Following error control method and system for swing shaft of numerical control machine tool

Abstract

The invention belongs to the technical field of numerical control, and particularly discloses a following error control method and system for a swing shaft of a numerical control machine tool, and the method comprises the steps: in the movement process of the machine tool, a numerical control system calculates the speed feed-forward quantity, the acceleration feed-forward quantity and the swing shaft gravitational torque feed-forward quantity at the current moment according to an instruction signal of a swing shaft i in a current control period k; sending the speed signal to a driver to obtain a feedback speed signal and a signal output by a current loop; and the LESO in the driver estimates the external disturbance quantity of the current control period based on the feedback speed signal and the signal output by the current loop, and feeds forward and compensates the external disturbance quantity into the current loop to complete the feed motion in the current control period. The following position error of the swing shaft of the numerical control machine tool is reduced, the following precision control level matched with a linear shaft is achieved, and then the final machining precision is improved.

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 a swing axis of a numerical control machine tool. Background Art

[0002] Precision is the most important performance indicator of CNC machine tools. With the emergence of five-axis linkage 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 forward more stringent requirements on the position following error control of the rotary axis of the 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 the servo control system error 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. At present, the 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 of the steady speed segment; applying acceleration feedforward to the current loop in the servo control can effectively improve the position following error of the variable speed segment; applying friction feedforward to the current loop in the servo control can further reduce the position following error of the reverse segment.

[0004] The above feedforward method works well for the linear ball screw feed axis of three-axis CNC machine tools, but has limited effect on the swing axis and rotary axis of the torque motor direct connection mechanism of five-axis machine tools. Especially for the swing axis whose motor driving torque is affected by posture changes and motion characteristics, the traditional velocity acceleration feedforward method cannot guarantee the control level of the position following error in the uniform speed section to be close to zero; at the same time, the traditional friction feedforward method requires the establishment of an accurate friction model, which involves many friction parameters and is difficult to identify, and is not suitable for swing axes that change with posture and motion characteristics. For five-axis linkage CNC machine tools, its processing accuracy is not only determined by the servo control system performance of the linear axis, but also by the servo performance of the linear axis, swing axis and rotary axis. When processing complex surfaces at high speed and high acceleration, the position following error and dynamic response performance of the swing axis are particularly important. 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 view of 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, which aims to reduce the following position error of the swing axis of the CNC machine tool and achieve a following accuracy control level matching that of the linear axis.

[0006] To achieve the above object, according to one aspect of the present invention, a following error control method for a swing 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 moment feedforward 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 in the current control cycle.

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

[0010]

[0011] Where z2 = δ i (k) is the external disturbance, z1 is the expansion state observation of LESO, 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 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:

[0013] Real-time acquisition of the multi-order 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 drive 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 moment feedforward 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 moment feedforward I gi (k) It is sent to the driver current loop feedforward register at the same time, 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 a swing axis of a numerically controlled machine tool is provided, comprising a processor, wherein the processor is used to execute the following error control method for the swing axis of a numerically controlled machine tool.

[0030] In general, the above technical solution conceived by the present invention has the following technical advantages compared with the prior art:

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

[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 the position, and adds a gravity torque feedforward model based on the change of the swing axis posture to the traditional feedforward model of the feed system dynamics; and considers the influence of the complexity of the traditional friction feedforward friction model and inaccurate parameter identification, and regards friction and other various nonlinear factors as external disturbances and compensates them 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 of the velocity acceleration feedforward and gravity torque feedforward of the present invention is implemented on the CNC system side, and the entire swing axis feedforward control process is completed through model calculation, bus transmission, and driver execution; while LESO is implemented on the servo side, which ensures the real-time estimation of the system state and avoids the influence of complex friction models and inaccurate parameter identification. Compared with the feedforward control method in the traditional servo drive, this method meets the calculation requirements of model modularization with the help of the high computing power of the CNC system, and provides users with higher flexibility and adaptability, effectively utilizing the software and hardware resources of the CNC system, and maximizing the control accuracy of the machine tool 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 of 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 the 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 purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain 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 can 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 numerically controlled 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 i (k) Further differential calculation to obtain the current command speed v i (k), command acceleration a i (k) Figure 2 The calculation formula is as follows:

[0046]

[0047] Where: i is the swing axis number, 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 speed 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), the calculation formula is as follows:

[0050]

[0051] Among them, h is the lead of the swing axis turntable, r is the resolution of the servo drive receiving the speed feedforward, K V is the speed feedforward gain coefficient (0%~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), the calculation formula is 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 gravity moment component m of the swing axis combined with the turntable can be calculated by the numerical control 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) It 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 needs to be timed 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 the speed acceleration feedforward control in the speed change section.

[0058] The above feedforward implementation method needs to ensure that the driver has a corresponding interface inside, and the feedforward amount needs to 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 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 in the current control cycle.

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

[0061]

[0062] Among them, β1 and β2 are the gains of LESO. LESO requires the machine tool commissioning personnel to configure the corresponding parameters according to the identified total inertia of the feed system.

[0063] The above steps (2) to (5) are the feedforward control implementation process within a 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 in 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 in which 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 only 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 protection scope of the present invention.

Claims

1. A following error control method for the swing axis of a CNC machine tool, It is 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 moment feedforward 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) 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 in the current control cycle.

2. The following error control method of the swing axis of a CNC machine tool according to claim 1, It is characterized in that The external disturbance δ i The calculation formula for (k) is: Among them, z 2 =δ i (k) is the external disturbance, z 1 is the expansion state observable of LESO, They are z 1 、z 2 The derivative of, J is the total moment of inertia of the motor and load, K t is the torque constant of the torque motor, β 1 and β 2 is the gain of LESO.

3. The following error control method of the swing axis of a CNC machine tool according to claim 1, It is 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-order 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 of the swing axis of a CNC machine tool according to claim 3, It is characterized in that The swing axis gravity moment 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 of the swing axis of a CNC machine tool according to claim 4, It is 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 of the swing axis of a numerically controlled machine tool according to claim 3, It is 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 drive receiving the speed feedforward, K V is the speed feedforward gain coefficient.

7. The following error control method of the swing axis of a numerically controlled machine tool according to claim 3, It is 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 of the swing axis of a numerically controlled machine tool according to claim 3, It is characterized in that 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 of the swing axis of a CNC machine tool according to any one of claims 1 to 8, It is characterized in that The current speed feedforward value V i (k), acceleration feedforward I ai (k) and the swing axis gravity moment feedforward 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 moment feedforward I gi (k) It is sent to the driver current loop feedforward register at the same time, and the speed feedforward V i (k) and acceleration feedforward I ai (k) Perform filtering processing.

10. A following error control system for the swing axis of a CNC machine tool. It is characterized in that It comprises a processor, and the processor is used to execute the following error control method of the swing axis of a CNC machine tool as described in any one of claims 1-9.

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