Capacitive flexible sensor

Abstract

A capacitive flexible sensor is disclosed. The flexible sensor has a pair of conductive electrodes on a flexible substrate that exhibit a variable mutual capacitance as a function of the amount of bending of the substrate, wherein contraction or stretching of the substrate brings the electrodes closer together or further apart. The invention also includes an electronic circuit configured to measure the capacitance of at least one of the conductive electrodes and output a digital value representative of the flexure, deformation or stress of the substrate.

Description

Technical Field

[0001] In some embodiments, the present invention relates to a flexible sensor for measuring the amount of deformation, deflection, or bending of a flexible substrate. Background Technology

[0002] Flexible sensors are used in several applications, particularly, but not exclusively, in human-machine interface devices and small portable electronic devices, as well as in innovative human-computer interfaces such as data gloves and rehabilitation devices. Many electrical properties of the material can be used to convert flexure into appropriate electrical signals.

[0003] Foldable devices are gaining increasing acceptance in the smartphone and general portable electronics markets, and their presence is expected to grow further. One of the key functions of these foldable devices is their ability to rearrange the display depending on the folded state. To this end, they typically rely on some form of detector that provides the folded state or, preferably, the folding angle. This can be achieved by Hall sensors and magnets located on opposite sides of the hinge; however, selecting the optimal location to neutralize or minimize the negative effects of magnetic interference is challenging.

[0004] Angle detectors are commonly used in many applications, including in the automotive industry, where angle sensors are increasingly used in hoods, trunks, doors, and seat mechanisms. Therefore, there is a need for inexpensive, robust angle sensors that are compatible with common electronic manufacturing techniques.

[0005] Known flexible sensors can utilize conductive ink or other piezoresistive materials in which the resistivity changes according to the flexible state or strain. Strain gauges can be considered an example of such sensors, although for very small deflections. At opposite ends of the bending range, flexible sensors on highly flexible substrates (such as plastic films) are used to capture large deflections and bending angles of 180° or greater.

[0006] Fiber-optic flexible sensors and capacitive sensors are also known. U.S. Patent 5,610,528 discloses a capacitive sensor in which each of two comb-shaped portions of conductive material is patterned on separate thin substrates. When the substrates are bent, the combs are stacked and slide relative to each other.

[0007] Capacitive sensors are commonly used in electronic devices as proximity sensors and touch input devices. U.S. Patent 11,082,550, in the name of the applicant, provides examples among many. Summary of the Invention

[0008] The object of this invention is to provide a device that at least partially overcomes the defects and limitations of the prior art.

[0009] According to the invention, these objectives are achieved by the subject matter of the appended claims. In its most common form, the innovative flexible sensor includes a pair of conductive electrodes on a flexible substrate, the pair of conductive electrodes exhibiting variable mutual capacitance depending on the amount of bending of the substrate, wherein contraction or stretching of the substrate causes the electrodes to move closer or further apart together. The invention also includes an electronic circuit configured to measure the capacitance of at least one of the conductive electrodes and output a digital value representing the deflection, deformation, or stress of the substrate. The sensing electrodes are preferably located on a common layer of the stacked substrates, but the invention will still function to some extent if the electrodes are placed on different layers, not too far apart.

[0010] The sensor of this invention can be used to detect simple elongation or shortening of a substrate, in which case its position within the substrate is not important. However, an important use case is the detection of bending. In this case, there will be a neutral plane in the substrate where its elements do not deform, and this neutral plane will generally be located in the middle of the thickness. Importantly, if bending detection is desired, the sensor is positioned away from the neutral plane. A separation of at least 15% of the total substrate thickness is desirable.

[0011] The shape of the sensing electrodes is unrestricted, but interlaced comb-like structures have provided satisfactory results. Their dimensions can be freely varied depending on the requirements. In many cases, it is desirable that the teeth of the comb-like structures be as fine and closely spaced as possible. In a typical FPC (flexible printed circuit) implementation of this invention, the teeth can have a width of 100–200 μm and will be separated by a space between 100 μm and 200 μm. For example, if the sensor is integrated into a flexible display, further processing can result in even finer teeth and spacing. Larger tooth widths and spacings are also possible if space permits.

[0012] In a simple implementation, the flexible sensor of the present invention can simply measure the self-capacitance of one electrode while the other electrode is grounded, and derive the deformation, buckling, or bending angle from it. In a variation, the flexible sensor of the present invention can also have additional electrodes for shielding against external influences. These electrodes can be permanently grounded or driven as active shields. A further combination of active shielding and permanently grounded external protection electrodes is also possible. Multiple additional electrodes are also advantageous because they allow for multiple measurements under different conditions and thus improve robustness against capacitance drift not caused by deformation, buckling, or bending. For this purpose, it is also preferable that the two combs forming the sensing electrodes do not share a common tooth width and are measured separately while the other electrode is grounded.

[0013] In the context of this disclosure, the term "flexible" refers to a material that can be bent and deformed under external force. When applied to printed circuits, it primarily refers to flexible circuits on polymer substrates (such as polyimide, polyester, liquid crystal polymers, etc.), but the invention can be extended to thin flexible layers of materials with higher elastic modulus (such as FR-4, metals, or ceramics). Similar considerations apply to flexible digital displays. Attached Figure Description

[0014] Exemplary embodiments of the present invention are disclosed in the specification and illustrated in the accompanying drawings, in which: Figure 1 The diagram schematically illustrates a portion of the flexible sensor of the present invention implemented in a flexible stacked structure; Figure 2 It is a cross-sectional view of a stacked structure, and Figure 3 It is the detail of interlaced comb-like patterns patterned on the stacked layers; Figure 4 This is a very simplified possible architecture of the electronic circuitry used in this invention; Figure 5 This is a simplified example of many possible variations of the circuit used in this invention to convert the self-capacitance of a general electrode into a digital value. Figure 6 The flexible detector of the present invention is schematically shown on a deformed substrate.

[0015] In the figures, prominent features are identified by reference numerals that are repeated throughout this document. The same reference numerals may be used to identify identical, similar, logically related, or technically equivalent distinguishing features. When many identical, similar, related, or equivalent features appear in the figures, some reference numerals may have been omitted to avoid clutter. Detailed Implementation

[0016] Figure 1 The following is a simplified representation and not to scale of a possible implementation of the invention of a stacked structure 80 comprising patterned electrodes on a multilayer flexible substrate. Structure 80 may be part of a flexible printed circuit or a flexible display and includes an arrangement of patterned electrodes, as disclosed later. The electrodes may be patterned metal layers (e.g., copper) or, in the case of flexible or foldable displays, patterned conductive and transparent layers, such as layers of indium tin oxide (ITO). Importantly, the patterned structure 80 is compatible with common manufacturing methods for printed circuits and displays; therefore, it can be integrated into many designs with minimal effort and cost. The various electrodes of the stacked structure 80 can be interconnected and connected to other electronic components using common traces and vias. This is only schematically shown in the figures.

[0017] Figure 1 Only the conductive electrodes of the stacked structure 80 are shown. The patterned dielectric flexible layer on which the electrodes are laid is not shown for simplification.

[0018] The paired electrodes 51 and 52 are located at the center of the multilayer structure 80. The electrodes are arranged such that the relative distance between them changes due to the stretching of one or more layers on which the electrodes are patterned when the substrate is folded. Preferably, the electrodes 51 and 52 are configured like two interlaced combs and are on the same stacked layer. However, other solutions are possible.

[0019] Sensing electrodes 51 and 52 are surrounded by a shielding ring 73 on the same layer, and vertically surrounded by shielding electrodes 72 and 74 on the corresponding upper and lower layers of the stack. The shielding ring, as well as the upper and lower shielding layers, are electrically interconnected, for example, by through-holes or by any other suitable means.

[0020] To further reduce the impact of external fields and interference, the shielding ring 73 is surrounded by a larger grounding ring 63, which is also repeated in the upper and lower layers by shielding rings 64 and 62. Additionally, an upper grounding plane 65 and a lower grounding plane 61 are stacked on their separate layers. The grounding planes and grounding rings can be permanently grounded or bound to a constant reference potential.

[0021] Electronic circuit 30 is connected to shielding electrodes and two sensing electrodes 51, 52, and is configured to determine the flexure or deformation digital signal capacitively, and to provide digital values ​​of flexure, deformation or angle at its output, as will be explained below.

[0022] Figure 2 The same structure is shown again out of scale in the cross-section. The structure comprises five layers of patterned conductors, labeled (A) to (E), on a flexible stack 41 that may be a portion of a printed circuit or a display screen. However, the number of layers is not a necessary feature of the invention, and other implementations may include more or fewer layers.

[0023] Sensing electrodes 51 and 52 are visible in the middle of layer (C), partially concealed behind it and surrounded by a shielding ring 73 on the same layer and shielding planes 72 and 74 on layers (B) and (D). The shielding ring and shielding planes are interconnected through vias and form an electrostatic barrier that protects the sensing electrodes 51 and 52 from external influences. They are connected to terminals of a readout circuit via traces (not shown), which is programmed to actively drive the shielding potential, as will be disclosed later.

[0024] Preferably, the electrostatic barrier is surrounded by grounding rings 62, 63, and 64 on layers (B), (C), and (D), and by grounding planes 61 and 65 on layers (A) and (E). These conductors can be permanently grounded for further protection against external influences.

[0025] exist Figure 1 In the design, the sensing electrodes are stylized as interlaced combs. Testing has determined that this configuration provides favorable sensitivity for buckling and deformation. Figure 3 Details of the structure of the interlaced comb-like structures that can be used in this invention are shown. The widths "a" and "b" of the teeth of comb-like structures 51 and 52 can be the same ( ) or different ( In a specific implementation on a printed circuit, the trace separation d can be 0.1 mm or less, up to the limit allowed by the manufacturing process, as needed, and the widths a and b can be between 0.1 mm and 10 mm or more. When the invention is applied to a flexible display, dimensions a, b, and d can be as small as the process allows, if desired. The overall width “w” and height “h” are not limited and, in many cases, can be significantly larger than shown in the drawings.

[0026] Figure 4 Electronic circuit 30 is shown. The detector has three sensing inputs CS0, CS1, and CS2, which can be connected to capacitive sensing electrodes 51 and 52 and a shielding electrode.

[0027] Internally, the capacitance detector 30 has an analog-to-digital converter 40 that converts the capacitance signal into a digital signal suitable for further processing in the digital processor 45. Preferably, a general converter is used to read the capacitance seen via the sensing electrodes and then via the multiplexer 35. However, this is not an absolute requirement.

[0028] Input unit 31 is configured to set the corresponding input terminal to a desired state selected from measurement state, ground state, and shielded state. In measurement state, the input potential is variable, following the variable voltage source in the capacitive sensor device, and the resulting charge change is sent to the ADC to determine the capacitance value. In ground state, the input is maintained at a constant voltage, either the ground voltage or offset by a fixed value. In shielded state, the voltage at the input follows the voltage of another input in measurement state, but charge change is ignored.

[0029] Figure 5A possible implementation of the input unit is shown in a very simplified form. The input unit causes the voltage at the input to follow a variable voltage source and can be used within the framework of this invention to convert capacitance to a digital value using ADC 40. This is provided for the completeness of this disclosure and by way of example; however, the invention also includes other structures that can provide the desired function for the input stage. Sensing electrodes are connected to the SC node, which is linked to the inverting input of amplifier 32, while another input is driven by a voltage source 36 that can provide a variable or constant voltage. Due to feedback, the SC node is a low-impedance node, and its voltage is the same as the voltage of voltage source 36 (possibly with a constant offset that is not significant in this application).

[0030] The grounding state of input node SC can be obtained by causing source 36 to generate a constant voltage. Input node SC is then virtually grounded. When source 36 generates a variable voltage, the measurement state and shielding state are obtained. Node SC then follows this voltage.

[0031] The capacitance of the sensing electrode connected to the SC node can be measured by inducing a series of steps at source 36. Capacitor 34, connected in the reaction loop, acts as an integrator, and the output of amplifier 32 will show a step proportional to the change in charge on the sensing electrode. The ratio between the charge of the step voltage at the output of amplifier 32, measurable as a step, and the known voltage step at source 36, is by definition the desired capacitance. Switch 33 is used to periodically discharge the reaction capacitor 34.

[0032] The output signal is digitized by ADC converter 40, preferably in sync with the pulses of source 36. Multiplexer 35, preamplifier 38, and offset correction 39 are not shown in this figure, but may be present.

[0033] Figure 6 This is a simplified representation of a flexible detector on a flexible deformable substrate 41. The bending imprinted on the substrate generates a strain field, which varies depending on the distance from the neutral sheet 43. Above the neutral sheet, the y-component of the strain is positive. The material is stretched. Beneath the sheet, Furthermore, the material is compressed. As shown, when the detector stack 80 is patterned on the neutral sheet, strain increases the gap between electrodes 51 and 52, and decreases their mutual capacitance. Conversely, relative bending will cause an increase in capacitance. For detectors below the neutral sheet, the effect will be reversed. It appears that a suitable location can usually be found by avoiding the neutral sheet when placing the sensing electrodes 51 and 52.

[0034] Advantageously, the detector of the present invention is configured to perform a number of measurements using electrodes in various modes to extract the deformation of the substrate. Table 1 summarizes some possible stages of the measurement and the corresponding settings of the input pins of the electronic circuit 30.

[0035] Not all stages and inputs shown in the table are necessary for the invention. In its simplest form, the present invention can be embodied by a circuit with a single sensing input that is permanently in the "measurement" mode, connected to one sensing electrode, while the other sensing electrode is grounded and does not provide an active shield. The electronic circuit can have only one input pin (CS0) in this case, and its operation will only include stage 2.

[0036] Additional stages and input pins increase accuracy and robustness. In stage 4, the roles of the sensing electrodes are swapped: the electrode that was measured in stage 2 is tied to ground, and the capacitance of the grounded electrode is now measured. This increases the robustness against spurious signals. The true buckling signal is expected to produce proportional capacitance changes in stage 1 and stage 4 that are related by a geometric factor. This requires an additional pin (CS1) in the electronic circuit.

[0037] The addition of a third input pin (CS2) opens the possibility of connecting the shield as an active shield in stages 2 and 4. This minimizes the background self-capacitance of the sensing electrodes and increases the relative capacitance change caused by buckling and deformation.

[0038] Stage 1 is the measurement of the self-capacitance of the shield electrode (connected to CS2), while the internal sensing electrodes (CS0 and CS1) are used as an active shield. The purpose of this measurement is to quantify the background contribution that is independent of the buckling or deformation of the substrate.

[0039] Stage 3 is the measurement of the capacitance of the two sensing electrodes (CS0 and CS1) when the shield is active. This is an alternative way to determine the background contribution that is independent of the buckling or deformation of the substrate.

[0040] According to the present invention, the electronic circuit can be configured as an angle detector that transmits a logic signal when the bending exceeds a predetermined angle or returns below that value. This can be done in a simple embodiment by checking the result M2 of the measurement in stage 2 against a pair of thresholds: d2 < M2 < u2. For additional robustness, the circuit can also perform a check to compare the stage 4 measurement M4 against a suitable threshold: d4 < M4 < u4 and only raise the logic signal when both criteria are met.

[0041] Stages 1 and 3 should not provide enhanced measurements during actual bending events, but rather react to unwanted external influences such as shocks, temperature drift, approaching bodies, noise, and electromagnetic interference. The electronic circuitry of this invention can be configured to suppress bending signals when the corresponding measurements M1 and M3 exceed their expected values. In addition to angle detection, this invention can also allow for the measurement of bending angles based on the values ​​of M2 and M4. This can be achieved by looking up a table or by calculation (possibly evaluating an expression that has been fitted to the response of the flexible sensor during calibration).

[0042] Reference numbers in the figure 30 Electronic circuit; 31 Front end; 32 Amplifier; 33 Reset switch; 34 Integrated capacitor; 35 Multiplexer; 36 Variable voltage source; 38 Analog processing; 39 Offset compensation; 40 A / D converter; 41 Substrate; 43 Neutral layer; 45 Digital processing; 51 First comb electrode; 52 Second comb electrode; 61 Ground surface; 62 Ground ring; 63 Ground ring; 64 Ground ring; 65 Ground surface; 72 Shielding surface; 73 Shielding ring; 74 Shielding surface; 80 Stacked structure.

Claims

1. A flexible sensor, comprising: Pairs of conductive electrodes are patterned on a flexible substrate, wherein the mutual capacitance between the electrodes is variable according to the amount of bending of the substrate. And electronic circuitry configured to measure the capacitance of at least one of the conductive electrodes and output a digital value representing the deflection, deformation, or stress of the substrate.

2. The flexible sensor as claimed in the preceding claim, wherein the conductive electrode is surrounded by one or more shielding electrodes, and wherein electronic circuitry is configured to maintain a constant potential difference between the one or more shielding electrodes and the conductive electrode whose capacitance is measured.

3. The flexible sensor as described in the preceding claim, wherein the flexible substrate is a layer of a flexible stacked assembly, and the shielding electrode comprises one or more of the following: - A shielding ring, patterned on a flexible substrate. - Shielding surfaces, which are patterned on layers of a printed circuit board above or below a flexible substrate.

4. The flexible sensor as claimed in any one of claims 2 to 3, comprising one or more grounding surfaces surrounding a shielding electrode.

5. The flexible sensor as claimed in any of the preceding claims, wherein the paired conductive electrodes are staggered combs having the same tooth width, or a first comb staggered with a second comb having a first tooth width, and a second comb having a second tooth width different from the first tooth width.

6. The flexible sensor as claimed in any of the preceding claims, wherein the electronic circuitry is configured to measure the capacitance of the first electrode in a pair during a first measurement phase, while the second electrode in the pair is held at a constant reference potential.

7. The flexible sensor of claims 6 and 2, wherein the electronic circuitry is configured to measure the capacitance of the second electrode in a second measurement phase, while the first electrode is connected to a constant reference potential, and the shielding electrode is maintained at a constant voltage difference with the second electrode.

8. The flexible sensor of claim 6, wherein the electronic circuitry is configured to measure the capacitance appearing on the shielding electrode in the third measurement phase, while the first and second inputs are maintained at a constant voltage difference from the shielding electrode.

9. The flexible sensor of claim 6, wherein the electronic circuitry is configured to measure the capacitance co-occurring on the first and second electrodes in a fourth measurement phase, while the shielding electrode is maintained at a constant voltage difference from the first and second electrodes.

10. The flexible sensor as described in any of the preceding claims, wherein the flexible substrate is a layer of flexible printed circuitry or a layer of foldable display.

Images