8.2 Black Spot module testing (2D testing)
Figure 8.2 (a)The testing configuration consists of two computer controlled translation rails that alter the distance between the hub enclosure and the testing beacon. The long translation rail allows the beacon to be moved towards the hub enclosure. (b) The hub enclosure is attached to the shorter rail and a rotational turret allows the angle of the hub enclosure to be varied by a computer.
During stationary testing, measurements were made using the configuration shown in Fig- ure 8.2a that consists of a beacon, two computer controlled linear rails, a rotational turret, and the hub. The beacon comprises three panels of 3 by 3 LEDs and is shown at the end of the longer of the two rails. The markers are placed in a regular pattern with a distance of 0.15m between adjacent markers. The angles between the two outer panels and the centre panel can be adjusted.
The two rails are oriented perpendicular to one another to provide a range of testing con- figurations. The long rail allows the beacon to be translated towards the hub enclosure. This is achieved by way of a stepper motor that allows precise movements to be made. The shorter rail translates the hub enclosure in a direction perpendicular to the long rail and is connected to another stepper motor. The hub sits on a turret whose angle can be adjusted by a third motor (Figure 8.2b).
The measurements discussed in the following subsections were taken at fixed distances. This distance refers to the distance between the test beacon and the centre of the hub en- closure, measured with a tolerance within ±1 cm.
Measure of centroid variation
The standard deviation of an ensemble of centroid coordinates is used as a measure of spread throughout these results. Figure 8.3a shows a plot of the distribution of centroids captured from a module. The plot has a Gaussian shape. Gaussian statistics are assumed for the centroid X and Y positions for the remainder of this chapter.