Ephrahim Garcia

Modeling of the slewing control of a flexible structure

This paper presents a formulation for the modeling of a single-link flexible beam that is undergoing slewing motion at an actively controlled pinned end, where the other end of the beam is free. This pinned end, or slewing axis, is of fixed orientation such that the beam rotates in the horizontal plane. A geared dc electric motor is connected to the beam at the slewing axis. A position and a velocity sensor are placed at this hinged location, and their proportional feedback provides a position control system about the slewing axis. The motor characteristics, gear ratio, and the position feedback constant determine an equivalent rotational spring constant, often called the servo stiffness. This paper generalizes the structures boundary conditions at the slewing axis to include the effects of the servo system. It is shown that the clamped-free beam assumption for the dynamics of the structure is a valid assumption if the ratio of the servo stiffness to beam flexibility is high. However, for moderate or low values of this ratio, the clamped-free beam leads to erroneous system models so that it becomes necessary to consider the effects of the driving servo on the dynamics of the flexible beam.

Advantages of Slewing an Active Structure

The slewing control of an active flexible structure is considered by examin ing the equations of motion of an integrated actuator/structure system containing both the actuator and structure dynamics. These equations are derived using a Hamiltonian ap proach. The system under consideration is a slewing flexible structure, a thin aluminum beam, torque driven by an armature controlled DC electric motor and actuated by a piece- wise distributed piezoceramic actuator. An improvement in performance is gained by 1) including the effects of motor actuator and beam dynamic interaction and 2) using a piezo electric device, layered on the structure, for direct vibration suppression of the structural dynamics. A comparison is made using simulations, to point out the advantages of slewing an active structure versus slewing a passive structure. This presents a multi input slewing control problem which is implemented using a standard linear quadratic regulator control design.

Passive and Active Control of a Complex Flexible Structure Using Reaction Mass Actuators

Passive and active control have been performed on a flexible structure possessing complex modal behavior. Specifically, the structure has closely spaced modes which created a “beat phenomena” in the structure’s vibrations. Reaction mass actuators (RMAs) were used to suppress the structural vibrations with both passive and active control schemes. The results of using both types of control systems were compared both analytically and experimentally. It was determined that, although passive tuning of the RMAs did suppress the structure’s vibrations, additionally applying active control to the optimally tuned RMAs did not significantly increase the system’s vibration suppression performance. However, by choosing the actuator’s characteristics from active control considerations, an active strategy utilizing a local velocity feedback controller suppressed structural vibrations and reduced the active system’s settling time to 20 percent of the settling time with passive damping. Both the passive and the active control were found to be a significant improvement over the structure’s open-loop response.