Position and attitude sensors
Model position detection system can be formed by optical sensors and helium-neon lasers outside the test chamber. The sensors are a combination of linear photodiodes. A typical sensor consists of 1024 photodiodes and has a linear dimension of 1 in., the precision of position detection is therefore about 25 mm. Laser beams, to illuminate the sets of linear photodiodes, are made plane through cylindrical lenses. The position and attitude of the model are obtained by the shadow of the model on the sensors and are measured by the number of illuminated photodiodes. Models are usually painted in matt black in order to avoid possible light reflections.
The number and location of optical sensors and lasers are functions of the number of degrees of freedom to be controlled. Figure 7.29 illustrates the schema of a system of position and attitude sensors in a balance with five degrees of freedom; lacking the sensitivity to roll, only tests on axially symmetric bodies are allowed. The longitudinal position is detected by sensor 2 through the partial shielding of the drag laser beam; the vertical position and the pitch angle from sensors 1 and 5 through the partial shielding of the horizontal laser beams, and finally the transverse position and the yaw angle by sensors 7 and 4 through the partial shielding of the vertical laser beams. Other detection systems may be an X-ray system or a television system.