U.S. patent application number 12/497385 was filed with the patent office on 2011-01-06 for camera-based heliostat tracking controller.
Invention is credited to DAN REZNIK.
Application Number | 20110000478 12/497385 |
Document ID | / |
Family ID | 43411955 |
Filed Date | 2011-01-06 |
United States Patent
Application |
20110000478 |
Kind Code |
A1 |
REZNIK; DAN |
January 6, 2011 |
CAMERA-BASED HELIOSTAT TRACKING CONTROLLER
Abstract
Systems and methods for a heliostat directing incident sun light
to a receiver based on an estimate or predicted receiver location
and an imaged sun location
Inventors: |
REZNIK; DAN; (New York,
NY) |
Correspondence
Address: |
MICHAEL BLAINE BROOKS, P.C.
P.O. BOX 1630
SIMI VALLEY
CA
93062-1630
US
|
Family ID: |
43411955 |
Appl. No.: |
12/497385 |
Filed: |
July 2, 2009 |
Current U.S.
Class: |
126/574 ;
126/576; 126/578 |
Current CPC
Class: |
F24S 23/77 20180501;
F24S 50/80 20180501; G01S 3/7861 20130101; F24S 30/452 20180501;
F24S 50/20 20180501; Y02E 10/47 20130101 |
Class at
Publication: |
126/574 ;
126/578; 126/576 |
International
Class: |
F24J 2/38 20060101
F24J002/38 |
Claims
1. A system for directing incident radiation from a source to a
target, the system comprising: a reflector for reflecting the
incident radiation, the reflector having an optical axis and at
least one angle of rotation; an imager attached to the reflector; a
tracking controller coupled to the imager; and one or more
actuators connected to the reflector and in communication with a
tracking controller; wherein the tracking controller comprises a
processor configured to: i) receive image data from the imager; ii)
detect a source projection onto the image sensor iii) determine an
estimated target location relative to the imager based on
predetermined target locations and at least one actuator position;
and iv) generate one or more actuator commands based on the
received image data and the estimated target location.
2. The system of claim 1, wherein the processor is further
configured to receive at least one of: a first actuator position
and a second actuator position; and determine an estimated target
location on the image sensor based on a set of predetermined target
locations and at least one received actuator position.
3. The system of claim 1 wherein the imager comprises a camera
having a single neutral density lens filter.
4. A heliostat for directing incident light to a receiver, the
heliostat comprising: a mirror for reflecting the incident light,
the mirror having an optical axis substantially perpendicular to
the mirror and at least one angle of rotation; an imager mounted to
the mirror; and a tracking controller in communication with the
imager; and one or more actuators in communication with the
tracking controller; wherein the tracking controller comprises a
processor configured to: i) locate one or more image points
corresponding to the center of the sun's projection on the imager;
ii) estimate at least one point on the imager corresponding to the
receiver based on at least one actuator position; and iii) generate
one or more actuator commands to at least one reflector angle
actuator to dispose the optical axis of the mirror between the one
or more image points corresponding to the sun based on the image
data from the imager and the at least one point corresponding to
the receiver based on at least one actuator position.
5. The system of claim 4 wherein the imager comprises a camera
having a single neutral density lens filter.
6. The heliostat of claim 4 wherein the tracking controller is
further configured to: receive at least one actuator motor
position; and determine an estimated target location based on a set
of predetermined target locations and the received at least one
actuator position.
7. A method of tracking the sun with a heliostat comprising an
imager mounted to a mirror, the method comprising: determining one
or more image points corresponding to the sun in a captured image;
estimating one or more image points corresponding to a receiver
based on a least one actuator position; actuating the mirror in one
or more angular directions to improve the antipodal arrangement of
the determined one or more image points corresponding to the sun
and the estimated one or more image points corresponding to the
receiver.
8. The method of claim 7 wherein the step of actuating the mirror
further comprises at least one command based in part on a search
based on a manhattan method.
9. The method of claim 7 further comprising: generating a lookup
table based on captured image points of a receiver and at least one
actuator position.
10. The method of claim 7 wherein the step of estimating one or
more image points corresponding to a receiver is based on a least
one actuator position and the generated lookup table based on
captured image points of a receiver and the at least one actuator
position.
Description
TECHNICAL FIELD
[0001] The invention generally relates to a technique for
configuring a heliostat to continuously reflect the sun onto a
desired target, called "tracking." In particular, the invention
relates to a system and method using a two dimensional imager
disposed on the heliostat to aim a mirror or other optical element
so as to continuously reflect the sun onto the aperture of a solar
receiver.
BACKGROUND
[0002] In some solar thermal power plants, numerous heliostats may
be employed to reflect light onto one or more receiver apertures.
The mirrors of each of the heliostats must be continually
repositioned in order to account for the relative motion of the
sun. Mirror orientation errors must be exceedingly small to
minimize spillage losses and achieve high concentration at the
receiver aperture.
[0003] An array of heliostats with two degrees of freedom, e.g.,
azimuth-elevation, or tilt-tilt, may be applied in power-tower
applications where the reflector or mirror of the heliostat may be
characterized as having an average or center directional vector
normal to the plane of a reflector. As part of a typical daylight
operation, each heliostat may be commanded, e.g., by an external
controller, to continuously reflect the sun onto a tower-mounted
receiver aperture. This may be achieved when said mirror normal
bisects the sun direction vector and receiver direction vector.
However, this requires a knowledge of sun and receiver positions,
e.g., in a fixed global coordinate system, and the forward
kinematic map which converts motor positions to mirror normal, in
the said coordinate system. The forward kinematic map will be a
function of both rigid body and internal mechanical parameters of
the heliostats, which in turn depend on the precise way a heliostat
is installed and/or manufactured. For example, an azimuth axis of
revolution may not be perfectly vertical, or the two axes of
rotation of a heliostat may not be perfectly perpendicular.
[0004] An open-loop system, such as the system described in U.S.
Pat. No. 4,564,275, to Stone, titled "Automatic Heliostat Track
Alignment Method," may attempt to first estimate forward kinematic
parameters of motor-to-normal mapping by a calibration phase, and
then use those estimates and the known sun and receiver positions,
e.g., via inverse kinematics, to achieve the bisection required for
tracking. On the other hand, a closed-loop system may or may not
include calibration to account for mounting and internal parameters
of the heliostat while it attempts to establish, continually or
continuously, a bisecting orientation via feedback. For example, a
prior art closed-loop method has been described which estimates the
output mirror orientation in real time, using an external camera
and computer vision algorithms, via a photogrammetric method. See
for example "Fast Determination of Heliostat Shape and Orientation
by Edge Detection and Photogrammetry," by M. Roger et al.,
SolarPACES, 2008.
[0005] A closed-loop heliostat control system may include a camera
that may be mounted rigidly to the heliostat mirror, with optical
axis of the camera substantially aligned with the mirror normal.
International Patent Application No. WO 2008/121335 A1 describes
how such a setup may be used for closed-loop sun tracking control
of the heliostat, namely, that from the camera's point of view, at
the bisecting orientation, both sun and receiver (imaged as blobs)
would appear symmetrically on the image sensor, and deviations
thereof could be corrected in closed-loop to maintain the tracking
orientation. In that system, the image sensor is required to image
both sun and receiver simultaneously as compact blobs; due to the
difference in brightness of said features, in one embodiment the
prior art describes a special type of split-filter able to
mask/attenuate a sub-region of the field of view, with a possible
rotating degree of freedom to address the sun's changing location
on the image sensor; another embodiment describes a Liquid Crystal
Diode (LCD)-shutter device which emulates in function to that of
the split-filter.
SUMMARY
[0006] The invention in one embodiment features a system for
directing incident radiation from the sun to a target. The system
includes a reflector for reflecting the sun's incident radiation;
an imager connected to the reflector, the imager having an aperture
(such as a pinhole or lens) and an imaging plane; a tracking
controller coupled to the imager; and one or more actuators
connected to the reflector and tracking controller. The tracking
controller is configured to receive image data from the imager;
determine a bisection error based on the image data; and orient the
reflector till the bisection property is achieved, and in general,
to preserve bisection continuously. The reflector may be a mirror
that redirects sunlight to a receiver based on image data from a
pinhole camera or other digital imager. In general, the optical
axis of the camera is substantially aligned with the vector normal
to the reflective surface so that the sun and the receiver appear
at antipodal points with respect to the center of the camera's
field of view. To increase tracking accuracy, however, the tracking
controller in some embodiments orients the mirror based on a
calibrated reference point that compensates for the deviation
between the mirror normal vector and the optical axis of the
camera, e.g., from a toleranced camera-to-mirror mounting process.
In this configuration, the mirror normal substantially bisects the
receiver and sun direction vectors with the receiver and sun imaged
at substantially antipodal positions with respect to a center or
reference origin (as determined by calibration) of the image
sensor. By orienting the mirror to maintain the antipodal
relationship of the sun and receiver, the heliostat may effectively
track the sun in closed-loop and in spite of largely unknown or
ignored global geometric information. Because each heliostat is
able to generate its own bisection error signal (e.g., deviation
from antipodality), they may independently execute tracking
operations with their own embedded tracking controller.
[0007] Embodiments of the present invention include control systems
and methods for continuously reflecting the sun onto a desired
target, such as a solar receiver aperture. In some embodiments, a
two-degree of freedom heliostat may include a camera rigidly
mounted to the mirror, with optical axis substantially aligned with
the heliostat normal. The camera may include a single
light-attenuating filter, e.g., a single neutral density lens
filter, and further include programmable gain and exposure
controls. The heliostat may include an actuator having an absolute
position that can be inferred from step counts with respect to a
home position by the controller and/or a relative or absolute
displacement encoder. The heliostat may include a controller
configured to receive and process image from the camera and set
gain and exposure controls of camera. The processor may be
configured to execute instructions to correlate receiver points,
spots or blobs captured by the camera with the two degrees of
freedom of the heliostat and input the correlated data into a
two-way table look-up. The processor may be further configured to
generate actuator commands based on the sun points, spots, or blobs
captured by the camera and based on the estimated receiver position
based on the interpolated position estimated of a two-way table
look-up.
[0008] An exemplary system of the present invention includes a
heliostat supporting a flat or curved mirror that may be oriented
along two angular degrees of freedom. The mirror's orientation may
be controlled by commands issued by an individual (on-board
microcontroller) or a remote central controller. Each degree of
freedom may be actuated by a motor whose position relative to an
origin or mechanical stop is known, e.g., counting the steps of a
stepper motor with respect to a home or zero position, or using a
relative or absolute encoder. Each heliostat of an array of
heliostats may include onboard memory storage, which may be always
powered, e.g., by on-board batteries, and/or a photovoltaic cell,
or may be otherwise non-volatile. In some embodiments, a heliostat
may be wired to a central controller and database, receiving power
from said sources. In some embodiments of the heliostat system and
method, an array of heliostats may be ground-mounted or otherwise
stationary, and disposed about a power tower having a tower-mounted
receiver and an aperture. The heliostat mirror is orientable along
two degrees of freedom, and it comprises an image sensor (IS),
e.g., a camera, which may be mounted rigidly to the heliostat
mirror. The camera may be disposed over the mirror or behind it. If
the camera is disposed behind the reflector, the reflector
preferably includes a viewing hole or light-conducting aperture.
The camera may be installed such that its optical axis is
substantially aligned with the mirror normal. If the mirror is
curved, the camera may be preferentially mounted at the mirror's
center or a substantially fixed point relative to rotational
motion, with optical axis along the normal at the mounting point.
In addition, the heliostat may include a filter, such as a single
neutral density filter, mounted over or integrated with the
camera's objective lens.
[0009] In some embodiments, the invention comprises a method of
tracking the sun with a heliostat, the heliostat including an
imager mounted, or otherwise disposed proximate to a mirror. The
method includes: (a) locating image features corresponding to the
sun in a captured image; (b) estimating one or more image locations
corresponding to a receiver based on a least one heliostat tilt
angle; (c) actuating the mirror toward an orientation at which the
vector normal to the mirror bisects the sun and the receiver
direction, i.e., said image feature and image location appear
substantially antipodal to the center of the image sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The present invention is illustrated by way of example and
not limitation in the figures of the accompanying drawings, and in
which:
[0011] FIG. 1A is a diagrammatic illustration of one of a plurality
of heliostats and a receiver for collecting and converting solar
energy, in accordance with one exemplary embodiment;
[0012] FIG. 1B is a diagrammatic illustration of exemplary rotation
angles of a heliostat;
[0013] FIG. 1C is a diagrammatic illustration of an exemplary image
sensor reference frame of a heliostat;
[0014] FIG. 1D is a diagrammatic illustration of a heliostat in
relation to an illuminated tower-mounted receiver;
[0015] FIG. 1E is a diagrammatic illustration of a table of
illuminated receiver location values recorded according to a
heliostat raster scan in the direction of the illuminated
receiver;
[0016] FIG. 2 is a cross-sectional view of a heliostat mirror with
integral imager, in accordance with one exemplary embodiment;
[0017] FIG. 3A is diagrammatic illustration of an image of the sun
and receiver acquired by the camera when properly aligned between
the sun and receiver, in accordance with one exemplary
embodiment;
[0018] FIGS. 3B through 3C are diagrammatic illustrations of an
image of the sun and receiver at various stages of misalignment of
the mirror normal vector with respect to the sun and receiver, in
accordance with one exemplary embodiment;
[0019] FIG. 4A is an exemplary functional block diagram of a system
embodiment of the present invention;
[0020] FIG. 4B is an exemplary block diagram of the building and
use of a table lookup of illuminated receiver locations;
[0021] FIG. 5 illustrates in a diagram the antipodal
regulation/solar tracking by a heliostat based on an estimated
location of a receiver;
[0022] FIGS. 6A, 6B, and 6C illustrate in a diagram antipodal
regulation/solar tracking by a heliostat based on an estimated
location of a receiver in the frame of the image sensor; and
[0023] FIG. 7 is a flowchart an exemplary method for tracking the
sun using the imager and corresponding tracking controller, in
accordance with one exemplary embodiment.
DETAILED DESCRIPTION
[0024] Illustrated in FIG. 1A is a diagrammatic representation of
one of a plurality of heliostats and a receiver for collecting and
converting solar energy 100 in what may be referred to as a solar
power plant that may be thermal or electric--depending on the type
of receiver. The heliostat 110 may be configured to track the sun
over the course of a day and reflect the incident light to the
receiver 150 where it may be converted to heat or electricity. The
heliostat includes a mirror 112, actuator and support structure
assembly 114 for changing the orientation of the mirror, and may
include a tracking controller to determine the appropriate
direction to aim the mirror and send commands to the appropriate
actuator. Throughout the day, the orientation of each mirror is
periodically adjusted about two degrees of freedom, e.g., two tilt
angles or an ordered pair of angles: an azimuth angle and an
elevation angle, to continually reflect light onto the receiver
150. The heliostat is preferably one of a plurality of heliostats
distributed in proximity to a tower 152 or other structure on which
the receiver 150 is mounted. The receiver 150 may include a
water-steam boiler, a molten salt system, a heat engine, one or
more photovoltaic cells, a biomass cooker, a water purification
system, or a combination thereof for generating electricity or
otherwise collecting the sun's energy.
[0025] The heliostat 110 tracks the sun or other radiation source
based on image data received from a two-dimensional imager 116,
e.g., a digital camera, rigidly attached or otherwise integrally
incorporated into the mirror of the heliostat. The camera 116
captures an image of the sun which may then be communicated to a
tracking controller. Under appropriate filtering and gain settings,
the sun will be imaged as a compact set of pixels--a "blob" on the
image sensor, and easily detectable by the tracking controller.
Based on an interpolated database lookup, the tracking controller
can estimate the location of the receiver, for example, in a planar
camera reference frame (e.g., Cartesian or polar), based sensed, or
commanded reflector orientation. On the prior art, e.g.,
International Patent Application No. WO 2008/121335 A1, the
receiver is assumed imageable simultaneously with the sun, e.g.,
via a split filter. Here the sun is presumed much brighter than the
receiver at all times so that under appropriate filtering, the
latter may not be imageable at all. The tracking controller may
then aim the mirror in the direction necessary to reflect incident
light onto the receiver or other target. To accomplish this, the
controller moves the mirror to an orientation where both sun and
estimated receiver locations on the image sensor are substantially
antipodal with respect to the center or an optionally calibrated
reference point in the sensor. This reference point--which
corresponds to the projection of the mirror normal vector onto the
imager at the aperture--represents the deviation between the
imager's optical axis and the mirror normal vector. As the sun
moves across the sky, the camera detects the shift in sun position
and drives the actuator system until the antipodal relationship
with respect to the estimated receiver position is re-established,
thereby providing a closed-loop tracking system. Because the sun
will be typically imaged as a blob, or a compact set of pixels, the
blob's centroid might be used to determine perfect antipodality.
Although the sun and estimate of the receiver appear precisely
antipodal if the normal to the imager plane coincides with the
mirror normal, it may be necessary to correct for any angular
offset between the normal to the imager plane N' and the mirror
normal N, e.g., due to installation misalignment, which causes the
calibrated reference point to shift away from the midpoint between
the sun and receiver bright spots even when the mirror normal
exactly bisects the angle between incident sunlight vector S and
the estimated receiver vector R.
[0026] Illustrated in FIG. 1B is a diagrammatic representation of a
heliostat 110 showing a rotation in the plane of the local level as
a rotation in azimuth (AZ) and a rotation perpendicular to the
local level and ordered after the azimuth rotation as a rotation in
elevation (EL). The two angular directions may be expressed in an
unordered pair of rotation angles without loss of generalization
for purposes of embodying the present invention.
[0027] Illustrated in FIG. 1C is a diagrammatic representation a
heliostat 110 showing an imaging array 160 (not to scale) having a
Cartesian, e.g., X-Y, reference frame by which a sensed point 172
of incident light 170 may be registered. Knowledge of the rotation
of said XY plane about the mirror normal is not relevant to the
present invention, but "X" may be generally aligned with the
horizontal, while "Y" is perpendicular to it along the mirror.
[0028] Illustrated in FIG. 1D is a diagrammatic representation a
heliostat 110 and a receiver 150 mounted on a tower 152 where a
light source 190 is disposed on the receiver 150 and sensed by the
image sensor 160 (not to scale) disposed proximate to the reflector
of the heliostat 110.
[0029] Illustrated in FIG. 1E is a diagrammatic representation of a
table of image sensor (X,Y) pairs that may be recorded as part of a
raster scan by the heliostat 110 relative to the illuminated
receiver 150 of FIG. 1D.
[0030] A cross-sectional view of the exemplary camera-based
tracking system is illustrated in FIG. 2. The tracking system
includes the imager 116, e.g., a narrow aperture, camera, optically
mounted to the back of the mirror 220 where it faces outward in a
direction between the source and target. The camera-based tracking
system may further include integrated control logic, i.e., a
tracking controller 226, for computing the tracking error and
driving the actuator assembly that orients the mirror. The imager
116 preferably has a view of the source and target by means of a
small aperture 223 in the mirror to admit light. The aperture 223
may be an actual opening or a section of glass where the reflective
metallization 222 has been removed by laser etching or machining,
for example. In some embodiments, a thin filter plate 224 is
mounted between the mirror 220 and camera 116 to suppress the
lateral spread of light and increase source/target image
resolution. In other embodiments, the camera is mounted to an edge
of the mirror, on the front face of the mirror, or cantilevered off
to the side of the mirror, for example. Suitable imagers include a
1/6 inch format CIF (352.times.288) or VGA (640.times.480), or
higher resolution, preferably small enough that it does not need a
large filter plate. The resolution of the imager need only be high
enough so that the center of the sun blob may be localized
accurately enough to allow the control logic to determine a smooth
path on which to actuate the mirror and achieve a light-reflection
accuracy required by the application (as may be dictated by
spillage and concentration requirements).
[0031] During assembly of the heliostat 110 before final assembly
of the imager 116 on the mirror 220, the imager may be positioned
on the mirror by (a) aligning the optical axis of the
camera--represented by normal vector 240--to the mirror's normal
vector perpendicular to the mirror plane, and (b) aligning the
optical axis of the camera with the center of the pinhole aperture.
The optical axis of the camera need not be precisely aligned with
the mirror normal vector since the deviation there between may be
determined and compensated using a calibration process. After
proper placement of the imager 116, the imager and tracking
controller 226 may be encapsulated with epoxy 228, potting
compound, or other sealant to hermetically seal the electronics and
camera behind the mirror, thereby protecting them from
environmental damage. The aperture may be filled with an optical
coupling agent to prevent an air gap from occurring between the
mirror glass 220 and filter plate 224. Before normal operation of
the heliostat, the precise position and orientation of the imager
with respect to the mirror may be determined and the calibrated
reference point may be uploaded to non-volatile memory in the
tracking controller.
[0032] During heliostat tracking operation, the camera 116
identifies the center of the sun's image on the sensor while the
tracking controller 226 predicts or estimates the location of the
receiver 150, which is presumably not imaged in the sensor due to a
much lower brightness. Given appropriate filtering, the sun is
easily identified as the brightest (or only) blob in the image. For
ease of processing, the image data may be thesholded into a binary
one; alternatively, the sensor may be gray-level (allowing for
sub-pixel center estimation) or black-and-white. Although the
receiver 150 may become bright when fully illuminated by a field of
heliostats, it will still be imaged faintly (if at all) on the
image sensor under a suitable filter. In the present invention, the
receiver location on the image sensor is estimated based on motor
position counts and a table of correlated receiver positions. Once
the tracking controller 226 identifies the locations on the image
sensor representing the sun and receiver, it updates the mirror's
elevation and/or azimuth angles so that the sun and receiver appear
symmetrically on the sensor with respect to its center, thus
placing the mirror in a bisecting orientation.
[0033] Illustrated in FIGS. 3A through 3C are diagrammatic
illustrations of the locations 300 of sun and receiver (if it were
visible) on the camera sensor at various stages of tracking or
misalignment. When the mirror is properly aligned as shown in FIG.
3A, the reference point or calibrated reference point 310 coincides
with the mid-point on the line 340 between the image of the sun 320
and estimated receiver point 330. The images of the sun 320 and
estimated receiver center 330 are therefore shown substantially
antipodal around the calibrated reference point 310 on the image
sensor. During daylight, the image course of the sun and the
estimate of the receiver spot trace out locii 322, 332 that are
instantaneously antipodal provided the mirror is continuously
bisecting.
[0034] When the mirror is improperly aligned, the reference point
or calibrated reference point 310 of the camera may be located off
the line 340 between the sun 320 and estimated receiver spot 330 as
shown in FIG. 3B (see point 310 along orthogonal line 350) or the
reference point or calibrated reference point 310 of the camera may
not be equidistant between the sun 320 and estimated receiver spot
330 as shown in FIG. 3C. To restore proper bisection, the mirror
may be actuated, for example, via closed-loop tracking, about its
two degrees of freedom separately or concurrently.
[0035] During assembly of a heliostat 110, a pick-and-place machine
may be used to locate the imager 116 on the mirror 220. Even a high
precision manufacturing process may result in a small deviation
between the imager and mirror. Although small, the difference
between the orientation of the imager and mirror may hamper the
ability of the heliostat to effectively redirect sunlight to a
receiver with the required angular accuracy. A calibration
procedure may be used to determine the precise difference between
the imager's optical axis and the mirror's normal as well as the
optical center of the imager and the optical center of the pinhole,
lens system or aperture, thereby providing the correction needed to
precisely locate the source and target from the image data acquired
by the camera 116.
[0036] FIG. 4A is a top level functional block diagram of a system
embodiment of the present invention where the controller 410 may
comprise a processor and/or circuitry 411 configured to read and
image data and locate features therein (e.g., blob detection and
center estimation), and storing said features in a table 413 of a
memory store 412. In addition the processor and/or circuitry 411
may be configured to generate commands to actuate motors based on
image features (e.g., the sun blob's center) and predicated
locations of the receiver. The commanded changes to the motor
positions, in this example are shown as an elevation command
(EL(c)) and an azimuth command (AZ(c)), and may cause the
respective elevation actuator 430 and azimuth actuator 440 to
change the mirror orientation 450. Alternatively, for a tilt-tilt
kinematic, two similar tilt commands T1(c) and T2(c) would be
available. A camera, image sensor 420 or a light-sensing array or
scanning array of the platform provides a location (e.g., on the
camera plane) of the receiver during the table building phase and a
location of the sun during the daylight tracking phase. In some
embodiments the commanded changes in motor position may be used to
retrieve predicted receiver positions via a table lookup 413. In
some embodiments, motor positions available in azimuth and
elevation from encoders 460 may be used to retrieve predicted
receiver positions via table lookup 413.
[0037] Accordingly, embodiments of the invention include control of
a heliostat during two phases. A first phase comprises sweeping
through various combinations of azimuth and elevation motor
position pairs and recording, said motor position pair, the sensed
position of a radiating and/or illuminated receiver where the
background lighting is sufficiently dim to permit the heliostat
light sensor or camera to sense the receiver as a point, spot, or
blob. For example, during nighttime, with a bright or pulsating
light positioned at the receiver aperture. A second phase comprises
daylight tracking of the sun spot, or blob in a fashion that places
the sun point complementarily equidistant from and collinear with
an estimated receiver location on the image sensor, where the
iteratively estimated receiver location may be retrieved via table
lookup by using the current motor position(s) as indices. For
example, the first phase may be embodied as a nighttime raster scan
of the heliostat motor position space under some resolution, and
the second phase may be embodied as the daytime sun-receiver
bisecting closed-loop control system, where the sun is imaged as a
non-saturating or blooming blob under a suitable filter and the
receiver spot may be inferred from interpolations of the data
obtained during the first phase and not be imaged at all. Because
the sun and receiver need not be imaged simultaneously during the
first or second phase, there is no need for a (moveable) split or
shutterable filter.
[0038] An exemplary first phase may presume the heliostat is
rigidly mounted to the ground or a stationary structure, with
mounting parameters not known or only known to some tolerance.
During nighttime, a relatively intense light source, e.g., bright
incandescent or halogen, may be disposed at the center of the
receiver aperture or target region. Preferably the size of the
light source disk may be very small compared to the minimum
heliostat distance and preferably smaller than, and centered at,
the receiver aperture. The light source must be sufficiently bright
to be suitably imaged by the image sensor (IS) under the filter
used for daytime tracking by adjusting the sensor's sensitivity to
high. Nighttime may be a preferred time for this exemplary first
phase of operation because all other objects may be masked by the
filter and by a decrease in brightness. Other bright objects in the
sky such as stars and moon may be avoided if their position is
approximately known, e.g., by a moon- or star-positioning
algorithm. Alternatively, the bright light may emit only a narrow
spectrum, attuned to a narrow band-pass filter on the camera. Still
alternatively, the bright light may be flashed at a very short duty
cycle, e.g. 0.1%, and at a very high brightness (1000.times.
stronger than normal), so as to overwhelm any other light sources.
In addition, the exact moment of flashing may trigger image
acquisition by mirror-mounted cameras.
[0039] FIG. 4B is a top level functional block diagram illustrating
during the first phase that receiver-platform (heliostat
reflector)-light sensor geometry 490 will yield measured
illuminated received positions (XR(m), YR(m)) that are stored in a
table 413 in records containing the measured (e.g., AZ(m), EL(m))
or commanded (e.g., AZ(c), EL(c)) positions in motor positions with
relative or absolute AZ/EL angles, or tilt pair angles. Some
embodiments may have step counts suitable for stepper motors
representative of the effects of angle changes. Some embodiments
may represent the two-way table as a two dimensional function of
angles. Accordingly a representation of the motor positions may be
applied in the second phase (the sun tracking phase) to generate
estimates of the receiver location in the camera of image sensor
space, i.e., (XR(e), YR(e)).
[0040] The exemplary pseudo-code of the first phase, when executed
by the controller or other processor, may instruct the heliostat
image sensor (IS) gain and exposure settings to be set high enough
to allow for the receiver-related light source to be imaged
preferably as a compact, non-saturating blob (compact pixel group).
The IS or the image it produces, may be at a gray level of
grayscale and/or may be subject to thresholding to produce a binary
representation. The relationship between the heliostat motor
positions and the receiver may be expressed as a two dimensional
interpolant, a neural network, or as a table lookup and an
interpolation which estimates the receiver location on the IS frame
based on commanded or measured heliostat motor positions. An
exemplary embodiment of the first phase to generate values for a
table look-up has the heliostat initially commanded to go to a zero
position, e.g., against a mechanical stop or limit or contact
switch. Next, a raster scan in the two dimensional space of motor
positions (reference is made to the pseudo-code below: th1, th2) of
the heliostat is initiated, with both motors stepped at some
resolution (reference is made to the pseudo-code below: th1step,
th2step). In some embodiments, the resolution may be given as a
number of steps where the relationship between step count and
mirror angular displacement need not be known. Referring to the
pseudo-code below, for each motor position pair (th1, th2) visited,
an image processing method IS.GetBlob( ) is invoked to return the
centroid of the receiver blob as imaged in the IS space that may be
a rectangular pixel array. Subpixel estimation of the blob centroid
may be used where the IS frame pair, i.e., the (x,y) pair, may be
registered in fixed-point or floating-point. Following this step,
the four-tuple (th1, th2, x, y) may be stored on an onboard memory
(OBM) as the procedure OBM.Store( . . . ). The specific
organization of the data store may be a linear or two-dimensional
array, a linked list, a hash table, a database, or a continuous
function. In addition, the motor positions (th1, th2) may be
embodied as integral step counts. As shown below, this data store
may be indexed by the pair (th1, th2), so the data store
organization may facilitate quick retrieval of the pair (x,y) based
on (th1,th2) as indices or keys, e.g., as with a hash table, a
dictionary or database.
[0041] Once the raster scan of the motor position space is
completed, the first phase may terminate. Accordingly, if the first
phase was not based on a post-daylight radiating receiver, the
light source of an illuminated receiver may be turned off and
removed from the receiver aperture. In a variant of a raster scan
pattern of equally-spaced raster lines comprising equally spaced
sample points, the (th1,th2) space may not be visited in uniform
fashion, but rather visited where parts of the (th1,th2) space are
sampled more finely than others. Such exemplary refined sampling
may better accommodate troughs in the Jacobian determinant of the
forward kinematic map. Alternatively, the (th1, th2) space may be
traversed along a spiral or any other continuous path which
provides coverage of the angular space under some resolution.
[0042] Exemplary pseudo-code representative of computer-readable
instructions for executing the first phase; in this example, the
table is constructed for the interpolated conversion of heliostat
motor positions to a receiver imaged position on the sensor:
TABLE-US-00001 Algm1: Position light source at receiver aperture;
IS.SetGain (HIGH); For (th1 = 0; th1 <= th1max; th1 += th1step)
For (th2 = 0; th2 <= th2max; th2 += th2step) { (x,y) =
IS.GetBlob( ); OBM.Store (th1, th2, x, y); } Remove light source
from receiver aperture;
[0043] In this example, the first phase may be executed at least
once and preferably in relative darkness with the exception of a
bright light source positioned at the receiver aperture, e.g., at
its center. The data store, in the case of heliostat-fixed memory,
the onboard memory (OBM) is preferably non-volatile so that power
to the memory store may be turned off for a period such as the
duration of nighttime, e.g., overnight, Accordingly, the memory
store may be referenced during the second phase, i.e., the solar
tracking phase, to support the estimation of a receiver location
point in the frame of reference of the image sensor (IS). In some
embodiments, the gain and exposure settings of the IS may be set to
high levels during the first phase and to low levels during a
second phase (below), so that the sun may be imaged as a
non-saturating or blooming blob through the same contiguous, i.e.,
non-split, filter used during the first phase.
[0044] Illustrated in FIG. 5 is a diagrammatic representation of a
heliostat 110 and a receiver 150 mounted on a tower 152 where a
point, spot, or blob 510 may be registered as the location of the
sun is tracked relative to a predicted location of the receiver 520
in order that the platform normal 240, N, in the plane of the sun,
predicted receiver location, and imaging sensor center point 560,
bisects the angle between the direction vector of the incident
sunlight 530 and the predicted location/direction vector 540 of the
receiver. For example, FIG. 6A shows a location in XY space, i.e.,
camera 2D space, of a sun blob 510, the reference center point 560
and the predicted location of the receiver 520. In this
illustration, antipodal symmetry is not met as indication by the
error values X.sub..epsilon. and Y.sub..epsilon.. The error values
are determined by the controller executing instructions of the
present invention and may cause the platform to move responsive to
platform commands from the controller as shown in FIG. 6B.
Accordingly, while the error values X.sub..epsilon. and
Y.sub..epsilon. are maintained within a threshold acceptable for
the given receiver-to-platform geometry, i.e., a threshold
consistent with a determined aiming accuracy, the antipodal
symmetry is approximately met as shown in FIG. 6C.
[0045] Alternatively, if during tracking the sun blob centroid and
the estimated receiver position are not found to be symmetric or
antipodal with respect to the center, a local search procedure,
such as a "manhattan" method may be executed to improve the
symmetry. For example, at a given motor position (az,el), the four
(or eight) neighboring "manhattan" motions can be executed,
(az+d,el),(az-d,el),(az,el+d),(az,el-d), and one may selected which
best improves antipodal symmetry.
[0046] The second phase, i.e., the tracking phase, may be initiated
with the heliostat being commanded to go to a zero position. Then a
loop ensues which varies the heliostat mirror orientation until the
IS registers a bright spot in its field of view (FOV) comparable to
an expected sun blob that may be determined by the predicate
IS.HasBlob( ). While the sun is in its daylight course, the
following set of exemplary operations may be performed by the
heliostat controller: [0047] (i) The location of the sun blob
(xsun, ysun) is retrieved by IS.GetBlob( ). Sub-pixel detection may
be used. By applying a modulo operator to accommodate any lost
steps, this exemplary subprocess may predictably track the current
(th1,th2) configuration due to the heliostat motion starting from a
known zero position. [0048] (ii) With the sun blob (xsun, ysun)
retrieved, the memory store can be queried for the position of the
receiver based on data stored during the first phase scan of and
indexed by the current motor positions (th1, th2). Optionally, the
controller may measure the motor positions, e.g., via encoders.
Because the scan in darkness may be accomplished at some finite
resolution, an exactly matching retrieval (x,y) pair associated
with the specific (th1, th2) may be rare. So, the function
OBM.Query(th1, th2) finds, via interpolation or regularization
(e.g., using closest neighbors to the query), the sub-pixel
receiver location (xrcv, yrcv) most likely associated with the
current (th1,th2) pair. Because the receiver (brightly lit or not
lit) will be faint or completely masked below the black level of
the camera when the latter gains are set to image a non-saturating
sun under the filter, OBM.Query( . . . ) this effectively allows
the control system to predict the receiver location based on the
data stored during the scan of the first phase. [0049] (iii) The
next step in the second phase includes generating commands to
adjust the (th1, th2) of the heliostat to achieve symmetry of the
visible sun blob and the estimated receiver blob. A closed-loop
tracking is illustrated by example by the pseudocode below where
the while loop which may invoke a Symmetric( ) predicate--may be a
simple check for a Euclidian distance from (xsun,ysun) to
(-xrcv,-yrcv), and a pseudo-coded adjustment body. This adjustment
may select one of the Manhattan motions (sets of motions in
straight lines at right angles to each other--akin to a
stair-stepping pattern) in the (th1,th2) space expected to produce
most improvement in symmetry. There may be, for example, four or
eight such motions tried. That particular selection, and indeed,
the entire sun tracking motion on a given day, may be stored, e.g.,
at the OBM, so as to guide and speedup the choices on the following
day(s). [0050] (iv) The process of the second phase may include a
rest or sleep step for a period of time, e.g., few seconds to
several tens of seconds (this depends on the maximum spillage
allowable, a typical value is 15 seconds), until the process
iterates to generate commands to reposition the heliostat mirror at
a position that restores the antipodal symmetry between the visible
sun and the predicted or estimated receiver as interpolated from
tuples registered and stored during the first phase.
[0051] Exemplary pseudo-code representative of computer-readable
instructions for executing the second phase; in this example,
antipodal symmetry is maintained by the closed-loop tracking of the
sun and the generation of estimated positions of the receiver based
on data of a two-way table lookup:
TABLE-US-00002 Algm2: IS.SetGain (LOW); Heliostat.GoHome ( ); While
( !IS.HasBlob( ) ) Vary heliostat angles to bring sun to view;
While ( SunIsUp( ) ) (xsun, ysun) = IS.GetBlob( ); (xrcv, rcv) =
OBM.Query(th1,th2); // Interpolation While ( !Symmetric(xsun, ysun,
xrcv, yrcv) ) Vary heliostat angles to improve symmetry; Sleep
(timeStep); }
[0052] Illustrated in FIG. 7 is an exemplary method for tracking
the sun using the imager and corresponding tracking controller. In
the morning, the tracking controller becomes active when the imager
detects (step 702) illumination above some predetermined threshold.
The tracking controller then begins listening for (1) a control
sequence (step 704) instructing the tracking controller as to the
proper tracking mode, for example, (2) calibration code (step 706)
instructing the tracking controller to initiate a calibration
sequence in which the relative position of the mirror and receiver
is precisely determined, and (3) setup code (step 708) instructing
the tracking controller to execute one or more configuration
operations before being activated. The control sequence,
calibration code, setup code, or combination thereof may be
transmitted to the particular tracking controller using a wired
system or wireless system including radio control (RC), infrared,
or optical transmission mode received via the imager or other
optical device. Alternatively, all the information may originate
from an on-board processor.
[0053] If the control sequence specifies an open-loop tracking
procedure, the decision block (test 710) is answered in the
affirmative and the tracking controller begins orienting (step 712)
the mirror based on a mathematical model prescribing the azimuth
and elevation angles of the sun over the course of a day. If a
closed-loop tracking mode is specified, then the decision block
(test 710) is answered in the negative and the heliostat controller
begins looking for the sun (step 714) in autonomous or local
fashion, e.g., by detecting the sun blob on the camera sensor. If
and when the sun is located (test 716), the heliostat initiates
(step 718) a closed-loop tracking operation using feedback based on
the camera image, and based on estimates of the receiver location,
to continually orient the mirror so as to maintain (step 720) the
sun and receiver at the antipodal points about the camera's center
axis, as described above in detail.
[0054] One of ordinary skill in the art will also appreciate that
the elements, modules, and functions described herein may be
further subdivided, combined, and/or varied and yet still be in the
spirit of the embodiments of the invention. In addition, while a
number of variations of the invention have been shown and described
in detail, other modifications, which are within the scope of this
invention, will be readily apparent to those of ordinary skill in
the art based upon this disclosure, e.g., the exemplary flowcharts
or processes described herein may be modified and varied and yet
still be in the spirit of the invention. It is also contemplated
that various combinations or subcombinations of the specific
features and aspects of the embodiments may be made and still fall
within the scope of the invention. Accordingly, it should be
understood that various features and aspects of the disclosed
embodiments can be combined with or substituted for one another in
order to form varying modes of the disclosed invention. Thus, it is
intended that the scope of the present invention herein disclosed
should not be limited by the particular disclosed embodiments
described above. Accordingly, the invention has been disclosed by
way of example and not limitation, and reference should be made to
the following claims to determine the scope of the present
invention.
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