Static and kinetic coefficients of friction have been determined in air for a range of microbearing designs suitable for use in micromotors. Both dry sliding and rolling friction using microspheres have been investigated using forces, contact areas and contact pressures typical of those expected in a particular design of double-stator axial-drive micromotor. Micromachined test specimens coated with polysilicon have been slid on a variety of substrate materials. It is found that the coefficients of friction for these small areas and loads are not constant and decrease with surface pressure for all ceramics except silicon dioxide. The coefficient of friction on aluminium remains constant through all the variations studied. Sliding of polysilicon on diamond-like carbon and single-crystal silicon exhibits the lowest static coefficients of friction of 0.42 and 0.35, respectively. The use of glass microspheres of diameter 40 μm for the rolling tests reveals effective static and kinetic coefficients of friction of 0.04 and 0.02, respectively. The electrostatic torques of the micromotor for applied stator voltages of 100 and 150 V determined using 3D finite-element analysis are compared with the friction torques for the bearings studied. The resultant motive torques suggest that a bushing design is the preferred option for this motor, since it results in both lower coefficients of friction and reduced electrostatic contact forces.

In microsystems engineering the understanding of tribological behaviour on the microscale is one of the key factors in the design of devices with moving parts such as micromotors.This study has investigated the friction coefficients of LIGA processed nickel rotor samples on an alumina substrate and how the measured friction was affected by wearing the nickel rotors. The nickel rotors are typical of those that would be used in electrostatic micromotors. The rotors were loaded only by their own mass, resulting in normal forces of as low as 10(-7) N. High coefficients of friction were measured for the smallest rotor samples.A relationship was found linking the mass of the rotor samples and the measured coefficient of friction, which showed, that for very small rotors and light loads, the friction coefficients can be very high. This is obviously a major concern for the miniaturisation of devices, if they are to work efficiently (or at all). The mass dependence relationship was investigated for increasing wear of the rotor samples and found to be less apparent with wear. It is suggested that this is a direct result of the increase in true contact area as wear progresses, resulting in a decreased sensitivity to the load.

This article presents a simple method to calculate the stator surface vibration of a unipolar four-phase hybrid stepping motor. The approximate radial magnetic force per pole is calculated by using a permeance model and its equivalent magnetic circuit. Based on the calculated radial magnetic forces and the measured stator mobility values, the radial vibration spectrum on top of a stator pole is es...

Friction has long since been outlined as one of the key areas where efficiency is lost in microdevices. In some cases it is the factor which dictates whether or not they function at all. The tribological characteristics of microsystems are generally more sensitive to a variety of physical, chemical and environmental effects than macroscopic cases. This study investigates frictional properties for microengineered rotors made out of nickel by the LIGA and LIGA related processes. The material properties and general morphology of these rotors would be similar to the rotors in micromotor and microengineered turbine systems. Preliminary results indicate a strong dependency of the friction coefficient on the normal load with the smallest structures and loads exhibiting the highest friction. This relation was found for both silver, alumina and nickel test substrates. The increase in the magnitude of the friction coefficient for the smallest structures has obvious implications for the efficient design of microdevices. A method for reducing the load dependency based on surface polishing of the rotors is discussed.

The fabrication of micro electro mechanical systems (MEMS) such as micromotors is mainly based on silicon and its compounds and their tribological behaviour plays a key role in the performance of such systems. In this paper the wear of MEMS-compatible materials has been investigated for a range of contact areas and contact forces typical of micro electro mechanical systems. Special test specimens incorporating a range of micromachined micro structures on their top surfaces were fabricated in order to simulate those conditions. The micro structures were coated with diamond-like carbon (DLC), silicon nitride, silicon dioxide, and doped polysilicon. A specimen-on-disc arrangement was used for the wear experiments and dead weight loading was applied. The results show that the wear rate of DLC and single-crystal silicon sliding on DLC decreases with increasing sliding distance whereas silicon dioxide and silicon nitride showed linear wear behaviour. The effect of contact morphology and contact pressure was investigated for doped polysilicon sliding on DLC. The results can be attributed to the differing mechanical and chemical properties of the materials leading to wear mechanisms ranging from asperity fracture to asperity deformation.

The design process for an electrostatic actuator for arterial plaque removal is outlined in this work. Measurements, aimed at assessing the torque levels required to successfully remove arterial blockages which are typical of patients suffering from atherosclerosis, are described in detail. On the basis of these results, a minimally invasive surgical system containing a microcutter powered by a electrostatically driven harmonic micromotor has been proposed to alleviate this type of condition. Force and torque predictions obtained from a finite-element model of the motor are presented, and shown to be compatible with the proposed application.

The effect of axial electrostatic forces on the rotor deflection of an axial-drive double-stator micromotor is studied with respect to the limits of stability against elastic rotor collapse using the finite-element method (FEM). Forces obtained from an electrostatic FE model have been applied to a structural FE model using the indirect coupled-field analysis approach. This technique may be applied to other MEMS structures with conducting components. A technical description is given of the design and the fabrication process of the axial-drive double-stator micromotor investigated. Two different axial rotor supports (bearing and bushing) are investigated. The results are discussed as a function of the applied voltage and rotor thickness considering polysilicon and aluminium as possible rotor materials. The effects on the stability of the rotor of supporting the rotor with an additional bushing, and of increasing the rotor thickness, are investigated quantitatively.

A cantilever microactuator made of a ceramic fibre was used to load LIGA processed microstructures by pushing them over different substrates. Static and kinetic friction coefficients in air were determined for the interfaces between the LIGA-processed microstructures and the different substrates. Microrotors disassembled from electrostatic micromotors made of Cu and Ni have been used for the experiments. The substrate materials were amorphous Al2O3 and sputtered Ag on Cr. The experiments were carried out in a clean environment under class 100 conditions. Different cleaning methods and surface treatments have been applied and the influence on the coefficient of friction studied. It was found that the cleanliness of the surface and adsorbed gas layers have a substantial effect on the static friction results. This dependence was higher for Cu than for Ni. The kinetic and static friction appeared to be lowest for sliding on the Ag substrate and sliding of Ni on sputtered Ag exhibited the lowest coefficients of friction.

Many applications for microengineered devices can be envisaged for actuators capable of doing work or transferring power. Millimetre order turbines are considered in this study for the development of torque and the possibilities for the delivery of work. A prototype microturbine, with overall thickness of less than a millimetre, was studied for its torque capabilities. The initial prototype was realized using precision mechanics although implementation of the turbine is planned using microengineering techniques. A viscous braking method was developed to measure the shaft torque of the turbine, demonstrating shaft coupling and the possibilities for power transfer. In order to validate the viscous braking method for torque measurement, a mechanical friction brake (dynamometer) was developed to compare the measurements obtained for a miniature electric motor of known characteristics. The results from this series of calibration experiments were then used to evaluate the performance of a microturbine prototype. The dynamometer torque measurements were found to closely agree with the manufacture's stated stall torque for the miniature motor of 1.8*10-4 N m. The viscous brake torque measurements were found to underestimate the motor torque by around 20% with slight variation related to the angular velocity of the shaft. Shaft torque measurements for the prototype microturbine were possible using the viscous brake but not the dynamometer. It was felt that 10-5 N m represented the lower limit for the dynamometer torque measurement while the viscous brake could address torques down to 10-8 N m. The fluid brake produced measurements of torque in the range of 10-7 N m for the microturbine. At this level only an order of magnitude accuracy is claimed because of some uncertainties with the fluid model used for the viscous brake torque calculation. The shaft torque range for the viscous brake was from 10-4 N m down to 10-8 N m; this might be extended by optimizing the fluid model.

The electrostatic forces occurring in a novel double stator axial-drive variable capacitance micromotor (VCM) are studied as a function of rotor-stator overlap, applied voltage, rotor support morphology, and rotor thickness. Analytical equations are developed using parallel plate assumptions, and results are compared with those obtained with 3D Finite Element Analysis (FEA) for tangential, axial, and radial electrostatic forces. The influence of the axial forces on the rotor deflections are studied using iterative indirect coupled field analysis, where the axial forces obtained from the electrostatic 3D FE model are iteratively applied to a structural FE model until stable rotor deflections are obtained. It was found that the axial forces, taking the rotor deflection into account, are twice as high as those obtained by analytical evaluation neglecting rotor deflections and about 70 times higher than the radial forces at a typical operating voltage of 100 V. Inclusion of bushing supports results in lower axial forces and decreases the influence of rotor tilt. Tangential forces likely to be exerted on the rotor at start-up are also examined and compared with analytical predictions. The study demonstrates that FEA provides more accurate results than analytical equations due to the geometry and field simplifications assumed in the latter. >

The fabrication of many micro electro mechanical systems (MEMS) is mainly based on silicon and its compounds and, for moving structures such as micromotors, tribological behaviour plays a key role in the performance. In this paper the wear of MEMS-compatible materials has been investigated for a range of contact areas and contact forces typical of MEMS. Special test specimens incorporating a range of micromachined micro structures on their top surfaces have been fabricated in order to simulate those conditions.These single crystal silicon (SCS) micro structures were coated with a range of materials used in MEMS; diamond-like carbon (DLC), silicon nitride (Si3N4), silicon dioxide (SiO2), and doped and undoped polysilicon. A specimen-on-disc arrangement (a development of the macroscopic pin-on-disc) was used for the wear experiments and dead weight loading was applied using micro loads specially calibrated for this purpose.The results show that the wear rates of DLC and SCS sliding on DLC decrease with increasing sliding distance whereas Si3N4 and SiO2 showed approximately linear wear behaviour. The effect of contact morphology and contact pressure was investigated for doped polysilicon sliding on DLC and for doped and undoped polysilicon sliding on Si3N4. The results can be attributed to the differing mechanical and chemical properties of the materials leading to wear mechanisms ranging from asperity fracture to asperity deformation.

This paper presents results from radial force and stator radial vibration calculations using the 3D finite element method. Because of the special construction of a hybrid stepping motor with both unipolar flux due to the rotor magnet and heteropolar fluxes due to the currents in stator poles it is treated as a 3D magnetostatics problem. The radial force in the air gap under each stator pole has been calculated by the Maxwell stress method. The variation of radial force under each pole with one rotor tooth pitch movement is analyzed by Fourier series to obtain the force frequency spectrum with magnitudes and phase angles. It is then multiplied by the corresponding measured stator mobility values to obtain the stator radial vibration. The results are compared with experimental data and have shown a reasonable agreement.

Power station acoustic noise assessment, which has experienced increased environmental awareness and subsequently more stringent legislation for a number of years, has received an added stimulus due to the recent advent of powerful measurement and analysis techniques including sound intensity and coherence. These experimental techniques are explained and results, for a generator unit, illustrate their value in providing a unique, correlated insight into noise problems. This includes noise quantification, full explanation of site sound pressure level in terms of the various influences and major noise source identification. These techniques are widely applicable and an invaluable aid to any industrial noise problem.

A simple model for calculating the stator surface vibration of a unipolar four-phase hybrid stepping motor is presented. The approximate radial magnetic force per pole is calculated using a permeance model and its equivalent magnetic circuit. Based on the calculated radial magnetic forces and the measured stator mobility values, the radial vibration spectrum on top of a stator pole is estimated. The results show a reasonable agreement between the calculated and measured radial vibration levels. This verifies the analytical approach which, due to the special structure and excitation of the motor, is significantly different from the technique commonly employed for vibration study of induction machines

The effect of tooth shape on radial forces inside the air-gap acting on the stator for a typical step motor tooth profile is investigated by a 3D finite element method. It is found that the radial force acting on the stator varies from high to low values due to rotor movement. The magnitude of the vibration depends on the tooth shape and the stator and rotor tooth to valley ratio and level of saturation. The results show that a rectangular tooth shape with a stator tooth to valley ratio of 1:1 and a rotor tooth to valley ratio of 1:3 is the best for reducing radial force variation for the cases studied.< >

A detailed knowledge of the distribution of forces in a micromechanical system such as a micromotor is of great value at the design stage of such devices. It is preferable to analyse these forces using simulations of the system before the design is built and tested, since the cost of fabrication of micromechanical systems is high. This paper presents results of three dimensional simulations of a practical design of a high aspect ratio micromotor. The analysis of the electrostatic fields in the micromotor geometry gives detailed profiles of the forces acting on the various microstructures. These results indicate high localisation of forces in certain areas of the micromotor. An example is given of the application of the force analysis results in the prediction of frictional effects on the rotor caused by the axial force exerted on it

The only effective treatment for cataract is the surgical removal of the cataract (i.e. lens). Successful and accurate removal of the central 4-7 mm of the anterior capsule is the key to a successful operation. Currently, this is done by a multiple puncture and scraping or tearing technique. This method leaves the edges of the circular aperture in a ragged state. The authors present an investigation into the feasibility of improving the present method of cataract surgery by using a special microactuator. The main cataract operation steps and the results of dynamic torques developed in a microactuator are presented.

Sound-field characterisation is of vital importance in evaluating and improving the reliability of sound-intensity measurements. This paper examines indicators of the sound-field characteristics and the consequent implications.for sound-intensity measurements. Results illustrate how the coherence between sound pressure and particle velocity, can classify, sound fields as diffuse multi-source, direct, or direct-plus-reverberant from one source. The measurements were conducted in a power station, which is typical of many commonly encountered multi-source, reverberant environments confronting the noise-control engineer. The value of sound-pressure/particle-velocity coherence is consistent with the form of acoustic field implied by the sound-power, vector-sound-intensity, and sound-pressure measurements. Some practical difficulties of sound-intensity measurement in complex environments are discussed, as is the identification of these by coherence and the standard sound-field indicators.

The results of an initial investigation into the feasibility of using micromachines for ophthalmic surgery are presented. The study concentrates on cataract surgery, and in particular the anterior capsulate method. The limitations on the machine size are that the micromachine must fit comfortably inside the anterior chamber of the eye and should not interfere with the cornea. Comparing the torque developed by a primitive machine to the torque required to cut through the lens of tissue of a pig's eye, it is found that sufficient torque could be developed under certain conditions.< >