| CPC B24B 49/02 (2013.01) [G05B 19/41875 (2013.01); G05B 2219/32368 (2013.01); G05B 2219/45161 (2013.01)] | 1 Claim |
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1. A digital twin control method for product processing quality, comprising the following steps:
step 1: constructing the digital twin control system for product processing quality
with product quality as a control objective, during actual processing, workpiece material interferes with a grinding wheel tool system to cause material deformation and final material removal, constructing the digital twin control system based on a control principle of moving part of a CNC (Computer Numerical Control) grinding machine, comprising physical entities, digital twin data, virtual entities, services and links between components, with the physical entities being moving entities of the CNC grinding machine, and accordingly constructing the digital twin control system for product quality;
analyzing the physical entities of corresponding moving parts of a grinding wheel and a workpiece, and connecting the grinding wheel with a CNC grinding machine guide rail through a grinding spindle in the CNC grinding machine to form a subsystem connected by grinding wheel—grinding spindle—X-axis of the CNC grinding machine; connecting the workpiece with the CNC grinding machine guide rail through a workpiece spindle or C-axis to form a subsystem connected by workpiece—workpiece spindle/C-axis—Z-axis of the CNC grinding machine; and connecting the above two subsystems together through a CNC grinding machine body, distributing in a CNC grinding machine coordinate system, moving in the CNC grinding machine through an instruction signal and achieving material removal to form the physical entities of the CNC grinding machine accordingly;
the virtual entities are real-time mapping of the physical entities; the physical entities of the CNC grinding machine move in the CNC system of the CNC grinding machine through the instruction signal: sending, by the CNC grinding machine, processing requirements of a product to the moving part of the CNC grinding machine in the form of NC codes; sending a control instruction signal by the moving part of the CNC grinding machine according to the processing requirements; converting, by a corresponding servo driver, the control instruction signal into a power signal of the corresponding moving parts of the CNC grinding machine and driving the corresponding moving parts of the CNC grinding machine to move; after the moving part of the CNC grinding machine moves, recording a real-time position of the moving part of the CNC grinding machine through a grating ruler and comparing with a control instruction signal of a servo controller, with a difference therebetween being a corresponding moving error; sending the moving error as a feedback signal to an IPC (Industrial Personal Computer) of the CNC grinding machine; adopting a corresponding PID (Proportional Integral Derivative) control strategy by the IPC of the CNC grinding machine according to the corresponding feedback signal; sending a corresponding control instruction signal to the servo driver again and driving the corresponding moving parts of the CNC grinding machine to move until the moving error corresponding to the feedback signal reaches the control accuracy of an algorithm; repeating an above control process by the digital twin control system to finally achieve the processing requirements of the corresponding product; creating the virtual entities completely corresponding to the physical entities accordingly, and achieving real-time monitoring and optimization of the physical entities through the digital twin data;
the services comprise key information related to the digital twin control system and provide real-time update of status changes in the physical entities and the virtual entities;
connection is a pipeline for transmitting the information between the physical entities and the virtual entities, and the physical entities provide real-time digital twin data for the virtual entities by using an information transmission technology;
the digital twin data comprises data from the CNC grinding machine entities, and status, performance and control data from moving status sensors of the CNC grinding machine;
achieving by the digital twin control system, real-time interaction between the virtual entities and the physical entities in a grinding process through the digital twin data and connections, and conducting real-time simulation and timely optimization, to achieve an accurate control of product quality in the grinding process;
step 2: acquiring real-time spatial data of the workpiece based on the digital twin control system
with product quality as the control objective, achieving a precision processing of the workpiece by driving relative motion between the workpiece and the grinding wheel through a CNC grinding machine moving system; achieving the processing of a cylindrical surface in combination with a linear feed motion of the X-axis through a rotational motion of the workpiece and a grinding spindle; achieving the processing of a plane in combination with the linear feed motion of the Z-axis through the rotational motion of the workpiece and the grinding spindle; achieving the processing of a rotary symmetric surface in combination with two-axis interpolation motion of the X-axis and the Z-axis through the rotational motion of the workpiece and the grinding spindle, and achieving a processing of a non-rotary symmetric surface through three-axis interpolation motion of the X-axis, the Z-axis and the C-axis;
extracting theoretical coordinates of each processing point through the digital twin control system accordingly based on that a surface formation of a CNC grinding machine product is related to point-to-point contact points between a tool and the workpiece during processing, and forming a spatial data distribution of the workpiece during processing through coordinate transformation, as shown in formula (1);
 where xactual, zactual, and cactual represent actual positions where XZC coordinates pass through in the CNC grinding machine coordinate system during grinding processing respectively, and X(t), Y(t), and Z(t) represent the spatial data distribution of workpiece coordinates respectively;
step 3: calibrating parameters of the digital twin system
during actual processing, a processing error is composed of two parts; one part is a processing principle error, and the other part is a dynamic error caused by a processing load, expressed as:
 where f represents a principle error of grinding processing, comprising a surface grinding mark caused by surface residual height resulting from grinding wheel speed Vs, workpiece speed Vw, feed rate Vf and grinding depth ap; g represents a principle error of CNC grinding machine motion, comprising a XZC guide rail accuracy EX, EZ and EC of the CNC grinding machine, and external interference u during processing; h represents elastoplastic deformation caused by an inherent characteristic E of the material under the action of a grinding force F, j represents elastic recovery, and w represents an influencing parameter of a degree of elastic recovery of the material;
under an ideal status, the processing course is a stationary process; when a planar workpiece is processed, a tool-workpiece contact point is on an ideal trajectory, a processing surface is in a pure flat status, and an error value is 0; if only the dynamic characteristics of the CNC grinding machine are considered, when the planar workpiece is processed, a grinding point of the CNC grinding machine fluctuates, and processing error value fluctuates; when difficult-to-process material is processed, due to the difficulty in processing of the material, the processing load is introduced during processing, causing fluctuation of the tool-workpiece contact point of the CNC grinding machine, and accordingly, the processing error is expressed as:
 where EX represents an influence of the grinding force F, the inherent characteristic E of the material, the grinding wheel speed Vs, the feed rate Vf, the grinding depth ap, and the external interference u on an X-axis error; EZ represents the influence of the grinding force F, the inherent characteristic E of the material, the workpiece speed Vw, the grinding depth ap and the external interference u on a Z-axis error; EC represents the influence of the grinding force F, the inherent characteristic E of the material, the workpiece speed Vw and the external interference u on a C-axis error;
by calibrating the parameters of the digital twin control system, establishing a correlation of the processing load—output voltage; wherein when the moving part of the CNC grinding machine is in a balanced position, a position error and output power are zero; during loading, a load on the moving part is increased, generating a forward load Fe+; at this time, the output power is less than an applied load, and a position Xe of the moving part deviates forwardly from an original instruction position; to ensure that the instruction position of the moving part is stable, the output power of the moving part is continuously increased, and a motor power is increased and maintains new stable power output; after the motor power is increased to be consistent with the external load, the position Xe returns to the original instruction position;
when the external load is removed, the load on the moving part is decreased; at this time, the output power of the motor is higher than that of the external load, and the position Xe of the moving part deviates reversely from the original instruction position; to ensure that the instruction position of the moving part is stable, the output power of the motor is decreased, and the power is decreased and maintains new stable power output; after the motor power is reduced to be consistent with the external load, the guide rail position Xe returns to the original instruction position;
accordingly, determining a relationship of the load, a current of a guide rail motor and the position error of the guide rail through load calibration, and then, obtaining real-time load change during processing according to the real-time changes of the current of the guide rail motor and the position error of the guide rail during grinding processing; extracting a real-time position signal of the CNC grinding machine through the digital twin control system, and judging a processing status of a workpiece surface through change conditions, under CNC grinding machine coordinates, of the position signals of the X-axis, the Z-axis and the C-axis under the CNC grinding machine coordinates, to obtain the processing quality of the workpiece:
 where xactual, zactual and cactual represent the actual positions where the X, Z and C coordinates pass through under the CNC grinding machine tool coordinate system during grinding processing respectively; xinstruction, zinstruction and cinstruction represent instruction positions of XZC coordinates in an NC program after discretized by the NC program of the CNC grinding machine respectively; and xpe, zpe and cpe represent influences on three XZC coordinates during grinding processing respectively;
accordingly, converting the real-time position signal of the CNC grinding machine into spatial coordinates:
 accordingly, controlling the product processing quality in real time through the digital twin control system;
step 4: obtaining a real-time processing quality of the product through the digital twin control system
achieving real-time monitoring and real-time control of the product processing quality according to the digital twin control system, then adjusting processing parameters in real time during processing, and changing the processing load to improve the product processing quality;
in the digital twin control system, regarding a difference between a theoretical position and a real-time position as a result of deviation of actual coordinate values from instruction coordinate values under the dynamic characteristics of the CNC grinding machine, a grinding parameter and material responses during processing:
 where X(F, E, Vs, Vf, ap, u) represents the influence of the grinding force F, the inherent characteristic E of the material, the grinding wheel speed Vs, the feed speed Vf, the grinding depth ap and the external interference u on deviation of the actual coordinate values of the X-axis from instruction coordinates; Z(F, E, Vw, ap, u) represents the influence of the grinding force F, the inherent characteristic E of the material, the workpiece speed Vw, the grinding depth ap and the external interference u on the Z-axis error; C(F, E, Vw, u) represents the influence of the grinding force F, the inherent characteristic E of the material, the workpiece speed Vw and the external interference u on the C-axis error;
extracting the error values of the moving part of the CNC grinding machine respectively to obtain error distribution results of product surface data:
 accordingly, achieving real-time control of the product processing quality.
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