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Miniatur-Laserscanner für mobile Anwendungen
Full text linkKurzfassung
Laserscanner-Systeme zur hochaufgelösten Umfelderfassung nutzen
momentan hauptsächlich
makroskopische Aktoren zur Strahlablenkung und unterliegen damit
Einschränkungen hinsichtlich
minimaler Baugröße und Unempfindlichkeit gegenüber
Erschütterungen. Hauptgrund ist die
Aktoreinheit, die meist große Spiegel- und Stellelemente und
damit große bewegte Massen enthält.
Eine deutliche Verbesserung verspricht der Einsatz von
Mikroaktoren. Bisherige Entwicklungen
ermöglichen meist jedoch nur kleine Scanwinkel um 10°. Große
Winkel über 50° erfordern
komplexe Aktorstrukturen, was mit aufwendiger Prozesstechnik und
hohen Kosten verbunden ist.
Die hier vorgestellte Arbeit versucht die Nachteile
makroskopischer Aktoren, sowie bisheriger Mikroaktoren,
durch die gezielte Anwendung sogenannter Smart Materials in
mikroaktorischen
Systemen zu beheben. Dazu wird zunächst ein neuartiger
Aktormechanismus auf Basis der ferromagnetischen
Formgedächtnislegierung Ni2MnGa eingesetzt. Dieser vereint die
Wirkmechanismen
des Ferromagnetismus und des Formgedächtniseffektes in einem
Dünnfilm-Bauteil. Den
erzeugten antagonistischen Kräften stehen sehr kleine
Rückstellkräfte gegenüber, wodurch sich
große Scanwinkel in einem breiten Frequenzbereich erzeugen
lassen. Weitere Mikroaktoren auf
der Grundlage von Smart Materials nutzen entweder
Dünnfilm-Verbundschichten mit reinem
Formgedächtniseffekt auf der Basis von NiTiCu oder mit rein
ferromagnetischen Eigenschaften.
In dieser Arbeit werden, ausgehend von den vorgestellten
Aktormechanismen, 1D-Mikroaktoren
mit einem Ablenkwinkel entwickelt und diese in einem weiteren
Schritt zu 2D-Mikroaktoren mit
zwei rechtwinklig zueinander stehenden Ablenkwinkeln erweitert.
Abschließend erfolgt die Entwicklung
eines 2D-Laserscanner-Systems bestehend aus einem 1D-Mikroaktor
kombiniert mit
einem FlugzeitEntfernungsmesssystem (TOF-System).
Das Design der 1D-Mikroaktoren basiert auf Doppelbiegebalken,
deren Strukturen auf eine möglichst
gleichmäßige Temperaturverteilung ausgelegt sind. Bei der
Herstellung kommt eine Spaltschweiß-
Technik zur Erzeugung kleiner und zuverlässiger elektrischer
Verbindungen zum Einsatz.
Verglichen mit anderen Wirkprinzipien, die meist eine passive
Feder-Rückstellung nutzen,
werden bei dem 1D-Mikroaktor aus Ni2MnGa große, nahezu
frequenzunabhängige optische Ablenkwinkel
von 120° bei 60 Hz oder 60° bei 180 Hz erzielt. Die Baugröße
liegt dabei im Millimeter-
Bereich, der Energiebedarf bei weniger als 100mW.
1D-Mikroaktoren aus der Verbundschicht mit
der Formgedächtnislegierung NiTiCu zeigen weit größere
Scanwinkel bis zu 170°, jedoch nur in
Resonanz bei 85 Hz.
Das Design der 2D-Mikroaktoren ist monolithisch ausgeführt und
benötigt nur einen Mikrospiegel
zur Strahlablenkung. 2D-Mikroaktoren aus Ni2MnGa lehnen an das
1D-Design an und kombinieren
Biegebewegungen mit einer zusätzlich hervorgerufenen
Kippbewegung. Erste Prototypen
zeigen in einem Frequenzbereich von 40 Hz bis 80 Hz bei
leistungsoptimierter Ansteuerung
2D-Scanmuster mit optischen Scanwinkeln von 38° x 50°. Eine
andere Variante besteht
aus zwei strukturoptimierten und mechanisch gekoppelten, im
90°-Winkel zueinander angeordneten
1D-Aktoren aus der NiTiCuVerbundschicht und erreicht maximale
optische Scanwinkel von
20° x 40°. Diese Winkel treten nur bei 25, beziehungsweise 85 Hz
in Resonanz auf und zeigen
eine große mechanische Kopplung.
Im letzten Teil der Arbeit wird ein 2D-Laserscanner-System zur
horizontalen Detektion von Entfernung
und Winkel hergestellt. Dies gelingt durch die Integration einer
Hochleistungs-Halbleiterlaserdiode,
eines Ni2MnGa 1D-Mikroaktors, eines optischen Winkelsensors und
eines TOFSystems.
Der Winkelsensor wird durch einen Strahlteiler und einen
positionsempfindlichen Photodetektor
(PSD) auf einer optischen Bank realisiert. Dabei stellen die
optischen Aperturen des
TOF-Empfängers große Anforderungen an die Strahlkollimation des
Lasers und die Fokussieroptik
im Detektorstrahlengang. Berücksichtigt werden muss weiterhin
die Abstimmung der komplexen
Hardware- und Software-Komponenten der Sensorik. Ein erster
Demonstrator zeigt eine
maximale Messentfernung von 30 m, die neben der Laserleistung
durch die Empfindlichkeit des
optischen Empfängers begrenzt wird. Die erzielbare Ortsauflösung
des Systems ist einerseits
durch die Auflösung des Winkelsensors von 0.5° vorgegeben und
wird weiterhin durch die zeitliche
Auflösung des TOF-Systems von etwa 1 ns bestimmt.
Miniature-Laserscanner for Mobile Applications
Abstract
Laserscanning systems for highly resolved sensing of the
environment are currently using macroscopic
actuators for beam deflection and, thus, are restricted with
respect to their size and insensitivity
to shock. Main reason is the actuation unit, comprising large
mirrors and actuators and,
thus, large moving masses. It is expected that the use of
microactuators will lead to a considerable
improvement. In most cases, however, previous developments
enable only small scanning angles
in the order of 10 deg. Large angles above 50 deg require
complex actuator structures resulting
in extensive processing technology and higher costs.
This work is an effort to resolve the disadvantages of
macroscopic as well as current microactuators
by the systematic implementation of so called smart materials in
microactuators. For this
purpose, first a novel actuation mechanism on the basis of the
ferromagnetic shape memory alloy
Ni2MnGa is introduced, combining the ferromagnetic and shape
memory effect in one thin film
component. The resulting antagonistic forces are opposed by
small reset forces, thus, allowing
large scanning angles in a wide frequency range. Further
microactuators on the basis of smart
materials make use of thin film composites with either pure
shape memory effect on the basis of
NiTiCu or pure ferromagnetic properties. Based on these
actuation mechanisms this work pursuits
the development of 1D microactuators with one deflection angle
and, in a further step, their
extension to 2D microactuators with two scanning directions
being at right angles with respect
to each other. Finally, a 2D laserscanner system is developed
consisting of a 1D microactuator
combined with a time-of-flight (TOF) system.
The designs of the 1D microactuators are based on double-beam
cantilever structures, which
are constructed for a temperature distribution as homogeneous as
possible. Fabrication technology
comprises a gap welding technology to generate small size and
reliable electrical contacts.
Compared to other actuation mechanisms, which mostly utilize a
passive reset spring, the 1D
microactuators of Ni2MnGa achieve large, nearly
frequency-independent optical scanning angles
of 120 deg at 60 Hz or 60 deg at 180 Hz. The size of the
actuator is in the millimeter-range, while
energy consumption is less than 100 mW. 1D microactuators of the
NiTiCu composite show even
larger scanning angles up to 170 deg but only at their resonance
frequency of 85 Hz.
The 2D microactuators are designed monolithically using one
micromirror for beam deflection.
The 2D microactuators of Ni2MnGa are based on the 1D design and
combine the bending motion
with an additionally excited tilting motion. First prototypes
show in the frequency range of 40 to 80
Hz at optimized driving power 2D scanning patterns with optical
scanning angles of 38 x 50 deg.
An other variant comprising of two mechanically coupled, at
right angles oriented 1D actuators of
the NiTiCu composite achieves optical scanning angles of 20 x 40
deg. These scanning angles
only occur at the resonance frequencies of 25 and 85 Hz,
respectively, and show a stronger
mechanical coupling.
In the final part of this work, a 2D laserscanner system for
horizontal detection of distance and
angle is built up. This is achieved by integration of a
high-power semiconducting laser diode, a
Ni2MnGa 1D microactuator, an angle sensor and a TOF system. The
angle sensor is realized
on an optical bench by a beam splitter and a position sensitive
detector (PSD). Thereby, the
optical apertures of the TOF detector are making high demands on
the beam collimation and
focussing optics in the detector beam path. Furthermore, the
adjustment of the complex software
and hardware components of the sensor system has to be taken
into account. A first demonstrator
shows a maximum range detection of 30 m which is limited,
besides the laser power, by the
receiver sensitivity. The achievable spatial resolution of the
system is on the one hand given by
the resolution of the angle sensor of 0.5 deg and in addition by
the time resolution of the TOF
system of about 1 ns
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