A tactile sensor with double closed-loop control and its control method

Abstract

The present invention belongs to the technical field of sensors and is a tactile sensor with a double closed-loop control, including a double closed-loop control system, a sensor measurement system, and a tactile sensor system model. The double closed-loop control system is provided with a sensor contact, an amplitude stabilization control system, and a frequency tracking system. The sensor measurement system is provided with a pressing device and a resonance frequency acquisition system. The pressing device controls the magnitude of the contact force through the spring deformation amount. The resonance frequency acquisition system is used to collect the initial frequency of the sensor contact and the system resonance frequency when touching the tissue to be measured, and is used to calculate the elasticity of the tissue to be measured according to the frequency offset. The present invention can reduce the initial resonance frequency of the sensor system, avoid the invasive damage to the contacted object, improve the system stability, and improve the measurement accuracy; the structure is simple, which is beneficial to the miniaturization of the sensor.

Description

Technical Field

[0001] The present invention belongs to the technical field of sensors, and particularly relates to a tactile sensor based on double closed-loop control and a control method. Background Art

[0002] Human beings perceive the world through touch, hearing, vision, etc. Touch is one of the important ways for humans to perceive external objects and information. Through tactile perception, humans can obtain the physical characteristics of the contact object and diverse information about the surrounding environment. Nowadays, a large number of visual sensors and auditory sensors have been studied and can be maturely applied, but relatively little progress has been made in tactile sensors. However, in the development of automation systems such as robots and virtual reality, there is an increasing need to combine information from various senses to achieve applications such as basic grasping operations, remote medical care, and minimally invasive surgical sensing. Although people have never stopped developing tactile sensors, the currently developed tactile sensors still have many technical problems to be solved when facing a series of high-level object attributes such as surface patterns, shapes, elasticity, and friction.

[0003] Currently, the elastic measurement principles of tactile sensors can be mainly divided into: static indentation method based on tissue compression deformation and dynamic measurement based on the resonance frequency shift of the system. Static measurement obtains the elastic information of an object by detecting the contact force, which has an intuitive physical meaning. However, the contact force applied to the tissue to be measured during the measurement process is too large, easily causing irreversible deformation or even damage to the tissue to be measured. Therefore, tactile sensors based on dynamic measurement principles have received increasing attention. A tactile sensor for hardness detection designed by Sadao Omata et al. uses a piezoelectric ceramic bimorph, including a driving element and a detecting element. A high-frequency alternating voltage signal is applied to the driving element through open-loop driving for excitation, and the detecting element is used to pick up the system resonance frequency signal in real time. When the sensor contacts the tissue to be measured, the resonance frequency shift of the sensor before and after contacting the tissue to be measured is fed back to reflect the elasticity of the tissue to be measured; Yoshinobu Murayama et al. use a similar principle, use the same sensor structure, adopt digital control, increase the number of elastic measurement units, and form an array sensor structure so that the tactile sensor can obtain tissue elastic information in a large range. When measuring human tissue that may contain a diseased tumor, the elastic distribution within the measurement area can be obtained, thereby obtaining information such as the location and size of the tumor mass; Shigenobu Muraoka et al. add a robotic cantilever and a mass block to the sensor, greatly reducing the resonance frequency of the sensor system but being unfavorable for miniaturization.

[0004] Looking at the above tactile sensors, there are the following problems that need to be solved urgently: 1. The open-loop drive method is unstable, the system fluctuates greatly, and the measurement accuracy is low; 2. Simplify the structure of the sensor to achieve miniaturization; 3. Reduce the initial system resonance frequency to reduce the influence of the mass effect. Summary of the Invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and propose a tactile sensor with a double closed-loop control and its control method. This tactile sensor adopts a double closed-loop control system to improve the system stability and measurement accuracy. Secondly, the structure of double-layer piezoelectric ceramics plus a silicone contact head is adopted, which can effectively reduce the system resonance frequency and avoid invasive damage to the tissue to be measured. Finally, the tactile sensor system adopts a self-excited oscillation circuit, which has a simple structure and is beneficial to the miniaturization of the sensor.

[0006] In order to achieve the above purpose, the present invention is realized through the following technical solutions:

[0007] A tactile sensor with double closed-loop control of the present invention includes:

[0008] A double closed-loop control system, which includes a sensor contact head, an amplitude stabilization control system, and a frequency tracking system. The amplitude stabilization control system adjusts the gain coefficient in a timely manner based on the change in the amplitude of the circuit voltage to keep the amplitude of the drive voltage stable and meet the amplitude condition of self-excited oscillation. The frequency tracking system makes the circuit oscillate at the system resonance frequency at all times by tracking the frequency of the self-excited oscillation circuit;

[0009] A sensor measurement system, which includes a pressing device and a resonance frequency acquisition system. The pressing device controls the magnitude of the contact force through the spring deformation amount to ensure sufficient contact with the tissue to be measured and that the contact force does not change during multiple measurements. The resonance frequency acquisition system is used to acquire the initial frequency of the system and the system resonance frequency when the sensor contact head touches the tissue to be measured;

[0010] A tactile sensor system model, which is derived according to the control model of the tactile sensor and the equivalent acoustic impedance when the sensor contact head touches the tissue to be measured, and is used to calculate the elasticity of the tissue to be measured according to the frequency offset. The frequency collected by the resonance frequency acquisition system can obtain the resonance frequency offset, which is used to deduce the elasticity of the tissue to be measured.

[0011] Further, the sensor contact head is sequentially provided with an elastic outer shell and a hemispherical contact head from the outside to the inside.

[0012] Further, the elastic outer shell is a plastic outer shell, and the hemispherical contact head is a silicone contact head.

[0013] Further, the amplitude stabilization control system includes an amplifier, an automatic gain controller, and a PI controller. The amplifier amplifies the feedback signal of the sensitive element. The PI controller is connected to the output end of the automatic gain controller for fine control. The automatic gain controller controls the gain according to the difference in amplitude between the voltage signal passing through the amplifier and the reference voltage signal, maintaining the stability of the driving voltage amplitude and making it meet the amplitude condition of self-excited oscillation.

[0014] Further, the automatic gain controller includes an amplitude detection circuit, a reference voltage, and a variable gain amplifier. The amplitude detection circuit detects the voltage amplitude signal passing through the amplifier, compares it with the reference voltage to obtain an error signal. When the error signal becomes larger, the gain of the variable gain amplifier decreases; when the error signal becomes smaller, the gain of the variable gain amplifier increases, achieving the purpose of automatic gain control.

[0015] Further, the frequency tracking system includes a phase-locked loop chip, a low-pass filter, and a phase shifter. The phase-locked loop chip tracks the output signal of the amplifier and changes with the change of the frequency of the amplifier output signal. The low-pass filter selects the frequency of the signal after the frequency is adjusted by the phase-locked loop and filters out the noise signal. The phase shifter adjusts the phase of the signal output by the low-pass filter to make it meet the self-excited oscillation condition of the system.

[0016] Further, the sensor measurement system includes a resonance frequency acquisition system and a pressing device. The resonance frequency acquisition system is connected to the sensitive element and is used to acquire the system resonance frequency when the sensor contact touches the tissue to be measured.

[0017] Further, the pressing device includes a pressing column and a spring. The pressing device controls the magnitude of the contact force by controlling the deformation amount of the spring, ensuring sufficient contact with the tissue to be measured and that the contact force does not change during multiple measurements.

[0018] Further, the sensitive element includes an oscillating piezoelectric ceramic and a detecting piezoelectric ceramic. The detecting piezoelectric ceramic is connected to the input end of the amplifier, and the oscillating piezoelectric ceramic is connected to the output end of the PI controller.

[0019] A control method for a tactile sensor system with double closed-loop control includes the following steps:

[0020] Step 1: Analyze the structure of the tactile sensor, establish an electromechanical coupling model of the sensor contact using finite element analysis, set different boundary conditions, perform simulation operations, analyze and record the physical variables under different contact forces and the frequency offset related to the elasticity of the object to obtain initial data, establish an algorithm model of the sensor elasticity-resonance frequency under different contact forces, conduct elasticity measurement experiments on multiple physical samples with known elasticity under different contact force conditions, change the contact force, record the physical variables under different contact force conditions and the corresponding relationship between elasticity and the system resonance frequency, and optimize the sensor elasticity-resonance frequency algorithm model;

[0021] Step 2: During measurement, bring the tactile sensor into contact with the surface of the tissue to be measured, adjust the contact point by moving the position of the sensor contact, ensure that the sensor contact is in normal contact with the tissue to be measured, and the resonance frequency acquisition system acquires the resonance frequency at this time. Through the circuit built in the resonance frequency acquisition system, detect whether the frequency of the tactile sensor changes before and after contact to ensure that the sensor contact is in full contact with the tissue to be measured. After determining full contact, continue to press the pressing device along the normal direction, and control the magnitude of the contact force through the spring compression amount in the pressing device to ensure that the sensor contact is in full contact with the tissue to be measured and the contact force meets the measurement requirements;

[0022] Step 3: Before contacting the tissue to be measured, the resonance frequency acquisition system acquires the initial resonance frequency of the system at this time. During measurement, ensure full contact and keep the contact state and contact force unchanged. The resonance frequency acquisition system acquires the resonance frequency of the system at this time. Import the contact force, the initial resonance frequency, and the resonance frequency of the system after contacting the object into the sensor elasticity-resonance frequency algorithm model to predict the surface elasticity of the tissue to be measured and obtain the elasticity information of the tissue to be measured. Specifically:

[0023] The acoustic impedance of the tissue to be measured can be expressed as:

[0024]

[0025] where r x is the acoustic impedance part of the object acoustic impedance z x , ω is the angular frequency of the sensor vibration, m x is the mass of the object, and k x is the surface elasticity of the object. If the contact radius is less than the wavelength of the shear wave, the acoustic impedance r x is expressed as:

[0026]

[0027] The mass term m x and the elasticity term k x are respectively expressed as:

[0028]

[0029] Wherein, E x , v x and ρ x are respectively the Young's modulus, Poisson's ratio and density of the tissue to be measured. S represents the contact area and the contact region is hemispherical, regarded as a frictionless contact. a 21 is a function of Poisson's ratio, and v is the Poisson's ratio of the sensor contact;

[0030] The resonance frequency f of the system under non-loaded conditions R is expressed as:

[0031]

[0032] Wherein, k and m are respectively the equivalent elasticity and mass of the sensor under non-loaded conditions. When the sensor contact touches the tissue to be measured, the contact impedance is Z x , then the resonance frequency becomes:

[0033]

[0034] where the mass m of the tissue to be measured x is much lower than the mass of the tactile sensor, so it can be ignored. The additional elasticity k x dominates. The change in the resonance frequency is expressed as:

[0035]

[0036] That is, Δf ∝ k x . Considering that when the sensor measures, the Poisson's ratio and density of the tissue to be measured hardly change, the above relationship is further expressed as:

[0037] Δf = c1E x + c2

[0038] Wherein, c1 and c2 are both constants. By calibrating the values of the constants c1 and c2 through experimental tests, the elasticity of the tissue to be measured can be obtained from the frequency offset.

[0039] The beneficial effects of the present invention are:

[0040] 1. The present invention can reduce the initial resonance frequency of the sensor system and avoid invasive damage to the tissue to be measured;

[0041] 2. The double closed-loop control system provides a sinusoidal drive signal with constant amplitude and frequency locking for the sensor, improves the system stability and measurement accuracy;

[0042] 3. The present invention adopts a self-excited oscillation circuit, which has a simple structure and is beneficial to the miniaturization of the sensor. Description of the Drawings

[0043] Figure 1It is a schematic diagram of the sensor contact structure in the present invention.

[0044] Figure 2 It is a schematic cross-sectional view of the sensor contact in the present invention.

[0045] Figure 3 It is a schematic top view of the sensor contact in the present invention.

[0046] Figure 4 It is a schematic diagram of the structure of the present invention.

[0047] Figure 5 It is a schematic diagram of the amplitude stabilization control system circuit of the present invention, where Figure a is a schematic diagram of the amplifier circuit, Figure b is a schematic diagram of the PI controller circuit, and Figure c is a schematic diagram of the automatic gain controller circuit.

[0048] Figure 6 It is a schematic diagram of the frequency tracking system circuit of the present invention; where Figure a is a schematic diagram of the phase-locked loop chip, Figure b is a schematic diagram of the low-pass filter circuit, Figure c is a schematic diagram of the phase shifter circuit, and the numbers represent the chip io interfaces.

[0049] Figure 7 It is a schematic diagram of the finite element simulation in the present invention.

[0050] Figure 8 It is a schematic diagram of the simulink simulation in the present invention; where Figure a is the contact theory model of the tactile sensor, and Figure b is the closed-loop control model of the sensor system.

[0051] Figure 9 It is a schematic diagram of the system control effect in the present invention.

[0052] Figure 10 It is the corresponding curve of the frequency offset and elastic modulus of the present invention.

[0053] Among them: 1 - sensitive element, 11 - detection piezoelectric ceramic, 12 - oscillation piezoelectric ceramic, 2 - sensor contact, 21 - elastic housing, 22 - hemispherical contact, 3 - pressing device, 31 - pressing column, 32 - spring, 4 - amplifier, 51 - amplitude detection circuit, 52 - reference voltage, 53 - variable gain amplifier, 6 - PI controller, 7 - phase-locked loop, 8 - low-pass filter, 9 - phase shifter, 10 - resonance frequency acquisition system. Specific embodiments

[0054] The following will disclose the embodiments of the present invention in the form of diagrams. For the sake of clarity, many practical details will be described together in the following narrative. However, it should be understood that these practical details should not be used to limit the present invention. That is to say, in some embodiments of the present invention, these practical details are not necessary. In addition, for the purpose of simplifying the diagrams, some well-known and commonly used structures and components will be shown in a simple schematic manner in the diagrams.

[0055] As Figure 4 shown, the present invention is a tactile sensor with a double closed-loop control, including a double closed-loop control system. The double closed-loop control system includes a sensor contact 2, an amplitude stabilization control system, and a frequency tracking system. The amplitude stabilization control system timely adjusts the gain coefficient based on the change in the amplitude of the circuit voltage of the self-excited oscillation circuit composed of the tactile sensor, so as to keep the amplitude of the driving voltage in the self-excited oscillation circuit composed of the tactile sensor stable and meet the self-excited oscillation amplitude condition. The frequency tracking system makes the circuit always oscillate at the system resonance frequency by tracking the frequency of the self-excited oscillation circuit of the tactile sensor.

[0056] As Figure 1 shown, the sensor contact 2 is successively provided with an elastic outer shell 21 and a hemispherical contact 22 from outside to inside. The elastic outer shell 21 is a plastic outer shell, and the hemispherical contact 22 is a silicone contact, which has good elasticity and corrosion resistance.

[0057] As Figure 5 shown, the amplitude stabilization control system includes an amplifier 4, an automatic gain controller, and a PI controller 6. As Figure 5 shown in a, the amplifier 4 amplifies the feedback signal of the sensitive element 1. As Figure 5 shown in b, the PI controller 6 is connected to the output end of the automatic gain controller for fine control. As Figure 5 shown in c, the automatic gain controller controls the gain according to the difference in amplitude between the voltage signal passing through the amplifier 4 and the reference voltage signal 52, so as to keep the amplitude of the driving voltage stable and make the self-excited oscillation circuit composed of the tactile sensor meet the self-excited oscillation amplitude condition. The automatic gain controller includes the amplitude detection circuit 51, the reference voltage 52, and a variable gain amplifier 53. The amplitude detection circuit 51 detects the voltage amplitude signal passing through the amplifier 4, compares it with the reference voltage 52 to obtain an error signal. When the error signal becomes larger, the gain of the variable gain amplifier decreases; when the error signal becomes smaller, the gain of the variable gain amplifier increases, achieving the purpose of automatic gain control. The variable gain amplifier 53 is a voltage-controlled gain amplifier, and the voltage signal is used as the control signal of the variable gain amplifier to control the amplitude of the circuit by adjusting the gain and output a stable-amplitude sinusoidal driving signal.

[0058] As Figure 6 shown, the frequency tracking system includes a phase-locked loop 7, a low-pass filter 8, and a phase shifter 9. As Figure 6 shown in a, the phase-locked loop 7 tracks the output signal of the amplifier 4 and changes with the change of the frequency of the output signal of the amplifier. As Figure 6 shown in b, the low-pass filter 8 selects the frequency of the signal after the frequency is adjusted by the phase-locked loop 7 and filters out the noise signal.Figure 6 As shown in Fig. c, the phase shifter 9 adjusts the phase of the signal output by the low-pass filter 8 to make the self-excited oscillation circuit composed of the tactile sensor satisfy the phase condition of system self-excited oscillation. The control effect of the double closed-loop control system is as Figure 9 shown, providing a sinusoidal AC signal with constant amplitude and locked frequency for the sensor.

[0059] The sensor measurement system includes a pressing device 3 and a resonance frequency acquisition system 10. The pressing device 3 includes a pressing column 31 and a spring 32. The pressing device controls the magnitude of the contact force by controlling the deformation amount of the spring; the resonance frequency acquisition system 10 is connected to the sensitive element 1 and is used to collect the system resonance frequency when the sensor contact 2 touches the tissue to be measured.

[0060] As Figure 2 shown, the sensitive element 1 includes a detection piezoelectric ceramic 11 and an oscillation piezoelectric ceramic 12. The detection piezoelectric ceramic 11 is connected to the input end of the amplifier 4, and the oscillation piezoelectric ceramic 12 is connected to the output end of the PI controller 6.

[0061] The tactile sensor system model is derived according to the control model of the tactile sensor (see Figure 8 ) and the equivalent acoustic impedance when the sensor contact touches the tissue to be measured, and is used to calculate the elasticity of the tissue to be measured according to the frequency offset.

[0062] According to the vibration theory of a finite rod, a contact theoretical model of the tactile sensor as shown in Figure 8 Fig. a is established, and the change in the resonance frequency caused by the contact between the tactile sensor system and an unknown object is converted into the influence of the impedance change in the circuit on its resonance frequency.

[0063] An elastic-resonance frequency algorithm model of the tactile sensor system is established by combining the sensor system model and the electromechanical coupling model established by finite element analysis. Among them, a finite element simulation software is used to model and simulate the sensor structure, and an axisymmetric model of the tactile sensor is established. As Figure 7 shown is the finite element simulation example of the sensor. An experimental measurement of a tissue with known elasticity is carried out to optimize the algorithm model of the sensor system, and it is used to calculate the elasticity of the tissue to be measured according to the resonance frequency offset of the system;

[0064] When measuring an object with a sensor system, record the initial system resonance frequency before the sensor system contacts the tissue to be measured. Since the resonance frequency of the tactile sensor system changes when the sensor tip 3 contacts the object, record the system resonance frequency at this time. The resonance frequency offset corresponds to the elastic information of the tissue to be measured. When contacting tissues with different elasticities, the resonance frequency offset of the sensor system will also change. During measurement, keep the contact state and contact force unchanged. The resonance frequency acquisition system acquires the system resonance frequency, and import the contact force, the initial resonance frequency, and the system resonance frequency after contacting the object into the algorithm model to obtain the elastic information of the tissue to be measured.

[0065] Specifically, the control method of the tactile sensor with double closed-loop control of the present invention includes the following steps:

[0066] Step 1: Analyze the structure of the tactile sensor, and use finite element analysis to establish an electromechanical coupling model of the sensor tip as Figure 7 shown. Set the force boundary conditions and electrical boundary conditions. The force boundary conditions include fixing the upper boundary of the sensor during modal analysis, applying a normal contact force to the sensor during static analysis, and fixing the lower boundary of the tissue to be measured. The electrical boundary conditions include setting the zero potential boundary and the positive electrical boundary. After modeling and setting the boundary conditions, perform mesh division on the model, and perform simulation analysis in the job module of the finite element simulation software. Analyze and record the physical variables under full contact and the frequency offset related to the elasticity of the object to obtain the initial data, and establish an algorithm model of sensor elasticity-resonance frequency. The curve of frequency offset and elastic modulus correspondence is as Figure 10 shown. Conduct elastic measurement experiments on multiple physical samples with known elasticities under different contact force conditions using the tactile sensor. Six test models with different elasticities were made of silicone. Before the experimental determination, the cylindrical indentation method was used to determine the Young's modulus of different test models. After completing the production of the silicone model and the calibration of the Young's modulus, use the tactile sensor to contact the test model, and record the corresponding resonance frequency offset at full contact. To ensure accuracy, take the average value after multiple measurements. During multiple measurements, control the spring compression amount to ensure that the contact force remains unchanged to obtain the correspondence between the Young's modulus and the frequency offset. As Figure 10 shown.

[0067] Step 2: During measurement, the tactile sensor is brought into contact with the surface of the tissue to be measured. The contact point is adjusted by moving the position of the sensor contact 2 to ensure that the sensor contact is in normal contact with the tissue to be measured. The resonance frequency acquisition system is connected to the host computer using a NI acquisition card. The resonance frequency signal is transmitted to the host computer through the NI acquisition card for calculation. The resonance frequency at this time is collected. Whether the frequency of the tactile sensor changes before and after contact is detected through the NI acquisition card built in the resonance frequency acquisition system to ensure that the sensor contact 2 is in full contact with the tissue to be measured. After determining full contact, the pressing device 3 is pressed further along the normal direction, and the magnitude of the contact force is controlled by the spring compression amount in the pressing device 3 to ensure that the sensor contact 2 is in full contact with the tissue to be measured and the contact force meets the measurement requirements;

[0068] Step 3: Before contacting the tissue to be measured, the resonance frequency acquisition system collects the initial resonance frequency of the system at this time. During measurement, full contact is made and the contact state and contact force are kept unchanged. The resonance frequency acquisition system collects the resonance frequency of the system at this time. The contact force, the initial resonance frequency, and the resonance frequency of the system after contacting the object are imported into the sensor elasticity-resonance frequency algorithm model to predict the elasticity of the surface of the tissue to be measured, and the elasticity information of the tissue to be measured is obtained.

[0069] Among them, the process of predicting the elasticity of the surface of the tissue to be measured is as follows:

[0070] The acoustic impedance of the tissue to be measured can be expressed as:

[0071]

[0072] where r x is the acoustic impedance part of the object acoustic impedance z x , ω is the angular frequency of sensor vibration, m x is the mass of the object, and k x is the surface elasticity of the object. If the contact radius is less than the wavelength of the shear wave, the acoustic impedance r x is expressed as:

[0073]

[0074] The mass m x of the tissue to be measured and the surface elasticity k x of the tissue to be measured are respectively expressed as:

[0075]

[0076] In the above formula, E x , v x and ρ x are respectively the Young's modulus, Poisson's ratio of the tissue to be measured, and the density of the tissue to be measured. S represents the contact area and the contact region is hemispherical, regarded as a frictionless contact, a 21 is a function of Poisson's ratio, and v is the Poisson's ratio of the sensor contact;

[0077] The resonance frequency f of the system under no-load conditions R is expressed as:

[0078]

[0079] where k and m are the equivalent elasticity and mass of the sensor under no load respectively. When the sensor contact touches the tissue to be measured, the contact impedance is Z x , then the resonance frequency becomes:

[0080]

[0081] where the mass m of the tissue to be measured x is much lower than the mass of the tactile sensor, so it can be ignored. The additional elasticity k x dominates. The change in the resonance frequency is expressed as:

[0082]

[0083] That is, Δf∝k x , considering that when the sensor measures, the Poisson's ratio and density of the tissue to be measured hardly change, so the above relational expression is further expressed as:

[0084] Δf = c1E x + c2

[0085] where c1 and c2 are both constants. By calibrating the values of the constants c1 and c2 through experimental tests, the elasticity of the tissue to be measured can be obtained from the frequency offset

[0086] The tactile sensor of the present invention can avoid invasive damage to the contacted object, and the double closed-loop control system provides a constant amplitude for the sensor, improving the system stability

[0087] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and deformations can be made, and these improvements and deformations should also be regarded as the protection scope of the present invention

Claims

1. A tactile sensor with double closed-loop control, characterized in that: The tactile sensor includes a dual closed-loop control system, a sensor measurement system, and a tactile sensor system model. The dual closed-loop control system includes a sensor contact (2), a stable amplitude control system, and a frequency tracking system. The stable amplitude control system adjusts the gain coefficient of the automatic gain controller in the stable amplitude control system in a timely manner based on the change in the circuit voltage amplitude of the self-excited oscillation circuit composed of the tactile sensor, so as to keep the driving voltage amplitude in the self-excited oscillation circuit composed of the tactile sensor stable and meet the self-excited oscillation amplitude condition. The frequency tracking system makes the self-excited oscillation circuit always oscillate at the resonant frequency of the tactile sensor by tracking the frequency of the self-excited oscillation circuit of the tactile sensor. The sensor measurement system, the sensor measurement system includes a pressing device (3) and a resonance frequency acquisition system (10). The pressing device (3) controls the contact force magnitude through the spring deformation amount, ensures sufficient contact with the tissue to be measured, and the contact force does not change during multiple measurements. The resonance frequency acquisition system (10) acquires the initial frequency of the tactile sensor and the resonance frequency of the tactile sensor when the sensor contact (2) touches the tissue to be measured. The tactile sensor system model is derived based on the control model of the tactile sensor and the equivalent acoustic impedance when the sensor contact (2) touches the tissue to be measured, and is used to calculate the elasticity of the tissue to be measured according to the frequency offset amount of tracking the frequency of the self-excited oscillation circuit.

2. The tactile sensor with double closed-loop control according to claim 1, characterized in that: The sensor contact (2) is sequentially provided with an elastic plastic shell (21) and a hemispherical silicone contact (22) from outside to inside.

3. The tactile sensor with double closed-loop control according to claim 1, characterized in that: The stable amplitude control system includes an amplifier (4), an automatic gain controller, and a PI controller (6). The amplifier (4) amplifies the feedback signal of the sensitive element (1). The PI controller (6) is connected to the output end of the automatic gain controller for control. The automatic gain controller controls the gain according to the difference in amplitude between the voltage signal passing through the amplifier (4) and the reference voltage (52) signal of the automatic gain controller, so that the self-excited oscillation circuit composed of the tactile sensor meets the self-excited oscillation amplitude condition.

4. A tactile sensor with double closed-loop control according to claim 3, characterized in that: The automatic gain controller includes a amplitude detection circuit (51), a reference voltage (52), and a variable gain amplifier (53). The amplitude detection circuit (51) detects the voltage amplitude signal passing through the amplifier (4), compares it with the reference voltage (52) to obtain an error signal. The reference voltage (52) is provided by voltage source voltage division. When the error signal becomes larger, the gain of the variable gain amplifier (53) decreases; when the error signal becomes smaller, the gain of the variable gain amplifier (53) increases.

5. A tactile sensor with double closed-loop control according to claim 4, characterized in that: The frequency tracking system includes a phase-locked loop (7), a low-pass filter (8), and a phase shifter (9). The phase-locked loop (7) tracks the output signal of the amplifier (4) and changes with the change of the frequency of the output signal of the amplifier (4). The low-pass filter (8) selects the frequency of the signal whose frequency has been adjusted by the phase-locked loop (7) and filters out the noise signal. The phase shifter (9) adjusts the phase of the signal output by the low-pass filter (8) so that the self-excited oscillation circuit composed of the tactile sensor satisfies the phase condition of system self-excitation oscillation.

6. The tactile sensor with double closed-loop control according to claim 3, characterized in that: The resonance frequency acquisition system (10) is connected to the sensitive element (1) and is used to acquire the system resonance frequency when the sensor contact (2) touches the tissue to be measured.

7. The tactile sensor with double closed-loop control according to claim 6, wherein: The sensitive element (1) includes a detection piezoelectric ceramic (11) and an oscillation piezoelectric ceramic (12). The detection piezoelectric ceramic (11) is connected to the input end of the amplifier (4), and the oscillation piezoelectric ceramic (12) is connected to the output end of the PI controller (6).

8. A tactile sensor with double closed-loop control according to claim 1, characterized in that: The pressing device (3) includes a pressing column (31) and a spring (32). The pressing device (3) controls the magnitude of the contact force by controlling the deformation amount of the spring (32).

9. A tactile sensor with double closed-loop control according to any one of claims 2-8, characterized in that: The control method of this sensor includes: Step 1: Analyze the structure of the tactile sensor, establish an electromechanical coupling model of the sensor contact using finite element analysis, set different boundary conditions, perform simulation operations, analyze and record the physical variables under different contact forces and the frequency offset related to the elasticity of the object, obtain the initial data, establish an elastic-resonance frequency algorithm model of the sensor under full contact. Conduct elastic measurement experiments on multiple physical samples with known elasticity under different contact force conditions, change the contact force, record the physical variables under different contact force conditions and the corresponding relationship between elasticity and the system resonance frequency, and optimize the elastic-resonance frequency algorithm model of the sensor. Step 2: During measurement, bring the tactile sensor into contact with the surface of the tissue to be measured, adjust the contact point by moving the position of the sensor contact (2) to ensure that the sensor contact is in normal contact with the tissue to be measured. The resonance frequency acquisition system acquires the resonance frequency at this time. Detect whether the frequency of the tactile sensor changes before and after contact through the NI acquisition card built in the resonance frequency acquisition system to ensure that the sensor contact (2) is in full contact with the tissue to be measured. After determining full contact, continue to press the pressing device (3) along the normal direction, and control the magnitude of the contact force through the compression amount of the spring in the pressing device (3) to ensure that the sensor contact (2) is in full contact with the tissue to be measured and the contact force meets the measurement requirements. Step 3: Before contacting the tissue to be measured, the resonance frequency acquisition system acquires the initial resonance frequency of the system at this time. During measurement, ensure full contact and keep the contact state and contact force unchanged. The resonance frequency acquisition system acquires the resonance frequency of the system at this time. Import the contact force, the initial resonance frequency, and the resonance frequency of the system after contacting the object into the elastic-resonance frequency algorithm model of the sensor to predict the surface elasticity of the tissue to be measured and obtain the elastic information of the tissue to be measured.

10. A tactile sensor with double closed-loop control according to claim 9, characterized in that: In Step 3, the process of predicting the surface elasticity of the tissue to be measured is as follows: Let the equivalent acoustic impedance when the tissue to be measured is... where r x is the acoustic impedance part of the equivalent acoustic impedance Z x , ω is the angular frequency of sensor vibration, m x is the mass of the tissue to be measured, k x is the surface elasticity of the tissue to be measured. If the contact radius is less than the wavelength of the shear wave, the acoustic impedance r x is expressed as: The mass m of the tissue to be measured x and the surface elasticity k of the tissue to be measured x are respectively expressed as: where E x , v x and ρ x are the Young's modulus, Poisson's ratio and density of the tissue to be measured respectively, S represents the contact area and the contact region is hemispherical, regarded as a frictionless contact, a 21 is a function of the Poisson's ratio, and v is the Poisson's ratio of the sensor contact tip; The resonance frequency f of the system under no-load conditions R is expressed as: Where k and m are the equivalent elasticity and mass of the sensor without load respectively. When the sensor contact touches the tissue to be measured, the resonance frequency becomes: where the mass \(m\) of the tissue to be measured x is much lower than the mass of the tactile sensor and is thus neglected. The additional surface elasticity \(k\) of the tissue to be measured x dominates. The change in the resonance frequency is expressed as: That is, Δf ∝ k x , so the above relationship can be further expressed as: Δf = c1E x + c2 Where c1 and c2 are both constants. By calibrating the values of constants c1 and c2 through experimental tests, the elasticity of the tissue to be measured can be obtained from the frequency offset.

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