A tactile sensor device

Abstract

A tactile sensor device, the device comprising sensing and driving electronics and a sensor array, each sensor having a driving coil electrically coupled to a sensor coil. The array has rows and columns of sensors. All the driving coils in each column are electrically connected in series and are driven by an AC constant current source through an analog multiplexer. All the sensing coils in each row are electrically connected in series and the inductive AC voltage is fed through an analog multiplexer to an AC amplifier. The amplified AC voltage is then fed to an amplitude demodulator to generate a DC signal that depends on the inductive coupling factor between the driving coil and the sensing coil of an individual sensor, which is selected by the intersection of an actively current-driven column and an actively sensing row.

Description

Technical Field

[0001] The subject matter of the present disclosure generally relates to tactile sensors and, more particularly, to tactile sensors (sensels) having sensing elements.

[0002] Background Art and Prior Art

[0003] Tactile sensors are used to detect mechanical forces, pressure, and other external environmental stimuli. Technologies that commonly use tactile sensors include robotics, computer hardware (e.g., human-machine interfaces (HMIs)), medical devices and systems, environmental monitoring systems, and security systems. Typical tactile sensors include a sensor array and electronic circuitry that measure the output of the sensors in response to changes in the physical interaction between the sensors and environmental stimuli. There are various types of sensors, including piezoresistive, piezoelectric, capacitive, and inductive. Each type of sensor has advantages and disadvantages.

[0004] Piezoresistive sensors operate based on the piezoresistive properties of semiconductors, where their resistance changes with the applied mechanical strain. The piezoresistivity of a semiconductor material is a function of the mobility of charge carriers, which itself varies proportionally with the volume of the material. Piezoresistive sensors are widely used due to their low manufacturing cost and high sensitivity. However, due to the nature of the transduction mechanism, the mobility of charge carriers in piezoresistive materials also varies with temperature. Therefore, a temperature compensation mechanism is required, which increases the complexity of the sensor and associated circuitry.

[0005] Piezoelectric sensors operate by converting an applied mechanical force into a voltage by developing a surface charge proportional to the applied mechanical force. Commonly used piezoelectric materials include polyvinylidene fluoride (PVDF) polymers, zinc oxide (ZnO), and lead zirconate titanate (PbZrTi03 or PZT). PVDF polymers are commonly used due to their flexibility, lightweight, high piezoelectric coefficient, dimensional stability, and chemical inertness. Piezoelectric sensors advantageously do not require a power source and can potentially be used in a wider range of applications. Additionally, some piezoelectric materials exhibit high sensitivity to very small deformations. However, the transduction mechanism is only applicable to detecting dynamically applied mechanical forces because if the applied mechanical force is constant, the output voltage of the sensor decays to zero over time. Additionally, when configured in an array, piezoelectric sensors suffer from crosstalk because the applied force tends to propagate to adjacent sensors, thus limiting the resolution. Additionally, piezoelectric sensors are prone to hysteresis.

[0006] A capacitive sensor has two conductive plates separated by a dielectric material. The capacitance of the sensor is inversely proportional to the distance between the two plates. The structure of the sensor deforms in response to a mechanical force or pressure, such that the distance between the plates changes, resulting in a change in capacitance. Capacitive sensors exhibit high sensitivity, good frequency response, high spatial resolution, low temperature sensitivity, and a large dynamic range. However, when configured in an array, capacitive sensors are vulnerable to noise and suffer from crosstalk, thus requiring complex signal processing and noise reduction circuits.

[0007] An inductive sensor has an electrical coil and an embedded metal target in front of the coil. When an external mechanical force displaces the metal target towards the coil, the eddy current in the metal target changes, and thus the inductance of the coil changes. The change in inductance is measured by an electronic circuit to determine the applied mechanical force. Inductive tactile sensors have been proposed, including, for example, "Design and Characterization of a Triaxial Soft Inductive Tactile Sensor" by Hongbo Wang et al. Although inductive tactile sensors have the potential advantage of being tolerant to fluids and contaminants, such as operating in an underwater environment, they have at least two drawbacks. The inductance response to changes in the applied mechanical force is relatively small, so the measurement requires relatively complex electronic circuits. In addition, the large number of electrical connections required between the sensor and the associated electronic circuit is complex and prone to noise and crosstalk. Furthermore, as the array scales proportionally in the number of sensors, these drawbacks become more pronounced, which limits the maximum size of practical arrays in actual applications. Summary of the Invention

[0008] All examples, aspects, and features mentioned in this document can be combined in any technically possible way.

[0009] According to some aspects, a tactile sensor device includes: a plurality of inductive sensors arranged in an array including rows and columns, each of the sensors including a drive coil and a sense coil, wherein the drive coils of the sensors in each column are connected in series, and the sense coils of the sensors in each row are connected in series; and a circuit configured to independently select a single sensor among the inductive sensors by activating the drive coil of the sensors in one of the columns and the sense coil of the sensors in one of the rows, wherein the sensor at the intersection of the activated column and row is selected, and the output of the selected sensor is measured by the circuit. In some embodiments, the circuit performs a scan by individually selecting and measuring the outputs of each of the sensors in the array in series. In some embodiments, the circuit generates a time series of measurement matrices corresponding to the positions of the corresponding sensors in the array. In some embodiments, the circuit includes a drive circuit connected to the drive coils of the sensors, the drive circuit including an AC current source and a demultiplexer. In some embodiments, the circuit includes a sense circuit connected to the sense coils of the sensors, the sense circuit including a multiplexer and an analog-to-digital converter. In some embodiments, the sensors are disposed on a printed circuit board (PCB), and a first compressible dielectric layer is disposed on a first side of the PCB. In some embodiments, a first deformable conductive shielding layer is disposed on the first compressible dielectric layer such that the first deformable conductive shielding layer is separated from the PCB by the first compressible dielectric layer. In some embodiments, a second compressible dielectric layer is disposed between a second side of the PCB and a second deformable conductive shielding layer. In some embodiments, a conductive target is disposed on a deformable dielectric film layer, the deformable dielectric film layer is disposed on the first compressible dielectric layer, and the first compressible dielectric layer is disposed on the first side of the PCB. In some embodiments, a second compressible dielectric layer is disposed between the second side of the PCB and a deformable conductive shielding layer. In some embodiments, the drive coils are disposed on a first printed circuit board (PCB), the sense coils are disposed on a second PCB, and the drive coils are separated from the sense coils by a compressible dielectric layer. In some embodiments, adjacent drive coils and adjacent sense coils are wound in opposite directions. In some embodiments, the sensors are arranged in groups, and wherein each conductive target partially covers a group of four sensors associated with the target, and an elastomeric layer is disposed between the sensors and the conductive target. In some embodiments, the circuit measures the forces applied on multiple axes based on the output measurements. In some embodiments, each conductive target has a truncated square shape.

[0010] According to some aspects, a tactile sensor includes: a sensor array; and circuitry configured to drive a selected group of the sensors of the array, where the group includes less than all of the sensors of the array and measure an output of one of the sensors of the selected group. In some embodiments, the outputs of the sensors of the array are measured in a time series.

[0011] According to some aspects, a method includes: having a tactile sensor that includes a sensor array, driving a selected group of the sensors of the array with a current source, where the group includes less than all of the sensors of the array; and measuring an output of one of the sensors of the selected group. Some implementations include measuring the outputs of the sensors of the array in a time series.

[0012] While advantages should not be considered limitations of aspects of the invention, some embodiments of multiplexed inductive tactile sensors may improve resistance to electromagnetic noise and the ability to function in the presence of liquids, contaminants, and other adverse environmental conditions. Additionally, array size scalability may be improved, and the necessary number of electrical connections within the sensor may be reduced. The reliability of the interconnection between the sensor array and control electronics may also be improved relative to prior designs.

[0013] Other aspects, features, and implementations may become apparent in view of the detailed description and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is an electrical diagram of a multiplexed inductive tactile sensor having a 4x4 sensor array, drive circuitry, and sense circuitry.

[0015] Figure 2A 、 2B and 3 show an 8x8 multiplexed inductive tactile sensor having a deformable conductive shielding layer and a compressible dielectric layer.

[0016] Figure 4A 、 4B and 5 show a multiplexed inductive tactile sensor having parallel conductive shielding layers and parallel compressible dielectric layers disposed on opposite sides of an FPCB.

[0017] Figure 6A 、 6B and 7 show a multiplexed inductive tactile sensor having a compressible dielectric layer between a conductive target array and an inductive sensor array.

[0018] Figure 8A and 8B show a multiplexed inductive tactile sensor having a conductive target and parallel conductive shielding and parallel compressible dielectric layers disposed on opposite sides of an FPCB.

[0019] Figure 9 Shows a multiplexed inductive tactile sensor, where the drive coil array and the sense coil array are separated by a compressible dielectric layer.

[0020] Figure 10 Is a system diagram of a multiplexed inductive tactile sensor with associated electronic circuitry.

[0021] Figure 11A And 11B Shows a coil array design with rows and columns of coils connected in series.

[0022] Figure 12 Shows a 4x4 sensor array implemented on a double-sided PCB with tightly coupled drive and sense coils, and where adjacent coils are wound in opposite directions.

[0023] Figure 13 Shows a sensor array PCB design that uses both sides of the PCB for sense and drive coil traces.

[0024] Figure 14 Shows the Figure 13 Design trace patterns on the top and bottom sides of the PCB, respectively.

[0025] Figure 15A , 15B , 16A and 16B show a multiplexed inductive tactile sensor that can sense mechanical forces applied along three axes (x, y, z), where the normal force is along the z-axis and the surface forces are along the x-axis and y-axis, respectively. Detailed Description

[0026] Figure 1Is an electrical diagram of a multiplexed inductive tactile sensor for measuring the position and force of a touch applied by an external object. The illustrated multiplexed inductive tactile sensor includes a 4x4 inductive sensor array 50, a drive circuit 52, and a sensing circuit 54. The drive circuit 52 includes an AC current source (4) and a demultiplexer (5). The sensing circuit 54 includes a multiplexer (7), an AC amplifier (8), an amplitude demodulator (9), and an analog-to-digital converter (ADC) (10). The sensor array includes sixteen sensors (23) disposed on a double-sided or multi-layer flexible or rigid printed circuit board (PCB, FPCB) (1). Each sensor (23) in the array (50) has a drive coil (2) inductively coupled to a corresponding sensing coil (3). All the drive coils (2) in each column (shown vertically) are connected in series electrically and are driven by an AC constant current source (4) through an analog demultiplexer (5). All the sensing coils (3) in each row (shown horizontally) are connected in series electrically, and the inductive AC voltage across the row is fed to an AC amplifier (8) through an analog multiplexer (7). The amplified AC voltage is fed to an amplitude demodulator (9) to generate a DC signal that depends on the inductive coupling factor between the drive coil and the sensing coil of an individual sensor, which is selected by the intersection of an active current-driven column and an active sensing row. The excitation of the drive coils connected in series in the sensor column by the AC constant current source (4) helps to avoid interference and inaccuracies when forces are applied to multiple sensors in the same column. Additionally, the inductive AC voltage on the sensing coils connected in series in any row will be affected only by the force applied to a specific sensor located at the intersection (crossing point) of the active drive column and the current sensing row because the AC amplifier (8) has a very high input impedance, so only a very small current flows into the inductive coil loop. Using the demultiplexer and multiplexer, individual sensor arrays are selected in series, and the DC signal of the selected sensor is measured. The sensors can be selected individually in any order or pattern such that each sensor is selected during a scan cycle.

[0027] Figure 2A , 2BFigures 3 and 8 show an 8x8 inductive tactile sensor including a compressible dielectric layer (12) and a deformable conductive shielding layer (15). The compressible dielectric layer (12) is disposed between the sensor array (23) and the deformable conductive shielding layer (15). The sensor array is formed on one or both sides of the FPCB (1). In response to an external object, such as a human finger, touching the deformable conductive shielding layer (15) and applying a normal force thereto, the deformable conductive shielding layer (15) deforms in the contact area, and adjacent portions of the dielectric layer (12) are compressed. Accordingly, the distance between the conductive shielding layer (15) and the sensors (23) near the contact area decreases in response to the applied mechanical force. Due to the secondary magnetic field generated by the induced eddy currents in the conductive shield, the change in distance reduces the inductive coupling factor between the drive coil and the sense coils of the sensors near the contact area. Consequently, the magnitude of the inductive AC voltage in the sense coil circuit decreases, which is detected when the controller scans those sensors (in series during one scan period when the sensors are selected at the intersection of an active drive coil row and a sense coil column and the output is measured). The compressible dielectric layer (12) is elastic and returns to its pre-contact planar shape when the mechanical force is removed.

[0028] Reference Figure 1 and 10 , the tactile sensor array controller (30) detects the change in the AC voltage magnitude through the AC amplifier (8), the amplitude demodulator (9), and the analog-to-digital converter (10), and stores the measurement result in the memory location corresponding to the drive coil row and the sense coil column of the selected sensor. After completing the scan cycle through all drive coil rows and all sense coil columns, a two-dimensional matrix is formed, where each matrix element represents the mechanical force applied to the corresponding sensor at the corresponding row and column coordinates. The sequential or periodic scanning of the drive coil rows and the sense coil columns generates a framed two-dimensional digital matrix over time for real-time tactile sensing.

[0029] Figure 4A , 4B5 shows a multiplexed inductive tactile sensor having a first deformable conductive shield layer (15) and a second deformable conductive shield layer (25), wherein the first deformable conductive shield layer (15) and the second deformable conductive shield layer (25) are connected in parallel with a sensor (23) and an FPCB (1) disposed therebetween. A first compressible dielectric layer (12) is disposed between the sensor (23) and the first deformable conductive shield layer (15). A second compressible dielectric layer (22) is disposed between the sensor (23) and the second deformable conductive shield layer (25). The controller electronics are configured to measure an inductive AC voltage change caused by a change in an inductive coupling factor between a drive coil and a sensing coil of the sensor when an external object applies a local mechanical force to the first conductive shield layer (15) and the first compressible dielectric layer (12). When an external object (such as a human finger) touches and exerts force on the deformable conductive shielding layer (15), both compressible dielectric layers (12 and 22) are compressed, making both conductive shielding layers (15 and 25) closer to the sensor array on the FPCB (1).

[0030] Figure 6A , 6B 7 show a multiplexed inductive tactile sensor having an array of conductive targets (6) made of a conductive material or film disposed on a deformable dielectric film (11) separated from the sensors (23) and the FPCB (1) by a compressible dielectric layer (12). Each sensor (23) is associated with and covered by a corresponding target (6) having a contact area larger or approximately the same size as the sensor. When an external object (such as a human finger) contacts and applies a mechanical force to the surface of the target array, the compressible dielectric layer (12) compresses and one or more targets (6) at the contact area move closer to each corresponding sensor they cover respectively. The reduced distance between these targets and their corresponding sensors reduces the inductive coupling factor between the drive coils and the sensing coils of those sensors, which reduces the inductive AC voltage on the sensing coils. Detection of this AC voltage change to measure position and force can be accomplished as already described above.

[0031] Figure 8A and 8B An embodiment of a multiplexed inductive touch sensor having a conductive target (6) disposed on a dielectric film (11) is shown, and a sensor array (23) is formed on one or both sides of an FPCB (1). A first compressible dielectric layer (12) is disposed between the sensor (23) and the deformable dielectric film (11), and the target (6) is disposed thereon. A second compressible dielectric layer (22) is disposed between a conductive shielding layer (25) and the sensor (23).

[0032] Figure 9An embodiment is shown in which an array of drive coils (2) and sense coils (3) is separated by a compressible dielectric layer (12). The drive coils (2) and sense coils (3) in each sensor (23) are disposed on separate double-sided FPCBs (16 and 17). The two FPCBs are separated by a compressible dielectric layer (12). When an external object touches and applies a normal force on the surface of the FPCB (16 or 17), the compressible dielectric layer (12) will be compressed, such that one or more of the sense coils (3) move closer to their corresponding drive coils (2), and thus, the inductive coupling factor between the sense and drive coils of those affected sensors will increase. The inductive AC voltage on the sense coils will increase and be detected as the tactile sensor signal as described above.

[0033] Figure 10 is a system diagram of a multiplexed inductive tactile sensor with an associated electronic circuit. The tactile sensor array controller (30) controls the analog demultiplexer (5) to sequentially drive the sensor rows with an AC constant current source. For each activated row, the controller controls the analog multiplexer (7) to feed the sense signals (inductive AC voltages) from each sensor column to the amplifier (8) and amplitude demodulator (9) for processing by the ADC (10) and conversion to the digital domain. The converted data is stored in the controller's memory to form a two-dimensional matrix with the scanned rows and columns to be used as the x and y coordinates of the matrix. The matrix is a representation of the force distribution applied to the tactile sensor array. The data can be preprocessed by the controller and then transmitted wirelessly to the host computer of the application system via data communication means such as USB, I2C or via Bluetooth (BT) or WIFI.

[0034] Figure 11A and 11B shows an 8x8 array of sensors in which the adjacent (adjacent in row or column) coils for both the drive coils and sense coils are wound in opposite directions (clockwise or counterclockwise as indicated by the arrows). The alternating winding directions help to reduce the electromagnetic noise picked up from the environment by the series-connected drive coils and series-connected sense coils, especially for large tactile sensor arrays with many sensor rows and columns. The noise voltage pickup is substantially eliminated by varying the alternating winding directions.

[0035] Figure 12 shows an embodiment of a 4x4 array of sensors disposed on a double-sided PCB with closely coupled drive coils and sense coils, where all adjacent coils (adjacent in row or column) are wound in opposite directions. The coil traces are only on one side of the flexible or rigid PCB, while the other side of the PCB is only used for interconnection. However, the coils can be wound in the same direction (clockwise or counterclockwise).

[0036] Figure 13 A flat coil array design pattern is shown, which uses both the top and bottom sides of a PCB to run coil traces, with the coil winding directions of adjacent coils alternating in each row and column. This implementation helps to maximize the possible number of turns in a limited PCB space, with all adjacent coils wound in opposite directions.

[0037] Figure 14 Shows the Figure 13 patterns of the top-side traces and bottom-side traces of the coil array design on a double-sided PCB. By repeating the design pattern along the row and column directions, sensor arrays of any size can be designed.

[0038] Figure 15A 、 15B 、16A and 16B show multiplexed inductive tactile sensors that can sense the applied mechanical force in three axes (x, y, z), where the normal force is along the z-axis and the surface forces are along the x-axis and y-axis respectively. The 8-sided electrically conductive targets (20) are associated with a set of four sensors (23). An elastomeric layer (12) is provided between the targets (20) and the sensors (23). Each of the conductive targets (20) has a truncated square shape. The truncated corners allow movement in the x-axis and y-axis relative to adjacent targets. Each target (20) covers the four sensors (23) associated with the target. The target array is embedded in an elastomeric material layer (12), as Figure 6A shown. The top surface of the elastomeric layer (21) covering the conductive targets can be intentionally made rough to increase surface friction.

[0039] In the absence of an external force applied to the target, the target (20) covers half of each of the four associated sensors (23) labeled S1 to S4 (see the target drawn with a dashed line). When a force in the positive direction is applied to the target (20) along the z-axis and x-axis, the elastomeric layer (12) is compressed and distorted, and the target (20) moves distances dz and dx along the z-axis and x-axis respectively (see the target drawn with a solid line). Assume that V1, V2, V3, and V4 are the inductive AC voltages of sensors S1, S2, S3, and S4 respectively. The inductance voltage of each sensing coil can be written as follows:

[0040] V1 = k1(x,y,z).Vs = k(x,y,z).Vs (a)

[0041] V2 = k2(x,y,z).Vs = k(-x,y,z).Vs (b)

[0042] V3 = k3(x,y,z).Vs = k(-x,-y,z).Vs (c)

[0043] V4 = k4(x,y,z).Vs = k(x,-y,z).Vs (d)

[0044] Here, ki(x,y,z) and k(x,y,z) are inductive coupling factors, which are functions of the target position, and Vs is the AC voltage across the drive coil. By taking the differentials of these coupling factor functions with respect to x, y, and z, we have:

[0045]

[0046] Set dkx = (dk1 - dk2 - dk3 + dk4), dky = (dk1 + dk2 - dk3 - dk4), and dkz = (dk1 + dk2 + dk3 + dk4), and further define the following

[0047] We note that dVx is proportional to dx, dVy is proportional to dy, and dVz is proportional to dz. Let us further use the function x = g x (Fx) to describe the x-axis displacement x as a function of the force Fx applied in the x-axis direction, which is a characteristic of the elastomer used. From equation (i), we have:

[0048]

[0049] Similarly, we have:

[0050]

[0051] And

[0052]

[0053] Therefore, dVx, dVy, and dVz can be used as measurements of the triaxial forces dFx, dFy, and dFz.

[0054] A tactile sensor using the sensor array described above can sense forces on three axes, namely the normal (z-axis) and the surface (x-axis and y-axis).

[0055] Several features, aspects, embodiments, and implementations have been described. However, it will be understood that various modifications and combinations can be made without departing from the scope of the inventive concept described herein. Therefore, these modifications and combinations are within the scope of the appended claims.

Claims

1. A tactile sensor device, comprising: a plurality of inductive sensors arranged in an array including rows and columns, each of the sensors including a helically wound drive coil and a parallel helically wound sense coil, wherein the parallel helical windings of the drive coil and the sense coil of each sensor are characterized in that they are closely coupled and helically wound together in parallel; and they are helically wound in the same clockwise or counterclockwise direction; and they are disposed on a first side of a printed circuit board; the drive coils of the sensors in each column are connected in series, and the sense coils of the sensors in each row are connected in series; and the drive coils of adjacent sensors in each column are helically wound in opposite directions; and the drive coils of adjacent sensors in each row are helically wound in opposite directions; and the sense coils of adjacent sensors in each row are helically wound in opposite directions; and the sense coils of adjacent sensors in each column are helically wound in opposite directions; and a circuit configured to independently select a single sensor among the inductive sensors by activating the drive coil of the sensors in one of the columns and the sense coil of the sensors in one of the rows, wherein the sensor at the intersection of the activated column and row is selected, and the output of the selected sensor is measured by the circuit.

2. The tactile sensor device according to claim 1, wherein the circuit performs a scan by individually selecting and measuring the outputs of each of the sensors of the array in series.

3. The tactile sensor device according to claim 2, wherein the circuit generates a measurement matrix corresponding to the positions of the corresponding sensors in the array.

4. The tactile sensor device according to claim 3, wherein the circuit generates a time series of the measurement matrix corresponding to the positions of the corresponding sensors in the array.

5. The tactile sensor device according to claim 1, wherein the circuit includes a drive circuit connected to the drive coil of the sensor, and the drive circuit includes an AC current source and a demultiplexer.

6. The tactile sensor device according to claim 1, wherein the circuit includes a sense circuit connected to the sense coil of the sensor, and the sense circuit includes a multiplexer and an analog-to-digital converter.

7. The tactile sensor device according to claim 1, wherein the sensors are disposed on the printed circuit board, and a first compressible dielectric layer is disposed on the first side of the printed circuit board.

8. The tactile sensor device according to claim 7, comprising a first deformable conductive shielding layer disposed on the first compressible dielectric layer such that the first deformable conductive shielding layer is separated from the printed circuit board by the first compressible dielectric layer.

9. The tactile sensor device according to claim 8, comprising a second compressible dielectric layer disposed between a second side of the printed circuit board and a second deformable conductive shielding layer.

10. The tactile sensor device according to claim 7, comprising a conductive target disposed on a deformable dielectric film layer, the deformable dielectric film layer being disposed on a first compressible dielectric layer, and the first compressible dielectric layer being disposed on the first side of the printed circuit board.

11. The tactile sensor device according to claim 10, comprising a second compressible dielectric layer disposed between the second side of the printed circuit board and a deformable conductive shielding layer.

12. The tactile sensor device according to claim 1 wherein the windings of the drive coil and the sense coil of each sensor are helically wound parallel to each other in the same direction and are also disposed on the second side of the printed circuit board.

13. The tactile sensor device according to claim 10, wherein the sensors are arranged in groups, and wherein each conductive target partially covers a group of four sensors associated with the target, and the first compressible dielectric layer is an elastomer layer disposed between the sensors and the conductive target.

14. The tactile sensor device according to claim 13 wherein the circuit measures the forces applied on three axes based on the outputs of four sensors of a selected group of sensors.

15. The tactile sensor device according to claim 14 wherein each conductive target has a truncated square shape.

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