## 6 Experimental Verification of the Mirror Alignment Method
### Experimental Alignment of the Plane Unit
#### 6.2.2 Results and Discussion
In the facet alignment experiment, the alignment accuracy of the metal plane mainly depended on the following factors: manufacture or installation error of the support-adjust structure (i.e., the position error of the ball-hinge center), positioning error of the feature points, spatial coordinate measurement errors of the feature points, and adjustment error of the bolt, therefore, the errors led to some deviation between the experimental results and theoretical values. To improve the accuracy of these methods in the application of mirror facet alignment method in a solar concentrator, control measures or reduction methods to minimize the above factors should be considered:  
(1) The mirror units are usually made from precision molds, so the position accuracy of the ball-hinge center and the geometrical accuracy of the reflected mirror should be guaranteed. Figure 10: The fitted equation of metal plane (Plate 1 and 2) before and after the adjustment using the three-rotation alignment method: (**a,b**) before adjustment, (**c,d**) after adjustment. (2) To reduce the positioning error of the feature points, a numerical control device could be used to accurately locate and attach the circular markers before the mirror manufacture, or three circular convex features could be included on the mirror unit surface in the initial design, such that the circular markers can be attached to the circular convex features after the mirror unit is made. Both methods could effectively control the positioning error of the feature points. (3) The coordinate measurements of the feature points is the only field measurement in the entire mirror facet alignment process. Measurement errors directly affect calculation accuracy of the mirror facet and adjustment values of the bolt leading to reduced facet alignment accuracy of mirror unit. Fortunately, the current commercialized photogrammetry system is well developed and highly accurate, and these commercial photogrammetry systems could be used to improve the measurement accuracy of spatial coordinates. (4) The adjustment error of the bolt includes the theoretical calculation error and the adjustment control error. Theoretical calculation errors are caused by the assumptions made in Equations (20) and (23) and the adjustment control error of the bolt is the deviation between the actual movement value and the theoretically calculated adjustment value. In Figure 11, the adjustment control error of the bolt is shown to be approximately \(\pm\)1.50 mm for the metal plane facet alignment experiment. Therefore, it is important to develop some type of device that can accurately control the adjustment of the bolt, or a new kind of support-adjust structure to conveniently and accurately control the bolt adjustment to be applied to the dish concentrator. A typical mirror slope error (i.e., root-mean-square error, RMS) form the solar dish concentrator is evaluated and they are 2.66 and 2.50 mrad according to [22] and [26], respectively. Some methods of mirror alignment for solar concentrator are reviewed by [PERSON] et al. [14], where the accuracy of mechanical alignment is as high as 1.5 mrad. According to [8], a typical mirror alignment error of the dish concentrator is 1.1 mrad. In this paper, the facet alignment accuracy was experimentally measured and found to be the range of 2.05-6.16 mrad and they are close to the mirror alignment accuracy given in the above literature. As such, the three-rotation and two-rotation alignment method proposed in this paper could effectively be used for mirror facet alignment in the dish concentrator system. Moreover, the alignment experiments in this work only adjusted the metal plate once. However there may be higher alignment accuracy if repeated adjustments were made.