1、PDF原文：http:/ Control of a Non-Orthogonal Reconfigurable Machine Tool Reuven KatzJohn YookYoram Koren Received: January 3, 2003； revised: September 16, 2003 Abstract Computerized control systems for machine tools must generate coordinated movements of the separately driven axes of motion in ord
2、er to trace accurately a predetermined path of the cutting tool relative to the workpiece. However, since the dynamic properties of the individual machine axes are not exactly equal, undesired contour errors are generated. The contour error is defined as the distance between the predetermined and ac
3、tual path of the cutting tool. The cross-coupling controller (CCC) strategy was introduced to effectively decrease the contour errors in conventional, orthogonal machine tools. This paper, however, deals with a new class of machines that have non-orthogonal axes of motion and called reconfigurable m
4、achine tools (RMTs). These machines may be included in large-scale reconfigurable machining systems (RMSs). When the axes of the machine are non-orthogonal, the movement between the axes is tightly coupled and the importance of coordinated movement among the axes becomes even greater. In the case of
5、 a non-orthogonal RMT, in addition to the contour error, another machining error called in-depth error is also generated due to the non-orthogonal nature of the machine. The focus of this study is on the conceptual design of a new type of cross-coupling controller for a non-orthogonal machine tool t
6、hat decreases both the contour and the in-depth machining errors. Various types of cross-coupling controllers, symmetric and non-symmetric, with and without feedforward, are suggested and studied. The stability of the control system is investigated, and simulation is used to compare the different ty
7、pes of controllers. We show that by using cross-coupling controllers the reduction of machining errors are significantly reduced in comparison with the conventional de-coupled controller. Furthermore, it is shown that the non-symmetric cross-coupling feedforward (NS-CC-FF) controller demonstrates th
8、e best results and is the leading concept for non-orthogonal machine tools. 2004 ASME Contributed by the Dynamic Systems, Measurement, and Control Division of THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS for publication in the ASME JOURNAL OF DYNAMIC SYSTEMS, MEASUREMENT, AND CONTROL. Manuscri
9、pt received by the ASME Dynamic Systems and Control Division January 3, 2003； final revision September 16, 2003. Associate Editor: J. Tu. Keywords:machine tool, cross-coupling controller, non-orthogonal, RMT 1 Introduction Currently manufacturing industries have two primary method
10、s for producing medium and high volume machined parts: dedicated machining systems (DMSs) and flexible manufacturing systems (FMSs) that include CNC machines. The DMS is an ideal solution when the part design is fixed and mass production is required to reduce cost. On the other hand, the FMS is idea
11、l when the required quantities are not so high and many modifications in the part design are foreseen. In contrast to these two extremes, Koren describes an innovative approach of customized manufacturing called reconfigurable manufacturing systems (RMS). The main advantage of this new approach is t
12、he customized flexibility in the system to produce a "part family" with lower investment cost than FMS. A typical RMS includes both conventional CNC machines and a new type of machine called the Reconfigurable Machine Tool . The Engineering Research Center (ERC) for Reconfigurable Machinin
13、g Systems (RMS) at the University of Michigan with its industrial partners has designed an experimental Reconfigurable Machine Tool (RMT) 。 This machine allows ERC researchers to validate many of the new concepts and machine tool design methodologies that have been already developed in the center. T
14、here are many types of RMTs. This paper describes an arch-type non-orthogonal multi-axis RMT machine 。 The economic justification of RMTs is given in section 2 of this paper. A contouring motion requires that the cutting tool moves along a desired trajectory. Typically, computerized control sy
15、stems for machine tools generate coordinated movements of the separately driven axes of motion in order to trace a predetermined path of the cutting tool relative to the workpiece. To reduce the contouring error, which is defined as the distance between the predetermined and the actual path, there h
16、ave been two main control strategies. The first approach is to use feedforward control in order to reduce axial tracking errors .however, they are limited when non-linear cuts are required. The other approach is to use cross-coupling control in which axial-feedback information is shared betwee
17、n the moving axes. The cross-coupling controller is used in addition to the conventional axial servo controller. At each sampling time, the cross-coupling controller calculates the current contour error and generates a command that moves the tool toward the closest point on the desired tool path. Th
18、is control strategy of the cross-coupling controller (CCC) effectively decreases the contour error. Advanced control methods have been applied to further improve the control properties of the original cross-coupling controller (CCC). An optimal CCC is suggested in , to improve the controller perform
19、ance when high contouring speeds were required. Another method to overcome the same problem for higher contour feedrates is addressed in , which uses adaptive feederate control strategy to improve the controller performance. The latest trend of cross-coupling controller improvement is the applicatio
20、n of fuzzy logic . All these methods, however, do not work for machines with non-orthogonal axes. Surface cut (e.g., a circular cut in the X-Y plane) on a 3-axis orthogonal milling machine requires a motion of two axes (e.g., X and Y). However, surface cuts in the non-orthogonal RMT require simultan
21、eous motion of all three axes. Therefore, in addition to the contour error, this motion creates another error, called the in-depth error, which is in the Z direction. This error affects the surface finish quality of the workpiece. While contouring, the tool tip of the RMT has not only to follow the
22、predetermined path, but also to control continuously the depth of cut. The simultaneous control of both errors, the conventional contour error and the in-depth error, requires a new control strategy since the standard CCC algorithms cannot be directly applied. In other words, the RMT control design
23、problem requires a new control approach that is able to correct simultaneously two types of cutting errors. This problem has not been addressed in the literature. In this paper, we describe three types of controllers aimed at reducing the contour and in-depth error simultaneously. First we inv
24、estigate a symmetrical cross-coupling (S-CC) controller, which unfortunately does not show good performance in reducing both errors. The poor performance is due to the conflicting demands in reducing the two errors and the lack of information sharing between the two pairs of axes (X-Y and Y-Z), whic
25、h are responsible for error compensation. To overcome this problem, the required motion information of one pair of axes is fed forward to the other. This idea results in two new controller types, symmetrical cross-coupling feedforward (S-CC-FF) controller and non-symmetrical cross-coupling feedforward (NS-CC-FF) controller. Finally, the influence of the reconfigurable angular position of the cutting tool on system stability is investigated.
