Jan Peirs

Assembly of microsystems

In the microworld, as well as in the macroworld, assembly is a crucial operation in the genesis of a product. This keynote paper focusses on the assembly problems occurring in the manufacturing cycle of microsystems. Scaling effects make that the assembly problems are different in the microworld. The different assembly operations and techniques, like manipulation by physical contact, non-contact manipulation, smart assembly techniques, and joining methods are thoroughly discussed. Finally, some relevant examples of micro-assembly systems and of assembled microproducts are given.

An implantable drug-delivery system based on shape memory alloy micro-actuation

Abstract Shape memory alloy actuators feature an extremely high power-to-volume ratio. This property is a major advantage for miniature applications. This paper describes implantable drug-delivery systems based on shape memory alloy micro-actuation. A first type is designed for use with solid drugs while a second design enables delivery of liquid drugs. The operating principle of the latter system is based on a precisely controlled discontinuous release from a pressurized reservoir. It is realized using a shape memory actuated microwave system. One dose can be controlled with an accuracy up to 5 μl. The system is remotely powered and controlled using a transcutaneous transformer. A refilling possibility based on transcutaneous injections is provided. The design of the valve is such that it can be mounted on a printed circuit board together with the other electrical components. Furthermore, the valve is optimized towards aspects like biocompatibility, low-cost production, lifetime, safety and minimal dimensions. The drug-delivery system is aiming at patients who need multiple injections each day over a long period of time. The current prototype could already reduce the number of injections by a factor of 200. By further miniaturization a reduction factor of 3000 could be obtained.

Design of miniature parallel manipulators for integration in a self-propelling endoscope

Abstract This paper presents two designs for a miniature robotic manipulator that has to be integrated into a self-propelling endoscope. The endoscope is meant to inspect and intervene in the human colon through which it moves by inchworm locomotion. Both manipulator designs are based on a 3-degree-of-freedom (dof) Stewart platform, either driven by hydraulic pistons or by electromagnetic motors. The hydraulic manipulator is 12 mm in diameter and 30 mm long. It has a stroke of 10 mm and tilts 30–35°. The system is designed to be used at pressures up to 10 bar at which each piston generates a force of 7 N. Piezoelectric and electromagnetic valves are developed that will be integrated into the manipulator. The electrical platform has three telescopic legs driven by a motor-spindle combination. This manipulator has a length and diameter of respectively 50 and 15 mm, and generates speeds up to 5 mm/s and forces up to 1.2 N per leg.

Development of an axial microturbine for a portable gas turbine generator

A miniature gas turbine is under development with the aim of generating electrical energy from fuel. This system consists of a compressor, combustion chamber, turbine and generator. The turbine is a single-stage axial impulse turbine (Laval turbine) with a rotor diameter of 10 mm, made of stainless steel using die-sinking electro-discharge machining. It has been tested with compressed air to speeds up to 160 000 rpm and generates a maximum mechanical power of 28 W with an efficiency of 18.4%. When coupled to a small generator, it generates 16 W of electrical power, which corresponds to an efficiency for the total system of 10.5%. The power density is mainly limited by the maximal speed of the ball bearings. The main losses are the blade profile losses and the exit losses. Higher speeds can considerably reduce the exit losses and therefore increase efficiency and power density. An improved turbine has been tested at temperatures up to 360 °C and generates up to 44 W of electrical energy with a total efficiency of 16%. A 20 mm diameter centrifugal compressor matching the pressure and flow characteristics of the turbine has been designed and is currently under construction.

Design of an advanced tool guiding system for robotic surgery

This paper describes an advanced tool guiding system for robot assisted surgery. It is offering two additional local degrees of freedom to a standard robotic tool guiding system like of the Zeus robotic surgery system. The tool guide is basically a tube that guides the inserted surgical instrument to the desired location. By adding two bending degrees of freedom at the tip, the developed system largely increases the maneuverability of the instrument. It consists of a micromachined superelastic tube driven by a conventional cable system. Main issue was the design of a flexible hinge system in superelastic NiTi that offers the desired bending angles (at least 90 degrees in both directions) and that is compatible with the material limits for long-time cycling. Based on extended FE calculations and experimental measurements the second-generation design offers 90 degrees bending in both directions. The improvement over previous devices is the combination of two degrees of freedom with a small diameter of only 5 mm. An additional advantage is the tool channel, which enables the use of different instruments with this single device.

Design of a shape memory actuated endoscopic tip

Abstract This paper describes two designs for an endoscopic tip, both actuated by shape memory alloys. The aim is to have 180° rotation available at the tip of the endoscope without any mechanical driving system connected to the surgeon. First, a modular actuator design is discussed. It consists of several ‘vertebras’ stapled together to a snake-like construction. The second prototype provides continuous motion and is based on a miniaturised planetary gear system.

Scale effects and thermal considerations for micro-actuators

This paper analyses scale effects for actuator types which are strong candidates for micro-actuation: shape memory alloys (SMAs), electrostatic, electromagnetic and piezoelectric actuators. SMA actuators are studied in detail and compared to the other actuator types on a basis of force, response time, and power density. Much of the analysis is based on thermal aspects and scaling effects on both convective and conductive heat transfer, which is a quite novel approach. Results of this thermal analysis are effects on surface temperature and maximum current density of microactuators.

Production and characterization of a hydraulic microactuator

In order to improve the power density of microactuators, recent research focuses on the applicability of fluidic actuation at the microscale. The main encountered difficulties in the development of small fluidic actuators are related to production tolerances and assembly requirements. In addition, these actuators tend to comprise highly three-dimensional parts, which are incompatible with traditional microproduction technologies. This paper presents accurate production and novel assembly techniques for the development of a hydraulic microactuator. Some of the presented techniques are widespread in precision mechanics, but have not yet been introduced in micromechanics. A prototype hydraulic microactuator with a bore of 1 mm and a length of 13 mm has been fabricated and tested. Measurements showed that this actuator is able to generate a force density of more than 0.23 N mm(-2) and a work density of 0.18 mJ mm(-3) at a driving pressure of 550 kPa, which is remarkable considering the small dimensions of the actuator.

High-speed bearings for micro gas turbines: stability analysis of foil bearings

Mesoscopic or microscopic gas turbines can be an interesting replacement for batteries as mobile energy supplies. A difficult consequence of small-scale turbomachinery is an increased rotor speed, in the order of 500 000 rpm and higher, turning bearing design into a challenging task. Air bearings are the only bearing type able to withstand the severe conditions of high speed and high temperature. However air bearings and more, in particular, aerodynamic bearings are prone to dynamic instabilities. Therefore unconventional bearing types such as foil bearings may present an interesting solution. This paper presents and discusses simulation techniques to predict the steady behaviour of a foil bearing. Furthermore, a method to calculate the dynamic properties is proposed. Using these dynamic stiffnesses and damping coefficients, a stability analysis is carried out. This analysis shows that, even without additional damping, a foil bearing is more stable than a rigid surface aerodynamic journal bearing with similar geometry but not as stable as is hitherto believed. However, due to its flexible nature, it is possible to improve the stability by simple means.

Micropower generation with microgasturbines: A challenge

Abstract This paper describes the development of a microgasturbine with a rotor diameter of 20 mm. The target electrical power output lies around 1 kW. The total system fits in a cylinder with a diameter of 95 mm and a length of 120 mm. The system contains the same components as a large gasturbine generator: compressor, recuperator, combustion chamber, turbine, and electrical generator. Major challenges are the high rotational speed (500 000 r/min), high turbine inlet temperature (1200 K), and the efficiency of the components. Because of the small dimensions, the flow through compressor and turbine is characterized by relatively low Reynolds numbers. The higher flow losses and inherently lower efficiency require a higher blade tip speed (524 m/s) than for large turbines (300-400 m/s). To minimize wear and frictional losses, the rotor is mounted on aerodynamic bearings. To withstand the high centrifugal stresses, a high-strength steel is used for compressor and shaft. The turbine is made of a Si3N4-TiN ceramic composite to withstand the combination of elevated stress and temperature.