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Introduction
To illustrate the problem and its scope I will use the analogy below:
More than likely you are reading this on a computer with a mouse attached to it. Over the last few decades the computer
mouse has become as ubiquitous as a pencil. One of the features that makes the mouse useful is that if your” mousing” surface
is flat then the mouse stays exactly where you put it until the user places their hand on it to move it again.
The mouse phenomenon that makes it so useful is due to several factors; the flat mousing surface, the fact that you are in a
gravity field that holds the mouse onto the surface, sufficient friction for the mouse not to be moved by small disturbances, and
the user knowing where the mouse is at all times even when not touching it.
Specific Statement of the Problem
The goal of this project is to repeat on a much smaller and more precise scale the general function of a computer mouse. The
precision optical platform is usually asked to carry a simple micro optical device such as a lens or mirror that is usually smaller
than 1 mm square and masses <1 gram. Usually speed of movement is not a factor but not always. In systems that use fast
steering mirrors (FASMs) there are a whole set of criteria such as momentum, jerk, overshoot and other issues that this project
will not address.
As will be shown below the current state of art is to use relatively large structures to achieve the sub μm movement desired.
The devices shown are some of the smallest devices. There are many larger ones that are used in manufacturing and
machining. These devices rely on a brute force and massive stability approach to achieve small movement.
The traditional way of achieving small movement has been the use of a drive screw that has an extremely fine pitch. The
French Screw shown below in figure 1 is an example of an early precision mechanism. It relied upon the fine screw for
adjustment of the cutting edge and the natural friction of a fine screw for stability.
Figure 1 Source ( English and American Tool Builders; by Joeseph Wickham Roe; Yale University Press 1914)
The screw thread was use for translating circular motion into linier motion for hundreds of years. The screw provided the press
for the wine press and the printing press.
Figure 2 (replica of the Gutenberg Printing press) Made by Urszula Sęczyk Region: kujawsko-pomorskie, Poland
In figure 2 it can be clearly seen that the “press’ is provided by a screw thread. As time progressed it was realized that the
screw thread could be used for moving loads very small distances.
Current Art
There are many different ways to achieve the movement in a 100μm X 100μm X-Y plane. A few of the more common ones are:
Piezo Electric Systems;
By using a piezoelectric effect to achieve the desired goal accuracy and travel can be achieved. The down fall of this in the past
has been that if there is a lapse in power the device goes to home and optical calibration has to start all over. The device in
figure 1 is a single stage so to achieve a viable X-Y device 2 stages would have to be stacked one upon the other, this would
achieve a device that would occupy about 1 inch cubed. This is a step in the right direction but is still quite large for the
purposes stated.
Figure 1
Flexure based systems;
By using various flexure devices to position the precision optical platform in the appropriate position the system can be quite
accurate. The system shown in figure 2 is repeatable to 0.1 μm but is still many orders of magnitude larger than its range. The
flexure based system meets most of the needs stated above except in an environment that is subject to vibration. Many times
secondary damping systems need to be introduced to stabilize the system. These type of devices are quite good in labartry
environments but due to size, expense, and “touchiness” they are not a practical solution for the real world.
Figure 2
The Friction Based system;
Using the fricative properties of two materials to hold the device in place against forces that would seek to dislodge it. This is
the system that will be explored at length in this project
Methods and Discussion
In the field of micro optics there are the same requirements for a precision optical platform. These requirements in general
are:
Stability;
the precision optical platform needs to stay exactly where it is put until an effort is made to
move it.
Repeatability;
the precision optical platform needs to be able to be moved and then moved back to the same
location within the range of motion throughout the life time of the system.
Orientation Independence;
many times it is unknown what orientation relative to the gravity field will be the final
orientation of the precision optical platform and the supporting structure and mechanism.
Small size;
just as there has been a revolution in microelectronics over the last quarter century there has
been a lesser revolution of micro optics. It is inadequate to have a 200 gram 5cm X 5cm X5 cm
cube supporting such small optics. It would be like tubes in your cell phone.
The friction based system is the system most closely analogous to the computer mouse example. There are many examples of
devices that are quite large when compared to the desired movement range. The scope of this project is to research possible
commercial solutions as well as various system concepts to achieve the desired outcome. By a friction based system the
meaning is a system in which the precision optical platform is held in place by friction and the normal force to that friction.
I will be using both the knowledge gained in Tribology, the design tools (Solidworks) and analysis tools (COSMOS, COMSOL,
Maple), to research and analyze a friction based precision optical platform. The goal is to define and designate the smallest
friction based precision optical platform available currently.
Conclusion
Bibliography
Current Art
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Newport Optics have a large variety of devices http://www.newport.com/Linear-TranslationStage-Guide/1006111/1033/content.aspx
This is a piezo system that actually has stability but to get X-Y movement the issue is
problematic (one dimension cannot support the other ) http://www.micromo.com/piezomotors.aspx
Faulhaber has been around for a number of years. Their ideas are good but their size and travel
are too great. http://www.youtube.com/user/faulhaberEN
Desirability
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From the National ignition Facility: https://lasers.llnl.gov/about/nif/seven_wonders.php
There are numerous small optical systems in this device
http://www.bizjournals.com/cincinnati/blog/2011/08/testing-out-krogers-new-tunnelscanner.html
Small optics were used in the shaping and metrology of the Hinode x-ray mirror .
http://solarb.msfc.nasa.gov/