Manipulator End and Method Based on Intelligent Sensing of Contact Stress of Ultrasonic Surface Waves

Abstract

The present invention relates to a manipulator end and method based on intelligent perception of ultrasonic surface wave contact stress, which is applicable to the real-time monitoring of local contact stress at the end of an industrial manipulator when grasping an object. It includes a special cylindrical or spherical end of the manipulator, ultrasonic transmitting and receiving probes, probe brackets, adjusting bolts and nuts, an ultrasonic transmitter-receiver, a data acquisition card, and a computer. Probe brackets are installed on the upper and lower planes of the manipulator end, and the adjusting bolts and nuts thereon are used to adjust and fix the positions of the probes; the probes, ultrasonic transmitter-receiver, data acquisition card, and computer are connected in sequence. During operation, the ultrasonic transmitter-receiver sends an electrical signal to excite the ultrasonic transmitting probe to generate surface waves; the surface waves propagate along the surface of the manipulator end, and the receiving probe receives the surface waves and converts them into electrical signals and sends them back to the ultrasonic receiver-transmitter; the data acquisition card samples and digitizes the emitted and received signals, and the computer realizes data storage and calculation analysis to obtain the change in the surface wave propagation time, realizing the real-time monitoring of the local contact stress at the end of the manipulator, enabling the originally cold industrial manipulator to have the ability to sense contact stress.

Description

Technical Field

[0001] The present invention relates to a manipulator end and method, and in particular to a manipulator end and method based on intelligent perception of ultrasonic surface wave contact stress, which is particularly suitable for industrial manipulators. Background Art

[0002] A manipulator is a new type of device developed in the process of mechanized and automated production. It can imitate some actions of a human arm, grasp or carry objects, and combines the advantages of both humans and machines in terms of structure and performance. With the development of robotics, the flexibility, accuracy, and speed of manipulators have been comparable to or even exceeded that of human hands. However, the cold manipulator does not have the same tactile sense as a human hand. How to endow the manipulator with a tactile sense like a human hand has become a hot issue in the field of robotic hands.

[0003] When a human hand grasps an object with a specific shape and material, it is necessary to consider whether the force between the fingers and the object will damage the object. Similarly, when an industrial manipulator grasps an object with high precision, easy deformation, or surface quality requirements, it is necessary to ensure that the object does not generate local residual stress, deformation, or damage due to the local contact stress during clamping. Therefore, it is very necessary to monitor the clamping contact stress at the end of the industrial manipulator in real time.

[0004] The strain gauge force sensor on the manipulator can be used to measure the clamping force or torque when the manipulator grasps an object, but the local stress condition at the contact part between the end of the manipulator and the object cannot be reflected. Although the robotic bionic skin has various tactile functions such as contact sense, pressure sense, and slip sense, it has problems such as limited strength, complex manufacturing process, and high cost, and is completely unsuitable for the situation where the objects to be grasped in industry are large in volume and heavy in weight. Summary of the Invention

[0005] Objective of the Invention: Aiming at the deficiencies of the above technologies, a manipulator end and method based on intelligent perception of ultrasonic surface wave contact stress are provided, which have a simple structure, reliable operation, and can effectively prevent the manipulator from damaging the object during clamping.

[0006] Technical Solution: A manipulator end based on intelligent perception of ultrasonic surface wave contact stress of the present invention includes a pair of mutually matching mechanical fingers as a clamping mechanism. The front end of the mechanical finger is a columnar end or a spherical end. It also includes a computer, a data acquisition card, and an ultrasonic transmitter-receiver that are sequentially connected by lines to an ultrasonic receiving probe and an ultrasonic transmitting probe above and below the manipulator end. A probe support is respectively provided above and below each mechanical finger. An ultrasonic receiving probe is provided above the manipulator end through the probe support, and an ultrasonic transmitting probe is provided below. The positions of the ultrasonic receiving probe and the ultrasonic transmitting probe in the probe support are adjustable.

[0007] Each of the described columnar ends has a hyperbolic structure formed by the combination of the cutting surfaces of the sides of two cylinders at the contact part with the clamped object. The hyperbolic structure is formed by intercepting a part of two juxtaposed cylinders. The upper and lower surfaces and the cylindrical surface of the cylinder formed by cutting the columnar end are transitioned with rounded corners. An ultrasonic receiving probe is provided at the center of the upper plane of the columnar end through a probe holder, and a transmitting probe is provided at the center of the lower plane through a probe holder. The surface wave emitted from the transmitting probe will be received by the ultrasonic receiving probe when propagating in any direction within the propagable angle, and the acoustic path of the sound wave propagating in each direction is the same. When the clamped surface of the object is a plane or a cylindrical surface, the described special columnar end is used for clamping, so as to monitor the line contact stress in real time.

[0008] Each of the described spherical ends has a partially cut structure of a sphere at the contact part with the clamped object. The shape of the clamping part of the spherical end is evolved from a sphere: two circular planes are symmetrically cut out on the sphere, and these two planes are transitioned with rounded corners and the spherical surface. A part of this spherical body is intercepted as the clamping part of the spherical end. An ultrasonic receiving probe is provided at the center of the upper plane of the spherical end through a probe holder, and a transmitting probe is provided at the center of the lower plane through a probe holder. The surface wave emitted from the ultrasonic transmitting probe will be received by the ultrasonic receiving probe when propagating in any direction within the propagable angle, and the acoustic path of the sound wave propagating in each direction is the same. When the clamped surface of the object is irregular, the described special spherical end is used for clamping, and the point contact stress can be monitored in real time.

[0009] The intersections of the upper and lower planes and the cylindrical surface or spherical surface of the described columnar end or spherical end are transitioned with rounded corners. If the propagation speed of the ultrasonic surface wave with a frequency of f in the end material is v, the minimum rounded corner radius R that can allow the surface wave to propagate directly without energy loss is min = 5v / f. For ultrasonic waves and ordinary steel, the rounded corner radius is at the millimeter level, which is just a size that is convenient to achieve. At the same time, a suitable minimum rounded corner radius R can be obtained by changing the end material and the ultrasonic frequency f. min 。

[0010] On the left and right sides of each of the described probe holders, there are respectively provided transverse bolt nuts for adjusting and fixing the left and right movement positions of the probe. At the rear of the probe holder, there is a longitudinal bolt nut for adjusting and fixing the front and rear movement positions of the probe. Above the probe holder, there is a pressing bolt nut for pressing the probe. By adjusting the above-mentioned transverse bolt nuts, longitudinal bolt nuts and pressing bolt nuts, the probe can be accurately maintained at the center of the fan-shaped plane.

[0011] For the four adjusting bolt nuts: the bolts in the probe pressing bolt nut, transverse bolt nut and longitudinal bolt nut are all double-headed spiral bolts. The fine-threaded heads are used to adjust the position of the probe, and the coarse-threaded heads are used to fix the position of the probe.

[0012] When adjusting the positions of the lateral bolt nuts and longitudinal bolt nuts, the relevant function waveforms of the surface wave emission signal and the received signal fed back by the computer are required for adjustment. First, replace the ultrasonic receiving probe with a cylindrical micro-probe that is convenient for determining the geometric center, and adjust the position of the ultrasonic transmitting probe so that there is only one maximum value near the maximum value of the relevant function waveform, which is the maximum value. After determining the position of the ultrasonic transmitting probe, replace it with the ultrasonic receiving probe and adjust the position so that there is also only one maximum value near the maximum value of the relevant function waveform, which is the maximum value. At this time, it indicates that both the ultrasonic transmitting probe and the ultrasonic receiving probe are located at the center of the fan-shaped plane.

[0013] Before fixing the ultrasonic receiving probe and the ultrasonic transmitting probe to the upper and lower planes at the end using the probe holder, it is necessary to first clean the mating surface between the probe and the upper and lower planes at the end, and then apply a sufficient amount of coupling agent on the side plane of the end to ensure full acoustic energy transfer between the ultrasonic receiving probe, the ultrasonic transmitting probe, and the end surface. Since the ultrasonic receiving probe and the ultrasonic transmitting probe need to maintain an accurate position for a long time, it is not convenient to disassemble and assemble frequently, and the manipulator often works in different postures, so the coupling agent selected is engine oil with good adsorption, high viscosity, and good coupling effect.

[0014] A working method for the end of a manipulator based on intelligent perception of ultrasonic surface wave contact stress, the steps of which are as follows:

[0015] When the cylindrical end or spherical end of the manipulator holds an object, the ultrasonic emission receiver generates an electrical signal of surface waves, which excites the ultrasonic transmitting probe to convert into surface waves and propagate along the surface of the end of the manipulator. The ultrasonic receiving probe then converts the received surface waves into electrical signals and transmits them back to the ultrasonic emission receiver.

[0016] There is line contact or point contact between the cylindrical surface or spherical surface of the cylindrical end or spherical end and the clamped object, and local contact stress is generated on the surface. Due to the acoustoelastic effect, the propagation speed of the surface wave will change in the stress area, and the time for the surface wave to propagate from the ultrasonic transmitting probe to the ultrasonic receiving probe will change.

[0017] The electrical signals emitted and received by the ultrasonic emission receiver are received by the data acquisition card, and the signals are sampled and digitized. The computer performs cross-correlation analysis on the emitted and received acoustic signals to obtain the propagation time of the surface wave within the acoustic path. Since the acoustic path of the surface wave is the same, the contact stress becomes the only influencing factor for the time difference. The time difference between when the acoustic wave propagates without stress and when it passes through the stress area has a linear relationship with the contact stress. Therefore, the corresponding contact stress can be obtained from the measured time difference through the pre-calibrated linear relationship. When the contact stress is too large or exceeds the rated value, a warning is issued or the clamping force on the object is stopped from increasing, preventing harmful residual stress, deformation, or even damage to the object during the process of the manipulator end holding objects with high precision, easy deformation, and surface quality requirements.

[0018] Use cross - correlation analysis to find the time difference Δt: The propagation time of sound waves within the acoustic path is obtained by using cross - correlation analysis of two acoustic wave signals. Denote x1(t) and x2(t) as the waveform signals of transmission and reception respectively. Since this acoustic wave signal is an energy - limited signal, the cross - correlation function of x1(t) and x2(t) is:

[0019]

[0020] Since x2(t) lags behind x1(t), τ only needs to take positive values. When R 12 (τ) reaches the maximum value R max the corresponding time τ is the propagation time of sound waves within the acoustic path. When the end of the manipulator does not hold an object, the contact stress at the end is zero. At this time, the cross - correlation function reaches the maximum value at τ = τ0, and τ0 is the propagation time of sound waves within the acoustic path without stress. When the end of the manipulator holds an object, contact stress will be generated at the end, and part of the sound waves will propagate from the stress area, and the propagation speed of the sound waves will change. At this time, the cross - correlation function will reach the maximum value at τ = τ1. Therefore, the time difference Δt is obtained as follows:

[0021] Δt = Δτ = |τ1 - τ0|

[0022] Since cross - correlation analysis is completed by a computer, the above - mentioned analog signals are all converted into discrete digital quantities through a data acquisition card for processing. Denote T s as the sampling interval of the data acquisition card. Denote the values of the nth sampling points of x1(t) and x2(t) as X1(n) and X2(n), then there are:

[0023] X1(n) = x1(nT s )

[0024] X2(n) = x2(nT s )

[0025] The cross - correlation function of digital signals can be expressed as:

[0026]

[0027] where m is a non - negative integer. From the above, when the manipulator does not hold an object, there is a non - negative integer m = m0 such that R 12 (m) reaches the maximum value; when the manipulator holds an object, there is a non - negative integer m = m1 such that R 12 (m) reaches the maximum value;

[0028] That is:

[0029] τ0 = m0T s

[0030] τ1 = m1T s

[0031] Δt = Δτ = |τ1 - τ0| = T s |m1 - m0|

[0032] According to the surface wave acoustoelastic theory, the change in the propagation time of surface waves over a certain acoustic path is a linear function of the surface stress of the elastic body. Therefore, the time difference Δt and the contact stress σ have the following relationship:

[0033] Δt = Kσ

[0034] Where the constant K is determined by the acoustoelastic coefficient of the material and the acoustic path length that generates the stress. Therefore, the contact stress σ can be linearly represented by the time difference Δt:

[0035]

[0036] Let be denoted as K', then we have:

[0037] σ = K'Δt

[0038] Therefore, by obtaining K', the linear relationship between the contact stress σ and the time difference Δt can be obtained. To improve the accuracy of obtaining the value of K', experimental calibration can be carried out. Since the contact stress is not equal within the contact width, in order to improve the protection of the clamped object and simplify the calculation, the maximum contact stress σ H is calibrated with the time difference Δt. To facilitate obtaining the maximum contact stress σ acting on the end clamping part during calibration H , the surface of the object applying force to the end clamping part is a plane, and this force is a normal pressure F perpendicular to the plane of the force-applying part n . Since the curvature radii of the curved surfaces of the end clamping parts are all known, the maximum contact stress σ can be calculated H . When calibrating the cylindrical end, from the contact stress calculation formula, the maximum linear contact stress σ generated H is:

[0039]

[0040] Where, E is the elastic modulus of the material, L is the contact length, and R1 is the radius of the cylinder.

[0041] Similarly, when calibrating the spherical end, the maximum point contact stress σ H is:

[0042]

[0043] Where, R2 is the radius of the sphere. For different magnitudes of F applied n , the corresponding σ can be obtained H, and at the same time, the corresponding Δt is obtained by the above-mentioned correlation analysis method. For each group of σ collected by experiments H and the corresponding Δt are used to plot the σ H —Δt relationship diagram, and a fitting straight line is drawn. The slope of the fitting straight line is K'. Thus, when the end of the manipulator is in use, the maximum contact stress σ H during clamping can be obtained from the time difference Δt: σ H = K'Δt.

[0044] Beneficial effects: The present invention uses ultrasonic surface waves to sense the contact stress when the industrial mechanical finger tip clamps an object, enabling the end of the industrial manipulator to have the ability to sense the contact stress like a human hand. When the contact stress is too large or exceeds the rated value, a warning or alarm signal is issued. This prevents the occurrence of harmful residual stress, deformation, or even damage to the object during the process of the manipulator clamping high-precision, easily deformable, and objects with surface quality requirements. The device has a simple structure, reliable operation, and effectively improves the reliability of the industrial manipulator.

[0045] Real-time monitoring of the contact stress enables the industrial manipulator to have the ability to sense the contact stress, avoiding the situation where excessive contact stress damages the object. In view of this, it is necessary to develop an intelligent contact stress sensing device for the end of an industrial manipulator when clamping an object. During the clamping process of the manipulator, real-time monitoring of the contact stress is carried out, enabling the cold industrial manipulator to have the ability to sense the contact stress like a human hand. When the contact stress is too large or exceeds the rated value, a warning or alarm signal is issued to prevent the object from being damaged due to excessive contact stress. Brief Description of the Drawings

[0046] Figure 1 is a schematic structural diagram of the end of the manipulator based on ultrasonic surface wave contact stress intelligent sensing according to the present invention;

[0047] Figure 2 is a schematic diagram of the columnar end structure of the end of the manipulator based on ultrasonic surface wave contact stress intelligent sensing according to the present invention;

[0048] Figure 3 is a schematic diagram of the spherical end structure of the end of the manipulator based on ultrasonic surface wave contact stress intelligent sensing according to the present invention;

[0049] Figure 4(a) is the principle of surface wave propagation between multiple surfaces at the end of the manipulator based on ultrasonic surface wave contact stress intelligent sensing according to the present invention;

[0050] Figure 4(b) is the principle of surface wave propagation between multiple surfaces at the end of the manipulator based on ultrasonic surface wave contact stress intelligent sensing according to the present invention;

[0051] Figure 5(a) Schematic diagram of the columnar end shape design of the robot end based on the intelligent perception of ultrasonic surface wave contact stress of the present invention;

[0052] Figure 5(b) Schematic diagram of the columnar end shape design of the robot end based on the intelligent perception of ultrasonic surface wave contact stress of the present invention;

[0053] Figure 5(c) Schematic diagram of the columnar end shape design of the robot end based on the intelligent perception of ultrasonic surface wave contact stress of the present invention;

[0054] Figure 6(a) Schematic diagram of the spherical end shape design of the robot end based on the intelligent perception of ultrasonic surface wave contact stress of the present invention;

[0055] Figure 6(b) Schematic diagram of the spherical end shape design of the robot end based on the intelligent perception of ultrasonic surface wave contact stress of the present invention;

[0056] Figure 6(c) Schematic diagram of the spherical end shape design of the robot end based on the intelligent perception of ultrasonic surface wave contact stress of the present invention;

[0057] Figure 6(d) Schematic diagram of the spherical end shape design of the robot end based on the intelligent perception of ultrasonic surface wave contact stress of the present invention;

[0058] Figure 7 Schematic diagram of the time difference Δt calculation of the cross-correlation function of the robot end based on the intelligent perception of ultrasonic surface wave contact stress of the present invention;

[0059] Figure 8 Schematic diagram of the probe holder adjustment and fixed probe structure of the robot end based on the intelligent perception of ultrasonic surface wave contact stress of the present invention.

[0060] In the figure: 1 - computer; 2 - data acquisition card; 3 - ultrasonic transmitter-receiver; 4 - ultrasonic transmitting probe; 5A - columnar end; 5B - spherical end; 6 - ultrasonic receiving probe; 7 - probe holder; 8 - compression bolt and nut; 9 - transverse bolt and nut; 10 - countersunk head screw; 11 - cylindrical pin; 12 - longitudinal bolt and nut. Detailed implementation manners

[0061] The present invention will be further described below with reference to the accompanying drawings:

[0062] As Figure 1As shown in the figure, the end of a manipulator based on intelligent sensing of ultrasonic surface wave contact stress according to the present invention mainly includes a computer 1, a data acquisition card 2, and an ultrasonic transmitting and receiving device 3 that are connected in sequence; both an ultrasonic transmitting probe 4 and an ultrasonic receiving probe 6 are connected to the interface of the ultrasonic transmitting and receiving device 3; the manipulator must use a special columnar end 5A or spherical end 5B; a probe support 7 is installed on the side plane of the finger tip to adjust and fix the position of the probe on the upper and lower planes of the end.

[0063] The end of a manipulator based on intelligent sensing of ultrasonic surface wave contact stress is characterized in that: it includes a pair of mechanical fingers that match each other as a clamping mechanism, the front end of the mechanical fingers is a columnar end 5A or a spherical end 5B, and it also includes a computer 1, a data acquisition card 2, and an ultrasonic transmitting and receiving device 3 that are connected in sequence by wires to the ultrasonic receiving probe 6 and the ultrasonic transmitting probe 4 above and below the end of the manipulator. A probe support 7 is respectively arranged above and below each mechanical finger. An ultrasonic receiving probe 6 is arranged above the end of the manipulator through the probe support 7, and an ultrasonic transmitting probe 4 is arranged below. The positions of the ultrasonic receiving probe 6 and the ultrasonic transmitting probe 4 in the probe support 7 are adjustable.

[0064] As Figure 2 shown in the figure, the contact part of each columnar end 5A with the clamped object is a hyperbolic structure composed of the cutting surfaces of the sides of two cylinders. The hyperbolic structure is formed by intercepting a part of two side-by-side cylinders. The upper and lower surfaces and the cylindrical surface of the cylinder cut from the columnar end 5A are transitioned with rounded corners; an ultrasonic receiving probe 6 is arranged at the center of the upper plane of the columnar end 5A through the probe support 7, and a transmitting probe 4 is arranged at the center of the lower plane through the probe support 7; the surface wave emitted from the transmitting probe 4 will be received by the ultrasonic receiving probe 6 when propagating in any direction within the propagable angle, and the sound paths of the sound waves propagating in each direction are the same; when the surface of the object to be clamped is a plane or a cylinder, the special columnar end 5A is used for clamping, so as to monitor the line contact stress in real time.

[0065] As Figure 3 shown in the figure, the contact part of each spherical end 5B with the clamped object is a partially cut structure of a sphere. The shape of the clamping part of the spherical end 5B is evolved from a sphere: two circular planes are symmetrically cut out on the sphere, and these two planes are transitioned with rounded corners and the spherical surface, and a part of this spherical body is intercepted as the clamping part of the spherical end 5B; an ultrasonic receiving probe 6 is arranged at the center of the upper plane of the spherical end 5B through the probe support 7, and a transmitting probe 4 is arranged at the center of the lower plane through the probe support 7. The surface wave emitted from the ultrasonic transmitting probe 4 will be received by the ultrasonic receiving probe 6 when propagating in any direction within the propagable angle, and the sound paths of the sound waves propagating in each direction are the same; when the surface of the object to be clamped is irregular, the special spherical end 5B is used for clamping, and the point contact stress can be monitored in real time.

[0066] On the left and right sides of each of the described probe brackets 7, there are respectively provided transverse bolt nuts 9 for adjusting and fixing the left - right movement position of the probe. Behind the probe bracket 7, there is a longitudinal bolt nut 12 for adjusting and fixing the front - back movement position of the probe. Above the probe bracket 7, there is a compression bolt nut 8 for pressing the probe. By adjusting the above - mentioned transverse bolt nut 9, longitudinal bolt nut 12, and compression bolt nut 8, the probe can be accurately maintained at the center of the fan - shaped plane.

[0067] For the four described adjusting bolt nuts: the bolts in the probe compression bolt nut 8, transverse bolt nut 9, and longitudinal bolt nut 12 are all double - headed spiral bolts. The fine - thread heads are used to adjust the position of the probe, and the coarse - thread heads are used to fix the position of the probe.

[0068] When adjusting the position of the ultrasonic transmitting probe 4 or ultrasonic receiving probe 6 with the transverse bolt nut 9 and longitudinal bolt nut 12, it is necessary to adjust according to the correlation function waveforms of the surface - wave emission signal and reception signal fed back by the computer 1. First, replace the ultrasonic receiving probe 6 with a cylindrical micro - probe that is convenient for determining the geometric center, and adjust the position of the ultrasonic transmitting probe 4 so that there is only one maximum value near the maximum value of the correlation function waveform, which is this maximum value. After determining the position of the ultrasonic transmitting probe 4, replace it with the ultrasonic receiving probe 6 again and adjust the position so that there is also only one maximum value near the maximum value of the correlation function waveform, which is the maximum value. At this time, it indicates that both the ultrasonic transmitting probe 4 and the ultrasonic receiving probe 6 are located at the center of the fan - shaped plane.

[0069] Before using the probe bracket 7 to fix the ultrasonic receiving probe 6 and ultrasonic transmitting probe 4 on the upper and lower end planes, it is necessary to first clean the bonding surface between the probe and the upper and lower end planes, and then apply a sufficient amount of coupling agent on the side plane of the end to ensure full acoustic energy transfer between the ultrasonic receiving probe 6, ultrasonic transmitting probe 4, and the end surface. Since the ultrasonic receiving probe 6 and ultrasonic transmitting probe 4 need to maintain an accurate position for a long time, are not convenient to disassemble and assemble frequently, and the manipulator often works in different postures, the coupling agent selected is engine oil with good adsorption, high viscosity, and good coupling effect.

[0070] A working method for the end of a manipulator based on intelligent perception of ultrasonic surface - wave contact stress, the steps of which are as follows:

[0071] When the cylindrical end 5A or spherical end 5B of the manipulator holds an object, the ultrasonic transmitter - receiver 3 generates an electrical signal of surface waves, which excites the ultrasonic transmitting probe 4 to convert into surface waves and propagate along the surface of the manipulator end. The ultrasonic receiving probe 6 then converts the received surface waves into electrical signals and transmits them back to the ultrasonic transmitter - receiver 3.

[0072] There is line contact or point contact between the cylindrical surface or spherical surface of the columnar end 5A or spherical end 5B and the clamp, and local contact stress is generated on the surface. Due to the acoustoelastic effect, the propagation speed of the surface wave will change in the stress area, and the time for the surface wave to propagate from the ultrasonic emission probe 4 to the ultrasonic reception probe 6 will change;

[0073] The electrical signals emitted and received by the ultrasonic emission and reception device 3 are received by the data acquisition card 2, and the signals are sampled and digitized; the computer 1 performs cross-correlation analysis on the emitted and received acoustic wave signals to obtain the propagation time of the surface wave within the acoustic path; since the acoustic path of the surface wave is the same, the contact stress becomes the only influencing factor of the time difference. The time difference between when the acoustic wave propagates without stress and when it passes through the stress area is linearly related to the contact stress. Therefore, the corresponding contact stress can be obtained from the measured time difference through the pre-calibrated linear relationship; when the contact stress is too large or exceeds the rated value, a warning is issued or the clamping force on the object is stopped from increasing, preventing the end of the manipulator from causing harmful residual stress, deformation or even damage to the object during the process of clamping objects with high precision, easy deformation and surface quality requirements.

[0074] As Figure 2 and Figure 3 shown, the shape of the clamping part of the special columnar end 5A or spherical end 5B evolves from a cylinder or a sphere. Designing the part in contact with the object into these two shapes is for the following two purposes:

[0075] Ⅰ) On the propagation surface, the surface wave emitted by the ultrasonic emission probe 4 can be received by the ultrasonic reception probe 6 along any direction of propagation. Because when the end contacts the object, due to reasons such as the object shape or the clamping position, the specific contact position between the clamping part and the object is not determined. Therefore, the propagation area of the surface wave must cover the entire surface of the clamping part that may contact the object, and the surface wave passing through the area must be received by the ultrasonic reception probe 6 to ensure that the received signal contains information about the contact stress.

[0076] Ⅱ) On the propagation surface, the acoustic path of the surface wave from the ultrasonic emission probe 4 to the ultrasonic reception probe 6 is the same along any direction. Since this device calculates the contact stress based on the time difference between when the surface wave propagates in the stress area and when there is no stress, ensuring that the acoustic path of the surface wave propagating along each direction is the same can make the contact stress the only influencing factor of the time difference.

[0077] The following will elaborate in detail on the shape design principle of the columnar end 5A and spherical end 5B in combination with the diagrams.

[0078] As shown in Fig. 4(a), when a surface wave propagates to the boundary of the medium surface, if the boundary is an edge, a reflected wave will be generated in the original plane and a refracted wave will also be generated on the side; as shown in Fig. 4(b), if the boundary of the transmission medium is a fillet with a radius of curvature greater than 5 wavelengths, the surface wave will hardly lose energy and continue to propagate; if the propagation speed of sound in the end material is v and the ultrasonic frequency is f, the minimum fillet radius R that can allow the surface wave to propagate directly without energy loss is min = 5v / f. For ultrasonic waves and ordinary steel, the fillet radius is in the millimeter range, which is just a size convenient for implementation. At the same time, the appropriate minimum fillet radius R can be obtained by changing the end material and the ultrasonic frequency f min , so as to realize the propagation of the surface wave between multiple end faces. The probe is placed on different planes, so that the probe can avoid the clamping space at the end

[0079] As Figure 2 shown in Fig. 5(a), Fig. 5(b), and Fig. 5(c), the shape of the clamping part of the columnar end 5A is evolved from a cylinder. As shown in Fig. 5(a), on the surface of the cylinder, the shortest path length from the center of one bottom surface to the center of the other bottom surface in any direction is equal, that is, the lengths of paths ①, ②, and ③ are L1 = L2 = L3 = 2R1 + h (where R1 is the radius of the cylinder and h is the height of the cylinder); as shown in Fig. 5(b), a fillet transition is made at the junction of the cylinder surface and the bottom surface, and the radius of curvature of each fillet surface is the same, so the lengths of paths ①, ②, and ③ still have L ' 1 = L ' 2 = L ' 3; as shown in Fig. 5(c), when a line contact stress is generated on the cylinder surface, due to the acoustoelastic effect, the surface wave propagating along path ② passes through the stress area, and the sound wave velocity will be different from paths ① and ③, and the corresponding propagation times are t1 = t2 ≠ t3 (whether the propagation time in the stress area becomes longer or shorter is determined by the sign of the acoustoelastic coefficient of the material). There will be a time difference Δt for the propagation along path ② compared to the stress-free propagation; as Figure 2 shown, a part of the columnar body in Fig. 5(c) is intercepted as the clamping part of the columnar end 5A; the surface of the columnar end 5A must be in line contact with the object, so it can be used to clamp an object with a flat or cylindrical surface

[0080] As Figure 3As shown in FIGS. 6(a), 6(b), 6(c), and 6(d), the shape of the clamping portion of the spherical end 5B evolves from a sphere. As shown in FIG. 6(a), on the spherical surface, the shortest path length from a point in any direction to the farthest point from this point is equal, that is, the lengths of paths ①, ②, and ③ are L1 = L2 = L3 = πR2 (where R2 is the radius of the sphere); as shown in FIG. 6(b), two planes are symmetrically cut out on the sphere. Since the obtained plane shapes are circular and parallel to each other, the shortest path length from the center of one circular plane to the center of the other circular plane in any direction on the surface is still equal, that is, the lengths of paths ①, ②, and ③ still have L'1 = L'2 = L'3; as shown in FIG. 6(c), a fillet transition is made at the junction of the circular plane and the spherical surface. Similarly, there is still L”1 = L”2 = L”3; as shown in FIG. 6(d), when point contact stress occurs on the spherical surface, similarly, the propagation times of the surface wave along paths ①, ②, and ③ are t1 = t2 ≠ t3, and there will be a time difference Δt in the propagation along path ② relative to the stress-free propagation; as Figure 3 shown, a part of the spheroid in FIG. 6(d) is intercepted as the clamping portion of the spherical end 5B; the surface of the spherical end 5B must be in point contact with the object, so it can be used to clamp an object with an irregular surface.

[0081] The cross-correlation analysis method is used to find the time difference Δt: Since the surface wave signal received by the ultrasonic receiving probe 6 has almost no difference in waveform from the surface wave signal emitted by the ultrasonic transmitting probe 4 except for a time delay and an amplitude reduction, and has a high degree of similarity, the propagation time of the sound wave in the sound path can be obtained through the cross-correlation analysis of the two sound wave signals. Denote x1(t) and x2(t) as the waveform signals of transmission and reception respectively. Since this sound wave signal is a signal with finite energy, the cross-correlation function of x1(t) and x2(t) is:

[0082]

[0083] Since x2(t) lags behind x1(t), it is only necessary to take positive values for τ. When R 12 (τ) reaches the maximum value R max the corresponding time τ is the propagation time of the sound wave in the sound path. When the manipulator does not clamp an object, the contact stress at the end is zero, as Figure 7 the solid line in shows the R 12 (τ) curve. At this time, the cross-correlation function reaches the maximum value at τ = τ0, and τ0 is the propagation time of the sound wave in the sound path without stress. When the manipulator clamps an object, contact stress will be generated at the end, and part of the sound wave will propagate from the stress area, and the propagation speed of the sound wave will change, as Figure 7 the dotted line in shows the R 12As shown by the (τ) curve (since the received acoustic wave energy decreases after contact with the object, the shape of the cross-correlation function curve changes), at this time, the cross-correlation function will reach its maximum value at τ = τ1 (the size relationship between τ0 and τ1 is determined by the sign of the acoustoelastic coefficient of the material), so the time difference Δt can be obtained:

[0084] Δt = Δτ = |τ1 - τ0|

[0085] Since cross-correlation analysis is completed by a computer, the above analog signals are all converted into discrete digital quantities through the data acquisition card 2 for processing. Denote T s as the sampling interval of the data acquisition card 2, and denote the values of the nth sampling points of x1(t) and x2(t) as X1(n) and X2(n), then there are:

[0086] X1(n) = x1(nT s )

[0087] X2(n) = x2(nT s )

[0088] The cross-correlation function of digital signals can be expressed as:

[0089]

[0090] where m is a non-negative integer. As known from the above, when the manipulator does not hold the object, there is a non-negative integer m = m0 such that R 12 (m) reaches its maximum value; when the manipulator holds the object, there is a non-negative integer m = m1 such that R 12 (m) reaches its maximum value. Then there are:

[0091] τ0 = m0T s

[0092] τ1 = m1T s

[0093] Δt = Δτ = |τ1 - τ0| = T s |m1 - m0|

[0094] According to the surface wave acoustoelastic theory: the change in the propagation time of surface waves over a certain acoustic path is a linear function of the surface stress of the elastic body. So the time difference Δt and the contact stress σ have the following relationship:

[0095] Δt = Kσ

[0096] where the constant K is determined by the acoustoelastic coefficient of the material and the acoustic path length that generates the stress. Therefore, the contact stress σ can be linearly represented by the time difference Δt:

[0097]

[0098] Substitute Denoted as K', we have:

[0099] σ = K'Δt

[0100] Therefore, by obtaining K', the linear relationship between the contact stress σ and the time difference Δt can be obtained. To improve the accuracy of obtaining the value of K', experimental calibration can be carried out. Since the contact stress is not equal within the contact width, in order to improve the protection of the clamped object and simplify the calculation, the maximum contact stress σ H is calibrated with the time difference Δt. At the same time, in order to conveniently obtain the maximum contact stress σ H acting on the end clamping part during calibration, the surface of the object applying force to the end clamping part is a plane, and this force is a normal pressure F perpendicular to the plane of the force-applying part n . Since the radius of curvature of the curved surface of the end clamping part is known, the maximum contact stress σ H can be calculated. As shown in Fig. 5(c), when calibrating the cylindrical end, from the contact stress calculation formula, the maximum linear contact stress σ H is:

[0101]

[0102] where E is the elastic modulus of the material, L is the contact length, and R1 is the radius of the cylinder.

[0103] Similarly: As shown in Fig. 6(d), when calibrating the spherical end, the maximum point contact stress σ H is:

[0104]

[0105] where R2 is the radius of the sphere. For different magnitudes of F n loaded, the corresponding σ H can be obtained, and at the same time, the corresponding Δt can be obtained by the above-mentioned correlation analysis method. Using each group of σ H collected by experiments and the corresponding Δt to make a σ H —Δt relationship graph, and draw a fitting straight line. The slope of the fitting straight line is K'. Thus, when the end of the manipulator is in use, the maximum contact stress σ H during clamping can be obtained from the time difference Δt:

[0106] σ H = K'Δt

[0107] Such as Figure 8As shown, each probe bracket is fixed on the upper and lower planes at the end by two countersunk head screws 10 and two cylindrical pins 11 distributed crosswise; four adjusting bolt nuts are respectively arranged on the probe bracket, namely two transverse bolt nuts 9 arranged on the left and right sides of the probe bracket for adjusting and fixing the left and right positions of the probe, a longitudinal bolt nut 12 arranged behind the probe bracket for adjusting and fixing the front and back positions of the probe, and a pressing bolt nut 8 arranged above the probe bracket for pressing the probe, which keep the probe precisely at the center of the fan-shaped plane.

[0108] The adjusting bolt is a double-headed screw, with the fine-threaded head for adjusting the position of the probe and the coarse-threaded head for fixing the position of the probe.

[0109] The debugging method and steps are as follows: First, use a cylindrical micro-probe convenient for determining the geometric center to replace the ultrasonic receiving probe 6 and adjust the position of the ultrasonic transmitting probe 4; as shown by the solid line R Figure 7 in the (τ) curve, when the correlation function R 12 (τ) reaches the maximum value at τ = τ0, there is only one maximum value nearby (i.e., this maximum value); after determining the position of the ultrasonic transmitting probe 4, replace it with the probe bracket 7 and adjust the position so that the cross-correlation function waveform resumes to be like the solid line R 12 shown in the (τ) curve; at this time, it indicates that both the ultrasonic transmitting probe 4 and the ultrasonic receiving probe 6 are located at the center of the fan-shaped plane. Figure 7 in the (τ) curve, as shown by the solid line R 12 (τ); at this time, it indicates that both the ultrasonic transmitting probe 4 and the ultrasonic receiving probe 6 are located at the center of the fan-shaped plane.

[0110] In addition, before the probe is fixed on the upper and lower planes at the end, the joint surface between the probe and the upper and lower planes at the end must be cleaned first, and then a sufficient amount of coupling agent is applied on the upper and lower planes at the end to ensure full acoustic energy transfer between the probe and the end surface; since the probe needs to maintain an accurate position for a long time, it is not convenient to disassemble and assemble frequently, and the manipulator often works in different postures, so the coupling agent selected is engine oil with good adsorption, high viscosity and good coupling effect.

Claims

1. A working method for the end of a manipulator based on intelligent perception of contact stress by ultrasonic surface waves, characterized in that: The end of the manipulator includes a pair of mutually matching mechanical fingers as a clamping mechanism. The front end of the mechanical fingers is a columnar end (5A) or a spherical end (5B). It also includes a computer (1), a data acquisition card (2), and an ultrasonic transmitter-receiver (3) that are connected in sequence by lines to an ultrasonic receiving probe (6) above the end of the manipulator and an ultrasonic transmitting probe (4) below it. The contact part of each columnar end (5A) with the clamped object is a hyperbolic structure composed of the cutting surfaces of the sides of two cylinders; an ultrasonic receiving probe (6) is provided at the center of the upper plane of the columnar end (5A) through a probe holder (7), and a transmitting probe (4) is provided at the center of the lower plane through a probe holder (7); when the clamped surface of the object is a plane or a cylinder, each columnar end (5A) is used for clamping, so as to monitor the line contact stress in real time. The contact part of each spherical end (5B) with the clamped object is a partial cutting structure of a sphere. An ultrasonic receiving probe (6) is provided at the center of the upper plane of the spherical end (5B) through a probe holder (7), and a transmitting probe (4) is provided at the center of the lower plane through a probe holder (7); when the clamped surface of the object is irregular, the spherical end (5B) is used for clamping to monitor the point contact stress in real time. On the left and right sides of each probe holder (7), there are respectively provided transverse bolt nuts (9) for adjusting and fixing the left and right movement positions of the ultrasonic receiving probe or the ultrasonic transmitting probe. Behind the probe holder (7), there is provided a longitudinal bolt nut (12) for adjusting and fixing the front and back movement positions of the ultrasonic receiving probe or the ultrasonic transmitting probe. Above the probe holder (7), there is provided a pressing bolt nut (8) for pressing the ultrasonic receiving probe or the ultrasonic transmitting probe. The bolts in the pressing bolt nut (8), the transverse bolt nut (9), and the longitudinal bolt nut (12) are all double-headed screw bolts. The fine-threaded screw heads of the double-headed screw bolts are used to adjust the positions of the ultrasonic receiving probe or the ultrasonic transmitting probe, and the coarse-threaded screw heads are used to fix the positions of the ultrasonic receiving probe or the ultrasonic transmitting probe. The steps of the working method are as follows: When the columnar end (5A) or the spherical end (5B) of the manipulator clamps an object, the ultrasonic transmitter-receiver (3) generates an electrical signal of an ultrasonic surface wave, which excites the ultrasonic transmitting probe (4) to emit an ultrasonic surface wave to propagate along the surface of the end of the manipulator. The ultrasonic receiving probe (6) then converts the received ultrasonic surface wave into an electrical signal and transmits it back to the ultrasonic transmitter-receiver (3). There is a line contact between the cylindrical surface of the columnar end (5A) and the object, or a point contact between the spherical surface of the spherical end (5B) and the object, and local contact stress is generated on the surface. The time for the ultrasonic surface wave to propagate from the ultrasonic transmitting probe (4) to the ultrasonic receiving probe (6) will change. The electrical signals transmitted and received by the ultrasonic transmitter-receiver (3) are received by the data acquisition card (2), and the signals are sampled and digitized. The computer (1) performs a cross-correlation analysis on the emitted and received ultrasonic surface wave signals to obtain the propagation time of the surface wave within the acoustic path. The corresponding contact stress is obtained from the measured time difference. When the contact stress exceeds the rated value, a warning is issued or the clamping force on the object is stopped from increasing.

2. The working method of the end of the manipulator based on intelligent perception of contact stress by ultrasonic surface waves according to claim 1, wherein: The cross-correlation analysis method is used to obtain the time difference Δt: The propagation time of the ultrasonic surface wave within the acoustic path is obtained by the cross-correlation analysis of the transmitted and received ultrasonic surface wave signals. Let x1(t) and x2(t) be the transmitted and received waveform signals respectively. Since x1(t) and x2(t) are energy-limited signals, the cross-correlation function of x1(t) and x2(t) is: Since x2(t) lags behind x1(t), τ only needs to take positive values. When R 12 (τ) reaches the maximum value R max the corresponding time τ is the propagation time of the ultrasonic surface wave within the acoustic path. When the end of the manipulator does not hold an object, the contact stress at the end is zero. At this time, the cross-correlation function reaches the maximum value at τ = τ0, and τ0 is the propagation time of the ultrasonic surface wave within the acoustic path without stress. When the end of the manipulator holds an object, a contact stress will be generated at the end, and part of the ultrasonic surface wave will propagate from the stress area, and the propagation speed of the ultrasonic surface wave will change. At this time, the cross-correlation function will reach the maximum value at τ = τ1. Therefore, the time difference Δt is obtained as follows: Δt = Δτ = |τ1 - τ0| Denote T s as the sampling interval of the data acquisition card (2). Denote the values of the n-th sampling points of x1(t) and x2(t) as X1(n) and X2(n), respectively. Then we have: X1(n) = x1(nT s ) X2(n) = x2(nT s ) The cross-correlation function of digital signals is expressed as: where m is a non - negative integer. When the end - effector of the manipulator does not hold an object, there exists a non - negative integer m = m0 such that R 12 (m) reaches the maximum value; When the end of the manipulator holds an object, there is a non - negative integer m = m1 such that R 12 (m) reaches the maximum value; That is: τ0 = m0T s τ1 = m1T s Δt = Δτ = |τ1 - τ0| = T s |m1 - m0| The time difference Δt and the contact stress σ have the following relationship: Δt = Kσ The contact stress σ is linearly represented by the time difference Δt: Denote as K', then we have: σ = K'Δt When calibrating the columnar end, according to the contact stress calculation formula, the maximum linear contact stress σ H is as follows: Where, E is the material elastic modulus of the clamping part, L is the contact length, and R1 is the cylinder radius; When calibrating the spherical end, the maximum point contact stress σ H is as follows: wherein, R2 is the radius of the sphere; for different loading magnitudes F n , the corresponding σ H can be obtained, and at the same time, the corresponding Δt is obtained by the above-mentioned correlation analysis method; for each group of σ H and the corresponding Δt, a σ H —Δt relationship graph is plotted, and a fitting straight line is drawn. The slope of the fitting straight line is K'; the maximum contact stress σ H when the end of the manipulator holds an object during use: σ H = K'Δt.

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