Patent Application
20160084530
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.
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.
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 m2). 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 m2), 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.
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.
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.
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.
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.
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.
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.
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.
The hard stop features may also facilitate a method of finding the home position of the heliostat, wherein the method comprises the steps of:
An improved AZ-EL heliostat assembly is described herein, with reference to
An embodiment of a heliostat drive assembly 10, as illustrated in
The elevation gear transmission, as illustrated in two perspectives in
The elevation output gear 8, as illustrated in
The planar faces of the elevation output gear may make contact with elevation hard stop features 25, as illustrated in
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.
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.
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.
A first embodiment of the azimuth output gear 31 is illustrated in
A second embodiment of the azimuth output gear 31 is illustrated in
The azimuth hard stop feature 34 may be formed on the underside of the azimuth gear housing cover 37, as illustrated in
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.
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.
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.
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.
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.
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.
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.
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.
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.
