Automatic predictive control mode switching method for numerical control machine tool

Abstract

The invention relates to the technical field of automatic switching of control modes of numerical control machine tools, and discloses an automatic switching method of predictive control modes for a numerical control machine tool. And then smooth automatic switching among different control modes is realized through a transition function. According to the method, the demand change of the system can be quickly responded, the problems of oscillation and impact caused by direct switching in a traditional method can be avoided, it is ensured that the system can still keep high-precision position control in the switching process, and the dynamic response speed and the working efficiency are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of automatic switching of control modes for numerically controlled machine tools driven by permanent magnet synchronous motors, and in particular to a method for automatically switching predictive control modes for numerically controlled machine tools. Background Art

[0002] The working process of a numerically controlled machine tool is generally as follows: After a series of processes are performed on the information recorded in digital and character coding mode by the computer in the numerical control device through the input device, commands are sent to the main spindle motor of the machine tool and the motors of the actuators such as the feed through the servo system and the programmable logic controller. The main body of the machine tool then, with the cooperation of the detection and feedback device, realizes automatic control of various actions required for workpiece processing, such as the movement trajectory, displacement amount, and feed speed of the tool relative to the workpiece, thereby completing the processing of the workpiece.

[0003] Permanent Magnet Synchronous Motor (PMSM) plays an important role in the field of numerically controlled machine tools due to its advantages such as high power factor, simple structure, and wide speed regulation range. Its control performance directly affects the overall performance of the system and the quality of products. However, traditional PMSM position control systems usually adopt a vector control strategy based on a PI controller. This traditional single position control mode is difficult to simultaneously consider the rapidity and stability of the operation of the numerically controlled machine tool system; especially in different stages such as startup, acceleration, constant speed, deceleration, and position approach under the commands of the numerically controlled machine tool, the permanent magnet synchronous motor cannot flexibly respond to multi-objective control requirements with a single control mode, resulting in insufficient control accuracy and dynamic response speed. In addition, traditional position control methods perform poorly in terms of dynamic response speed, especially in stages such as startup, acceleration, and deceleration of the permanent magnet synchronous motor, where the system response speed is slow and large position errors are likely to occur.

[0004] The switching of the control mode of the permanent magnet synchronous motor (PMSM) in a numerically controlled machine tool directly affects the machining accuracy and efficiency. However, the existing basic control mode switching in numerical control has the following problems:

[0005] 1. Fixed switching points: Traditional methods set fixed switching points (such as speed thresholds) based on experience, without considering dynamic working conditions such as load fluctuations and temperature changes, resulting in inaccurate switching times (error > 15%).

[0006] 2. Uneven switching process: Directly switching the control mode (such as from constant torque to constant power) easily causes sudden changes in torque and speed, resulting in a decrease in the surface quality of the machining (roughness increases by 20% - 30%).

[0007] 3. Static mode evaluation: No dynamic evaluation model is established, and it is impossible to judge the optimal control mode in the current operation stage in real time (such as low-speed high-precision vs. high-speed high-efficiency).

[0008] 4. High computational complexity: The complex switching algorithm relies on a high-performance controller and is difficult to run in real time in an embedded system (computation delay > 50 ms).

[0009] Therefore, those skilled in the art urgently need a method to meet the automatic switching of the predictive control mode of CNC machine tools. Summary of the Invention

[0010] The purpose of the present invention is to solve the above problems and design an automatic switching method for the predictive control mode of CNC machine tools.

[0011] To achieve the above object, the technical solution of the present invention is an automatic switching method for the predictive control mode of CNC machine tools, including:

[0012] Construct a multi-objective optimization model to evaluate the optimal control mode in different operation stages;

[0013] Based on the fuzzy logic algorithm, determine the switching points in different operation stages of the permanent magnet synchronous motor;

[0014] Use the transition function to achieve smooth switching of the control mode in different operation stages of the permanent magnet synchronous motor.

[0015] The multi-objective optimization model is:

[0016]

[0017] In the formula, T e is the electromagnetic torque (Nm), calculated by i sd and i sq measured by the current sensor, and its calculation formula is:

[0018]

[0019] In the formula, p is the number of pole pairs, and its function is to convert the electrical angular velocity into the mechanical angular velocity; L d is the d-axis inductance; L q is the q-axis inductance; i sd is the d-axis current; i sq is the q-axis current; ψ m is the permanent magnet flux linkage, generally a constant;

[0020] ω e is the electrical angular velocity (rad / s), calculated by measuring the rotational speed n with an encoder, and its calculation formula is:

[0021]

[0022] P loss is the total loss (W), and its calculation formula is:

[0023]

[0024] In the formula, R s is the stator resistance, k h is the hysteresis loss coefficient and k e is the eddy current loss coefficient, and B is the magnetic induction intensity;

[0025] w1, w2, w3 are weight coefficients, which are dynamically adjusted according to the processing tasks.

[0026] The function of the multi-objective optimization model: calculate the J value in real time, evaluate the applicability of constant torque (J torque ), constant power (J power ) and other modes under the current working conditions; select the mode with the minimum J as the optimal control mode.

[0027] The determination process of the switching points in different operating stages of the permanent magnet synchronous motor is as follows:

[0028] 1) Input variables, the variables include: speed error e ω , torque error e T and load rate η; where, e ω = ω ref - ω e , e T = T ref - T e , where, T max is the maximum electromagnetic torque, T ref is the reference torque, and ω ref is the reference speed;

[0029] 2) Establish a fuzzy rule base:

[0030] IF e ω is large AND e T is small, then THEN the switching priority is high;

[0031] IF η is high AND e ω is small, then THEN the switching priority is low;

[0032] 3) Output the variable F switch as the switching flag. If F switch is 0, then do not switch. If F switch is 1, then switch.

[0033] The transition function is:

[0034] uout =(1 - α)u old +αu new (2)

[0035] Wherein, u old is the current mode control quantity, U new is the target mode control quantity, α is the transition coefficient, which linearly increases from 0 to 1 with time, and the transition time T trans = 0.1 s.

[0036] Compared with the prior art, the technical solution of this application has the following advantages:

[0037] 1. Through multi-mode predictive control and smooth self-switching technology, the present invention application can quickly respond to the demand changes of the system, select the most suitable control mode at different operation stages, significantly improve the dynamic response speed, and reduce the position error and overshoot;

[0038] 2. Compared with the existing method, the switching timing of the method of the present invention application is more accurate, and the dynamic switching point error is reduced to 5%-6%, which is better than 10%-20% of the prior art;

[0039] 3. Compared with the existing method, the method of the present invention application has higher processing quality. During the machining process of the numerically controlled machine tool, it can ensure that the surface roughness of the component is reduced by 25%-50%, the contour accuracy is improved by 60%, and the machining efficiency is effectively improved, and the efficiency can be increased by 18.8%-28.6%;

[0040] 4. The method of the present invention application has strong adaptability and can be applied to various machining tasks, such as turning, milling, grinding, etc. During the production and machining process, it can ensure the smooth switching of different control modes and effectively suppress the impact during switching. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 is a flowchart of a method for automatically switching predictive control modes for a numerically controlled machine tool according to the present invention;

[0042] Figure 2 is a graph comparing the effects in Embodiment 1 of the method of the present invention;

[0043] Figure 3 is a graph comparing the effects in Embodiment 2 of the method of the present invention;

[0044] Figure 4 is a graph comparing the effects in Embodiment 3 of the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0045] The present invention will be specifically described below with reference to the accompanying drawings, as Figures 1-4 shown;

[0046] A method for automatically switching predictive control modes for numerically controlled machine tools, comprising:

[0047] Constructing a multi-objective optimization model to evaluate the optimal control modes at different operating stages;

[0048] Determining the switching points at different operating stages of a permanent magnet synchronous motor based on a fuzzy logic algorithm;

[0049] Utilizing a transition function to achieve smooth switching of control modes at different operating stages of the permanent magnet synchronous motor.

[0050] The operating stages of the permanent magnet synchronous motor include:

[0051] Acceleration stage: The stage where the permanent magnet synchronous motor accelerates from a stationary state to the maximum rotational speed;

[0052] Constant speed stage: The stage where the permanent magnet synchronous motor operates at the maximum rotational speed;

[0053] Deceleration stage: The stage where the permanent magnet synchronous motor decelerates from the maximum rotational speed to a low speed;

[0054] Low speed approaching stage: The stage where the permanent magnet synchronous motor approaches the target position from a low speed.

[0055] The control modes include:

[0056] Predictive current control: Calculating future current values through a multi-objective optimization model and selecting the optimal voltage vector to enable the motor current to quickly track the set value;

[0057] Predictive speed control: Calculating future speed values through a multi-objective optimization model and selecting the optimal voltage vector to enable the motor speed to quickly track the set value;

[0058] Predictive position control: Calculating future position values through a multi-objective optimization model and selecting the optimal voltage vector to enable the motor position to quickly track the set value.

[0059] The multi-objective optimization model is:

[0060]

[0061] In the formula, T e is the electromagnetic torque (Nm), calculated by i sd and i sq measured by a current sensor, and its calculation formula is:

[0062]

[0063] In the formula, p is the number of pole pairs, whose function is to convert the electrical angular velocity into the mechanical angular velocity; L d is the d-axis inductance; Lq is the q-axis inductance; i sd is the d-axis current; i sq is the q-axis current; ψ m is the permanent magnet flux linkage (constant);

[0064] ω e is the electrical angular velocity (rad / s), calculated by measuring the rotational speed n with an encoder, and its calculation formula is:

[0065]

[0066] P loss is the total loss (W), including the copper loss P cu and the iron loss P fe , and:

[0067]

[0068] In the formula, R s is the stator resistance, k h is the hysteresis loss coefficient and k e is the eddy current loss coefficient, and B is the magnetic induction intensity;

[0069] w1, w2, w3 are weight coefficients, dynamically adjusted according to the processing task (for example, w2 increases in the high-precision mode and w3 increases in the high-efficiency mode).

[0070] The function of the multi-objective optimization model: Calculate the J value in real time, evaluate the applicability of constant torque (J torque ), constant power (J power ) and other modes under the current working conditions; Select the mode that minimizes J as the optimal control mode..

[0071] The determination process of the switching points at different operating stages of the permanent magnet synchronous motor is as follows:

[0072] 1) Input variables, the variables include: speed error e ω , torque error e T and load rate η; Among them, e ω = ω ref - ω e , e T = T ref - T e , Among them, T max is the maximum electromagnetic torque, T ref is the reference torque, ω ref is the reference speed;

[0073] 2) Establish a fuzzy rule base:

[0074] IF e ω is large AND eT If it is small, THEN switch to a high priority;

[0075] IF η is high AND e ω is small, THEN switch to a low priority;

[0076] 3) Output variable F switch As the switching flag, if F switch is 0, then do not switch; if F switch is 1, then switch;

[0077] It should be noted that the functions of determining the switching point include:

[0078] Dynamically adjust the switching point to adapt to load fluctuations and changes in processing tasks;

[0079] Avoid premature or late switching caused by a fixed switching point.

[0080] The transition function is:

[0081] u out = (1 - α)u old + αu new (2)

[0082] In the formula, u old is the current mode control quantity (such as i sd in the constant torque mode), U new is the target mode control quantity (such as i sq in the constant power mode), α is the transition coefficient, which linearly increases from 0 to 1 with time, and the transition time T trans = 0.1 s.

[0083] Example 1;

[0084] Application object: CNC lathe;

[0085] Processing task: Turning the outer diameter of shaft parts;

[0086] Operating conditions characteristics:

[0087] Low-speed stage: High-precision positioning (constant torque mode),

[0088] High-speed stage: High-efficiency cutting (constant power mode);

[0089] Switching process:

[0090] Low-speed stage:

[0091] Mathematical model evaluation: J torque < J power , select the constant torque mode.

[0092] Control objective: Keep the torque Te = 10 Nm, rotational speed ω e = 100 rad / s.

[0093] Determination of dynamic switching point:

[0094] Fuzzy logic input: e ω = 5 rad / s, e T = 1 Nm, η = 0.8;

[0095] Output: F switch = 1 (trigger switching).

[0096] High-speed stage:

[0097] Smooth switching: The transition coefficient α increases from 0 to 1, and the control variable u out transitions from the constant torque mode to the constant power mode.

[0098] Control objective: Maintain power P = 5 kW, rotational speed ω e = 500 rad / s.

[0099] Effect comparison of Example 1:

[0100] Index Traditional method This application Enhanced effect Switching timing error 15％ 5％ 66.7％↓ Surface roughness Ra 3.2μm Ra 2.4μm 25％↓ Processing efficiency 80％ 95％ 18.8％↑

[0101] Example 2;

[0102] Application object: CNC milling machine;

[0103] Machining task: Complex surface milling;

[0104] Operating condition characteristics:

[0105] Low-speed stage: High-precision contour machining (constant torque mode),

[0106] High-speed stage: Rapid removal of stock (constant power mode);

[0107] Switching process:

[0108] Low-speed stage:

[0109] Mathematical model evaluation: J torque < J power , select the constant torque mode;

[0110] Control objective: Maintain torque T e = 15 Nm, rotational speed ω e = 200 rad / s.

[0111] Determination of dynamic switching point:

[0112] Fuzzy logic input: e ω = 10 rad / s, e T= 2 Nm, η = 0.7;

[0113] Output: F switch = 1 (trigger switching).

[0114] High-speed stage:

[0115] Smooth switching: The transition coefficient α increases from 0 to 1, and the control variable u out transitions from the constant torque mode to the constant power mode.

[0116] Control objective: Maintain the power P = 7 kW and the rotational speed ω e = 800 rad / s.

[0117] Effect comparison of Example 2:

[0118]

[0119]

[0120] Example 3;

[0121] Application object: CNC grinding machine;

[0122] Processing task: High-precision surface grinding;

[0123] Operating conditions characteristics:

[0124] Low-speed stage: High-precision grinding (constant torque mode),

[0125] High-speed stage: Quick dressing of the grinding wheel (constant power mode);

[0126] Switching process:

[0127] Low-speed stage:

[0128] Mathematical model evaluation: J torque < J power , select the constant torque mode;

[0129] Control objective: Maintain the torque T e = 5 Nm, the rotational speed ω e = 50 rad / s.

[0130] Determination of the dynamic switching point:

[0131] Fuzzy logic input: e ω = 2 rad / s, e T = 0.5 Nm, η = 0.6.

[0132] Output: F switch = 1 (trigger switching).

[0133] High-speed stage:

[0134] Smooth switching: The transition coefficient α increases from 0 to 1, and the control variable u out transitions from the constant torque mode to the constant power mode.

[0135] Control objective: Keep the power P = 3 kW and the rotational speed ω e = 300 rad / s.

[0136] Effect comparison of Embodiment 3:

[0137]

[0138] The above technical solutions only reflect the preferred technical solutions of the technical solutions of the present invention. Some changes that those skilled in the art may make to some parts thereof all reflect the principles of the present invention and fall within the protection scope of the present invention.

Claims

1. A method for automatically switching predictive control modes for a CNC machine tool, characterized in that: include: Construct a multi-objective optimization model to evaluate the optimal control mode at different operation stages; Determine the switching points of permanent magnet synchronous motors at different operating stages based on fuzzy logic algorithms; Transition function is used to realize smooth switching of control modes of permanent magnet synchronous motor in different operating stages.

2. A predictive control mode automatic switching method for a CNC machine tool according to claim 1, characterized in that: The multi-objective optimization model is: Where, T e is the electromagnetic torque, ω e is the electrical angular velocity, P loss is the total loss (W), P loss Including copper loss P cu and iron loss P fe , w1, w2, w3 are weight coefficients.

3. The method for automatically switching the predictive control mode for a CNC machine tool according to claim 1, characterized in that: The switching point determination process of the permanent magnet synchronous motor in different operating stages is as follows: 1) Input variables, including: speed error e ω , torque error e T and load factor η; 2) Establish a fuzzy rule base: IFe ω Big AND e T Small, then the THEN switching priority is high; If IFη is high AND eω is small, then THEN switching priority is low; 3) Output variable F switch As a switching flag, if F switch If it is 0, it will not switch. switch to 1).

4. The method for automatically switching the predictive control mode for a CNC machine tool according to claim 1, characterized in that: The transition function is: you out =(1-α)u old +au new (2) In the formula, u old is the current mode control quantity, U new is the target mode control quantity, and α is the transition coefficient.

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