Abstract:
A solar energy system drive apparatus includes: a rotor including a first circumferential surface and having a first radial dimension from a center axis of the rotor; a stator that is axially aligned with the rotor about the center axis and includes a second circumferential surface and having a second radial dimension from the center of the rotor, wherein the second radial dimension is different than the first radial dimension; and a plurality of planetary members arranged about the first and second circumferential surfaces of the rotor and stator respectively, wherein each of the planetary members has a stepped circumferential surface comprising a third surface and a fourth surface such that the third circumferential surface is in contact with the first circumferential surface and has a third radial dimension from a central planetary axis of the planetary member.

Description:
TECHNICAL BACKGROUND 
       [0001]    This disclosure relates to systems and methods for solar energy system management. 
       BACKGROUND 
       [0002]    Solar energy management, collection, and use can often help alleviate energy problems around the world. In particular, solar energy systems (e.g., photovoltaic (“PV”) systems) that generate electrical energy from solar energy can reduce dependence on fossil fuels or other power generation techniques. Additionally, solar energy may be used to generate heat that can subsequently be used in power generation systems. In some cases, solar energy collection systems may include multiple heliostats that reflect solar energy to a receiver. The receiver may then use the reflected solar energy for one or more purposes. In some instances, heliostats are tracking mirrors, which reflect and focus sunlight onto a distant target, such as the receiver. 
         [0003]    Heliostats typically move precisely and slowly throughout the day to track the sun. Heliostats must also be very stable when acted upon by external forces, especially wind, otherwise they can direct light off-target, reducing field efficiency. Thus, the drive-mechanism for heliostats must produce a slow, precise, high-torque rotation. In some cases, these requirements are often met by multi-stage gearboxes with high reduction ratios, torsional stiffness, and minimal backlash. Such gearboxes, however, can be notoriously expensive. Further, such precise mechanisms often require substantial protection from environmental elements, such as dust, rain, moisture, and otherwise, e.g., requiring a hermetically sealed enclosure in order to operate correctly over time. 
       SUMMARY 
       [0004]    In one general embodiment, a drive apparatus includes: a rotor including a first circumferential surface and having a first radial dimension from a center axis of the rotor; a stator that is axially aligned with the rotor about the center axis and includes a second circumferential surface and having a second radial dimension from the center of the rotor, wherein the second radial dimension is different than the first radial dimension; a plurality of planetary members arranged about the first and second circumferential surfaces of the rotor and stator respectively, wherein each of the planetary members has a stepped circumferential surface comprising a third surface and a fourth surface such that the third circumferential surface is in contact with the first circumferential surface and has a third radial dimension from a central planetary axis of the planetary member; and the fourth circumferential surface is in contact with the second circumferential surface and has a fourth radial dimension from the planetary axis of the planetary member that is different than the third radial dimension; and a driver configured to rotate the plurality of planetary members about the center axis, wherein each planetary member rotates about the member&#39;s planetary axis through contact between the second circumferential surface of the stator and the fourth circumferential surface of the planetary member and wherein said rotation of the planetary members imparts rotation of the rotor about the center axis through contact between the third circumferential surfaces of the planetary members and the first circumferential surface of the rotor. 
         [0005]    In another general embodiment, a method for operating a solar energy collection system includes: rotating a plurality of planetary members about a first circumferential surface of a stator that is axially aligned about a central axis with a rotor that has a second circumferential surface, the stator and the rotor having different radial dimensions, each of the planetary members includes a stepped circumferential surface comprising a third circumferential surface and a fourth circumferential surface at different radial distances from a central planetary axis of the planetary member, wherein the third circumferential surface is in contact with the first circumferential surface of the stator and each planetary member rotates about the central planetary axis at a first rotational speed while the planetary member rotates about the stator; imparting a torque to the rotor during rotation of the plurality of planetary members about their respective planetary axes, the second circumferential surface of the rotor in contact with the fourth circumferential surfaces of the plurality of planetary members; rotating the rotor about the central axis in response to the torque applied to the rotor from the plurality of planetary members at a second rotational speed that is less than the first rotational speed; and rotating at the second rotational speed a solar energy member that is coupled to the rotor, the solar energy member having a surface facing toward the Sun, wherein solar rays from the Sun are incident on the surface. 
         [0006]    In another general embodiment, a solar energy system includes: a solar energy member having a first surface facing toward the Sun, wherein solar rays from the Sun are incident on the first surface; a drive assembly coupled to the solar energy member, the drive assembly including a rotor having a first circumferential surface and having a first radial dimension from a center axis at a center of the rotor; a stator that is axially aligned with the rotor about the center axis, the stator having a second circumferential surface and having a second radial dimension from the center axis, wherein the second radial dimension is different than the first radial dimension; a plurality of planetary members arranged about the first and second circumferential surfaces of the rotor and stator respectively, wherein each of the planetary members has a stepped circumferential surface including a third surface and a fourth surface such that the third circumferential surface is in contact with the first circumferential surface and has a third radial dimension from a central planetary axis of the planetary member; and the fourth circumferential surface is in contact with the second circumferential surface and has a fourth radial dimension from the planetary axis of the planetary member that is different than the third radial dimension; and a driver configured to rotate the plurality of planetary members about the center axis, wherein each planetary member rotates about the member&#39;s planetary axis through contact between the second circumferential surface of the stator and the fourth circumferential surface of the planetary member and wherein said rotation of the planetary members imparts rotation of the rotor about the center axis through contact between the third circumferential surfaces of the planetary members and the first circumferential surface of the rotor. The system also includes a controller configured to control movement of the solar energy member with the drive assembly in accordance with movement of the Sun. 
         [0007]    In one or more specific aspects of one or more of the general embodiments, at least one of the contact between the second circumferential surface of the stator and the fourth circumferential surface of the planetary member, and the contact between the third circumferential surfaces of the planetary members and the first circumferential surface of the rotor may include frictional contact. 
         [0008]    In one or more specific aspects of one or more of the general embodiments, at least one of the contact between the second circumferential surface of the stator and the fourth circumferential surface of the planetary member and the contact between the third circumferential surfaces of the planetary members and the first circumferential surface of the rotor may include geared contact. 
         [0009]    In one or more specific aspects of one or more of the general embodiments, the driver may be coupled to at least one of the planetary members. 
         [0010]    In one or more specific aspects of one or more of the general embodiments, the system may further include a harness assembly including: at least one substantially rigid member connecting axles disposed through respective centers of two planetary members; and a biasing member connecting axles disposed through respective centers of two planetary members, wherein the biasing member urges the axles together. 
         [0011]    In one or more specific aspects of one or more of the general embodiments, the driver may be coupled to the harness assembly and configured to rotate the harness assembly about the center axis to rotate the planetary members about the center axis. 
         [0012]    In one or more specific aspects of one or more of the general embodiments, the biasing member may include a spring. 
         [0013]    In one or more specific aspects of one or more of the general embodiments, the harness assembly may include first and second plates having centers aligned with the center axis, and wherein the rotor and stator are sandwiched between the first and second plates. 
         [0014]    In one or more specific aspects of one or more of the general embodiments, at least one of the first and second plates may include a circumferential surface having a geared surface. 
         [0015]    In one or more specific aspects of one or more of the general embodiments, the system may further include a gear coupled to the driver and interfacing the geared surface, wherein the gear is configured to transfer rotational force from the driver to the one of the first and second plates through the geared surface. 
         [0016]    In one or more specific aspects of one or more of the general embodiments, the first and second plates may include a circumferential surface in frictional contact with the planetary member such that rotation of the plate causes rotation of the planetary member about the member&#39;s central planetary axis. 
         [0017]    In one or more specific aspects of one or more of the general embodiments, the plurality of planetary members may include three planetary members. 
         [0018]    In one or more specific aspects of one or more of the general embodiments, at least one of the rotor or stator may be made of or include a formed concrete disk. 
         [0019]    In one or more specific aspects of one or more of the general embodiments, at least one of the rotor or stator contact surfaces may include a vehicular tire tread. 
         [0020]    In one or more specific aspects of one or more of the general embodiments, the driver may be an electric motor. 
         [0021]    In one or more specific aspects of one or more of the general embodiments, the second radial dimension may be smaller than the first radial dimension by a predetermined differential, and the fourth radial dimension may be larger than the third radial dimension by the predetermined differential. 
         [0022]    In one or more specific aspects of one or more of the general embodiments, the first radial dimension may be smaller than the second radial dimension by a predetermined differential, and the third radial dimension may be larger than the fourth radial dimension by the predetermined differential. 
         [0023]    In one or more specific aspects of one or more of the general embodiments, the predetermined differential may be approximately 0.1 inches; and the third radial dimension may be approximately 3 inches; and the second radial dimension may be between approximately 24 inches and approximately 36 inches. 
         [0024]    In one or more specific aspects of one or more of the general embodiments, the system may include a bearing disposed between the rotor and the stator. 
         [0025]    In one or more specific aspects of one or more of the general embodiments, the system may further include a spider assembly having: multiple spokes coupled to axles disposed through respective centers of two planetary members; and a web member coupled to the spokes and having an aperture operable to allow a shaft rigidly coupled to the rotor to pass through. 
         [0026]    In one or more specific aspects of one or more of the general embodiments, imparting a torque to the rotor during rotation of the plurality of planetary members about their respective planetary axes may include imparting a torque to the rotor during rotation of the plurality of planetary members about their respective planetary axes through frictional contact between the second and fourth circumferential surfaces. 
         [0027]    In one or more specific aspects of one or more of the general embodiments, imparting a torque to the rotor during rotation of the plurality of planetary members about their respective planetary axes may include imparting a torque to the rotor during rotation of the plurality of planetary members about their respective planetary axes through geared contact between the second and fourth circumferential surfaces. 
         [0028]    In one or more specific aspects of one or more of the general embodiments, rotating a plurality of planetary members about a first circumferential surface of a stator that is axially aligned about a central axis with a rotor that has a second circumferential surface may include: rotating a harness assembly coupled to at least one of the planetary members about the central axis, the harness assembly having at least one substantially rigid member connecting axles disposed through respective centers of two planetary members; and a biasing member connecting axles disposed through respective centers of two planetary members, wherein the biasing member urges the axles together; and rotating, in response to the rotation of the harness assembly, at least one of the planetary members about the central axis. 
         [0029]    In one or more specific aspects of one or more of the general embodiments, rotating a harness assembly coupled to at least one of the planetary members about the central axis may include: rotating at least one of first and second plates having centers aligned with the central axis, the rotor and stator sandwiched between the first and second plates. 
         [0030]    In one or more specific aspects of one or more of the general embodiments, rotating, in response to the rotation of the harness assembly, at least one of the planetary members about the central axis may include imparting a torque to the planetary member through frictional contact between the at least one of the first and second plates and the planetary member. 
         [0031]    In one or more specific aspects of one or more of the general embodiments, rotating, in response to the rotation of the harness assembly, at least one of the planetary members about the central axis includes: imparting a torque to the planetary member through geared contact between the at least one of the first and second plates and the planetary member. 
         [0032]    In one or more specific aspects of one or more of the general embodiments, rotating a plurality of planetary members about a first circumferential surface of a stator that is axially aligned about a central axis with a rotor that has a second circumferential surface may include rotating only one of the planetary members about its central planetary axis with a driver; imparting a torque on the other planetary members through contact between the respective third circumferential surfaces of the other planetary members and the first circumferential surface of the stator; and rotating the other planetary members about the central axis in response to the torque. 
         [0033]    In one or more specific aspects of one or more of the general embodiments, the first surface of the solar energy member may be a reflective surface configured to reflect the solar rays toward a solar energy receiver. 
         [0034]    In one or more specific aspects of one or more of the general embodiments, the first surface of the solar energy member may be a solar panel that includes one or more photovoltaic cells. 
         [0035]    In one or more specific aspects of one or more of the general embodiments, the controller may be configured to control at least one of azimuthal movement of the solar energy member and elevational movement of the solar energy member. 
         [0036]    In one or more specific aspects of one or more of the general embodiments, the drive assembly may be a first drive assembly configured to adjust position of the solar energy member in azimuth in response to commands received from the controller, the system further including a second drive assembly substantially identical to the first drive assembly and configured to adjust a position of the solar energy member in elevation in response to commands received from the controller. 
         [0037]    In one or more specific aspects of one or more of the general embodiments, the drive assembly may be exposed to an outside environment during normal operation. 
         [0038]    In one or more specific aspects of one or more of the general embodiments, the drive assembly may further include a spool coupled to the rotor, and the system further may include: a cable coupled to the spool at a first end and coupled to the solar energy member at a second end opposite the first end, wherein rotation of the rotor about the center axis causes rotation of the spool to effect one of reeling in a portion of the cable around the spool or releasing a portion of the cable from the spool such that the solar energy member is rotated based on the rotation of the spool. 
         [0039]    In one or more specific aspects of one or more of the general embodiments, the controller may be configured to control the driver at a first angular speed to compensate for slippage between at least one of a frictional contact between the third circumferential surface and the first circumferential, and a frictional contact between the fourth circumferential surface and the second circumferential surface. 
         [0040]    In another general embodiment, a drive apparatus includes: a rotor comprising a first circumferential surface and having a first radial dimension from a center axis of the rotor; a stator that is axially aligned with the rotor about the center axis and includes a second circumferential surface and having a second radial dimension from the center of the rotor, wherein the second radial dimension is different than the first radial dimension; a plurality of planetary members arranged about the first and second circumferential surfaces of the rotor and stator respectively, wherein each of the planetary members has an outer surface in contact with the first and second circumferential surfaces; and a driver configured to rotate the plurality of planetary members about the center axis, wherein each planetary member rotates about the member&#39;s planetary axis through contact between the second circumferential surface of the stator and the outer surface of the planetary member and wherein the rotation of the planetary members imparts rotation of the rotor about the center axis through contact between the outer surfaces of the planetary members and the first circumferential surface of the rotor. At least a portion of at least one of the plurality of planetary members is adapted to be substantially sealed against an outside environment during normal operation of the apparatus, and at least a portion of at least one of the rotor and stator are adapted to be exposed to the outside environment during normal operation of the apparatus. 
         [0041]    In one or more specific aspects of one or more of the general embodiments, at least one of the planetary members may include an axle such that a portion of the axle is disposed through a bore of the planetary member, and the portion of the planetary member adapted to be substantially sealed against the outside environment comprises the portion of the axle disposed through the bore. 
         [0042]    In one or more specific aspects of one or more of the general embodiments, the stator may be axially aligned with the rotor about the center axis such that the stator is mounted adjacent a bottom surface of the rotor. 
         [0043]    In one or more specific aspects of one or more of the general embodiments, the first radial dimension from the center axis of the rotor may be greater than the second radial dimension from the center of the rotor. 
         [0044]    In one or more specific aspects of one or more of the general embodiments, the rotor may include a conduit disposed on an upper surface of the rotor and adapted to direct a flow of fluid away from the center of the rotor. 
         [0045]    In one or more specific aspects of one or more of the general embodiments, at least one of the contact between the second circumferential surface of the stator and the outer surfaces of the planetary members, and the contact between the first circumferential surface of the rotor and the outer surfaces of the planetary members may be frictional contact. 
         [0046]    In one or more specific aspects of one or more of the general embodiments, at least one of the rotor and stator may be constructed of a moisture resistant material. 
         [0047]    In one or more specific aspects of one or more of the general embodiments, all of the rotor and the stator may be adapted to be exposed to the outside environment during normal operation of the apparatus. 
         [0048]    Various implementations of a solar energy system drive assembly according to the present disclosure may include one or more of the following features and/or advantages. For example, the drive assembly may provide a relatively high gear ratio to drive a solar energy member at precise rotational speeds potentially with high torque. Further, the drive assembly may be more efficiently manufactured from common materials without extensive machining as compared to conventional gear boxes. In addition, the drive assembly may be configured to rotate the solar energy member about an azimuthal axis, as well as an elevational axis, in order to track in response to the movement of the Sun. As another example, the drive assembly may operate with relatively little protection from environmental elements, and avoid the expense of having to hermetically seal the entire assembly into a housing. 
         [0049]    These general and specific aspects may be implemented using a device, system or method, or any combinations of devices, systems, or methods. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0050]      FIG. 1  illustrates an example embodiment of a solar energy system including a drive assembly for rotating a solar energy member; 
           [0051]      FIGS. 2A-2B  illustrate top and side views of an example embodiment of a drive assembly; 
           [0052]      FIG. 3  illustrates a side view of another example embodiment of a drive assembly; 
           [0053]      FIG. 4  illustrates a side view of another example embodiment of a drive assembly; 
           [0054]      FIGS. 5A-5B  illustrate top and side views of another example embodiment of a drive assembly; 
           [0055]      FIG. 6  illustrates another example embodiment of a solar energy system including a drive assembly for rotating a solar energy member; 
           [0056]      FIG. 7  illustrates an example method for operating a solar energy system including a drive assembly for rotating a solar energy member; 
           [0057]      FIGS. 8A-8B  illustrate top and side views of another example embodiment of a drive assembly; and 
           [0058]      FIG. 9  illustrates another example embodiment of a solar energy system including a drive assembly for rotating a solar energy member. 
       
    
    
     DETAILED DESCRIPTION 
       [0059]    In some embodiments, a drive assembly includes a fixed stator and a rotor free to rotate about a central axis coincident with the centers of the rotor and stator. The stator and rotor have distinct radial dimensions that differ by a predetermined amount. Multiple planetary members are arranged around the outer circumferential surfaces of the rotor and stator. Each of the planetary members includes a stepped outer circumferential surface such that one portion of the planetary member having a first radial dimension is in contact with the fixed stator while a second portion of the member having a second radial dimension is in contact with the rotor. The first and second radial dimensions of the planetary members differ by the predetermined amount. Rotational movement imparted to at least one of the planetary members by a driver causes rotation of the members about the central axis, as well as about their planetary axes due to the contact with the fixed stator. Torque is imparted to the rotor by the contact between the planetary members and the rotor, thereby causing the rotor to rotate. The rotational speed of the planetary members may be much greater than that of the rotor, thereby providing a relatively high gear ratio for the rotor. In some implementations, such a drive assembly may be utilized with a solar energy system to, for example, rotate one or more solar energy members about an axis. 
         [0060]    In some embodiments, a solar energy system includes a drive assembly, which rotates a solar energy member (e.g., heliostat mirror, PV panel) about an axis with an effectively high gear ratio. For instance, the drive assembly may include a fixed stator and rotor concentrically mounted about a bearing but with the stator and rotor having different radial dimensions. A number of planetary members are mounted about circumferential surfaces of the rotor and stator and have stepped circumferential surfaces (i.e., at least two different radial dimensions) to contactingly interface with the rotor and stator. As power is applied to a driver of the drive assembly to rotate the planetary members about a central axis of the rotor and stator, the contacting interface between the fixed stator and planetary members causes rotational movement of the planetary members about their respective planetary axes. During rotation of the planetary members about their respective planetary axes, torque is imparted to the rotor, causing rotational movement of the rotor about the central axis at a speed less than (and in some cases, much less than) the rotational speed of the planetary members about their respective planetary axes. A solar energy member rigidly coupled to the rotor may thus rotate at the reduced speed as well. 
         [0061]      FIG. 1  illustrates an example embodiment of a solar energy system  100  including a drive assembly for rotating a solar energy member  125 . In the illustrated embodiment, the solar energy system  100  includes a support member  105 , a base  110  mounted on the support member  105 , a drive assembly  115  mounted on the base  110 , a solar energy member support  120  mounted on the drive assembly  115 , and a solar energy member  125  coupled to the solar energy member support  120 . Solar energy system  100 , as illustrated, may collect or reflect solar energy from a remote source (e.g., the Sun or other solar energy source) while rotatably tracking the source under varying environmental conditions. For example, in some embodiments, the solar energy system  100  may be a heliostat that tracks (e.g., rotates to change its azimuth and/or pivots to change its elevation) the Sun in order to receive and reflect solar energy from the Sun to a solar energy collector (receiver) located remote from the heliostat. Alternatively, the solar energy system  100  may be a PV system where the solar energy member  125  is a PV panel that tracks the Sun to receive and collect solar energy to, for example, produce electricity. In any event, in some instances, the solar energy system  100  may be one of many systems  100  installed within a field or array that operate to collect and/or reflect solar energy provided by the remote source. 
         [0062]    Changes in azimuth of the solar energy member  125  refers to rotation of the solar energy member  125  about a vertical axis, i.e., rotation about a vertical axis coincident with an axis through a centerline of the support member  105  (an “azimuthal axis”). Changes in elevation of the solar energy member  125  refers to changes in the angle between the direction the solar energy member  125  is pointing and a local horizontal plane, i.e., changes in the up-down angle (rotation about an “elevational axis”). The solar energy member  125  is mounted to the support  120  such that rotation about the azimuthal axis and rotation about the elevational axis within desired ranges to account for tracking the Sun throughout the course of day and throughout the days of a year are permitted. For example, in some implementations, a bearing at the interface  122  can operate to rotate the solar energy member  125  about the elevational axis. 
         [0063]    The support member  105 , as illustrated, is substantially vertical in orientation and may be mounted orthogonal to a terranean surface  135 . The support member  105 , in some embodiments, may be a wooden post, such as a cylindrical wooden post treated for exposure to varying environmental conditions (e.g., moisture, heat, and otherwise). Alternatively, the support member  105  may be any suitable material, such as stainless steel, painted ferrous steel, formed concrete, or otherwise, that may be secured in a substantially vertical position and support the solar energy member  125 . 
         [0064]    The illustrated support member  105  may be secured and/or attached to a footer  130  at one end of the member  105 . In some embodiments, the footer  130  may be a concrete foundation installed to a particular depth below the terranean surface  135 , thereby forming a cantilevered beam with the support member  105 . Alternatively, however, the footer  130  may be supported by the terranean surface  135  without being installed or anchored below the surface. For example, the footer  130  may be a structure that can support the member  105  in a substantially vertical position under the weight of the solar energy member  125  (both static weight and dynamic weight during movement of the solar energy member  125 ). For example, the footer  130  can be a block or mass of concrete or other material (e.g., glass reinforced plastic) that includes an aperture or other recess for installation of the support member  105  therein. Further, in some embodiments, the footer  130  may not be installed to support the support member  105  and instead, the support member  105  may be inserted into a post hole formed in the terranean surface  135 . 
         [0065]    The base  110  is mounted or coupled to the support member  105  and, typically, provides a transitional member between the support member  105  and the drive assembly  115 . In some embodiments, the base  110  may be eliminated or replaced with another form of transitional member. 
         [0066]    The drive assembly  115  is coupled to the support member  105  through the base  110  and, in the illustrated embodiment, may rotate one or more components of the solar energy system  100  in order to, for example, allow the solar energy member  125  to track the Sun during its azimuthal and elevational movement. For instance, in some embodiments, the drive assembly  115  (as more fully described in reference to  FIGS. 2-7 ) may include a substantially fixed stator (e.g., a disc, cylindrical stator, or otherwise), a rotor (e.g., a disc, cylindrical stator, or otherwise) mounted concentrically on top of the stator and free to rotate with respect to the stator, and multiple (e.g., two or more, and preferably three or more) planetary members formed with stepped exterior, circumferential surfaces. Further, as described more fully below, the drive assembly  115  may include one or more drive mechanisms and a harness assembly for rotating the rotor and planetary members about respective axes of rotation. In some embodiments, the drive assembly  115  may, therefore, provide for controlled rotation to a predetermined gear ratio of one or more components of the solar energy system  100 , such as the solar energy member  125 . 
         [0067]    Although illustrated enclosed in a container in  FIG. 1 , the drive assembly  115  may be substantially open to environmental conditions, such as rain, snow, dust, and otherwise, whether during operation or while idle. For example, one or more components of the drive assembly  115  (described in more detail below) may not be sealed (e.g., hermetically) in a housing or container. Alternatively, and as shown in  FIG. 1  for example, the drive assembly  115  may be substantially contained in an enclosure (e.g., weatherproof container). 
         [0068]    The solar energy member support  120  is mounted or coupled to the drive assembly  115  and provides structural support for the solar energy member  125 . Although one embodiment of the solar energy member support  120  is illustrated in  FIG. 1 , the solar energy member support  120  may take many different configurations not shown here without departing from the scope of the present disclosure. In the illustrated embodiment, the solar energy member support  120  may be mounted to the drive assembly  115  through a common, rotatable shaft, such that rotation of the drive assembly  115  (or components of the drive assembly  115  such as a rotor) also rotates the shaft and, therefore, support  120 . As the solar energy member support  120  may be rigidly coupled to the solar energy member  125 , the solar energy member  125  may also rotate (e.g., about an azimuthal axis). In the illustrated embodiment, the solar energy member support  120  is mounted to the drive assembly  115  via the interface  122 . The interface  122  may provide a cradle bearing (e.g., a goniometric bearing) that allows the support  120  to “rock” back and forth to adjust the elevational position of the solar energy member  125 . 
         [0069]    In the illustrated embodiment, the solar energy member  125  may be a heliostat mirror, which receives and reflects solar energy incident on a surface of the member  125  toward a remote location, such as a solar energy receiver. Alternatively, however, the solar energy member  125  may be another solar energy device, such as a PV panel. In any event, the solar energy member  125 , typically, is substantially planar or can be curved and includes at least one surface that receives and reflects (i.e., a heliostat mirror) or receives and absorbs (i.e., a PV panel) solar energy. Although illustrated in  FIG. 1  as a single, monolithic panel, the solar energy member  125  may be split into two (or more) equally or unequally-sized panels that together form the solar energy member  125 . 
         [0070]      FIGS. 2A-2B  illustrate top and side views of various components of an example embodiment of a drive assembly  200 . The drive assembly  200  can be configured to rotate a solar energy member, such as the solar energy member  125  shown in  FIG. 1 , although the drive assembly  200  can also be used in other implementations. In some embodiments, the illustrated components of drive assembly  200  may be used in or with the drive assembly  115  shown in  FIG. 1 . Generally, the drive assembly  200  may receive a rotational torque at a first rotational speed (i.e., angular speed, ω) and transform the first rotational speed to a second rotational speed according to a predetermined ratio. The second rotational speed, which in some embodiments may be less than, or much less than, the first rotational speed, may be applied to a rotor and/or shaft. The rotor and/or shaft may be coupled to, for example, a solar energy member in order to rotate the solar energy member at the second rotational speed. 
         [0071]    As illustrated, the drive assembly  200  includes a cylindrical rotor  205  and a cylindrical stator  230  having centers coincident with a central axis  201 ; multiple planetary members  210  having respective planetary axes  212  arranged around respective circumferential surfaces of the rotor  205  and stator  230 ; and a harness assembly  215  coupled to the planetary members  210 . The illustrated drive assembly  200  also includes a driver  240  coupled to one of the planetary members  240 . 
         [0072]    As illustrated, the rotor  205  and stator  230  are cylindrical disks having respective centerlines coincident with the central axis  201  therethrough. The rotor  205 , as illustrated, has a radius Rr, which is slightly larger than a radius of the stator  230 , which has a radius, Rs. The rotor  205  may be mounted concentrically on top of the stator  203  (e.g., via a shaft), and the rotor  205  is free to rotate with respect to the stator  230 , which is rotationally fixed. The rotor  205  and stator  230  may be constructed to be very nearly the same size (i.e., (Rr−Rs)&lt;&lt;Rr). For example, the difference in radial dimensions of the rotor  205  and the stator  230  may be less than approximately 5% of Rr. In some aspects, one or both of the rotor  205  and the stator  230  may be formed, concrete discs, having relatively large tolerances (e.g., plus or minus 1 mm). Along with having such tolerances, one or both of the rotor  205  and the stator  230  may be formed from weather resistant (e.g., moisture resistant, thermal resistant) material, such that exposure to environmental conditions (during operation or otherwise) may not substantially affect the rotor  205  and stator  230 . 
         [0073]    In the illustrated embodiment, a bearing  235  is disposed between the rotor  205  and the stator  230 . The bearing  235  may provide for reduced friction movement of the rotor  205  as it rotates with respect to the stator  230 . For example, the bearing  235  may be a thrust bearing formed using matched circular grooves in the faces of the rotor  205  and the stator  230  with rollers (e.g., polyurethane balls) which roll in the grooves. Alternatively, or additionally, the bearing  235  may be a lubricating material that reduces a frictional force generated during rotation of the rotor  205 . 
         [0074]    The planetary members  210  are arranged around respective outer circumferential surfaces of the rotor  205  and the stator  230 , and, as illustrated, have stepped circumferential outer surfaces such that an upper portion of a planetary member  210  has a smaller radial dimension than a radial dimension of a lower portion of the planetary member  210 . That is, the planetary members  210  may be stepped cylindrically shaped members (solid, hollow or otherwise) with two, concentric steps. The steps have slightly different radii, Rpr and Rps, where, for example, Rpr is smaller than Rps. The difference between the radii of the steps may be equal or substantially equal to the difference in radial dimensions of the rotor  205  and the stator  230  (Rr and Rs, respectively). In the illustrated embodiment, the planetary members  210  are constructed such that Rps and Rpr are both much smaller than the radial dimensions of the rotor  205  and the stator  230  (i.e., Rpr&lt;Rps&lt;&lt;Rr). For example, the planetary members may be less than approximately ¼ the radius of the rotor  205  and stator  230  and, in some aspects, much less. In some implementations, the planetary members  210  are wheels having a stepped outer surface as described above. 
         [0075]    In the illustrated embodiment, each planetary member  210  includes a lower planetary member surface  216  and an upper planetary member surface  218 . The surfaces  216  and  218  are circumferential and are opposed, respectively, with a stator circumferential surface  232  and a rotor circumferential surface  207 . The lower planetary member surface  216  is in contact (e.g., frictional) with the stator circumferential surface  232 , while the upper planetary member surface  28  is in contact (e.g., frictional) with the rotor circumferential surface  207 . As discussed above, the lower planetary member surface  216  has a radial dimension, Rps, from the planetary axis  212 , while the upper planetary member surface  218  has a radial dimension, Rpr, from the planetary axis  212 . 
         [0076]    As illustrated, each planetary member  210  includes an axle  214  disposed through the planetary axis  212  of the planetary member  210 . In some implementations, the axle  214  may be sealed (e.g., hermetically) through the planetary member  210  such that moisture, dust, or other substance is substantially prevented from intrusion therein. 
         [0077]    In the implementation shown, the drive assembly  200  also includes a harness assembly  215 . The harness assembly  215  includes linkages  220  coupled to axles  214  of the planetary members  210 . The harness assembly  215  also includes a biasing member  225  coupled between two of the planetary members  215 . The biasing member  225 , which, in some aspects, may be a spring, a bungee, or other biasing element, urges the axles  214  of the respective planetary members  210  together. The respective planetary members  210  may, therefore, be urged together to maintain contact of the planetary members  210  against the rotor  205  and stator  230 . As illustrated, the harness assembly  215  is free to rotate with respect to both the stator  230  and the rotor  205  about the central axis  201 . 
         [0078]    As illustrated in  FIG. 2B , the driver  240  is coupled to the axle  214  of one of the planetary members  210 . While more than one of the planetary members  210  may be coupled to a driver (e.g., an electric motor) and driven (e.g., rotated) about its planetary axis  212 , one driver  240  may be coupled to one planetary member  210  and utilized for operation of the drive assembly  200 . In other implementations, there may be multiple drivers  240 . 
         [0079]    In operation, the driver  240  may rotate the planetary member  210  to which it is coupled, which in turn rotates the harness assembly  215 . Alternatively, the driver may be configured to directly rotate the harness assembly  215  coupled to the planetary members  210 , at a first rotational speed. In any event, the harness assembly  215  carries the planetary members  210  along as it rotates, keeping the planetary members  210  in firm contact with the rotor  205  and stator  230  (i.e., keeping contact between the upper planetary member surface  218  and rotor circumferential surface  207 , and between the lower planetary member surface  216  and the stator circumferential surface  232 ). As the harness assembly  215  rotates about the central axis  201 , the planetary members  210  also rotate about their own respective planetary axes  212  due to the rolling contact between the lower planetary member surface  216  and the fixed stator circumferential surface  232 . As the planetary members  210  rotate, they are also in rolling contact with the rotor  205  and impart a torque to the rotor  210 . Due to the difference in radial dimensions between rotor  205  and the stator  230  (Rr&gt;Rs), the rotor  205  may be rotated with the rotation of the harness assembly  215 , but at a second rotational speed that is substantially reduced compared to the first rotational speed. Further, the rotor  205  may be rotated with a substantially-multiplied torque. The net result is an effective gear ratio, with the harness assembly  215  serving as an input and the rotor  205  serving as an output. While not shown, a shaft rigidly coupled to the rotor  205  will thus rotate at the second rotational speed as well. Further, any structure rigidly coupled to the shaft (directly or indirectly), such as a solar energy member, will also rotate at the second rotational speed. 
         [0080]    In some aspects, the effective gear ratio described above may be predetermined and/or calculated with reference to the first rotational speed and the relative dimensions of the rotor  205 , stator  230 , and planetary members  210 . For example, the second rotational speed may be calculated according to the following equation: 
         [0000]      ω R =GR*ω PM    [Equation 1],
 
         [0000]    where ω R  is the second rotational speed (i.e., the rotational speed of the rotor  205 ); GR is the gear ratio, and ω PM  is the first rotational speed (i.e., the rotational speed of the harness assembly  215  about the central axis  201 ). The gear ratio, GR, may be calculated according to the equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     GR 
                     = 
                     
                       
                         E 
                         
                           r 
                           + 
                           E 
                         
                       
                       * 
                       
                         
                           R 
                           + 
                           r 
                         
                         R 
                       
                     
                   
                   , 
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
         [0000]    where E is the difference between the radial dimensions of the rotor  205  and the stator  230 ; R is the radial dimension, Rr, of the rotor  205 ; and r is the radial dimension of the upper planetary member surface  218 , Rpr, of the planetary members  210 . 
         [0081]    In one embodiment, the dimensions may be as follows: R is approximately 12 inches; r is approximately 3 inches; and E is approximately 0.1 inches. Accordingly, the gear ratio, GR, according to Equation 2 is approximately 1/24 (e.g., with geared surfaces and close to this value with frictional contact surfaces). Given a steady state first rotational speed of the harness assembly  215 , the second rotational speed may be calculated per Equation 1 above. 
         [0082]      FIG. 3  illustrates a side view of another example embodiment of a drive assembly  300 . In some implementations, the drive assembly can be used for rotating a solar energy member, such as the solar energy member  125  shown in  FIG. 1  (or other solar energy members), although other uses for the drive assembly  300  are possible. As illustrated, the drive assembly  300  includes a cylindrical rotor  305  (with rotor circumferential surface  307 ) and a cylindrical stator  330  (with stator circumferential surface  332 ) having centers coincident with a central axis  301 ; multiple planetary members  310  having respective planetary axes  312  and axles  314  there through; and a harness assembly  315  with one or more linkages  320  coupled to the planetary members  310 . The illustrated drive assembly  300  also includes a driver  340  coupled to one of the planetary members  340  and/or the harness assembly  315 . The illustrated planetary members  310  include an upper planetary member surface  318  and a lower planetary member surface  316 . Further, a bearing  335  may be disposed between the rotor  305  and the stator  330 . 
         [0083]    The foregoing components of the drive assembly  300  may be substantially similar to similarly named components illustrated in  FIGS. 2A-2B . However, as illustrated, the stator circumferential surface  332  and lower planetary member surface  316  are geared surfaces that mesh together during rotation of the planetary members  310  about the stator  330 . Thus, rather than utilizing a frictional contact between surfaces  316  and  332  (for example, as shown in  FIG. 2B ), a geared interface is utilized. In addition, although not shown in  FIG. 3 , a geared interface may be utilized between the planetary members  310  and the rotor  305 . For example, the upper planetary member surface  418  and rotor circumferential surface  407  may be geared (i.e., have gear teeth disposed about the circumferential surfaces), such that torque is imparted to the rotor  305  by the planetary members  310  through the geared interface. 
         [0084]      FIG. 4  illustrates a side view of another example embodiment of a drive assembly  400 . In some implementations, the drive assembly  400  can be used for rotating a solar energy member, such as the solar energy member  125  (or other solar energy members), although other uses for the drive assembly  400  are possible. As illustrated, the drive assembly  400  includes a cylindrical rotor  405  (with rotor circumferential surface  407 ) and a cylindrical stator  430  (with stator circumferential surface  432 ) having centers coincident with a central axis  401 ; multiple planetary members  410  having respective planetary axes  412  and axles  414  therethrough; and a harness assembly  415  with one or more linkages  420  coupled to the planetary members  410 . The foregoing components of the drive assembly  400  may be substantially similar to similarly named components illustrated in  FIGS. 2A-2B  and  3 . As illustrated, the stator circumferential surface  432  and lower planetary member surface  416  are geared surfaces that mesh together during rotation of the planetary members  410  about the stator  430 . Thus, rather than utilizing a frictional contact between surfaces  416  and  432 , a geared interface is utilized. 
         [0085]    The illustrated drive assembly  400  also includes a driver  440  and a chain drive  450 . As illustrated, the chain drive  450  interfaces with the lower planetary member surface  416  (which is geared) and is disposed about the planetary members  410 . The driver  440  (e.g., an electric motor) includes a gear  445  disposed on a shaft of the driver  440 , which may rotate as the shaft of the driver  440  rotates. As the gear  445  also interfaces with the chain drive  450 , rotation of the gear  445  will impart rotation to the chain drive  450 , and thus, the planetary members  410 . Alternatively, in some embodiments, the chain drive  450  may be eliminated and the gear  445  may interface directly with the lower planetary member surface  416 . 
         [0086]    In operation, the driver  440  can be powered (e.g., by electricity or otherwise) to rotate the gear  445 , which in turn rotates the planetary members  410  (e.g., through the chain drive  450  or otherwise) at a first rotational speed about the central axis  401 . As described above, as the planetary members  410  rotate about their respective planetary axes  412  and the central axis  401 , a torque is imparted to the rotor  405 , thereby causing the rotor  405  to rotate at a second rotational speed. The second rotational speed, as described above, is a function of the first rotational speed and the specific gear ratio, GR. 
         [0087]      FIGS. 5A-5B  illustrate top and side views, respectively, of another example embodiment of a drive assembly  500 . In some implementations, the drive assembly  500  can be used for rotating a solar energy member, such as solar energy member  125  or other solar energy member, although other uses of the drive assembly  500  are possible. As illustrated, the drive assembly  500  includes a cylindrical rotor  505  (with rotor circumferential surface  507 ) and a cylindrical stator  530  (with stator circumferential surface  532 ) having centers coincident with a central axis  501 ; multiple planetary members  510  having respective planetary axes  512  and axles  514  therethrough; and a bearing  535  disposed between the rotor  505  and the stator  530 . The foregoing components of the drive assembly  500  may be substantially similar to similarly named components illustrated in  FIGS. 2A-2B . 
         [0088]    In addition, drive assembly  500  includes a driver  540  coupled to a gear  545 , a top plate  560  disposed on top of the rotor  505 , a bottom plate  570  disposed below the stator  530 , and a shaft  575  connecting the top and bottom plates  560  and  570  through apertures formed in the rotor  505 , the stator  530 , and the bearing  535 . As illustrated, the top plate  560  is a substantially cylindrical disc with a center coincident with the central axis  501  and having apertures  580  allowing the axles  514  of the planetary members  510  to protrude upwardly therethrough. In some embodiments, the apertures  580  may be formed such that incremental radial movement (e.g., about 1 centimeter) of the axles  514  (e.g., towards the central axis  501 ) is allowed. The top plate  560  and apertures  580  may otherwise constrain the axles  514  to rotate about the central axis  501  during rotational movement of the top plate  560  (as explained below). 
         [0089]    The top plate  560  includes an outer circumferential surface  550 , which, as illustrated, is geared. The outer circumferential surface  550  thus meshes with the gear  545  such that rotational movement of the gear  545  (e.g., during operation of the driver  540 ) is imparted to the top plate  560 . Alternatively, the outer circumferential surface  550  and gear  545  may be formed for frictional contact therebetween. 
         [0090]    The bottom plate  570 , as illustrated,  560  is a substantially cylindrical disc with a center coincident with the central axis  501  and is rigidly coupled to the top plate  560  via the shaft  575 . The bottom plate  570 , as illustrated, may provide increased structural rigidity to the drive assembly  500  and, in some aspects, may help seal the drive assembly  500  against environmental conditions. During rotational movement of the top plate  560 , therefore, the bottom plate  570  also rotates with the same rotational velocity. Of course, in some embodiments, the bottom plate  570  may have a geared outer surface and the driver  540  may drive the bottom plate  570  via the gear  545  (e.g., in place of or in addition to driving the top plate  560 ). 
         [0091]    In one example operation, the driver  540  rotates the gear  545 , which interfaces with the outer circumferential surface  550 . The rotation of the gear  545  rotates the top plate  560  at a first rotational speed. As the top plate  560  rotates about the central axis  501 , the axles  514  also rotate about the central axis  501  as they are rotationally urged by the top plate  560 . As the axles  514  are rotatably urged around the central axis  501 , the planetary members  510  are also rotated around the central axis  510  while in contact (e.g., frictional, geared, or otherwise) with the rotor  505  and the stator  530 . In some aspects, the top plate  560  may also include a mechanism for pressing the planetary members  510  forcefully into contact with the stator  530  and the rotor  510 , such as, for example, a spring or other source of elastic tension. As described above, as the planetary members  510  rotate about their respective planetary axes  512  in contact with the fixed stator  530 , a torque is imparted by the planetary members  510  to the rotor  505 . The imparted torque on the rotor  505  forcibly rotates the rotor  505  about the central axis at a second rotational speed that is different (i.e., lower and, preferably, much lower) than the first rotational speed. A structure rigidly coupled (directly or indirectly) with the rotor  505 , such as a solar energy member, also rotates at the second rotational speed. 
         [0092]      FIG. 6  illustrates another example embodiment of a solar energy system  600  including a drive assembly for rotating a solar energy member  605 . In some embodiments, the solar energy system  600  may include two (or more) drive assemblies to rotate the solar energy system  600  about at least two axes of rotation. For example, system  600  includes a first drive assembly  625  that rotates the solar energy member  605  about an azimuthal axis  650  and a second drive assembly  610  that rotates the solar energy member  605  about an elevational axis  645 . One or both of the drive assemblies  610  and  625  may be substantially similar to one of the drive assemblies described above with respect to  FIGS. 2A-2B ,  3 ,  4 , and  5 A- 5 B. For example, drive assembly  610  may be one of the drive assemblies described above but flipped on its side. Thus, a shaft  615  coupled to a rotor of the drive assembly  610  is coincident with the axis  645 . Rotation of the shaft  615 , therefore, rotates the solar energy member  605  through varying elevational positions. While illustrated as enclosed within a housing, either or both of the drive assemblies  610  and  625  may be exposed (substantially or completely) to an outdoor environment during operation. 
         [0093]    As illustrated, the solar energy system  600  also includes a support member  630 , a support  620 , an interface  622 , a first controller  635 , and a second controller  640 . The support member  630  and the support  620  may, in some embodiments, be substantially similar to the same components described in reference to  FIG. 1 . As illustrated, the solar energy member  605  is split down a centerline of the member  605  coincident with the azimuthal axis  650 , with the second drive assembly  610  mounted between and coupled to first and second portions of the solar energy member  605 . Alternatively, the solar energy member  605  may be divided into more portions, different sized portions, or left as a single panel. The solar energy member  605  may be flat as shown or may have a curved surface that faces toward the Sun. 
         [0094]    The first controller  635  is communicably coupled with the first drive assembly  625  and operably controls the first drive assembly  625  to rotate the solar energy member  605  about the azimuthal axis  650 . For example, the first controller  635  may receive and/or measure various data (e.g., Sun location, time of day, wind speed, solar receiver location, or otherwise) and algorithmically determine an optimal azimuthal position of the solar energy member  605 . The first controller  635  may then transmit signals to the first drive assembly  625  to operate the assembly  625  (e.g., operate a driver of the drive assembly  625 ) to rotate the solar energy member  605  to the optimal azimuthal position. 
         [0095]    In similar fashion, the second controller  640  is communicably coupled with the second drive assembly  610  and operably controls the second drive assembly  610  to rotate the solar energy member  605  about the elevational axis  645 . For example, the second controller  640  may receive and/or measure various data (e.g., Sun location, time of day, wind speed, solar receiver location, or otherwise) and algorithmically determine an optimal elevational position of the solar energy member  605 . The second controller  640  may then transmit signals to the second drive assembly  610  to operate the assembly  610  (e.g., operate a driver of the drive assembly  610 ) to rotate the solar energy member  605  to the optimal elevational position. In some implementations, the first controller  635  and the second controller  640  are implemented together as a single controller that is configured to carry out the operations described above. 
         [0096]    While  FIG. 6  shows one example embodiment of the solar energy system  600 , others are within the scope of the present disclosure. For example, the solar energy system  600  may only include one of the two drive assemblies  610  and  625 . Alternatively, there may be more than two drive assemblies; for example, there may be a separate drive assembly for each panel of the solar energy member  605 . In addition, while two controllers  635  and  640  are shown, there may be fewer or more controllers operably controlling the drive assemblies  610  and  625 . In addition, the controllers  635  and  640  may be located at the respective drive assemblies  625  and  610 ; at a remote location (e.g., at a facility designed to control an array of solar energy systems  600 ); or integral to the drive assemblies themselves, to name but a few locations. In some implementations, one or both of the controllers  635  and  640  provide signals to continuously adjust the position of the solar energy member  605 . In other implementations, signals are provided at predetermined intervals (e.g., every 15 minutes). In other implementations, signals are provided at predetermined intervals to account for predictable conditions (e.g., the position of the Sun) and on an as-needed basis to account for unpredictable conditions (e.g., being moved off target by a wind force). Other techniques can be used to determine when and by how much to adjust the position of the solar energy member  605 . 
         [0097]      FIG. 7  illustrates an example method  700  for operating a solar energy system including a drive assembly for rotating a solar energy member. In some embodiments, method  700  may be used, for example, to operate solar energy system  100 , solar energy system  600 , or any other solar energy system. Method  700  may begin at step  702 , when power is supplied to a driver of the solar energy system. In some embodiments, the driver, which may be similar, for example, to driver  240  (e.g., an electric motor) may receive a command to start from a controller. For instance, a controller, such as controller  635  or controller  640  shown in  FIG. 6 , may algorithmically determine that a solar energy member of the solar energy system may need to rotate (e.g., to track elevational or azimuthal movement of the Sun). 
         [0098]    Next, the driver may rotate one or more planetary members of a drive assembly of the solar energy system about a stator at a first rotational speed at step  704 . For example, as described above, the driver may directly rotate one planetary member (such as planetary member  210 ) about a central axis of the drive assembly, thereby generating rotational movement of the planetary members about their respective planetary axes while in contact (e.g., frictional, geared or otherwise) with the fixed stator. Alternatively, the driver may rotate a harness assembly coupled to the planetary members about the central axis of the drive assembly, thereby generating rotational movement of the planetary members about the central axis. In addition, the driver may rotate a plate coupled to the planetary members, as described above with respect to  FIGS. 5A-5B . And as described more fully below, a driver may rotate a spider assembly coupled to the planetary members to generate rotational movement of the planetary members about a central axis. 
         [0099]    In step  706 , a torque is imparted to a rotor of the drive assembly through contact (e.g., frictional, geared, or otherwise) with the rotating planetary members. As described above, the rotor may be free to rotate relative to the stator, and may have a slightly larger radial dimension than the stator. The planetary members, therefore, may have a stepped cylindrical outer surface, such that contact between the planetary members and the rotor and stator is maintained during rotation of the planetary members about the central axis. As the planetary members rotate about their respective planetary axes and the stator, the torque may be imparted to the rotor by the rolling contact between the planetary members and rotor. 
         [0100]    In step  708 , the rotor may rotate about the central axis due to the imparted torque. As described above, the rotor rotates at a second rotational speed which is different (e.g., less and often much less) than the first rotational speed of the planetary members about their respective planetary axes. At step  710 , the solar energy member rigidly coupled to the rotor (e.g., via a shaft) also rotates at the second rotational speed. In some embodiments, the driver may receive power for a set time duration in order to rotate the solar energy system a predetermined angular distance (e.g., elevationally or azimuthally). For instance, the controller may be pre-programmed with data regarding the second rotational speed as a function of the gear ratio (as described above) and an input speed of the driver (i.e., the first rotational speed); thus, the controller may be pre-programmed or may pre-determine an angular distance travelled of the solar energy member per time unit as a function of the first rotational speed. The controller may, therefore, pre-determine a time duration in which the driver is powered in order to meet a desired angular rotation distance of the solar energy member. Once power to the driver has been applied for the pre-determined time duration, the driver may be stopped, thereby stopping angular rotation of the solar energy member coupled to the rotor of the drive assembly. Of course, the foregoing is an example operation and other operations that accomplish desired angular movement of the solar energy member have been described herein and are within the scope of this disclosure. 
         [0101]      FIGS. 8A-8B  illustrate top and side views of various components of another example embodiment of a drive assembly  800 . The drive assembly  800  can be configured to rotate a solar energy member, such as the solar energy member  125  shown in  FIG. 1 , although the drive assembly  800  can also be used in other implementations. In some embodiments, the illustrated components of drive assembly  800  may be used in or with the drive assembly  115  shown in  FIG. 1 . Generally, the drive assembly  800  may receive a rotational torque at a first rotational speed and transform the first rotational speed to a second rotational speed according to a predetermined ratio. The second rotational speed, which in some embodiments may be less than, or much less than, the first rotational speed, may be applied to a rotor and/or shaft. The rotor and/or shaft may be coupled to, for example, a solar energy member in order to rotate the solar energy member at the second rotational speed. 
         [0102]    As illustrated, the drive assembly  800  includes a cylindrical rotor  805  and a cylindrical stator  830  having centers coincident with a central axis  801 ; multiple planetary members  810  having respective planetary axes  812  arranged around respective circumferential surfaces of the rotor  805  and stator  830 ; and a spider assembly  815  coupled to the planetary members  810 . The illustrated drive assembly  800  also includes a driver  840  coupled to one of the planetary members  840  and a bearing  835  disposed between the rotor  805  and the stator  830 . The cylindrical rotor  805 , cylindrical stator  830 , planetary members  810 , bearing  835 , and driver  840  may be substantially similar to those components described above with respect to  FIGS. 2A-2B . 
         [0103]    As illustrated, each planetary member  810  includes an axle  814  disposed through the planetary axis  812  of the planetary member  810 . In some implementations, the axle  814  may be sealed (e.g., hermetically) through the planetary member  810  such that moisture, dust, or other substance is substantially prevented from intrusion therein. In the implementation shown, the drive assembly  800  also includes a spider assembly  815 . The spider assembly  815  includes spokes  820  coupled to axles  814  of the planetary members  810  and also coupled to a web member  817 . The planetary members  810  may be rigidly or semi-rigidly held against the rotor  805  and stator  830  by the spokes  820 . 
         [0104]    As illustrated, the spider assembly  815  is free to rotate with respect to both the stator  830  and the rotor  805  about the central axis  801 . The spider assembly  815 , as illustrated, may fit over a shaft  850  that is coupled to the rotor  850  with little or no contact with the shaft  850 . For example, the web member  817  may include an aperture therethrough that allows the shaft  850  to pass through the web member  817  with little or no contact. Thus, rotation of the spider assembly  815  may be decoupled from rotation of the rotor  805  (and shaft  850 ) such that the spider assembly  815  and rotor  805  may rotate at different speeds about the central axis  801 . 
         [0105]    As illustrated in  FIG. 8B , the driver  840  is coupled to the axle  814  of one of the planetary members  810 . While more than one of the planetary members  810  may be coupled to a driver (e.g., an electric motor) and driven (e.g., rotated) about its planetary axis  812 , one driver  840  may be coupled to one planetary member  810  and utilized for operation of the drive assembly  800 . In other implementations, there may be multiple drivers  840 . 
         [0106]    In operation, the driver  840  may rotate the planetary member  810  to which it is coupled, which in turn rotates the spider assembly  815 . Alternatively, the driver may be configured to directly rotate the spider assembly  815  coupled to the planetary members  810 , at a first rotational speed. In any event, the spider assembly  815  carries the planetary members  810  along as it rotates, keeping the planetary members  810  in firm contact with the rotor  805  and stator  830  (i.e., keeping contact between the upper planetary member surface  818  and rotor circumferential surface  807 , and between the lower planetary member surface  816  and the stator circumferential surface  832 ). As the spider assembly  815  rotates about the central axis  801 , the planetary members  810  also rotate about their own respective planetary axes  812  due to the rolling contact between the lower planetary member surface  816  and the fixed stator circumferential surface  832 . As the planetary members  810  rotate, they are also in rolling contact with the rotor  805  and impart a torque to the rotor  810 . Due to the difference in radial dimensions between rotor  805  and the stator  830  (Rr&gt;Rs), the rotor  805  may be rotated with the rotation of the spider assembly  815 , but at a second rotational speed that is substantially reduced compared to the first rotational speed. Further, the rotor  805  may be rotated with a substantially-multiplied torque. The net result is an effective gear ratio, with the spider assembly  815  serving as an input and the rotor  805  serving as an output. While not shown, a shaft rigidly coupled to the rotor  805  will thus rotate at the second rotational speed as well. Further, any structure rigidly coupled to the shaft (directly or indirectly), such as a solar energy member, will also rotate at the second rotational speed. 
         [0107]    As illustrated, in  FIG. 8A , the rotor  820  includes two fluid conduits  813  disposed on an upper surface of the rotor  805 . In some embodiments, the fluid conduits  813  may be weep channels, funneling liquid and other debris away from the central axis  801  and towards the rotor circumferential surface  807 . For example, in some embodiments, the drive assembly  800  (like other illustrated drive assemblies disclosed herein) may be exposed to an outdoor environment during operation. For instance, all or part of the rotor  805 , the stator  830 , the planetary members  810 , and other components of the drive assembly  800  may operate without any enclosure while exposed to, for example, rain, snow, sleet, and other environmental elements. In some aspects, the only portions of the drive assembly  800  that may be sealed (substantially or completely) against the outdoor environment are the axles  814  disposed through the planetary members  810 . During periods of inclement weather during operation of the drive assembly, moisture, such as rain, snow, or sleet, may accumulate on the upper surface of the rotor  805 . Such moisture may gather (e.g., by a graded surface on the upper surface of the rotor  805 ) in the fluid channels  813  and ultimately may be funneled off the rotor  805 . Further, debris within the fluid and other debris, such as dirt, sand, and other debris, may also be funneled off the rotor  805  by the fluid channels  813 . In some embodiments, the fluid channels  813  may slope away from the central axis  801  and towards the rotor circumferential surface  807 . 
         [0108]      FIG. 9  illustrates another example embodiment of a solar energy system  900  including a drive assembly for rotating a solar energy member  925 . In the illustrated embodiment, the solar energy system  900  includes a support member  905 , a base  910  mounted on the support member  905 , a azimuthal bearing  915  mounted on the base  910 , a solar energy member support  920  mounted on the azimuthal bearing  915  via an interface  922 , and a solar energy member  925  coupled to the solar energy member support  920 . The support member  905 , base  910 , solar energy member support  920 , interface  922 , and solar energy member  925  may be substantially similar to those same components described above with reference to  FIG. 1 . 
         [0109]    Solar energy system  900 , as illustrated, may collect or reflect solar energy from a mote source (e.g., the Sun or other solar energy source) while rotatably tracking the source under varying environmental conditions. For example, in some embodiments, the solar energy system  900  may be a heliostat that tracks (e.g., rotates to change its azimuth and/or pivots to change its elevation) the Sun in order to receive and reflect solar energy from the Sun to a solar energy collector (receiver) located remote from the heliostat. Alternatively, the solar energy system  900  may be a PV system where the solar energy member  925  is a PV panel that tracks the Sun to receive and collect solar energy to, for example, produce electricity. In any event, in some instances, the solar energy system  900  may be one of many systems  900  installed within a field or array that operate to collect and/or reflect solar energy provided by the remote source. 
         [0110]    The solar energy member  925  is mounted to the support  920  such that rotation about the azimuthal axis and rotation about the elevational axis within desired ranges to account for tracking the Sun throughout the course of day and throughout the days of a year are permitted. For example, in some implementations, a bearing at the interface  922  can operate to facilitate reduced friction rotation of the solar energy member  925  about the elevational axis. Further, the azimuthal bearing  915  may operate to facilitate reduced friction rotation of the solar energy member  925  about the azimuthal axis. 
         [0111]    System  900  includes one or more drive assemblies  950  that operate to exert a force on the solar energy member  925  in order to, for instance, rotate the member  925  about one or both of the azimuthal and elevational axes. As illustrated, two drive assemblies  950  are coupled to eye-hooks  970  of the solar energy member  925  via cables  965 . Alternatively, more or fewer drive assemblies  950  may be used. Further, although illustrated as coupled to corners of the solar energy member  925 , the drive assemblies  950  may be coupled to other portions of the solar energy member  925  in addition to, or rather than, the corners. Moreover, while illustrated as mounted on a terranean surface  925 , the one or more drive assemblies  950  may be mounted in other locations while still coupled to the solar energy member  925 . For instance, the drive assemblies  950  may be mounted to the solar energy member  925  itself and coupled to the terranean surface  935  via the cables  965 . One or more of the drive assemblies  950  may be substantially similar to one of the foregoing drive assemblies described above. 
         [0112]    In the illustrated embodiment, the solar energy member  925  may be a heliostat mirror, which receives and reflects solar energy incident on a surface of the member  925  toward a remote location, such as a solar energy receiver. Alternatively, however, the solar energy member  925  may be another solar energy device, such as a PV panel. In any event, the solar energy member  925 , typically, is substantially planar or can be curved and includes at least one surface that receives and reflects (i.e., a heliostat mirror) or receives and absorbs (i.e., a PV panel) solar energy. Although illustrated in  FIG. 9  as a single, monolithic panel, the solar energy member  925  may be split into two (or more) equally or unequally-sized panels that together form the solar energy member  925 . 
         [0113]    The drive assemblies  950  may be substantially similar to one or more of the drive assemblies described with reference to  FIGS. 2A-2B ,  3 ,  4 ,  5 A- 5 B, and  8 . As illustrated, the drive assemblies  950  also include a driver  960  that, as described above, may ultimately rotate a shaft rigidly coupled to a rotor of the drive assembly  950  at a predetermined gear ratio. Each of the illustrated drive assemblies  950  include a spool  975  coupled to the shaft of the drive assembly  950 . The spool  975  may rotate at the same angular speed as the shaft in order to reel in the cable  965  and/or release additional cable  965 . As the cable  965  is reeled in and/or released, the solar energy member  925  may be rotated (e.g., azimuthally and/or elevationally) a particular amount depending on the length of cable  965  that is reeled in and/or released. 
         [0114]    In operation, one or both of the drive assemblies  950  may receive a command to rotate the solar energy member  925  and/or determine that the solar energy member  925  should be moved (e.g., to track the position of the Sun). The driver  960  may then be powered to rotate one or more components of the drive assembly  950 , such as, for example, one or more planetary members, a harness assembly, a spider assembly, or a plate, as described above. Rotation of the driver  960  ultimately rotates a rotor of the drive assembly  950  that is rigidly coupled with the spool  975 . As the spool  975  rotates, the cable  965  reels in or is released, depending on the rotational direction of the spool  975 . If the cable  965  is reeled in, tension is exerted on the cable  965  and pulls the solar energy member  925  towards the drive assembly  950 . The solar energy member  925  therefore rotates about one or both of the azimuthal or elevational axes depending on the position of the drive assembly  950 . If the cable  965  is released, tension is released on the cable  965  and the solar energy member  925  may, for example, rotate towards a default position (e.g., due to a biasing force) and/or rotate due to tension placed on the solar energy member  925  due to another drive assembly  950 . The solar energy member  925  therefore rotates about one or both of the azimuthal or elevational axes depending on the position of the drive assembly  950 . Rotation is facilitated by the azimuthal bearing  915  and/or the elevational bearing of the interface  922 . 
         [0115]    In some embodiments, two or more drive assemblies  950  may operate in concert to rotate the solar energy member  925 . For example, in order to rotate the solar energy member  925  about the azimuthal axis, one or more drive assemblies  950  may operate to release cable  965  towards the solar energy member  925  while another drive assembly  950  (or multiple assemblies) may operate to reel in cable  965 . Further, although two drive assemblies  950  are illustrated in  FIG. 9 , there may be more or fewer drive assemblies. Moreover, in some embodiments, certain drive assemblies  950  may operate to rotate the solar energy member  925  about the azimuthal axis while other drive assemblies  950  may operate to rotate the solar energy member  925  about the elevational axis independent of the azimuthal rotation. 
         [0116]    A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, while some embodiments have been described and/or illustrated in terms of heliostats, other solar energy members, such as PV panels, may also be utilized in accordance with the present disclosure. In addition, a controller of a solar energy system, such as the first controller  635  and/or the second controller  640  of the solar energy system  600 , may be operable to control a motor at a particular angular speed while compensating for slippage in a planetary gear drive that uses friction (as opposed to all-geared) components. Further, method  700  may include less steps than those illustrated or more steps than those illustrated. In addition, the illustrated steps of method  700  may be performed in the order illustrated or in different orders than that illustrated. 
         [0117]    As another example, while certain embodiments of a drive assembly illustrated herein include a rotor mounted above a stator, other embodiments of a drive assembly may have a stator mounted above (i.e., on top of) a rotor. Further, while the illustrated embodiments of a drive assembly include a rotor with a larger radial dimension than a stator, other embodiments may include a stator with a larger radial dimension than a rotor. In such embodiments, the value E in equation (2) is negative and then the rotation of the (smaller) rotor will be in the opposite rotational direction as the planetary members of the drive assembly. 
         [0118]    As another example, in some embodiments, a drive assembly (such as the drive assemblies  200 ,  300 ,  400 ,  500 ,  800 , and otherwise) may include a feedback system to provide feedback on a position of a solar energy member, such as a heliostat mirror or PV cell. For example, the feedback system may provide a drive assembly (e.g., a controller or other processor of a drive assembly) signals or data regarding a pointing angle (e.g., elevational or azimuthal) of the solar energy member. The drive assembly may then operate to correct the pointing angle by, for instance, moving the solar energy member that is coupled to the drive assembly. For example, certain drive assemblies may utilize frictional contact between certain components, such as between one or more planetary members and a rotor and stator. In such systems (and others), there may be backlash or slippage, among other issues, thereby making a gear ratio imprecise. Moreover, after significant use of such components, wear and tear may increase or cause slippage, thereby further exacerbating the issue. In such cases, the feedback system may ensure that the drive assembly operates to move the solar energy member to a desired or correct pointing angle (elevational and/or azimuthal). In one aspect, the feedback system may include a laser reader or scanner operable to detect one or more markings or pattern engraved or marked on an upper surface of the rotor. Through operation of the laser scanner, the feedback system may more precisely determine the pointing angle of the solar energy member coupled to the rotor and control the drive assembly accordingly. In another aspect, the feedback system may include a sensor located at a solar energy receiver, which measures solar energy intensity reflected toward the receiver by one or more solar energy members. Based on the measured solar energy intensity, the feedback system may determine whether the pointing angle of the solar energy member is optimal and control the drive assembly (e.g., to move the solar energy member) accordingly. These two aspects may be combined, of course, and other types or techniques for feedback systems may be utilized, as appropriate. Accordingly, other implementations are within the scope of the following claims.