Neil M. White

Energy harvesting vibration sources for microsystems applications

This paper reviews the state-of-the art in vibration energy harvesting for wireless, self-powered microsystems. Vibration-powered generators are typically, although not exclusively, inertial spring and mass systems. The characteristic equations for inertial-based generators are presented, along with the specific damping equations that relate to the three main transduction mechanisms employed to extract energy from the system. These transduction mechanisms are: piezoelectric, electromagnetic and electrostatic. Piezoelectric generators employ active materials that generate a charge when mechanically stressed. A comprehensive review of existing piezoelectric generators is presented, including impact coupled, resonant and human-based devices. Electromagnetic generators employ electromagnetic induction arising from the relative motion between a magnetic flux gradient and a conductor. Electromagnetic generators presented in the literature are reviewed including large scale discrete devices and wafer-scale integrated versions. Electrostatic generators utilize the relative movement between electrically isolated charged capacitor plates to generate energy. The work done against the electrostatic force between the plates provides the harvested energy. Electrostatic-based generators are reviewed under the classifications of in-plane overlap varying, in-plane gap closing and out-of-plane gap closing; the Coulomb force parametric generator and electret-based generators are also covered. The coupling factor of each transduction mechanism is discussed and all the devices presented in the literature are summarized in tables classified by transduction type; conclusions are drawn as to the suitability of the various techniques.

Design and fabrication of a new vibration-based electromechanical power generator

A device is described for generating electrical power from mechanical energy in a vibrating environment. The design utilises an electromagnetic transducer and its operating principle is based on the relative movement of a magnet pole with respect to a coil. The approach is suitable for embedded remote microsystems structures with no physical links to the outside world. Simulation, modelling and test results following fabrication of a first prototype have demonstrated that generation of practical amounts of power within a reasonable space is possible. Power generation of more than 1 mW within a volume of 240 mm3 at a vibration frequency of 320 Hz has been obtained.

Thick-film sensors: Past, present and future

A review of thick-film sensors is presented. The evolution of the technology to a successful enabling mechanism for solid-state sensors is described. Many examples of applications in the major signal domains (mechanical, radiant, thermal, magnetic and chemical) are cited. The important characteristics of the technology relating to hybrid circuits, support structures and primary sensing elements are illustrated. The future of thick-film sensors is discussed in the light of the rapid development of new materials for sensors that have emerged over recent years.

A novel thick-film piezoelectric micro-generator

The use of alternative electrical energy sources to batteries is of particular significance to remote sensor systems. A vibration-powered micro-generator, based on a screen printed piezoelectric material, is proposed for this purpose. Theoretical and experimental results show that 2 µW can be generated for a vibration frequency of only 80 Hz. The device is not optimized and significant improvements are envisaged in the future.

A Silicon Microfluidic Ultrasonic Separator

Ultrasonic standing waves can be used to generate forces on particles within a fluid. Such forces have a number of potential applications in microfluidic devices. This paper describes a device that provides filtration on a microfluidic scale. It is microfabricated and uses ultrasound in the megahertz frequency range to concentrate particles at a node within the flow. It offers the possibility of a functional equivalent of a centrifugal separator for microfluidic systems. It is constructed using silicon and Pyrex, and hence is compatible with established microfabrication techniques. The modelling, design, fabrication and control of the device are discussed.

A Novel Micropump Design with Thick Film Piezoelectric Actuation

A new design for a silicon-based micropump is described. Passive cantilever valves are produced by boron etch stop and fusion bonding. Tests of these valves show good performance, as no flow could be detected in the reverse direction. Initial experiments on a thick-film screen printed piezoelectric membrane actuator were undertaken. A study of suitable inks for electrodes on different insulation layers on silicon yielded silicon dioxide and cermet gold ink as the most satisfactory combination. Deflection measurements of a mm PZT (lead zirconate titanate) - bimorph membrane gave movement at an applied voltage of 100 V. A quasi-static simulation package of the flow through a micropump is also presented. The valve action is simulated using ANSYS coupled with FLOW3D. The piezoelectric membrane deflection is simulated with ANSYS. A differential equation for the combined actuation of membrane and valves is solved numerically with Maple. Pump rates of up to and a maximum backpressure of up to 70 kPa for a driving voltage of 40 V have been modelled using bulk values for PZT-5H. A pump rate of up to and a maximum backpressure of up to 35 kPa at 100 V driving voltage are predicted using thick-film parameters extracted from the measurements.

Thick-film printing of PZT onto silicon

This paper describes a way of screen-printing PZT (lead-zirconate-titanate) onto silicon substrates. Initial experiments of printing a PZT-ink on a native oxide-covered Si-substrate showed contamination of the silicon substrate by lead diffusion during the firing stage of the film. This occurred also on thin-film deposited silicon oxide or silicon nitride layers. The effect of this is potentially catastrophic as conduction occurs between areas required to be electrically isolated by the thin film. The problem was solved by using a screen-printed barrier layer of IP211 (Heraeus), which also reduces the diffusion and prevents the conduction.

Processing of PZT piezoelectric thick films on silicon for microelectromechancial systems

Thick-film printed lead zirconate titanate (PZT) structures can be combined with micromachined silicon structures and offer relatively large actuation forces compared to alternative techniques. This paper describes the initial investigation into the compatibility issues of micromachining silicon wafers with PZT layers printed on the surface. It assesses the effect of many standard photolithography and micromachining upon the printed PZT layer. In particular the adhesion of the printed layer to the substrate and its internal structure have been studied after exposure to each process. Standard photolithography using positive resists has been found to destroy the adhesion of the platinum electrode and an alternative masking technique using a thick-film printed dielectric polymer has been developed. Aluminium top electrodes have been fabricated using this masking technique combined with ion beam milling. Finally many standard micromachining etching processes have been carried out on a range of silicon substrates incorporating platinum electrode/thick-film PZT structures. Wet silicon and silicon dioxide etches were found to be unsuitable since they also attacked the adhesion of the bottom electrode. Plasma etching processes appear to be well suited for the combination of materials since there is a wide range of etches available that do not affect the PZT. These can therefore be used for the micromachining of the silicon substrate after thick-film processing greatly expanding the range of applications suited to this combination of technologies.

A Novel Thick-Film Piezoelectric Slip Sensor for a Prosthetic Hand

The ability to mimic the tactile feedback exhibited by the human hand in an artificial limb is considered advantageous in the automatic control of new multifunctional prosthetic hands. The role of a slip sensor in this tactile feedback is to detect object slip and thus provide information to a controller, which automatically adjusts the grip force applied to a held object to prevent it from falling. This system reduces the cognitive load experienced by the user by not having to visually assess the stability of an object, as well as giving them the confidence not to apply unnecessarily excessive grip forces. A candidate for such a sensor is a thick-film piezoelectric sensor. The method of fabricating a thick-film piezoelectric slip sensor on a prototype fingertip is described. The construction of experimental apparatus to mimic slip has been designed and analyzed to allow the coefficient of friction between the fingertip and the material in contact with the fingertip to be calculated. Finally, results show that for a coefficient of friction between the fingertip and grade P100 sandpaper of approximately 0.3, an object velocity of 0.025plusmn0.008 ms-1 was reached before a slip signal from the piezoelectric sensor was able to be used to detect slip. It is anticipated that this limiting velocity will be lowered (improved) in the intended application where the sensor electronics will be powered from a battery, connections will be appropriately screened, and if necessary a filter employed. This will remove mains interference and reduce other extraneous noise sources with the consequence of an improved signal-to-noise ratio, allowing lower threshold values to be used in the detection software

Experimental investigation into the effect of substrate clamping on the piezoelectric behaviour of thick-film PZT elements

This paper details an experimental investigation of the clamping effect associated with thick-film piezoelectric elements printed on a substrate. The clamping effect reduces the measured piezoelectric coefficient, d33, of the film. This reduction is due to the influence of the d31 component in the film when a deformation of the structure occurs, by either the direct or indirect piezoelectric effect. Theoretical analysis shows a reduction in the measured d33 of 62%, i.e. a standard bulk lead zirconate titanate (PZT)-5H sample with a manufacturer specified d33 of 593pC/N would fall to 227.8pC/N. To confirm this effect, the d33 coefficients of five thin bulk PZT-5H samples of 220µm thickness were measured before and after their attachment to a metallized 96% alumina substrate. The experimental results show a reduction in d33 of 74% from 529pC/N to 139pC/N. The theoretical analysis was then applied to existing University of Southampton thick-film devices. It is estimated that the measured d33 value of 131pC/N of the thick-film devices is the equivalent of an unconstrained d33 of 345pC/N.