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Scanning stages are used for precise positioning in a variety of advanced manufacturing processes, and must deliver high accelerations/decelerations at motion reversals to achieve high throughput. The resulting inertial forces cause excessive motor heating and residual vibration of the stage, both of which must be mitigated to preserve positioning accuracy. A novel scanning stage, having a permanent magnet based device that actively provides assist forces to the motor to simultaneously reduce motor heating and residual vibration, is presented. The active assist device is optimally designed to be versatile, allowing it to provide adequate assist for scan trajectories involving the widest range of scan strokes/positions. A basis–spline-based method is proposed for optimal control of the device, to maximize the assist it provides for scan trajectories with varying scan strokes/positions. Experiments conducted with a prototype stage involving scanning trajectories commonly employed in silicon wafer processing are used to demonstrate up to 60% and 34% reductions in heat and vibration-induced errors, respectively, when compared to cases without the proposed active assist device. [ABSTRACT FROM AUTHOR]

Ultra-precision manufacturing (UPM) machines are used to fabricate and measure complex parts having micrometer-level features and nanometer-level tolerances/surface finishes. Therefore, random vibration of the machine due to ground excitations and residual vibration stemming from onboard disturbances must be mitigated using vibration isolation systems. A long-standing rule of thumb in vibration isolation system design is to locate the isolators in such a way that the vibration modes of the isolated machine are decoupled. However, prior work by the authors has demonstrated that coupling vibration modes of passively-isolated UPM machines could provide conditions for drastic reduction of residual vibrations compared to decoupling. The authors’ analysis was based on the restrictive assumption that the isolated machine was modally damped. The key contribution of this paper is in investigating the effect of mode coupling on the residual vibrations of UPM machines with non-proportional (NP) damping—which is more realistic than modal damping. It also analyzes the effects of mode coupling on the reduction of ground vibrations (i.e., transmissibility). The analyses reveal that, even though NP damping changes the vibration behavior of the machine compared to modal damping, mode coupling still provides ample opportunities to reduce residual vibration and transmissibility. Guidelines for properly designing a UPM machine to best exploit mode coupling for vibration reduction are provided and validated through simulations and experiments. Up to 40% reduction in residual vibration and 50% reduction in transmissibility are demonstrated. [ABSTRACT FROM AUTHOR]

Wafer scanning stages must deliver high accelerations/decelerations at motion reversals to achieve high productivity. The resulting inertial forces cause vibration of the machine frame and overheating of the linear motor actuators, thus diminishing the accuracy and increasing the cost of the stages. The novel stage design presented in this paper uses magnetic repulsion to provide assistive forces to the linear motors during acceleration/deceleration to reduce actuation force requirements and overheating. Vibration is reduced by transmitting the assistive forces to the ground, not the machine frame. 66% and 55% reduction in vibration and heat, respectively, are demonstrated using a prototype stage. [ABSTRACT FROM AUTHOR]

Abstract: Ultra-precision manufacturing (UPM) machines are used to fabricate and measure complex parts having micrometer-level features and nanometer-level tolerances/surface finishes. Consequently, low-frequency residual vibrations that occur during the motion of the machines’ axes must be mitigated. A long-standing rule of thumb in vibration isolation system design is to locate the isolators in such a way that all vibration modes are decoupled. This paper uses the 2D dynamics of a passively isolated system to show that coupling the vibration modes of the isolated system by altering the location of the isolators provides conditions which allow for the drastic reduction of residual vibrations. An objective function which minimizes residual vibration energy is defined. Perturbation analyses of the objective function reveal that the recommended practice of decoupling the vibration modes more often than not leads to sub-optimal results in terms of residual vibration reduction. The analyses also provide guidelines for correctly locating the isolators so as to reduce residual vibrations. Simulations and experiments conducted on a passively isolated ultra-precision machine tool are used to validate the findings of the paper; a 5-fold reduction of the dominant residual vibrations of the machine tool is achieved without sacrificing vibration isolation quality (i.e., transmissibility). [Copyright &y& Elsevier]

Highlights • Inversion-based feedforward control technique for stable LTI systems is proposed. • Technique enables tracking of long-duration trajectories online in small batches. • It achieves excellent tracking control for minimum and non-minimum phase systems. • It is used for online compensation of vibration-induced errors of a 3D printer. • Significant improvements in printing accuracy at higher printing speeds are achieved. Abstract A limited-preview filtered B-spline (FBS) approach for minimizing errors in tracking a desired trajectory is presented. In the full-preview FBS approach, the feedforward control input to a stable linear system, with or without non-minimum phase zeros, is decomposed into B-spline basis functions with unknown coefficients; the basis functions are forward filtered using the (modeled) dynamics of the system, and their coefficients selected to minimize tracking errors of the entire trajectory in one batch. Instead, this paper proposes the use of a receding horizon to recursively compute unknown coefficients that minimize tracking errors for small batches (subsets) of the trajectory at a time, by exploiting the local property of B-splines. This allows optimal control signals to be determined at much lower computational cost compared to full-preview FBS, thus enabling online implementation on real-time controllers. The adverse effects of limited preview on tracking accuracy, relative to full preview, are analyzed, and limited-preview FBS is shown in numerical examples to preserve the versatility of full-preview FBS in tracking systems irrespective of their zero locations. The practicality and effectiveness of the limited-preview FBS approach are demonstrated by employing it for online feedforward compensation of tracking errors caused by structural vibrations of a stepper-motor-driven 3D printer. Alleviation of vibration-induced surface waviness and layer-to-layer registration errors, without sacrificing print speed, are demonstrated. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

As a result of the push for sustainable machine designs, efforts are constantly being made to reduce the mass/inertia of moving machine components so as to minimize material usage and energy consumption. However, the reduction of structural stiffness that often accompanies such efforts gives rise to unwanted vibrations which must be effectively mitigated to ensure satisfactory performance of the designed machine. The ball screw mechanism (BSM) is commonly used in machines for motion and force transmission. Recent research has shown that, due to the coupling introduced by the nut, a torque applied to the shaft of a ball screw mechanism causes undesirable lateral (bending) vibrations of the screw, which adversely affect the fatigue life and positioning accuracy of ball screw-driven machines. In this paper, an analysis of the stiffness matrix connecting the screw to the nut is used to show that the entry/exit angle of the balls and the lead angle of the screw have the greatest influence on the coupling between the torsional and lateral directions. An objective function is proposed to minimize the static coupling between the applied torque and lateral deformations of the screw. The existence of local minima in the objective function is shown to be dependent on the cyclical characteristics of cross-coupling terms in the screw-nut interface stiffness matrix as a function of the entry/exit angle of the balls. Moreover, the sensitivity of the local minima to other nut/screw parameters is shown to highly depend on the lead angle. Simulations conducted on the finite element (FE) model of a single-axis ball screw-driven machine demonstrate that the optimally selected entry/exit angles result in a significant reduction of the low-frequency torque-induced vibrations of the machine compared to the unoptimized case, particularly when the lead angle is small. The proposed method is therefore suitable for reducing the torque-induced lateral vibrations of ball screws without increasing the diameter (i.e., inertia) of the screw, thus leading to more sustainable designs of ball screw-driven machines. [ABSTRACT FROM AUTHOR]