Optics: measuring and testing – Velocity or velocity/height measuring – With light detector
Reexamination Certificate
2001-09-28
2002-12-10
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
Velocity or velocity/height measuring
With light detector
C250S23700G
Reexamination Certificate
active
06493068
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to optical flow measurement or computation and, more particularly, to an optic flow sensor incorporating an improved photoreceptor array.
2. Description of the Related Art
As set forth in my earlier U.S. Pat. No. 6,020,953, the subject matter of which is incorporated herein by reference, the term “optical flow” generally refers to the motion of texture seen by an agent (such as an animal or a robot) as a result of relative motion between the agent and other objects in the environment. It is well known that animals, especially insects, use information from the optical flow for depth perception and to move about in an environment without colliding into obstacles. Robotics and machine vision researchers have borrowed from these ideas in biology to build matching vision systems that successfully use optical flow for depth perception and obstacle avoidance. These successes verify that optical flow can indeed be used for depth perception and obstacle avoidance in real systems. In a “neuromorphic” approach, computations are performed with analog or mixed-mode circuitry which exploit the physical dynamics inherent in VLSI circuitry and often mimic biological structures.
In the above-identified patent, there is disclosed a feature tracking linear optic flow sensor which includes photoreceptor array which responds to light from the visual field focussed thereon. In my U.S. Pat. No. 6,194,695 B1, the subject matter of which is also incorporated by reference, a number of different ways are discussed of implementing a photoreceptor array for linear optic flow measurement. By using a combination of electronics and optics, the individual photoreceptors have visual field response functions that are shaped like a fuzzy elongated rectangle. These photoreceptors are also arranged in a linear array so that the direction of the array (the sensor orientation vector or SOV) is perpendicular to the long axis of that rectangle. The patent discusses two general ways in which the elongated photoreceptor receptive field can be created. The first way is to use a phototransistor (photoreceptor) the active area of which is rectangular. In this embodiment, the photoreceptors of the array are located on a focal plane chip. A lens or pinhole focuses an image of the environment or visual field onto the focal plane chip. Thus, the photoreceptors sample a rectangular section of the image, and, therefore, of the visual field. This rectangular response function can be made slightly blurry through the use of optical smoothing by either placing the lens slightly out of focus or by using an iris with a graded transmission function.
The second way to achieve an elongated rectangular photoreceptor receptive function is to use photoreceptors the active areas of which are substantially point-like and to also use an iris with an elongated rectangular shape. With such an arrangement, all the light striking the photoreceptor will be from an elongated rectangular part of the visual field.
Each of these two ways of forming elongated photoreceptor receptive fields has a potential shortcoming. The main shortcoming of the first pinhole-camera version is that a very small amount of light strikes the photoreceptor. This results in a very small current flowing through the photoreceptor. Because of this, the parasitic capacitance between the photoreceptor (phototransistor) and ground takes more time to charge or discharge. This effectively slows down the photoreceptor so that only slowly changing textures can be detected. Faster intensity changes due to faster optic flow are merely filtered out. This same effect is observable in the second version of the sensor in which point photoreceptors are used with rectangular irises. However, the effective cutoff frequency is higher because more light is let in by the rectangular iris and less parasitic capacitance needs to be overcome in the phototransistors, which are physically smaller.
Considering the above-mentioned shortcoming in a more rigorous fashion, reference is made to
FIGS. 1 and 2
wherein
FIG. 1
shows a minimal photoreceptor circuit for analysis and
FIG. 2
shows a linearized version of the photoreceptor circuit for AC analysis. As shown in
FIG. 1
, the basic photoreceptor circuit
10
consists of a PNP phototransistor
12
and a diode-connected MOSFET
14
. The phototransistor
12
connects to ground (or the substrate) an amount of current proportional to the total light striking its surface. The diode-connected MOSFET
14
converts the photoreceptor current into a voltage. Typically MOSFET
14
is wide enough so that it is biased in the subthreshold region. When a diode-connected MOSFET is in the subthreshold region, the current flowing through it is of the form
I
diode
=I
s
exp(
kV
gs
) (Eq. 1)
where I
s
is dependent on the MOSFET geometry (and fabrication process), V
gs
is the gate to source voltage, and k is dependent on the fabrication process. Thus, the voltage drop across MOSFET
14
is a logarithmic function of the phototransistor current. Typical k values are on the order of 50-100 mV. Accordingly, as a result of the logarithmic compression, several orders of magnitude of light intensity can be handled by the photoreceptor circuit
10
.
As indicated above,
FIG. 2
shows a linearized version of the photoreceptor circuit for a specific operating phototransistor current loop. This linearization is performed in exactly the same manner as is used to analyze simple transistor amplifier circuits. The phototransistor is modeled as a current source I
in
in parallel with a capacitance C
p
. The current source i
in
represents or refers to the deviation in current from the operating point current I
diode
. The capacitance C
p
represents or refers to the parasitic capacitance between the phototransistor and the substrate. The diode-connected MOSFET
14
can be reduced to a conductance g
m
, since the gate is connected to the drain. This transconductance g
m
is computed from Eq. 1 by linearizing about the operating point current I
diode
as follows:
g
m
≡
dI
diode
dV
gs
=
I
s
k
exp
(
kV
gs
)
g
m
=
I
s
k
exp
(
k
1
k
ln
I
diode
I
s
)
(
by
substituting
in
Eq
.1
solved
for
V
gs
)
g
m
=
I
s
k
exp
(
ln
I
diode
I
s
)
=
I
s
k
I
diode
I
s
=
kI
diode
(
Eq
.
2
)
Thus, it will be seen that the transconductance g
m
is proportional to the operating point current I
diode
Parasitic capacitances in MOSFET
14
are neglected here, but if included would be incorporated into C
p
.
The bandwidth of the photoreceptor circuit is determined by the conductance g
m
and capacitance C
p
, which together form a simple RC low-pass filter of cutoff frequency:
f
c
=
g
m
2
π
C
p
.
(
Eq
.
3
)
Eq. 2 shows that the conductance g
m
increases for higher light levels. Thus the cutoff frequency of photoreceptor circuit is higher for higher light levels than for low light levels. For light levels associated with pinhole cameras, the cutoff frequency can be on the order of a fraction of a Hertz. It is noted that according to the model under consideration, the cutoff frequency is independent of photoreceptor area. If the photoreceptor area is doubled, then g
m
and C
p
both double, and thus the cutoff frequency remains constant.
The cutoff frequency is much higher for photoreceptor versions or embodiments using a lens to focus an image of the environment onto the focal plane. This is because a lens gathers a large amount of light and focuses it onto the photoreceptors. Thus the value of g
m
is increased by up to several orders of magnitude, which increases the bandwidth by an equivalent amount. The main disadvantage of this approach is that a lens is required to gather
Buczinski Stephen C.
Mills, III John G.
The United States of America as represented by the Secretary of
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