Comparison of micro-dispensing performance between micro-valve and piezoelectric printhead

In micro-dispensing applications, printhead activation mechanism, its design and operating parameters are integrated together to affect the droplet generation process. These factors give each printhead advantages and limitations over the others in specific fabrication. Hence, multiple printheads on micro level fabrication are preferred to perform multi-material dispensing task. In this paper, the mechanisms of two commonly used micro printheads, solenoid actuated micro-valve and piezoelectric printhead are discussed. Comprehensive experiments are conducted to characterize their performance and the results are analyzed so as to explore optimal droplet formation condition. With regards to the operational parameters’ influence on droplet formation, micro-valve is investigated in terms of pressure, and operational on time, and piezoelectric printhead is investigated based on pulse amplitude, and width of driving pulse. Nozzle size, a key design parameter in the two printheads, is also studied according to its influence on dispensing capability. To facilitate dispenser selection, the two printheads are compared based on droplet size, droplet stability, droplet velocity, and dispensing viscosity of successful ejection. Other factors such as chemical compatibility, time consumption in determining optimal condition and reliability of dispensing process are also reported to play an essential role in this selection. Our investigation on the relationship between related parameters and dispensing performance will not only benefit dispenser selection in multi-material dispensing application, but also build a solid background to develop multiple printhead system for fabrication of bioengineering components.     

Effects of trapped air bubbles on frequency responses of the piezo-driven inkjet printheads and visualization of the bubbles using synchrotron X-ray

The reliability of drop formation is one of the key factors for the successful commercialization of inkjet printing applications. However, when the air bubble is entrapped from the nozzle exit, it leads to stop jetting of the droplets immediately. It has been known that the trapped air bubbles inside the chamber prevent a printhead from stable jetting. In this study the synchrotron X-ray has been used to visualize the air bubbles in the flow field inside of the printhead undergoing the standard jetting with the firing frequency of 1–20 kHz. An air pocket of bubble was formed repeatedly and reproduced well through the tested printheads at a certain jetting condition. To see the effects of the bubbles on the dynamic characteristics of the piezoelectric printheads, the piezoelectric velocity on the top of the pressure chamber was measured with a laser vibrometer. When the bubble was trapped at the nozzle exit orifice, the piezo velocity signals showed significantly different frequency peaks appearing in the spectrum and the high frequency components were identified with the frequency response measurements.     

Investigation to minimize heater burnout in thermal thin film print heads

A test system to improve the reliability of a printhead was developed and various printheads was tested. We developed a thermally driven monolithic inkjet printhead comprising dome-shaped ink chambers, thin film nozzle guides and omega-shaped heaters integrated on the top surface of each chamber. In a durability test of an inkjet printhead, the test system automatically detects a heater failure using a Wheatstone bridge circuit. Various models were designed and tested to develop a more reliable printhead. Three design parameters of the thickness of heat-transmission layer, reinforcement layer, and insulation layer were investigated in the test. Specially, the reinforcement layer was introduced to improve the lifetime of printhead. The thicker the heat-transmission layer as well as the insulation layer, the longer the life of the printhead. The lifetime of a heater with a reinforcement layer was longer than the lifetime of a heater without a reinforcement layer.     

A restructured artificial bee colony optimizer combining life-cycle,local search and crossover operations for droplet property prediction in printable electronics fabrication

For printable electronics fabrication, a major challenge is the print resolution and accuracy delivered by a drop-on-demand piezoelectric inkjet printhead. In order to meet the challenging requirements of printable electronics fabrication, this paper proposes a novel restructured artificial bee colony optimizer called HABC for optimal prediction of the droplet volume and velocity. The main idea of HABC is to develop an adaptive and cooperative scheme by combining life-cycle, Powell’s search and crossover-based social learning strategies for complex optimizations. HABC is a more biologically-realistic model that the reproduce and die dynamically throughout the foraging process and the population size varies as the algorithm runs. With the crossover operator, the information exchange ability of the bees can be enhanced in the early exploration phase while the Powell’s search enables the bees deeply exploit around the promising area, which provides an appropriate balance between exploration and exploitation. The proposed algorithm is benchmarked against other four state-of-the-art bio-inspired algorithms using both classical and CEC2005 test function suites. Then HABC is applied to predict the printing quality using nano-silver ink. Statistical analysis of all these tests highlights the significant performance improvement due to the beneficial combination and shows that the proposed HABC outperforms the reference algorithms.     

Experimental-based feedforward control for a DoD inkjet printhead

Markets demand continuously for higher quality, higher speed, and more energy-efficient professional printers. Drop-on-Demand (DoD) inkjet printing is considered as one of the most promising printing technologies. It offers many advantages including high speed, quiet operation, and compatibility with a variety of printing media. Nowadays, it has been used as low-cost and efficient manufacturing technology in a wide variety of markets. Although the performance requirements, which are imposed by the current applications, are tight, the future performance requirements are expected to be even more challenging. These print requirements are related to the jetted drop properties, namely, drop velocity, drop volume, drop velocity consistency, productivity, and reliability. Meeting these performance requirements is restricted by several operational issues that are associated with the design and the operation of inkjet printheads. Major issues that are usually encountered are residual vibrations and crosstalk among ink channels. These result in a poor printing quality for high-speed printing. The main objective is to design a feedforward control strategy such that variations in the velocity and volume of the jetted drops are minimized. In this article, an experimental-based feedforward control scheme is proposed to improve the performance of a professional inkjet printer.     

Fabrication of bio-inspired composite coatings for titanium implants using the micro-dispensing technique

Bio-inspired composite coating, which resembles the unique nano-structured composite bone tissue, is greatly needed in biomedical implant applications. The target of this coating is to create a continuous transition from tissue to implant surface, and modulate the biological response between the implant and host tissue. Our study on this bio-inspired composite coating is to fabricate organic–inorganic composite coatings (OICCs) and functionally graded coatings (FGCs). In this paper, a few commonly used coating methods have been evaluated on their capabilities on OICCs and FGCs fabrication. Compared with other available methods, the drop-on-demand (DoD) micro-dispensing technique enables us not only to flexibly fabricate multi-material coating at micron scale, but also to reliably construct multi-layer structures with varied material property distribution within a layer and/or among layers. This DoD micro-dispensing technique has been characterized based on three type of biomaterials (hydroxyapatite, titanium oxide and collagen) and dispensing parameters. The micro-fabrication process has been further investigated with regards to its capabilities on OICCs and FGCs. The fabricated samples on titanium substrate are characterized in terms of material distribution, adhesion, layer thickness and uniformity. The results show that the DoD micro-dispensing technique is capable to fabricate OICC and FGC samples in a single process. A comprehensive study on fabrication process and biological test will be conducted in the next stage.     

Stochastic optimization of ferroelectric ceramics for piezoelectric applications

An optimization model and numerical framework is developed to identify the optimal microstructure of ferroelectric (FE) materials. Piezoelectricity in polycrystalline ceramic FEs differs significantly from that of single crystals because of the presence of crystallites (grains) possessing crystallographic axes aligned imperfectly. The polarization and for that matter the piezoelectric properties of FEs are inextricably related to the orientation of individual crystallographic grains that constitute the polycrystal. The orientation of the grains fall in a Gaussian distribution after the poling process. The orientation distribution parameters, (viz., the standard deviation σ and mean μ) hence, would play the crucial role in the effective piezoelectric properties of FE polycrystals. We have applied a stochastic optimization combined with homogenization to determine the optimal distribution parameters which dictates the orientation distribution (texture) of an ideal ferroelectric polycrystal. This procedure would be used to find out the optimum piezoelectric properties characterised by the piezoelectric coefficients e_ijke_{\it ijk}. The method is applied with an objective to maximize the piezoelectricity of ferroelectric BaTiO₃ which in turn maximizes the electromechanical coupling. Convergence studies are made in order to arrive at a meaningful representative volume element for the computation. Focussing on the polar component of piezoelectric coefficient, e ₃₃, the optimization course is perfected. Apparent enhancement of piezoelectric coefficient e_ijke_{\it ijk} is observed in an optimally oriented BaTiO₃ single crystal. In polycrystalline ceramics, optimal grain configurations that shows better piezoelectric performance than polar ferroelectrics are derived. Our optimization model provides designs for materials with better piezoelectric performance, which would stimulate further studies involving materials possessing higher spontaneous polarization.     

A neurofuzzy-based quality-control system for fine pitch stencil printing process in surface mount assembly

Surface mount assembly defect problems can cause significant production-time losses. About 60% of surface mount assembly defects can be attributed to the solder paste stencil printing process. This paper proposes a neurofuzzy-based quality-control system for the fine pitch stencil printing process. The neurofuzzy approach is used to model the nonlinear behavior of the stencil printing process. Eight control variables are defined for process planning and control, including stencil thickness, component pitch, aperture area, snap-off height, squeegee speed, squeegee pressure, solder paste viscosity, and solder paste particle size. The response variables are the volume and height of solder paste deposited. The values of the response variables provide indicators for identifying potential quality problems. A 3^8–3 fractional factorial experimental design is conducted to collect structured data to augment those collected from the production line for neurofuzzy learning and modeling. Visual basic programming language is then used for both rule retrieval and graphical-user-interface modeling. The effectiveness of the proposed system is illustrated through a real-world application.     

Fabrication of chamber for piezo inkjet printhead by SU-8 photolithography technology and bonding method

The chamber is an important part of the inkjet printhead. However, the present fabrication methods of chamber suffer from a low alignment resolution between nozzle plates and piezoelectric structure and residual SU-8 removing problems during chamber fabricating process. In this paper, a SU-8 chamber was fabricated by using ultraviolet (UV) photolithography and SU-8 thermal bonding method. By this method, the infilling problem of the chamber during thermal bonding process was solved, and low alignment resolution problem of conventional UV exposure system during assembly process was avoided. The thickness of the SU-8 nozzle plate was optimized, and the influence of bonding parameters on the deformation of chamber was analyzed. The simulation results show that the optimal thickness of the SU-8 nozzle plate is 40 μm and the optimal bonding parameters are bonding temperature of 50 °C, bonding pressure of 160 kPa and bonding time of 6 min. The tensile test results show the bonding strength of the SU-8 chamber is 2.1 MPa by using the optimized bonding parameter.     

High throughput micro droplet generator array controlled by two-dimensional dynamic virtual walls

A chamber-free two-dimensional-array micro droplet generator has been realized by precise time-delayed control of micro bubble arrays as virtual chamber walls. Droplets can be ejected out by the bubbles around the ejection site in specific configuration of excitation, thus replacing physical chamber walls for pressure preservation. The micro droplet generator array was fabricated by heater lithography and direct nozzle formation on a laminated SU-8 dry film without any solid chamber wall among heaters. The nozzle density of this compact droplet generator can be five to ten times higher than that of commercial inkjet printheads in one-dimensional formats. The volume and initial speed of the generated droplets was 3.6–5.7 pL and 14–15 m/s, respectively, meeting the standard of commercial printheads. The micro droplet generator is free of satellite droplets due to the precise meniscus control. The analyzed data shows the meniscus undergoes a “push–pull–push” progress which effectively cuts the liquid column short. The refilling time of the innovative micro droplet generator was estimated to be 0.296 μs from the simplified chamber model, and it was one-tenth of the commercial printheads. In addition, the frequency response was estimated to be higher than 20 kHz by observing the meniscus fluctuation condition. Finally, a 3 × 5 heater array was used to generate two droplets simultaneously, which shows that the crosstalk problem can be eliminated by precise time-delayed control. An interlacing operation was also proposed to address the large array control algorithm. To summarize, a 330-dpi monolithic micro droplet generator prototype has been proposed for high speed and large 2D format printing.     

LED packaging by ink‐jet microdeposition of high‐viscosity resin and phosphor dispersion

Abstract— An ink‐jet‐printing method applied to the microdeposition of high‐viscosity resin, including optimization of phosphor dispersion for light‐emitting‐diode (LED) packaging was examined for the first time. An ultrasonic ink‐jet‐printing method was used, in which ink droplets are ejected by a focused ultrasonic beam from a nozzle‐less printhead. To fabricate white LEDs, high‐viscosity phosphor‐dispersed resin was deposited to form an encapsulant dome. Two types of methods to control phosphor sedimentation for color uniformity were examined; one is heating the lead frame during the resin deposition, and the other is hydrophobic surface treatment of the lead frame base enabling the fabrication of a small encapsulant dome. For light direction control, a silicone micro lens was deposited on an encapsulant dome using the ink‐jet method. The results show that ultrasonic ink‐jet printing is an applicable technique to optimize and modify on‐demand optical characteristics of LED devices.     

High-resolution long-array thermal ink jet printhead fabricated by anisotropic wet etching and deep Si RIE

This paper describes the fabrication and characterization of a thermal ink jet (TIJ) printhead suitable for high speed and high-quality printing. The printhead has been fabricated by dicing the bonded wafer, which consists of a bubble generating heater plate and a Si channel plate. The Si channel plate consists of an ink chamber and an ink inlet formed by KOH etching, and a nozzle formed by inductively couple plasma reactive ion etching (ICP RIE). The nozzle formed by RIE has squeezed structures, which contribute to high-energy efficiency of drop ejector and, therefore, successful ejection of small ink drop. The nozzle also has a dome-like structure called channel pit, which contributes to high jetting frequency and high-energy efficiency. These two wafers are directly bonded using electrostatic bonding of full-cured polyimide to Si. The adhesive-less bonding provided an ideal shaped small nozzle orifice. Use of the same material (Si substrate) in heater plate and channel plate enables the fabrication of high precision long printhead because no displacement and delamination occur, which are caused by the difference in thermal expansion coefficient between the plates. With these technologies, we have fabricated a 1" long printhead with 832 nozzles having 800 dots per inch (dpi) resolution and a 4 pl. ink drop volume.     

Wind turbine integrated structural and LPV control design for improved closed-loop performance

An iterative redesign algorithm is proposed to integrate the design of the structural parameters and a linear parameter-varying (LPV) controller for a three-bladed horizontal-axis wind turbine. The LPV controller is designed for an eighth-order lumped model of the wind turbine consisting of blades, drive-train and the tower. The lumped model response is matched with detailed open-loop numerical simulations using the Fatigue, Aerodynamics, Structures and Turbulence (FAST) code. The controller is scheduled in real-time based on the mean wind speed to account for the varying system dynamics. The objective is to track the operating trajectory meanwhile minimise the H _∞ performance index from the wind turbulence to the controlled output vector consisting of pitch angle, blade tip deflection, and the generator speed and torque. Sensitivity analysis of the closed-loop performance index with respect to the structural parameters of the system is examined. The integrated design problem is formulated as an iterative sequential controller/structure redesign to obtain the structural parameters and controller matrices corresponding to a local optimal performance index. Each step of the iterative procedure is formulated as a linear matrix inequality (LMI) optimisation problem that can be solved efficiently using available LMI solvers. The evolution of the structural parameters and performance index through the integrated design is illustrated. The FAST closed-loop simulations for two selected designs with the smallest values of the performance index demonstrate the improved performance of the overall system through the integrated structure/control redesign in both minimising the effect of the wind disturbance on the generator output power, and reducing the structural loads on the wind turbine.     

E-beam lithography of nano-interdigital transducers on insulating and semiconducting substrates

This work describes the micro-fabrication process developed to manufacture nano-interdigital transducers (nano-IDTs) to be used in surface acoustic wave applications. The combination of electron-beam (e-beam) lithography and lift-off process is shown to be effective in fabricating IDT finger patterns with a line width below 100 nm and good yield. It is also shown how a very thin organic anti-static layer can be used to avoid charge accumulation on the resist layer during e-beam lithography, which is easy to occur on insulating piezoelectric substrates and results in e-beam deflection. However, it is also shown how the use of this anti-static layer is not required with the insulating piezoelectric layer resting on a semiconducting substrate such as highly doped silicon. The effect of the e-beam dose on insulating and semiconducting layers is also discussed.     

Robust Speaker Verification with Principal Pitch Components

We are presenting a new method that improves the accuracy of text dependent speaker verification systems. The new method exploits a set of novel speech features derived from a principal component analysis of pitch synchronous voiced speech segments. We use the term principal pitch components (PPCs) or optimal pitch bases (OPBs) to denote the new feature set. Utterance distances computed from these new PPC features are only loosely correlated with utterance distances computed from cepstral features. A distance measure that combines both cepstral and PPC features provides a discriminative power that cannot be achieved with cepstral features alone. By augmenting the feature space of a cepstral baseline system with PPC features we achieve a significant reduction of the equal error probability of incorrect customer rejection versus incorrect impostor acceptance. The proposed method delivers robust performance in various noise conditions.     

Manufacture of microfluidic chips using a gap-control method based on traditional 3D printing technique

The solution to the commercialization of polymer microfluidic chips lies in the development of a low-cost and concise method. We present in this paper a gap-control method for obtaining low cost microfluidic chips on PMMA (polymethyl methacrylate) sheets based on traditional 3D printing technique—fused deposition modeling. The influence of 3D printing parameters such as printing temperature, printing speed, wire flow rate and initial layer thickness on printing quality is studied by experiments. The effect of O₂ plasma parameters such as chamber power and treatment time on the adhesion strength between printed PLA (polylactic acid) structures and PMMA substrate is investigated. The dye filling tests demonstrate that there is no blocking or leakage over the entire microchannels. With this newly developed technology, low-cost and large scale microfluidic chips can be fabricated, which allows commercial manufacturing of microchannels over large areas.

     

Design of adaptive robust guaranteed cost controller for wind power generator

According to the increasing requirement of the wind energy utilization and the dynamic stability in the variable speed variable pitch wind power generation system, a linear parameter varying (LPV) system model is established and a new adaptive robust guaranteed cost controller (AGCC) is proposed in this paper. First, the uncertain parameters of the system are estimated by using the adaptive method, then the estimated uncertain parameters and robust guaranteed cost control method are used to design a state feedback controller. The controller’s feedback gain is obtained by solving a set of linear matrix inequality (LMI) constraints, such that the controller can meet a quadratic performance evaluation criterion. The simulation results show that we can realize the goal of maximum wind energy capture in low wind speed by the optimal torque control and constant power control in high wind speed by variable pitch control with good dynamic characteristics, robustness and the ability of suppressing disturbance.     

线段二维裁剪与绘制的算法性能分析

分析与讨论了几种线段二维裁减和绘制算法,为了提高图形绘制的精确度和计算速度,通过实验在Windows平台上利用C语言和DirectDraw接口实现了这些算法。针对不同的实验参数,对各个算法性能作出了比较。分析结果显示：在实现过程中可以对直线裁剪的累计误差处理,对整数除法的四舍五入处理等细节改良,从而使得绘制的图形更加精确,付出的代价非常小。实验结果表明,采用最优树裁剪算法对线段进行二维裁剪并利用步距长度片算法进行绘制,能够高效完成显示任务,结果具有较好的参考价值。     

Effects of line length,number of lines,line spacing,and font size on reading performance of Chinese text in virtual reality environment

The increasing importance of virtual reality (VR) necessitates reading in VR environments. Nonetheless, there is a lack of research on the factors influencing reading performance (speed and accuracy) in VR. The purpose of this study is to investigate the influence of line length, number of lines, line spacing, and font size on the response time and accuracy of a Chinese comprehension task in VR. Thirty-two participants were required to read Chinese passages and answer 32 multiple-choice questions in VR. The results indicated that line spacing significantly affected reading speed. Meanwhile, the interaction between the number of lines and line spacing had a significant impact on reading accuracy. To improve reading speed and accuracy, it is advisable to use single-line spacing, 20 characters per line, and ensure that the vertical distance created by the number of lines and line spacing does not exceed the optimal field of view.