Cross-Reference to Related Applications

[0001] This patent application claims benefit to and priority of Luxembourg Patent

Application No. LU101144 filed on 1 March 2019 entitled“Optical Sensor”.

Field of the Invention

[0002] The invention relates to an optical pressure sensor for the location of a pressure point.

Prior Art

[0003] Pressure-sensitive touch systems are known in the art. For example, US Patent Application No. US 2014/0210770 (Chen et al, assigned to Corning Incorporated) teaches a pressure sensitive pad for sensing the occurrence of a touch event based on pressure applied at a touch location. The pressure sensitive pad includes a light source system and a detector system that are located at the edges of an optical waveguide. Pressure applied at a touch location on the optical waveguide causes the optical waveguide to bend or flex and leads to a change in the optical paths of the optical waveguide. The changes are detected and used to determine whether a touch event occurred as well as the time evolution of the touch event.

[0004] Another patent application, US Patent Application No. US 2010/0253650 (Dietzel et al, assigned to TNO) teaches an optical pressure sensor for measuring a force distribution and not a single touch event. The optical pressure sensor has a deformable opto-mechanical layer and its light-responsive properties depend on the degree of deformation of the opto mechanical layer.

[0005] European Patent Application Nr. 0 377 549 teaches (Keinermann) a variety of method and devices for measuring physical parameter by converting a fraction of the intensity of a so- called interrogating light into a positive optical signal with wavelengths and/or light propagation modes different from those of the interrogating light. In one aspect shown in the EP‘549 application, the sensor has a single fluorescent layer at a sensor pad which is illuminated at two different wavelengths in order to determine the magnitude of the pressure or force applied to the sensor pad. EP’ 549 fails, however, to teach the determination of a position at which the force is applied.

[0006] US Patent No. US 6,965,709 (Weiss, assigned to Sandia Corp) teaches an optical position sensor using a fluorescent waveguide. A small excitation source side-pumps a localized region of fluorescence at an unknown position along the fluorescent waveguide. Measurement of the attenuated intensity of the fluorescent waveguide and comparison with calibrated response tables or a mathematics method enables the position of the excitation source to be identified. The waveguide could have a non-uniform variation of the

concentration of the fluorescing material within the waveguide, but the US’709 document does not teach the use of more than one fluorescing material.

Summary of the Invention

[0007] An optical pressure sensor for the detection and measurement of an applied pressure point is disclosed. The optical sensor comprises a first optical waveguide and a second optical waveguide separated by a compressible spacer. A layer of at least two fluorescent dyes is arranged on the second optical waveguide in differing concentrations at differing locations or through a geometrical arrangement along the length of the second optical waveguide.

[0008] In operation, exciting light, i.e. light at an appropriate wavelength for exciting the fluorescent dyes, is coupled into the first optical waveguide of the optical pressure sensor through one of a first end or a second end of the optical pressure sensor. Any pressure placed on the first optical waveguide will cause the first optical waveguide to bend or deflect at a location and the exciting light is coupled into the optical pressure sensor and causes fluorescent emission from the dyes on the second optical waveguide. The fluorescent light is captured by the second optical waveguide and leads to changes in the amplitude and spectrum of the light in the second optical waveguide. The relative intensities of the fluorescent light can be detected by a detector and knowing the position and concentration of the two fluorescent dyes on the second optical waveguide, the location of the pressure point can be simply calculated

[0009] It is not necessary to perform any mathematical calculations such as a Fourier transformation of the spectrum to determine the pressure point, or to use calibrated look-up tables. The colour spectrum of light transmitted through and/or reflected from the optical pressure sensor will enable the determination of the pressure point by simply measuring the relative intensities of the fluorescent light from the different fluorescent dyes.

[0010] In a first aspect of the optical pressure sensor, the concentration of one of the two fluorescent dyes increases, for example linearly, from the first end of the optical pressure sensor to the second end of the optical pressure sensor and, similarly, in a same aspect or a second aspect, the amount of the another one of the two fluorescent dyes decreases, for example linearly, from the first end of the optical pressure sensor to the second end of the optical pressure sensor. This enables simple calibration of the optical pressure sensor. The fluorescent dyes used include but are not limited to organic dyes like the classes of perylenes, coumarins, metal organic and inorganic dyes as well as fluorescent particles like quantum dots.

[0011] The compressible spacer can be made of a hydrogel.

[0012] The optical pressure sensor has further a light source, such as a laser or LED light producing the exciting light. The wavelength of the exciting light depends on the fluorescent dyes used. In one non-limiting example the exciting light is green or blue light.

[0013] A method of operation of the optical pressure sensor is also disclosed in this document. The method comprises illuminating with light at least one of the first end or the second end of the first optical waveguide and coupling the light from the first optical waveguide into fluorescent dyes to cause fluorescing in at least two fluorescent dyes. The light is coupled at the position at which the first optical waveguide is deflected by the application of pressure. The fluorescent light from the at least two fluorescent dyes is coupled by a second optical waveguide and detected by a detector. The detected fluorescent light is analysed in a processor to determine the relative intensities of the fluorescent light from the different fluorescent dyes.

Summary of the Drawings

[0014] Fig. 1 shows a bottom view of an optical pressure sensor. [0015] Fig. 2 shows a side view of the optical pressure sensor. [0016] Fig. 3 shows a flow diagram with the operation of the optical device.

Detailed Description of the Invention

[0017] The invention will now be described on the basis of the drawings. It will be understood that the embodiments and aspects of the invention described herein are only examples and do not limit the protective scope of the claims in any way. The invention is defined by the claims and their equivalents. It will be understood that features of one aspect or embodiment of the invention can be combined with a feature of a different aspect or aspects and/or embodiments of the invention.

[0018] An optical pressure sensor 10 for the measurement of an applied pressure to a first surface, e.g. an upper surface 12 is shown in Figs. l and 2.

[0019] The optical pressure sensor 10 comprises a first optical waveguide 20 and a second optical waveguide 30. The first optical waveguide 20 and the second optical waveguide 30 are separated by a compressible spacer 40. The first optical waveguide 20 and the second optical waveguide 30 are multimode waveguides made from a polymer. Different polymers are used as optical waveguides, and these include but are not limited to polymethylmethacrylate (PMMA), Polystyrene (PS), Acrylic Copolymers or cyclic olefin polymers (COP) and copolymer (COC), or transparent silicone like polymethylsiloxane (PDMS).

[0020] The first optical waveguide 20 has a top surface 12 and includes no fluorescent dyes.

In use the first optical waveguide 20 carries the exciting light substantially uniformly over the length of the optical pressure sensor 10. The multimode nature of the first optical waveguide 20 means that coupling to the device can be achieved by simple means, as multimode coupling to a structure with dimensions greater than 100 pm is easier than coupling to single mode structures with dimensions in the order of ~10 pm. In one aspect of the optical pressure sensor 10, green light is used because green light has less loss along the first optical waveguide made of PMMA.

[0021] The bottom surface 14 of the optical pressure sensor 10 is coated with a layer 35 of a pair of fluorescent dyes A and B. More fluorescent dyes could be used, and the number is not limiting of the invention. The pair of fluorescent dyes A and B have wavelengths which can be separated from one another on optical excitation. Non-limiting examples of such pairs include Coumarin-1 (absorption maximum at 354 nm and emission in range of 420-470 nm) and Coumarin-6 (absorption maximum at 458 nm and emission in range of 500-560 nm) or Perylene (with an absorption between 410-434 nm and emission in range of 436-465 nm) and a different Perylene (with an absorption maximum at 524 nm and emission in range of 539 nm).

[0022] It can be seen from Fig. 1 that the pair of fluorescent dyes A and B are arranged such that the fluorescent dyes form a substantially triangular shape elongated along the length of the optical pressure sensor 10. At first end 16 of the optical pressure sensor 10, the layer 35 is comprised substantially solely of the fluorescent dye A. At the second end 18 of the optical pressure sensor 10, the layer 35 is comprises substantially solely of the fluorescent dye B. In the middle 19 along the length of the optical pressure sensor 10, there is approximately a SO SO mixture of the fluorescent dye A and the fluorescent dye B. The choice of the fluorescent dyes A and B will depend on the colour of the light in the first optical waveguide 20.

[0023] In one aspect of the optical pressure sensor 10, the compressible spacer 40 is made of a hydrogel, such as but not limited to glycerine. The hydrogel is chosen because it is highly compressible and therefore enables the thickness of the optical pressure sensor 10 to be varied over a large interval. On compression, fluid in the hydrogel moves to the edges of the optical pressure sensor 10 and returns when the pressure is released.

[0024] A light source 50 for the exciting light, such as but not limited to a laser or LED emitting excitation light for the organic dyes used, is coupled to at least one of the first end 16 or the second end 18 of the first optical waveguide 20 of the optical pressure sensor 10 and a detector 60 coupled to at least one of the first end 16 or the second end 18 of the second optical waveguide 30 of the optical pressure sensor 10.

[0025] Applying a pressure to the top surface 12 of the optical pressure sensor 10 means that the blue or green light from the first optical waveguide 10 is coupled through the

compressible layer 40 into the second optical waveguide 30 and the layer 35 with the fluorescent dyes. The blue or green light causes the fluorescent dyes in the layer to fluoresce and the fluorescent light from the fluorescent dyes is captured by the second optical waveguide 30 and passed to the detector 60 which records the relative intensities of the fluorescent light. The relative intensity indicates the position along the optical pressure sensor 10 at which the pressure is applied, as the amount of the fluorescent dyes A and B differs along the optical pressure sensor 10.

[0026] The detector 60 can be either a spectrometer or two colour-sensitive photodiodes which can pass the signals recorded to be processed by a processor 70.

[0027] Fig. 3. shows the optical pressure sensor 10 in operation. In a first step 310, light from the light source 50 is coupled into the first optical waveguide 20. The light is transmitted along the first optical waveguide in step 320. Any pressure applied to the surface at a position of the first optical waveguide 20 means that the first optical waveguide 20 is bent or deflected in step 325 and light in the first optical waveguide 20 at the position is coupled into the compressible spacer 40 and thence to the layer 35 of fluorescent dyes.

[0028] The fluorescent light from the fluorescent dyes is captured in step 330 and will be transmitted along the second optical waveguide 30, as shown by the arrows, to the detector 60. As disclosed above, the position of the application of the pressure in step 330 will change the optical properties of the optical pressure sensor 10 and lead to changes in the spectrum of the detected light. This change of optical properties is processed and analysed in the processor 70 in step 340. The processor 70 stores a previously defined calibration table 75 which is used to determine the location and strength of the applied pressure, which is output in step 350.

Reference Numerals

10 Optical pressure sensor

12 Top surface

14 Bottom Surface

16 First end

18 Second end

19 Middle

20 First optical waveguide

22 Pressure position

30 Second optical waveguide

35 Layer of at least two fluorescent dyes

40 Compressible spacer

50 Light source

60 Detector

70 Processor

75 Calibration Table