Multi-mode touch sensor based on cross array design

Abstract

The invention discloses a multi-mode tactile sensor based on cross-shaped array design, the sensor is provided with five sensing units which are arranged in a cross-shaped array form, and each sensing unit is composed of a stress loading layer, a piezoelectric sensing layer, an isolation layer, a piezoresistive sensing layer and an electrode layer. The area of a perceptible region of the sensor is increased due to array type distribution, and the spatial resolution of the sensor is improved. Wherein the piezoresistive sensing layer and the piezoelectric sensing layer respectively simulate the characteristics that an SA receptor in human skin senses low-frequency pressure and an FA receptor senses dynamic vibration. The piezoresistive sensing unit and the electrode unit form a planar resistance structure, so that the sensor is sensitive to the direction of a loading force, and the design can be used for representing the magnitude and direction of the loading force, thereby realizing three-dimensional static force detection. The piezoelectric sensing unit utilizes the direct piezoelectric effect of a PVDF piezoelectric film to realize tiny vibration detection and is used for representing high-frequency vibration when the piezoelectric sensing unit is in contact with an object.

Description

Technical Field

[0001] The present invention relates to the technical field of multimodal tactile sensors, and in particular to a multimodal tactile sensor based on a cross array design. Background Art

[0002] Touch is one of the five human senses. Humans acquire environmental information through touch and use this tactile information to better adapt to their surroundings. This information is primarily acquired through human skin, whose tactile perception is highly complex. This is reflected in its multi-input, multi-output nature, meaning that human skin can simultaneously perceive multiple stimuli and respond differently to each. This characteristic relies on the unique structure of the skin, which contains a rich array of mechanoreceptors. SA (solw adaptive) and FA (fast adaptive) mechanoreceptors can respond differently to different external stimuli. SA receptors are sensitive to static pressure, allowing them to detect continuous physical stimuli and perceive pressure distribution. In contrast, FA receptors respond to dynamic pressure or vibration and can be used for scenarios such as texture differentiation. Skin, composed of the epidermis, dermis, and subcutaneous tissue, possesses a certain degree of softness, enabling it to adapt to a variety of objects and surfaces. The texture distribution of the skin's surface makes it sensitive to friction in different directions, allowing it to perceive forces in multiple directions. Therefore, in order for tactile sensors to achieve complex perceptual characteristics similar to those of the skin, they need to be optimized in many aspects, including sensor structure design, material selection, signal processing, and data analysis, so as to improve the measurement accuracy of the sensor, more accurately feedback tactile information, and achieve a high degree of perception of the environment.

[0003] Although research on tactile sensors is relatively mature, limitations remain in many areas. For example, research on single-dimensional (normal force) tactile sensors is relatively mature. However, in order for a tactile sensor to detect actual contact force with an object, it must not only detect surface forces perpendicular to the surface but also horizontal tangential forces. Traditional rigid tactile sensors are inherently inflexible and inextensible, making them limited to detecting specific points or smooth surfaces and unable to operate in environments with unusual surface topography. In the selection of sensitive materials, the limitations of their physical properties can lead to poor performance in terms of sensitivity, detection range, and response speed. Material aging and susceptibility to environmental influences can also lead to poor repeatability and stability. Furthermore, most current tactile sensors can only detect a single stimulus type, and single-modality sensors may not be able to meet diverse requirements in complex environments. Therefore, designing a tactile sensor that not only exhibits excellent sensing performance but also excels in multiple aspects, such as detection range, modality, and dimension, is crucial to meet the demands of complex scenarios. Summary of the Invention

[0004] The purpose of the present invention is to improve the detection performance of a tactile sensor through structural design and material selection, and to overcome the various limitations of existing single-modal tactile sensors.

[0005] In order to achieve the above purpose, the specific technical solutions of the present invention are as follows:

[0006] A multimodal tactile sensor based on a cross array design. The sensor consists of five sensing units arranged in a cross array structure.

[0007] The perception unit consists of five layers, each layer is composed in order from top to bottom, among which,

[0008] The first layer is a stress loading layer, in which stress loading units are arranged in a cross-shaped array, with a total of five units. The stress loading units are prism-shaped protrusion structures;

[0009] The second layer is the piezoelectric sensing layer, using PVDF film with piezoelectric effect as the piezoelectric material to simulate the FA receptors in the skin of human fingertips that are sensitive to high-frequency vibrations. Double-sided conductive copper foil is attached to the upper and lower surfaces of the PVDF piezoelectric film as a conductive material, acting as a connecting wire. Five piezoelectric units are arranged directly below each stress loading unit.

[0010] The third layer is an isolation layer made of flexible and stretchable TPU material;

[0011] The fourth layer is the piezoresistive sensing layer, which is made of a composite film made by uniformly mixing MXene solution and bacterial cellulose. It has a piezoresistive effect and is used to simulate the characteristics of the SA receptors in the human fingertip skin to detect low-frequency pressure;

[0012] The fifth layer is the electrode layer, which consists of five electrode units. Each electrode unit consists of four semicircular electrodes of the same shape and size, which are axially symmetrically distributed above, below, left and right of the middle circular common electrode with the same spacing. They are distributed directly below the piezoresistive sensing layer, and each electrode unit is located directly below the stress loading unit. The electrode layer uses the four semicircular electrodes of the top electrode unit to share row and column lines with one semicircular electrode in the three electrode units in the second row, and the remaining unshared electrodes are connected by a separate lead design. This connection method reduces the number of leads and reduces the space occupied by the electrode layer to a certain extent, thereby reducing the overall size of the sensor.

[0013] Furthermore, the composite film was prepared by uniformly mixing 5 mg / mL MXene solution and bacterial cellulose (BC) dispersion with a solid content of 8% in a ratio of 2:1.

[0014] Furthermore, the composite film and a single electrode unit form a planar resistor structure. The structural design of the electrode unit makes the piezoresistive sensing unit sensitive to the direction of the loading force. The electrode unit outputs four voltage signals, and the cross-shaped array formed by the electrode units outputs 20 voltage signals for characterizing three-dimensional static force. The output signal of the piezoelectric unit outputs an electrical signal through a 0.28mm wire. The output signals of the five piezoelectric units of the sensor do not affect each other, characterizing the vibration generated when contacting an object, and the isolation layer is used to isolate the coupling interference between the piezoelectric sensing layer and the piezoresistive sensing layer, so that the output signals of the piezoresistive sensing layer and the piezoelectric sensing layer are independent of each other.

[0015] Furthermore, the sensor's piezoresistive sensing layer and electrode layer are bonded using conductive glue, and the TPU film encapsulates the upper and lower surfaces of the piezoelectric sensing layer; silicone adhesive is used to bond the upper surface of the piezoelectric sensing layer to the stress loading layer and the lower surface to the isolation layer, and finally, hot melt adhesive is used to seal the sensor edge.

[0016] Furthermore, the inclination angle of the pyramid of the stress loading layer is 25 degrees, and the ratio of the thickness of the pyramid protrusion on the stress loading layer to the thickness of the bottom base thereof is 4:1.

[0017] Furthermore, the radius of each electrode unit is 0.9 mm, the distance between the semicircular electrode and the middle circular common electrode is 0.75 mm, and the distance between adjacent electrode units is 1.3 mm.

[0018] Furthermore, the stress loading layer uses Sylgard 184 PDMS (Dow Corning) as the stress loading layer material. PDMS and curing agent are mixed in a mass ratio of 10:1. This ratio balances the elastic modulus and elongation at break, meeting the mechanical requirements of the flexible tactile sensor for the loading layer.

[0019] The multimodal tactile sensor based on a cross array design of the present invention has the following advantages over traditional tactile sensors:

[0020] (1) The present invention uses piezoresistive and piezoelectric materials to simulate SA and FA sensors, respectively, to achieve multimodal detection, enabling simultaneous detection of three-dimensional static force and vibration. Static force provides feedback on the pressure of the contacting object, while vibration detection can sense dynamic changes in the object, such as surface texture, object movement, or vibration characteristics, thereby enhancing comprehensive sensing capabilities. (2) The cross-shaped array distribution design enables detection over a larger area. The cross-shaped design conforms to the characteristics of the fingertip area and is easy to integrate into the fingertip, effectively covering multiple areas and improving the comprehensiveness and accuracy of detection.

[0021] (3) The piezoresistive sensing unit is a composite film made of MXene and BC. MXene, as a transition metal carbonitride material, has high electrical conductivity and excellent mechanical properties, and can effectively conduct and respond to external pressure changes. BC has good biocompatibility and flexibility, and can provide good mechanical support and toughening. The MXene and BC composite can combine the high electrical conductivity of MXene with the high strength and flexibility of BC, so that the composite material has excellent mechanical strength and flexibility while maintaining good electrical properties, and exhibits higher sensitivity and stability under pressure changes.

[0022] (4) The design of the semicircular electrode optimizes the electric field distribution, improves the uniformity, stability and directional sensitivity of the signal, and reduces noise interference.

[0023] (5) By reducing the number of independent traces and introducing a common bus structure, the electrode wiring is optimized. This electrode structure design reduces the number of signal lines. In addition, this design without flying wires can easily use other processing technologies (such as screen printing, photolithography, deposition, etc.) to make electrode layers on stretchable materials, making a more suitable tactile sensor.

[0024] (6) Compared with truncated cones, hemispherical structures, and other structures, the prism-shaped loading unit can better guide the external pressure distribution along the loading layer, thereby avoiding excessive local stress concentration. This uniform distribution helps improve the accuracy and stability of the sensor's response to force or pressure.

[0025] (7) The inclination angle of the prism of the stress loading layer is set to 25 degrees, and the ratio of the prism protrusion thickness to the lower base thickness is 4:1. The edge of the prism can achieve more concentrated force transmission, and has better mechanical strength after actual testing. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 An exploded view of a multimodal tactile sensor array structure according to an embodiment of the present invention;

[0027] Figure 2 Schematic diagram of the equivalent of a single piezoresistive sensing unit of a multimodal tactile sensor in an embodiment of the present invention;

[0028] Figure 3 (a) is a schematic diagram of a single stress loading unit of a multimodal tactile sensor without loading force according to an embodiment of the present invention;

[0029] Figure 3 (b) is a schematic diagram of a normal force applied to a single stress loading unit of a multimodal tactile sensor according to an embodiment of the present invention;

[0030] Figure 3(c) is a schematic diagram of a single stress loading unit of a multimodal tactile sensor applying a Y-axis tangential force in an embodiment of the present invention;

[0031] Figure 4 This figure shows the changes in the four equivalent resistances of a single stress loading unit of the multimodal tactile sensor in an embodiment of the present invention when forces in the directions of +X, -X, +Y, -Y, +Z, and -Z are applied;

[0032] Figure 5 This is a schematic diagram of the positive piezoelectric effect principle of the piezoelectric sensing unit of the multimodal tactile sensor in an embodiment of the present invention;

[0033] Figure 6 Schematic diagram of the lead connection of the electrode layer of the multimodal tactile sensor in an embodiment of the present invention;

[0034] Figure 7 Schematic diagram of the output signal measurement principle of a single piezoresistive sensing unit of a multimodal tactile sensor in an embodiment of the present invention;

[0035] Explanation of the marks in the figure: 1. Stress loading layer; 2. Piezoelectric sensing layer; 3. Isolation layer; 4. Piezoresistive sensing layer; 5. Electrode layer. DETAILED DESCRIPTION

[0036] In order to gain a deeper understanding of the design structure, principle, and function of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific implementation methods.

[0037] The sensor of the present invention is composed of five sensing units, which are arranged in a cross-shaped array structure; the sensing unit comprises a five-layer structure, namely a stress loading layer 1, a piezoelectric sensing layer 2, an isolation layer 3, a piezoresistive sensing layer 4, and an electrode layer 5. Figure 1 , each layer is arranged in order from top to bottom, among which,

[0038] The first layer is the stress loading layer 1. The stress loading units are arranged in a cross-shaped array. There are five units in total. The stress loading unit is a prism-shaped protrusion structure. The inclination angle of the prism-shaped protrusion of the stress loading layer 1 is set to 25 degrees. The ratio of the thickness of the prism-shaped protrusion to the thickness of the lower base is 4:1. This design makes the conduction force of the prism edge more concentrated, and has better mechanical strength after actual testing.

[0039] When the stress loading layer 1 is subjected to stress, the piezoelectric sensing layer and the isolation layer 3 will move in the direction of the loading force. At this time, the piezoresistive sensing layer 4 will deform with the stress, and the plane resistance formed by the piezoresistive sensing layer 4 and the electrode layer 5 will change accordingly. The structural design of a single electrode unit is equivalent to four piezoresistors R1, R2, R3, and R4, as shown in Figure 2. Figure 2 As shown, the four electrodes are sensitive to forces in different directions, and the force components in each direction are calculated by the difference between the output signals. Figure 3 ,When loading forces in different directions are applied to the loading unit, the loading layer will experience different displacements and deformations;

[0040] When the sensor is in the initial state without any external force, such as Figure 3 As shown in (a), since no pressure is applied, the pressure-sensitive material does not deform, and since the sensor electrode structure is designed to be symmetrically distributed, the four equivalent resistance values ​​remain unchanged;

[0041] When the sensor is subjected to a normal force in the vertical downward direction, such as Figure 3 As shown in (b), the stress loading layer 1 is slightly displaced downward as a whole. Due to the symmetrical distribution design of the electrodes, the piezoresistive materials at the four electrode positions are subjected to the same loading force, so the resistance values ​​of the four equivalent resistors decrease by the same amount;

[0042] Figure 3 (c) is a schematic diagram of a horizontal tangential force applied to the sensor surface. The tangential force squeezes the stress-loading layer 1 in the direction of the tangential force and produces a slight displacement. At this time, the pressure-sensitive material is subjected to a spatial force, which causes the pressure-sensitive material of the sensor to produce different deformations in different directions. The pressure gradually increases from the negative direction to the positive direction of the y-axis. Therefore, the resistance value of R2 changes the least. Due to the symmetrical distribution, the resistance values ​​of R1 and R3 decrease by the same amount, and the resistance value change is greater than that of R2. The pressure-sensitive material at R4 is subjected to the largest loading force, so the resistance value of R4 changes the most.

[0043] refer to Figure 4 , lists the changes of four equivalent resistances when loading +X, -X, +Y, -Y, +Z, -Z direction forces respectively, R min R is the resistance value when the applied stress is large. mid R is the resistance value when the applied stress is small. max is the resistance value when no stress is applied. The four equivalent resistances change in the other loading directions in a similar manner. When an external force acts on the sensor, the planar resistance formed by the four electrodes and the piezoresistive material will produce different responses depending on the magnitude and direction of the loading force.

[0044] refer to Figure 6The electrode layer 5 uses four semicircular electrodes in the top electrode unit to share row and column lines with one semicircular electrode in the second row of three electrode units, and the remaining unshared electrodes are connected by a separate lead design method, so as to reduce the number of leads within a limited space. The electrode layer 5 is processed by printing the metal material into a pre-designed pattern. When the piezoresistive material is subjected to pressure, it contacts the bottom electrode layer 5 to form a planar resistance structure, and the resistance changes with the change of pressure. The metal leads of the semicircular and circular electrodes are used to transmit electrical signals. The resistance change is converted into a voltage change through the measurement circuit, thereby reflecting the actual applied pressure. Its specific measurement principle is referenced in Figure 7 The piezoresistive sensing unit is equivalent to four resistors and a common potential terminal. The common potential terminal is connected to a 5V voltage. The four output resistors are connected to the analog input port of the data acquisition card. The resistance of each piezoresistive unit is connected in series with a high-precision voltage divider resistor. To ensure measurement accuracy, the resistance value of the voltage divider resistor should match the resistance of each piezoresistive unit. When a 5V voltage is applied, ensure that the voltage across the voltage divider resistor is large enough so that the subsequent circuit can accurately detect the voltage. After the voltage is divided by the voltage divider resistor, a piezoresistive unit outputs four voltage signals. The entire piezoresistive array unit can output 20 voltage signals.

[0045] The piezoelectric sensing layer 2 detects vibration based on the positive piezoelectric effect of PVDF material. Figure 5 When a piezoelectric material is deformed by an external force, the centers of positive and negative charges within the material shift relative to each other, resulting in a distribution of equal charges of opposite polarity on the upper and lower surfaces. The amount of charge released by the piezoelectric material is proportional to the magnitude of the external force F; a greater F results in a greater charge. Therefore, the direct piezoelectric effect can be used to convert dynamic force into an electrical signal, and the magnitude of the applied force can be inferred by measuring the output voltage. This makes it suitable for detecting rapidly changing and high-frequency forces.

[0046] The piezoelectric sensing layer 2 uses a PVDF film with a piezoelectric effect as a piezoelectric material. Double-sided conductive copper foil is attached to the upper and lower surfaces of the PVDF piezoelectric film as a conductive material, which acts as a connecting wire. A wire with a diameter of 0.28 mm is used as a piezoelectric signal output signal wire. The upper and lower surfaces of each piezoelectric material are connected by conductive copper foil. The two wires of each unit are output as the output signal of the charge conversion and amplification circuit. The weak charge signal generated by the piezoelectric unit is converted into a voltage signal and the voltage is amplified by a certain multiple through filtering and other operations to finally obtain a measurable voltage signal. The output signals of the five piezoelectric units do not affect each other. Due to the effect of the isolation layer 3, the output signals of the piezoelectric unit and the piezoresistive unit are independent of each other. Each piezoelectric sensing unit is arranged directly below each stress loading unit, and the entire piezoelectric array can output five differential piezoelectric signals;

[0047] The multimodal tactile sensor based on a cross-shaped array design outputs piezoresistive signals using the electrode layer 5 to transmit electrical signals, while the piezoelectric signals are transmitted through wires. The piezoresistive and piezoelectric transmission circuits are independent of each other, and an isolation layer is used to block the influence between the piezoresistive and piezoelectric sensing layers.

[0048] The piezoresistive sensing layer 4 is made by uniformly mixing a 5 mg / mL MXene solution with an 8% solid content BC dispersion in a 2:1 ratio. This ratio exhibits good performance in terms of detection sensitivity, stability, mechanical strength, and other properties.

[0049] Stress loading layer 1 uses Sylgard 184 PDMS (Dow Corning) as the stress loading layer material. PDMS and curing agent are mixed in a mass ratio of 10:1. This ratio balances the elastic modulus and elongation at break, meeting the mechanical requirements of the flexible tactile sensor for the loading layer.

[0050] The radius of each electrode is 1mm, the spacing is 0.75mm, the spacing between adjacent electrode units is 1.3mm, and each piezoelectric unit is distributed directly below each loading unit with a spacing of 4.5mm. The thickness of the piezoelectric film is 28μm, the height of the stress loading layer prism is 2mm, the thickness of the prism bottom is 0.5mm, and the overall size of the sensor is 2.79cm×2.79cm. The response time of the piezoresistive unit is 55ms, and the recovery time is 20ms. The response time of the piezoelectric unit is less than 10ms, and there is slight oscillation in the recovery stage.

[0051] The embodiments described above are only preferred embodiments of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them. Those skilled in the art should understand that, based on the technical principles and core designs disclosed in the present invention, any non-substantial modifications or equivalent replacements such as material replacement, structural deformation, parameter adjustment, etc. in the specific implementation manner within the scope of protection defined by the claims of the present invention, all fall within the scope of protection of the present invention. The scope of protection of the present invention is not limited by the specific details described in the embodiments, and any simple modifications, equivalent changes and modifications to the embodiments in accordance with the technical essence of the present invention should be covered within the scope of protection of the claims of the present invention. If the relevant technical solutions do not deviate from the design purpose and technical principles of the present invention, and can achieve the same or similar technical effects, no matter what means are used to implement them, they shall be deemed to fall within the scope of protection of the present invention.

Claims

1. A multimodal tactile sensor based on a cross array design, characterized in that: The sensor consists of five sensing units arranged in a cross-shaped array structure; The perception unit consists of five layers, each layer is composed in order from top to bottom, among which, The first layer is a stress loading layer (1), wherein the stress loading units are arranged in a cross-shaped array, and there are five units in total. The stress loading units are prism-shaped protrusion structures; The second layer is a piezoelectric sensing layer (2), which uses a PVDF film with a piezoelectric effect as a piezoelectric material to simulate the characteristics of the FA receptors in the skin of the human fingertips that are sensitive to high-frequency vibrations; double-sided conductive copper foils are attached to the upper and lower surfaces of the PVDF piezoelectric film as conductive materials, acting as connecting wires, and five piezoelectric units are arranged directly below each stress loading unit; The third layer is an isolation layer (3), which is made of a flexible and stretchable TPU material; The fourth layer is a piezoresistive sensing layer (4), which is made of a composite film made by uniformly mixing MXene solution and bacterial cellulose, and has a piezoresistive effect, which is used to simulate the characteristics of the SA receptor in the human fingertip skin to detect low-frequency pressure; The fifth layer is the electrode layer (5), which is composed of five electrode units. Each electrode unit is composed of four semicircular electrodes of the same shape and size, which are axially symmetrically distributed at the upper, lower, left and right positions of the middle circular common electrode with the same spacing, and are distributed directly below the piezoresistive sensing layer (4). Each electrode unit is located directly below the stress loading unit. The electrode layer (5) uses a design method in which the four semicircular electrodes of the top electrode unit share row and column lines with one semicircular electrode of the three electrode units in the second row, and the remaining unshared electrodes are connected by separate leads.

2. The multimodal tactile sensor based on a cross array design according to claim 1, characterized in that: The composite film was made by uniformly mixing 5 mg / mL MXene solution and 8% solid content bacterial cellulose dispersion in a ratio of 2:

1.

3. The multimodal tactile sensor based on a cross array design according to claim 1, characterized in that: The composite film and a single electrode unit form a planar resistor structure. The electrode unit outputs four voltage signals. The cross-shaped array of electrode units outputs twenty voltage signals for characterizing three-dimensional static force. The output signal of the piezoelectric unit is output as an electrical signal through a wire. The output signals of the five piezoelectric units of the sensor do not affect each other and represent the vibration generated when contacting an object. The isolation layer (3) is used to isolate the coupling interference between the piezoelectric sensing layer (2) and the piezoresistive sensing layer (4), so that the output signals of the piezoresistive sensing layer (4) and the piezoelectric sensing layer (2) are independent of each other.

4. The multimodal tactile sensor based on a cross array design according to claim 1, characterized in that: The sensor's piezoresistive sensing layer (4) and electrode layer (5) are bonded using conductive adhesive, and a TPU film encapsulates the upper and lower surfaces of the piezoelectric sensing layer (2); a silicone adhesive is used to bond the upper surface of the piezoelectric sensing layer (2) to the stress loading layer (1), and the lower surface to the isolation layer (3); and finally, a hot melt adhesive is used to seal the sensor edge.

5. The multimodal tactile sensor based on a cross array design according to claim 1, characterized in that: The prism inclination angle of the stress loading layer (1) is 25 degrees, and the ratio of the prism protrusion thickness on the stress loading layer (1) to the bottom base thickness thereof is 4:

1.

6. The multimodal tactile sensor based on a cross array design according to claim 1, characterized in that: The radius of each electrode unit is 0.9 mm, the distance between the semicircular electrode and the middle circular common electrode is 0.75 mm, and the distance between adjacent electrode units is 1.3 mm.

7. The multimodal tactile sensor based on a cross array design according to claim 1, characterized in that: The stress loading layer (1) uses Sylgard 184 type PDMS as the stress loading layer material, and the PDMS and the curing agent are mixed in a mass ratio of 10:1.

Images