june 2011 - minifaros.eu · usual technique has been to ... barcode scanners, laser print-ers,...
TRANSCRIPT
issue 3 June 2011
Editorial
Welcome to the MiniFaros EC funded project third
newsletter. MiniFaros is con-tinuing successfully its ac-tivities. The work performed
so far has been dissemi-nated for the first time to
the wider public through two major events in Europe: the ITS European Congress
that took place in Lyon on June 8-9, 2011 and the
AMAA Conference specializ-ing on Microsystems for Automotive applications.
Minifaros featured 3 papers in the Conference achieving
thus a very strong repre-sentation to that particular conference that took place
in Berlin on June 29-30, 2011.
In this Newsletter various articles containing among
others information on the project advancements that
were presented in the past Conferences as well as up-dates on the core research
items are included. More in-formation can be found on
the project website (www.minifaros.eu), while Minifaros has also a page on
Facebook as a supplemen-tary communication chan-
nel. Enjoy reading.
Editorial 1
TDC11 (Time-to-
Digital Converter) (J.
Kostamovaara) 2
Omnidirectional
lenses for low cost la-
ser scanners
(M. Aikio ) 3
MEMS mirror for low
cost laser scanners (U.
Hofmann) 4
News and Events 6
MiniFaros Consortium
7
Inside this issue:
TDC11 (Time-to-Digital Converter)
functionality and performance now
verified
One of the project goals is to develop a multi-channel
time-to-digital converter integrated circuit, which measures the time intervals
between the emitted laser pulse and several succes-
sive echoes related to the transmitted pulse (TSP1-TSP3 in Fig.1). Moreover, the de-
vice is to measure the widths of the received ech-
oes, which can then be used for the walk error compensation (TW1-TW3 in
Fig. 1). The timing walk (dependence of the timing
moment on the echo ampli-tude, see Fig. 1.) is the
main source of systematic error in pulsed time-of-
flight laser radars. In fact, the accurate multi-channel TDC techniques to be de-
veloped enable in principle the realization of new
“multiple-threshold time-domain” RF/high-speed op-tical pulse detection princi-
ples and circuits. The latter make it possible to detect
with picosecond accuracy the time position of the re-ceived pulse over a wide
dynamic amplitude range exceeding that of the re-
ceiver. It is also believed that the use of these tech-
June 2011
niques may result in reduc-tion in power consumption
and complexity relative to the levels available with tra-ditional high-speed synchro-
nous receiver sampling and AD conversion techniques.
This CMOS time-to-digital converter (TDC11) developed
by the University of Oulu team has now been realized
and tested with respect to the main performance pa-rameters. The main perform-
ance parameters of the TDC11 are its measurement
precision (sigma value of the distribution of the single shot measurement results for a
constant time interval), measurement accuracy and
drift. The measurement pre-cision of the developed de-vice is shown in Figure 2 and
demonstrates a single shot precision of better than 10ps.
The TDC11 is capable of measuring also “negative time intervals” (time inter-
vals where stop signal pre-ceeds the start signal, which
may well be the case in a practical laser radar at short
measurement distances due
to the electronic delay in the
start pulse gen-eration). The
accuracy is at the level of a few pico sec-
onds and partly limited by the
performance of the measure-ment arrangement.
The temperature drifts of the TDC11 with respect to the
start-stop time interval and stop pulse width are shown in Fig. 3 indicating a drift of
~0,3ps/°C and ~0,4ps/°C, respectively.
The measurement results ver-
ify the operation of the TDC in different circumstances with
the state-of-the art perform-ance. Time interval measure-
ment is stable with respect to
variations in temperature and operating voltage, and the low internal jitter in the
delay lines makes it possible to use a low frequency exter-
nal crystal as a reference. A measurement precision bet-ter than about 10ps is
achieved over the whole temperature range (C).
Figure 1: TDC11 measurement ap-
proach.
Figure 2: Single shot measurement precision
Figure 3: Start-stop and Stop pulse width drifts
of the TDC11 in relation to Temperature (C)
June 2011
There is a need for small sensors that provide 360-
degree field of view in intelli-gent vehicle applications. The usual technique has been to
use a catadioptric system where a conical shaped mir-
ror is placed in front of a camera, providing 360-degree horizontal field of
view and a few decades of degrees of vertical view. The
downside of these kinds of systems has been their size, usually ranging around 20
centimetres. A so-called om-nidirectional lens can fold the
optical path inside the lens decreasing the volume re-quirements considerably,
while still providing compara-tive optical performance. In
this work, two different om-nidirectional lens systems are presented, more common
type of this lens images a whole surrounding scenery to
an image sensor, providing instant 360-degree field of view. The other lens can se-
lect a known position from the 360-degree scenery, and
provide an undistorted image of it. The other application
for this type of lens is a laser scanner that necessitates di-rection selectivity.
The general objective of the
current Minifaros-project is to replace a large rotating mirror from laser scanners
with a MEMS mirror. Instead of imaging a whole scenery
reflected at the lens, a rotat-ing mirror is used to select a portion of the scenery to be
imaged on the sensor – or to be measured with a laser
scanner. This kind of lens is new and no prior art work has been published. The
working principle of the
lens is shown in Figure 4, and one manufactured
lens is shown in Figure 5. A biaxial laser scanner
consisting of two lenses as shown in Figure 5 was
constructed, and the per-formance was evaluated. The divergence of the
sensor was 30 milliradians with a detector of diame-
ter 200 µm. The signal to noise ratio allowed the us-age of the sensor up to 10
metres, with a black dif-fuse target. Expanding
the measurement distance from this is one of the objec-
tives in Minifaros project. Omnidirectional vision and
sensor systems are important in autonomous vehicle opera-
tions if the amount of sensors needs to be reduced. By using a large field of view sensor,
there is no need to have mul-tiple sensors in a vehicle.
However, one constraint on using them has been the size, manufacturing tolerances and
the price of the resulting sys-tem.
The type of omnidirectional lens presented just above al-
lows also imaging of the sur-rounding scenery without
distortion, if multiple expo-sures are taken and the ava-lanche photo diode is re-
placed with a small image sensor.
The second important factor to be considered is the price
of the sensor and related op-tics. The omnidirectional
lenses are roughly 40 to 50 millimetres in diameter and
are made of plastic to allow for easier serial production of this type of optics. In serial
production when the produc-tion volume approaches hun-
dreds of thousands of pieces per year, the price for a sin-gle omnidirectional lens is
around several cents. In Minifaros project, the omnidi-
rectional lens is used in a la-ser scanner application (LIDAR) to prevent and miti-
gate the consequences of ve-hicle accidents.
Omnidirectional lenses for low cost laser scanners
Figure 4: A sketch of an omnidirectional lens that
has a beam direction capa-bility .
Figure 5: A manufactured om-nidirectional lens which is
used in conjunction with a beam steering mirror.
June 2011
LIDAR sensors are becoming
increasingly interesting for
the realization and improve-
ment of driver assistance
systems like pre-crash safety
systems, intersection assis-
tant, lane change assistant,
blind spot assistant, parking
assistant or traffic jam assis-
tant. A wide angular range
and high angular resolution
are key-features that scan-
ning LIDAR systems offer.
Existing scanning LIDAR sys-
tems use bulky servo motors
for rotation of a large aper-
ture scanning mirror making
it difficult to demonstrate the
required sensor dimensions
and sensor costs for a series
automotive product. But cost
reduction and a higher level
of miniaturization seem to be
possible by introduction of
MEMS technology. The con-
cept and the design of a low
cost two-axis MEMS scanning
mirror that aims at replacing
the bulky and expensive con-
ventional laser scanner in an
automotive LIDAR sensor ap-
plication is presented.
The key feature of the low-
cost LIDAR sensor is an om-
nidirectional lens that inte-
grates several reflective and
refractive functions within
one single component like
the lens presented in the
previous article. Omnidirec-
tional scanning is achieved
by first collimating the diver-
gent laser beam by passing
the refractive centre area of
the omnidirectional lens. The
collimated beam then im-
pinges on a 2-axis MEMS
scanning mirror.
The tilted mirror reflects the
beam back to propagate
trough the lens again. After
passing two internal reflec-
tions at two reflective lens
facets the beam exits the om-
nidirectional lens almost per-
pendicular to the optical axis
of the incoming divergent la-
ser beam. According to the
cylindrical symmetry of the
overall configuration the laser
beam can be scanned within
the whole range of 360 de-
grees. The optical concept re-
quires a two-axis MEMS scan-
ning mirror which performs a
circular scan at a constant tilt
angle of 15 degrees resulting
in a cylinder symmetric optical
deflection of 30 degrees. In
order to enable a long meas-
urement range of up to 80
metres the optical configura-
tion requires a mirror diame-
ter of 7mm.
MEMS mirror design
MEMS scanning mirrors have
been used in many different
applications as for instance
barcode scanners, laser print-
ers, endoscopes, laser scan-
ning microscopes or laser pro-
jection displays. Typically
MEMS mirrors have a mirror
aperture size within the range
of 0.5 to 2 millimetres. There
are two major reasons for the
limitation of MEMS mirrors to
such small dimensions:
Firstly, static and dynamic
mirror deformations rapidly
increase with increasing mir-
ror diameter and secondly,
the very low driving forces of
MEMS actuators usually do
not allow a reasonable tilt
angle of high inertia mirrors.
Hence, to design and fabri-
cate a 2D-MEMS scanning
mirror with an outstanding
mirror size of 7 mm and a
large mechanical tilt angle of
+/-15 degrees is a challenge.
Static and dynamic mirror
deformation
The optical conception of the
LIDAR sensor requires that
deformation of the MEMS
mirror plate does not exceed
+/-500 nanometres. Defor-
mations can be caused by
stress gradients within the
layers which the mirror is be-
ing made of. Typically the
uppermost reflective layer
introduces mechanical stress
that deforms the mirror to
some extent. But more often
deformation is predominantly
caused by the MEMS mirror
dynamics. The dynamic mir-
ror deformation is known to
scale proportional to the fifth
power of mirror diameter.
This scaling law indicates
that to keep the deformation
of a mirror of 7 millimetres
and tilt angle of 15 degrees
sufficiently low the thickness
of the mirror needs to be
correctly adjusted. For a
more detailed investigation
on how different mirror ge-
ometries may effect the dy-
MEMS mirror for low cost laser scanners
June 2011
namic mirror deformation fi-
nite element analysis (FEA)
was carried out. Three differ-
ent types of mirrors were
simulated: 1) a mirror plate
having a standard thickness
of 80 microns (typical MEMS
device layer thickness), 2) a
mirror plate identical to first
type but additionally rein-
forced by a 500 micron thick
and 200 microns wide stiff-
ening ring underneath the
mirror plate, 3) a solid mirror
plate with a thickness of 580
microns. For each type of
mirror the simulation of mir-
ror deformation was per-
formed for four different di-
ameters (figure 6).
The FEA showed that a 7mm-
mirror with a standard thick-
ness of 80 microns would ex-
perience unacceptably large
deformations exceeding +/-6
microns. Considerable reduc-
tion of mirror deformation to
only +/-1.2 microns can be
achieved by a narrow but
500 microns thick reinforce-
ment ring underneath the
mirror. Finally a solid mirror
plate with a thickness of 580
microns achieved the best re-
sult and showed a minimized
mirror deformation of only +/-
0.2 microns. Thus, further de-
sign assessments and simula-
tions only considered the two
reinforced mirror types.
Driving concept and fabri-
cation process
In principle electromagnetic
actuation would enable to
achieve the highest driving
forces and hence would be the
first choice for actuation of
such a high inertia MEMS mir-
ror. But the attractiveness is
lowered by the fact that it
requires mounting of large
permanent magnets on chip
level resulting in a too large
and too expensive scanning
device. A compact and cost
effective solution is an elec-
trostatically driven MEMS
mirror since the whole device
can be produced completely
on wafer level including her-
metic packaging. Figure 7
shows a two-axes MEMS scan-
ning mirror electrostatically
actuated by stacked vertical
comb drives.
To drive such a large MEMS
mirror with an aperture size of
7millimetres to the required
large tilt angles of +/-15 de-
grees it is necessary to apply
resonant actuation because it
allows to achieve higher oscil-
lation amplitudes. However, if
the MEMS mirror works in
standard atmosphere damp-
ing by gas molecules is so
high that even resonant ac-
tuation is not sufficient to
achieve the required scan
angles. To meet the require-
ments of large mirror size
and large tilt angle it is nec-
essary to minimize damping.
This can be achieved by
packaging the 2D-MEMS
scanning on wafer level in a
miniature vacuum environ-
ment. This allows the MEMS
mirror to accumulate driving
energy over many thousand
oscillation cycles.
The low-cost LIDAR MEMS
scanning mirror will be fabri-
cated in a dual layer thick
polysilicon process. Wafer
bonding techniques will be
applied to permanently pro-
tect each MEMS mirror
against contamination by
particles, fluids or gases. A
titanium getter will be inte-
grated into each MEMS scan-
Figure 6: Calculated mir-ror deformation versus
mirror diameter for three different mirror geome-tries.
Figure 7: Typical gimbal-mounted two-axes MEMS
scanning mirror electro-statically driven by stacked vertical comb
drives
June 2011
ner cavity in order to achieve
a permanent miniature vac-
uum environment.
Suspension concept
The standard design to allow
a MEMS mirror to scan a la-
ser beam in two dimensions
is a gimbal mounted device.
But the optical concept of the
targeted low-cost LIDAR sen-
sor requires a circular scan
trajectory and the MEMS mir-
ror has to provide two per-
pendicular scan axes that
have identical scan fre-
quency. Practically, this is
difficult to be achieved using
a gimbal mounted mirror de-
sign. For that reason a com-
pletely different design was
chosen which eliminates the
need for an outer gimbal
frame. Instead of suspending
the mirror by two torsional
beams the mirror plate is
movably kept by three long
and circular bending beams.
This allows achieving an ad-
vantageous ratio of mirror di-
ameter and chip size which is
an important factor for a low
cost scanner. Because of a
considerably lower total mass
with respect to a gimbal mir-
ror design such a tripod de-
sign shows higher robustness.
Finite element analysis has
shown that mechanical stress
in the bending beams can be
kept sufficiently low to enable
the required tilt angle of 15
degrees. Regardless of the
three beams which are spa-
tially separated by angles of
120 degree the mirror builds
two perpendicular tilt axes
(two eigenmodes) that have
almost identical resonant fre-
quencies. In comparison with
a gimbal mounted mirror de-
sign the tripod approach
shows a considerably lower
number of parasitic eigen-
modes. Different variants of
such a tripod MEMS mirror
design will be fabricated cov-
ering a range of scan fre-
quency of 600Hz to 1.6kHz.
This scan frequency depends
on the stiffness of the three
suspensions and by the mo-
ment of inertia which is dif-
ferent for a solid reinforced
mirror and for the ring rein-
forced mirror. The whole 360
degree scenery is thus
scanned at a rate of 600Hz
or higher.
News and Events
Minifaros managed to participate in two major conferences this period, initiating thus success-
fully the dissemination of its mid-term results.
ITS in Europe, Lyon France, June 8-9, 2011
Minifaros was represented by Florian Ahlers (SICK) to the special session “SS 42 / Avoiding ac-
cidents by enhanced perception and active interventions: a look into the future of intelligent
vehicles ” organized jointly by the IP interactIVe and
Minifaros. A presentation about the novel laser scan-
ners and its applications
AMAA 2011—15th International Forum on
Advanced Microsystems for Automotive
Applications
Minifaros had a strong presence featuring 4 papers ac-
companied by the respective presentations. Presenta-
tions were very attractive to the audience consisting of
key stakeholders from the automotive companies and
suppliers.