In order to realize software locating processing, the primary problem that must be solved is how to quickly and accurately obtain the actual state of the workpiece that is fixed on the workbench without being accurately positioned. For milling, drilling, and boring, the key to workpiece locating is to find the true relationship between the machining coordinate system (the coordinate system fixed to the workpiece) and the design coordinate system (the coordinate system determined by the process design).
The relationship between the machining coordinate system and the design coordinate system can be represented by a homogeneous transformation matrix T. In order to obtain T, an intuitive consideration is to take m measurement points Pi on the workpiece, and find m corresponding points Qi on its CAD model, and then construct an objective function J=Emi=1|TPi-Qi|2, Finally, the transformation matrix T is determined according to the principle of least squares. However, the intuitive solution process is difficult to achieve, and the main difficulties are as follows.
First, there is a basic requirement for this solution process, that is, the corresponding point Qi of the measurement point Pi in the workpiece CAD model must be found. This requirement can be satisfied in the case of small-range locating, because in this case, the offset between the machining coordinate system of the workpiece and the design coordinate system is small, and Qi is near Pi, according to the value of Pi, It is possible to determine which surface of the CAD model the Qi belongs to, which gives the correspondence between the Pi and the surface of the CAD model. However, in the case of large-scale locating, due to the large offset between the machining coordinate system and the design coordinate system, the correspondence information between the Pi and the CAD model surface is lacking. At this time, it is difficult to infer Qi from the value of Pi. Which surface of the CAD model belongs to, that is, the CAD surface Si corresponding to Pi cannot be determined, and thus it is impossible to find the point Qci closest to Pi by Si, and to find Qi by an iterative algorithm.
Second, in order to know the correspondence between the measured points and the surface of the CAD model in the case of large-scale positioning, it is not enough to rely solely on the information of the above m measuring points, and a large amount of information about the surface contour of the workpiece must be collected. Due to the measurement speed requirements, conventional contact measuring devices are difficult to perform this task, and other high-precision fast measuring instruments are not popular because of their high price, so other approaches must be taken.
In order to solve the above difficulties encountered in large-scale locating, a hierarchical locating scheme combining macroscopic and microscopic can be adopted. The basic idea of ​​this scheme is to decompose the transformation matrix T into two parts: the macro coarse transform matrix TR and the micro fine transform matrix TF, that is, T=TF*TR, and then obtain TR and TF respectively, and finally obtain T. In this hierarchical scheme, the first level seeks TR as a large-scale macro rough seek position, and the accuracy requirement is not very high, and fast is the main goal pursued. To this end, firstly, the economical contactless sensor is used to quickly obtain the macroscopic contour information of the workpiece randomly fixed in the machining area of ​​the machine tool, and then the large-scale coarse transformation matrix TR is determined by the method of pattern recognition and the like.
Use the obtained TF to correct the relationship between the workpiece coordinate system and the design coordinate system, and then repeat the above process to obtain a more accurate Qi and obtain a more accurate TF. In this way, after a certain number of iterations, the TF satisfying the accuracy requirement can be obtained, thereby completing the locating task.
(Finish)
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