FORCE SENSOR AND METHOD FOR OPERATING A FORCE SENSOR

Abstract

A force sensor with the following features is provided: - at least one emitter (2) which emits a first polarized electromagnetic radiation (12) during operation, - a photoelastic layer (3) with a photoelastic material (3') that is birefringent under mechanical stress, wherein the photoelastic layer (3) is traversed by the first polarized electromagnetic radiation (12) during operation, so that a second polarized electromagnetic radiation (14) is generated, - at least a second polarizer (4) that polarizes the second polarized electromagnetic radiation (14) such that a third polarized electromagnetic radiation (15) is generated during operation, and - at least one detector (5) which detects the third polarized electromagnetic radiation (15) during operation. Furthermore, a method for operating a force sensor is provided.

Description

[0001] A force sensor and a method for operating a force sensor are provided.

[0002] The aim is to provide an improved force sensor and an improved method for operating a force sensor.

[0003] These objectives are achieved with a force sensor having the features of claim 1 and a method for operating a force sensor having the steps of claim 15.

[0004] Improved developments and embodiments of the force sensor and the method for operating a force sensor are specified in the dependent claims.

[0005] According to one embodiment, the force sensor comprises at least one emitter that emits a first polarized electromagnetic radiation during operation. The first polarized electromagnetic radiation comprises or consists, for example, of wavelengths in the visible or infrared range. In particular, the first polarized electromagnetic radiation is linearly polarized. For example, a polarization plane of the first, for instance, initial, polarized electromagnetic radiation forms a polarization angle of approximately 45° with an optical axis of the force sensor. For example, the polarization plane of the first, for instance, initial, polarized electromagnetic radiation forms a polarization angle of 45° with a principal axis of the load induced by an external force to be measured. In particular, the electromagnetic radiation propagates within the force sensor along the optical axis.

[0006] According to a further embodiment, the force sensor comprises a photoelastic layer with a photoelastic material that is birefringent under mechanical stress. In particular, the birefringent photoelastic material separates an incident beam of electromagnetic radiation, such as the first polarized electromagnetic radiation, into an ordinary beam of electromagnetic radiation and an extraordinary beam of electromagnetic radiation that is polarized perpendicular to the ordinary beam of electromagnetic radiation, especially in the case that the incident beam of electromagnetic radiation forms a predetermined angle with the birefringent photoelastic material.

[0007] The birefringent material has different refractive indices for the electromagnetic radiation of the ordinary and extraordinary rays. In particular, the photoelastic material is isotropic when no mechanical stress is applied, and therefore not birefringent. In other words, the photoelastic material changes its optical properties when a mechanical stress is applied.

[0008] In particular, the photoelastic, birefringent material generates an ordinary beam of electromagnetic radiation that is linearly polarized and an extraordinary beam that is also linearly polarized but perpendicular to the ordinary beam of electromagnetic radiation. Specifically, the ordinary and extraordinary electromagnetic beams are oriented perpendicular to the direction of propagation of the electromagnetic radiation. For example, the ordinary and extraordinary electromagnetic beams pass through the photoelastic, birefringent material unchanged except for a relative difference in an optical path.

[0009] In particular, the birefringence of the photoelastic material and the orientation of this birefringence allow the identification of two orthogonal polarizations - in this case linearly polarized electromagnetic radiation, oriented perpendicular to each other and perpendicular to the direction of propagation: where the polarization along the optical axis of the material is called ordinary and the other extraordinary - which traverse the medium unchanged, apart from a relative difference in the optical path length traveled between the two orthogonal states.

[0010] In particular, during operation, the photoelastic layer is traversed by the first polarized electromagnetic radiation, generating a second polarized electromagnetic radiation. Specifically, the first polarized electromagnetic radiation traverses the photoelastic layer along the optical axis of the force sensor. The second polarized electromagnetic radiation has, for example, the same wavelength as the first polarized electromagnetic radiation. If mechanical stress is applied to the photoelastic material of the photoelastic layer, the second polarized electromagnetic radiation differs from the first polarized electromagnetic radiation, particularly in its polarization.If no mechanical stress is applied to the photoelastic material of the photoelastic layer, the second polarized electromagnetic radiation does not differ from the first polarized electromagnetic radiation, particularly not in its polarization. However, the wavelength of the electromagnetic radiation passing through the photoelastic layer is not altered by the photoelastic material of the photoelastic layer when a mechanical stress is applied to it.

[0011] The second type of polarized electromagnetic radiation is, for example, circularly or elliptically polarized electromagnetic radiation with two linearly polarized components of the electromagnetic radiation that are perpendicular to each other.

[0012] According to a further embodiment, the force sensor comprises at least one second polarizer that polarizes the second polarized electromagnetic radiation such that a third polarized electromagnetic radiation is generated during operation. In particular, the third polarized electromagnetic radiation also has the same wavelength as the first polarized electromagnetic radiation and / or the second polarized electromagnetic radiation. Specifically, the polarization of the third polarized electromagnetic radiation is the same as the polarization of the second polarized electromagnetic radiation. For example, the second polarizer is a 45° polarizer, and the third polarized electromagnetic radiation is linearly polarized electromagnetic radiation with a plane of polarization that forms a polarization angle of 45° with the optical axis.

[0013] According to another embodiment, the force sensor comprises at least one detector that detects the third polarized electromagnetic radiation during operation. The at least one detector is, for example, a photodiode.

[0014] In particular, the force sensor includes - at least one emitter that emits the first polarized electromagnetic radiation during operation, - the photoelastic layer with the photoelastic material which is birefringent under mechanical stress, wherein the photoelastic layer is traversed by the first polarized electromagnetic radiation during operation, so that the second polarized electromagnetic radiation is generated, - at least one second polarizer that polarizes the second polarized electromagnetic radiation in such a way that the third polarized electromagnetic radiation is generated during operation, and - at least one detector that detects the third polarized electromagnetic radiation during operation.

[0015] In particular, the emitter, the photoelastic layer, the second polarizer and the detector are arranged along the optical axis of the force sensor.

[0016] The force sensor is based, in particular, on the idea that the photoelastic material of the photoelastic layer becomes birefringent under the influence of mechanical stress, such as that exerted by a human user pressing a contact surface of the force sensor with their finger. When linearly polarized light, for example, first polarized electromagnetic radiation, passes through the mechanically stressed and therefore birefringent photoelastic layer, the first polarized electromagnetic radiation changes its polarization. After passing through the photoelastic layer, the electromagnetic radiation is referred to in this disclosure as second polarized electromagnetic radiation.

[0017] In particular, the first polarized electromagnetic radiation, which is linearly polarized and has a plane of polarization that includes a polarization angle of 45° with the optical axis of the force sensor, becomes elliptically or circularly polarized electromagnetic radiation, which has a further linearly polarized component perpendicular to the plane of polarization of the incident wave of the first polarized electromagnetic radiation, when it passes through the photoelastic layer under mechanical stress.

[0018] The second polarizer then polarizes the second polarized electromagnetic radiation to generate the third polarized electromagnetic radiation. Specifically, the second polarizer imparts the same polarization to the second polarized electromagnetic radiation as to the first. For example, the third polarized electromagnetic radiation, like the first, is linearly polarized electromagnetic radiation with a plane of polarization that forms a polarization angle of 45° with the optical axis of the force sensor.

[0019] When no mechanical stress is applied to the photoelastic layer, the electromagnetic radiation detected by the detector corresponds to a maximum value of the detector's electrical signal, particularly since the polarization of the first polarized electromagnetic radiation remains unchanged as it passes through the photoelastic layer. In this case, the first, second, and third polarized electromagnetic radiations are identical. When mechanical stress is applied to the photoelastic layer, the second polarized electromagnetic radiation acquires an additional component of linearly polarized electromagnetic radiation perpendicular to the polarization plane of the incident first electromagnetic radiation.

[0020] The second polarizer transmits only the component of the second polarized electromagnetic radiation with the same polarization as the first polarized electromagnetic radiation, particularly when both polarizers are aligned parallel. Force detection is also possible, and in a similarly optimal configuration, when the second polarizer is perpendicular to the first polarizer, except that the electrical signal from the detector is minimal in the absence of a mechanical force. Therefore, if the potassium-elastic layer alters the polarization of the electromagnetic radiation, the electromagnetic radiation, and in particular the intensity of the electromagnetic radiation detected by the detector, generally decreases, and the electrical signal from the detector also diminishes.It is also possible that if the potassium-elastic layer changes the polarization of the electromagnetic radiation, the electromagnetic radiation, and in particular the intensity of the electromagnetic radiation detected by the detector, will be sinusoidally altered depending on the applied mechanical force.

[0021] According to a further embodiment, the force sensor comprises a contact surface configured to apply a mechanical force and / or a mechanical pressure resulting from the mechanical force to the photoelastic layer. For example, the contact surface is mechanically connected to the photoelastic layer or applied directly to the photoelastic layer. The mechanical force is exerted on the contact surface, for example, by a human user, to trigger a contact event.

[0022] In particular, the force sensor is designed to detect touch events from a human user, such as those triggered by pressing the contact surface with a finger. Specifically, the mechanical force exerted on the contact surface induces mechanical stress within the photoelastic layer. This mechanical stress, resulting from the mechanical force, alters the polarization of the first polarized electromagnetic radiation passing through the photoelastic layer, and this alteration is detected by the sensor as described above.

[0023] According to a further embodiment of the force sensor, the emitter comprises an LED chip that emits electromagnetic radiation during operation. In particular, the LED chip comprises an epitaxial semiconductor layer sequence with an active zone that generates electromagnetic radiation during operation. The LED chip is, for example, mounted in a housing. Specifically, the active zone of the LED chip generates unpolarized isotropic electromagnetic radiation emitted by the LED chip. In particular, the emitter further comprises a first polarizer that polarizes the electromagnetic radiation emitted by the LED chip during operation such that the first polarized electromagnetic radiation is generated. The first polarizer is, for example, also enclosed by the housing and arranged downstream of the LED chip in the direction of radiation.

[0024] The emitter is, for example, a so-called side-looker, in which a principal extent plane of the epitaxial semiconductor layer sequence of the LED chip is arranged transversely, and in particular perpendicularly, to a mounting surface of the housing. Specifically, the principal extent plane of the epitaxial semiconductor layer sequence is perpendicular to the optical axis of the force sensor.

[0025] According to another embodiment of the force sensor, the emitter comprises a semiconductor laser that emits the first polarized electromagnetic radiation. In particular, it is possible that the emitter comprising the semiconductor laser emitting the first polarized electromagnetic radiation is free of a first polarizer. Rather, the semiconductor laser emits the first polarized electromagnetic radiation intrinsically. It is also possible that, in addition to the semiconductor laser emitting the electromagnetic radiation, the emitter includes an additional first polarizer to generate the first polarized electromagnetic radiation. It is also possible that the laser emits non-polarized electromagnetic radiation with a plane of polarization that forms a polarization angle of 45° with the optical axis of the force sensor. Such a laser is particularly combined with a λ / 4 plate.

[0026] It is also possible that the emitter includes an additional optical element for collimating the electromagnetic radiation emitted by the emitter.

[0027] According to another embodiment of the force sensor, the photoelastic layer is applied to a substrate. For example, the emitter and / or the detector and / or the first polarizer and / or the second polarizer are also applied to the substrate. The substrate is, for example, a printed circuit board (PCB). The circuit board includes, for example, functional electronic elements, such as transistors, which are configured for controlling the force sensor during operation.

[0028] The photoelastic material is, in particular, an amorphous and / or isotropic polymer material that exhibits a linear relationship between the applied mechanical stress and the degree of birefringence. Specifically, the birefringence increases with increasing mechanical stress. The difference between the two different refractive indices n ORD , n EXT The photoelasticity of both ordinary and extraordinary electromagnetic radiation increases linearly with the applied mechanical stress. An example of a photoelastic material is polycarbonate (PC).

[0029] It is also possible that the photoelastic material exhibits intrinsic mechanical stress without external influence. In this case, the intrinsic mechanical stress is preferably known and described, for example, by a mathematical tensor. The influence of the intrinsic mechanical stress can then be taken into account by a suitable algorithm.

[0030] The photoelastic material is, for example, a birefringent material with circular stress, a compact optical rotator. This compensates for hysteresis of the photoelastic material during operation of the force sensor.

[0031] According to another embodiment of the force sensor, the emitter and the detector are arranged on a connecting line, in particular on a straight connecting line.

[0032] In particular, the straight connecting line is parallel to the optical axis of the force sensor. Furthermore, in this embodiment, at least one structure is arranged on the connecting line between the emitter and the detector on a major surface of the photoelastic layer. In particular, the major surface of the photoelastic layer extends along the substrate. For example, the major surface of the photoelastic layer is parallel to the substrate.

[0033] For example, the structure is applied to the substrate onto which the photoelastic layer is deposited. For instance, at least one structure might be a dummy resistor applied to the substrate, a printed circuit board (PCB). The dummy resistor has the same structure as a functional resistor, but is not electrically connected. The dummy resistor is particularly easy to manufacture on the PCB along with other functional electronics. The structures can be produced, for example, by additive manufacturing or in-mold electronics (IME). It is also possible for the structures to be cast onto the substrate. Specifically, two, several, or a multitude of structures are applied along the interface.

[0034] According to another embodiment, the structure penetrates the photoelastic layer when mechanical force is applied. This increases the mechanical stress of the photoelastic layer. It is also possible for the structure to partially penetrate the photoelastic layer without any mechanical stress being applied. In this case, the structure penetrates further into the photoelastic layer when mechanical force is applied. In this way, the mechanical stress within the photoelastic layer is increased at the location of the structure. The mechanical stress is thus directed along the structure and concentrated at that location.If the force sensor comprises multiple structures, the structures are arranged on the connecting line, and the stress exerted on the photoelastic layer by the mechanical force is concentrated along the connecting line, resulting in increased sensitivity of the force sensor.

[0035] The force sensor, for example, has exactly one emitter and exactly one detector arranged opposite each other on a straight connecting line, in particular with a photoelastic layer and a second polarizer on the straight connecting line. In this case, the force sensor is in particular a zero-dimensional force sensor, like an analog push button.

[0036] According to a further embodiment, the force sensor comprises a plurality of emitters and a plurality of detectors. In particular, the emitters are arranged in a first line and a second line, wherein the first line and the second line run transversely, for example perpendicularly, to each other. In particular, both the first line and / or the second line are straight lines. In this embodiment, the detectors are arranged in a third line and a fourth line, which also run transversely, in particular perpendicularly, to each other. The third line and / or the fourth line are also preferably straight lines.

[0037] In particular, the emitters and detectors can be configured as described above.

[0038] For example, the first and second lines run along two adjacent edges of the photoelastic layer. Similarly, the third and second lines also extend along two adjacent edges of the photoelastic layer. In this way, a two-dimensional force sensor is realized.

[0039] According to another embodiment of the force sensor, the emitters of one line and the detectors of another line are arranged opposite each other. For example, the first and third lines are arranged opposite each other, and the second and fourth lines are arranged opposite each other. It is also possible for the first and fourth lines to be arranged opposite each other, and for the second and third lines to be arranged opposite each other.

[0040] The connecting lines between an emitter and a detector preferably run parallel to each other and / or to at least one edge of the photoelastic layer. In particular, a second polarizer is arranged between an emitter and a detector, preferably on the straight connecting line between the emitter and the detector. In particular, each connecting line between an emitter and a detector is an optical axis of the force sensor.

[0041] According to another embodiment of the force sensor, each emitter is arranged along a connecting line opposite a detector, and at least one structure is arranged on the connecting line between the emitter and the detector on a major surface of the photoelastic layer. In this way, an alignment and concentration of the mechanical stress along the connecting line can be achieved, which increases the sensitivity of the force sensor.

[0042] According to a further embodiment, the force sensor comprises a rib with a principal direction of extension. In particular, the rib is configured to exert an amplified mechanical force on the photoelastic layer. Specifically, the mechanical force exerted by the human user on a contact surface, for example to trigger a contact event, is amplified by the rib. In particular, the rib is movable to transmit the amplified mechanical force to the photoelastic layer. In particular, the rib is arranged offset from at least one or all optical axes of the force sensor. In particular, the principal direction of extension is transverse to one or all optical axes of the force sensor.

[0043] When a mechanical force is applied to the rib, the mechanical stress at the base of the rib, towards which the electromagnetic radiation is directed, is particularly increased.

[0044] According to another embodiment of the force sensor, the rib has or consists of a photoelastic material. For example, the rib has the same material as the photoelastic layer, such as polycarbonate, or consists of the same material as the photoelastic layer.

[0045] According to a further embodiment, the force sensor comprises at least two emitters that, during operation, emit polarized electromagnetic radiation with different wavelengths. In particular, the at least two emitters emitting polarized electromagnetic radiation with different wavelengths can distinguish between high and low mechanical forces exerted on the photoelastic material. Specifically, due to the sinusoidal intensity-force curve, the received intensity does not uniquely determine a force value, and a second wavelength provides more information to extrapolate the force from an intensity measurement. Furthermore, the sensitivity of the force sensor can be improved.

[0046] The force sensor described so far can be operated using the method described below. All features and embodiments described in connection with the force sensor can also be implemented in the method for operating a force sensor, and vice versa.

[0047] According to one embodiment, the force sensor operated by the method comprises at least one emitter, a photoelastic layer with a photoelastic material that is birefringent under mechanical stress, at least one second polarizer and at least one detector.

[0048] According to one embodiment of the method, the emitter is operated in such a way that it emits a first polarized electromagnetic radiation.

[0049] According to another embodiment, a second polarized electromagnetic radiation is generated from the first polarized electromagnetic radiation as it passes through the photoelastic material.

[0050] According to another embodiment of the method, the second polarized electromagnetic radiation is polarized by the second polarizer, so that a third polarized electromagnetic radiation is generated.

[0051] According to another embodiment of the method, the third polarized electromagnetic radiation is detected by the detector, whereby the detector generates an electrical signal.

[0052] According to another embodiment, the method comprises the following steps: - Operating the emitter to emit initial polarized electromagnetic radiation, - Generating the second polarized electromagnetic radiation from the first polarized electromagnetic radiation by passing the photoelastic material through the photoelastic layer, - Polarizing the second polarized electromagnetic radiation by the second polarizer, so that the third polarized electromagnetic radiation is generated, - Detecting the third polarized electromagnetic radiation through the detector, with the detector generating the electrical signal.

[0053] Preferably, the above steps should be carried out in the order given.

[0054] According to another embodiment of the method, a mechanical force is applied to the photoelastic layer, causing the photoelastic material to be birefringent. In particular, the mechanical force is applied to the photoelastic layer by a human user, for example, with a finger. The application of this mechanical force creates mechanical stress within the photoelastic material, which makes it birefringent.

[0055] According to another embodiment of the method, the electrical signal of the detector decreases when a mechanical force is applied to the photoelastic layer, causing the photoelastic material to become birefringent.

[0056] According to a further embodiment of the method, the force sensor comprises a plurality of emitters and a plurality of detectors, wherein the emitters are arranged in a first line and a second line, the first and second lines being perpendicular to each other. Furthermore, the detectors are arranged in a third line and a fourth line, which are also perpendicular to each other. In this embodiment of the method, the emitters are switched on and off. In this way, the location of a touch event triggered by a human user can be determined.

[0057] Further advantageous embodiments and developments of the force sensor and the method for operating a force sensor result from the exemplary embodiments described below in connection with the figures.

[0058] The Fig. 1A and Fig. Figure 1B shows schematic views of a force sensor according to an exemplary embodiment.

[0059] Fig. Figure 2 schematically shows a flowchart of a method for operating a force sensor according to an exemplary embodiment.

[0060] The Fig. Figure 3 shows a schematic view of a force sensor according to a further embodiment.

[0061] The Fig. Figures 4 to 6 show schematic views of a force sensor according to a further embodiment.

[0062] The Fig. Figure 7 shows a schematic view of a force sensor according to a further embodiment.

[0063] The Fig. Figure 8 schematically shows a normalized intensity I of the electromagnetic radiation as a function of a pressure P.

[0064] The Fig. Figure 9 schematically shows an example of a normalized intensity of a third polarized electromagnetic radiation detected by the detector, as a function of a linear phase delay according to an embodiment.

[0065] The Fig. Figure 10 schematically shows the path taken by the x-component and the y-component of a polarization vector of electromagnetic radiation.

[0066] The Fig. Figures 11 to 13 show schematic views of a force sensor according to a further embodiment.

[0067] Identical or similar elements, as well as elements with the same function, are designated with the same reference symbols in the figures. The figures and the proportions of the elements depicted in the figures are not to be considered as being to scale. Rather, individual elements, especially layers, may be enlarged for the purpose of better illustration and / or understanding.

[0068] The force sensor according to the exemplary embodiment of the Fig. 1A and Fig. 1B comprises a carrier 1, for example, a printed circuit board. Mounted on the carrier 1 are an emitter 2, a photoelastic layer 3, a second polarizer 4, and a detector 5. The photoelastic layer 3 and the second polarizer 4 are arranged along a straight connecting line 6 between the emitter 2 and the detector 2. The straight connecting line 6 runs parallel to an optical axis 7 of the force sensor. Furthermore, the electromagnetic radiation travels along the optical axis 7 within the force sensor. The detector 5 is, for example, a photodiode. In particular, the detector 5 is a side-accepting detector and / or comprises side-accepting optics.

[0069] The photoelastic layer 3, for example, has a thickness between 1 millimeter and 10 millimeters inclusive.

[0070] The emitter 2 comprises an LED chip 8 arranged in a housing 9. The LED chip 8 includes an epitaxial semiconductor layer sequence with an active zone 10 that generates electromagnetic radiation. The electromagnetic radiation generated in the active zone 10 of the LED chip 8 and emitted by the LED chip 8 is, in particular, isotropic unpolarized electromagnetic radiation. Furthermore, the emitter 2 includes a first polarizer 11 that polarizes the electromagnetic radiation from the LED chip 8 such that a first polarized electromagnetic radiation 12 is generated, which has a polarization plane 13 with a polarization angle α. P approximately 45° with the optical axis 7 of the force sensor.

[0071] In this case, a main extension plane of the epitaxial layer sequence of the LED chip 8 is arranged perpendicular to the optical axis 7 of the force sensor (side-looker design). During operation, the emitter 2 emits a first polarized electromagnetic radiation 12. During operation of the force sensor, this first polarized electromagnetic radiation 12 passes through the photoelastic layer 3, generating a second polarized electromagnetic radiation 14.

[0072] The second polarized electromagnetic radiation 14 passes through the second polarizer 4, so that a third polarized electromagnetic radiation 15 is generated. The second polarizer 4 is also a 45° polarizer, meaning that the second polarizer 4 only transmits the component of the second polarized electromagnetic radiation 14 with a polarization plane 13 that has a polarization angle α. PThe third polarized electromagnetic radiation 15 is detected by the detector 5, which is arranged downstream of the second polarizer 4 along the optical axis 7 in one direction of radiation from the emitter 2.

[0073] The photoelastic layer 3 comprises or consists of a photoelastic material 3', for example polycarbonate. Fig. Figure 1A shows a state of the force sensor in which no mechanical force F is exerted on the photoelastic material 3' of the photoelastic layer 3 via a contact surface 16. Therefore, the photoelastic material 3' of the photoelastic layer 3 is not under mechanical stress and has no birefringent properties. Rather, the photoelastic material 3' of the photoelastic layer 3 has isotropic optical properties and only a single refractive index for the first polarized electromagnetic radiation 12 passing through the photoelastic material 3'.

[0074] When the first polarized electromagnetic radiation 13 passes through the photoelastic layer 3, its polarization is therefore not changed. Consequently, the first polarized electromagnetic radiation 12 and the second polarized electromagnetic radiation 14 have the same polarization.

[0075] Since the second polarizer 4 emits only linearly polarized electromagnetic radiation with a polarization plane 13 that has a polarization angle α P enclosing at approximately 45° with the optical axis 7 of the force sensor, the intensity of the third polarized electromagnetic radiation 15 detected by the detector 5 does not differ significantly from the intensity of the first polarized electromagnetic radiation 13, and an electrical signal generated by the detector 5 is maximal.

[0076] Fig. Figure 1B shows a different state of the force sensor according to Fig. 1A. In the state of the force sensor according to Fig. 1B A mechanical force F is exerted on the contact surface. The contact surface 16 is arranged above the photoelastic material 3' of the photoelastic layer 3 and is configured to exert the mechanical force F on the photoelastic material 3', so that a mechanical stress is induced in the photoelastic layer 3.

[0077] When the mechanical force F is applied to the contact surface 16, the photoelastic material 3' of the photoelastic layer 3 is deformed by the pressure P due to the mechanical force F, and mechanical stress is exerted on the photoelastic material 3' of the photoelastic layer 3. When the mechanical stress acts on the photoelastic material 3' of the photoelastic layer 3, the photoelastic material 3' becomes birefringent.

[0078] As in Fig. As shown schematically in Figure 1B, the polarization of the first polarized electromagnetic radiation 12, which passes through the photoelastic material 3' of the photoelastic layer 3, is changed such that the second polarized electromagnetic radiation 14 has a different polarization than the first polarized electromagnetic radiation 12. In particular, the second polarized electromagnetic radiation 14 is elliptically polarized and has two different linearly polarized components that are perpendicular to each other.

[0079] The second polarizer 4 transmits only the component of the second polarized electromagnetic radiation 14, which is linearly polarized and has a polarization plane 13 with a polarization angle α. P of 45° with an optical axis 7 of the force sensor. Therefore, the third polarized electromagnetic radiation 15, which is transmitted by the second polarizer 4, in which in Fig. The state of the force sensor shown in 1B has a lower intensity than in the state shown in Fig. Figure 1A shows the state of the force sensor. Consequently, the detector 5, which detects the third polarized electromagnetic radiation 15, generates in the state of Fig. 1B a smaller electrical signal than in the state of the force sensor according to Fig. 1A.

[0080] The force sensor according to the embodiment of the Fig. 1A and Fig. 1B is a one-dimensional force sensor, for example an LED push button.

[0081] The method for operating a force sensor according to the exemplary embodiment of the Fig. 2 is, for example, set up to connect a force sensor as already mentioned in connection with the Fig. 1A and Fig. 1B is described as being operated.

[0082] In a first step S1, an emitter 2 is operated to emit a first polarized electromagnetic radiation 12.

[0083] In a second step S2, a second polarized electromagnetic radiation 14 is generated from the first polarized electromagnetic radiation 12 by passing through a photoelastic layer 3 with a photoelastic material 3'. Depending on whether a mechanical force F acts on the photoelastic material 3' or not, the polarization of the second polarized electromagnetic radiation 14 differs from the first polarized electromagnetic radiation 12.

[0084] In a third step S3, the second polarized electromagnetic radiation 14 is polarized by the second polarizer 4 in such a way that a third polarized electromagnetic radiation 15 is generated.

[0085] In a fourth step S4, the third polarized electromagnetic radiation 15 is detected by the detector 5, which generates an electrical signal. If no mechanical stress is applied to the photoelastic material 3' of the photoelastic layer 3, the third polarized electromagnetic radiation 15 has the same polarization as the first polarized electromagnetic radiation 12 and the second polarized electromagnetic radiation 14, and the electrical signal of the detector 5 has a maximum (see Fig. 1A). If a mechanical stress is applied to the photoelastic material 3' of the photoelastic layer 3, the third polarized electromagnetic radiation 15 has a lower intensity than the first polarized electromagnetic radiation 12 and the electrical signal of the detector is reduced.

[0086] In comparison to the force sensor according to the Fig. 1A and Fig. 1B comprises the force sensor according to the exemplary embodiment of the Fig. 3 a multitude of emitters 2 and a multitude of detectors 5. In particular, the force sensor of the Fig. 3 a two-dimensional force sensor.

[0087] The emitters 2 feature LED chips 8 that emit electromagnetic radiation, as already mentioned in connection with the Fig. 1A and Fig. Figure 1B explains. The LED chips 8 are arranged in two LED arrays 8' in a first straight line 17 and a second straight line 17' along two adjacent edges of a photoelastic layer 3 comprising a photoelastic material 3'. First polarizers 11 are arranged between the photoelastic layer 3 and each LED array 8'.

[0088] The detectors 5 are arranged as two photodiode arrays 5' in a third straight line 18 and a fourth straight line 18' at two adjacent edges of the photoelastic layer 3. In particular, an LED array 8' is arranged at an edge of the photoelastic layer 3 opposite a photodiode array 5'. Each emitter 2 is arranged opposite a detector 5 along a connecting line 6. Parallel to the connecting line 6, an optical axis 7 of the force sensor is arranged.

[0089] Furthermore, the force sensor according to Fig. 3 second polarizers 4, which are arranged between the edge of the photoelastic layer 3 and each photodiode array 5'.

[0090] During operation of the force sensor, electromagnetic radiation passes through the photoelastic material 3' of the photoelastic layer 3 and the polarizers 4, 11 and is detected by the detectors 5, as already described in connection with the one-dimensional force sensor of the Fig. 1A and Fig. 1B explains, with the difference that the force sensor is after Fig. 3 comprises several emitters 5 that emit first polarized electromagnetic radiation 12, and the same number of detectors 5 that detect third polarized electromagnetic radiation 15.

[0091] Furthermore, the force sensor comprises a plurality of structures 19 arranged on a major surface of the photoelastic layer 3. The structures 19 are arranged on the straight connecting line 6 between an emitter 2 and a detector 5. The structures 19 can be arranged on a major surface of the photoelastic layer 3 facing a support 1 or on an opposite major surface of the photoelastic layer 3 facing away from the support 1.

[0092] When a mechanical force F is applied to the main surface of the photoelastic layer 3, for example via a contact surface 16 (not shown), the structures 19 penetrate the photoelastic layer 3 and direct or concentrate the mechanical stress resulting from the mechanical force F onto the straight connecting line 6 between the emitter 2 and the detector 5, and thus onto an optical axis 7 running parallel to the connecting line 6. In this way, the efficiency and sensitivity of the force sensor can be increased.

[0093] In comparison to the force sensor according to the exemplary embodiment of the Fig. 1A, Fig. 1B and Fig. 3 The force sensor according to the exemplary embodiment of the Fig. 4, Fig. 5 and Fig. 6 a movable output lever arm 20. The movable output lever arm 20 is connected in this case via a lever 21 with two pivot points 22 to a contact surface 16 for force transmission ( Fig. 5) and an output lever arm 20. Springs 23 are arranged at the corners of the contact surface 16 to automatically reset the contact surface 16 after a contact event. The lever 21 transmits the mechanical force F acting on the contact surface 16, for example by the finger of a human user, to the output lever arm 20, resulting in a mechanical stress on the photoelastic material 3' of the photoelastic layer 3.

[0094] In particular, the mechanical force F can be increased with the help of the lever 21. Specifically, if the contact surface 16 is lowered by Δx1, the output lever arm 20 is lowered by a distance Δx2 and the mechanical force F is increased if Δx1 > Δx2 ( Fig. 5 and Fig. 6) The output lever arm 20 is also made of a photoelastic material 3', such as polycarbonate.

[0095] The force sensor according to the exemplary embodiment of the Fig. The device 7 comprises two emitters 2 that emit a first polarized electromagnetic radiation 12 of different wavelengths λ1 and λ2. Specifically, each emitter 2 comprises an LED chip 8, one of which emits isotropic electromagnetic radiation of a first wavelength λ1 and the other of which emits isotropic electromagnetic radiation of a second wavelength λ2. The isotropic electromagnetic radiation from the LED chips 8 is polarized by a first polarizer 11 such that the first polarized electromagnetic radiation 12 of wavelengths λ1 and λ2 is generated. The first polarizer is a 45° polarizer, and the first polarized electromagnetic radiation 12 is linearly polarized electromagnetic radiation with a plane of polarization that forms a polarization angle of 45° with an optical axis 7 of the force sensor. Specifically, the optical axis 7 of the force sensor is a rotational axis of the force sensor.

[0096] The two LED chips 8 are arranged at the edge of a photoelastic layer 3, which has a contact surface 16 on one of its main surfaces. As the first polarized electromagnetic radiation 12 passes through the photoelastic layer 3, a second polarized electromagnetic radiation 14 with two different wavelengths λ1, λ2 is generated. Depending on whether a mechanical force F acts on the contact surface 16 or not, the polarization of the second polarized electromagnetic radiation 14 with the two different wavelengths λ1, λ2 is either the same or different from the polarization of the first polarized electromagnetic radiation 12.

[0097] A detector 5 is arranged at another edge of the photoelastic layer 3, opposite the emitters. A second polarizer 4 is arranged between the photoelastic layer 3 and the detector 5. This polarizer polarizes the second polarized electromagnetic radiation 14, which has two different wavelengths λ1 and λ2, such that a third polarized electromagnetic radiation 15, also with two different wavelengths λ1 and λ2, is generated. The second polarizer is a 45° polarizer, and the third polarized electromagnetic radiation 15 is linearly polarized electromagnetic radiation with a polarization plane that forms a polarization angle of 45° with the optical axis 7 of the force sensor.

[0098] The detector 5 detects the third polarized electromagnetic radiation 15 with the two different wavelengths λ1, λ2.

[0099] The polarization of electromagnetic radiation can be described using a retarding Jones matrix.

[0100] A polarization vector of the first polarized electromagnetic radiation 12 is proportional to

[11]

[0101] The change in polarization of the first polarized electromagnetic radiation 12 as it passes through the photoelastic layer 3 is described by the Retarder-Jones matrix: e−iη(λ)2[100eiη(λ)] where λ is the wavelength of the electromagnetic radiation, in this case the wavelengths of the first polarized electromagnetic radiation 12, and η is the retardation of the photoelastic layer 3.

[0102] The polarization vector of the second polarized electromagnetic radiation 14 is given by: e−iη(λ)2[100eiη(λ)]

[11] ∝[1eiη(λ)].

[0103] Therefore, the intensity I(η(λ)) of the third polarized electromagnetic radiation 15 detected by the detector 5 depends on the retardation η of the photoelastic material 3' of the photoelastic layer 3 and the wavelength λ of the third polarized electromagnetic radiation 15.

[0104] If a mechanical force F is exerted on the contact surface 16 with area A, a pressure P=F / A is exerted on the photoelastic layer 3, leading to a mechanical stress in the photoelastic layer 3. Due to the mechanical stress in the photoelastic layer 3, the stress tensor ε changes, and the influence of the photoelastic layer 3 on the first polarized electromagnetic radiation changes accordingly. In other words, the retardation of the photoelastic layer q(ε,λ) is a function of the stress tensor ε and the wavelength λ.

[0105] In particular, the intensity I of the third polarized electromagnetic radiation detected by the detector depends on the wavelength λ and the pressure P. Fig. Figure 8 schematically shows the normalized intensity I as a function of the pressure P for two different wavelengths λ1, λ2. As can be seen, it is possible to distinguish between high and low pressure exerted on the photoelastic layer 3 by using electromagnetic radiation with two different wavelengths λ1, λ2.

[0106] The theoretical performance of the photoelastic material 3' can be estimated using the Retarder-Jones matrix. The Retarder-Jones matrix for the photoelastic layer 3 is a linear phase Retarder-Jones matrix with horizontal loading and is as described above: e−iη2[100eiη] where η is the retardation of the photoelastic layer. From this, the intensity of the detector I can be determined. det depending on the retardation η, derive: I(det)∝cos2η / 2

[0107] The Fig. 9 shows the detector intensity I det depending on the retardation η of the force sensor (straight line) and the values ​​of cos 2 η / 2 (stars) accordingly, while the Fig. Figure 10 shows the polarization points of the electromagnetic radiation.

[0108] The force sensor according to the exemplary embodiment of the Fig. Figures 11 to 13, like the force sensors already described, comprise an emitter 2 with an LED chip 8 and a first polarizer 11. The emitter 2 emits a first polarized electromagnetic radiation. The force sensor also comprises a photoelastic layer 3, a second polarizer 4, and a detector 5 ( Fig. 11).

[0109] The photoelastic layer 3 comprises at least one rib 24 on a base surface 25. In this case, the photoelastic layer 3 comprises two ribs 24 on the base surface 25. A schematic sectional view of the photoelastic layer 3 with the two ribs 24 is shown as an example in Fig. 12 shown.

[0110] The ribs 24 extend from the base surface 25 of the photoelastic layer 3 in two opposite directions. Furthermore, the ribs 24 are formed in one piece with the photoelastic layer 3. In particular, the ribs 24 comprise the same material as the photoelastic layer 3 or are formed from the same material. In particular, the ribs 24 comprise or consist of a photoelastic material, such as polycarbonate.

[0111] Furthermore, the ribs 24 have a principal direction of extension that runs transversely to an optical axis 7 of the force sensor and, in particular, to a direction of propagation of the electromagnetic radiation within the force sensor. In other words, the ribs 24 are arranged offset from the optical axis 7 of the force sensor and / or from the direction of propagation of the electromagnetic radiation within the force sensor. The ribs 24 form an acute angle α a with the base surface 25 of the photoelastic layer. Furthermore, the ribs 24 form an obtuse angle αo with a base surface 26 of the photoelastic layer 3.

[0112] In particular, the ribs 24 are movable. If a mechanical force F is exerted on the ribs 24, for example by pressing on the photoelastic layer 3 with a finger, the ribs 24 spread laterally and additional mechanical stress is induced within the photoelastic layer 3 in the regions 27 about the acute angles αa and in the regions 28 about the obtuse angles αo. In particular, mechanical compression is induced in the regions 27 about the acute angle αa, while mechanical strain is induced in the regions 28 about the obtuse angle αo. Thus, when the mechanical force F acts on the contact surface 16, the mechanical stress is increased, especially where the electromagnetic radiation strikes ( Fig. 13).

[0113] The present application claims priority over US application 63 / 544,770, the disclosure content of which is hereby incorporated by reference.

[0114] The invention is not limited to the description of exemplary embodiments. Rather, the invention encompasses every new feature and every combination of features, in particular every combination of features of the claims, even if the feature or combination of features itself is not expressly stated in the claims or the exemplary embodiments. Reference sign 1 carrier 2 emitters 3 photoelastic layer 3' photoelastic material 4 second polarizer 5 Detector 5' photodiode array 6 straight connecting lines 7 optical axis 8 LED chips 8' LED array 9 cases 10 active zones 11 first polarizer 12 first polarized electromagnetic radiation 13 Polarization plane 14 second polarized electromagnetic radiation 15 third polarized electromagnetic radiation 16 Contact area 17 first straight line 17' second straight line 18 third straight line 18' fourth straight line 19 Structure 20 Output lever arm 21 levers 22 Pivot point 23 spring 24th rib 25 floor area 26 Base area Area 27, 28 α P Polarization angle F mechanical force P print Area S1, S2, S3, S4 Procedure step a acute angle αo obtuse angle oα QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] US 63 / 544,770

[0113]

Claims

[1] Force sensor comprising: - at least one emitter (2) which emits a first polarized electromagnetic radiation (12) during operation, - a photoelastic layer (3) with a photoelastic material (3') that is birefringent under mechanical stress, wherein the photoelastic layer (3) is traversed by the first polarized electromagnetic radiation (12) during operation, so that a second polarized electromagnetic radiation (14) is generated, - at least a second polarizer (4) that polarizes the second polarized electromagnetic radiation (14) in such a way that a third polarized electromagnetic radiation (15) is generated during operation, and - at least one detector (5) that detects the third polarized electromagnetic radiation (15) during operation. [2] Force sensor according to the preceding claim, further comprising a contact surface (16) which is configured to exert a mechanical force (F) on the photoelastic layer (3). [3] Force sensor according to any of the preceding claims, wherein the emitter (2) comprises: - an LED chip (8) which emits electromagnetic radiation during operation, and - a first polarizer (11) that polarizes the electromagnetic radiation emitted by the LED chip (8) during operation, so that the first polarized electromagnetic radiation (12) is generated. [4] Force sensor according to one of the preceding claims, wherein the emitter (2) comprises a semiconductor laser which emits the first polarized electromagnetic radiation (12). [5] Force sensor according to one of the preceding claims, wherein the photoelastic layer (3) is applied to a support (1). [6] Force sensor according to one of the preceding claims, wherein the photoelastic material (3') is an amorphous and / or isotropic plastic material which exhibits a linear dependence between the applied mechanical stress and a degree of birefringence. [7] Force sensor according to one of the preceding claims, wherein the emitter (2) and the detector (5) are arranged on a connecting line (6), and at least one structure (19) is arranged on the connecting line (6) between the emitter (2) and the detector (5) on a principal surface of the photoelastic layer (3). [8] Force sensor according to the preceding claim, wherein the structure (19) penetrates the photoelastic layer (3) when the mechanical force (F) is applied. [9] Force sensor according to one of the preceding claims, comprising a plurality of emitters (2) and a plurality of detectors (5), wherein - the emitters (2) are arranged in a first line (17) and a second line (17'), wherein the first line (17) and the second line (17') run perpendicular to each other, - the detectors (5) are arranged in a third line (18) and a fourth line (18') that run perpendicular to each other. [10] Force sensor according to the preceding claim, wherein - the first line (17) and the third line (18) are arranged opposite each other, - the second line (17') and the fourth line (18') are arranged opposite each other. [11] Force sensor according to the preceding claim, wherein the emitters (2) of a line (17, 17') and the detectors (5) of a line (18, 18') are arranged opposite each other. [12] Force sensor according to the preceding claim, wherein - each emitter (2) is arranged opposite a detector (5) on a connecting line (6), and - at least one structure (19) is arranged on the connecting line (6) between the emitter (2) and the detector (5) on the main surface of the photoelastic layer (3). [13] Force sensor according to one of the preceding claims, further comprising a rib (24) with a principal extension direction, wherein - the rib (24) is movable to exert an increased force on the photoelastic layer (3), and - the rib (24) is arranged offset from an optical axis (7) of the force sensor. [14] Force sensor according to one of the preceding claims, comprising at least two emitters (2) which, during operation, emit first polarized electromagnetic radiation (12) with different wavelengths (λ1,λ2). [15] Method for operating a force sensor comprising: - at least one emitter (2), - a photoelastic layer (3) made of a photoelastic material (3') which is birefringent under mechanical stress, - at least one second polarizer (4), - at least one detector (5) wherein the method comprises the following steps: - Operating the emitter (2) to emit a first polarized electromagnetic radiation (12), - Generating a second polarized electromagnetic radiation (14) from the first polarized electromagnetic radiation (12) by passing through the photoelastic layer (3) which comprises the photoelastic material (3'), - Polarizing the second polarized electromagnetic radiation (14) by the second polarizer (4) so ​​that a third polarized electromagnetic radiation (15) is generated, - Detecting the third polarized electromagnetic radiation (15) by the detector (5), wherein the detector (5) generates an electrical signal. [16] Method according to the preceding claim, wherein the electrical signal of the detector (5) decreases when a mechanical force (F) is applied to the photoelastic layer (3) so that the photoelastic material (3') becomes birefringent. [17] Method according to one of claims 15 to 16, wherein the force sensor comprises a plurality of emitters (2) and a plurality of detectors (2), wherein - the emitters (2) are arranged in a first line (17, 17') and in a second line (17, 17'), wherein the first line (17, 17') and the second line (17, 17') run perpendicular to each other, - the detectors (5) are arranged in a third line (18, 18') and a fourth line (18, 18') that run perpendicular to each other, and the emitters (2) during the process can be switched on and off.

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