Contact Free Micromotion Devices

Overview

Our group focuses on the following topics:

  • Passive and Active magnetic bearings
  • Electrostatic actuators

Contact-free suspension provided by magnetic bearings (passive or active) or electrostatic bearings have a number of advantages over conventional bearings. These advantages include elimination of lubrication, friction-free operation and the ability to operate at higher speeds. For these reasons, magnetic bearings are used in applications with high rotor speeds and high power density. The electrostatic glass actuator is a promising novel concept for synchronous propulsion of glass rotors. This concept represents an alternative to electromagnetic solutions, featuring a higher degree of simplicity and compactness. Possible applications are very compact disk drives for handheld devices and precise positioning of optical elements.

2D Inclinometer

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Conventional inclinometers are limited by the manufacturing precision of mechanical parts. These mechanism also presents non-linearities like friction. To eliminate these problems, a levitated inertial mass can be used to measure inclination. In addition, the system can be used (with the same basic embodiment) for other high precision and small measuring instruments.

A 2D inclinometer has been built using a diamagnetic disc surrounded with an aluminum crown, which is levitated over permanent magnets. Although the diamagnetic forces are very weak, it is possible to levitate a piece of diamagnetic material over an optimized arrangement of permanent magnets. The position of the disc is stabilized horizontally using electrostatic bearings.

Three Axis High Speed Micro Active Motor Magnetic Bearing

Fast rotation is needed more and more in many industrial applications. Classical examples are compact devices like scanners, gyroscopes, centrifuge units, spinning rotor gauges and so on. The key to achieve high rotating speeds is to have contact-less bearings. Active magnetic bearings (AMBs) allow the levitation of a body by magnetic forces with no mechanical contact.

Actually, the record of rotational speed has been reached already in 1946 by Jesse Beams with a magnetically suspended steel sphere spinning at over 20 million rpm before bursting under the influence of the centrifugal field. The experiment was done with a 0.8 mm diameter solid steel ball in vacuum.

This research project is based on J. Beams experiment and includes the realization of a small device in order to suspend and spin a sub-millimetric spherical rotor (down to 0.4 mm diameter). The rotor is stabilized with a three axis active magnetic bearing and it is spun with a two-phase induction motor. Until now, small spherical rotors with diameter down to 0.5 mm have been levitated and spun. The maximum reported speed with a 1 mm diameter rotor is 180’000 rpm in air and 2’880’000 rpm in vacuum.

Electrostatic Glass Motor

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The physical principle that governs the electrostatic glass actuator is the relatively long relaxation time of charges in the glass (in the order of several tens of seconds). Net charge densities can be arranged by non-contact charge induction by applying an electrostatic field. After the creation of areas of positive and negative polarity, a moving electrostatic field can move the glass rotor.

The actuator behaves much like a well know electret device, but it has the advantage that no electret-polymer has to be applied to the rotor. Moreover, applying a sinusoidal moving field for propulsion, a sinusoidal charge distribution will appear on the rotor. This distribution leads to a perfect smooth synchronous movement, a feature unmatched by electromagnetic or piezoelectric alternatives.

The electrostatic glass motor consists basically of a glass rotor “sandwiched” by one or two electrodes. The electrodes form a three-phase array (phase-shifted excitation) in order to produce a moving electrostatic field. This excitation may be sinusoid or step waves.

Differential Delay Line for the Search of Extra-Solar Planets

/webdav/site/lsro/shared/MICRO/MICRO_ddl.jpgA mechanical system has been developed, which allows the compensation of the perturbations from earth atmosphere
during observations with the Very Large Telescope Interferometer (VLTI) situated at Paranal, Chile
The current step is now the integration of eight of such systems into the heart of the VLTI.

Self-sensing (Sensorless) Active Magnetic Bearing

The concept of self-sensing or sensorless active magnetic bearings is to eliminate the position sensors and estimate the position of the levitated object by measuring the bearing coil current. The main interest of this approach is to reduce production costs and hardware complexity.

Though the idea of self-sensing itself is not new and research has been carried out for years in this topic, it still remains a challenge. Many self-sensing methods have been proposed in literature, however they are all very delicate to realize.

The goal of this project is to investigate the realization of a self-sensing active magnetic bearing suitable for industrial use. In order to do that, this work explores a self-sensing configuration based on the current amplitude modulation. This approach is applied to an active magnetic bearing test rig using reference sensors.

Hybrid and monolithic piezoelectric actuators and sensors

High precision actuation and sensing using smart materials is the main purpose of this research activity. Our focus is the design, modeling and fabrication of piezoelectric based actuators and sensors in domains such as:

  • Astronomy
  • Aerospace
  • Biology
  • Medicine
  • Optics
  • Electronics

Commerialized piezoelectric materials such as ceramics PZT and polymers PVDF are used in this research work. Other electroactive polymers elastomers, ferroelectric polymers, ionic polymers and others exhibiting interesting properties in terms of response under electric field are also targeted in this work. moreover, polymer nanocomposites with potential piezoelectric capabilities are also investigated and implemented as smart materials.

For the specifics of the required design, shape, connectivity and size, different techniques going from clean room to laser cutting are considered. The resulting actuators and sensors are mainly monolithic structures where the piezoelectric response is amplified by a bimoph design. Hybrid designs where ceramics and polymers are combined to provide a particular performance are also a part of our investigations.

Micromanipulation of cells and tissues in life science is a potential field for these investigations. The capabilities of polymers as biocompatible and mechanically strong makes them excellent candidates for this kind of tasks. Recent results of our work in the field of characterisation of cells indicate the feasibility and performance of PVDF actuators. Also laser optics demanding switching and lens motion control capabilities in the kHz range is another activity in our laboratory.