Fast Tool Servo Research Articles

Integrative design and processing of a conformal optical window pair in a cylindrical isolator.

In order to perform the flow visualization of a shock train structure by the schlieren imaging method in the cylindrical isolator, to the best of our knowledge, a novel integrative design and processing scheme of an aluminum alloy pipe with an acrylic conformal optical window pair are proposed. The optical ray tracing and wavefront correction methods were applied to design the inner cylindrical surfaces and outer aspherical cylindrical surfaces of the optical window pair for parallel light correction based on the conjoint analysis with the processing capability. Under the tolerance analysis and the optimization of the machining path, the integrative model was fabricated on a three-axis computer numerical control machine using two-axis turning and fast tool servo machining. The wavefront aberration (peak-to-valley value) and wavefront aberration (RMS) of the optical window pair were corrected within 12.189 and 2.658λ (λ=632.8nm) in the observation area which met the requirements of high-precision schlieren observation.

Robustness evaluation of control algorithms for a long-stroke fast tool servo

Fast tool servo (FTS) is an effective freeform surface machining technology in precision machining. The robustness of the FTS control algorithm is an important factor influencing the quality of machining. In this paper, an advanced PID control algorithm and a hybrid control algorithm are tested on a Lorentz force FTS. A mathematical simulation model is built according to the system characteristics. The model is verified by the system identification model and used for the simulation of the system's motion under disturbance. Simulation results show that the advanced PID control results in more significant differences in tracking error, amplitude error, and phase errors than the hybrid control. Four machining experiments are designed and conducted. The motion profile results from simulations and experiments show that the hybrid control (<0.5% tracking error) has better robustness than advanced PID control (>1.5% tracking error). In addition, the hybrid control exhibits rapid response speed. From the 3D profile of the machined microstructured surface, the hybrid control helps to achieve better form accuracy in the workpiece than the advanced PID control.

Dynamic performance of a long-stroke fast tool servo system

A fast tool servo (FTS) system can be used to efficiently manufacture optical freeform surfaces. This paper investigates the dynamic performance of an FTS system driven by a voice coil motor and guided by air bearings. A simulation model and testing platform are developed to evaluate the load capacity and stiffness of the air bearings. The mechanical dynamic performance of the designed FTS, including modal and harmonic analyses, is assessed using finite element analysis. A nonlinear relation between air-bearing stiffness and mechanical bandwidth is obtained. The working dynamic performance is tested through system runout, tracking performance, and closed-loop tests. Quantitative relations between air-bearing stiffness and the mechanical and working bandwidths are established and analyzed. Machining experiments verify the feasibility of the FTS system with 31.05 N/μm stiffness air-bearings.

Fabrication of multi-channel filter array mold based on fast tool servo turning

To realize a multi-channel integrated filter on a single substrate, a new preparation method is proposed in this paper, which was to adjust the thickness of the space layer based on the Fabry–Pérot interferometer theory. Optical thickness adjustment is achieved by integrating multispectral channels on a single substrate that is fabricated combined with coating, nanoimprinting, and single point diamond turning. A multi-channel filter is integrated on a mold and machined once, and three kinds of multi-channel array filter molds are fabricated utilizing fast tool servo turning. The accuracy of each channel is analyzed for the machine accuracy of the mold, which affords the key to this method. Although there exists a slight oscillation error on the surface of each channel, the maximum error is less than 4 nm and the ratio of error and differentials designed is less than 7%. The results indicate that the processing method of the multi-channel filter mold is feasible. As far as this technology is concerned, it provides technical support for mass manufacturing of large-scale multi-channel array filters.

Modeling, design and control of normal-stressed electromagnetic actuated fast tool servos

The normal-stressed electromagnetic actuated fast tool servo (FTS) plays an important role in modern ultra-precision manufacturing due to its high actuation force density and fast dynamics response. In order to promote the design and control of the normal-stressed electromagnetic actuated FTS, this paper proposes a comprehensive dynamics model based on the principles of magnetic equivalent circuits, which can be described as a Hammerstein-like structure. The electromagnetic phenomena, including hysteresis, magnetic saturation, and eddy currents, as well as the coupling between electromagnetic and mechanical dynamics are explicitly represented in the model. Based on a simplified version of this model, an integrated optimal design method for simultaneously determining actuator and mechanism parameters of the FTS is developed with full consideration of various practical physical limitations, which is then systematically verified through the finite element analysis (FEA). Afterwards, the comprehensive model of the assembled FTS prototype is identified and experimentally verified with a two-step identification strategy. Finally, a dual-loop control scheme is employed to improve tracking accuracy while suppressing the hysteresis and lightly damped resonance. Both open-loop and closed-loop performance of the FTS prototype validate the feasibility and effectiveness of the proposed model and methods.

Tool path generation and optimization for freeform surface diamond turning based on an independently controlled fast tool servo

Diamond turning based on a fast tool servo (FTS) is widely used in freeform optics fabrication due to its high accuracy and machining efficiency. As a new trend, recently developed high-frequency and long-stroke FTS units are independently driven by a separate control system from the machine tool controller. However, the tool path generation strategy for the independently controlled FTS is far from complete. This study aims to establish methods for optimizing tool path for the independent control FTS to reduce form errors in a single step of machining. Different from the conventional integrated FTS control system, where control points are distributed in a spiral pattern, in this study, the tool path for the independent FTS controller is generated by the ring method and the mesh method, respectively. The machined surface profile is predicted by simulation and the parameters for the control point generation are optimized by minimizing the deviation between the predicted and the designed surfaces. To demonstrate the feasibility of the proposed tool path generation strategies, cutting tests of a two-dimensional sinewave and a micro-lens array were conducted and the results were compared. As a result, after tool path optimization, the peak-to-valley form error of the machined surface was reduced from 429 nm to 56 nm for the two-dimensional sinewave by using the ring method, and from 191 nm to 103 nm for the micro-lens array by using the mesh method, respectively.

A novel long-stroke fast tool servo system with counterbalance and its application to the ultra-precision machining of microstructured surfaces

This paper presents a novel long-stroke fast tool servo (FTS) system with counterbalance and its application to the diamond machining of microstructured surfaces. The FTS system is driven by a voice coil motor and guided by air bearings. A hybrid control algorithm which combines PID control, sliding mode control and feed-forward control was specifically designed for the system and ensures that it has less than 1% tracking error and achieves ± 1 mm stroke and 105 Hz bandwidth. The counterbalance is achieved by the symmetric arrangement of two of the same FTS systems. System vibration decreased significantly from ± 3 μm to ± 0.145 μm when the counterbalance was working. Machining experiments were conducted using the FTS system on a diamond turning machine. A microstructured surface simulation model was built and information about the workpiece surface tested and collected. When the counterbalance was working, error in the machine axes decreased from micron-level to nano-level and the surface roughness of the microstructured surface was also reduced. The system achieves ± 1 mm (±0.5 mm) stroke at 30 Hz (40 Hz) with 0.35% (0.68%) tracking error during machining processes, and the depth error of the machined microstructured surface was measured at 1.4% (2.3%).

High-performance control of fast tool servos with robust disturbance observer and modified [formula omitted] control

Fast tool servo (FTS) is an effective technique for the generation of complicated surfaces in modern precision manufacture industry. In this paper, a novel three-degree-of-freedom (3-DOF) control scheme is proposed for the FTS to achieve good tracking and disturbance rejection performance. Specifically, the 3-DOF control scheme contains a hysteresis compensation term, a modified H∞ feedback control term, a robust disturbance observer (RDOB) term and a feedforward control term. Firstly, a hysteresis compensator is developed based on the modified Prandtl–Ishlinskii (MPI) model to eliminate the intrinsic hysteresis nonlinearity so as to identify the linear dynamic model of the FTS system. A modified mixed-sensitivity-based H∞ feedback control is then proposed to simultaneously provide an excellent damping and tracking performance with a deliberately designed input weighting function. A RDOB is designed for non-minimum phase (NMP) system based on all-pass factorization of the linear dynamics model and the standard H∞ control framework to compensate uncertain external disturbances as well as unmodeled dynamics while guaranteeing the robust stability of closed-loop system. To remove phase delay and enlarge control bandwidth, a closed-loop-inversion feedforward controller is synthesized by integrating a zero-phase error tracking control (ZPETC) with a finite impulse response (FIR) filter. A series of comparative experiments are conducted on a self-developed flux-biased normal-stressed electromagnetic actuated FTS. The experimental results consistently validate the effectiveness and superiority of the proposed scheme in terms of outstanding tracking performance and disturbance rejection.

Machining control of non-axisymmetric aspheric surface based on piezoelectric fast tool servo system

Non-axisymmetric aspheric machining is increasingly important in photoelectric industry. The machining process requires an external feeding device mounted to a machine tool to achieve fast-tool servo (FTS) function. In this study, a piezoelectric (PZT, Lead zirconate titanate) FTS was used to perform non-axisymmetric aspheric machining by control methods. In the beginning, system identification of the PZT tool holder was conducted to obtain the mathematical model of the system and then the simulations were performed for various controllers, including the PID (Proportional-integral-derivative), sliding mode control (SMC), non-singular terminal sliding mode control (NTSMC), and adaptive NTSMC. The simulation results showed that the NTSMC and the adaptive NTSMC had excellent control performances with the tracking errors of 0.0 and 3.1 nm, respectively. Then, control practices without machining were performed with the PV (Peak-to-valley) tracking errors of 0.96 μm, 0.77 μm, and 0.95 μm obtained by the PID controller, NTSMC, and adaptive NTSMC, correspondingly. The large PV errors were found due to the low sampling rate of position commands. Finally, experiments of non-axisymmetric aspheric machining with the implemented controllers were performed for a workpiece of oxygen-free copper (OFC) having 5 mm diameter. The measured results of the machined surfaces were with the PV form errors of 7.2 μm, 2.8 μm, and 3.8 μm in corresponding to the usual machining, using NTSMC and adaptive NTSMC. The effectiveness of the implemented controllers was demonstrated, and the NTSMC showed the best performance. Future works are to examine the repeatability of various controllers with a large hardware memory size and improved environment.

Research on Compound PID Control Strategy Based on Input Feedforward and Dynamic Compensation Applied in Noncircular Turning.

The fast tool servo (FTS) control strategy is the control core of high-speed noncircular turning. This method should ensure high-speed and precision positioning and have the corresponding anti-interference ability in the micro-stroke motion with dynamic changes of tool feed and load. Most of the previous FTS control studies used the repetitive control and speed feedforward control strategy, which achieved promising results under ideal machining conditions. However, this strategy showed some defects in the real-world complex and changeable working conditions such as time-varying cutting force, intermittent cutting and fluctuating machine spindle speed. This paper proposed and implemented a compound proportional integral derivative control strategy based on input feedforward and dynamic compensation in noncircular turning. This technique successfully met the motion requirements of the high responsiveness of micro-stroke in noncircular turning and overcame disturbances from complex time variations of the cutting force, intermittent cutting case of the product and fluctuations of machine spindle speed. According to the findings, the machining tracking error was less than ±2 µm. Experimental results demonstrated the excellent tracking performance and machining effect of this control strategy.

Triaxial Fast Tool Servo Using Hybrid Electromagnetic–Piezoelectric Actuation for Diamond Turning

A high-performance triaxial fast tool servo (FTS) with the hybrid electromagnetic–piezoelectric actuation and the hybrid parallel–serial-kinematic structure is reported. Featuring the balanced and uniform actuation, in this article, a novel axis-symmetric linearized reluctance actuator is proposed to generate the planar motion in parallel, and the piezoactuated vertical motion is then serially carried by the planar motion within a limited space. Verified by the finite-element analysis, a two-stage design strategy is developed to optimally determine the multiphysical system parameters for the triaxial FTS, assisted by an analytical model of the electromagnetic circuit as well as the mechanical mechanism. As for the trajectory tracking, the loop-shaping tuned PID controller with a feedforward compensator is employed for each axis, and a damping controller is additionally designed for the planar motion. Finally, both open-loop and closed-loop performance of the prototype are carefully demonstrated.

Investigation of control algorithm for long-stroke fast tool servo system

Fast tool servo (FTS) is an efficient and reliable method in precision machining for fabricating freeform surfaces or microarrays with sub-micrometric form accuracy. In this paper, a Lorentz force FTS is designed where the voice coil motor is located inside the slide, and four air bearings are used as support components. Three different control algorithms, namely conventional PID control, advanced PID control with velocity/acceleration feed-forward (FF) and sliding mode control (SMC) are implemented in the system, and corresponding Simulink simulation models are built including for both mechanical and electrical systems. The results show that advanced PID and SMC can reduce phase error and overshoot, and tracking error can be controlled at 3.13% at 50 Hz. A new hybrid control algorithm (PID + SMC + FF) is developed, with system tracking error subsequently decreased to 0.871% at 50 Hz. In addition, with a suitable compensation method, the steady state tracking error is further decreased to 0.029%. Consistent results from testing with signals of different input frequency also indicate the general effectiveness of the algorithm.

Ultra-precision turning method efficient for optical freeform surfaces with a hybrid slow-fast tool servo.

The machining of freeform surfaces is a current research hotspot: A slow tool servo (STS) has limitations in machining accuracy and efficiency for large steep freeform surfaces. Most fast tool servo (FTS) tools are limited by their stroke and cannot manufacture freeform surfaces with a large sag. We propose a hybrid slow-fast tool servo method that combines STS and FTS to machine large steep freeform surfaces by decomposing the freeform surfaces and simultaneously turning efficiently with STS and FTS. Experimental studies were undertaken to fabricate a saddle surface. Meanwhile, a variable feedrates tool path was designed and applied to further improve the machining efficiency. The results show that this method can improve the processing efficiency by 47.5%. The arithmetic mean of surface roughness (Ra) is 2-4 nm, and the peak-to-valley (PV) value is 0.4780 µm at the hollows and 0.3884 µm at the swells.

Model Reference Adaptive Control of Cross-Coupling Hysteresis in Piezoceramics With Dynamic Loads

During the ultraprecise cutting of micro-structure surface with fast tool servo (FTS), the hysteresis of piezoelectric actuators (PEAs) are affected by dynamic voltage excitations and real-time cutting force, which declines the servo accuracy and cutting performance. In this paper, for a multi-input-single-output (MISO) cutting system, a cross-coupling rate-dependent Prandtl-Ishlinskii (CRPI) model is proposed and identified for the dynamic hysteresis of PEAs under dynamic voltage excitation and external loads. A model reference adaptive control method is then presented to eliminate the positioning nonlinearity of PEAs. The hysteresis modeling accuracy is discussed and the adaptive controller is validated through experiments.

Trajectory Tracking Control of a Fast Tool Servo System Driven by Maxwell Electromagnetic Force

Trajectory Tracking Control of a Fast Tool Servo System Driven by Maxwell Electromagnetic Force

Optimal design of high frequency and high precision fast tool servo system

Optimal design of high frequency and high precision fast tool servo system

Feasibility study of single-crystal silicon ductile-regime turning via fast tool servo

Due to the material properties of single-crystal silicon, it is difficult to achieve high-efficiency processing in conventional turning. Fast tool servo (FTS) turning has emerged as a favorable technology for processing brittle materials. This paper investigates the tool radius, feed speed, and cutting depth effects on the maximum undeformed chip thickness (MUCT). A high-frequency low-coupling FTS system is designed for processing experiments. Effects of the tool rake angle, tool radius, cutting speed, and vibration frequency on the critical undeformed chip thickness (CUCT) are explored in this paper. Experimental results indicate FTS technology can increase the CUCT from 80 nm to 580 nm. Surface and subsurface morphology tests of conventional and FTS single-crystal silicon cutting verify that FTS technology can obtain an optical surface roughness of 5 nm and lower subsurface damage. Moreover, FTS turning can reduce tool wear. Lastly, through Raman spectroscopy analysis, the amorphous phase remaining on the machined surface is lower when FTS turning is employed.

Fabrication of Sinusoidal Microstructures on Curved Copper Surface by Ultra-Precision Diamond Cutting with a Rotary B-Axis and Fast Tool Servo System

While curved surface microstructures have wide applications in optical components and devices, how to achieve high machining accuracy of the microstructures is crucial for their applications. In the present work, we fabricate sinusoidal modulation microstructures on a curved copper surface by ultra-precision diamond cutting, with the combination of a rotary B-axis and a fast tool servo system. Specifically, tool path planning, together with the consideration of a curved, sinusoidal surface meshing and tool tip arc segmentation compensation, is carried out. Preliminary cutting experiments are firstly carried out on a homemade four-axis ultra-precision lathe, which demonstrates the advantages of additionally applying the rotary B-axis in suppressing burr formations and over-cutting phenomenon over the sole utilization of the fast tool servo system. Subsequent experiments are carried out to evaluate the effects of feed rate and the number of sampling points on the machining accuracy of the microstructures under the combination of a rotary B-axis and a fast tool servo system. With the optimized machining parameters, sinusoidal modulation microstructures, which have a wavelength of 700.6 μm, a peak-to-valley of 18.7 μm, a surface roughness of 18.9 nm and a deviation of profile tolerance of 4.326 μm, are successfully fabricated on a curved copper surface with a face radius of 10 mm and a curvature radius of 500 mm.

Comprehensive Design Method of a High-Frequency-Response Fast Tool Servo System Based on a Full-Frequency Error Control Algorithm.

With the development of optoelectronic information technology, high-performance optical systems require an increasingly higher surface accuracy of optical mirrors. The fast tool servo (FTS) based on the piezoelectric actuator is widely used in the compensation machining of high-precision optical mirrors. However, with the low natural frequency of mechanical structures, hysteresis of the piezoelectric actuators, and phase delay of the control systems, conventional FTS systems face problems such as a low working frequency and a large tracking error. This study presents a method for the design of a high-performance FTS system. First, a flexure hinge servo turret with a high natural frequency was designed through multi-objective optimization and finite element simulations. Subsequently, a composite control algorithm was proposed, targeting the problems of hysteresis and phase delay. The modified Prandtl–Ishlinskii inverse hysteresis model was used to overcome the hysteresis effect and a zero-phase error tracker was designed to reduce the phase error. The experimental results reveal that the tracking error of the designed FTS system was <10% in the full frequency range (0–1000 Hz).

Fast tool machining and hot embossing for the manufacture of diffractive structured surfaces

Nowadays, highly functional optical elements are part of day-to-day use by most of us, whether in form of camera systems in mobile phones or as security features on banknotes and identification documents. The conventional production of diffractive optical elements is mainly covered by lithographic processes. Nevertheless, due to long production times, this process is very expensive when it comes to mass production and the structuring of large areas. For this reason, a novel manufacturing process for structured moulds was developed by the Leibniz Institute for Materials Engineering (IWT). This involves a diamond turning process in combination with a high frequency nano Fast Tool Servo (nFTS) to generate the negative diffractive structure on a metal surface. Afterwards, the structured surface can be replicated in large numbers in a hot embossing process and thus increase market share. In recent work, this process has been investigated in cooperation with Karlsruhe Institute of Technology. The surfaces reproduced in this process meet the highest requirements in terms of precision and shape accuracy of the structure elements. Moreover, the optical functionality of such structures applied as security holograms was successfully demonstrated.