CONTROL OF THERMAL DEFORMATIONS OF SPHERICAL MIRROR SEGMENT

Control of thermal deformations of a thin hexagonal spherical mirror s egment using discrete and distributed actuators is presented. To deter mine the effectiveness of the actuators in controlling the thermal def ormations of the mirror segment, a comparative study is conducted usin g two different models of the mirror-actuator system: 1) the mirror mo unted on kinematic supports and controlled by piezoelectric strips bon ded to the rear surface of the mirror and 2) the mirror mounted on for ce actuators, which are used to support the mirror as well as to contr ol the surface deformations of the mirror. The performance of evenly d istributed strips and that of strips placed at near-optimal locations obtained using heuristic integer programming are also compared. Both t he force actuators and the piezoelectric strips are found to be equall y effective in controlling the surface deformations of the mirror. A m ajor drawback of the force actuators is the increase in the overall we ight of the system, which is undesirable for space applications. On th e other hand, the piezoelectric strips are very lightweight, and hence a large number of such strips can be used to control the surface dist ortions of the mirror, without imposing a weight penalty. The piezoele ctric strips appear to be promising candidates for static shape contro l of flexible structures in space.