A tactile sensor with self-calibration function and direction detection

Abstract

The application discloses a tactile sensor with self-calibration function and direction detection. The application comprises a force detection module and a self-calibration module in the force detection module. The force detection module is two different types of silicon beams, which are a silicon beam for measuring z-axis stress and a silicon beam for measuring x / y-axis stress. The self-calibration module comprises piezoelectric material and electrodes; the piezoelectric material is located above the two different types of silicon beams respectively, and the inverse piezoelectric effect is used to drive the silicon beam to generate mechanical strain, so as to realize self-calibration of the tactile sensor. The shape of the silicon beam for measuring x / y-axis stress in the application is a circular arc instead of a traditional straight beam, which can generate greater mechanical strain under the same stress, thereby expanding the sensitivity of the tactile sensor, and the direction of the external force acting on the tactile sensor is detected through the trend of the output resistance change. The application can also calibrate and compensate errors of the tactile sensor through the self-calibration module.

Description

Technical Field

[0001] This invention relates to the field of tactile sensor technology and MEMS self-calibration devices, and more particularly to a tactile sensor with self-calibration function and directional detection. Background Technology

[0002] With the continuous development of artificial intelligence, robotics, and intelligent sensing technology, tactile sensors have broad prospects for application in robotics. Robots are no longer limited to a single working mode; people expect robotics technology to achieve complex human-computer interactions, simulate the sensory mechanisms of human skin, and realize diverse human behavioral patterns.

[0003] Based on their working principle, sensors can be classified into capacitive sensors, piezoresistive sensors, and piezoelectric sensors. Compared with other types of tactile sensors, piezoresistive tactile sensors have the advantages of simple structure, suitability for miniaturization, high sensitivity, strong anti-interference ability, and high frequency response. Piezoresistive sensors detect force signals by utilizing the change in resistance generated by the deformation of a piezoresistive silicon element. In robotics applications, piezoresistive silicon elements are typically embedded in elastic materials to provide soft contact with objects.

[0004] Tactile sensors are susceptible to interference from ambient temperature and noise, leading to reduced measurement accuracy and impacting robot performance. To ensure measurement reliability, devices are calibrated before use to guarantee accuracy. To address this issue, self-calibration technology has emerged in recent years to compensate for manufacturing defects, temperature drift, and aging, improving measurement accuracy in practical applications and enabling high-performance, high-precision tactile sensors. To achieve self-calibration, a combination of piezoelectric actuation and force sensors is typically used. Utilizing the inverse piezoelectric effect of piezoelectric materials, electrical signals are converted into mechanical strain. The tactile sensor output is compared to the piezoelectric actuation output, and the correlation is used to calibrate and compensate the tactile sensor.

[0005] In existing technologies, MEMS-based tactile sensors can basically meet the practical application needs of robotic hands, achieving high-sensitivity tactile sensors. Taking a MEMS-based tactile sensor proposed in Japan in 2016 as an example, a silicon beam is fabricated using a 20µm thick SOI wafer. Three types of piezoresistors, sensitive only to stress in their own direction, are formed by doping the surface and sidewalls of the silicon beam. This tactile sensor essentially achieves self-decoupling. Compared to human fingertip touch, while the above-mentioned tactile sensor can perceive tactile information of objects, there is still significant room for improvement in sensitivity, and it cannot determine the directionality of stress. Considering the complexity of the measurement environment and the high-precision requirements of robotic hands in practical applications, and addressing the limitations of traditional calibration platforms—which are large, immobile, complex to operate, labor-intensive, and prone to errors—a self-calibrating tactile sensor with directional detection is proposed. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a tactile sensor with self-calibration function and directional detection.

[0007] The present invention includes a force detection module and a self-calibration module located within the force detection module.

[0008] The force detection module consists of two different types of silicon beams: one for measuring z-axis stress and the other for measuring x / y-axis stress. The silicon beams for measuring x / y-axis stress are arranged orthogonally in the x / y-axis directions.

[0009] The middle part of the silicon beam used to measure x / y axis stress is arc-shaped in the x / y axis direction. When subjected to stress in a certain direction, mechanical strain is generated, and the resistance of the varistor increases / decreases. When subjected to stress in the opposite direction, mechanical strain is generated, but the change trend of the varistor resistance is opposite, thus determining the directionality of the stress.

[0010] The silicon beam used to measure z-axis stress is a straight beam. When subjected to stress in the z-axis direction, the mechanical strain generated is much greater than that in the x / y-axis direction, thus exhibiting decoupling capability.

[0011] The two different types of silicon beams generate mechanical strain under the action of external force. The mechanical strain converts the force signal into an electrical signal through the piezoresistor above the silicon beam, and is finally output through the amplification circuit.

[0012] The self-calibration module includes piezoelectric materials and electrodes; the piezoelectric materials are located above two different types of silicon beams, and the inverse piezoelectric effect is used to drive the silicon beams to generate mechanical strain in order to achieve self-calibration of the tactile sensor.

[0013] The beneficial effects of this invention are as follows: This tactile sensor, while meeting the requirements of small size and high precision, integrates a self-calibration function, including a force detection circuit module and a self-calibration module. In the force detection module, the silicon beam measuring x / y axis stress is arc-shaped rather than a traditional straight beam, which can generate greater mechanical strain under the same stress, thereby increasing the sensitivity of the tactile sensor. Furthermore, the direction of the external force acting on the tactile sensor is detected by the trend of the output resistance change. The self-calibration module can perform self-calibration testing on the tactile sensor. In cases where the tactile sensor ages or is affected by environmental factors, causing output drift, the self-calibration module can perform self-calibration and error compensation. Attached Figure Description

[0014] Figure 1 The above is a schematic diagram of the tactile sensor structure.

[0015] Figure 2 The image shows a schematic diagram of the self-calibration structure of a silicon beam used to measure z-axis stress in a tactile sensor.

[0016] Figure 3 This is a schematic diagram of the z-axis self-calibration electrode distribution.

[0017] Figure 4 This is a schematic diagram of the self-calibration structure of the silicon beam used to measure the x / y axis stress in the tactile sensor.

[0018] Figure 5 This is a schematic diagram of the x / y axis self-calibration electrode distribution.

[0019] Figure 6 This is a schematic diagram illustrating the principle of x / y axis directionality detection.

[0020] Figure 7 This is a flowchart of the manufacturing process for the self-calibration part of the tactile sensor. Detailed Implementation

[0021] The tactile sensor designed in this invention can detect the magnitude of three-dimensional forces, including z-axis stress, x-axis stress, and y-axis stress. The directionality of the stress can be determined by observing the resistance change trend of the silicon beam caused by mechanical strain. The silicon beam is driven using the inverse piezoelectric effect of lead zirconate titanate (PZT), and the output under external force is compared to determine whether the tactile sensor needs calibration. If calibration is required, the output is compensated, thereby achieving a self-calibration function.

[0022] The specific technical solution of this invention is as follows:

[0023] The tactile sensor includes a force detection module and a self-calibration module. The force detection module of the tactile sensor consists of two different types of silicon beams: a silicon beam for measuring z-axis stress and a silicon beam for measuring x / y-axis stress. The silicon beams for measuring x / y-axis stress are orthogonally arranged in the x / y-axis directions, and both types of silicon beams are fixed at both ends.

[0024] The middle part of the silicon beam used to measure x / y axis stress is arc-shaped in the x / y axis direction. When subjected to stress in a specific direction, it can generate a large mechanical strain. At this time, the resistance of the varistor increases / decreases. When subjected to opposite stress, the mechanical strain generated is basically the same, but the trend of the change in the resistance of the varistor is opposite, so the directionality of the stress can be determined.

[0025] The silicon beam used to measure z-axis stress is a traditional straight beam. When subjected to z-axis stress, the mechanical strain generated is much greater than that of x / y-axis stress, thus exhibiting good decoupling capability.

[0026] The two different types of silicon beams generate minute mechanical strain under the action of external force. The mechanical strain converts the force signal into an electrical signal through the piezoresistor above the silicon beam, and is finally output through the amplification circuit.

[0027] The self-calibration module in the tactile sensor comprises two parts: a piezoelectric material and electrodes. The piezoelectric material is positioned above two different types of silicon beams, utilizing the inverse piezoelectric effect to drive mechanical strain in the silicon beams, thereby achieving the self-calibration function of the tactile sensor. The external force is a z-axis stress, x-axis stress, or y-axis stress applied to the surface of the tactile sensor.

[0028] This invention employs a piezoresistive sensing principle. Phosphorus ions are doped onto the upper surface of the silicon beam measuring z-axis stress and the upper surface and sidewalls of the silicon beam measuring x / y-axis stress using a thermal diffusion method, forming a piezoresistive layer approximately 100 nm thick. The piezoresistors are connected via metal leads to form a Wheatstone bridge circuit, converting the force signal into an electrical signal output. To achieve the directional detection function of the tactile sensor, it is assumed that the tactile sensor is subjected to stress in the positive x / y-axis direction. In this case, the resistance change output of the bridge is positive; when the x / y-axis stress becomes negative, the resistance change output of the bridge becomes negative.

[0029] This invention utilizes the inverse piezoelectric effect of piezoelectric materials. By pre-polarizing the piezoelectric material, it can generate mechanical strain when an electric field is applied in a specific direction, and this mechanical strain disappears as the electric field disappears. The selected piezoelectric material is PZT, which has advantages such as high driving accuracy, low power consumption, and small size. It can be well integrated with tactile sensors to meet the practical application requirements of tactile sensors in robot hands and achieve self-calibration functionality.

[0030] This invention utilizes the piezoelectric actuation function of PZT to achieve self-calibration of a tactile sensor. After pre-polarization, when a voltage is applied to the entire upper electrode of the PZT film, the silicon beam will vibrate perpendicular to the PZT direction. Alternatively, when voltage is applied to specific regions of the upper electrode (e.g., two electrodes connected in parallel), applying voltages of the same magnitude but opposite signs to each electrode will cause the silicon beam to vibrate parallel to the PZT direction. The resistance change of the piezoresistor caused by PZT vibration is compared with the resistance change caused by external force. If the resistance changes are consistent, the device is functioning well and requires no calibration. If the resistance changes are inconsistent, the device is affected by external factors, resulting in errors. The tactile sensor can be compensated for by the relationship between force and output and voltage and output in the initial state, thus achieving the self-calibration function of the tactile sensor.

[0031] Furthermore, the silicon beams are of two types. One type is flat, 50 μm wide and 20 μm high, used to measure z-axis stress. The other type has a width much smaller than its thickness, 10 μm wide and 20 μm high, orthogonally arranged in the x / y axis directions, used to measure x / y axis stress. The silicon beam for measuring x / y axis stress is designed in an arc shape, which can generate a larger mechanical strain under the same stress. When facing stress in the opposite direction, the arc beam generates basically the same strain, but the output trend is completely opposite, thus enabling directional detection.

[0032] Furthermore, phosphorus ion doping is used to form varistors on the surfaces and sides of the two different types of silicon beams. An Au layer deposited on the surface of the tactile sensor is patterned and used as leads to connect the various varistors, forming a Wheatstone bridge circuit for detecting changes in resistance under external force. An insulating layer is deposited above the doped layer using plasma-enhanced chemical vapor deposition (PECVD) to insulate the doped layer from the Pt layer, ensuring the varistors are unaffected by the Pt electrode and providing conditions for PZT growth. A PZT thin film is grown on top of the two different types of silicon beams. The inverse piezoelectric effect of PZT enables the device's self-calibration function. A Pt electrode is grown on top of the PZT thin film; after patterning, different voltages are applied, allowing the beams to move in different directions under different voltage driving.

[0033] Example:

[0034] Figure 1This is a schematic diagram of the tactile sensor structure. Area A is the self-calibration module (the specific structure of this module is not shown, only its location within the tactile sensor is indicated). The tactile sensor only contains the force detection module; the self-calibration module is located in area A. This tactile sensor consists of four relatively independent detection units: two pairs of silicon beams for measuring z-axis stress, one silicon beam for measuring x-axis stress, and one silicon beam for measuring y-axis stress.

[0035] Figure 2 This diagram illustrates the self-calibration structure of a silicon beam for measuring z-axis stress in a tactile sensor. Area B shows a magnified cross-section of the silicon beam. The self-calibration module, consisting of PZT and upper and lower electrodes, sits atop the beam. 1a is a polyelastomer (PDMS), serving as the soft contact between the object and the tactile sensor. 2a, 2b, and 2c are the monocrystalline silicon portions of the tactile sensor. 2b and 2c form the silicon beam, 400µm long and 50µm wide. The PZT grown on top of the beam enables self-calibration. 2a forms the silicon substrate, fixing the ends of the beam and providing support for the chip. 3a and 3b are varistors, formed on the upper surface of the silicon beam through phosphorus ion doping. 4a, 4b, and 4c are insulating layers. 4b and 4c are the insulating layers between the varistor layer and the lower electrode in the self-calibration module. Their purpose is to ensure that the varistor is unaffected by the voltage applied by the PZT for device calibration, allowing for effective measurement of the varistor's resistance change under a specific voltage drive. 5a, 5b, 5c, and 5d are the driving electrodes of the PZT. 5a and 5d serve as the lower electrode of the PZT, grounded, while 5b and 5c serve as the upper electrode of the PZT, capable of connecting to a voltage range of 0-20V. 6a and 6b are PZT layers. Applying voltage to the upper and lower surfaces of the PZT drives it to move along the z-axis. Based on the relationship between the output resistance change and the applied voltage, the self-calibration function of the silicon beam for measuring z-axis stress can be achieved. 7a and 7b are the lead parts of the tactile sensor. The deposited metal layer is patterned, and the lead is connected to two varistors through a specific connection method. At the same time, two external fixed resistors are connected to form a Wheatstone bridge. The four resistors have the same resistance value. The ground and power supply are connected at the intersection of the varistor and the fixed resistor. The output is in the middle of the two varistors and the two fixed resistors, which can obtain the resistance change of the silicon beam under external force deformation.

[0036] Figure 3 The diagram shows the distribution of the self-calibrating electrodes along the z-axis. The self-calibrating module is located above the silicon beam used to measure z-axis stress. The length of the PZT film is half that of the silicon beam used to measure z-axis stress, and the width is the same as that of the silicon beam. Utilizing the inverse piezoelectric effect of PZT, the lower electrode of PZT is grounded, and a positive voltage is applied to the upper electrode of PZT. The piezoelectric material drives the silicon beam to move downward. At this time, the motion trend generated by the inverse piezoelectric effect of PZT is the same as the motion trend of the tactile sensor under z-axis stress.

[0037] Figure 4 This diagram illustrates the self-calibration structure of the silicon beam used to measure x / y axis stress in a tactile sensor. Area C shows a magnified cross-section of the silicon beam. Similarly, the self-calibration module is located above the silicon beam and consists of PZT and upper and lower electrodes. 1b is a polymorphic elastomer (PDMS), serving as a soft contact between the object and the tactile sensor. 2d, 2e, and 2f are the single-crystal silicon portions of the tactile sensor. 2e and 2f are the silicon beams, 350µm long and 10µm wide, with a circular arc shape in the middle. The mechanical strain trend generated by the silicon beams allows for the direction detection of stress in the tactile sensor. The circular arc beam design allows for greater mechanical strain under the same external force, thereby increasing the sensitivity of the tactile sensor. The PZT film grown on top enables the self-calibration function of the silicon beam. 2d is the silicon substrate, which fixes the two ends of the silicon beam and provides support for the chip. 3c and 3d are varistors, formed by phosphorus ion doping on the upper surface and sidewalls of the silicon beam. 4d, 4e, and 4f are insulating layers. 5e, 5f, 5g, 5h, and 5i are the driving electrodes of the PZT, with 5f and 5g serving as the lower electrode grounded, and 5e, 5h, and 5i serving as the upper electrode, capable of withstanding voltages from 0 to ±20V. In region C, the two Pt electrodes 5h and 5i divide the silicon beam into two parallel silicon beams, with the applied voltages on the two electrodes being of the same magnitude but opposite signs. 6c and 6d are PZT layers; applying voltage to the upper and lower surfaces of the PZT drives it to achieve translation in the x / y axis directions. The self-calibration function of the x / y axis beam can be achieved based on the relationship between the output resistance change and the applied voltage. 7c and 7d are the lead wires of the tactile sensor.

[0038] Figure 5 This is a schematic diagram of the x / y axis self-calibration electrode distribution. Area D is a schematic diagram of the cross-section of the silicon beam for measuring x / y axis stress. The self-calibration module is located above the silicon beam. The length of the PZT is half that of the silicon beam for measuring x / y axis stress, and the width is the same as that of the silicon beam for measuring x / y axis stress. Utilizing the inverse piezoelectric effect of the PZT, the lower electrode of the PZT is grounded, and the upper electrode of the PZT is evenly divided into two electrodes, which can be regarded as two parallel silicon beams. Applying voltages of the same magnitude but opposite signs to the two electrodes causes the piezoelectric material to drive the silicon beam to produce translation (x / y axis movement). At this time, the movement trend generated by the inverse piezoelectric effect of the PZT is the same as the movement trend of the tactile sensor under x / y axis stress.

[0039] Figure 6This diagram illustrates the principle of x / y-axis directionality detection. Regions E and F show the distribution of the piezoresistors. The working principle is as follows: When the tactile sensor is not subjected to external force, its output is 0, and the two piezoresistors have the same resistance. Assuming the tactile sensor is subjected to stress in the positive x / y direction, one silicon beam (from left to right) is compressed, decreasing its resistance, while the other is stretched, increasing its resistance. At this time, the voltage output of the bridge circuit decreases, and the total resistance change decreases. When the x / y-axis stress becomes negative, one silicon beam (from left to right) is stretched, increasing its resistance, while the other is compressed, decreasing its resistance. The voltage output of the bridge circuit increases, and the total resistance change also increases. In other words, the tactile sensor can determine the direction of the applied external force by observing the trend of the output change.

[0040] In this example, the self-calibration principle of the tactile sensor works as follows: When piezoelectric actuation is used to simulate external force, the silicon beam with the self-calibration module exhibits the same motion trend as under the corresponding external force. The deformation of the silicon beam under the external force is converted into a resistance change output by the piezoresistor through a bridge circuit. By comparing the two resistance changes, it is determined whether the tactile sensor has errors due to environmental or other external factors. Assuming the two resistance changes are different, the output of the tactile sensor can be compensated by the correspondence between voltage and resistance change in the initial state of the tactile sensor (self-calibration module) and the correspondence between force and resistance change (force detection module), thereby achieving the self-calibration function.

[0041] Figure 7 The fabrication process flow diagram for the silicon beam portion of a self-calibrating tactile sensor is shown below.

[0042] (a) Prepare a p-type SOI wafer with a top silicon layer thickness of 20 μm, a bottom silicon layer thickness of 300 μm, and an intermediate oxide layer thickness of 2 μm, and clean it thoroughly. Use KOH etching solution to thin the top silicon layer to a suitable thickness;

[0043] (b) Using plasma reactive ion etching (ICP-RIE), holes for sidewall doping are etched, with a diameter of approximately 70 μm.

[0044] (c) An n-type varistor is formed on the upper surface and sidewalls of SOI using a thermal diffusion method. The ion used is phosphorus ion, and the thickness of the doped layer is about 100 nm.

[0045] (d) A metal layer is deposited on the front side of the wafer to form a mask pattern, which is used as a tactile sensor lead. Au is selected as the deposited metal, and the thickness of the metal layer is about 50 nm.

[0046] (e) An insulating layer is deposited using PECVD to isolate the PZT electrode from the doped layer, and then a PZT sandwich structure is deposited in sequence, consisting of a lower electrode, PZT, and an upper electrode, wherein the metal used for the PZT electrode is Pt.

[0047] (f) The upper electrode, PZT and lower electrode are etched sequentially to form a self-calibrating structure that can be used for silicon beams, wherein the PZT and the lower electrode share a mask.

[0048] (g) The upper silicon layer is etched using ICP-RIE to form a beam structure, and the insulating layer and Au layer are etched at the same time to form leads, forming a Wheatstone bridge circuit.

[0049] (h) A second insulating layer is deposited on the top surface of the chip to bring out all the chip leads and PZT electrodes from the top layer.

[0050] (i) Etch through holes at the positions corresponding to the leads and electrodes in the insulating layer. The best shape for the through holes is circular, with a diameter of about a few micrometers.

[0051] (j) Deposit and pattern the top layer metal, fill the vias with the deposited metal, and electrically connect Au and Pt. The deposited metal is Au, and the metal layer thickness is approximately 50 nm.

[0052] (k) Deep ion reaction is used to etch the underlying silicon from the back side, and HF vapor is used to etch the silicon dioxide layer to release the beam structure;

[0053] In this example, the deposited metal layers are divided into Au and Pt layers. The Au layer can serve as the lead for the tactile sensor, while the Pt layer can only serve as the electrode for PZT. The tactile sensor chip is fabricated using an SOI wafer. The intermediate silicon oxide layer of the SOI wafer enables self-stopping DRIE etching, precisely achieving the dimensions of each sensitive part.

[0054] In summary, the selected tactile sensor features high sensitivity, miniaturization, and self-decoupling. The direction of the applied external force can be determined by observing the trend of the output change. To enhance the sensor's sensitivity, the x / y-axis silicon beam is designed with an arc shape, making it easier to generate mechanical strain under external forces. Furthermore, to meet the application requirements of tactile sensors in complex environments in the robotics field, a self-calibration function is added. This function uses the inverse piezoelectric effect of PZT to drive the silicon beam to generate mechanical strain, performing self-calibration of the tactile sensor and compensating for errors that may occur.

Claims

1. A tactile sensor with self-calibration function and directional detection, comprising a force detection module and a self-calibration module located within the force detection module, characterized in that: The force detection module consists of two different types of silicon beams: a silicon beam for measuring z-axis stress and a silicon beam for measuring x / y-axis stress, wherein the silicon beams for measuring x / y-axis stress are orthogonally arranged in the x / y-axis directions. The middle part of the silicon beam used to measure x / y axis stress is arc-shaped in the x / y axis direction. When subjected to stress in a certain direction, mechanical strain is generated, and the resistance of the varistor increases / decreases. When subjected to stress in the opposite direction, mechanical strain is generated, but the change trend of the varistor resistance is opposite, thus determining the directionality of the stress. The silicon beam used for measuring z-axis stress is a straight beam. When subjected to stress in the z-axis direction, the mechanical strain generated is much greater than that in the x / y-axis direction, thus exhibiting decoupling capability. The two different types of silicon beams generate mechanical strain under the action of external force. The mechanical strain converts the force signal into an electrical signal through the piezoresistor above the silicon beam, and is finally output through the amplification circuit. The self-calibration module includes piezoelectric materials and electrodes; the piezoelectric materials are located above two different types of silicon beams, and the inverse piezoelectric effect is used to drive the silicon beams to generate mechanical strain in order to achieve self-calibration of the tactile sensor.

2. A tactile sensor with self-calibration function and directional detection according to claim 1, characterized in that: Both types of silicon beams are fixed at both ends.

3. A tactile sensor with self-calibration function and directional detection according to claim 1, characterized in that: The piezoelectric material used is PZT.

4. A tactile sensor with self-calibration function and directional detection according to claim 3, characterized in that: The PZT on the silicon beam is in the form of a thin film, with an insulating layer between it and the silicon beam. The length of the PZT film is half that of the silicon beam, and the width is the same as that of the silicon beam.

5. A tactile sensor with self-calibration function and directional detection according to claim 1, 3, or 4, characterized in that: The electrodes include an upper electrode and a lower electrode. There is one upper electrode on the silicon beam for measuring z-axis stress and two upper electrodes on the silicon beam for measuring x / y-axis stress.

6. A tactile sensor with self-calibration function and directional detection according to claim 5, characterized in that: The voltages applied to the two upper electrodes on the silicon beam for measuring x / y axis stress are the same in magnitude but opposite in sign.

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