A method for predicting the drilling resistance of lunar soil based on axis offset

Abstract

The present invention proposes a method for predicting the drilling resistance of lunar soil based on axis offset, belonging to the technical field of lunar soil exploration. For the analysis of the movement of the drilling tool and the soil body, the feeding movement, rotary movement and chip removal process of the drilling tool are first defined, and an equivalent model is established; then based on the reaming mechanism, the soil expansion stage from elastic, elastoplastic to fully plastic is analyzed to predict the change of resistance during the drilling process; furthermore, according to the agitation mechanism, the soil interaction generated by the movement of the drilling tool is analyzed to predict the agitation resistance on the surface of the drilling tool; ignoring the friction between the samples collected inside the drilling tool and the inner wall of the drill pipe, a drilling resistance model is constructed; finally, the rationality and accuracy of the model are verified through experiments. The present invention can adapt to different drilling and mining conditions and soil conditions; at the same time, it provides a basic guarantee for subsequent research on lunar soil parameter identification, adaptive drilling and mining control, etc., and helps to promote the development of lunar exploration technology.

Description

Technical Field

[0001] The present invention belongs to the technical field of lunar soil exploration, and specifically, relates to a method for predicting the drilling resistance of lunar soil based on axis offset. Background Art

[0002] The drilling resistance is the resultant force generated in the vertical direction due to the complex interactions such as cutting, extrusion, and friction between the drill tool and the soil during the process of planetary drilling and sampling. Predicting and evaluating the drilling resistance during the drilling and sampling process is the prerequisite for ensuring the efficient and reliable completion of the sampling task, and also the basis for conducting research on planetary drilling and sampling technologies such as identifying soil parameters and highly adaptive unmanned automatic control.

[0003] During the process of lunar soil drilling, the slender drill pipe will inevitably undergo axis offset, thus changing the force condition of the drill tool. Regarding the research on the formation mechanism of drilling resistance and the resistance prediction model, there are currently mainly two research methods: the discrete element method and the analytical method. Among them, the discrete element method has been widely used in recent years because it can describe the granular characteristics of lunar soil. Liu Tianxi et al. constructed a discrete element constitutive model that can reflect the mechanical characteristics such as the large internal friction angle and small cohesion of lunar soil according to the structural characteristics of lunar soil particles with multiple pores and corners, analyzed the motion field and stress field distribution of lunar soil particles under different drilling and sampling conditions, and also analyzed the influence of the distribution of large particles such as rocks on the load during the sampling process. Jiang Mingjing et al. added van der Waals forces to the discrete element particle contact model to simulate the cohesive interaction in the lunar environment and numerically studied the cone penetration test in lunar soil. Chen et al. used DEM to reveal the motion and stress characteristics of the transported particles in the working screw conveyor and established a simplified dynamic model, which can well explain the proportional relationship between the maximum transport rate and the screw conveyor speed in the experiment. In addition, the discrete element method has also been applied to the simulation study of the interaction behavior between other lunar exploration mechanisms and lunar soil, and even to the analysis of the stability of small-diameter horizontal tunnels on the lunar surface. Although the discrete element method can intuitively simulate the motion of lunar soil particles and provide support for the research on the mechanism of machine-soil interaction at the mesoscopic level, the massive inter-particle operations lead to low discrete simulation efficiency and long time consumption, which is not conducive to the subsequent research on real-time identification and inversion of lunar soil state.

[0004] However, the research progress of the analytical method is much slower than that of the discrete element method. The difficulty lies in the need to fully consider the motion characteristics of the drill string during the drilling and extraction process, and establish a cross-scale correlation between the mesoscopic particle state and the macroscopic soil mechanical behavior by combining the mechanical properties of lunar soil. Some scholars considered the damage effect of the drill bit cutting edge on the soil, and based on the passive earth pressure theory, constructed a failure zone of lunar soil in front of the cutting edge, and calculated the drilling load through the force and geometric relationships in the failure zone. Other scholars analyzed the mutual influence relationship between the geometric structure parameters of the drill pipe and the rotary torque by constructing a lunar soil movement model in the spiral groove of the drill pipe, and realized the optimization of the drill pipe structure design. However, the above models and related tests only focused on the change of the rotary torque during the drilling process, while ignoring the drilling resistance. Tang integrated the shear dilation model under the rotary cutting action of lunar soil, the coring mechanics model under the silo effect, and the migration model of drill cuttings in the spiral groove to construct an adaptive drilling strategy based on specific drilling samples. However, the drill bit shape used is not the double-row stepped configuration adopted in the lunar exploration project, and there are essential differences in the formation mechanism of the drilling resistance. Zhang et al. established a drilling load model based on the Coulomb-Mohr soil mechanics law under quasi-static conditions to predict the drilling resistance and rotary torque. However, the equivalent self-tapping effect of the drill pipe in the model is too strong, resulting in zero penetration resistance after drilling to a certain depth, which is contrary to the test results. Zhou et al. successively constructed a semi-empirical prediction model of drilling resistance based on ground test data and a theoretical prediction model of drilling resistance based on the equivalent analytical solution of the expansion problem of the lunar soil column hole during the penetration process of the drill bit at the mesoscopic level. In the former, the interaction between the machine and the soil is simplified as a mapping of the change in soil density, lacking a deeper analysis of the formation mechanism of the drilling resistance, and the fitting model is too dependent on test samples. There is a large deviation between the calculation results of the latter model and the test results, and further optimization is needed. In addition, Tian et al. constructed a drill-soil interaction model based on the virtual soil pile method and analyzed the relationship between the geotechnical properties and the velocity frequency response when impacting the lunar regolith. Chen et al. proposed a multi-state control strategy for autonomous lunar drilling, using support vector machine (SVM) and continuous wavelet transform to identify the drilling medium and interface online respectively. However, this method lacks the discussion and analysis of the mechanism of interaction between the machine and the soil. In addition, for the problem of deep hole drilling with a long diameter ratio of the borehole greater than 10, due to the influence of factors such as the installation accuracy of the drill string and the force deformation of the drill string during the drilling process, the phenomenon of drill string axis inclination is common. When the axis of the drill string deviates, it will cause a change in the attitude of the drill string during operation, affect the acting direction of the surface load of the drill string, and thus affect the drilling resistance. The mechanical complexity of the lunar unmanned automated drilling and extraction mechanism is high, with a high risk of failure, and the motor load of the detector is very limited. Therefore, the change in the drilling resistance caused by the deviation of the drill string axis has a decisive impact on the smooth completion of the drilling and extraction task.In summary, although the research on the analytical method is difficult and the research progress is relatively slow, the analytical model is the basic guarantee for subsequent research on lunar soil parameter identification, adaptive drilling control, etc. Therefore, it is very necessary and meaningful to explore the formation mechanism of drilling resistance and construct a corresponding analytical model for resistance prediction. Summary of the Invention

[0005] In view of the above problems, the present invention proposes a method for predicting lunar soil drilling resistance based on axis offset. The drilling process under the inclined state of the drill tool is decomposed and equivalent. Based on the equivalent analytical solution of the soil column hole expansion problem under the rotary state and the equivalent solution of the additional soil pressure under the inclined attitude of the drill tool, a drilling resistance prediction model is constructed. The influence of main soil parameters on the prediction results is analyzed through simulation examples, and a simulated lunar soil ground drilling test is carried out to verify the model.

[0006] The present invention is realized through the following technical solutions:

[0007] A method for predicting lunar soil drilling resistance based on axis offset: The method specifically includes the following steps:

[0008] Step 1: Analyze the motion of the drill tool and the soil body, define the feeding motion, rotary motion and chip removal process of the drill tool, and establish an equivalent model;

[0009] Step 2: Based on the hole expansion mechanism, analyze the soil body expansion stages from elastic, elastoplastic to fully plastic, and calculate the corresponding stress, displacement and frictional resistance to predict the resistance change during the drilling process;

[0010] Step 3: According to the agitation mechanism, analyze the soil body interaction generated by the drill tool motion, equivalently calculate the soil pressure and consider the change of soil body density to predict the agitation resistance on the surface of the drill tool;

[0011] Step 4: Based on Steps 1 to 3, ignoring the friction between the internal sample collection of the drill tool and the inner wall of the drill pipe, construct a drilling resistance model;

[0012] Step 5: Verify the accuracy of the model constructed in Step 4.

[0013] Furthermore, in Step 1,

[0014] The feeding motion is that the whole drill tool moves vertically downward. At this time, the soil body around the drill bit is extruded by the drill bit and moves radially, being in a compressed state. At the same time, the soil body below the drill bit will enter the drill pipe through the sampling hole; Therefore, the feeding motion causes a relative displacement between the drill tool and the soil body, and the surface of the drill tool will be subject to a frictional resistance in the opposite direction of the motion;

[0015] The rotary motion is that the drill tool rotates around its axis. At this time, the cutting edge on the drill bit will destroy the soil body structure, making the soil body around the drill bit become loose and move circumferentially under the drive of the drill bit;

[0016] Under the combined action of feeding and rotation, the soil at the edge of the drill bit will be transported to the lunar surface by the drill pipe threads.

[0017] However, the fixed-axis rotation of the drilling tool is an interference motion caused by errors. Due to the slender structure of the drill pipe, this rotation will be amplified at the drill bit. When the drilling tool rotates around a fixed axis, it will continuously stir the soil, causing the surface of the drilling tool to be subjected to the extrusion load of the soil, and this load has the characteristic of periodic change. That is, the motion of the drilling tool is equivalently decomposed into a vertically downward motion and a fixed-axis rotation with a fixed end.

[0018] When the drilling and mining conditions remain unchanged, it is assumed that the soil flow velocity generated by the rotation of the drilling tool is the same as the rotation speed, that is, the lunar soil and the drilling tool are relatively stationary in the circumferential direction. At the same time, it is assumed that the lunar soil carried on the drill pipe threads starts to be filled at the drill bit and is transported upward with a stable powder discharge amount. The flow characteristics of the lunar soil under the rotation action are equivalently regarded as the change of the macroscopic mechanical properties of the lunar soil.

[0019] Furthermore, in step 2, the vertically downward motion of the drilling tool is equivalently regarded as the process of pile penetration. Then, the pressure load received by the surface of the drilling tool is equivalently regarded as the expansion pressure load at the hole wall during the penetration process.

[0020] Step 2.1: Elastic expansion stage

[0021] Calculate the stress distribution function: Derive the radial normal stress and tangential normal stress, involving the reaming pressure, initial pressure, Poisson's ratio, and the angular velocity of soil rotation.

[0022] Calculate the displacement distribution function: Describe the displacement distribution during the elastic deformation of the soil related to the elastic modulus of the soil.

[0023] Step 2.2: Elastic-plastic expansion stage

[0024] Plastic zone and elastic zone: The soil deformation is divided into a plastic zone and an elastic zone, and the boundary radius and initial position of the plastic zone are determined.

[0025] Based on the Coulomb-Mohr yield criterion, determine the soil yield condition, introduce the internal friction angle and cohesion, derive the stress distribution function of the plastic zone, solve the stress distribution function of the elastic zone, and describe the displacement distribution of the outer boundary of the soil and within the plastic zone based on the differential equilibrium equation, stress-strain relationship, and boundary conditions of the elastic zone.

[0026] Step 2.3: Complete plastic expansion stage;

[0027] When the soil enters the complete plastic state, the elastic-plastic boundary moves to the outer boundary of the soil, and the soil enters the complete plastic state. Derive the stress distribution function, displacement distribution function, and hole diameter expansion relationship in the complete plastic stage.

[0028] Step 2.4: Calculate the reaming friction resistance: Specifically, it is the friction resistance between the drill string and the soil layer.

[0029] Further, in Step 3, when there is a very small deviation angle between the axis of the drill string and the feed rate, the rotational movement of the drill string will cause a fixed-axis rotation of the same frequency. Ignoring the surface threads and the protrusions of the drill bit, the shape of the drill string is simplified to a slender round rod. Assuming that the lunar soil around the drill string is in a state of limit equilibrium at any moment during this rotation process, the interaction between the lunar soil and the drill string can be equivalently calculated by the earth pressure of the retaining wall.

[0030] Step 3.1: Calculate the equivalent earth pressure; Based on the theory of earth pressure of the retaining wall, describe the equivalent passive earth pressure coefficient and the equivalent active earth pressure coefficient on the surface of the drill string.

[0031] Step 3.2: Calculate the unit weight distribution; Based on the influence of the drill string rotation, analyze the change of the soil density during the fixed-axis rotation of the drill string, and deduce the unit weight distribution function.

[0032] Step 3.3: Calculate the vertical component of the earth pressure load on the surface of the drill string, and calculate the agitation resistance according to the equivalent passive earth pressure coefficient and the equivalent active earth pressure coefficient.

[0033] Further, in Step 4, the samples collected inside the drill string will rub against the inner wall of the drill pipe. However, due to the low density of the samples, the pressure generated on the inner wall of the drill pipe is small. Therefore, this friction load is ignored, and the drilling resistance received by the drill string during the drilling process is regarded as the resultant force in the vertical direction of all the loads on the part of the surface interacting with the lunar soil.

[0034] Further, in Step 5, the equivalent elastic modulus, equivalent cohesion, equivalent internal friction angle, and equivalent earth pressure coefficient of the soil are set respectively for experimental verification.

[0035] A lunar soil drilling resistance prediction system based on axis offset:

[0036] The prediction system includes the following drill string and soil analysis module, reaming mechanism module, agitation mechanism module, drilling resistance construction module, and verification module:

[0037] The drill string and soil analysis module defines the feed motion, rotational motion, and chip removal process of the drill string, and establishes an equivalent model.

[0038] The reaming mechanism module analyzes the soil expansion stage from elastic, elastoplastic to fully plastic, and calculates the corresponding stress, displacement, and friction resistance to predict the resistance change during the drilling process.

[0039] The agitation mechanism module analyzes the interaction of the soil generated by the movement of the drill string, equivalently calculates the earth pressure, and considers the change of the soil density to predict the agitation resistance on the surface of the drill string.

[0040] The drilling resistance construction module: ignoring the friction between the samples collected inside the drill string and the inner wall of the drill pipe, constructs a drilling resistance model;

[0041] The verification module is used to verify the accuracy of the model.

[0042] An experimental system for a lunar soil drilling resistance prediction system based on axis offset:

[0043] The experimental system consists of a drill string motion simulation test bench, a control cabinet, and a test chamber.

[0044] Among them, the drill string motion simulation test bench mainly includes a drill string, moving slide rails in the x, y, and z directions, a rotary motor, and a pressure sensor; the control cabinet includes two parts: control buttons and a parameter setting panel; the test chamber is used to fill simulated lunar soil;

[0045] The drill string structure used in the drilling and mining test is a double-row 8-tooth stepped configuration. The drill bit has 8 cutting edges, which can continuously cut and destroy the soil below the drill string through the rotary motion of the drill string, turning it into loose particles; the inside of the drill pipe is hollow, and when the drill string makes a feeding motion, the loose soil directly below the drill string can be collected into the drill pipe cavity; the drill pipe is wound with double-row threads, which drives the soil particles on the drill pipe wall to move upward and be discharged to the soil surface during the drilling process.

[0046] An electronic device includes a memory and a processor. The memory stores a computer program, and when the processor executes the computer program, the steps of the above method are implemented.

[0047] A computer-readable storage medium is used to store computer instructions, and when the computer instructions are executed by a processor, the steps of the above method are implemented.

[0048] Advantages of the present invention

[0049] The present invention not only considers the mechanical property parameters of the soil, such as elastic modulus, cohesion, internal friction angle, and earth pressure coefficient, but also considers the inclination state of the drill string axis, which provides a more comprehensive analysis for more accurately predicting the drilling resistance.

[0050] The present invention decomposes the drilling process into two processes: the drill bit penetrating the soil and the drill pipe stirring the soil, and constructs a new drilling resistance prediction model based on the equivalent analytical solution of the soil column hole expansion problem under the rotary state and the equivalent solution of the additional earth pressure under the inclined posture of the drill string. Sensitivity analysis is carried out on the main soil mechanical property parameters in the model to further understand the influence degree of each parameter on the drilling resistance.

[0051] The present invention verifies the rationality and accuracy of the model through simulation example analysis and simulated lunar soil surface drilling and mining tests, ensuring the practicability and reliability of the model.

[0052] The technical solution of the present invention has strong adaptability and versatility, and can adapt to different drilling and production working conditions and soil conditions; at the same time, it provides a basic guarantee for subsequent research on lunar soil parameter identification, adaptive drilling and production control, etc., and helps to promote the development of lunar exploration technology. Brief Description of the Drawings

[0053] Figure 1 It is a schematic diagram of the present invention equivalenting the flow characteristics of lunar soil under the rotary action to the change of the macroscopic mechanical properties of lunar soil.

[0054] Figure 2 It is a schematic diagram of the hole of the cylindrical finite-rotation soil in the elastoplastic expansion stage.

[0055] Figure 3 It is a schematic diagram of the hole-expanding frictional resistance of each layer of soil.

[0056] Figure 4 It is a schematic diagram of the equivalent earth pressure of each layer of soil.

[0057] Figure 5 It is the horizontal section of the i-th layer of soil where the drilling tool is located.

[0058] Figure 6 It is the drilling resistance curve within 200 s when the equivalent elastic modulus of the soil is set to 150 kPa, 250 kPa, 350 kPa, the equivalent cohesive force is 20 kPa, the equivalent internal friction angle is 40°, and the equivalent earth pressure coefficient is 15.

[0059] Figure 7 It is the pressure change at the hole wall during the hole-expanding process of the single-layer soil with different elastic moduli of the 1000th layer of soil when the equivalent elastic modulus of the soil is 150 kPa, 250 kPa, 350 kPa, the equivalent cohesive force is 20 kPa, the equivalent internal friction angle is 40°, and the equivalent earth pressure coefficient is 15.

[0060] Figure 8 It is the drilling resistance curve within 200 s when the equivalent cohesive force of the soil is set to 10 kPa, 15 kPa, 20 kPa, the equivalent elastic modulus is 250 kPa, the equivalent internal friction angle is 40°, and the equivalent earth pressure coefficient is 15.

[0061] Figure 9 It is the pressure change at the hole wall during the hole-expanding process of the single-layer soil with different elastic moduli of the 1000th layer of soil when the equivalent cohesive force of the soil is 10 kPa, 15 kPa, 20 kPa, the equivalent elastic modulus is 250 kPa, the equivalent internal friction angle is 40°, and the equivalent earth pressure coefficient is 15.

[0062] Figure 10When the equivalent internal friction angles of the soil are 40°, 50°, and 60° respectively, the equivalent elastic modulus is 250 kPa, the equivalent cohesion is 20 kPa, and the equivalent earth pressure coefficients are 15, the drilling resistance curve within 200 s.

[0063] Figure 11 When the equivalent internal friction angles of the soil are 40°, 50°, and 60° respectively, the equivalent elastic modulus is 250 kPa, the equivalent cohesion is 20 kPa, and the equivalent earth pressure coefficients are 15, the pressure change at the hole wall during the reaming process of a single-layer soil under different elastic moduli of the 1000th layer of soil.

[0064] Figure 12 The equivalent mixing resistance change curves within 200 s under three internal friction angle conditions.

[0065] Figure 13 When the equivalent earth pressure coefficients of the soil are 9, 12, and 15 respectively, the equivalent elastic modulus is 250 kPa, the equivalent cohesion is 20 kPa, and the equivalent internal friction angle is 40°, the drilling resistance curve within 200 s.

[0066] Figure 14 Schematic diagram of the test system of the present invention.

[0067] Figure 15 Schematic diagram of the entry trajectory when the drill tool is inclined.

[0068] Figure 16 Schematic diagram of the experimental results when the equivalent internal friction angle of the soil is set to 40°, the equivalent elastic modulus is 250 kPa, the equivalent cohesion is 20 kPa, the equivalent earth pressure coefficient is 15, and the inclination angle of the drill tool is set to 0.74°.

[0069] Figure 17 Frequency amplitude curve after Fourier transform for frequency domain analysis of the test results. Detailed implementation manners

[0070] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0071] The experimental methods used in the following embodiments are all conventional methods unless otherwise specified. The materials, reagents, methods, and instruments used, unless otherwise specified, are all conventional materials, reagents, methods, and instruments in this field, and those skilled in the art can obtain them through commercial channels.

[0072] In conjunction withFigures 1 to 17 , the present invention proposes a lunar soil drilling resistance prediction system based on axis offset. By simplifying the interaction between the drill tool and the lunar soil, the drilling process is equivalent to the process of soil column hole expansion and inclined drill rod stirring the soil. The influence of soil mechanical property parameters (such as elastic modulus, cohesion, internal friction angle, and earth pressure coefficient) on the drilling resistance is analyzed, and the rationality and accuracy of the model are verified through ground tests.

[0073] Through the simplification and equivalence of the interaction process between the drill tool and the lunar soil during the drilling and mining process, based on the equivalent analytical solution of the soil column hole expansion problem under the rotary state and the equivalent solution of the equivalent additional earth pressure under the inclined attitude of the drill tool, a drilling resistance prediction model is constructed. The sensitivity of different soil property parameters in the model to the drilling resistance prediction results is analyzed, and the accuracy of the drilling resistance prediction model is verified through simulated lunar soil drilling and mining tests, and the following conclusions are obtained:

[0074] 1) The drilling process is simplified into two processes: the drill bit penetrating the soil and the drill rod stirring the soil, and the rotary process of the soil is equivalent to the change of soil mechanical properties. The equivalent analytical solutions of the finite soil column hole expansion problem under the rotary state and the equivalent additional earth pressure during the soil stirring process are solved respectively. Combining the two finally constructs a drilling resistance prediction model.

[0075] 2) Analyze the influence of the main soil mechanical property parameters in the model on the drilling resistance prediction results, including elastic modulus, cohesion, internal friction angle, and earth pressure coefficient. Cohesion and earth pressure coefficient can significantly change the magnitude of the drilling resistance. The larger the values of cohesion and earth pressure coefficient, the greater the drilling resistance. Although the elastic modulus and internal friction angle also have an impact on the drilling resistance, the impact is very limited.

[0076] 3) Conducted simulated lunar soil ground drilling and mining tests. The comparison between the test and simulation results shows that the drilling resistance prediction model constructed by the present invention can describe the change law of the drilling resistance and verifies the accuracy of the model.

[0077] Step 1, analysis of the movement of the drill tool and the soil; by equating the drilling process to the processes of soil column hole expansion and inclined drill rod stirring the soil, it lays the foundation for subsequent mathematical derivation and simulation analysis.

[0078] During the drilling and sampling process, the drill tool is fixedly connected by a chuck and moves forward and rotates with the motor. However, the existence of assembly errors will cause a small angle between the axis of the drill tool and the vertical direction, resulting in the drill tool rotating around the vertical axis while rotating. Therefore, the actual trajectory of the drill bit center is a helix, as Figure 1 shown.

[0079] Define the feeding motion: The feeding motion is the overall vertical downward movement of the drill string. At this time, the soil around the drill bit is squeezed by the drill bit and moves radially, being in a compressed state. Meanwhile, the soil below the drill bit will enter the drill pipe through the sampling hole. Therefore, the feeding motion causes relative displacement between the drill string and the soil, and the surface of the drill string will be subject to frictional resistance in the opposite direction of the motion.

[0080] Define the rotary motion: The rotary motion is the drill string rotating around its axis. At this time, the cutting edges on the drill bit will destroy the soil structure, making the soil around the drill bit loose and moving circumferentially under the drive of the drill bit.

[0081] Analyze the chip removal process: Under the combined action of feeding and rotation, the soil at the edge of the drill bit will be transported to the lunar surface by the drill pipe threads. This chip removal process reduces the density of the soil around the drill string and makes the drilling easier. However, the fixed-axis rotation of the drill string is an interference motion caused by errors. Due to the slender structure of the drill pipe, this rotation will be amplified at the drill bit. When the drill string rotates around a fixed axis, it will continuously stir the soil, causing the surface of the drill string to be subject to the extrusion load of the soil, and this load has the characteristic of periodic change.

[0082] Discretize the drilling process into N + 1 time points starting from the initial time of 0 and ending at time T, with a time interval of When t = 0, the drill string and the lunar soil are in the initial state. Then, the time point corresponding to the simulation step s is t s = s·Δt, s = 1, 2, …, N. The motion of the drill string within the time interval Δt is equivalently decomposed into a vertical downward motion and a fixed-axis rotation with a fixed end, as Figure 1 shown. The downward movement distance Δh and the rotated angle Δφ can be expressed as:

[0083]

[0084] where: v is the feeding speed; n is the rotary speed.

[0085] Meanwhile, the soil is discretely stratified with Δh as the unit depth, and the drilling depth corresponding to the s-th step is h = s·Δh.

[0086] When the drilling and mining conditions remain unchanged, assume that the soil flow velocity generated by the rotation of the drill string is the same as the rotary speed, that is, the lunar soil is relatively stationary with the drill string in the circumferential direction. Meanwhile, assume that the lunar soil carried on the drill pipe threads starts to be filled from the drill bit and is transported upward with a stable chip removal amount. Then, the flow characteristics of the lunar soil under the rotary action can be equivalently considered as the change in the macroscopic mechanical properties of the lunar soil, such as elastic modulus, cohesion, earth pressure coefficient, etc.

[0087] Step 2: Analysis of the reaming mechanism; Use the equivalent model to analyze the influence of soil mechanical property parameters (such as elastic modulus, cohesion, internal friction angle, earth pressure coefficient) on the drilling resistance, and provide key input parameters for the model.

[0088] Analyze the equivalent penetration process: The vertical downward movement of the drill string can be equivalent to the pile penetration process. Then, the pressure load on the surface of the drill string can be equivalent to the expansion pressure load at the hole wall during the penetration process. For a homogeneous and isotropic thick-walled cylindrical soil mass with an inner diameter of a0, an outer diameter of b0, and a density of ρ, according to the existing technology, the relationship between the hole expansion pressure p1 and the hole diameter a can be derived in the elastic stage, elastoplastic stage, and fully plastic stage.

[0089] Step 2.1, Elastic expansion stage:

[0090] Calculate the stress distribution function: Derive the radial normal stress and tangential normal stress, which involve the hole expansion pressure, initial pressure, Poisson's ratio, and the angular velocity of soil rotation.

[0091] The stress distribution function when the soil undergoes elastic deformation is:

[0092]

[0093] In the formula: σ r is the radial normal stress; σ θ is the tangential normal stress; p1 is the hole expansion pressure; p0 is the initial pressure; ν is Poisson's ratio; ω = 2πn is the angular velocity of soil rotation.

[0094] The displacement distribution function is: Describe the displacement distribution when the soil undergoes elastic deformation, which is related to the elastic modulus of the soil.

[0095]

[0096] In the formula: E is the elastic modulus of the soil.

[0097] Step 2.2, Elastoplastic expansion stage:

[0098] As the hole diameter expands, the soil deformation will be divided into two parts: the plastic zone and the elastic zone, as Figure 2 shown. Let the radius of the elastoplastic zone boundary be c, and the initial position of the material point corresponding to this boundary be c0.

[0099] When the soil yields, it satisfies the Coulomb-Mohr yield criterion:

[0100]

[0101] In the formula: is the internal friction angle, and C is the cohesion. To simplify the subsequent formulas, denote Then the stress distribution function in the plastic zone can be expressed as:

[0102]

[0103] In the formula:

[0104] The stress distribution function in the elastic zone can be solved according to the differential equilibrium equation in the elastic zone, the stress-strain relationship, and the boundary conditions at r = b:

[0105]

[0106] Where: The displacement at the outer boundary of the soil mass satisfies:

[0107]

[0108] The displacement relationship at the boundary is:

[0109]

[0110] The displacement distribution function in the plastic zone:

[0111]

[0112] Where: The three coefficients are respectively:

[0113]

[0114] By introducing an infinite series, the hole diameter expansion relationship in the plastic zone can be obtained as:

[0115]

[0116] Where: The infinite series of order j

[0117] Step 2.3: Fully plastic expansion stage

[0118] When the soil mass enters the fully plastic state, the elastic-plastic boundary moves to the outer boundary of the soil mass. Substituting c = b into equations (5), (7), and (11), the stress distribution function, displacement distribution function, and hole diameter expansion relationship in the fully plastic stage can be obtained respectively.

[0119] Step 2.4: Calculate the reaming friction resistance: Specifically, it is the friction resistance between the drill string and the soil layer, which involves the equivalent friction coefficient and the reaming pressure.

[0120] When the drilling time is t s , the drill string interacts with the soil mass in layer s, as Figure 3 shown.

[0121] From the expansion state of the i-th layer of soil mass, the friction resistance f i between this layer of soil mass and the drill string is:

[0122] ​

[0123] where: μ is the equivalent friction coefficient between the soil and the surface of the drilling tool, p i is the hole expansion pressure when the hole diameter expands to a i , that is, the radial compressive stress at the hole wall of the i-th layer of soil θ i is the angle between the normal vector of the drilling tool surface in contact with the i-th layer of soil and the radial direction.

[0124] Therefore, the frictional resistance F1 generated by the equivalent hole expansion process at this moment can be obtained (s) as follows:

[0125]

[0126] Step 3: Study the agitation mechanism, analyze the deviation angle between the axis of the drilling tool and the feed rate, and equivalently calculate the interaction of the soil around the drilling tool through the earth pressure of the retaining wall to further refine the prediction of the drilling resistance.

[0127] When there is a very small deviation angle θ e between the axis of the drilling tool and the feed rate, the rotary motion of the drilling tool will inevitably cause a fixed-axis rotation of the same frequency. Ignoring the surface threads and the protrusions of the drill bit, the shape of the drilling tool is simplified to a slender round rod. Assuming that the lunar soil around the drilling tool is in a limit equilibrium state at any moment during this rotation process, the interaction of the lunar soil on the drilling tool can be equivalently calculated through the earth pressure of the retaining wall.

[0128] Step 3.1: Calculate the equivalent earth pressure

[0129] Passive earth pressure and active earth pressure: Describe the equivalent passive earth pressure load and equivalent active earth pressure load received on the surface of the drilling tool, involving the equivalent earth pressure coefficient and the unit weight of the soil.

[0130] Taking the initial state when the center of the drill bit is on the soil surface plane and the feed rate is along the vertical direction, after a time of Δt, the center of the drill bit generates a displacement of Δh in the vertical direction. Then for the soil with a unit depth of Δh, when drilling to the t s moment, the drilling tool interacts with s layers of soil in total, and the total drilling depth is h = s·Δh, as Figure 4 shown.

[0131] According to the motion attitude of the drill string, it can be known that at any moment, the soil on one side of the drill string is in a passive extrusion state, while the soil on the other side is in an active sliding state. Therefore, the surface of the drill string is equivalent to a retaining wall. When the drill string stirs the soil, the surface of the drill string will be subjected to the equivalent earth pressure exerted by the soil. Based on the earth pressure theory of retaining walls, the soils in different states on both sides of the drill string apply equivalent passive earth pressure loads and equivalent active earth pressure loads to the surface respectively. The loads form an angle δ (equivalent friction angle, related to the shape of the drill string and the properties of the soil) with the surface normal, but the passive earth pressure acts below the horizontal section, while the active earth pressure acts above the horizontal section.

[0132] For the i-th layer of soil, the drilling depth is h i = i·Δh, (i = 1, 2, …, s), and the equivalent earth pressure load p i on the surface of the drill string is:

[0133]

[0134] In the formula: K is the equivalent earth pressure coefficient, which is determined by the state of the soil; γ is the unit weight of the soil.

[0135] Step 3.2: Calculate the unit weight distribution

[0136] Influence of drill string rotation: Analyze the change of soil density during the fixed-axis rotation of the drill string, and deduce the unit weight distribution function, which involves the distribution functions of the cross-sectional position of the drill string and the length of the soil in the acting area.

[0137] During the fixed-axis rotation of the drill string, the density of the soil around the drill string is constantly changing. The horizontal section of the i-th layer of soil (depth h i ) is as shown in Figure 5 . Among them, the cross-section of the acting area is a circle with O as the center and b as the radius. Establish a polar coordinate system for the cross-section of the acting area with O as the pole and the Ox axis as the polar axis.

[0138] If the drill string is inclined but the feeding speed direction during drilling and mining is still along the axis of the drill string, then the horizontal cross-section position of the drill string at time t s and depth h i is recorded as the initial position. Under the condition of a very small inclination angle, the horizontal elliptical cross-section of the drill string can be approximated as a circular cross-section. Establish a polar coordinate system with the center O i of this cross-section as the pole, and the polar axis Ox i coincides with the polar axis Ox of the acting area. However, when there is a deviation angle θ e between the feeding speed and the axis of the drill string during drilling and mining, the polar axis Ox i of the drill string cross-section rotates around the pole O of the acting area at an angular velocity ω under the combined action of rotation and fixed-axis rotation. The horizontal cross-section position of the drill string at time t s and depth h iThe horizontal cross-section at this position is denoted as the current position. At this time, the polar axis x of the drill string cross-section i The rotated angle is ωt s .

[0139] From the geometric relationship, it is easy to obtain t s The depth h at time t i The distance from the center of the drill string cross-section at this position to the center of the action area, that is, the radius of the circular motion of the cross-section center:

[0140] |OO i | = l d sinθ e -(s - i)Δh tanθ e ≈[l d -(s - i)Δh]θ e (15)

[0141] In the formula: l d Is the total length of the drill string,

[0142] The rotation of the drill pipe will cause the radial soil at the angle θ in the action area to move from point A to point A'. The void ratio of the soil in this radial direction changes, that is, the soil unit weight changes, resulting in a change in the soil pressure on the surface of the drill string here. By equivalently calculating the soil unit weight from the average radial density of the soil in the action area, then at time t s The distribution function of the soil unit weight of the i-th layer of soil is:

[0143]

[0144] In the formula: Is the distribution function of the length of the soil in the action area when the drill string cross-section is at the initial position; Is the distribution function of the length of the soil in the action area when the drill string cross-section is at the current position, and ρ0 is the initial average density of the soil in the action area.

[0145] From the geometric relationship, it is easy to obtain the distribution functions of the lengths of the soils in the two action areas as follows:

[0146]

[0147] Substitute Equation (17) into Equation (16), and the distribution function of the soil unit weight can be obtained.

[0148] From the distribution function of the soil length, it can be seen that when the rotated angle ωt of the drill string s Is an integer multiple of 2π, the position of the drill string cross-section at this moment coincides with the initial position. When the rotated angle ωt of the drill string s Is not an integer multiple of 2π, the position of the drill string cross-section at this moment must intersect with the initial position at two points. Let the polar axis O of the current position of the drill string cross-section i x iThe included angle with the polar axis Ox of the scope plane is φ s , φ s ∈(0, 2π), then by solving equation (17), the polar angles of the two intersection points can be obtained as follows:

[0149]

[0150] Therefore, the connection line of the two intersection points will surely divide the acting area into two equal parts on average. When the polar angle is such that, in this area, the radial length of the soil mass shrinks and the unit weight increases, and the soil taking in this area is in a passive extrusion state. When the polar angle is such that, in this area, the radial length of the soil mass increases and the unit weight decreases, and the soil taking in this area is in an active sliding state.

[0151] Step 3.3: Calculate the stirring resistance

[0152] Calculate the component of the soil pressure load on the surface of the drill string in the vertical direction, which involves the equivalent passive earth pressure coefficient and the equivalent active earth pressure coefficient.

[0153] When the drill string stirs the soil mass, the surface of the drill string is subjected to the action of the soil pressure load, and the set of the components of this load in the vertical direction is the stirring resistance F2. At the depth h i the resistance on the surface of the drill string is:

[0154]

[0155] In the formula: K1 is the equivalent passive earth pressure coefficient; K2 is the equivalent active earth pressure coefficient.

[0156] Denote the equivalent earth pressure coefficient as K0 = K1 - K2, then the stirring resistance F2 at time t s is:

[0157]

[0158] Step 4: Based on the analysis results of Steps 1, 2, and 3, construct a drilling resistance model:

[0159] There will be friction between the samples collected inside the drill string and the inner wall of the drill pipe. However, due to the low density of the samples, the pressure on the inner wall of the drill pipe is small. Therefore, ignoring this frictional load will not affect the analysis of the drilling resistance. Most scholars also ignore the interaction between the samples and the drill string when studying this problem. Therefore, during the drilling process, at time t s the drilling resistance F on the drill string is the resultant force in the vertical direction of all the loads on the surface of the part interacting with the lunar soil, that is:

[0160]

[0161] Step 5: Simulation Examples and Analysis of Parameter Sensitivity Analysis of Model

[0162] The parameters mainly involved in the equivalent model for the mechanical properties of soil include elastic modulus, cohesion, internal friction angle, and earth pressure coefficient. It is necessary to analyze the influence of these coefficients on the change of drilling resistance, which plays a very important role in the analysis of drilling mechanism, model optimization, and subsequent inversion of soil properties, etc.

[0163] According to the existing technology, the preparation indexes of simulated lunar soil, and the test measurement data, the equivalent soil parameters in the simulation are set as shown in Table 1, and the geometric parameters of the drill are shown in Table 2. During the drilling process, the penetration rate is 1.5 mm / s and the rotation rate is 15 r / min.

[0164] Table 1 Equivalent Soil Parameters

[0165]

[0166] Table 2 Geometric Parameters of the Drill

[0167]

[0168] Step 5.1 Set the Elastic Modulus

[0169] Set the equivalent elastic modulus of the soil to be 150 kPa, 250 kPa, and 350 kPa respectively, the equivalent cohesion to be 20 kPa, the equivalent internal friction angle to be 40°, and the equivalent earth pressure coefficient to be 15.

[0170] The drilling resistance curve within 200 s is as Figure 6 shown Figure 6 The middle dash line is the upper boundary of the drilling resistance formed by connecting the peak points of the drilling resistance, and the dotted line is the lower boundary of the drilling resistance formed by connecting the valley points of the drilling resistance. It can be found from the figure that the smaller the elastic modulus, the greater the growth rate of the drilling resistance, and the greater the drilling resistance generated at the same drilling depth. However, the increase in the drilling resistance is very limited. The boundary value of the drilling resistance when the elastic modulus is 250 kPa is greater than the boundary value of the drilling resistance when the elastic modulus is 300 kPa, and the difference between the upper and lower boundaries of the two is within 23 N. The boundary value of the drilling resistance when the elastic modulus is 250 kPa is greater than the boundary value of the drilling resistance when the elastic modulus is 300 kPa, and the difference between the two is only within 12 N. This result shows that the elastic modulus has an impact on the change of the drilling resistance, but the impact is very limited, and the greater the elastic modulus, the less obvious the impact on the drilling resistance.

[0171] Since the elastic modulus is only related to the equivalent reaming process, the change in the drilling resistance is caused by the change in the reaming friction resistance generated during the equivalent reaming process. The magnitude of the reaming friction resistance is determined by the pressure at the hole wall. Therefore, the pressure change at the hole wall during the reaming process of a single-layer soil with different elastic moduli is analyzed. Taking the 1000th layer of soil as an example, the equivalent reaming radius of this layer of soil increases from 7 mm to 17.5 mm during the drilling process, and the pressure at the hole wall is as Figure 7 shown. When the elastic modulus is 250 kPa, the hole wall begins to yield when the hole diameter expands to 7.72 mm. When the elastic modulus is 300 kPa, the hole diameter expands to 7.60 mm. When the elastic modulus is 350 kPa, the hole diameter expands to 7.52 mm. At this time, the pressure at the hole wall for all three cases is 20.7 kPa. It can be seen that the increase in the elastic modulus causes the soil to enter the yield state earlier, but the initial yield stress when the hole wall begins to yield remains unchanged. After entering the yield stage, the hole wall pressure first increases with the expansion of the hole diameter. After reaching the ultimate strength, it instead decreases with the expansion of the hole diameter. And the larger the elastic modulus, the smaller the hole wall pressure when the soil expands to the maximum hole diameter. However, the difference in the hole wall pressure at the end of the expansion is not significant. Therefore, the influence of the elastic modulus on the drilling resistance is very limited.

[0172] Step 5.2 Set the cohesion

[0173] Set the equivalent cohesion of the soil to be 10 kPa, 15 kPa, and 20 kPa respectively, the equivalent elastic modulus to be 250 kPa, the equivalent internal friction angle to be 40°, the equivalent earth pressure coefficient to be 15 m, and the drilling resistance curve within 200 s is as Figure 8 shown.

[0174] Figure 8 The drilling resistance curve within 200 s in [Figure] shows that the change in cohesion has a very obvious influence on the drilling resistance. The greater the cohesion, the greater the growth rate of the drilling resistance, and the greater the drilling resistance generated at the same drilling depth. At the end of the simulation, the drilling resistance boundary value with a cohesion of 15 kPa is 118 N larger than the drilling resistance boundary with a cohesion of 10 kPa, and the drilling resistance boundary value with a cohesion of 20 kPa is 143 N larger than the drilling resistance boundary with a cohesion of 15 kPa.

[0175] Similar to the elastic modulus, the cohesion also acts on the hole wall pressure generated during the equal reaming process. Similarly, the pressure change at the hole wall during the reaming process of the 1000th layer of soil is analyzed, as Figure 9As shown in the figure. When the cohesion is 10 kPa, 15 kPa, and 20 kPa respectively, the hole wall begins to yield when the hole diameter expands to 7.36 mm, 7.54 mm, and 7.73 mm respectively. At this time, the pressures at the hole walls of the three are 11.27 kPa, 16.06 kPa, and 20.86 kPa respectively. This indicates that the greater the cohesion, the larger the reaming radius when the soil reaches the yield condition, the greater the corresponding initial yield stress, and the greater the ultimate strength after the soil yields. In addition, after reaching the ultimate strength, the hole wall pressure decreases as the reaming radius continues to increase, and the greater the cohesion, the faster the rate of decrease of the hole wall pressure. However, since the geometry of the drill tool determines the maximum reaming radius, after the soil stops expanding, the differences in the hole wall pressures corresponding to different cohesions are obvious. To sum up, the magnitude of cohesion has an obvious impact on the magnitude of the drilling resistance predicted by the model. The greater the cohesion, the faster the rate of increase of the drilling resistance, and the greater the resistance generated at the same drilling depth.

[0176] Step 5.3 Set the internal friction angle

[0177] Set the equivalent internal friction angles of the soil to be 40°, 50°, and 60° respectively, the equivalent elastic modulus to be 250 kPa, the equivalent cohesion to be 20 kPa, and the equivalent earth pressure coefficients to be 15. The drilling resistance curve within 200 s is as Figure 10 shown.

[0178] Figure 10 The drilling resistance curve within 200 s in the figure shows that the change in cohesion has a relatively small impact on the drilling resistance, and the increase in the internal friction angle can only slightly change the rate of increase of the drilling resistance. The boundary value of the drilling resistance with an internal friction angle of 50° is less than the drilling resistance boundary with an internal friction angle of 40° within 118 s, and then greater than the drilling resistance boundary with an internal friction angle of 40°; the boundary value of the drilling resistance with an internal friction angle of 60° is less than the drilling resistance boundary with an internal friction angle of 50° within 79 s, and then greater than the drilling resistance boundary with an internal friction angle of 50°. Although the drilling resistance decreases with the increase of the internal friction angle in the early stage of drilling and increases with the increase of the internal friction angle in the later stage, the difference between the boundary values does not exceed 9 N in the early stage and does not exceed 32 N in the later stage, which is very small compared to the overall drilling resistance.

[0179] Taking the soil layer at the 1000th layer as an example, analyze the change of the hole wall pressure during the equivalent reaming process, as Figure 11As shown. It can be seen from the figure that under different internal friction angle conditions, the hole wall starts to yield when it expands to 7.72 mm, and the initial yield stress is 20.86 kPa. After the soil starts to yield, the larger the internal friction angle, the smaller the stress increment. The larger the reaming radius corresponding to the ultimate yield stress, and the smaller the corresponding ultimate yield stress. After reaching the ultimate strength, the larger the internal friction angle, the slower the rate of decrease of the hole wall stress with the expansion of the hole diameter. Since the drilling resistance is the result of the combined action of multiple layers of soil, with the increase of the drilling depth, the overall drilling resistance will inevitably show an increasing trend. From the change of the hole wall pressure, it can be seen that when the internal friction angle is larger, the increment of the drilling resistance is smaller. However, the change of the hole wall pressure caused by the change of the internal friction angle is extremely limited. Therefore, although the increase of the internal friction angle will lead to the decrease of the equivalent reaming friction resistance, the difference in the drilling resistance under different conditions is not obvious.

[0180] The internal friction angle is not only related to the equivalent reaming process but also to the equivalent mixing process. The change of the internal friction angle will cause the change of the equivalent friction angle in the equivalent mixing process, changing the acting direction of the equivalent earth pressure on the surface of the drill tool. Figure 12 The curves of the equivalent mixing resistance change within 200 s under three internal friction angle conditions are given. It can be seen that the larger the internal friction angle, the greater the equivalent mixing resistance generated, and the greater the fluctuation amplitude of the resistance.

[0181] Step 5.4 Set the earth pressure coefficient

[0182] Set the equivalent earth pressure coefficients of the soil to 9, 12, and 15 respectively, the equivalent elastic modulus to 250 kPa, the equivalent cohesion to 20 kPa, and the equivalent internal friction angle to 40°. The drilling resistance curve within 200 s is as Figure 13 shown. Since the equivalent earth pressure coefficient can directly change the magnitude of the equivalent mixing resistance, the magnitude of the drilling resistance will also be affected by the change of the equivalent earth pressure coefficient. The larger the earth pressure coefficient, the greater the growth rate of the drilling resistance, the greater the drilling resistance generated at the same drilling depth, and the greater the amplitude of the generated resistance fluctuation.

[0183] Example: Ground test results and analysis:

[0184] (1) Test system

[0185] The drilling and mining process is simulated through the ground simulation test system. The test system consists of three parts: a drill tool movement simulation test bench, a control cabinet, and a test box, as Figure 14 shown. Among them, the drill tool movement simulation test bench mainly includes parts such as the drill tool, moving slide rails in the x, y, and z directions, a rotary motor, and a pressure sensor; the control cabinet includes two parts: control buttons and a parameter setting panel; the test box is used to fill simulated lunar soil.

[0186] The drill string structure used in the drilling and mining test is a double-row 8-tooth stepped configuration, which is similar to the drill string structure used by the Chang'e-5 probe. The drill bit has 8 cutting edges, which can continuously cut and break the soil under the drill string through the rotary motion of the drill string, turning it into loose particles. The inside of the drill pipe is hollow. When the drill string makes a feeding motion, the loose soil directly below the drill string can be collected into the cavity of the drill pipe. In addition, the drill pipe is surrounded by double-row threads, which can drive the soil particles on the drill pipe wall to move upward and be discharged to the soil surface during the drilling process. There will be no excessive drilling resistance due to being too tight between the drill pipe and the soil, ensuring the smooth progress of the drilling and mining process.

[0187] In order to more clearly observe the mechanism of the interaction between the machine and the soil and minimize interference factors as much as possible, a homogeneous small-particle foundation is used to simulate lunar soil for the drilling and mining simulation test. The main component of the simulated lunar soil used in the test is Cenozoic alkaline olivine basalt, and its particle size distribution is shown in Table 4. After measurement, the total mass of the test soil is 50.1 kg, and the density of the test simulated lunar soil is controlled at 1.65 ± 0.1 g / cm 3 .

[0188] Table 3 Particle Size Distribution of Simulated Lunar Soil

[0189]

[0190]

[0191] The installation inclination angle of the drill string is 1°. However, due to the small angle, its accuracy cannot be guaranteed during installation. Therefore, the actual drill string inclination angle is calibrated using the trajectory traced by the small circular motion of the drill pipe on the soil surface when the drill bit initially enters the soil. As Figure 15 shown, the motion trajectory of the maximum diameter section of the drill bit is the circle formed by the raised soil in the figure. After measurement, the diameter of the trajectory is 49.16 mm, so the diameter of the circular trajectory formed by the movement of the drill string center is 49.16 - 35 = 14.16 mm. Therefore, according to the drill string dimensions, the drill string inclination angle in the test is

[0192] (2) Test Results and Analysis:

[0193] The test drilling depth is set to 300 mm, the penetration rate is 1.5 mm / s, and the rotary rate is 15 r / min. After the experiment starts, the data acquisition system collects data such as the position, speed, and drilling resistance of the drill string end at a time interval of 1 s. The drilling resistance curve obtained from the test is shown by the dotted line in Figure 16 . The test results show that the drilling resistance is close to zero at the initial stage of drilling. When the drill bit completely enters the soil, the drilling resistance begins to increase as the drilling depth increases. Moreover, the drilling resistance has fluctuations, and the amplitude increases with the increase of the drilling depth.

[0194] The equivalent internal friction angle of the soil mass is set to be 40°, the equivalent elastic modulus is 250 kPa, the equivalent cohesion is 20 kPa, and the equivalent earth pressure coefficient is 15. And the inclination angle of the drill tool is set to be 0.74°. The prediction results of the model are shown by the solid line in Figure 16. Compared with the test results, at the initial stage of drilling, the simulation results are significantly greater than the test results. This is because at the beginning of drilling, the surface soil mass is in a completely free state. Under the extrusion of the drill bit, the soil particles move and bulge around, without imposing constraints on the drill tool. The interaction between the soil mass and the drill tool is very weak, so the actual resistance is close to zero. However, from the analysis of the entire drilling process, the prediction results of the model and the test results have an approximate increasing trend and fluctuation characteristics.

[0195] The test results are analyzed in the frequency domain. After Fourier transform, the frequency amplitude curve is as Figure 17 shown. Since the drilling resistance generally shows an increasing trend with time, there is a drift characteristic in the frequency domain signal, and there is a relatively large amplitude near 0 Hz. After ignoring the drift point, a relatively obvious frequency spectrum line can be obtained at a frequency of 1.49 Hz, and a frequency spectrum line can also be obtained at a frequency of 2.99. This shows that there are indeed periodic fluctuations in the drilling resistance, and the fluctuation frequency is consistent with the rotary frequency of the drill tool, which is 1.5 Hz.

[0196] In summary, the test results confirm the rationality of the formation mechanism of the drilling resistance and also verify that the drilling resistance prediction model constructed based on the mechanism can accurately predict the drilling resistance.

[0197] The above has introduced in detail a method for predicting the lunar soil drilling resistance based on axis offset proposed by the present invention, and has elaborated on the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A method for predicting lunar soil drilling resistance based on axis offset, characterized in that: The method specifically comprises the following steps: Step 1: Analyze the movement of drilling tools and soil, define the feeding movement, rotary movement and chip removal process of drilling tools, and establish an equivalent model; Feed motion is the vertical downward movement of the drill as a whole. At this time, the soil around the drill bit is squeezed by the drill bit and moves radially, which is in a compressed state. At the same time, the soil under the drill bit enters the drill rod through the sampling hole. Therefore, the feed motion causes a relative displacement between the drill bit and the soil, and the surface of the drill bit is subject to friction resistance in the opposite direction of the movement. Rotational motion is the rotation of the drill bit around its axis. At this time, the cutting edge on the drill bit will destroy the soil structure, making the soil around the drill bit loose and move in the circumferential direction under the drive of the drill bit. The chip removal process is as follows: under the combined action of feeding and rotation, the soil at the edge of the drill bit will be transported to the lunar surface by the drill rod thread. However, the fixed-axis rotation of the drill bit is an interference motion caused by errors. Due to the slender structure of the drill rod, the rotation will be magnified at the drill bit. The drill bit will continue to stir the soil during the fixed-axis rotation, causing the surface of the drill bit to be subjected to the extrusion load of the soil. This load has the characteristics of periodic variation. That is, the movement of the drill bit is equivalently decomposed into vertical downward movement and fixed-axis rotation at the end. When the drilling and mining conditions remain unchanged, it is assumed that the soil flow velocity generated by the rotation of the drill bit is the same as the rotation velocity, that is, the lunar soil and the drill bit are relatively stationary in the circumferential direction; at the same time, it is assumed that the lunar soil carried on the drill rod thread is filled from the drill bit and transported upward, and the powder discharge is stable, and the flow characteristics of the lunar soil under the action of rotation are equivalent to the changes in the macroscopic mechanical properties of the lunar soil; Step 2: Based on the hole expansion mechanism, analyze the soil expansion stages from elasticity, elastoplasticity to complete plasticity, and calculate the corresponding stress, displacement and friction resistance to predict the resistance change during drilling; Step 3: According to the stirring mechanism, the deviation angle between the drill axis and the feed speed is analyzed, the interaction between the soil around the drill is calculated by equivalent earth pressure of the retaining wall, and the stirring resistance on the drill surface is predicted; Step 4: Based on steps 1 to 3, ignoring the friction between the collected samples inside the drilling tool and the inner wall of the drill pipe, a drilling resistance model is constructed; Step 5: Verify the accuracy of the model constructed in step 4.

2. The method for predicting lunar soil drilling resistance according to claim 1, characterized in that: In step 2, the vertical downward movement of the drill bit is equivalent to the pile penetration process, so the pressure load on the drill bit surface is equivalent to the expansion pressure load on the hole wall during the penetration process; Step 2.1: Elastic expansion phase Calculate stress distribution function: derive radial normal stress and tangential normal stress, involving expansion pressure, initial pressure, Poisson's ratio, and soil rotation angular velocity; Calculate the displacement distribution function: describe the displacement distribution during elastic deformation of the soil related to the elastic modulus of the soil; Step 2.2: Elastic-Plastic Expansion Stage Plastic zone and elastic zone: soil deformation is divided into plastic zone and elastic zone, and the boundary radius and initial position of the plastic zone are determined; Based on the Coulomb-Moore yield criterion, the yield condition of the soil is determined, the internal friction angle and cohesion are introduced, the stress distribution function of the plastic zone is derived, the stress distribution function of the elastic zone is solved, and the displacement distribution outside the soil boundary and in the plastic zone is described based on the differential equilibrium equation of the elastic zone, the stress-strain relationship and the boundary conditions; Step 2.3: Complete plastic expansion stage; When the soil enters the fully plastic state, the elastic-plastic boundary moves to the outer boundary of the soil body, and the soil enters the fully plastic state; the stress distribution function, displacement distribution function and aperture expansion relationship of the fully plastic stage are derived; Step 2.4: Calculate the friction resistance of hole expansion: specifically, the friction resistance between the drilling tool and the soil layer.

3. The method for predicting lunar soil drilling resistance according to claim 2, characterized in that: In step 3, when there is a very small deviation angle between the drill axis and the feed speed, the rotation of the drill will cause a fixed axis rotation at the same frequency. The surface threads and the drill bit protrusion are ignored, and the shape of the drill is simplified to a slender round rod. It is assumed that the lunar soil around the drill is in a limit equilibrium state at any time during the rotation process. At this time, the interaction of the lunar soil with the drill can be equivalently calculated by the earth pressure of the retaining wall. Step 3.1: Calculate the equivalent earth pressure; Based on the earth pressure theory of retaining wall, describe the equivalent passive earth pressure coefficient and the equivalent active earth pressure coefficient on the drilling tool surface; Step 3.2: Calculate the bulk density distribution; based on the influence of drill bit rotation, analyze the change of soil density during the fixed axis rotation of the drill bit and derive the bulk density distribution function; Step 3.3: Calculate the vertical component of the earth pressure load on the drill bit surface, and calculate the stirring resistance based on the equivalent passive earth pressure coefficient and the equivalent active earth pressure coefficient.

4. The method for predicting lunar soil drilling resistance according to claim 3, characterized in that: In step 4, the samples collected inside the drill tool will rub against the inner wall of the drill rod, but due to the low density of the samples, the pressure on the inner wall of the drill rod is small. Therefore, the friction load is ignored and the drilling resistance encountered by the drill tool during the drilling process is regarded as the resultant force in the vertical direction of all loads on the surface of the part that interacts with the lunar soil.

5. The method for predicting lunar soil drilling resistance according to claim 4, characterized in that: In step 5, the equivalent elastic modulus, equivalent cohesion, equivalent internal friction angle and equivalent earth pressure coefficient of the soil are set for experimental verification.

6. A prediction system for executing the method for predicting lunar soil drilling resistance based on axis offset as described in any one of claims 1 to 5, characterized in that: The prediction system includes the following drilling tool and soil analysis module, hole expansion mechanism module, stirring mechanism module, drilling resistance construction module and verification module: The drill tool and soil analysis module defines the feed motion, rotation motion and chip removal process of the drill tool, and establishes an equivalent model; the feed motion is the vertical downward movement of the drill tool as a whole, at which time the soil around the drill bit is squeezed by the drill bit and moves radially, in a compressed state, and at the same time the soil under the drill bit enters the drill rod through the sampling hole; therefore, the feed motion causes a relative displacement between the drill tool and the soil, and the surface of the drill tool is subject to friction resistance opposite to the direction of movement; Rotational motion is the rotation of the drill bit around its axis. At this time, the cutting edge on the drill bit will destroy the soil structure, making the soil around the drill bit loose and move in the circumferential direction under the drive of the drill bit. The chip removal process is as follows: under the combined action of feeding and rotation, the soil at the edge of the drill bit will be transported to the lunar surface by the drill rod thread. However, the fixed-axis rotation of the drill bit is an interference motion caused by errors. Due to the slender structure of the drill rod, the rotation will be magnified at the drill bit. The drill bit will continue to stir the soil during the fixed-axis rotation, causing the surface of the drill bit to be subjected to the extrusion load of the soil. This load has the characteristics of periodic variation. That is, the movement of the drill bit is equivalently decomposed into vertical downward movement and fixed-axis rotation at the end. When the drilling and mining conditions remain unchanged, it is assumed that the soil flow velocity generated by the rotation of the drill bit is the same as the rotation velocity, that is, the lunar soil and the drill bit are relatively stationary in the circumferential direction; at the same time, it is assumed that the lunar soil carried on the drill rod thread is filled from the drill bit and transported upward, and the powder discharge is stable, and the flow characteristics of the lunar soil under the action of rotation are equivalent to the changes in the macroscopic mechanical properties of the lunar soil; The hole expansion mechanism module analyzes the soil expansion stages from elastic, elastoplastic to fully plastic, and calculates the corresponding stress, displacement and friction resistance to predict the resistance change during drilling; The stirring mechanism module analyzes the deviation angle between the drilling tool axis and the feed speed, calculates the interaction of the soil around the drilling tool through the equivalent calculation of the retaining wall earth pressure, and predicts the stirring resistance on the drilling tool surface; The drilling resistance building module: ignoring the friction between the collected samples inside the drilling tool and the inner wall of the drill pipe, and building a drilling resistance model; The verification module is used to verify the accuracy of the model.

7. A test system for realizing the lunar soil drilling resistance prediction system based on axis offset as described in claim 6, characterized in that: The test system consists of three parts: a drill motion simulation test bench, a control cabinet and a test box. The drilling tool motion simulation test bench mainly includes drilling tools, moving slide rails in the x, y, and z directions, a rotary motor, and a pressure sensor; the control cabinet includes two parts: control buttons and parameter setting panel; the test box is used to load simulated lunar soil; The drill tool structure used in the drilling test is a double-row 8-tooth stepped configuration. The drill bit has 8 cutting edges, which can continuously cut and destroy the soil below the drill tool through the rotary motion of the drill tool, turning it into loose particles; the drill rod is hollow inside, and the loose soil directly below the drill tool can be collected into the drill rod cavity when the drill tool is feeding; the drill rod is surrounded by double rows of threads, which drive the soil particles on the drill rod wall to move upward and discharge them to the soil surface during the drilling process.

8. An electronic device comprising a memory and a processor, wherein the memory stores a computer program, and the processor implements the steps of the method according to any one of claims 1 to 5 when executing the computer program.

9. A computer-readable storage medium for storing computer instructions, wherein the computer instructions, when executed by a processor, implement the steps of the method according to any one of claims 1 to 5.

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