Showing papers by "Chia-Hsiang Menq published in 1997"

Precision motion control of a magnetic suspension actuator using a robust nonlinear compensation scheme

Abstract: This paper presents a robust nonlinear compensation algorithm for realizing large travel in magnetic suspension systems suffering from parameter variations and external disturbance forces. A geometric feedback linearization technique that utilizes the complete nonlinear description of the electromagnetic field distribution is employed to obtain large travel. Robustness to uncertainties in the feedback linearized system is achieved through the development of a discrete-time delay-control-based compensation algorithm. In comparison to previous developments, the new scheme removes the constraints of triangularity conditions in compensation of unmatched uncertainties. The performance of this algorithm is experimentally investigated on a magnetic suspension system. In each of the experiments, the controller is designed using the approximate nonlinear model of the system, which is significantly different from the actual plant model. For a fixed set of gains, the robust nonlinear controller accurately stabilizes the system for a large range of ball positions. In trajectory tracking performance evaluation, the controller provides tracking accuracies that are of the same order of magnitude as the accuracy of the position sensor. Finally, when the suspended ball is impressed with an external disturbance force, the controller provides adequate model regulation and rejection of disturbance forces, demonstrating high stiffness control. The experimental results, therefore, verify the consistent performance of the algorithm in realizing large travel in spite of parameter variations and external disturbances.

Integrated planning for precision machining of complex surfaces. Part 1: Cutting-path and feedrate optimization

Abstract: In order to achieve higher productivity and product quality simultaneously for sculptured surface productions, two advanced strategies are proposed for machining planning, namely a cutting-path-adaptive-feedrate strategy and a control surface strategy. In the cutting-path-adaptive-feedrate strategy, machining time is reduced by cutting along low-force-low-error machining directions and by maximizing feedrates. In the control surface strategy, machining errors are minimized by using a compensated control surface based on predicted machining errors. In part 1 of this paper, the cutting-path-adaptive-feedrate strategy, which improves the productivity of sculptured surface machining when subjected to both force and dimensional constraints, is described. In this proposed strategy, a new machining-planning aid called a maximum feedrate map is developed. In this map, the maximum allowable feedrates, subjected to the specified constraints, at each control point along various machining directions, are determined using a surface generation model. These local maximum-feedrate boundaries indicate the acceptable range of feedrates that a part programmer can use in the NC programming. In addition, the maximum feedrate map also provides the part programmer an important aid in selecting the cutting directions. In order to illustrate the application of the maximum feedrate map and to examine the capability of the proposed cutting-path-adaptive-feedrate strategy in improving the productivity of sculptured surface machining, simulation studies of a two-dimensional curved surface are performed and the results are presented in this paper. The applications of the proposed strategy to real three-dimensional complex surfaces (e.g. a turbine blade die) along with experimental verifications are presented in part 2 of this paper. In part 3 of this paper the control surface strategy and its applications to the finish milling of three-dimensional complex surfaces are discussed.

Modeling of Friction Contact and Its Application to the Design of Shroud Contact

Abstract: Designers of aircraft engines frequently employ shrouds in turbine design. In this paper, a variable normal load friction force model is proposed to investigate the influence of shroudlike contact kinematics on the forced response of frictionally constrained turbine blades. Analytical criteria are formulated to predict the transitions between stick, slip, and separation of the interface so as to assess the induced friction forces. When considering cyclic loading, the induced friction forces are combined with the variable normal load so as to determine the effective stiffness and damping of the friction joint over a cycle of motion. The harmonic balance method is then used to impose the effective stiffness and damping of the friction joint on the linear structure. The solution procedure for the nonlinear response of two-degree-of-freedom oscillator is demonstrated. As an application, this procedure is used to study the coupling effect of two constrained forces, friction force and variable normal load, on the optimization of the shroud contact design.

Integrated planning for precision machining of complex surfaces—III. Compensation of dimensionai errors

Abstract: This paper presents an approach that compensates machining errors in ball-end milling processes using the control-surface strategy. In the proposed approach, machining errors induced by cutter deflections are predicted from a surface generation model so that actual machining experiments and dimensional inspections are not required. Therefore, the proposed approach can be integrated into an integrated framework for machining planning in which prediction and compensation of dimensional errors take place in the process development phase rather than in the manufacturing phase of the production cycle. Based on the predictive capability of the surface generation model, a sensitivity function is defined so as to characterize the variations of the machining errors when perturbing the control surface. Using the defined sensitivity function, a compensated control surface can be constructed, based on which new cutting paths can be planned and “near-zero” surface dimensional errors can be accomplished. In order to verify the proposed strategy, actual production of a turbine blade die using a CNC machining center was conducted. By using the control-surface strategy with the sensitivity function approach, the maximum surface dimensional error is reduced from ± 340 to ± 10 μm.

Error compensation for sculptured surface productions by the application of control-surface strategy using predicted machining errors

Abstract: In this paper, the principle and effectiveness of the control-surface strategy in machining-error compensation for end milling processes are studied. Using this strategy, two new approaches, namely the direct compensation approach and the sensitivity function approach, are proposed. When compared to existing approaches, there are two major improvements in the proposed approaches. First, machining errors caused by tool deflection are estimated from a developed surface generation model. This eliminates the time and costs required to design and conduct the actual machining experiments and dimensional inspections. Second, the effectiveness of the proposed approaches is improved either by increasing the number of times the strategy is been used or by selecting the appropriate shifted distance based on the estimated machining-error curve. The effectiveness of the proposed error-compensation approaches is verified from simulations and experimental results for a 2D sculptured surface. By using computer aided design tool, this surface generation model can be easily applied to the problems in which the designed surfaces are complex 3D sculptured by considering more complicated chip geometry model. These proposed approaches can also be integrated into an integrated framework for machining path planning in which prediction and compensation of dimensional errors take place in the process development phase rather than in the manufacturing phase of the production cycle.

Integrated planning for precision machining of complex surfaces. Part 2: Application to the machining of a turbine blade die

Abstract: The applications of the proposed cutting-path-adaptive-feedrate strategy, described in part 1 of this paper, to real three-dimensional complex surfaces are examined. Its applications to the aerospace, automobile, and die-mold industries are of particular interest. As an example, the proposed strategy is used in planning the machining of a turbine blade die. This process consists of rough milling (using the stock material), and the subsequent semi-finish and finish milling operations. Computer simulations along with experimental verifications are presented in this paper.