pressure between the upper and lower hemispheres of the inflatable structure housing the reflective membrane.
With respect to goal 5 of the Project Goals, encouraging results were obtained on pointing accuracy and beam shape stability in the presence of environmental factors, particularly winds and gusts. Temperature effects also had no adverse impacts on pointing accuracy and beam shapes, to the extent that ambient and heliostat internal (upper and lower hemisphere) temperatures varied during the test period.
With respect to goal 6 of the Project Goals, the heliostat prototype successfully survived some adverse environmental conditions including wind gusts up to 30 mph, and precipitation in the form of rain, snow and light hail. Survival in high winds and survival under severe winter conditions remains to be established by future prototype testing efforts.
In summary, the construction and testing of a subscale prototype of a lightweight, low-cost inflatable structure supported heliostat has been successfully accomplished, and has satisfactorily met the Project Goals. The sections below provide additional detail on the testing done to satisfy each of the project goals.
The aiming system uses stepper motors and polyester reinforced neoprene toothed belts to drive the heliostat in azimuth and elevation. Our tests indicate that the aiming system provides accurate and repeatable aiming of the heliostat. To verify repeatability indoor and outdoor tests were performed. In the indoor test, targets were secured to walls and ceiling at a range of pointing angles. A laser pointer was rigidly attached to the mirror support ring, and aimed in the same direction as the mirror (Figure 9a). The heliostat was commanded by the control software to point at each target in turn, and the initial spot on the target was marked. This cycle was repeated multiple times in each direction to observe how closely the laser beam matched the original marked spot on successive cycles. Within the accuracy that the spot positions could be measured, there was no deviation from the original spots on successive trials. The outdoor test consisted of using the aiming system to keep the heliostat’s focused beam tracked on the target. Figure 9b shows the test setup for outdoor testing. The center of the target is 57 ft from the heliostat mirror. The target squares are one foot on a side. Note that the target color was changed to white after this picture was taken, for better visibility. Figure 9c shows the beam focused on the target – the heliostat was able to track the focused beam on the center of the target. In addition, tests were done in which the heliostat beam was centered on the target, then commanded to a position well off the target, then commanded back to the center of the target. In these tests, the beam returned to the same position on the target within the accuracy that could be visually observed.
The absolute accuracy of the aiming system could not be tested to a high level of precision due to the requirement to measure angles and positions to very small tolerances. It is our belief based on the repeatability tests described above and the simple geometry of the heliostat that the absolute pointing accuracy will meet the requirements. If, however, small and consistent errors in absolute pointing accuracy do occur, then software corrections could be introduced to correct them.
November 24, 2002