U.S. patent application number 14/494567 was filed with the patent office on 2016-03-24 for heliostat mechanical stop and method of finding heliostat home position.
This patent application is currently assigned to eSolar. The applicant listed for this patent is eSolar. Invention is credited to Jason Blair, Ulrik Pilegaard, Alexander Sonn, Maximilian Zavodny.
Application Number | 20160084530 14/494567 |
Document ID | / |
Family ID | 55525434 |
Filed Date | 2016-03-24 |
United States Patent
Application |
20160084530 |
Kind Code |
A1 |
Pilegaard; Ulrik ; et
al. |
March 24, 2016 |
HELIOSTAT MECHANICAL STOP AND METHOD OF FINDING HELIOSTAT HOME
POSITION
Abstract
A drive assembly for a heliostat and a method of determining the
drive's home position are described. The drive assembly comprises
at least one mechanical hard stop for impeding the motion of an
output gear of at least one gear transmission driven by a motor.
Each hard stop may be built into the housing of a gear transmission
or may be made integral with an output gear. The method for
determining the drive assembly's home position comprises the steps
of iteratively actuating an output gear towards, and then away
from, a hard stop feature while monitoring the input torque of the
driving motor. When the motor input torque is registered to be
higher than a predetermined trip torque, the stepper motor is
commanded to stop and the home position of the drive assembly is
determined.
Inventors: |
Pilegaard; Ulrik; (Woodland
Hills, CA) ; Blair; Jason; (La Crescenta, CA)
; Sonn; Alexander; (Pasadena, CA) ; Zavodny;
Maximilian; (Pasadena, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
eSolar |
Burbank |
CA |
US |
|
|
Assignee: |
eSolar
Burbank
CA
|
Family ID: |
55525434 |
Appl. No.: |
14/494567 |
Filed: |
September 23, 2014 |
Current U.S.
Class: |
126/714 ;
126/601; 126/606 |
Current CPC
Class: |
F24S 30/452 20180501;
F24S 40/00 20180501; F24S 2030/134 20180501; F24S 2050/25 20180501;
F24S 50/20 20180501; Y02B 10/20 20130101; F24S 2025/018 20180501;
Y02E 10/47 20130101 |
International
Class: |
F24J 2/38 20060101
F24J002/38; F24J 2/54 20060101 F24J002/54 |
Claims
1. A heliostat drive assembly comprising: a drive chassis; a drive
shaft connected to the drive chassis; a reflector channel connected
to the drive chassis; a first gear transmission comprising a first
output gear, wherein the first gear transmission is configured to
rotate said drive shaft via the first output gear about a first
axis; a second gear transmission comprising a second output gear,
wherein the second gear transmission is configured to rotate said
reflector channel via the second output gear about a second axis; a
motor controller configured to actuate the first and second gear
transmissions; a first stop feature configured to cease the motion
of the first output gear; and a second stop feature configured to
cease the motion of the second output gear.
2. The heliostat drive assembly of claim 2, wherein the first and
second stop features comprise mechanical stops.
3. The heliostat drive assembly of claim 2, wherein the first and
second stops comprises flat surfaces.
4. The heliostat drive assembly of claim 1, wherein the first stop
is made integral with the drive chassis.
5. The heliostat drive assembly of claim 2, wherein the first
output gear comprises at least one protrusion.
6. The heliostat drive assembly of claim 5, wherein the first
output gear has an annulus shape and comprises an inner ring and an
outer ring, wherein the outer ring has a larger diameter than the
inner ring and a gap is present between the inner ring and the
outer ring.
7. The heliostat drive assembly of claim 6, wherein said
protrusions are formed on the inner ring or on the outer ring.
8. The heliostat drive assembly of claim 7, wherein said
protrusions are collinear with the center of the first output
gear.
9. The heliostat drive assembly of claim 8, wherein said
protrusions have planar faces.
10. The heliostat drive assembly of claim 1, wherein the second
stop is made integral with the drive chassis.
11. The heliostat drive assembly of claim 1, wherein the second
output gear comprises a tooth region having an arc segment between
120 and 150 degrees.
12. The heliostat drive assembly of claim 2, wherein the second
output gear further comprises at least one planar surface that
makes flush contact with the second stop.
13. The heliostat drive assembly of claim 1, wherein the motor
controller has local memory storage.
14. A method of determining the home position of a heliostat drive
assembly, which comprises the steps of: (a) utilizing a motor
controller having local memory storage to actuate at least one
motor to drive at least one gear transmission having an output
gear, wherein the output gear is configured to rotate a drive
assembly about an axis and wherein the output gear is actuated in
the direction of a hard stop feature; (b) measuring the input
torque of the at least one motor with a torque sensor while
simultaneously driving the at least one gear transmission with the
motor; (c) measuring the step position of the at least one motor
with the motor controller while simultaneously driving the at least
one gear transmission with the motor; (d) commanding the motor to
cease operation upon exhibiting a predetermined condition; (e)
recording the current motor step position to the memory of the
motor controller; and (f) defining the recorded motor position as
the home stop position in the motor controller memory.
15. The method of determining the home position of a heliostat
drive assembly of claim 14, further comprising the steps of: (g)
actuating the stepper motor to move the output gear away from the
stop feature a predetermined number of steps; (h) repeating steps
(a) through (f); and (i) repeating steps (g) and (h) a
predetermined number of times.
16. The method of determining the home position of a heliostat
drive assembly of claim 14, wherein the predetermined condition of
step (d) is the registering by the torque sensor of a higher input
torque to the respective gear transmission than a predetermined
trip torque.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] The present application claims priority to and the benefit
of U.S. Provisional Patent Application No. 61/881,373, filed on
Sep. 23, 2013, the entire disclosure of which is incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] This invention relates generally to heliostats having
reflectors configured to redirect sun light to a target or
receiver. In particular, the invention relates to a mechanical stop
of a gear train configured to limit the rotation of a heliostat
drive assembly and the method of finding the home position of a
heliostat using said mechanical stop.
[0003] In Concentrating Solar Power (CSP) plants, arrangements of
heliostats reflect sunlight toward a receiver mounted atop a tower
containing a working fluid. One type of receiver transfers incident
radiant energy to the working fluid to produce high-pressure,
high-temperature steam through the means of a heat exchanger or a
phase change of the working fluid itself. The working fluid can be
water, air, or a salt material heated to a molten state. The output
steam can facilitate a variety of applications, such as electrical
power generation, enhanced oil recovery, and desalination.
Heliostats are generally mounted on the ground in an area facing or
surrounding the receiver tower. Each heliostat has a reflector: a
rigid reflective surface, such as a mirror, that tracks the sun
through the actuation of a heliostat drive mechanism about at least
one axis. Sun-tracking involves orienting the reflector throughout
the day so as to optimally redirect sunlight from the sun toward
the receiver and maintain the desired temperature of the working
fluid.
[0004] One approach to constructing a heliostat field is to utilize
a small amount of comparatively large heliostats (e.g., greater
than between 50 and 150 m.sup.2). In such a power plant, having a
fewer number of heliostats may necessitate the manufacture of very
precise, and thus very expensive, components for the positioning of
the reflective surfaces. Another approach, however, is to use a
large amount of comparatively small heliostats (e.g., between 1 and
10 m.sup.2), such as with reflective surfaces that measure between
1 and 3 m on each side. Such an approach may be more efficient at
redirecting sun light because there are more individually
adjustable reflective surfaces. In addition, smaller heliostats may
be cheaper to produce and easier to assemble, decreasing
installation time and operations costs. However, a plant comprising
more heliostats will necessarily require the same amount of
additional drive assemblies, increasing the number of repeated
steps during installation. Accordingly, there is a need for
heliostat assemblies that are both economical to manufacture and
efficient to install.
[0005] One problem with controlling the heliostats during plant
operation is that sun-tracking must be precise, and the orientation
of the reflective surface must be within a certain prescribed
angular tolerance at all times. This orientation must be precise
because accurate positioning of the reflectors is necessary to
ensure that the requisite amount of light is targeted onto the
receiver to maintain plant efficiency. A necessary condition for
accurate heliostat positioning may be for the heliostat controller
to always be apprised of the heliostat's current orientation. The
orientation position may be determined by noting any deviation from
the heliostat's zero position, or "home." Defining a home position
may also be useful for establishing a default orientation to which
the heliostat may be reset in the event of a system restart or a
reboot of a controller processor.
[0006] Additionally, wind and other environmental factors may apply
loads to the reflector that may move it away from its preferred
orientation at a given point in time while tracking the sun.
Manufacturing tolerances between the components of the heliostat
may also contribute to backlash, undesirable movement and
non-linearity in the drive systems. These effects may undesirably
result in a greater amount of variation between the expected and
the actual reflector orientation. Such variation and external
loading may cause a heliostat to attempt to traverse a range of
motion beyond its design capabilities. For example, if the
heliostat controller processor registers the initial orientation of
the heliostat incorrectly, the heliostat may attempt to rotate
about an axis farther than its mechanical constraints would
normally allow. Consequently, inconsistency in the known heliostat
orientation and anomalous motion may lead to excessive wear and
tear to mechanical parts or even catastrophic failure. A
conventional solution to this problem is to incorporate electronic
limit switches that signal the motor controllers to cease operation
when the switches are actuated. The heliostat drive assembly may be
configured such that the limit switches are actuated when the
heliostat has moved to a precise position, thereby establishing
limits to the heliostat's range of motion. However, these limit
switches may be unreliable or may malfunction, making heliostats
vulnerable to failure and inaccurate positioning. Additionally,
limit switches present an added cost to heliostat manufacturing in
terms of both additional component parts and power consumption.
SUMMARY OF THE INVENTION
[0007] Improved heliostat drive assemblies are described herein,
wherein the assemblies are configured to reduce the likelihood of
mechanical component failure through the inclusion of mechanical
stops in at least one gear train. A method of finding a heliostat
home position is further described wherein the method comprises the
steps of sensing when a heliostat has reached the limits imposed by
a mechanical stop and then defining the heliostat's present
orientation as a home position. The improved drive assemblies
thereby reduce the risk of catastrophic failure resulting from
drives actuating beyond their prescribed range of motion and
improve the efficiency of their operation by facilitating a stable
and findable home position.
[0008] A heliostat drive assembly may comprise a drive chassis, a
drive shaft connected to the drive chassis, a reflector channel
connected to the drive chassis, a first gear transmission
comprising a first output gear, wherein the first gear transmission
is configured to rotate said drive shaft via the first output gear
about a first axis, a second gear transmission comprising a second
output gear, wherein the second output gear transmission is
configured to rotate said reflector channel via the second output
gear about a second axis, a motor controller configured to actuate
the first and second gear transmissions, a first stop feature
configured to cease the motion of the first output gear of the
first gear transmission, and a second stop feature configured to
cease the motion of the output gear of the second gear
transmission.
[0009] Each of the heliostat drive assemblies may have at least one
axis of rotation for an attached reflector. For example, the
heliostat drive assemblies may have two axes--an elevation axis and
an azimuth axis, hence an "AZ-EL Heliostat." The drive may have a
prescribed range of motion about both axes that may be set
according to its spatial dimensions and required functionality.
This range of motion may be imposed by the locations of the
mechanical hard stops relative to the traversable arc length of the
outermost gears in the respective azimuth and elevation
transmissions. When the drive controller is supplied a command to
rotate the drive and an attached reflector about either the azimuth
or elevation axes, the motors initiate the rotation of gears in
said transmissions. In both the azimuth and elevation gear
transmissions, there may be an output gear provided that works to
rotate a shaft about which the drive or reflector rotates. When
this output gear makes contact with a mechanical hard stop, the
motor starts to output a higher torque in the face of resistance.
When the motor torque becomes higher than a predetermined threshold
torque, the motor may be stopped, preventing further rotation of
the drive in the present direction about the given axis.
[0010] The reflector may be mounted to the drive via a reflector
channel that may be actuated by the elevation gear transmission to
rotate about the elevation axis. In an exemplary embodiment, the
reflector maybe rotated in an arc no greater than 90 degrees from a
home position about the elevation axis. The hard stop thus prevents
the reflector from rotating so far about the elevation axis that it
is brought into contact with the drive housing, which could cause
damage to the mirror.
[0011] At least one of the azimuth and elevation transmissions may
be configured such that there is a generally predetermined minimum
biasing force on the one of the axes in a predetermined rotational
direction. By having at least one of the axes biased in a
predetermined rotational direction, the ability of wind loads and
other external forces to cause undesirable motion, such as
backlash, maybe significantly reduced and even eliminated. In an
exemplary embodiment, the biasing force maybe provided by a spring,
which may be tightened to a predetermined torsion. Excess loosening
of the bias springs may result in insufficient biasing force for
ameliorating the effects of backlash. Additionally, excess
tightening of the bias springs may unduly impede the motion of the
drive as it rotates opposite the direction of the bias force as
well as result in dangerous mechanical failure if the spring is
wound too tightly. The hard stop thus prevents the reflector from
rotating about the azimuth or elevation axes in such a way as to
cause the bias springs to loosen or tighten beyond acceptable
torsion limits.
[0012] The hard stop features may also facilitate a method of
finding the home position of the heliostat, wherein the method
comprises the steps of: [0013] (a) utilizing a motor controller
having local memory storage to actuate at least one motor to drive
at least one gear transmission having an output gear, wherein the
output gear is configured to rotate a drive assembly about an axis
and wherein the output gear is actuated in the direction of a hard
stop feature; [0014] (b) measuring the input torque of the at least
one motor with a torque sensor while simultaneously driving the at
least one gear transmission with the motor; [0015] (c) measuring
the step position of the at least one motor with the motor
controller while simultaneously driving the at least one gear
transmission with the motor; [0016] (d) commanding the motor to
cease operation upon exhibiting a predetermined condition; [0017]
(e) recording the current motor step position to the memory of the
motor controller; and [0018] (f) defining the recorded motor
position as the home stop position in the motor controller
memory.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a perspective view of a drive assembly for a
heliostat, showing an azimuth gear housing with a cover and
configured to rotate the drive assembly about an azimuth axis and
an elevation gear housing with a cover and configured to rotate a
reflector channel about an elevation axis;
[0020] FIG. 2 is a side view of the drive assembly of FIG. 1
showing the elevation gear housing with the cover removed and an
elevation gear transmission, wherein the elevation gear
transmission comprises an output elevation gear;
[0021] FIG. 3 is a perspective view of the elevation gear
transmission removed from the elevation gear housing;
[0022] FIG. 4 is a perspective view of the output elevation gear of
FIG. 2;
[0023] FIG. 5 is a perspective view of the drive assembly of FIG. 1
showing the elevation gear housing with the cover and elevation
gear transmission removed;
[0024] FIG. 6 is a top view of the drive assembly of FIG. 1,
showing the azimuth gear housing with the cover removed and an
azimuth gear transmission comprising an output azimuth gear;
[0025] FIG. 7 is a perspective view of a first embodiment of the
output azimuth gear of FIG. 5;
[0026] FIG. 8 is a perspective view of a second embodiment of the
output azimuth gear of FIG. 5; and
[0027] FIG. 9 is a perspective view of the bottom of the azimuth
gear housing cover and a hard stop feature made integral with the
cover.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] An improved AZ-EL heliostat assembly is described herein,
with reference to FIGS. 1-9. The exemplary heliostat assembly is
advantageously configured to provide for a mechanical stop that
ceases movement outside of a prescribed range of motion, as
described in greater detail herein.
[0029] An embodiment of a heliostat drive assembly 10, as
illustrated in FIG. 1 comprises a chassis 1 and two gear
transmissions (not shown), wherein each gear transmission may be
enclosed in a respective gear housing. The chassis may comprise a
single element or a plurality of elements bonded together or
connected via fasteners. The gear housings may be made integral
with the chassis and may comprise, for example, an azimuth gear
housing 2 and an elevation gear housing 3. The azimuth gear housing
2 may enclose an azimuth gear transmission (not shown) for
actuating the heliostat drive chassis about an azimuth axis. The
azimuth axis may be defined as being orthogonal to the ground and
passing through the center of an azimuth drive shaft 4. The
elevation gear housing 3 may enclose an elevation gear transmission
(not shown) for actuating a reflector channel 5 about an elevation
axis. The elevation axis may be defined as orthogonal to the
azimuth axis and co-linear with the center of an elevation output
gear 8 (see FIG. 2). The elevation and azimuth gear transmissions
may be driven by motors 7 that receive power from an internal or
external power source. A reflector (not shown) may be mounted to
the reflector channel 5 via fastener slots 6. The drive assembly
may further comprise an azimuth torsion spring 9 that surrounds the
azimuth drive shaft 4. The torsion spring may be configured to
supply a biasing force on the azimuth output gear to prevent
backlash in the azimuth gear transmission.
[0030] The elevation gear transmission, as illustrated in two
perspectives in FIG. 2 and FIG. 3, may comprise a worm gear 11
driven by an input motor 7, wherein the worm gear interfaces with a
first cluster spur gear. The first cluster spur gear may comprise
primary and secondary gears 12 and 13 that are connected to each
other or fixedly mounted to the same shaft, wherein the primary
gear 12 has a larger diameter than the secondary gear 13, and
wherein the primary gear is oriented between the secondary gear and
the elevation gear housing cover (the cover has been removed in the
illustrations of FIGS. 2 and 3). The secondary gear 13 of the first
cluster spur gear may interface with a primary gear 14 of a second
cluster spur gear, wherein the second cluster spur gear also may
comprise primary and secondary gears 14 and 15 that are connected
to each other or fixedly mounted to the same shaft, wherein the
primary gear has a larger diameter than the secondary gear. The
secondary gear 15 of the second cluster spur gear interfaces with
the elevation output gear 8, wherein the elevation output gear
works to rotate the reflector channel 5 about the elevation axis.
The elevation gear transmission may be enclosed within elevation
gear housing 3 and may be sealed by a housing cover (not shown)
which may be fastened to the housing via mounting screw holes 16.
An elevation torsion spring 17 may be connected to the elevation
output gear 8. The torsion spring may induce a biasing force
opposite a direction of rotation to reduce backlash in the
elevation gear transmission.
[0031] The elevation output gear 8, as illustrated in FIG. 4, may
comprise an arc segment of gear teeth 18, wherein the arc segment
may be between 120 and 150 degrees. The center face of the output
gear 8 may be outfitted with a plurality of bolt holes 19, through
which fasteners may connect the output gear to the reflector
channel 5, and a first spring retaining aperture 20, through which
a first tip of an elevation torsion spring 17 (see FIG. 3) may be
held. The output gear additionally comprises planar faces 21 at
either end of the gear teeth arc segment.
[0032] The planar faces of the elevation output gear may make
contact with elevation hard stop features 25, as illustrated in
FIG. 5. The hard stop features serve to define the full range of
motion of the output gear and cease any further movement of the
heliostat in the elevation axis beyond the limits imposed
thereupon. FIG. 5 displays the elevation gear housing 3 with all
internal components removed. The drive chassis and elevation gear
housing may be assembled from interconnected parts or formed from a
single piece of material by, for example, die casting. The
elevation gear housing 3 may comprise gear shaft bosses 22 and 23
which support the gear shafts upon which cluster spur gears 13 and
14 are mounted, respectively. The elevation gear housing may
additionally comprise a second spring retaining aperture 24 through
which a second tip of the elevation torsion spring 17 (see FIG. 3)
may be held. The elevation gear housing 3 may additionally comprise
elevation hard stop features 25, wherein the elevation heard stop
features are planar surfaces formed into the side wall of the
housing. The elevation hard stop features 25 cease movement of the
elevation output gear 8 about the elevation axis when the planar
faces 21 of the elevation output gear contact the planar surfaces
of the hard stop feature.
[0033] FIG. 6 illustrates an open view of the azimuth gear housing
2 which houses the azimuth gear transmission. The azimuth gear
housing and the drive chassis may be assembled from interconnected
parts or formed from a single piece of material by, for example,
die casting. The azimuth gear transmission may comprise an input
motor 7 that receives power from an internal or external power
source. The input motor 7 may drive a worm gear 26 mounted on a
shaft, wherein the worm gear interfaces with a primary gear 27 of a
first azimuth cluster spur gear, the first azimuth cluster spur
gear additionally comprising a secondary gear 28 that is connected
to the primary gear 28. The primary and secondary gears of the
first azimuth cluster spur gear may be connected to each other or
may be fixedly mounted to the same shaft. The primary gear 27 may
have a larger diameter than the secondary gear 28, and the
secondary gear 28 may be oriented between the primary gear 27 and
the azimuth gear housing cover (the cover has been removed in the
illustrations of FIG. 6). The secondary gear 28 of the first
azimuth cluster spur gear may interface with a primary gear 29 of a
second azimuth cluster spur gear, wherein the second azimuth
cluster spur gear may additionally comprise a secondary gear 30.
The primary and secondary gears of the second azimuth cluster spur
gear may be connected to each other or may be fixedly mounted to
the same shaft. The primary gear 29 may have a larger diameter than
the secondary gear 30. The secondary gear 30 of the second azimuth
cluster spur gear may interface with an azimuth output gear 31,
which works to rotate the heliostat drive shaft 4 about the azimuth
axis.
[0034] The azimuth output gear 31 may comprise a hollow center
defining an annulus, an outer ring 32 and an inner ring 33, wherein
the inner ring 33 has a smaller diameter than the outer ring 32 and
a gap may be present between the inner ring and the outer ring. The
azimuth output gear 31 may further comprise at least one protrusion
35 made integral with one or both of the inner and outer rings,
wherein the protrusions are positioned within the gap between the
outer ring 32 and the inner ring 33. The protrusions may be formed
to exhibit shapes such as a triangle, trapezoid or rectangle, and
may be formed as part of the gear itself or consist of additional
parts that are attached to the gear. The protrusions may be formed
from any material suitable for withstanding the force of impinging
upon an azimuth hard stop feature 34, such as, but not limited to,
steel or a steel alloy. The azimuth output gear 31 interfaces with
the azimuth drive shaft 4 by insertion of the drive shaft into the
hollow center of the gear. The azimuth drive shaft 4 may be held in
place by, for example, a press fit, and may be rotatable by
actuation of the azimuth output gear 31.
[0035] The azimuth gear housing 2 may be sealed by an azimuth gear
housing cover (not shown) which may be fastened to the housing via
mounting screw holes and comprises an azimuth hard stop feature 34.
The azimuth hard stop feature 34 ceases movement of the azimuth
output gear 31 about the azimuth axis when the planar face of the
protrusions 35 come into contact with the azimuth hard stop feature
34. In this way, the position of the protrusions and the azimuth
hard stop feature define the full range of motion of the azimuth
output gear 31. In the present embodiment the azimuth output gear
31 may actuate through a maximum travel arc of 350 degrees from the
home position.
[0036] In an alternative embodiment, the interface between the
protrusion 35 and the azimuth hard stop feature 34 on the azimuth
gear housing cover may incorporate a third part, an intermediate
follower (not shown). The intermediate follower may comprise a
second protrusion set along the arc of the azimuth output gear on
either the inner or outer ring. The intermediate follower may be
structured such that the azimuth output gear 31 may actuate through
a cycle of motion greater than 360 degrees before contacting the
azimuth hard stop feature 34.
[0037] A first embodiment of the azimuth output gear 31 is
illustrated in FIG. 7. The azimuth output gear may be installed
onto the end of the azimuth drive shaft 4 via, for example, a press
fit. The azimuth output gear 31 comprises teeth that may interface
with the secondary gear 30 of the second azimuth cluster spur gear
(as shown in FIG. 6). The azimuth output gear 31 may have a
rectangular protrusion 35 formed along the circumference of the
outer ring 32 or the inner ring 33. In this way the protrusion may
make contact with the planar face of the azimuth hard stop feature
34.
[0038] A second embodiment of the azimuth output gear 31 is
illustrated in FIG. 8. In this second embodiment the azimuth output
gear 31 may comprise two triangular protrusions 36, wherein the one
protrusion is formed along the circumference of the outer ring 32
and the other along the inner ring 33. The two protrusions 36 may
be aligned such that they are facing each other. In this way both
protrusions may make contact with the planar faces of the azimuth
hard stop feature 34.
[0039] The azimuth hard stop feature 34 may be formed on the
underside of the azimuth gear housing cover 37, as illustrated in
FIG. 9. The azimuth gear housing cover 37 may comprise gear shaft
bosses 38 and 39 which support the gear shafts upon which the
helical cluster spur gears 28 and 29 rotate, respectively.
Additionally, the azimuth gear housing cover may further comprise
an azimuth hard stop feature 34 having azimuth hard stop contact
surfaces 40. The azimuth hard stop contact surfaces 40 are formed
so as to make flush contact with the flat sides of the protrusions
35 of the azimuth output gear 31. The azimuth hard stop contact
surfaces 40 and the protrusions 35 may comprise a variety of
compatible shapes such that the sides of the protrusions can make
flush contact with the hard contact surfaces. The azimuth hard stop
feature 34 may further comprises ribs 41, which brace the hard stop
surfaces 40 from excessive forces applied by the act of actuating
the drive shaft to the limits of its range of motion. The azimuth
hard stop contact surfaces 40 may be formed at an angle relative to
the ribs 41. The angle of the azimuth hard stop contact surfaces 40
may be chosen such that the at least one protrusion 35 contacts the
hard stop contact surface with as much surface area as
possible.
[0040] The azimuth and elevation hard stops may help to mitigate
catastrophic failures, which may result when the heliostat actuates
through a range of motion that is greater than desired. Such
failures may include potentially over-winding or unwinding the
torsion bias springs, tangling or stretching power and
communication cables, or damaging the reflector due to undesired
contact with obstacles or the drive chassis. Unwinding the torsion
springs reduce their effectiveness in preventing gear backlash, and
over-winding of the torsion springs may produce unsafe conditions
if the springs were to suddenly release. If the power and
communication cables become excessively tangled or wound, they may
become damaged and prevent operation of the heliostat. Therefore
the azimuth and elevation hard stop features reduce the risk of
performance degradation and prolong the lifespan of the device.
[0041] The present embodiment sets forth a compact heliostat drive,
which may, for example, be sized to fit inside a square meter
(without the attached reflector). Because the gear transmissions
may have high gear ratios between 1600:1 to 2000:1, such as 1800:1,
the drive motors may be low power units and still deliver the
requisite torque, minimizing energy consumption of the units and
helping to lower operation costs. Even at the present gear ratio,
the small size of the heliostats reduces the need for a very robust
mechanical hard stop, and so the present hard stop design may be
easier to manufacture using reduced material quantities while still
preventing physical breakage.
[0042] The presence of a hard stop feature at the outputs of both
the azimuth and elevation gear transmissions provide numerous
additional advantages, especially with regards to defining a "home
stop" position. The heliostat input motors may be activated by
motor controllers that may be housed within the drive chassis, the
drive azimuth shaft or located external to the drive chassis. The
motor controllers may have local memory storage capabilities and
may be programmed to detect when the motor input torque is above a
predetermined threshold value. This threshold value is called the
"trip torque" and may be defined as a torque higher than the range
of torque values exhibited by the motor during normal operation and
lower than the "stall torque", the torque at which the motor stops
rotating. As an output gear interfaces with a hard stop feature in
the azimuth or elevation gear transmissions, resistance from the
planar surfaces of the hard stop introduce a region of gear torsion
as the entire transmission "winds up". This condition may be
detected by the electronics of the motor controller board using
software to monitor the input torque of the stepper motors using at
least one torque sensor.
[0043] When the measured torque has reached or gone higher than the
trip torque, the motor controller software may then command the
motor to cease operation to prevent excessive torsion in the gears.
Stopping the motor prior to it reaching a stalled condition
maintains the commanded position of the drive without losing motor
steps, providing for a more accurate determination of the
heliostat's orientation. The trip torque level may be set low
enough such that the output gear pressing against the surfaces of
the hard stop feature will result in detection of the "winding up"
condition, but not so low as to result in false positives from
external forcing, such as light winds or other environmental
externalities. Additionally, the motor step position at the hard
stop location may be associated with a "home stop" position,
providing a marker in the controller memory by which all future
movements of the heliostat away from the home position may be
measured. By forming the hard stop to be immutable and of a fixed
position, the home position measurement may be repeatable and
stable over many movement cycles.
[0044] To improve the accuracy and repeatability of the home stop
determination, the drive motor controller software may further
comprise a method for setting the home stop position after repeated
movements of the output gears into the hard stop feature. The
method of finding the hard stop may comprise a first step of
actuating the heliostat drive to traverse its range of motion
towards the home position. The motor controller software may then,
upon detecting that the motor torque has become greater than the
trip torque, which would be indicative of having contacted the hard
stop and begun the "winding up" of the gear train, record the
current motor step position and then command the motor to actuate
the gears in the opposite direction, effectively backing away from
the hard stop by a small number of steps. The output gear may then
be moved back into the hard stop, repeating the steps of sensing
the motor input torque levels with respect to the predetermined
trip torque and then recording the "home" step position.
[0045] After a set number of iterations of backing away from the
hard stop and contacting it again, the plurality of measured home
step positions of the motor may be analyzed to determine a best
guess for the true home position. This method of analysis may
comprise, for example, the step of taking an arithmetic mean of all
the home step positions measured during the homing attempt and
setting the true home position to a calculated value. The analysis
could also comprise a method of eliminating data samples obtained
during the settling phase of the windup region. The method of
analysis may further comprise a step of eliminating erroneous
samples generated by premature sensing of an input torque higher
than the trip torque or stop conditions exhibited by the drive
prior to reaching the hard stop. For instance, if upon encountering
a high motor input torque condition, the drive subsequently
advances further than its current position by a predetermined
number of steps, this may indicate that the first high motor input
torque location was not at the true home position. The motor
controller may then work to move the heliostat drive in the present
axis to a newly calculated step position. This operation may make
the home stop determination much more repeatable by filtering out
false positives.
[0046] The threshold for detecting erroneous home positions may
take into account acceptable variation between homing attempts.
Incorrect home stop measurements may occur due to external
applications of torque from environmental externalities or user
error. The method of determining the home position may also include
the step of recording the step position variance between home
attempts and performing statistical analysis on the data set to
ascertain if the home position is changing over time or has
suddenly shifted. This analysis could prompt the motor controller
software to recalculate the home position using the same or
modified data set, initiate a new homing event, or flag the
heliostat unit for inspection or repair.
[0047] Various combinations and/or sub-combinations of the specific
features and aspects of the above 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 may be combined with or substituted for one another in
order to form varying modes of the disclosed invention. Further it
is intended that the scope of the present invention herein
disclosed by way of examples should not be limited by the
particular disclosed embodiments described above.
* * * * *