Summary of the 3D solar-array tracking joint design for solar-array tracking.

The joint is designed around the primary goal of carrying an 820 ft.lb torque load while providing tracking capability to the solar arrays (wrist pitched to 90 degrees while rotating fully). The following figures provide an overview of the design as a CAD model.

Fig. 1: CAD Rendering of solar-array tracking joint

Fig 2 (a-d): Solar-Array Tracking Joint at 90 degree pitch at locations in full rotation

In the figure above, the Solar-Array is removed for clarity. The location in fig. 2b shows the joint in the highest-load configuration. Joint limiters can be seen in these figures to mechanically limit joint rotation (Fig. 2b may show this most clearly). The actuation system is currently demonstrated as a combination Maxon EC-90 Flat motor and HDC-1M harmonic drive gear head. Numerous configurations for actuation are available, this one is shown for representation only.

The next set of figures demonstrate general workspace sizes for the current joint design. With a firmer set of requirements and more detailed design, this workspace can be scaled to a reasonable degree.

Fig 3: Wire-frame Solar-Array Tracking Joint with overall dimensions

Key metrics for the initial design of the 3D solar-array tracking mechanism are listed in the following table:

Table I: Summary of 3D solar-array tracking mechanism

Component / Description / Mass/ Workspace
Motor Drive: / Maxon EC90 Flat Motor / 1.5 lb
Drive reduction / Harmonic Drive HDC 1M / 2.6 lb
Bearings / Bronze bushings, ¾, 3/8 ID
Shaft / 440c SS
Link components / 7075 Al
Mechanism weight * / (without actuation) / 14.75 lb
3D Mechanism weight / (with actuation, includes a 20% factor) / 32.5 lb
Working Volume* / 5.5 inch radius cylinder
Height is selectable / 5.5 inch cylinder
Stowed Volume* / 3 high x 7 radius cylinder (inches) / 7x3 inch cylinder

* without actuators

Analysis and design of the structural and actuation components of the 3D solar-array tracking mechanism are based initially on a forward dynamic load analysis tool that incorporates the inverse kinematics, load specifications and kinetostatic analysis to give all loads within the mechanism as a function of position within its workspace. The following graphs (fig. 4) represent some of the bearing loads and motor torques required in the worst-case loading scenario. The maximum of these loads was selected as requirements for actuator and bearing design.

Fig. 4: Motor and bearing loads for worst-case loading condition