A novel approach for placement of sensors and actuators in control of flexible space structures is developed. Using an approximation of the control forces and output measurements by spatially continuous functions, the approach follows a nonlinear programming technique to determine optimal locations for sensors and actuators. Two different criteria are considered for the placement of sensors and actuators. The first criterion optimizes the location of the sensors and actuators in order to move the transmission zeros of the system farther to the left of the imaginary axis. The second criterion, however, places the sensors and actuators to optimize a function of the singular values of the Hankel matrix, which includes both measures of controllabili ty and observability. Moreover, the effect of actuator dynamics in the placement of sensors and actuators is investigated. ENSOR and actuator placement is an integral part of a control design, particularly in the control of flexible struc- tures. Although engineering judgment and trial-and-error analysis are quite often used to determine the optimal locations of sensors and actuators, there have been various attempts to develop systematic means of achieving it. Among a few of these, Chen and Seinfeld1 used an optimization- based ap- proach wherein optimal locations of measurement outputs are computed from a finite set of possible locations with the aid of integer programming. Skelton and Chiu2 have considered an order reduction approach such that the inputs and outputs are reduced from a large set so as to make the smallest possible perturbation in a performance metric. Horta and Juang3 pro- posed a sequential linear programming approach for truss structures in which optimal numbers as well as locations of actuators are obtained from a large set of actuators. A com- mon point of reference in most of these approaches is the use of a performance metric. To date, various measures of con- trollability and observability,45'7 controllability and spill- over,6 and energy-based metrics2 have been considered as per- formance metrics in the literature. Of recent efforts, Maghami and Joshi8 provide a method in which the discrete nature of the sensor and actuator placement problem is transformed into a continuous nonlinear programming problem with the aid of functional approximation of the control forces and output measurements. Lim9 proposes an approach for placing actua- tors and sensors that is based on projecting eigenvectors into the intersection subspace of the controllability and observabil- ity subspaces for each sensor-actuator pair. In this paper, a new approach for sensor and actuator place- ment in the control of flexible space structures is developed. The approach follows the development of Ref. 8, which re- duces the problem of sensor-actuator positioning to a solution of a nonlinear programming optimization. The discrete con- trol forces and output measurements are approximated by spatially continuous functions in order to avoid the disconti- nuity problems caused when actuators and sensors are recon-