A following error control method and system for a swing axis of a CNC machine tool

Abstract

The present invention belongs to the field of numerical control technology and specifically discloses a following error control method and system for the swing axis of a numerically controlled machine tool. The method includes the following steps: during the movement of the machine tool, the numerical control system calculates the current velocity feedforward, acceleration feedforward, and swing axis gravity torque feedforward based on the command signal of the swing axis i in the current control cycle k, and sends the calculated value to the driver to obtain the feedback velocity signal and the signal output by the current loop; the LESO inside the driver estimates the external disturbance in the current control cycle based on the feedback velocity signal and the signal output by the current loop, and feeds the disturbance forward into the current loop to complete the feed motion in the current control cycle. The present invention reduces the following position error of the swing axis of the numerically controlled machine tool, achieving a following accuracy control level that matches that of a linear axis, thereby improving the final processing accuracy.

Description

Technical Field

[0001] The present invention belongs to the field of numerical control technology, and more specifically, relates to a following error control method and system for an oscillating axis of a numerically controlled machine tool. Background Art

[0002] Precision is the most important performance indicator of CNC machine tools. With the emergence of five-axis machining of spatial surfaces and the urgent need to improve machining efficiency, the proportion of errors caused by the dynamic characteristics of machine tools in the total error continues to increase. This puts more stringent requirements on the position following error control of the rotary axis of five-axis CNC machine tools and the improvement of servo dynamic response performance.

[0003] At present, feedforward control methods have been widely used in the field of three-axis CNC machine tools. Feedforward control is a control strategy that actively compensates for servo control system errors based on the prediction of the dynamic model of the machine tool feed system. It can effectively reduce the position following error of the machine tool feed motion and improve the dynamic response performance of the machine tool. The current three-axis CNC machine tool feed system widely uses ball screw transmission mechanisms. The commonly used feedforward control methods based on the dual-inertia dynamic model of the feed system are: speed acceleration feedforward based on NC instructions and friction feedforward control methods. Applying speed feedforward to the speed loop in the servo control can effectively reduce the position following error in the steady speed section; applying acceleration feedforward to the current loop in the servo control can effectively improve the position following error in the variable speed section; applying friction feedforward to the current loop in the servo control can further reduce the position following error in the reverse section.

[0004] The aforementioned feedforward method works well for the linear ball screw feed axes of three-axis CNC machine tools, but has limited effectiveness for the swing and rotary axes of the torque motor direct-connect mechanisms of five-axis machine tools. In particular, for swing axes, where the motor drive torque is affected by changes in posture and kinematic characteristics, traditional velocity and acceleration feedforward methods cannot guarantee near-zero control of position following error during the constant speed period. Furthermore, traditional friction feedforward methods require the establishment of a precise friction model, involving numerous friction parameters that are difficult to identify, making them unsuitable for swing axes that vary with posture and kinematic characteristics. For five-axis CNC machine tools, machining accuracy is determined not solely by the servo control system performance of the linear axes, but rather by the combined servo performance of the linear, swing, and rotary axes. Therefore, the position following error and dynamic response performance of the swing axis are particularly important when machining complex curved surfaces at high speeds and high accelerations. Therefore, in order to meet the precision requirements of five-axis CNC machine tools in high-end manufacturing industries such as aerospace, shipbuilding, and high-precision instruments, it is urgent to improve the feedforward control method of the swing axis of five-axis CNC machine tools to achieve a following precision control level that matches the linear axis, thereby improving the final processing accuracy. Summary of the Invention

[0005] In response to the above defects or improvement needs of the prior art, the present invention provides a following error control method and system for the swing axis of a CNC machine tool, the purpose of which is to reduce the following position error of the swing axis of the CNC machine tool and achieve a following accuracy control level that matches that of the linear axis.

[0006] To achieve the above object, according to one aspect of the present invention, a method for controlling the following error of an oscillating axis of a CNC machine tool is proposed, comprising the following steps:

[0007] During the movement of the machine tool, the CNC system calculates the current velocity feedforward value V according to the command signal of the swing axis i in the current control cycle k. i (k), acceleration feedforward I ai (k) and the swing axis gravity torque feedforward value I gi (k), and send it to the driver to get the feedback speed signal v back_i (k) and the current loop output signal I back_i (k);

[0008] The LESO inside the drive is based on the feedback speed signal v back_i (k) and the current loop output signal I back_i (k), estimate the external disturbance δ of the current control cycle i (k) and feedforwards it into the current loop to complete the feed motion within the current control cycle.

[0009] As a further preferred embodiment, the external disturbance δ i The calculation formula for (k) is:

[0010]

[0011] Where z2 = δ i (k) is the external perturbation, z1 is the LESO expansion state observation, are the derivatives of z1 and z2 respectively, J is the total moment of inertia of the motor and load, K t is the torque constant of the torque motor, β1 and β2 are the gains of LESO.

[0012] As a further preferred embodiment, the command signal includes the command position θ i (k), command speed v i (k) and command acceleration a i (k); Obtaining the command signal comprises the following steps:

[0013] Real-time acquisition of the multi-level continuous command position θ of the swing axis i after interpolation within the current control cycle k i (k), and then according to the command position θ i (k) Calculate the current command speed vi (k) and command acceleration a i (k).

[0014] As a further preferred embodiment, the swing axis gravity moment feedforward value I gi The calculation formula for (k) is:

[0015]

[0016] Among them, m AC is the mass of the AC joint turntable, g is the acceleration of gravity, l is the gravity moment arm of the AC joint turntable; K t is the torque constant of the torque motor.

[0017] As a further preferred embodiment, m AC gl is the fixed component of the gravity moment of the swing axis joint turntable. The machine tool is pre-set to run a specific motion trajectory, and m is calculated by the CNC system. AC The value of gl.

[0018] As a further preferred embodiment, the speed feedforward value V i The calculation formula for (k) is:

[0019]

[0020] Among them, h is the lead of the swing axis turntable, r is the resolution of the servo driver receiving the speed feedforward, K V is the speed feedforward gain coefficient.

[0021] As a further preferred embodiment, the acceleration feedforward value I ai The calculation formula for (k) is:

[0022]

[0023] Where J is the total moment of inertia of the motor and load, K I is the current feedforward gain coefficient, K t is the servo motor torque constant.

[0024] As a further preferred embodiment, the command speed v i (k) and command acceleration a i The calculation formula for (k) is:

[0025]

[0026] Among them, ΔT is the interpolation period of the CNC system, θ i (k-1), v i (k-1) are the command position and command speed within the control cycle k-1 respectively.

[0027] As a further preferred method, the current speed feedforward value V i (k), acceleration feedforward I ai (k) and the swing axis gravity torque feedforward value I gi (k) Sent to the driver, specifically:

[0028] The current speed feedforward value V i (k) After timing optimization, the acceleration feedforward value I ai (k) and the swing axis gravity torque feedforward value I gi (k) At the same time, it is sent to the driver current loop feedforward register and the speed feedforward V i (k) and acceleration feedforward I ai (k) Perform filtering processing.

[0029] According to another aspect of the present invention, a following error control system for an oscillating axis of a numerically controlled machine tool is provided, comprising a processor configured to execute the following error control method for an oscillating axis of a numerically controlled machine tool.

[0030] In general, the above technical solutions conceived by the present invention have the following technical advantages compared with the existing technology:

[0031] 1. Compared with the traditional velocity and acceleration feedforward control method, the present invention adds the swing axis gravity torque feedforward, thereby obtaining the external disturbance and feedforward compensating it into the current loop, thereby improving the dynamic response performance of the machine tool swing axis feed system. On the basis of the traditional linear axis feedforward method, the position following error of the machine tool swing axis is reduced, and the following error control level can be achieved at the millidegree level (within 0.005°), thereby improving the machining accuracy of high-precision dual-turntable five-axis CNC machine tools.

[0032] 2. Compared with the traditional velocity acceleration feedforward + friction model feedforward method, the present invention takes into account the dynamic characteristics of the feed system in which the gravity torque of the swing axis changes with position. In the traditional feed system dynamics feedforward model, a gravity torque feedforward model based on the change of the swing axis posture is added; and the influence of the complexity of the traditional friction feedforward friction model and inaccurate parameter identification is taken into account. The friction force and various other nonlinear factors are regarded as external disturbances and compensated through the linear extended state observer (LESO), thereby expanding the feedforward calculation model and realizing high-precision control based on the dynamics of the swing axis.

[0033] 3. The calculation process for velocity acceleration feedforward and gravity torque feedforward in this invention is implemented on the CNC system side, completing the entire swing axis feedforward control process through model calculation, bus transmission, and driver execution. LESO, on the other hand, is implemented on the servo side, ensuring real-time estimation of system states and avoiding the impact of complex friction models and inaccurate parameter identification. Compared to feedforward control methods within traditional servo drives, this method leverages the high computing power of the CNC system to meet the computational requirements of model modularization, while providing users with greater flexibility and adaptability. It effectively utilizes the CNC system's software and hardware resources, maximizing the control accuracy of the machine tool's swing axis feed system. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a flow chart of a following error control method for a swing axis of a CNC machine tool according to an embodiment of the present invention;

[0035] Figure 2 (a)-(c) are schematic diagrams of command signals required by the feedforward calculation model according to an embodiment of the present invention;

[0036] Figure 3 This is a block diagram of the following error control principle of the swing axis of a CNC machine tool according to an embodiment of the present invention;

[0037] Figure 4 (a) and (b) are comparison diagrams of the reduction effect of position following error by the embodiment of the present invention and the traditional linear axis velocity acceleration feedforward and friction feedforward control methods. DETAILED DESCRIPTION

[0038] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0039] The embodiment of the present invention provides a method for controlling the following error of a swing axis of a CNC machine tool, such as Figure 1 and Figure 3 As shown, the following steps are included:

[0040] (1) The system parameters required for the feedforward model of the machine tool swing axis turntable are obtained in advance, as shown in Table 1.

[0041] Table 1. Related parameters required for the feedforward model

[0042]

[0043]

[0044] (2) During the movement of the machine tool, the CNC system obtains the command position θ in the current control cycle in real time i (k), and calculate the current command speed v i (k), command acceleration a i (k).

[0045] Specifically, after the machine tool is started, it is necessary to obtain the command position signal that has been interpolated and planned by the CNC system and meets certain speed and acceleration constraints, and then the multi-order continuous command position signal θ after interpolation by the CNC system is obtained. i (k) Further differential calculation is performed to obtain the current command speed v i (k), command acceleration a i (k), such as Figure 2 The calculation formula is as follows:

[0046]

[0047] Where: i is the axis number of the swing axis, k is the kth control cycle, and ΔT is the interpolation cycle of the CNC system.

[0048] (3) The command signal (command position θ i (k), command speed v i (k) and command acceleration a i (k)) is input into the feedforward model to calculate the current velocity feedforward value V i (k), acceleration feedforward U ai (k) and gravity torque current feedforward I gi (k).

[0049] Specifically, after obtaining the command speed and command acceleration, calculate the speed feedforward value V i (k) and acceleration feedforward I ai (k), calculated as follows:

[0050]

[0051] Among them, h is the lead of the swing axis turntable, r is the resolution of the servo driver receiving the speed feedforward, K V is the speed feedforward gain coefficient (0% to 200%), K I is the current feedforward gain coefficient (0% to 200%), K t is the servo motor torque constant.

[0052] After obtaining the command position, it is necessary to further calculate the swing axis gravity torque feedforward value I gi (k), calculated as follows:

[0053]

[0054] Among them, m AC is the mass of the AC joint turntable, g is the acceleration of gravity, l is the distance between the center of mass of the AC joint turntable and the axis of the A axis, that is, the gravity arm, sinθ i (k) is the gravity moment component related to the position, K t is the torque constant of the torque motor.

[0055] It should be noted that the machine tool is pre-set to run a specific motion trajectory, and the oscillating axis combined turntable gravity moment component m can be calculated by the CNC system. AC gl.

[0056] (4) The current speed feedforward value V i (k) After timing optimization, the acceleration feedforward value I ai (k) and gravity torque current feedforward I gi (k) is sent to the driver (specifically the driver current loop feedforward register) at the same time, and the speed feedforward V i (k) and acceleration feedforward I ai (k) Filtering is performed in the driver to obtain the feedback speed signal v back_i (k) and the current loop output signal I back_i (k).

[0057] Specifically, since the servo speed loop feedback loop and feedforward loop usually have filtering links with different cutoff frequencies, the speed feedforward amount needs to be time-adjusted in the CNC system before it is sent down to compensate for the phase inconsistency caused by the filter links of the feedforward and feedback loops, thereby ensuring the accuracy of speed acceleration feedforward control in the speed change section.

[0058] The above feedforward implementation method must ensure that the driver has a corresponding interface inside, and the feedforward amount must be converted according to the speed and current calculation units inside the driver to ensure the accuracy of the feedforward control.

[0059] (5) The LESO inside the driver is based on the feedback speed signal v back_i (k) and the current loop output signal I back_i (k), estimate the external disturbance δ of the current control cycle i (k) and feedforwards it into the current loop to complete the feed motion within the current control cycle.

[0060] Specifically, the disturbance z2 observed by LESO is δ i (k) is calculated as follows:

[0061]

[0062] Where β1 and β2 are the gains of LESO. LESO requires the machine tool commissioning personnel to configure the corresponding parameters based on the identified total inertia of the feed system.

[0063] The above steps (2) to (5) are the feedforward control implementation process within one control cycle (i.e., for the swing axis i in the control cycle k). During the machining process of the machine tool, the above feedforward control process can effectively reduce the position following error of the swing axis combined with the turntable within the entire motion cycle, and realize the near-zero following error control in the uniform speed section and the millidegree level (within 0.005°) following error control in the variable speed section.

[0064] Compared with the traditional linear axis feedforward method, the specific effects of the present invention are as follows: Figure 4 As shown, it can be seen that the feedforward method of the present invention has a significant effect on reducing the maximum following error of the swing axis, especially improving the situation where the following error in the uniform speed section changes with the position, and achieving a near-zero control level of the following error in the uniform speed section.

[0065] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for controlling the following error of a swing axis of a CNC machine tool, characterized in that: The steps include: During the movement of the machine tool, the CNC system calculates the current velocity feedforward value V according to the command signal of the swing axis i in the current control cycle k. i (k), acceleration feedforward I ai (k) and the swing axis gravity torque feedforward value I gi (k), and send it to the driver to get the feedback speed signal v back_i (k) and the current loop output signal I back_i (k); The LESO inside the drive is based on the feedback speed signal v back_i (k), the signal I output by the current loop back_i (k) and the swing axis gravity torque feedforward value I gi (k), estimate the external disturbance δ of the current control cycle i (k) and feedforwards it into the current loop to complete the feed motion within the current control cycle.

2. The following error control method for the swing axis of a CNC machine tool according to claim 1, characterized in that: The external disturbance δ i The calculation formula for (k) is: Where z2 = δ i (k) is the external perturbation, z1 is the LESO expansion state observation, are the derivatives of z1 and z2 respectively, J is the total moment of inertia of the motor and load, K t is the torque constant of the torque motor, β1 and β2 are the gains of LESO.

3. The following error control method for the swing axis of a CNC machine tool according to claim 1, characterized in that: The command signal includes a command position θ i (k), command speed v i (k) and command acceleration a i (k); Obtaining the command signal comprises the following steps: Real-time acquisition of the multi-level continuous command position θ of the swing axis i after interpolation within the current control cycle k i (k), and then according to the command position θ i (k) Calculate the current command speed v i (k) and command acceleration a i (k).

4. The following error control method for the swing axis of a CNC machine tool according to claim 3, characterized in that: The swing axis gravity torque feedforward value I gi The calculation formula for (k) is: Among them, m AC is the mass of the AC joint turntable, g is the acceleration of gravity, l is the gravity moment arm of the AC joint turntable; K t is the torque constant of the torque motor.

5. The following error control method for the swing axis of a CNC machine tool according to claim 4, characterized in that: m AC gl is the fixed component of the gravity moment of the swing axis joint turntable. The machine tool is pre-set to run a specific motion trajectory, and m is calculated by the CNC system. AC The value of gl.

6. The following error control method for the swing axis of a CNC machine tool according to claim 3, characterized in that: The speed feedforward V i The calculation formula for (k) is: Among them, h is the lead of the swing axis turntable, r is the resolution of the servo driver receiving the speed feedforward, K V is the speed feedforward gain coefficient.

7. The following error control method for the swing axis of a CNC machine tool according to claim 3, characterized in that: The acceleration feedforward value I ai The calculation formula for (k) is: Where J is the total moment of inertia of the motor and load, K I is the current feedforward gain coefficient, K t is the servo motor torque constant.

8. The following error control method for the swing axis of a CNC machine tool according to claim 3, wherein: The command speed v i (k) and command acceleration a i The calculation formula for (k) is: Among them, ΔT is the interpolation period of the CNC system, θ i (k-1), v i (k-1) are the command position and command speed within the control cycle k-1 respectively.

9. The following error control method for the swing axis of a CNC machine tool according to any one of claims 1 to 8, characterized in that: The current speed feedforward value V i (k), acceleration feedforward I ai (k) and the swing axis gravity torque feedforward value I gi (k) Sent to the driver, specifically: The current speed feedforward value V i (k) After timing optimization, the acceleration feedforward value I ai (k) and the swing axis gravity torque feedforward value I gi (k) At the same time, it is sent to the driver current loop feedforward register and the speed feedforward V i (k) and acceleration feedforward I ai (k) Perform filtering processing.

10. A following error control system for a swing axis of a CNC machine tool, characterized in that: The method comprises a processor configured to execute the following error control method for the swing axis of a CNC machine tool according to any one of claims 1 to 9.

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