A flexible six-axis force sensor based on piezoelectric sensors

Abstract

The application discloses a flexible six-dimensional force sensor based on a piezoelectric sensor. The six-dimensional force sensor comprises a stepped circular table, a base and a pressure sensor; the stepped circular table is composed of coaxially arranged upper and lower circular tables, and the diameter of the lower circular table is larger than that of the upper circular table; the lower surface of the lower circular table is in butt joint with the upper surface of the base to form a circular table structure, the upper surface of the base is provided with a cross beam boss, and the lower surface structure of the lower circular table is matched with the cross beam boss; a plurality of pressure sensors are arranged at the butt joint of the stepped circular table and the base. The piezoelectric sensor is embedded in the space in the three-axis direction by using flexible silica gel material, the signals of the embedded piezoelectric sensors in different directions of each node are collected, the real-time measurement of six-dimensional force is realized, the whole sensor is made of flexible material, the traditional rigid material is abandoned, the application occasions are widened, and the sensor can be more closely attached to the human skin as a wearable sensor.

Description

Technical Field

[0001] This invention belongs to the field of sensors, specifically relating to a flexible six-dimensional force sensor based on a piezoelectric sensor. Background Technology

[0002] With the development of artificial intelligence and wearable electronics, the demand for highly sensitive, high-resolution, and highly reliable physical sensors is increasing. Different types of wearable sensors have been developed to meet these diverse needs, including strain sensors, pressure sensors, and temperature sensors. These sensors convert various mechanical movements or temperature variations into detectable signal changes, making them suitable for personal health monitoring, human-computer interaction, and wearable devices. An ideal physical sensor should be able to distinguishably detect multi-axis motion without interfering with external variables.

[0003] Piezoelectric sensors are sensors that can convert transient mechanical energy into electrical energy, playing an important role in detecting dynamic stimuli such as vibration, sound waves, and human movement.

[0004] A six-dimensional force sensor is a sensor that measures pressure components in three directions (X, Y, and Z) and torque components in three directions. Most existing six-dimensional force sensors are rigid beam-type frame structures, with pressure and torque sensors installed at various nodes in different directions. However, these sensors are mostly cast from rigid materials during manufacturing, requiring high precision during installation and thus having significant limitations. Summary of the Invention

[0005] To address the issues of high stiffness, demanding installation requirements, and limited application of existing six-dimensional force sensors, this invention provides a flexible six-dimensional force sensor based on piezoelectric sensors. It employs flexible silicone material to embed piezoelectric sensors in three spatial axes. By collecting signals from the pre-embedded piezoelectric sensors in different directions at each node, real-time measurement of six-dimensional force is achieved. The sensor as a whole is made of flexible material, abandoning traditional rigid materials, thus broadening its application scenarios and making it a more wearable sensor that fits snugly against the human skin.

[0006] The present invention proposes a flexible six-dimensional force sensor based on a piezoelectric sensor. The entire sensor is made of flexible silicone material and includes a stepped frustum and a crossbeam base. Pressure sensors are respectively disposed on the upper surface and opposite sides of the crossbeam base. The pressure sensors are piezoelectric sensors, which are made by doping barium titanate (BaTiO3, BTO) nanoparticles, an inorganic piezoelectric material, with dopamine modification and encapsulation into a piezoelectric polymer P (VDF-TrFE). A flexible composite piezoelectric film is then prepared by a casting method. Gold electrodes are deposited on the upper and lower surfaces of the film using an electrode evaporation process. The gold electrodes on the upper and lower surfaces are connected to an external detection device through copper wires.

[0007] I. A Flexible Six-Dimensional Force Sensor Based on Piezoelectric Sensors

[0008] It includes a stepped frustum, a base, and pressure sensors. The stepped frustum consists of an upper frustum and a lower frustum arranged coaxially, with the lower frustum having a larger diameter than the upper frustum. The lower surface of the lower frustum and the upper surface of the base are joined together to form a frustum structure. The upper surface of the base is provided with a crossbeam boss, and the structure of the lower surface of the lower frustum is adapted to the crossbeam boss. Multiple pressure sensors are provided at the joint between the stepped frustum and the base.

[0009] Three pressure sensors are installed on each of the four arms of the cross beam boss. The three pressure sensors are located on the top surface and two sides of each arm of the cross beam, respectively.

[0010] The upper truncated cone has a radially extending channel that passes through it; a stainless steel wire passes through the channel, and the outer diameter of the stainless steel wire is fitted with the inner diameter of the channel with a clearance.

[0011] The pressure sensor is a piezoelectric sensor used to convert external stimuli into collectable voltage signals; the piezoelectric sensor is obtained by depositing electrodes on a piezoelectric composite film and then encapsulating it.

[0012] The process of using vapor deposition electrodes for piezoelectric composite thin films involves depositing gold electrodes on the upper and lower surfaces of the film, respectively, and connecting the gold electrodes on the upper and lower surfaces to external testing equipment via copper wires.

[0013] Dopamine-modified encapsulated inorganic piezoelectric material (BTO nanoparticles) was incorporated into piezoelectric polymer P (VDF-TrFE), and then a piezoelectric composite film was prepared by the casting method.

[0014] P(VDF-TrFE) is a copolymer of VDF (polyvinylidene fluoride) and trifluoroethylene (TrFE) and has excellent ferroelectric and electrostatic properties.

[0015] The piezoelectric composite film is prepared by:

[0016] Step 1) Disperse 100 nm diameter inorganic piezoelectric material barium titanate (BaTiO3, BTO) nanoparticles in a 0.01 mol·L⁻¹ solution. -1 After ultrasonic stirring in a dopamine aqueous solution for 1 hour, the mixture is uniformly coated with the dopamine solution and then dried in an oven at 70°C to obtain modified BTO powder.

[0017] Step 2) The modified BTO powder is added to the organic solvent N,N-dimethylformamide (DMF), and ultrasonicated for 1 hour to disperse BTO evenly in the solvent to obtain a BTO solution.

[0018] Step 3) Dissolve the piezoelectric polymer P (VDF-TrFE) powder in DMF and mix it with the BTO solution from Step 2). After magnetic stirring for 2 hours, a stable suspension is formed.

[0019] Step 4) Pour the suspension into a 9cm diameter petri dish and place it in an 80℃ oven for 12 hours to evaporate the solvent;

[0020] Step 5) Finally, anneal at 120℃ for 2 hours to remove residual solvent and improve crystallinity, to obtain a BTO / P (VDF-TrFE) piezoelectric composite film with a thickness of 20-30μm.

[0021] II. Spatial Six-Dimensional Force Measurement Method Using Flexible Six-Dimensional Force Sensors

[0022] Method 1:

[0023] 1) Calibration of a flexible six-dimensional force sensor:

[0024] A spatial six-dimensional force is applied to a flexible six-dimensional force sensor as a calibration force. The calibration voltage output by each piezoelectric sensor is collected using a data acquisition card, and a mapping matrix between the spatial six-dimensional force and the calibration voltage output by all piezoelectric sensors is constructed.

[0025] 2) When the flexible six-dimensional force sensor is subjected to pressure and torsional torque, the overall deformation of the flexible six-dimensional force sensor is converted into a voltage change signal of the piezoelectric sensor for output; the magnitude of the six-dimensional force on the flexible six-dimensional force sensor is calculated by back-calculating the mapping relationship matrix and the output voltage signal.

[0026] Method 2:

[0027] 1) Constructing a DNN deep neural network: Apply a spatial six-dimensional force as a calibration force to a flexible six-dimensional force sensor, and use a data acquisition card to collect the output calibration voltage of each piezoelectric sensor; use the applied six-dimensional force and the output calibration voltage signal as a sample set, divide the sample set into a training set and a test set, and input them into the DNN deep neural network for training to obtain the trained DNN deep neural network;

[0028] 2) When the flexible six-dimensional force sensor is subjected to pressure and torsional torque, the overall deformation of the flexible six-dimensional force sensor is converted into a voltage change signal of the piezoelectric sensor for output; the output voltage signal is input into the trained DNN deep neural network, and the magnitude and direction of the six-dimensional force on the flexible six-dimensional force sensor are obtained from the output.

[0029] The beneficial effects of this invention are:

[0030] The advantages of choosing a piezoelectric sensor as the sensing unit in this invention are: (1) it can detect dynamic stimuli in real time; (2) it is a passive element; the piezoelectric sensor is embedded between flexible silicone bumps, which eliminates the traditional rigid materials, reduces the overall size of the sensor, and uses the deformation of the bumps caused by pressure and torsional torque to convert it into a voltage change signal of the piezoelectric sensor for output. It can collect dynamic stimulation signals, has a simple structure, low cost, and a wide range of applications.

[0031] Compared to ordinary pressure sensors that can only display single-dimensional pressure signals and cannot explain the influence of forces and torques in other dimensions of space, this invention, through a DNN-decoupled sensor, can accurately identify the magnitude and direction information of six-dimensional forces in space. Attached Figure Description

[0032] Figure 1 This is a structural diagram of the flexible six-dimensional force sensor of the present invention.

[0033] Figure 2 This is a front view of the flexible six-dimensional force sensor of the present invention;

[0034] Figure 3 This is a side view of the flexible six-dimensional force sensor of the present invention;

[0035] Figure 4 A schematic diagram of a flexible base with twelve piezoelectric sensors attached;

[0036] Figure 5 A physical model of the voltage formation mechanism of a piezoelectric sensor;

[0037] Figure 6 A comparison graph showing the signal changes of the piezoelectric sensor output voltage at different frequencies;

[0038] Figure 7 This is a graph showing the signal variation of the piezoelectric sensor's output voltage under different pressures at the same frequency.

[0039] In the figure: stepped frustum (1), base (2), pressure sensor (3). Detailed Implementation

[0040] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0041] like Figure 1 , Figure 2 and Figure 3As shown. This invention includes a stepped silicone frustum 1, a crossbeam silicone base 2, and a pressure sensor 3. The upper part of the stepped silicone frustum 1 has a small square hole with a side length of 0.4 mm. The purpose of this small square hole is to allow the addition of a square stainless steel wire, or to customize its size to match the required components. The load is applied to the entire sensor through the stainless steel wire or the added component. Its advantage lies in integrating the load-applying device with the sensor, greatly reducing the overall size of the sensor and eliminating the complex operation of external bushings.

[0042] The pressure sensor 3 is a piezoelectric sensor that can convert external stimuli into collectable voltage signals. It is obtained by encapsulating a self-made piezoelectric thin film. The entire six-dimensional force sensor encapsulates twelve pressure sensors, which are respectively embedded on opposite sides of the silicone base, parallel and aligned with each other.

[0043] Piezoelectric sensors are obtained by using evaporation electrode deposition and encapsulation processes on piezoelectric thin films. In order to match the twelve piezoelectric sensing elements with the stepped frustum and cross beam base, they are cut into small cubes of 0.8mm x 0.8mm. Electrodes are deposited on each small cube individually to ensure that they do not interfere with each other. The twelve small cubes are arranged in a spatial arrangement at the node positions of the cross beam base as sensing units.

[0044] The stepped silicone frustum 1 serves as the force-bearing point. External loads are transmitted to the entire six-dimensional force sensor through the connector at the small square hole on its upper part. The cross-beam silicone base 2 acts as the constraint end. After the silicone frustum is stimulated by external forces, the external force or torque is transmitted to the pressure sensor. The piezoelectric sensor generates a voltage signal change due to the compression between the flexible silicone frustum and the silicone base.

[0045] like Figure 4 As shown, twelve piezoelectric sensors are spatially arranged and bonded to a flexible silicone base with silicone adhesive. The base is divided into four symmetrical parts. The signal changes detected by the four sensors on the upper surface correspond to the magnitude of the external positive pressure. The eight sensors on the sides will generate different numbers of signal changes depending on the external stimuli.

[0046] To effectively improve the actual measurement accuracy of the sensor, a static calibration experiment is required before testing. Sensor calibration involves applying external forces and torques to a force-measuring platform, acquiring the corresponding measurement branch voltages using a data acquisition card, and transmitting the data to a computer. After data processing, a mapping matrix is ​​obtained between the applied six-dimensional force and the measurement branch output voltages. Based on this mapping matrix, the actual six-dimensional force value can be calculated. A certain deviation will exist between this value and the calibrated force; the magnitude of this deviation reflects the actual measurement accuracy of the sensor.

[0047] This invention utilizes a deep neural network (DNN) to decouple a six-dimensional force sensor. Because the six-dimensional force input and output voltage signal of the sensor are affected by external environmental and internal sensor factors, the sensor system is nonlinear. To reduce actual measurement errors, a nonlinear fitting method (DNN) is used to establish a nonlinear mapping relationship between the nonlinear input and output. The main steps of establishing the neural network are as follows: 1) Define the model structure; 2) Initialize the model parameters; 3) Perform forward and backward propagation steps to learn the parameters.

[0048] The piezoelectric sensor of this invention is a passive sensor; it generates its own electricity and does not require an external power source. The sensitive element of the piezoelectric sensor is made of a piezoelectric material, which generates a charge on its surface when subjected to force. This charge is amplified by a charge amplifier and a measuring circuit, and after impedance transformation, it becomes an electrical output proportional to the applied external force.

[0049] The advantages of choosing a piezoelectric sensor as the sensing unit are: (1) it can detect dynamic stimuli in real time; (2) it is a passive element. The piezoelectric voltage of a flexible piezoelectric sensor exhibits different frequency response characteristics under pressure loading at different frequencies. The piezoelectric voltage increases with the increase of the dynamic stimulus loading frequency, showing frequency-sensitive characteristics. As the frequency further increases, the piezoelectric voltage gradually stabilizes, showing frequency-insensitive characteristics. Since the instruments required for measurement are usually active elements (wires, circuits, or elastic modules), they must be separated from the corresponding support, and there is an unknown amount of friction between the wires and the support, resulting in a relatively large measurement deviation. Choosing a piezoelectric sensor can effectively reduce the error.

[0050] like Figure 5 As shown, the schematic diagram in the middle illustrates the formation mechanism of piezoelectric voltage: the application of mechanical pressure causes the dipoles to realign, thereby generating an internal piezoelectric potential. Figure 5 The right side shows the response time of the piezoelectric sensor; the pressure can be applied within a 6ms measurement time. For example... Figure 6 and Figure 7 As shown, the piezoelectric sensor, as the sensitive unit of a six-dimensional force sensor measuring device, exhibits different sensitive behaviors at different frequencies when its output voltage is generated after being stimulated by external stimuli. From Figure 6 As can be seen from (a) in the diagram, under the same pressure conditions, the higher the frequency, the higher the output voltage. Meanwhile, from... Figure 6 As can be seen in (b), the piezoelectric output voltage remains stable as the frequency further increases from 200Hz to 300Hz. Choosing a piezoelectric sensor as the sensing element offers significant advantages for detecting external dynamic frequency stimuli.

Claims

1. A flexible six-axis force sensor based on piezoelectric sensors, characterized by It comprises a stepped silica gel round table (1), a cross beam silica gel base (2) and a pressure sensor (3); The stepped silica gel round table (1) is composed of coaxially arranged upper and lower round tables, and the diameter of the lower round table is larger than that of the upper round table; the lower surface of the lower round table is in vertical connection with the upper surface of the cross beam silica gel base (2) to form a round table structure, and the upper surface of the cross beam silica gel base (2) is provided with a cross beam boss, and the lower surface structure of the lower round table is matched with the cross beam boss; A plurality of pressure sensors (3) are arranged at the joint of the stepped silica gel round table (1) and the cross beam silica gel base (2); Three pressure sensors (3) are arranged on each of the four arms of the cross beam boss, and the three pressure sensors (3) are respectively located on the top surface and the two side surfaces of each arm of the cross beam; The pressure sensor (3) is a piezoelectric sensor; the piezoelectric sensor is obtained by evaporating electrodes on a piezoelectric composite film and then packaging; The preparation method of the piezoelectric composite film is as follows: Step 1) dispersing 100 nm particle size inorganic piezoelectric material barium titanate nanoparticles into 0.01 mol-L -1 After 1 h of ultrasonic stirring in the dopamine aqueous solution, the nanoparticles were uniformly coated with the dopamine solution, and the modified BTO powder was obtained after drying in an oven at 70 °C. Step 2) After the modified BTO powder is added to the organic solvent N, N-dimethylformamide, ultrasonic is applied for 1 h to uniformly disperse the BTO in the solvent to obtain a BTO solution; Step 3) The piezoelectric polymer powder is dissolved in DMF, and mixed with the BTO solution of step 2) to form a stable suspension after magnetic stirring for 2 h; Step 4) The suspension is poured into a culture dish, and then placed in an oven at 80 ºC for 12 h to volatilize the solvent; Step 5) Finally, annealing treatment is carried out at a temperature of 120 ºC for 2 h to remove residual solvent to improve the crystallinity, and a piezoelectric composite film with a thickness of 20-30 µm is obtained.

2. The flexible six-axis force sensor based on piezoelectric sensors according to claim 1, characterized in that, The upper round table is provided with a channel penetrating through the upper round table along the radial direction; a stainless steel wire is penetrated through the channel, and the outer diameter of the stainless steel wire is matched with the inner diameter of the channel.

3. A space six-dimensional force measurement method using the flexible six-dimensional force sensor of any one of claims 1-2, characterized in that, 1) Calibrating the flexible six-dimensional force sensor: a space six-dimensional force is applied to the flexible six-dimensional force sensor as a calibration force, a data acquisition card is used to collect the calibration voltages output by each piezoelectric sensor, and a mapping relationship matrix between the space six-dimensional force and the calibration voltages output by all piezoelectric sensors is constructed; 2) When the flexible six-dimensional force sensor is subjected to pressure and torque, the deformation of the flexible six-dimensional force sensor as a whole is converted into a voltage change signal of the piezoelectric sensor for output; The size of the six-dimensional force received by the flexible six-dimensional force sensor is calculated by backstepping according to the mapping relationship matrix and the output voltage signal.

4. A space six-dimensional force measurement method using the flexible six-dimensional force sensor of any one of claims 1-2, characterized in that, 1) Constructing a DNN deep neural network: a space six-dimensional force is applied to the flexible six-dimensional force sensor as a calibration force, a data acquisition card is used to collect the output calibration voltages of each piezoelectric sensor; the applied six-dimensional force and the output calibration voltage signal are used as a sample set, the sample set is divided into a training set and a test set, and the DNN deep neural network is trained by inputting the training set and the test set into the DNN deep neural network to obtain a trained DNN deep neural network; ​ 2) When the flexible six-dimensional force sensor is subjected to pressure and torsional moment, the deformation of the whole flexible six-dimensional force sensor is converted into the voltage change signal of the piezoelectric sensor for output; the output voltage signal is input into the trained DNN deep neural network, and the size and direction of the six-dimensional force received by the flexible six-dimensional force sensor are output.

Images