surface of the elastomeric membrane; the first and second strain gauges being configured to deform causing a change in their electrical resistances on application of positive or 5 negative pressure force which deforms the elastomeric membrane; wherein an output electric signal is provided, the output electric signal being determined from the difference in electrical resistances between the first and second strain gauges. In a preferred embodiment, the set of strain gauges may further comprise a 10 third strain gauge attached to the first surface of the elastomeric membrane and a fourth strain gauge attached to the second surface of the elastomeric membrane. In such an embodiment, the fourth strain gauge may be opposite the first strain gauge (i.e. in the same position on the membrane but on the other side) and the third strain gauge may be opposite the second strain gauge. 15 Preferably, a first output signal is determined from the difference in electrical resistances between the first and second strain gauges (i.e. strain gauges diagonally positioned with respect to each other, that is on opposing sides and in different positions on the membrane), and a second output signal is determined from the difference in electrical resistances between the third and 20 fourth strain gauges (i.e. another pairof diagonally positioned strain gauges. An overall output signal may be determined from the difference between the first and second output signals. Advantageously, the sensors may be electrically connected in a Wheatstone bridge configuration. This allows the sensor to maximise its sensitivity to 25 pressure changes, whilst simultaneously cancelling out extraneous environmental effects such as temperature variations and electrical noise. This provides precision measurement of both normal (positive) and vacuum (negative) pressures, which is crucial in a number of applications, for example in applications involving fluid dynamics. The sensor can be particularly beneficial in the automotive and aerospace industries for the characterisation of pressure profiles, which are key to optimising aerodynamic designs, panel rigidity and noise analysis. In marine engineering, the sensor’s precise measurements can inform the development 5 of vessels designed to withstand various pressure forces, enhancing hydrodynamic efficiency, or to provide high-resolution under pressure maps. . The sensor can be used for industrial machinery, where pressure monitoring is integral to the control and improvement of fluid handling systems, and in environmental science for the study of atmospheric and underwater pressure 10 dynamics affecting diverse ecosystems. The sensor can be used in the construction sector for evaluating the impact of pressure forces on structural integrity. Furthermore, in the renewable energy industry, the sensor can be employed to analyse and enhance the aerodynamic performance of wind turbine blades under varying pressure conditions. 15 By facilitating a detailed comprehension of both normal and vacuum pressure forces, the sensor enables the fine-tuning of designs and the maximisation of operational efficiencies across these diverse domains. The sensor is a bidirectional sensor, meaning that it can measure both positive and negative pressures. 20 The surfaces of the elastomeric membrane and the strain gauges may undergo compression or tension due to the applied positive or negative pressure. The electrical resistances of the strain gauges change in response to the positive or negative pressure applied to the sensor. In embodiments where only two strain gauges are provided, the strain gauges 25 may be arranged to form a potential divider between a positive and negative supply on one arm of a Wheatstone bridge, with a pair of fixed resistors forming another potential divider between the positive and negative supply, to form another arm of the Wheatstone bridge. The output of the bridge, and the overall output signal, is the measured voltage between the centre points of each 30 potential divider. The fixed resistors may preferably have a resistance of between 10 kQ and 15 kQ. In embodiments where four strain gauges are provided, preferably it is diagonally opposite strain gauges (i.e. strain gauges on opposing sides and on 5 different positions on the surface) which form each potential divider of the Wheatstone bridge. The output electrical signal may be a voltage signal. An amplifier may be provided for amplifying the output electric signal. This ensures that even minor changes in electrical resistance, which is indicative of 10 slight pressure variations, are accurately converted into measurable electric signals. For example, the use of an instrumentation amplifier to magnify the electric signal at a high gain further refines the measurement capability, ensuring that even the most minute pressure variations are captured with a high degree of accuracy. Such precision is particularly advantageous for full-scale 15 deflection measurements, which require only a small electric signal change e.g. a small voltage difference. The amplified electric signal is proportional to the overall alteration in electrical resistances of the strain gauges. The electric is used as an indicator of the extent of the deformation or strain experienced by the elastomeric membrane 20 due to the applied pressure. The amplified electric signal may be converted to a pressure value equivalent to the applied pressure. The amplifier may be configured to amplify the electric signal by a gain of around 1000. 25 The first and third strain gauges may be spaced apart from each other on the first surface of the elastomeric membrane. The second and fourth strain gauges may be spaced apart from each other on the second surface of the elastomeric membrane. The strain gauges may be in-plane strain-sensitive strain gauges. Due to the capability of the in-plane strain gauges to capture the variation in its length, it is possible to translate the deformation due to pressure variations as variation of strain in the upper and lower surfaces of the sensor. 5 When there is a plurality of sets of strain gauges, the sets of strain gauges may be spaced apart from each other on the elastomeric membrane. When there is a plurality of sets of strain gauges, each set of strain gauges in effect forms an independent pressure sensor at a different position on the membrane. Each set of strain gauges may be configured to measure pressure 10 as described, and to send pressure data to a common data line. When there is a plurality of sets of strain gauges, the sets may be connected together. In some embodiments, one or more multiplexers may connect the plurality of sets of strain gauges together. The one or more multiplexers may allow the collection or reading of data for all the sets of strain gauges. 15 The or each set of strain gauges may include an identifier. The elastomeric membrane may have an elastic modulus of around 0.1 MPa to 3 MPa. This allows a flexible of low modulus membrane to be provided. The elastomeric membrane may have a thickness of around 0.1 mm to 0.5 mm. The strain gauge may be formed on a sheet material with a thickness of around 20 0.01 mm to 0.1 mm. According to a second aspect of the present invention, there is provided a method for measuring pressure applied to a bidirectional pressure sensor according to the present invention, the method comprising the steps of: applying positive or negative pressure to the first or second surface of 25 the elastomeric member of the sensor; and measuring the output electrical signal. The bidirectional pressure sensor may be fixed to a host substrate, material or surface before applying the positive or negative pressure. The output electric signal may be a voltage signal. The output electric signal may be amplified. The overall electric signal may be 5 amplified by a gain of around 1000. The electric signal may be converted to a pressure value equivalent to the pressure applied to the sensor. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, and to show more clearly 10 how it may be carried into effect, reference will now be made by way of example only to the accompanying drawings, in which: Figure 1 shows a perspective view of a first embodiment of a bidirectional pressure sensor; Figure 2 shows a perspective view of a second embodiment of a bidirectional 15 pressure sensor; Figure 3a shows a cross-sectional view of the bidirectional pressure sensor of Figure 1 under positive pressure; Figure 3b shows a cross-sectional view of the bidirectional pressure sensor of Figure 1 under negative pressure; 20 Figure 4 shows circuit diagram of the bidirectional pressure sensor of Figure 1; and Figure 5 shows a circuit diagram of a third embodiment of the bidirectional pressure sensor. DESCRIPTION OF PREFERRED EMBODIMENTS Referring firstly to Figure 1, a first embodiment of a bidirectional pressure sensor for measuring positive or negative pressures is indicated generally at 10. The sensor comprises an elastomeric membrane 12 and a set 14 of strain 5 gauges attached or fixed to the elastomeric membrane. The set of strain gauges includes first 14a, second 14b, third 14c and fourth 14d strain gauges connected together. In this embodiment, the strain gauges 14a, 14b, 14c, 14d are identical to each other. In this embodiment, the elastomeric membrane 12 and each of the strain 10 gauges 14a, 14b, 14c, 14d are provided as thin rectangular sheets, but the strain gauges have a smaller overall dimension and thickness than the elastomeric membrane 12. The strain gauges 14a, 14b, 14c, 14d extend in a direction parallel to each other, and parallel to the length of the elastomeric membrane 12. 15 The elastomeric membrane 12 is a flexible semi-incompressible material. This allows the elastomeric membrane 12 to stretch or compress when positive or negative pressure is applied to the sensor 10. In this embodiment, the elastomeric membrane is a substantially transparent material. The elastomeric membrane 12 can be made from rubber or silicone, for example. The thickness 20 of the elastomeric membrane 12 can be between 0.1 mm to 0.5 mm. The dimensions of each strain gauge 14a, 14b, 14c, 14d can range between 1 mm by 1 mm to 4 mm by 4 mm. The thickness of each strain gauge can be 14a, 14b, 14c, 14d between 0.01 mm to 0.1 mm. The first 14a and third 14c strain gauges are spaced apart and attached or fixed 25 to a first surface 12a of the elastomeric membrane 12. In the Figures 1 to 3b, the first surface 12a is viewed as a top surface of the elastomeric membrane 12. The second 14b and fourth 14d strain gauges are spaced apart and attached or fixed to a second surface 12b of the elastomeric membrane 12. In the Figures 1 to 3b, the second surface 12b is viewed as a bottom surface of 30 the elastomeric membrane 12. The first 12a and fourth 12d strain gauges are disposed directly opposite each other. The second 12b and third 12c strain gauges are disposed directly opposite each other. The arrangement and connection of the strain gauges 14a, 14b, 14c, 14d with respect to each other forms a Wheatstone bridge configuration. The 5 Wheatstone bridge configuration increases the sensitivity of the sensor '10 to pressure changes and allows the sensor 10 to precisely measure the applied positive or negative pressures. It also allows extraneous environmental effects such as temperature variations and electrical noise to also be cancelled out at the same time. 10 The first surface 12a or the second surface 12b of the elastomeric membrane 12 can be .fixed to a host substrate, material or surface. In this embodiment, each strain gauge 14a, 14b, 14c, 14d is an in-plane strain sensitive strain gauge. Each strain gauge comprises a metallic element made from copper-nickel alloy and arranged in a zig-zag pattern of parallel lines, 15 which is surrounded by an insulating backing made from polyamide. However, in other embodiments, the strain gauges may be made from a different material. When positive or negative pressure force is applied to the elastomeric membrane 12, the elastomeric membrane 12 deforms, and the strain gauges 14a, 14b, 14c, 14d change length at the same time. Figures 3a and 3b show a 20 cross-sectional view of the elastomeric membrane 12, the first strain gauge 14a and the fourth strain gauge 14d. When a positive pressure is applied as shown in Figure 3a, the elastomeric membrane 12 bends downwards. The first strain gauge 14a experiences compression wherein the first strain gauge 14a decreases in length, and the fourth strain gauge 14d experiences tension 25 wherein the fourth strain gauge 14c increases in length or is stretched. When negative pressure is applied as shown in Figure 3b, the opposite happens. The elastomeric membrane 12 bends upwards, the first strain gauge 14a is stretched, and the fourth strain gauge 14d is compressed. The deformation i.e. the change in length of each strain gauge 14a, 14b, 14c, 30 14d or specifically the change in length of the metallic element of each strain gauge alters the electrical resistance of each strain gauge 14a, 14b, 14c, 14d. A voltage source 16 is connected to the sensor 10, as shown in the circuit diagram of the sensor 10 in Figure 4. This allows a voltage differential or a voltage signal to be measured or outputted from the change in electrical resistances of the strain gauges 14a, 14b, 14c, 14d. 5 Once pressure has been applied to the sensor 10, a first voltage differential or voltage signal is measured from the difference in resistances between the first 14a and second 14b strain gauges. A second voltage differential or voltage signal is measured from the difference in resistances between the third 14c and fourth 14d strain gauges. In other words, the two voltage differentials are 10 obtained from the diagonal pairs of strain gauges forming two parallel voltage dividers. An overall voltage differential or voltage signal is then measured from the difference between the first and second electric signals. The overall voltage signal is measured at the output of the Wheatstone bridge arrangement, i,e, between the centre points of the two voltage dividers between 15 the positive and negative supply 16. This is shown most clearly in the circuit diagram of the sensor in Figure 4. A connector 15 is provided to the sensor 10 which sends the overall voltage differential to an amplifier. The amplifier amplifies the overall voltage differential, and ensures that even minor changes in resistance, which are indicative of 20 slight pressure variations, are accurately converted into measurable electric signals. This conversion process is both sensitive and specific, enabling the detection of differential pressures with exceptional accuracy and reliability, thus translating the physical deformation of the sensor 10 into quantifiable electrical outputs that can be easily analysed and interpreted for various applications. 25 The amplified overall voltage signal provides an indication of the extent of strain experienced by the elastomeric membrane 12 due to the applied pressure. The amplified overall voltage differential is proportional to the pressure applied to the sensor 10, which can then be processed and quantified. In Figure 2, a second embodiment of the sensor is generally indicated at 100 30 with a plurality of sets of strain gauges. Each set of strain gauges effectively forms a sensor as described above and shown in Figure 1. Many sensors are disposed on the same elastomeric membrane. In this embodiment, the sensor has sixteen sets 102, 104, 106, 108, 110, 112, 114, 116, 118, 120, 122, 124, 126, 128, 130, 132 of strain gauges provided in the same configuration. The sets are substantially equally spaced apart from each other and are fixed to a 5 larger elastomeric membrane 134. The plurality of sets of strain gauges allows the applied pressure to be measured at different points of the elastomeric membrane 12. Each set of strain gauges includes four strain gauges arranged in the same way as the four strain gauges in the first embodiment. Each set of strain gauges thereby forms a Wheatstone bridge configuration. 10 The sets of strain gauges can be connected together using different methods, such as multiplexers in a distributed bus system where each sensor cell sends its data to a common data line. Other connectivity methods may be implemented to ensure the collection of data for all the sets of strain gauges. If multiplexers are used to read a plurality of sets of strain gauges, collection of 15 data is possible with a smaller number of input / output (I / O) lines. Each set of strain gauges can be activated and read one at a time in rapid succession, which reduces the complexity of the wiring but requires more sophisticated control electronics. In the case of the bus connection system, each set of strain gauges has a 20 unique identifier, and the bus system uses a protocol to prevent data collision between the sets of strain gauges. This allows for the collection of distributed pressure information over the surface or structure of interest. In Figure 5, a circuit diagram of a third embodiment of the sensor is generally indicated at 200. The arrangement of the sensor is similar to the first 25 embodiment. However, in this embodiment, only first and second strain gauges 214a, 214b are provided in a half-bridge arrangement. Fixed resistors 214c, 214d form the other side of the Wheatstone bridge, i.e. the second voltage divider.. In this embodiment, only the first and second 214a, 214b strain gauges change in resistance when pressure is applied to the elastomeric membrane. Overall, the bidirectional pressure sensor of the present invention is able to provide a more precise measurement of positive or negative pressure compared to traditional or currently available pressure sensors. The embodiments described above are provided by way of example only, and 5 various changes and modifications will be apparent to persons skilled in the art without departing from the scope of the present invention as defined by the appended claims.

Claims

1. A bidirectional pressure sensor comprising:an elastomeric membrane having a first surface and a second surface; andone or more sets of strain gauges attached to the elastomeric membrane, the 5 or each set of strain gauges including: a first strain gauge attached to the firstsurface of the elastomeric membrane, and a second strain gauge attached to the second surface of the elastomeric membrane;the first and second strain gauges being configured to deform causing a change in their electrical resistances on application of positive or negative pressure 10 force which deforms the elastomeric membrane;wherein an output electric signal is provided, the output electric signal being determined from the difference in electrical resistances between the first and second strain gauges.

2. A sensor as claimed in claim 1, in which the set of strain gauges further 15 comprises a third strain gauge attached to the first surface of the elastomericmembrane, and a fourth strain gauge attached to the second surface of the elastomeric membrane.

3. A sensor as claimed in claim 2, in which the fourth strain gauge is disposed on the elastomeric membrane directly opposite the first strain gauge, and the third 20 strain gauge is disposed on the elastomeric membrane directly opposite thesecond strain gauge.

4. A sensor as claimed in any of the preceding claims, in which the electric signal is a voltage signal.

5. A sensor as claimed in any preceding claim, in which an amplifier is provided 25 for amplifying the electric signal.

6. A sensor as claimed in claim 5, in which the amplifier is configured to amplify the electric signal by a gain of between 800 and 1200.

7. A sensor as claimed in claim 5 or claim 6, in which the amplified electric signal is converted to a pressure value which is equivalent to the applied pressure.

8. A sensor as claimed in claim 2, in which the first and third strain gauges are spaced apart from each other on the first surface of the elastomeric membrane, and the second and fourth strain gauges are spaced apart from each other on the second surface of the elastomeric membrane.5 9. A sensor as claimed in any preceding claim, in which the strain gauges are inplane strain sensitive strain gauges.

10. A sensor as claimed in any preceding claim, in which when there is a plurality of sets of strain gauges, and the sets are spaced apart from each other on the elastomeric membrane.10 11. A sensor as claimed in any preceding claim, in which when there is a pluralityof sets of strain gauges, and each set is configured to send electric signal data to a common data line.

12. A sensor as claimed in any preceding claim, in which when there is a plurality of sets of strain gauges, and one or more multiplexers are provided for 15 collection or reading data from each set of strain gauges.

13. A sensor as claimed in any preceding claim, in which the or each set of strain gauges includes an identifier.■ । .

14. A sensor as claimed in any preceding claim, in which an voltage source is connected to the sensor.20 15. A sensor as claimed in any preceding claim, in which the elastomericmembrane has an elastic modulus of around 0.1 MPa to 3 MPa.

16. A sensor as claimed in any preceding claim, in which the elastomeric membrane has a thickness of around 0.1 mm to 0.5 mm.

17. A sensor as claimed in any preceding claim, in which the strain gauge is a sheet 25 material with a thickness of around 0.01 mm to 0.1 mm.

18. A method for measuring pressure applied to a bidirectional pressure sensor as claimed in any preceding claim, the method comprising the steps of:applying positive or negative pressure to the first or second surface of the elastomeric membrane of the sensor; andmeasuring the output electric signal.

19. A method as claimed in claim 18, further comprising the step of fixing the sensor to a host substrate, material or surface.

20. A method as claimed in claim 18 or 19, in which the electric signal is a voltage 5 signal.

21. A method as claimed in claim 20, further comprising the step of amplifying the electric signal.

22. A method as claimed in claim 21, in which the electric signal is amplified by a gain of between 800 and 1200.10 23. A method as claimed in any one of claims 18 to 22, further comprising the stepof converting the electric signal to a pressure value equivalent to pressure applied the sensor.

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