Development of a Piezo-based tilting platform for mirrors to enhance the process of Laser-Beam Polishing.
Haupt, Conrad ; Bliedtner, Jens ; Rinck, Stephan 等
1. INTRODUCTION
Manual quartz glass polishing is a process which eliminates
smallest grinding scratches and flaws from the surface of high quality
work pieces (GRU). For optics applications it is used to decrease the
surface roughness to values of 0.1 to 0.01[micro]m, which is smaller
than the wave length of visible light (KRA). A modern method for
polishing high quality surfaces is the so called Laser-Beam Polishing,
which provides several advantages towards manual polishing. The process
implies high speed, great accuracy and the possibility to be automated,
of course resulting in special demands towards the process parameters
and the necessary equipment.
After a number of previous experiments on polishing quartz glass
samples by Laser-Beam Polishing, a need for a purpose-adapted scanner
system developed. The current market provides out of the box two-axis
scanner systems with high performance at high price. As the demands
towards the field of application are different, the current situation
offers a great incentive to building a new, customized scanner system.
The settled task is to create a Piezo-driven two-axis laser scanner
system for Laser-Beam Polishing.
2. REQUIREMENTS AND SETUP
A prepared list of requirements is the basis for all further
development. The type of hinge and the lever between the mirror and the
actuator, as well as the realization of the minimum overall travel of
the laser-beam, pose the main challenges towards the construction design
of the scanner system.
Starting off with the desired feed rate of the laser of around 2
m/s and the average stroke of the Piezo-Actuator of about 1/1000th of
its original length, a first specification of constructional details can
be compiled.
An experimental setup includes 2 linear movable axes for the
processing of a plane sample. Previous setups used one scanner-axis and
one external axis, so that the scanner realizes the reciprocating motion and e.g. a machine-table adds a steady orthogonal motion. A zigzag kind
of pattern is the result (HEC). To enhance speed and control of the
process and the regularity of the polishing pattern, a two axis scanner
system is set to be the construction design aim.
The experimental setup, as shown in Fig. 1, illustrates the
focussing of the laser-beam towards the work piece.
[FIGURE 1 OMITTED]
Nevertheless the laser-beam remains defocused at the surface of the
work piece. The adjustability of the focus point of the laser-beam is an
important criterion at the design of a versatile laser scanner system.
In this assembly, the focussing is put into practice by adding a
shiftable concave mirror, placed previous to the actual scanner system.
Current research points out a two-phase process--first phase
polishing and second phase cleaning. The so called cleaning concerns the
removal of the sublimate layer of the pre-polished surface to provide
the required surface quality for Laser-Beam Polishing.
The implementation of requirements into an assembly turns out to be
very complex. This is why the construction design had to be modified
until the presented simplified setup was complete. Every single part,
except standard parts, can be produced at the University's
production area, which is a great advantage in time and cost.
3. CONSTRUCTION DESIGN DETAILS OF THE SCANNER SYSTEM
The basic scanner system consists of two tilting tables (Fig.2),
each rotating independently around an axis with both axes crossing in
one pivot point. The two tables' axes are arranged orthogonally to
ensure a manageable controlling effort. According to the intended motion
for the laser-beam diversion, one table is designed to realize a
step-by-step inclination whereas the second table performs a
reciprocating motion.
[FIGURE 2 OMITTED]
Fig. 2 also displays the two different working principles. One
table is driven by two actuators at parallel feed and the other table is
driven by one actuator and reset by a compression spring. If required,
the spring can easily be removed and replaced by one of different
strength.
The first table, driven by two actuators, creates the basis for the
second table. A special arrangement of the tables and actuators shall
avoid a mutual impact of both motions towards another. Considering the
moment of inertia, the second table is designed to be much lighter than
the first one, as it's placed further away from the axis and
additionally carries the laser mirror.
The applied silicon mirror (as seen in Fig. 3) has to provide a
high reflection rate at low weight. This is to ensure, that the tilting
system as well as the Piezo actuators are not influenced by the heat of
the partly absorbed incoming laser-beam and still enable the fast
tilting motion.
To adjust and adapt the scanner system efficiently, the holding
fixture is designed to be shifted and remounted in 3 orthogonal axes.
For adjustment and development reasons the Piezo actuators are preset by
detachable screws, secured with locked nuts.
Also the control unit has a major effect on the functioning of the
designed system. A special system of electronic components is developed
by co-workers to control and trigger the actuators as an interface
between the controlling personal computer and the working Piezo stacks.
The Piezo actuators work at Voltages between -20 and 150 Volts and
make high demands on the control unit and the prepared power supply.
[FIGURE 3 OMITTED]
A protection against overvoltage is another necessary point of
interest and will also be part of the control system. The dimensions of
the scanner system prototype are about 40 mm x 40 mm x 65 mm (WxDxH) and
it consists of around 30 single parts, mainly made of stainless special
steel.
4. CONCLUSION AND PERSPECTIVES
The task to create a Piezo-driven two-axis laser scanner system
evolves into a complex and time-consuming project, and as production and
testing are not complete, there are further problems to be solved.
The used Piezo actuators are multilayer stack design actuators,
which consist of thin layers of ceramics combined with metal electrodes,
stacked and glued together. As nearly every actuating device, Piezo
actuators experience hysteresis effects as well as mechanical and
electrical losses regarding voltage and reciprocation (JAN). Those
effects are generally known, but must be actively defined, compensated
and counteracted by the control unit.
Also effects of friction and abrasion have to be taken seriously.
Here the advantage of the designed construction is that every single
part of it can be unmounted and remounted without resulting in
tremenduous effort of reconfiguration. Single parts or even assemblies
can be replaced by new designs if necessary or utile.
The whole scanner system is designed to be versatile and still
provide specific preferences for its main application, the Laser-Beam
Polishing.
Small size and low weight are qualifications which make the scanner
system generally applicable to different working environments, such as
laboratories, industrial use or even clean rooms.
It provides virtually unlimited capabilities in the field of laser
applications. The maximum tilt can be expanded by the use of longer
Piezo stacks. Accuracy and speed can both be enhanced by optimization of
weight and the use of different materials, such as aluminum or titanium.
Even the size of the installed mirror can be adapted without or at
least through minor constructional changes. A reconfiguration of the
motion parameters will have to be done accordingly.
Since a personal computer provides and controls most calculations,
there is the possibility that more complex laser-beam trails will be
realized through further development of the software and the interface
system.
Next to Laser-Beam Polishing, applications such as laser cutting,
laser marking or other short-time process laser applications are
possible alternatives for future use.
5. REFERENCES
Gruenwald, Franz. (1985). Fertigungsverfahren in der Geraetetechnik
(Manufacturing Methods in Apparatus Technology), Hanser Verlag, ISBN:
3-446-14195-2, Munich
Hecht, K. & Bliedtner, J. (2008). Finishing quartz glass
surfaces with laser radiation: Analysis of the parameters for process
optimisation/automation, The 19th International DAAAM Symposium 2008,
Intelligent Manufacturing and Automation
Janocha, Hartmut. (1992). Aktoren: Grundlagen und Anwendungen
(Actuators: basic knowledge and use), Springer Verlag Berlin Heidelberg,
ISBN: 3-540-54707-X, Berlin
Krause, W. & Gerlach, G. (1996). Fertigung in der Feinwerk-und
Mikrotechnik: Verfahren, Werkstoffe, Gestaltung(Manufacturing in
Microtechnology), Hanser Verlag, ISBN: 3-446-18226-8, Munich &
Vienna
HAUPT, C[onrad]; BLIEDTNER, J[ens]; RINCK, S[tephan] & HECHT,
K[erstin] *
* Supervisor, Mentor
