Zero-backtracking trajectory re-planning method and system for composite material machining

Abstract

This invention relates to the field of CNC machining technology, specifically to a method and system for zero-backtrack trajectory replanning in composite material machining. The method and system include: pre-planning a theoretical trajectory and constructing an allowable error band; acquiring a continuous cutting force signal and calculating its time-varying gradient; calculating a dynamic equilibrium point within the allowable error band based on the cutting force gradient; when the gradient is greater than zero, the equilibrium point shifts along the tangential direction to avoid the material; when the gradient is less than zero, the point shifts inward along the normal direction to feed the material; and correcting the tool target position to the equilibrium point to generate a replanned trajectory, ensuring that the dot product of the tool speed and the feed direction is always non-negative. This invention uses the cutting force gradient to drive the dynamic equilibrium point for non-backtracking tangential active avoidance, which can suppress fiber tearing and delamination, while simultaneously improving machining stability and tool life.

Description

Technical Field

[0001] This invention relates to the field of CNC machining technology, and in particular to a zero-backtrack trajectory replanning method for composite material cutting. Background Technology

[0002] In fields such as aerospace, automotive, and wind power, composite materials such as glass fiber and carbon fiber have anisotropic internal fiber distribution, which causes the cutting force to fluctuate drastically with the relative angle between the tool and the fiber direction. Especially in complex scenarios where the spindle needs to continuously adjust its position within the range of 0 to 180 degrees to complete vertical milling, side milling, and inclined surface machining, sudden changes in cutting force can easily cause tool breakage, workpiece delamination, or surface tearing.

[0003] To address this issue, a detection-reverse compensation control mode is commonly adopted: the tool path is pre-planned, and the actual tool position is acquired in real time through a laser displacement sensor or vision system. After comparing it with the planned position, a servo drive system outputs a reverse compensation pulse to forcibly pull the tool center point back to the theoretical path. While this approach has proven effective in the precision machining of isotropic materials such as metals, it is unsuitable for machining composite materials. This is because reverse compensation means the tool must temporarily leave the current cutting point and retract. For composite materials with inherently weak interlayer bonding, this retraction can tear the fiber bundles on the machined surface, damaging the interlayer bond and exacerbating delamination and burrs—defects that were originally intended to be addressed. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the problem of easy fiber tearing and aggravation of delamination and burrs when cutting composite materials, and to provide a zero-backtrack trajectory replanning method and system for composite material cutting, which uses the cutting force gradient to drive the dynamic equilibrium point to make non-backtracking tangential active avoidance, thereby suppressing fiber tearing and delamination, while improving processing stability and tool life.

[0005] In a first aspect, to solve the above-mentioned technical problems, the present invention provides a zero-backtrack trajectory replanning method for composite material machining, comprising: The theoretical trajectory of the tool-workpiece contact point is planned in advance, and the allowable error band is constructed according to the machining tolerance of composite materials; The continuous cutting force signal during the cutting process of the tool along the theoretical trajectory is obtained, and the gradient of the cutting force change with time is calculated. Based on the current cutting force gradient, a dynamic equilibrium point is calculated within the allowable error band; the calculation method of the dynamic equilibrium point includes: if the cutting force gradient is greater than zero, the equilibrium point is shifted along the tangent direction of the current feed path toward the direction of decreasing cutting force; if the cutting force gradient is less than zero, the equilibrium point is shifted along the normal direction of the current feed path toward the inside of the workpiece. Under the premise that the current direction of motion remains unchanged, the tool target position at the next moment is corrected to the equilibrium point to generate a replanning trajectory; Wherein, the dot product of the velocity vector of the center point of the tool in the workpiece coordinate system and the unit vector of the current feed direction is greater than or equal to zero.

[0006] Preferably, the method for calculating the dynamic equilibrium point further includes: if the cutting force amplitude exceeds a safety threshold, the equilibrium point is shifted toward the outside of the workpiece along the tangent direction of the current feed path; if the cutting force amplitude is lower than a stability threshold, the equilibrium point converges toward the theoretical trajectory direction.

[0007] Preferably, when the cutting force gradient is greater than zero and the equilibrium point shifts along the tangential direction causing the tool center point to exceed the allowable error band, the shift is decomposed into a tangential direction component and a normal direction component: the tangential direction component is the remaining distance in the tangential direction from the current tool position to the boundary of the allowable error band; the normal direction component is the offset towards the outside of the workpiece; wherein, the normal direction component is activated only when the remaining distance in the tangential direction is less than the desired tangential offset.

[0008] Preferably, determining the radial dimension of the allowable error band includes: setting a basic radial dimension, calculating an adjustment coefficient based on the angle between the fiber direction of the composite material and the cutting direction, and determining the radial dimension of the current point based on the basic radial dimension and the adjustment coefficient; wherein, when the angle is in the range of 60° to 120°, the adjustment coefficient is greater than 1; otherwise, the adjustment coefficient is equal to 1.

[0009] Preferably, the radial dimension of the allowable error band is related to the angle between the fiber direction of the composite material and the current cutting direction; wherein, when the angle is in the range of 60° to 120°, the radial dimension of the allowable error band is greater than the radial dimension when the angle is outside this range.

[0010] Preferably, the allowable error band is discretized into multiple trajectory points distributed along the theoretical trajectory, and each trajectory point stores the three-dimensional coordinates of the point and the corresponding radial dimension; during the machining process, linear interpolation is performed between adjacent trajectory points according to the current tool position to obtain the radial dimension of the current point.

[0011] Preferably, calculating the gradient of the cutting force relative to time includes: continuously acquiring cutting force signals at a fixed sampling frequency, constructing a sliding window of length N to store the original cutting force of the most recent N sampling points; performing an arithmetic average on the original cutting force within the sliding window to obtain the filtered cutting force; and performing a first-order backward difference on the filtered cutting force to obtain the cutting force gradient at the current moment.

[0012] Preferably, the length N of the sliding window is inversely proportional to the current rotational speed of the spindle.

[0013] Secondly, to solve the above-mentioned technical problems, the present invention also provides a zero-backtrack trajectory replanning system for composite material machining, comprising: The cutting tool, mounted on the spindle, is used to machine the workpiece; The force sensing module is used to acquire continuous cutting force signals between the tool and the workpiece during the cutting process and to calculate the gradient of the cutting force change with time. The allowable error curve construction module is used to construct allowable error bands based on the processing tolerances of composite materials; The dynamic equilibrium point calculation module is used to calculate a dynamic equilibrium point within the allowable error band based on the current cutting force gradient. The calculation method of the dynamic equilibrium point includes: if the cutting force gradient is greater than zero, the equilibrium point is shifted towards the direction of decreasing cutting force along the tangent direction of the current feed path; if the cutting force gradient is less than zero, the equilibrium point is shifted towards the inside of the workpiece along the normal direction of the current feed path. The zero-backtrack trajectory replanning module is used to correct the tool target position at the next moment to the equilibrium point while keeping the current direction of motion unchanged. The system is configured such that the dot product of the velocity vector of the center point of the tool in the workpiece coordinate system and the unit vector of the current feed direction is greater than or equal to zero.

[0014] Preferably, the allowable error curve construction module is configured to: determine the radial dimension of the allowable error band based on the angle between the fiber direction of the composite material and the current cutting direction; wherein, when the angle is in the range of 60° to 120°, the radial dimension of the allowable error band is greater than the radial dimension when the angle is outside this range.

[0015] Compared with the prior art, the above-described technical solution of the present invention has the following advantages: First, the cutting tool does not rigidly follow the theoretical trajectory, but adjusts the dynamic equilibrium point according to the cutting force state within the allowable error band. This flexible constraint allows the cutting tool to obtain sufficient degrees of freedom to avoid obstacles while maintaining the machining accuracy level.

[0016] Secondly, by using the cutting force gradient as the control basis, a targeted response to the anisotropic properties of composite materials is achieved. The gradient reflects the trend of force change. Before the cutting force reaches the dangerous peak and damage occurs, the tool is actively guided to deviate along the tangential direction to avoid the high resistance zone, so that defects such as tool chipping and fiber tearing are suppressed before they occur, effectively filtering out transient impacts during the machining process and keeping the cutting process stable.

[0017] Secondly, the tangential offset completely eliminates the backlash component, ensuring the tool always moves unidirectionally along the feed direction. Simultaneously, by adjusting the relative position of the tool to the fiber direction, it seeks a cutting path with less resistance. When the gradient is greater than zero, the tangential offset causes the tool to actively avoid high-resistance areas; when the gradient is less than zero, the normal inward offset allows the tool to appropriately increase the depth of cut to maintain efficiency when resistance decreases. Neither offset involves backward movement, thus replacing reverse backlash with tangential avoidance. This ensures the tool always advances forward by cutting the fibers, preventing secondary scraping of the machined surface and preserving the integrity of the fiber bundle. This fundamentally suppresses delamination and burr defects commonly found in composite material processing.

[0018] In summary, the zero-backtrack trajectory replanning method and system for composite material machining described in this invention uses the cutting force gradient to drive the dynamic equilibrium point to perform non-backtracking tangential active avoidance, thereby suppressing fiber tearing and delamination, while improving machining stability and tool life. Attached Figure Description

[0019] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0020] Figure 1 This is a schematic diagram of the composite material cutting and processing equipment in a preferred embodiment of the present invention; Figure 2 This is a flowchart of a zero-backtrack trajectory replanning method for composite material machining in a preferred embodiment of the present invention; Figure 3 This is a structural block diagram of the zero-backtrack trajectory replanning system for composite material machining according to the present invention.

[0021] The diagram in the instruction manual is labeled as follows: 2. Cutting tool; 4. Machine base; 6. Drive mechanism; 8. Servo controller. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0023] The purpose of this invention is to overcome the problem of easy fiber tearing, delamination and burrs when cutting composite materials, and to provide a zero-backtrack trajectory replanning method and system for composite material cutting. The cutting force gradient drives the dynamic equilibrium point to make non-backtracking tangential active avoidance, thereby suppressing fiber tearing and delamination, while improving machining stability and tool life.

[0024] Reference Figure 1 The image shows a composite material cutting and machining equipment, including a cutting tool 2, a machine base 4, a drive mechanism 6, and a servo controller 8. The cutting tool 2 is installed at the end of the machine base 4, and the servo controller 8 controls the drive mechanism 6 to drive the cutting tool 2 to cut and machine the workpiece.

[0025] In isotropic materials such as metals, the cutting force is mainly related to the cutting parameters (depth of cut, feed rate, and rotational speed). Regardless of the angle of the tool, the mechanical response of the material is basically the same. However, for workpiece 1, which is a glass fiber or carbon fiber product, the internal fibers are anisotropically distributed. When the tool cuts along the fiber direction (angle ≈ 0°), the cutting force is small and stable, and the fibers are continuously cut. When the tool cuts laterally (angle ≈ 90°), the cutting force suddenly increases by 2-5 times, and the fibers are pushed and pulled rather than cut. When the tool turns from 0° to 90°, the cutting force does not increase linearly, but rises sharply after a certain critical angle.

[0026] Composite materials, including glass fiber and carbon fiber, are anisotropic, which causes the cutting force to fluctuate drastically with slight changes in the tool angle or fiber direction. This is especially true in complex scenarios where the spindle needs to continuously adjust its position within the range of 0 to 180 degrees to complete vertical milling, side milling, and inclined surface machining. Sudden changes in cutting force can easily lead to tool breakage, workpiece delamination, or surface tearing.

[0027] Example 1: Refer to Figure 2 As shown, this embodiment of the invention discloses a zero-backtrack trajectory replanning method for composite material machining, comprising: S100: Pre-plan the theoretical trajectory of the contact point between the tool and the workpiece, and construct the allowable error band based on the machining tolerance of the composite material; S200: Acquire the continuous cutting force signal during the process of the tool cutting the workpiece along the theoretical trajectory, and calculate the gradient of the cutting force change with time; S300. Based on the current cutting force gradient, calculate a dynamic equilibrium point within the allowable error band. The calculation method for the dynamic equilibrium point includes: if the cutting force gradient is greater than zero, the equilibrium point is shifted along the tangent direction of the current feed path toward the direction of decreasing cutting force; if the cutting force gradient is less than zero, the equilibrium point is shifted along the normal direction of the current feed path toward the inside of the workpiece. S400: Under the premise that the current direction of motion remains unchanged, correct the tool target position to the equilibrium point at the next moment and generate a replanning trajectory; In this context, the dot product of the velocity vector of the tool's center point in the workpiece coordinate system and the unit vector of the current feed direction is always greater than or equal to zero.

[0028] In the specific scenario of implementing step S100, the theoretical trajectory is a predefined spatial curve in the CAD / CAM system. The allowable error band is a three-dimensional spatial region centered on this theoretical trajectory, and its radial dimension is determined according to the machining tolerance of the composite material. For example, for aerospace-grade composite parts, the machining tolerance is usually ±0.1mm, so the basic radial dimension of the allowable error band can be set to 0.1mm; for general industrial parts, it can be set to 0.5mm.

[0029] The allowable error band is constructed as follows: A normal plane is taken at regular intervals (e.g., 0.5mm) along the theoretical trajectory. Within each normal plane, a circle is drawn with the intersection of the theoretical trajectory and the normal plane as the center and the radial dimension as the radius. The envelope of all these circles constitutes the allowable error band. The tool center point is allowed to move freely within this spatial region, without being forced to strictly follow every point on the theoretical trajectory. During machining, the boundary information of the corresponding allowable error band is queried based on the current tool position to determine whether subsequent movements meet the requirements.

[0030] In specific implementation step S200, a strain gauge or piezoelectric force sensor is installed on the spindle or tool holder to continuously acquire the cutting force signal F(t) at a fixed sampling frequency f (e.g., 1000Hz). Let the sampling period be T, and the original cutting force value acquired at the k-th sampling time is F[k].

[0031] To extract reliable gradient information from noisy signals, the original cutting force signal is filtered to obtain the filtered cutting force value F'[k]. Then, a first-order backward difference is performed on the filtered cutting force value to calculate the cutting force gradient G[k] = (F'[k] - F'[k-1]) / T at the current moment. The unit of the calculated cutting force gradient G[k] is Newtons per second. The positive or negative sign of the cutting force gradient reflects the trend of the cutting force: G[k] > 0 indicates that the cutting force is increasing, G[k] < 0 indicates that the cutting force is decreasing, and G[k] = 0 indicates that the cutting force is stable.

[0032] In the specific implementation step S300, the core of this step lies in the correlation between the dynamic equilibrium point calculation rule and the sign of the current cutting force gradient. If the current cutting force gradient is greater than zero, it indicates that the cutting force is increasing, and the tool is facing the risk of increased resistance. At this time, the equilibrium point shifts along the tangent direction of the current feed path towards the direction of decreasing cutting force. The tangent direction is the current feed direction, determined by the tangential vector of the theoretical trajectory at the current point. The specific offset distance can be determined proportionally based on the magnitude of the cutting force gradient: the larger the gradient magnitude, the faster the cutting force increases, and the larger the offset distance; the smaller the gradient magnitude, the smaller the offset distance. Within the allowable error band, this offset action moves the tool forward from the current path position to a new position, thereby changing the relative angle between the tool and the composite material fiber direction, and finding a cutting path with less cutting resistance.

[0033] If the current cutting force gradient is less than zero, it indicates that the cutting force is decreasing and the tool is entering a region of reduced resistance. At this time, the equilibrium point shifts towards the inside of the workpiece along the normal direction of the current feed path. The normal direction is perpendicular to the machined surface, and "towards" the inside of the workpiece indicates a deeper position in the material. This shift causes the tool to appropriately increase the depth of cut, thereby maintaining machining efficiency in a region of lower cutting resistance. Regardless of whether the cutting force gradient is greater than or less than zero, the calculated dynamic equilibrium point must be located within the allowable error band established in step S100; that is, the distance between the equilibrium point and the theoretical trajectory does not exceed the radial dimension of the allowable error band corresponding to that point.

[0034] In the specific implementation step S400, within each control cycle, the dynamic equilibrium point calculated in step S300 is used as the tool target position for the next moment, and a drive command is generated and sent to the servo drive system to drive the spindle or worktable to move.

[0035] It should be noted that, under the premise that the current direction of motion remains unchanged, this means that the macroscopic feed direction of the tool does not reverse. The tool always moves forward along the overall direction from the starting point to the ending point and will not retreat. Mathematically, this constraint is reflected in the fact that the dot product of the velocity vector v of the tool's center point and the unit vector u of the current feed direction is always greater than or equal to zero, i.e., v·u≥0. This constraint ensures that the tool never retreats or detaches from the workpiece during the entire machining trajectory. In the specific implementation, after calculating the candidate corrected displacement vector in each control cycle, the system projects it onto the current feed direction. If the projection value is negative, the candidate vector is rejected, thus ensuring the validity of the no-retreat constraint from the algorithm's underlying level.

[0036] The implementation details and mechanisms of the key technical features are explained in further detail below.

[0037] Traditional CNC machining requires the tool center point to strictly follow every point on the theoretical trajectory. This rigid constraint can ensure accuracy in the machining of isotropic materials, but it becomes a burden in the machining of composite materials: when the theoretical trajectory happens to pass through the transverse cutting zone of the fiber, the tool will be forced to take a high-resistance path, resulting in a sharp increase in cutting force, tool chipping, and workpiece delamination.

[0038] The present invention introduces an allowable error band, transforming the rigid trajectory into a flexible space. The tool does not need to rigidly follow every theoretical coordinate, but rather autonomously adjusts its actual path within a preset tolerance space based on the cutting force. As long as the final product's geometric accuracy is within acceptable limits, minor trajectory deviations during the process are acceptable, enabling the system to obtain sufficient degrees of freedom for avoidance while maintaining machining accuracy.

[0039] The following explanation of the dynamic equilibrium point calculation method in step S300 is based on the anisotropic properties of composite materials.

[0040] When the cutting force gradient is greater than zero, the tool is considered to be entering a high-resistance zone. This high-resistance zone typically corresponds to a transverse cutting area where the angle between the fiber direction and the cutting direction is close to 90°. Within this zone, the tool primarily bears the bending and tensile loads of the fibers, resulting in a significant increase in cutting force. At this point, the purpose of offsetting along the tangential direction is to change the relative position of the tool and the fibers. Since the fiber direction of composite materials is continuously changing in space, after the tool moves a small distance along the tangential direction, the angle of the fiber direction it faces will change accordingly, potentially moving from the transverse cutting zone to the fiber-aligned cutting zone, thus significantly reducing cutting resistance. Simultaneously, the tangential offset maintains the tool's unidirectional movement along the feed direction, without generating a retraction component, thus preventing secondary scratching of the machined surface.

[0041] When the cutting force gradient is less than zero, the tool is considered to be entering a low-resistance zone. This low-resistance zone typically corresponds to a longitudinal cutting region where the angle between the fiber direction and the cutting direction is close to 0°. Within this region, the tool can smoothly cut the fibers, and the cutting force is small and stable. At this point, shifting the tool inward along the normal direction towards the workpiece aims to appropriately increase the depth of cut. Because the resistance in the current cutting region is low, increasing the depth of cut will not lead to chipping; instead, it can improve material removal rate and maintain machining efficiency.

[0042] Furthermore, it's crucial to clarify the fundamental difference between cutting force gradient and position deviation: position deviation represents an error that has already occurred, acting as lagging information; while the cutting force gradient reflects an ongoing trend, acting as forward-looking information. In composite material machining, the abrupt changes in cutting force due to anisotropy often occur on the order of milliseconds, making it impossible for position deviation detection to respond before damage occurs. The cutting force gradient, however, directly measures the rate of force change, enabling it to predict risks and trigger avoidance actions before the cutting force reaches a dangerous peak, before tool deviation occurs, and before damage develops.

[0043] In summary, the zero-backtrack trajectory replanning method for composite material machining described in this embodiment of the invention uses the cutting force gradient to drive the dynamic equilibrium point to perform non-backtracking tangential active avoidance, thereby suppressing fiber tearing and delamination, while improving machining stability and tool life.

[0044] Based on the above embodiments, tangential avoidance is performed when the cutting force gradient is greater than zero, and normal feed is performed when the cutting force gradient is less than zero. In actual machining, there are two special cases. The first is when the cutting force amplitude has exceeded the safety threshold. In this case, regardless of whether the cutting force gradient is positive or negative, the tool is already overloaded and the cutting load needs to be forcibly reduced. The second case is when the cutting force amplitude is below the stability threshold. In this case, the machining state is very safe, and it is a favorable time to converge the tool to the theoretical trajectory and restore machining accuracy.

[0045] To address these two scenarios, the dynamic equilibrium point calculation method in this embodiment further includes: when the cutting force amplitude exceeds the safety threshold, the equilibrium point shifts outward along the tangent direction of the current feed path. The safety threshold is a preset absolute upper limit value, which physically represents the maximum allowable cutting force for normal tool operation. In one specific implementation, the safety threshold is determined based on the rated cutting force data provided by the tool manufacturer, for example, 120% of the rated cutting force.

[0046] When the current cutting force amplitude exceeds the safety threshold, it indicates that the tool is overloaded and at risk of chipping or breakage. At this point, regardless of whether the cutting force gradient is greater than or less than zero, a safety maneuver is forcibly executed. The direction of the safety maneuver is an offset towards the outside of the workpiece along the tangent of the current feed path. "Towards the outside of the workpiece" refers to reducing the depth of cut or width of cut.

[0047] In the specific implementation, this direction can be decomposed into two components: forward along the tangential direction and outward along the normal direction (tool lifting), but the non-negative tangential component is prioritized to satisfy the non-backoff constraint. This offset action continues until the cutting force amplitude falls below the safety threshold. Once the cutting force amplitude falls below the safety threshold, the system exits the risk avoidance mode and resumes the dynamic equilibrium point calculation dominated by the cutting force gradient.

[0048] When the cutting force amplitude is lower than the stability threshold, the equilibrium point converges towards the theoretical trajectory. The stability threshold is a preset lower limit value, which physically means that the cutting force is low enough and the machining state is safe enough to begin restoring machining accuracy. In one specific implementation, the stability threshold is set to 30% to 50% of the safety threshold. For example, if the safety threshold is 300N, the stability threshold can be set to 120N.

[0049] When the current cutting force amplitude is detected to be below the stability threshold, it indicates that the tool is under a light load and the machining process is very stable. This is a favorable time to converge towards the theoretical trajectory. The convergence method is as follows: within each control cycle, the equilibrium point moves one step towards the theoretical trajectory direction. This step size is proportional to the current deviation distance and the convergence coefficient is less than 1. The convergence process continues until the cutting force amplitude rises again above the stability threshold. It should be noted that the convergence action must be performed under the premise of satisfying the non-backoff constraint, that is, the convergence path is along the tangential or normal direction, without generating a negative component in the feed direction.

[0050] In a preferred embodiment, the safety threshold and the stability threshold are not fixed, but dynamically adjusted according to the current machining conditions. For example, when the spindle speed is high, the upper limit of the allowable cutting force is relatively low, and the safety threshold is lowered accordingly; when the tool wear is severe, the safety threshold is also lowered accordingly. The stability threshold is adjusted proportionally to the safety threshold, maintaining the relative relationship between the two.

[0051] Furthermore, when the cutting force gradient is greater than zero and the offset of the equilibrium point along the tangential direction causes the tool center point to exceed the allowable error band, the offset is further decomposed into a tangential direction component and a normal direction component. Specifically, let the current tool center point position be P1, the corresponding point position on the theoretical trajectory be P0, and the radial dimension of the current allowable error band be R. First, calculate the vertical distance d = |P1-P0| from the current tool center point to the theoretical trajectory; the vertical distance d should be less than or equal to the radial dimension R. Then, calculate the remaining distance r in the tangential direction: .

[0052] The desired tangent offset calculated based on the cutting force gradient is D = K × (|G| × Tctrl), where K is the tangent offset ratio coefficient, and |G| × Tctrl represents the change in cutting force within the current control cycle. If D ≤ r, the tangent offset is executed normally without activating the boundary detour strategy; if D > r, it indicates that the desired offset exceeds the remaining space, and the boundary detour strategy needs to be activated.

[0053] The value of K is 0.005 to 0.02 mm / N. For glass fiber reinforced composites (GFRP), the overall cutting force is less than that of carbon fiber, but the fluctuation is relatively gentle, and the value of K is 0.01 to 0.03 mm / N. For high modulus carbon fibers (such as aerospace-grade T800 and T1000), the material is brittle and has a high risk of chipping, requiring a more sensitive avoidance response. K is usually 0.015 to 0.03 mm / N.

[0054] The specific execution method of the boundary detour strategy is as follows: The tangential component value Dtan is the remaining distance r from the current tool position to the boundary of the allowable error zone in the tangential direction, i.e., Dtan = r. This component moves the tool along the tangential direction to the boundary position of the allowable error zone, making full use of all the remaining space in the tangential direction. The normal component value Dn is the offset towards the outside of the workpiece; the role of this component is to help reduce the cutting load through a small tool lifting motion when there is no remaining space in the tangential direction, while allowing the tool to slide along the boundary of the allowable error zone instead of forcibly crossing the boundary. The normal component is activated only when the remaining distance in the tangential direction is less than the desired tangential offset, i.e., it only produces a non-zero value when D > r.

[0055] Ultimately, the equilibrium point is located at P1' = P1 + Dtan×u1 + Dn×u2, where u1 is the unit vector in the current feed direction and u2 is the unit vector in the normal direction pointing out of the workpiece (i.e., the tool lifting direction). This equilibrium point is located on the boundary of the allowable error zone (the tangential direction has reached the boundary), and is slightly offset outward from the workpiece, allowing the tool to reduce the cutting load by slightly lifting itself during the edge-sliding process.

[0056] In a preferred embodiment, after the boundary detour strategy is activated, the change in the cutting force gradient is continuously monitored; if the cutting force gradient has decreased significantly after several control cycles (e.g., dropped to less than 50% of the original value), an attempt is made to exit the boundary detour mode and restore normal tangent offset control.

[0057] The present invention's embodiments transform excess avoidance requirements into minute normal lifting movements, enabling the tool to continue performing avoidance functions while remaining within the boundaries, thus satisfying boundary constraints and minimizing cutting force.

[0058] In the above embodiment, the allowable error band is a three-dimensional spatial region constructed based on the machining tolerance of the composite material. Furthermore, the cutting characteristics of the composite material vary significantly with the angle between the fiber direction and the cutting direction. Specifically, when the angle is within the range of 60° to 120°, the tool is in the transverse cutting zone, the cutting force is large and fluctuates violently, which can easily cause delamination and tearing, requiring a larger tolerance space. When the angle is outside this range, the tool is in the forward or reverse cutting zone, the cutting force is relatively stable, and a stricter trajectory constraint can be maintained.

[0059] Based on this, in the embodiments of the present invention, the anisotropic allowable error band is further entered, and the radial dimension of the allowable error band is adjusted according to the angle between the fiber direction and the cutting direction.

[0060] Specifically, the theoretical trajectory is a predefined spatial curve in the CAD / CAM system. The allowable error zone is a three-dimensional spatial region centered on this theoretical trajectory, with its radial dimension varying along the trajectory. Specifically, it is determined by the angle θ(s) between the fiber direction and the cutting direction at the current machining position. When θ(s) is within the range of 60° to 120°, the radial dimension is set to be larger than the radial dimension outside this range. For example, for aerospace-grade composite parts with a basic machining tolerance of ±0.1mm, the radial dimension is 0.1mm in the safe zone (θ∈[0°,60°)∪(120°,180°]) and 0.2mm in the dangerous zone (θ∈[60°,120°]). This allows the tool to have greater operating space in the transverse cutting zone, allowing for larger avoidance offsets; and maintains stricter trajectory constraints in the forward or reverse cutting zones, ensuring the geometric accuracy of the final product.

[0061] In one specific implementation, the radial dimension of the allowable error band is determined as follows: First, a base radial dimension Rbase is set, which is determined based on the processing tolerance of the composite material, for example, 1 to 1.5 times the tolerance value. Then, an adjustment factor k(θ) is calculated based on the angle θ between the fiber direction and the cutting direction. When θ∈[60°,120°], the adjustment factor k(θ) is greater than 1, typically between 1.5 and 2.0, for example, 2.0; when θ∈[0°,60°)∪(120°,180°], the adjustment factor k(θ) is equal to 1. The radial dimension of the current point is R = Rbase × k(θ).

[0062] In another specific implementation, the adjustment coefficient k(θ) adopts a continuous function form, for example, k(θ) = 1 + sin²θ. When θ = 90°, k(θ) = 2.0, and when θ = 0° or 180°, k(θ) = 1.0. This function is continuous and smooth across the entire angular range, requiring no transition interval, and meets the requirement of a larger radial dimension in the transverse cutting zone and a smaller radial dimension in the forward cutting zone.

[0063] Furthermore, the allowable error band can be discretized into multiple trajectory points distributed along the theoretical trajectory to enable rapid lookup and real-time calculation in the CNC system. Specifically, a series of discrete points are taken along the theoretical trajectory at certain step sizes (e.g., 0.5 mm or 1 mm), and each discrete point stores its three-dimensional coordinates and corresponding radial dimension. In one embodiment, the maximum distance between adjacent trajectory points does not exceed half of the minimum radial dimension of the allowable error band to ensure interpolation accuracy. For example, if the minimum radial dimension is 0.1 mm, the distance between adjacent points does not exceed 0.05 mm. During machining, the system finds the two nearest adjacent trajectory points based on the current tool position, performs linear interpolation between these two points, and obtains the radial dimension of the current point in real time.

[0064] In a preferred embodiment, each trajectory point stores not only its three-dimensional coordinates and radial dimension, but also its cumulative arc length (the distance from the trajectory start point along the theoretical trajectory to that point). This allows for rapid determination of the projection point on the theoretical trajectory based on the current tool position during machining, and then interpolation can be performed on adjacent trajectory points.

[0065] The embodiment of this invention allows the radial dimension of the error band to be correlated with the angle between the fiber direction and the cutting direction. When the angle is between 60° and 120° in the transverse cutting zone, the radial dimension is automatically enlarged to provide the tool with greater clearance to cope with severe cutting force fluctuations and the risk of delamination. When the angle is outside this range, a smaller radial dimension is maintained to ensure machining accuracy in the forward cutting zone, achieving an adaptive balance between accuracy requirements and clearance needs.

[0066] The radial dimension of the current point is determined by setting the basic radial dimension and multiplying it by an adjustment factor related to the included angle. This keeps the ratio of the radial dimensions of the dangerous area to the safe area constant, ensuring a consistent safety margin logic regardless of changes in the basic machining tolerances.

[0067] The continuous theoretical trajectory is discretized into trajectory points that store three-dimensional coordinates and corresponding radial dimensions. During the machining process, the radial dimension at any position is obtained in real time through linear interpolation. This allows the anisotropic error band to be deployed in an engineered manner on the existing CNC hardware platform. The current allowable boundary can be obtained quickly in each control cycle, thereby effectively guiding the calculation of dynamic equilibrium points and maintaining the geometric accuracy of the final product while improving cutting stability.

[0068] In the aforementioned embodiments, the cutting force gradient is the core basis for solving the dynamic equilibrium point. The original cutting force signal inevitably contains high-frequency noise, such as vibration interference during the cutting process, electromagnetic noise, and electronic noise from the sensor itself. The embodiments of this invention further employ a gradient calculation method combining sliding window arithmetic mean filtering and first-order backward difference, and adaptively adjust the sliding window length inversely proportional to the spindle speed to obtain a stable and reliable cutting force gradient signal across the entire speed range.

[0069] Specifically, the cutting force signal is continuously acquired at a fixed sampling frequency f. In one specific embodiment, a piezoelectric force sensor is installed on the spindle or tool holder to acquire the cutting force signal at a sampling frequency of f=1000Hz, with a sampling period T=0.001 seconds. Let the original cutting force value acquired at the k-th sampling time be F[k].

[0070] A sliding window of length N is constructed to store the original cutting force values ​​of the N most recent sampling points. That is, the data stored in the window is {F[k-N+1], F[k-N+2], ..., F[k]}. The length N of the sliding window is a preset parameter, and its value affects the filtering effect: the larger N is, the smoother the filtering but the slower the response to signal changes; the smaller N is, the faster the response but the weaker the noise suppression capability. In one specific implementation, N is set to 10. In each sampling period, the window slides forward one position, discarding the oldest data and adding the latest data.

[0071] To extract reliable gradient information from noisy signals, the original cutting force signal is filtered to obtain the filtered cutting force value F'[k]. Then, a first-order backward difference is performed on the filtered cutting force value to calculate the cutting force gradient G[k] = (F'[k] - F'[k-1]) / T at the current moment. The unit of the calculated cutting force gradient G[k] is Newtons per second. The cutting force gradient value reflects the instantaneous rate of change of the cutting force: G[k] > 0 indicates that the cutting force is increasing, G[k] < 0 indicates that the cutting force is decreasing, and G[k] = 0 indicates that the cutting force is stable.

[0072] In a preferred embodiment, the length N of the sliding window is inversely proportional to the current spindle speed. The physical basis of this design is that the effective frequency components of the cutting force signal are directly related to the spindle speed: the higher the speed, the higher the cutting frequency per tooth, the faster the signal changes, and a smaller N is needed to ensure a fast response; the lower the speed, the slower the signal changes, and a larger N can be used to obtain better noise suppression.

[0073] Specifically, let the spindle speed be n (in revolutions per minute) and the number of tool teeth be z, then the frequency per tooth is f. tooth =n×z / 60. Sliding window length N = floor(β×f / f)tooth ), where β is a proportionality constant, ranging from 2 to 5; floor() indicates rounding down.

[0074] For example, let f = 1000 Hz, n = 1000 rpm, z = 4, then f tooth =1000×4 / 60≈66.7Hz, take β=3, then N =floor(3×1000 / 66.7) = floor(45) = 45. When the speed decreases to 500rpm, f tooth =500×4 / 60≈33.3Hz, N = floor(3×1000 / 33.3) = floor(90) = 90. When the speed increases to 2000rpm, f tooth =2000×4 / 60≈133.3Hz, N = floor(3×1000 / 133.3) = floor(22.5) = 22. In this way, the length of the sliding window automatically adapts to changes in the spindle speed, maintaining a relatively consistent frequency response characteristic of the filter across the entire speed range.

[0075] In another specific implementation, the length N of the sliding window can also be dynamically adjusted according to the cutting state. For example, when the absolute value of the cutting force gradient is large (indicating a transient impact process), N is decreased to improve the response speed; when the cutting force gradient is small (indicating a steady-state cutting process), N is increased to enhance noise suppression.

[0076] The cutting force gradient G[k] calculated above is used as the input for solving the dynamic equilibrium point in step S300 to determine the offset direction and calculate the offset distance.

[0077] The present invention employs an arithmetic mean filter on the original cutting force signal using a sliding window, effectively suppressing high-frequency noise such as cutting vibration and electromagnetic interference, resulting in a smooth and continuous filtered cutting force signal. Based on this, a first-order backward differential calculation is performed to obtain a stable and reliable cutting force gradient value, avoiding frequent jumps in gradient sign and violent oscillations of the dynamic equilibrium point caused by noise. By making the sliding window length inversely proportional to the spindle speed, the system can obtain an accurate cutting force gradient signal within a wide speed range continuously adjustable from 0° to 180° of the spindle, thereby achieving stable operation of the entire zero-backtrack trajectory replanning method.

[0078] Example 2: Refer to Figure 3 As shown, this embodiment of the invention discloses a zero-backtrack trajectory replanning system for composite material machining, comprising: The cutting tool, mounted on the spindle, is used to machine the workpiece; The force sensing module is used to acquire continuous cutting force signals between the tool and the workpiece during the cutting process and to calculate the gradient of the cutting force change with time. The allowable error curve construction module is used to construct allowable error bands based on the processing tolerances of composite materials; The dynamic equilibrium point calculation module is used to calculate a dynamic equilibrium point within the allowable error band based on the current cutting force gradient. The calculation method of the dynamic equilibrium point includes: if the cutting force gradient is greater than zero, the equilibrium point is shifted towards the direction of decreasing cutting force along the tangent direction of the current feed path; if the cutting force gradient is less than zero, the equilibrium point is shifted towards the inside of the workpiece along the normal direction of the current feed path. The zero-backtrack trajectory replanning module is used to correct the tool target position at the next moment to the equilibrium point while keeping the current direction of motion unchanged. The system is configured such that the dot product of the velocity vector of the center point of the tool in the workpiece coordinate system and the unit vector of the current feed direction is always greater than or equal to zero.

[0079] Preferably, the allowable error curve construction module is configured to: determine the radial dimension of the allowable error band based on the angle between the fiber direction of the composite material and the current cutting direction; wherein, when the angle is in the range of 60° to 120°, the radial dimension of the allowable error band is greater than the radial dimension when the angle is outside this range.

[0080] The embodiments of the present invention are used to implement the zero-backtrack trajectory replanning method for composite material cutting in Embodiment 1. Both have the same inventive concept and the same beneficial effects, which will not be repeated here.

[0081] In summary, the zero-backtrack trajectory replanning method and system for composite material machining described in this invention uses the cutting force gradient to drive the dynamic equilibrium point to perform non-backtracking tangential active avoidance, thereby suppressing fiber tearing and delamination, while improving machining stability and tool life.

[0082] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A zero-retreat trajectory replanning method for composite material machining, characterized in that, include: The theoretical trajectory of the tool-workpiece contact point is planned in advance, and the allowable error band is constructed according to the machining tolerance of composite materials; The continuous cutting force signal during the cutting process of the tool along the theoretical trajectory is obtained, and the gradient of the cutting force change with time is calculated. Based on the current cutting force gradient, calculate a dynamic equilibrium point within the allowable error band; The method for calculating the dynamic equilibrium point includes: if the cutting force gradient is greater than zero, the equilibrium point is shifted along the tangent direction of the current feed path toward the direction of decreasing cutting force; if the cutting force gradient is less than zero, the equilibrium point is shifted along the normal direction of the current feed path toward the inside of the workpiece. Under the premise that the current direction of motion remains unchanged, the tool target position at the next moment is corrected to the equilibrium point to generate a replanning trajectory; Wherein, the dot product of the velocity vector of the center point of the tool in the workpiece coordinate system and the unit vector of the current feed direction is greater than or equal to zero.

2. The zero-retreat trajectory replanning method for composite material machining according to claim 1, characterized in that, The method for calculating the dynamic equilibrium point further includes: if the cutting force amplitude exceeds the safety threshold, the equilibrium point is shifted towards the outside of the workpiece along the tangent direction of the current feed path; if the cutting force amplitude is lower than the stability threshold, the equilibrium point converges towards the theoretical trajectory direction.

3. The zero-retreat trajectory replanning method for composite material machining according to claim 1, characterized in that, When the cutting force gradient is greater than zero and the equilibrium point shifts along the tangential direction, causing the tool center point to exceed the allowable error band, the shift is decomposed into a tangential direction component and a normal direction component: The tangential direction component is the remaining distance in the tangential direction from the current tool position to the boundary of the allowable error band; The normal direction component is taken as an offset towards the outside of the workpiece; wherein, the normal direction component is activated only when the remaining distance in the tangential direction is less than the desired tangential offset.

4. The zero-backtrack trajectory replanning method for composite material machining according to any one of claims 1-3, characterized in that, The radial dimension of the allowable error band is related to the angle between the fiber direction of the composite material and the current cutting direction; wherein, when the angle is in the range of 60° to 120°, the radial dimension of the allowable error band is greater than the radial dimension when the angle is outside this range.

5. The zero-backtrack trajectory replanning method for composite material machining according to claim 4, characterized in that, Determining the radial dimension of the allowable error band includes: setting a basic radial dimension, calculating an adjustment coefficient based on the angle between the fiber direction of the composite material and the cutting direction, and determining the radial dimension of the current point based on the basic radial dimension and the adjustment coefficient; wherein, when the angle is within the range of 60° to 120°, the adjustment coefficient is greater than 1; otherwise, the adjustment coefficient is equal to 1.

6. The zero-retreat trajectory replanning method for composite material machining according to claim 4, characterized in that, The allowable error band is discretized into multiple trajectory points distributed along the theoretical trajectory. Each trajectory point stores the three-dimensional coordinates of the point and the corresponding radial dimension. During the machining process, linear interpolation is performed between adjacent trajectory points based on the current tool position to obtain the radial dimension of the current point.

7. The zero-retreat trajectory replanning method for composite material machining according to claim 1, characterized in that, Calculate the gradient of the cutting force with respect to time, including: The cutting force signal is continuously acquired at a fixed sampling frequency, and a sliding window of length N is constructed to store the original cutting force of the most recent N sampling points; The original cutting force within the sliding window is arithmetically averaged to obtain the filtered cutting force. The first-order backward difference is performed on the filtered cutting force to obtain the cutting force gradient at the current moment.

8. The zero-retreat trajectory replanning method for composite material machining according to claim 7, characterized in that, The length N of the sliding window is inversely proportional to the current rotational speed of the spindle.

9. A zero-retreat trajectory replanning system for composite material machining, characterized in that, include: The cutting tool, mounted on the spindle, is used to machine the workpiece; The force sensing module is used to acquire continuous cutting force signals between the tool and the workpiece during the cutting process and to calculate the gradient of the cutting force change with time. The allowable error curve construction module is used to construct allowable error bands based on the processing tolerances of composite materials; The dynamic equilibrium point calculation module is used to calculate a dynamic equilibrium point within the allowable error band based on the current cutting force gradient. The method for calculating the dynamic equilibrium point includes: if the cutting force gradient is greater than zero, the equilibrium point is shifted along the tangent direction of the current feed path toward the direction of decreasing cutting force; if the cutting force gradient is less than zero, the equilibrium point is shifted along the normal direction of the current feed path toward the inside of the workpiece. The zero-backtrack trajectory replanning module is used to correct the tool target position at the next moment to the equilibrium point while keeping the current direction of motion unchanged. The system is configured such that the dot product of the velocity vector of the center point of the tool in the workpiece coordinate system and the unit vector of the current feed direction is greater than or equal to zero.

10. The zero-backtrack trajectory replanning system for composite material machining according to claim 9, characterized in that, The allowable error curve construction module is configured to determine the radial dimension of the allowable error band based on the angle between the fiber direction of the composite material and the current cutting direction; wherein, when the angle is in the range of 60° to 120°, the radial dimension of the allowable error band is greater than the radial dimension when the angle is outside this range.

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