A STRETCHABLE BIDIRECTIONAL CAPACITIVE PRESSURE SENSOR AND METHOD OF USE

Abstract

A stretchable bidirectional capacitive pressure sensor (20) comprising: a first elastomeric sheet (22) made from a dielectric material, with a series of conductor lines (221) located on or in the first elastomeric sheet; a second elastomeric sheet (28) made from a dielectric material, with a series of conductor lines (261) located on or in the second elastomeric sheet; wherein the conductor lines of the first elastomeric sheet are substantially orthogonal to the conductor lines of the second elastomeric sheet; a microstructure comprising a plurality of elastomeric pillars (241) made from a dielectric material, disposed between the elastomeric sheets; wherein the microstructure is bonded to both the first and second elastomeric sheets so that the bidirectional sensor can register positive and negative pressure by the movement of the first and second elastomeric sheets. A further aspect of the invention discloses a method of collecting data related to fluid flow over an object by using a two-dimensional capacitive pressure sensor.

Claims

1. A stretchable bidirectional capacitive pressure sensor comprising: a first elastomeric sheet made from a dielectric material, with a series of conductor lines located on or in the first elastomeric sheet; a second elastomeric sheet made from a dielectric material, with a series of conductor lines located on or in the second elastomeric sheet; wherein the conductor lines of the first elastomeric sheet are substantially orthogonal to the conductor lines of the second elastomeric sheet; a microstructure comprising a plurality of elastomeric pillars made from a dielectric material, disposed between the elastomeric sheets; wherein the microstructure is bonded to both the first and second elastomeric sheets so that the bidirectional sensor can register positive and negative pressure by the movement of the first and second elastomeric sheets.

2. A stretchable bidirectional capacitive pressure sensor as defined in claim 1 wherein the series of conductor lines in each of the first and second elastomeric sheets are formed from carbon nanotubes.

3. A stretchable bidirectional capacitive pressure sensor as defined in claim 1 wherein each elastomeric pillar is located at a crossing point between a conductor line of the first elastomeric sheet and a conductor line of the second elastomeric sheet.

4. A stretchable bidirectional capacitive pressure sensor as defined in claim wherein the elastomeric material of each of the first and second elastomeric sheets is a polydimethylsiloxane polymer.

5. A stretchable bidirectional capacitive pressure sensor as defined in claim 1 wherein the elastomeric material of the microstructure is polydimethylsiloxane polymer.

6. A stretchable bidirectional capacitive pressure sensor as defined in claim 1 wherein each of the elastomeric sheets have a stretchable electrode connected to the series of conductor lines and located on or in the elastomeric sheet.

7. A stretchable bidirectional capacitive pressure sensor as defined in claim 6 wherein each stretchable electrode is a serpentine electrode.

8. A stretchable bidirectional capacitive pressure sensor as defined in claim 6 wherein the stretchable electrodes are copper electrodes.

9. A stretchable bidirectional capacitive pressure sensor as defined in claim 1 wherein at least one of the elastomeric sheets comprises an adhesive layer to allow attachment to an object.

10. A stretchable bidirectional capacitive pressure sensor as defined in claim 1 wherein the plurality of pillars are substantially evenly spaced within an area.

11. A stretchable bidirectional capacitive pressure sensor as defined in claim 10 wherein the area is a square.

12. A stretchable bidirectional capacitive pressure sensor as defined in claim 11 where the square is 4 mm by 4 mm.

13. A stretchable bidirectional capacitive pressure sensor as defined in claim 1 wherein dimensions of each pillar in the plurality of pillars are substantially similar.

14. A stretchable bidirectional capacitive pressure sensor as defined in claim 1 wherein the elastomeric sheets comprises a laminate structure.

15. A method of collecting data related to fluid flow over an object, comprising the steps of: attaching at least one stretchable two-dimensional capacitive pressure sensor to an area of the object; subjecting the object to a fluid flow; and, recording, from the stretchable two-dimensional pressure sensor, data indicative of pressure over the area of the object.

16. A method of analysing fluid flow over an object as defined in claim 15 wherein the stretchable two-dimensional capacitive pressure sensor is a stretchable two-dimensional bidirectional capacitive pressure sensor.

17. A method of analysing fluid flow over an object as defined in claim 15 wherein the stretchable two-dimensional capacitive pressure sensor is a stretchable two- dimensional bidirectional capacitive pressure sensor that comprises a first elastomeric sheet made from a dielectric material, with a series of conductor lines located on or in the first elastomeric sheet; a second elastomeric sheet made from a dielectric material, with a series of conductor lines located on or in the second elastomeric sheet; wherein the conductor lines of the first elastomeric sheet are substantially orthogonal to the conductor lines of the second elastomeric sheet; a microstructure comprising a plurality of elastomeric pillars made from a dielectric material, disposed between the elastomeric sheets; wherein the microstructure is bonded to both the first and second elastomeric sheets so that the bidirectional sensor can register positive and negative pressure by the movement of the first and second elastomeric sheets.

18. A method of analysing fluid flow as defined in claim 15 further comprising the step of placing the object within a wind tunnel or a water tank.

19. A method of analysing fluid flow as defined in claim 15 wherein the at least one stretchable two-dimensional capacitive pressure sensor is within an array of sensors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made by way of example only to the accompanying drawings, in which:

[0025] FIG. 1 shows an exploded schematic view of a first embodiment of a bidirectional pressure sensor according to the first aspect of the present invention;

[0026] FIG. 2 shows a plan view of the bidirectional pressure sensor according to the first embodiment of the present invention;

[0027] FIG. 3 shows a magnified view of the bidirectional pressure sensor in FIG. 2;

[0028] FIG. 4 shows an example of the microstructure of the sensor of FIG. 1;

[0029] FIGS. 5a, 5b and 5c together show a side view of a stretchable bidirectional capacitive pressure sensor with multiple microstructures being deformed as a result of external forces according to the first aspect of the present invention; FIG. 5a shows the array of stretchable bidirectional capacitive pressure sensors under positive pressure; FIG. 5b shows the array of stretchable bidirectional capacitive pressure sensors under negative pressure; and FIG. 5c shows the array of stretchable bidirectional capacitive pressure sensors under rest conditions.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0030] FIG. 1 shows an exploded-view of an example embodiment of the first aspect of the invention. The stretchable bidirectional capacitive pressure sensor 20 comprises a first elastomeric sheet 22, a microstructure 24, and a second elastomeric sheet 26. The first elastomeric sheet 22 and second elastomeric sheet 26 are made from a dielectric material. A series of parallel running conductor lines 221 is located on or in the first elastomeric sheet 22. The microstructure 24 comprises a plurality of elastomeric pillars 241 made from a dielectric material. Each pillar in the present embodiment is a cuboid, preferably a rectangular cuboid with a width of approximately 6 μm and a height less than 300 μm. Although not apparent from the exploded view in FIG. 1, two opposite faces of the cuboid pillars are bonded, either indirectly or directly, to the first elastomeric sheet 22 and the second elastomeric sheet 26. A series of parallel running conductor lines 261 are located on or in the second elastomeric sheet 26. The series of conductor lines of the first elastomeric sheet 221 are orthogonal to the series of conductor lines of the second elastomeric sheet 261.

[0031] The conductor lines in FIG. 1 appear solid, the skilled person will recognise this is not indicative of the type of material used. In preferable embodiments the conductor lines are formed from carbon nanotubes, however any conductive material which is known to the skilled person as being flexible and deformable while maintaining its electrical properties would be suitable.

[0032] Each of the elastomeric sheets 22 & 26 are laminate structures with a PDMS laminate layer. The second elastomeric sheet 26 includes an adhesive layer which allows the sensor 20 to be attached to an object. Numeral 263 in the figure indicates a peel-off backing as is commonly used to protect adhesive layers until used. Both the first elastomeric sheet and the second elastomeric sheet are essentially identical in construction. Therefore, when the stretchable bidirectional capacitive pressure sensor 20 is constructed the elastomeric sheets may be orthogonally disposed.

[0033] Both elastomeric sheets include a stretchable electrode located on an edge, in the current embodiment this is a serpentine electrode 28 made from copper. Each serpentine electrode 28 is connected to the ends of all the conductor lines in the series on its respective sheet. Furthermore, the serpentine electrode of the first elastomeric sheet is perpendicular to the serpentine electrode of the second elastomeric sheet.

[0034] FIGS. 2 and 3 both show the stretchable bidirectional capacitive pressure sensor according to the first embodiment of the present invention when viewed from the top. In the present embodiment, the elastomeric sheets and pillars are translucent or transparent allowing for both series of conductor lines and serpentine electrodes to be seen. However, in other embodiments the elastomeric sheets may be opaque. The stretchable bidirectional capacitive pressure sensor 20 comprises a first elastomeric sheet 22 having a series of conductor lines 221, a microstructure, and a second elastomeric sheet having a series of conductor lines 261. Each elastomeric sheet comprises a series of conductor lines 221 and 261. The series of conductor lines in the first elastomeric sheet are orthogonal to the series of conductor lines in the second elastomeric sheet. This provides for crossing points 30, or apparent intersections, between the conductor lines. Although not visible in FIG. 2 or FIG. 3, the microstructure comprises a plurality of pillars bonded either directly or indirectly to the elastomeric sheets. Each pillar is located and bonded at a crossing point 30. The magnified view of the stretchable bidirectional pressure sensor provided in FIG. 3 shows in broken lines the approximate outline of two pillars located and bonded at crossing points. The serpentine electrodes 28 are connected to each conductor line in a series of conductor lines.

[0035] FIG. 4 shows an example of the microstructure used in the first embodiment of the invention, it also shows an example of an elastomeric pillar used in the microstructure. The microstructure 40 is made from a dielectric elastomer and comprises a plurality of elastomeric pillars, such as pillar 42, which will be located at a crossing point between conductor lines and bonded to elastomeric sheets. The microstructure is an array of evenly spaced and identically sized pillars. Each individual pillar of the microstructure 40 has substantially similar, or identical, dimensions and the pillars are substantially evenly spaced from one another. The number of pillars, inter-pillar spacing, dimension and shape is selected based on the required sensor specification (resolution, sensitivity, or etc), for example a larger number of smaller pillars may increase the resolution of the sensor while maintaining the required sensitivity to both positive and negative pressure. The overall size and shape of the microstructure determines the effective detection area, or sensing area, of the sensor. In the current embodiment each pillar 42 has a plurality of faces with two opposite faces providing the surface for bonding to the elastomeric sheets.

[0036] FIGS. 5a, 5b & 5c together show a stretchable bidirectional capacitive pressure sensor having an array of sensing areas formed by a plurality of microstructures according to the first aspect of the present invention. In the current embodiment, the stretchable bidirectional capacitive pressure sensor comprises a first elastomeric sheet made from a dielectric material, a second elastomeric sheet made from a dielectric material and an array of elastomeric microstructures disposed between the elastomeric sheets. Each of the elastomeric sheets comprises a PDMS layer 52 and a further elastomeric layer 54 made of a dielectric material. The array of microstructures comprises at least two microstructures 58, each having a plurality of pillars bonded, directly or indirectly, to the elastomeric sheets. Each microstructure is disposed between an electrode set 56 & 60 formed on or in the first and second elastomeric sheets. Each electrode in the electrode set comprises a series of conductor lines which are orthogonal to the conductor lines in the opposing electrode in the same electrode set. As in the embodiment shown in FIGS. 1 to 3 and described above, each pillar is located and bonded at a crossing point formed between the orthogonal conductor lines in an electrode set. The combination of a pillar and crossing point forms a pixel. Each microstructure has a plurality of pillars and crossing points (pixels) which allow for multiple readings to be taken, forming a sensing area.

[0037] FIG. 5a shows the array 50 under a compressive force (shown by force arrows) acting on the first elastomeric sheet and second elastomeric sheet. The plurality of pillars within each microstructure 58 are deformed through compression which results in a decrease in distance between the electrode sets 56 & 60.

[0038] FIG. 5b shows a stretchable bidirectional capacitive pressure sensor 50 under an expansive force (shown by force arrows), such as a negative pressure, acting on both the first and second elastomeric sheets. The plurality of pillars in each microstructure are deformed through tension, resulting in an increase in the distance between each electrode set 56 & 60.

[0039] FIG. 5c shows a stretchable bidirectional capacitive pressure sensor 50 at rest with no external forces acting upon either the first elastomeric sheet or the second elastomeric sheet. Therefore, there is no deformation to the plurality of pillars.

[0040] The sensing mechanism for the stretchable bidirectional pressure sensor of the first aspect of the present invention and second aspect of the present invention use a capacitive sensing mechanism. At least one pillar of the microstructure is deformed by an external force applied to either the first elastomeric sheet and second elastomeric sheet. As discussed in relation to FIGS. 5a and 5b, the pillar can deform through compression or tension. This deformation causes a change in capacitance because the distance between the conductor lines change. The capacitance of each pillar and conductor line crossing point is calculated by equation 1.

[00001]$\begin{array}{cc}C=\frac{{\mathrm{.Math.}}_0{\mathrm{.Math.}}_{r}A}{L}& \left[1\right]\end{array}$

Where the capacitance (C) is inversely proportional to the distance between the orthogonal conductor lines (L), and directly proportional to the area formed by conductor lines at the crossing point (A), relative permittivity of the dielectric material (ϵ.sub.r) and the permittivity in a vacuum (ϵ.sub.0). By calculating the change in capacitance, it is possible to calculate the location and intensity of the force.

[0041] Bidirectional pressure sensors according to embodiments of the first aspect of the present invention remain functional when substantially stretched because of the various materials used, such as the elastomers and stretchable conductors. Similarly, the pressure sensors used in embodiments of the second aspect of the present invention remain functional when substantially stretched because of the materials used.

[0042] The second aspect of the invention is provided by a stretchable two-dimensional capacitive pressure sensor, or an array of sensors, attached to an object allowing for the collection of data related to fluid flow over the object. The stretchable two-dimensional pressure sensor is a capacitive type of sensor. The stretchable sensor is attached, preferably by means of an adhesive, to an area of the object, for example a portion of its surface. Once the stretchable sensor has been attached to the object it is subjected to fluid flow and data indicative of the pressure of the object surface is recorded. This recorded data can be used immediately in analysing fluid flow, or stored for later use. The fluid flow over the stretchable capacitive pressure sensor, or array of sensors, creates an external force which acts on an elastomeric sheet. This elastomeric sheet deforms a dielectric material located between two electrodes. The deformation in the dielectric material changes the distance between the two electrodes which changes the capacitance.

[0043] Another embodiment of the second aspect of the present invention uses a stretchable bidirectional capacitive pressure sensor. This sensor comprises two elastomeric layers containing electrodes, with an electrode separation layer made from dielectric material. The electrode separation layer is bonded, either directly or indirectly, to both elastomeric layers. The bonding allows for an external force, such as a positive or negative pressure, to be translated into the separation layer undergoing compression or tension.

[0044] Another embodiment of the second aspect of the present invention uses a stretchable two-dimensional bidirectional capacitive pressure sensor described in the first aspect of the present invention.

[0045] The use of a stretchable pressure sensor, particularly one with multiple electrodes in each sensing layer, allows for measurements to be taken over an area. These measurements can be used to create a map of the forces. Unlike the traditional means of measuring pressure, such as pressure taps, the use of a stretchable pressure sensor allows for the pressure within an area to be measured. This provides results similar to that of computational methods, such as CFD, which can also calculate pressure within an area of the model.

[0046] The embodiments described above are provided by way of example only, and 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.