Two Uni-axial Scanning Mirrors Vs One Bi-axial Scanning Mirror
Laser deflection module
Laser projectors include a deflection module which steers a laser light beam, across two
axes, at high frequencies.
The horizontal scan dimension is implemented by using a micro-mirror coupled to a
spring. The micro-mirror and spring are designed so that their natural resonance
frequency is in accordance with the required horizontal scan rate. An actuator is
used to keep the mirror moving in a constant harmonic motion.
The slower vertical scan frequency could be similarly implemented. The
perpendicular deflection axis can be actuated on linear steps or on a harmonic
resonative motion mode.
Laser deflection mechanism
Two laser deflection mechanism designs are known today:
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A Bi-Axial mirror – a single mirror tilting in orthogonal axes using a mechanism of
two perpendicular gimbals. |
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A set of two Uni-Axial mirrors – a pair of mirrors, each tilting on one axis and
configured to generate the movement in two orthogonal dimensions. |
Mirror's actuation
These mirror elements, MEMS based, might be actuated linearly or into their resonance
state by four types of actuators – Thermal, Piezo-electric, Electrostatic and
Electromagnetic. Only the last two enable sufficient displacement to achieve the proper
resolution.
An Electromagnetic actuator consumes significant current. It consists of a miniature coil
engine which requires the add-on of the coil micro element, its interconnection and
more – these obviously impact on its weight, size, heat dissipation and require delicate
as well as expensive process' workmanship.
The Electrostatic actuator requires an electric field. It is manufactured in a single
process, together with the mirror on the same wafer (without any further extra weight or
substrate's add-ons).
The preferred electrostatic engine
The electrostatic engine is the preferred actuation mechanism for such tiny units due to
its low power consumption (minimal current), no add-on weight or inertia implications, low
heat dissipation implications as well as due to its simple production process and cost.
However, it can not be fully exploited in the Bi-Axial design.
In a Bi-Axial design, an electrostatic actuator is used for the critical fast moving internal
axis only, since the externally slow engine has to bear the physical dimensions (size and
weight) of the 'internal' fast axis engine.
In most cases, the requirements from the external engine rule out the selection of such
an electrostatic actuation mechanism for the other axis as well. This constraint leads to
the usage of the electro magnetic alternative which is heavier, uses more power,
dissipates more heat, and is more costly.
The Uni-Axial mirrors mechanism simplifies the requirements from the actuation
engines and enables the deployment of electrostatic engine on both axes – positively
impacting both on its production yield, cost and projection qualities.
Image resolution
and spot size
Image resolution is an important user requirement with respect to the final image quality
obtained from a projector. It defines the required minimum pixel size at the projected
screen.
The effective laser spot size on the screen, and the related effective resolution of the
projector, depends mainly on the scanning mirrors' size and the maximum
tilting/scanning angles:
Scan angle - Larger scan angle accommodates higher resolution.
Mirror size - Larger mirror accommodates larger beam diameter which allows the
generation of smaller pixels on the screen.
Any required change of the mirror diameter on the Bi-Axial approach is by far more difficult to implement due to its
immediate implication on weight and production complexity.
Higher flatness
enables higher
resolution
The flatness of a mirror is achieved more easily on two distinctive elements (Uni-Axial
mirrors) rather than on a bi-axial mirror. Higher flatness enables the generation of a
smaller laser screen spot/ beam waist thus obviously allows higher resolution at higher
production yields at a lower cost.
Mirror size and fluctuation in production of Bi-Axial mirrors might cause mechanical
distortions which necessarily affect both axes – without taking into account the
additional assembly required and the add-on mounting, which makes its manufacturing
even more complicated.
The simplified separate production, of the two mirror elements, secures better yields
than one Bi-Axial mirror.
There are strict requirements for the mirror's flatness since it directly affects its reflective
qualities – surface deformation/aberration results in image blurring and lower contrast.
Static and
dynamic
deformations:
There are two types of mirror's flatness deformations: static and dynamic. The static
deformation's source is due to the production process and the original raw wafer quality
while the dynamic deformation results from the mirror tilting movement and its
mechanical structure and parameters. A sufficiently flat static mirror plane might
become severely deformed in its dynamic mode, due to mechanical fluttering, while
turned on.
Cross-axial dynamic effects (in the Bi-Axial mirror mechanism) are thought to induce
additional mirror plane deformations, due to the obvious correlation of the two axes.
The dynamic rigidity of the rotating plane increases with its decreasing size, however
the image's resolution requires an increase of the mirror's size.
As with the actuation related consideration, also from production yield aspects, this
mirror size design conflict is best addressed by simplifying the reflective mechanism and
optimizing each of the reflective axis separately – each with its own Uni-Axial mirror.
'Pin cushion'
effect
The two scanning mirrors approaches (Bi-Axial Vs two Uni-Axial) also differ with their
sensitivity to the 'Pin cushion' effect.
The Pin Cushion effect is caused due to a spherical projection onto a plane surface,
which results in curved lines in both axes.
In the case of the two Uni-Axial mirrors design, the two single axis mirrors are arranged
so that the horizontally drawn lines are straight. The vertical Pin Cushion effect is
corrected in this direction by modulating the lasers accordingly. The laser source is
simply turned off at the right and left sides of the projected image's line.
In comparison, handling the Pin Cushion effect on both the horizontal and vertical axes
generated by the Bi-Axial mirror design might require a much higher reduction of the
effective display screen. This would be done by blocking the distorted areas along all
the four edges, in both axes, thus wasting available projection power.
Summary
Taking into account actuation methods and consequences, the need for high
resolutions, production yields, optical aspects as well as projection power efficiencies, it
is clear that the two Uni-Axial scanning mirrors approach presents the optimal solution
for mobile personal projectors. On the other hand the simplified optical design of a
single Bi-Axial mirror results in bow-tie distortions and costs which are difficult to reduce
over shed its 'simplicity'.
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