Precision Assembly Technologies for Mini and Micro Products

This paper describes the study of a gripper using the surface tension effects to pick and place the 0.5mm or 0.3mm diameter balls of a millimetric watch bearing. Two liquid supply strategies have been tested (pressure drive and tip dip). The effects of the coating (through the measurement of the contact angles), the presence of an internal channel and the size of the gripper have been studied. Analytical and numerical force models have been developped and validated thanks to a test bed allowing the measurement of the developped force (typically of the order of 100 µN) with a resolution of 1 µN. A complete pick and place cycle has been performed using the 0.5mm diameter gripper. Such a test has still to be done in the future with the 0.3mm diameter gripper.

Optical micro parts, such as glass fibres, require handling and alignment accuracies down to the sub micrometer range. Addressing this task, one aim of the Fraunhofer IPT is the development of new concepts of active gripper systems. In this context a highly integrated, adaptive, rugged and economical gripper system particularly for accurate handling and alignment of flexible micro parts down to the sub-micron level has been developed. This gripper system can be used on conventional robot systems for carrying out micro-assembly operations. The robot system does the pre-positioning, the tolerances necessary for the micro-assembly are subsequently realized directly at the tip of the gripper with the gripper integrated multi-axes system. Positioning systems that achieve the required positioning increments in the sub micron range are already existent. However, the problem of such systems is that they are normally highly sensitive against mechanical impact and extremely cost intensive. In this paper the development of a highly robust gripper-integrable axes system and its integration in a novel gripper design with a multi-axis adjustment system is presented.

This paper presents the design, fabrication and characterization of a device able to exchange the tip part (so-called the tools) of a two fingered microgripper. The principle of this tool changer is based on the use of a thermal glue whose state (liquid or solid) is changed by heating or cooling. Several kinds of pairs of tools have been designed. The suitable pair of tools can be chosen according to the size, shape and material of the object to manipulate. The tool changer enables one to perform a sequence of elementary micromanipulation tasks (i.e. an assembly sequence) by using only one gripper mounted on only one manipulator. The tool changer has been automated and successfully tested in air and in the vacuum chamber of a Scanning Electron Microscope (SEM). It brings flexibility to the micromanipulation cell and contributes to reduce the costs, the used space and experimentations time for micro-manipulations in the SEM. The assembly of a ball bearing (the balls are 200 µm. in diameter) has been successfully tested using the microgripper equipped with the tool changer in a SEM. This tool changer has been designed for a microgripper but can be easily adapted to lots of other kinds of systems.

The paper describes the modeling and experimental investigation of the application issues of electrostatic based micro-gripping. The design of an electrostatic gripping system for both grip force measure and pickup and place experiment is presented. A finite element model is made to study the gripping environment and process related features. The design and validation of the model are provided. Investigation of the influences that the gripping process gives out to and may receive from the operating environment is discussed. The preliminary result shows that grounded objects that come into the gripping area do not influence the gripping action significantly. Some real gripping actions are performed. Discussions on the scope of gripping environment and process related features are raised on basis of gripping experiments and observations. The phenomena of charging and discharging on the gripper isolator may add difficulties to gripping control. The investigation is concluded in a form of Process Data Sheet. The research draws an insight view on the application criteria of electrostatic gripping technology. Both advantages of the gripping principle

e.g

. flexible in terms of part dimension and geometry, and restrictions of the application are illustrated.

The paper propose a new approach in order to design a generic microparts feeder. The method based on a classification scheme allows to emerge the principal characteristics of each studies solutions. The different criteria take into account the specifities of the micro world and moreover the main characteristics for the feeding functions. Thus, we analyse three systems and confront them to find the generic and flexibility aspects.

The need of fully automated microassembly systems place specific functional requirements on the design and fabrication of critical elements such as grippers, sensors, manipulators and feeders. A new microfeeder design is proposed based on contactless pneumatic distributed manipulation. By cooperation of dynamically programmable microactuators, a number of feeding functions and even some elementary assembly operations can be achieved.

The paper describes the development of a micro-parallel-SCARA robot adapted in size to MEMS products. The degree of miniaturization is optimized concerning a smaller structure but high accuracy in a workspace dimensioned to chip card size. The robot supports the mostly used four degrees of freedom with a base area of less than 150 × 150 mm

2

. It is the result of a cooperative project between the Institute of Machine Tools and Production Technology at the Technical University of Braunschweig and Micromotion GmbH. This company is an innovative manufacturer of miniaturized zero-backlash gears and actors, which are used as main drives of the robot.

In the future the assembled parts become smaller and the precision demands increase in many cases. Grippers are one factor of the solution to make a competitive automation assembly system. Grippers have to be flexible and also capable of holding parts without dropping them during accelerations. Other demands for a gripper for mini and micro assembly applications are, among other things, accuracy and good force/size ratio. By using servo grippers also in high-precision applications for miniature-size parts, a noticeably amount of time will be saved. This paper presents two different kinds of developed two-fingered servo grippers with parallel moving mechanics, as well as the developed test systems for servo grippers and some test results.

In this paper the development of a macro parallel robot is presented in which conventional bearings are replaced by pseudo-elastic flexure hinges. The robot consists of a spatial parallel structure with three translational degrees of freedom and is driven by three linear direct drives. The structure has been optimized with respect to workspace and transmission ratio. Additionally, in simulations with the FEA tool ANSYS different geometrical arrangements and combinations of flexure hinges have been investigated with respect to the dynamic behavior of the compliant mechanism. Due to the symmetrical character of the structure and the optimized design of the combined flexure hinges the structure is very stiff. The experimental measured repeatability of the compliant robot is below 0.3 µrn.

This paper presents a test environment enabling the study of factors affecting on the success of a robotic precision assembly work cycle. The developed testing environment measures forces and torques occurring during the assembly, and uses a system based on machine vision to measure the repeatability of work piece positioning. The testing environment is capable of producing exactly known artificial positioning errors in four degrees-of-freedom to simulate errors in work-piece positioning accuracy. The testing environment also measures the total duration of the robot work cycle as well as the durations of all essential phases of the work cycle. The testing environment is best suited for light assembly operations and has measurement ranges of ±36 N and ±0.5 Nm and the vision system has a field-of-view mm.

The latter part of this paper presents the results of the research done in order to find out how some selected factors affect the assembly forces of robotic assembly. These factors include work piece and process parameters such as work piece material and design (chamfered/straight), positioning tolerances, and robot insertion motion speed.

To recognize geometric objects of components in assembly processes, in particular of microelectronic or micro system components, nowadays the use of vision systems is preferred. These systems are working very fast but the results depend on ambient conditions, especially of light settings. They deliver 2D object information referring to the image plane of a camera. In most sensor guided assembly systems, additional to vision systems laser displacement sensors are implemented to get information about the third dimension, the components height. In this paper a scanning method is presented which enables object recognition by using a laser positioning sensor and to use the achieved measuring values for a sensor guided assembly process.

The trend to miniaturization cannot be overseen. The use of very small electronic and electro-optical components in a variety of consumer and investment goods is leading to an increasing demand for small-scale servo actuators for micro assembly applications in production equipment. The previous generation of micro gears and micro actuators was not suited to this type of application, because of unacceptable accuracy.

The Micro Harmonic Drive® gear was introduced into the market in 2001 as the world’s smallest backlash-free micro gear. It is manufactured using a modified LIGA process, called Direct-LIG. This allows the cost-effective production of extremely precise metallic gear components. In the meantime this gear has been implemented in a range of miniaturized servo actuators, which provide zero backlash, excellent repeatability and long operating life.

In addition to the above-mentioned advantages this innovative product also features a central hollow shaft. This allows the design engineer to pass an optical fiber, a laser beam or media such as fluids, compressed air or vacuum along the central axis of the servo actuator. This greatly simplifies the design of machines for micro assembly applications in the semi conductor, consumer goods, medical and optical fields. In this paper we will describe the development history, key features and applications of this innovative drive solution.

Because of intensification of the international market situation the industry is forced to extend the product range and to shorten the period of time between the model changes in order to put new products on the market. Standards that allow generating product and resource variants by creating reusable structures (templates) have to be worked out to support the simultaneous engineering process. The challenge is the parallelisation of the product development and the production planning as far as possible to generate a robust production. In process planning as a part of assembly planning, the assembly processes and their sequence have to be defined and the motion path and speed of an assembly process can be visualized and optimized by Digital Mock-Up (DMU). Realising an early connection between product design and assembly planning new strategies have to be developed.

This paper presents a morphological classification of approaches to the assembly of hybrid microsystems or hybrid MEMS. The need for the presented classification comes from the fact that at the moment only limited well structured knowledge is available on how to assemble hybrid MEMS. The classification is based and evaluated on the basis of cases found in literature. The scheme is used in a case study to analyze the assembly process of die encapsulation.

This paper discusses the problems of micro-scale part manipulation and assembly. Parts less than 10×10×10 cm3 in size need special attention when they are designed. There is a need for handling tools, feeding systems, and fastening methods that resemble macro-scale industrial systems but are of a special nature. No effort has been made prior to this presentation to collect the data in any systematic form for designers. The Finnish

DFA-Tool

software version 3.0 is the first attempt to try to formulate this problematic in a systematic and suitable presentation for product designers.

This paper presents the tolerance budgeting for the coarse assembly step in a novel coarse-fine strategy for micro-assembly. The final assembly step is to be performed using MEMS-based functionality that remains part of the product. Total tolerance build-up due to the coarse assembly and dimensional inaccuracies has been determined for the alignment of a single mode optical fibre to a laser diode, a challenging problem in optical telecommunication.

Producing an automated assembly and test machine is a lengthy process consisting of numerous stages. It can involve several iterations through some or, very occasionally, all of these stages. There is one major similarity linking these stages; the requirement for consistent and accurate information. This unifying requirement makes communication a crucial, but often overlooked, aspect of producing an assembly automation machine.

It is often the case that the simplest and most reliable method of communication is a visual representation or “Visualisation”. Visualisation can and does take many forms; anything from sketches on napkins to detailed 3D computer images. All formats have benefits as well as drawbacks.

This paper looks at the role of visualisation and its importance within the production of an automated assembly and test machine. It also further develops the issues and problems faced by the assembly automation industry as well as looking in greater detail at the potential solutions and how these are being developed and expanded on by European research projects.

Conventional machining methods have been developed to meet the standards of ultra precision machining. Special milling processes utilizing monocrystalline diamond tools, the so-called fly-cutting processes, are used successfully to manufacture highly precise microstructures with an optical surface finish. In micro assembly often positioning accuracies of only a few micro meters are needed. An approach of the Fraunhofer IPT is to achieve these accuracies using passive alignment strategies. In this paper, the ultra precision machining of the v-groove structures as well as their passive alignment capacities for micro assembly tasks are presented.

In this paper we present a new concept of miniaturized production line for the assembly of small components. The concept is based on miniature robotic cells that work together. A miniature parallel robot has been developed for this purpose and is presented in this paper.

Miniaturization and mechatronics are important technological trends of the last years and will be gaining importance in the future. The micro-mechatronical products which have already been put on the market are currently designed to meet the abilities of existing production techniques and are manufactured as mass products in specialized and capital intensive plants. In order to make miniaturized mechatronical systems more attractive for the market, their production must be made possible in small and medium volumes at acceptable costs. A holistic approach for the conception of a scalable micro production which consists of the development of an integrated planning process as well as the creation of a flexible production platform with general process modules is presented and demonstrated considering as example the realization of a scalable production for a micro-mechatronical actuator. The approach is characterized by a high level of flexibility considering joining technologies as well as small space requirements. A reduction of capital costs for production technology and a step wise extension of the production are achieved.

In today’s increasingly volatile and dynamic global markets it is increasingly important to react to changing market demands and reduce the time-to-market. The design and re-design of assembly systems has a significant impact on the product development time. This paper reports on the effort that has been put into developing an assembly process decomposition and modular assembly equipment configuration methodology that takes advantage of the current trend towards modular equipment solutions and is expected to reduce design time and improve the design process integration. A general framework for the proposed methodology has been outlined and an ontology for the design of modular assembly systems is being discussed.

The Eupass project (www.eupass.org) proposes to develop assembly systems, especially in the field of ultra precision assembly, with evolvable skills to fit a wide range of products being assembled by applying a “Zero-Engineering” approach for the switch-over from one product to another. The Eupass partners are currently developing a machine line network with new and additional machine and device discovery mechanisms, synchronization and data exchange capabilities. By means of a common behavioral model, machines and devices connected to a network can jointly be discovered, commanded and execute any programmed activity in a collaborative way — developing evolvable skills exceeding the skill set of the singular machines.

The microfactory paradigm means the miniaturisation of manufacturing systems according to the miniaturisation of products. Some benefits are the saving of material, energy and place. A vision based solution to the problem of supervision of microfactories is proposed. It consists in synthetising a high resolution global view of the work field and real time inlay of local image in this background. The result can be used for micromanipulation monitoring, assistance to the operator, alarms and others useful informations displaying.

The paper is dealing with the development of a new class of multipurpose

multi-degrees-o-freedom

(MDOF)

Positioning Systems

(PS) to be used as

precision devices

in a cell manufacturing desktop environment. Based on a structural investigation and systematization with imposed criteria, PM with 2dof actuators on the base was proposed. In its first modular design concept, each of the members

-Positioning units

(Pu), having at least 2DOF motion capabilities over a large workspaces, are symmetric parallel mechanisms (PM) architectures, fully actuated, by in-parallel linear-linear (L-L)

actuation modules(aM)

. The kinematics and design aspects for the representative cases — 2, 4 and 6 DOF Pu, by using planar motors and scissors (and/or, parallelograms) as

transfer of motion modules(TofM)

are presented, as they exhibit, both, an increased stiffness and speed features, respectively.

Assembly of high precision products is often done manually. The main reasons are the complexity of automation and the production volumes that often remain small to medium. Watches, medical devices and sensors are some examples of products requiring high precision assembly: often expensive products with high margins. It is interesting to notice that achieving higher assembly yield allows for relative quick pay-back of equipment. This is also the reason why western European countries remain competitive in this field. In this paper, the important points to remember when selecting a solution to efficiently assist operators in their assembly tasks are highlighted. Good assistance should lead to higher yields, higher throughputs and better quality. One should take into account assembly processes and their difficulties, as well as production volume and economic profitability.

This article presents a case study in the design of a modular semi-automated reconfigurable assembly system using life cycle cost analysis methodology. To ensure that an assembly system is appropriately designed, system measurement schemes should be established for determining and understanding design effectiveness. Understanding life cycle costs is the first step toward increasing profits. The authors are developing an analysis tool that integrates Overall Equipment Efficiency (OEE), Cost of Ownership (COO), and other analysis methods to improve the design of flexible, modular reconfigurable assembly systems. The development is based on selected industrial standards and the authors’ own experience in modular assembly system design and simulation. The developed TCO (Total Cost of Ownership) methodology is useful in system supplier and end-user communication and helps in trade-off analysis of the system concepts.

This article shows the consequences of using parts with scrap in an automatic assembly system. The scrap is the percentage of parts that do not reach the required conformity and will cause either a defective assembly or a jam in the system. The two consequences considered here are extra cost and loss of productivity. To show this, we will focus on one part assembly operation anywhere in the process, which consists of feeding, assembling the part and inspecting the resulting sub-assembly.

We will show that the effect of scrap can be huge and depends strongly on how the bad part is detected. 4 cases are considered: 1.The parts are sorted before feeding 2.The bad parts jam in the feeder 3.The bad parts jam while assembling 4.The sub-assembly with bad parts is ejected after inspection. By evaluating each case in 3 examples, we will show that case 3 is often the worst. We will also define the conditions where it is better to sort the parts first (1), let them jam in the feeder (2) or sort the sub-assemblies (4).

The use of bonding as a joining technique is gaining more and more importance in micro manufacturing. There is a demand for systems which are capable of reliably dispensing tiny amounts of adhesive, especially under industrial conditions. By integrating sensors into the dispensing system or checking process results there is the possibility to become aware of the system specific problems and options of improvement. Methods and systems of investigation have been developed and realized at the Fraunhofer IPA.

Packaging is the last process of microsystem manufacturing. There are mainly two kinds of packages: plastic or metallic. The two main components of the package (base and cover) may either be glued or soldered. Each of these techniques has its advantages and drawbacks, and the choice should be driven by the functionality of the microsystem.

The advantage of gluing is that it is quite an easy production process. The drawback is that glue, like all polymers, is not hermetic on the long term. This is a problem when the package should protect the microsystem from oxygen or water vapour, an example being OLED displays (Organic Light Emitting Diodes).

The advantage of soldered metallic packages is that they are hermetic. The drawback is that the welding process takes place in an oven, and that the whole microsytem is heated to the fusion temperature of the joint. This is a problem for many microsystems, typically biomedical MEMS and OLEDS again.

The LPM-EPFL is working on the development of a two-part sealed packaging. One part of the package is metallic, the other part is made of glass.

The goal of the project is to soft solder the two parts of the package by the mean of a laser diode. The advantages of the laser soft soldered joint are:

its water and air-tightness in regard to glue or a plastic joint

the possibility to heat only the solder joint, and not inside the package.

The main difficulties encountered in the development of microscale fluidic pumping systems stem from the fact that these systems tend to comprise highly three-dimensional parts, which are incompatible with traditional microproduction technologies. Regardless of the type of pumping principle, most of the hydraulic systems contain valves and in particular a one-way valve. This paper presents the design and modelling of an ortho-planar one-way microvalve. The main advantages of such a valve are that it is very compact and can be made from a single flat piece of material. An analytical model of the spring deflection has been developed and compared to FEM. A prototype with a bore of 1.5 mm has been build using a micro EDM (electro discharge machining) machine and also tested.

Mechatronic competences represent a strong component in Microsystems Technologies, but very especially in Microhandling and -assembly, a field with challenging requirements. The Research and Training Network “Advanced Methods and Tools for Handling and Assembly in Microtechnology” (ASSEMIC) addresses this research field at a European scale. This paper presents aspects of the ASSEMIC project and some results achieved in its frame.