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Imitation three-axis control method of virtual axis CNC machine tool (1)
Source: Bearing network time: 2013-06-07
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Through analysis, it is found that the virtual axis machine tool is generally based on the Stewart channel principle; the reasonable motion scale of the rotating coordinates is much smaller than the conventional five-axis CNC machine tool (generally only 20 to 30 degrees; and the five-axis machine tool can reach more than 90 degrees). ; and the increase of the rotation angle will greatly reduce the useful working space of the machine tool. Although the composite layout can expand the corner size; but the layout is messy; it is difficult to ensure high rigidity; therefore; the general virtual axis machine is not suitable for processing large-scale, multi- Coordinated moving parts. But from another point of view; in practice production, the chaotic parts that require multi-coordinate machining are a small amount; the dominant position is still the processing of general custom parts. Thus; how to use the layout of the virtual axis machine Features; carry forward its advantages in the high-speed and efficient processing of custom parts; it will be more practical.
The basic idea of ​​the virtual axis machine tool imitation three-axis control method is; imitate the control method of the existing three-coordinate CNC machine tool; control the six-degree-of-freedom movement of the virtual axis machine tool; from the external characteristics; make the virtual axis machine tool and the convention three Coordinate CNC machine tool equivalent. This way; not only the existing sophisticated three-coordinate active programming system can be directly used for the six-degree-of-freedom virtual axis machine tool; and the spindle unit can only perform translational motion by imitation three-axis control; The working space of the virtual axis machine tool; to make it carry forward more effect. In addition; through the imitation of three-axis control; can also usefully reduce the clutter of the control system; then significantly reduce the cost of the machine; Promote applications on a large scale.
2 Virtual axis machine tool for the advantages of conventional machining A typical layout of the virtual axis machine; the layout can be reduced to a so-called "six-bar channel layout". The detailed meaning is; will be six variable length drive rod (referred to as the drive rod) One end is fixed to a static channel (such as a foundation or a machine tool layout); the other end of the drive rod is coupled to the moving channel; that is, coupled to the spindle unit. Thus; adjusting the length of the six driving rod; the spindle and the tool can be related to the workpiece The required feed motion. The feed system is precisely manipulated by the control system; the workpieces that meet the requirements can be machined.
In view of the unparalleled advantages of the custom CNC machine tools, the virtual axis machine tools are necessary for the completion of high-speed, high-precision machining; therefore, they are used as efficient processing equipment for custom parts; to maximize their advantages.
3 The basic principle of imitation three-axis control is that there is no guide rail guided in the fixed direction in the virtual axis machine tool; the tool movement axes X, Y, Z, etc. required for NC machining are not real; therefore, even if only three-dimensional tool movement is required (Stress stability is only azimuth change); it is also necessary to perform six degrees of freedom control on the moving channel.
The imitation three-axis control method is a control method of the three-axis numerical control machine tool based on the layout characteristics of the virtual axis machine tool. The starting point is: when the custom part is processed by the virtual axis machine tool; the tool installed in the main shaft only needs to be made. Three-dimensional translational motion; its posture is a fixed value. Thus; although the spindle unit fixed with the moving channel has six degrees of freedom of motion; but only three translational degrees of freedom are touched by real-time accounting. For this purpose, the tool center or end face is used. The coordinates Xm, Ym, Zm in the machine coordinate system indicate the tool orientation; and the displacement is calculated in real time through the three-coordinate interpolation algorithm. Together, the tool coordinate system located in the middle of the tool center or end face is established; its coordinate axis Xt, Yt, and Zt are parallel to the Xm, Ym, and Zm axes of the machine coordinate system. The rotation angle of the tool coordinate system around the Xm, Ym, and Zm axes indicates the posture of the moving channel; and it is set to a fixed value. The real-time accounting and real-time manipulation of the motion of the three channels of Xm, Ym and Zm are carried out; the real-time control of the transition of the moving channel around the Xm, Ym and Zm axes is carried out; Degree manipulation; and then complete the three-coordinate linkage control of the tool movement. Because this method does not require real-time accounting of the dynamic channel posture; this way; not only can reduce the amount of accounting of the true and false mapping and linkage manipulation; can also be six degrees of freedom The operation of the virtual axis machine tool is classified into the field of the conventional three-coordinate S-controlled machine tool control; the sophisticated three-coordinate control method is used to control the new machine tool.
The layout of the virtual axis machine tool is known; because the directly controllable controlled amount in the machine tool is the length Li (i = 1, 2, ...; 6) of the six drive rods supporting the spindle components; that is, the practical motion axis of the machine tool ( Referred to as the real axis); therefore, the motion of the moving channel should be fully controlled; then the precise control of the tool motion track should be completed; the moving channel motion command (virtual axis command) should be converted to the real axis space to perform; The active inverse mapping from the real axis space to the imaginary axis space is completed.
The operation process of the system is: firstly; the tool motion track is generated in real time according to the input information given by the part NC program; that is, the expected motion amount of the tool along the Xm, Ym, Zm coordinates in the virtual axis space is solved, and then; Converting the expected amount of motion of the virtual axis to the motion command value of the six-drive rod, and finally; decoupling and following the length of each drive rod; making the actual length and the desired length common; and implicitly through the machine layout Virtual inverse mapping; you can get the tool motion track that meets the instruction requirements; and ensure that the tool pose is a given constant value.
4 imaginary axis space tool motion orbit generation tool motion orbit generation mission is to convert the tool path given by the part NC program (several curves in the virtual axis space independent of time and machine characteristics) into time and machine characteristics (such as plus Deceleration characteristics, etc.) Associated discretized tool motion orbits. The solution process is as follows:
The mathematical model is established to ensure the accuracy of the orbital generation; the parametric direct interpolation algorithm is used in the imitation three-axis manipulation. The key is to establish a parametric mathematical model for the imputed curve:
x=f1(u)
y=f2(u)
z=f3(u) (1)
In the formula u parameter; u∈[0,1] demand for real-time orbital accounting does not touch the function accounting; only need to add, subtract, multiply and divide the number of times to complete.
For example, regarding circular interpolation; the detailed method of equation (1) is: (2) the matrix of M constants in the formula; when the interpolation points are located in one to four quadrants; the values ​​are different: r radius of the arc; orbit The accounting can be affirmative; that is, the calculation of the coordinates of each orbital point is based on the origin of the model coordinates; then the stacking errors can be eliminated; the speed and accuracy of the imputed accounting are usefully ensured.
The acceleration/deceleration control is to satisfy the requirements of the machine acceleration and deceleration characteristics of the generated tool movement track; the optimal acceleration/deceleration curve can be determined according to the dynamic characteristics of the machine tool; and it is stored in the control system. During the operation of the system; Several program segments; parsing the change trend of the feed rate; determining the desired feed rate F, then reading the feedrate override K on the operation panel; and using it to correct the F; the policy feed rate Fnew; Fnew=K . F, further; compare Fnew with the current feed speed Fold; and calculate the instantaneous feed rate Fk (mm/min) of the current sampling period according to the acceleration and deceleration characteristic curve of the machine tool.
Speed ​​and error control Because interpolation is not a static calculation; it is necessary to make the interval between the interpolation point and the previous interpolation point satisfactory for the feed rate and acceleration and deceleration; together with these two points The error between the interpolated straight line segment and the interpolated curve is within the given tolerance scale. For this reason, the instantaneous feed rate is required to be the control policy; the interpolated straight line segment length Dtk is made with the constraint error as the constraint condition. Control.
The method is as follows:
Firstly; the instantaneous feed rate Fk given by the acceleration/deceleration calculation; the expected chord length in the sampling period at the time (the length of the interpolated straight line segment without binding) is calculated by the following formula: (3) where Dt1 is the desired chord length; mm T sampling period; ms Then; according to the error of the sampling interpolation, the bound chord length is calculated: (4)
e Intermittent error between the interpolated orbit and the desired orbit
r The radius of curvature of the desired orbit at the interpolation point is final; the value of Dtk is determined according to the relative size of Dt1 and Dt2. That is, if the expected chord length Dt1 is smaller than the bound chord length Dt2, then the length of the interpolated straight line segment is Dtk=Dt1; Otherwise take Dtk=Dt2.
The mission of imputing orbital accounting interpolation orbit calculation is: in each sampling period; according to the length of the interpolated straight line segment Dtk obtained above; the coordinate value of the current point on the interpolation orbit is calculated in real time. The accounting process is as follows:
Firstly, according to the following relationship between the parameter increments Du and Dt, the Du of the interpolation period is obtained: (5) where the rate of change of the arc length of the du/ds parameter is higher due to the interpolation frequency; one sampling period The length of the middle arc is very close to the length of the chord; therefore, in practice accounting, du/ds≈Du/Dt. can be used to take an increment of Du; find the corresponding Dt; then the desired du/ds can be obtained.
Although this approximation indicates a small effect on the feed rate; it does not have any effect on the accuracy of the interpolated orbit. In the sampling interpolation; the orbital accuracy is the main contradiction; the coordinate calculation of the interpolation point is necessary to be sure and accurate; The accuracy of the speed of the interpolation point along the orbit is in an unnecessary position; it can be promised to have small errors. The results obtained thus ensure the accuracy of the orbit and improve the speed of accounting.
Then; calculate the value of the sampling period parameter at that time: uk=uk-1+Du (6) Finally; substitute uk into equation (1); then calculate the coordinate value of the current point on the interpolation orbit xk; yk; zk Repeat the above process until the end of the interpolation; you can get the entire discretized interpolation track.
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Features
1. In the cable laying process, it can support the cable and change the sliding friction into rolling friction between cable and support, which means it can reduce pulling force.
2. Straight pulling roller is used in the straight line segment of cable laying. Generally put one roller every 2-3 meter.
3, Corner pulling roller is used in the turning point of cable laying. The amount of corner pulling roller in the turning point is determined by the side pressure and cable bending radius.
Usually used in laying path of cable tray, well head, upper and down slope and control cable.
Notes: When operating, the roller should be fixed in ground or other supports.
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