Position and strain feedback control for shaping an active truss
The design requirements for future aircraft demand increased performance and multimission capability. Active, in-flight, aerodynamic surface shaping is currently being investigated to increase the flight envelope of tomorrow's aircraft. The primary application of this research is to aid the evaluation of active airfoil and wing shape control using an active truss. This specific research addresses the effect of replacing position feedback with strain feedback for shape control of an active truss. There are two research objectives: a comparison of multiple-beam shape control using position and strain feedback, and to confirm a system displacement model through experimentation.
The experimental setup uses two cantilevered aluminum T-sections attached by one to three-ball screw actuators. The actuators have been pinned at each beam location to allow rotation during extension, while also allowing the effect of a variable number of actuators to be examined. The three-actuator arrangement enables independent specification of three positions along the two beams; therefore, the system is a multiple input-multiple output (MIMO) system. The controller consists of three pulse-width modulated amplifiers converted for use with a position feedback loop. A model is developed which consists of a spring-actuator arrangement with the equivalent spring constants corresponding to the beam deflection equations.
The system model is first examined based on the results of the actuator length feedback experiments. The model is confirmed for the single actuator system. The joint gap accounts for the inconclusive results in confirming the multiple actuator displacement models. The next set of experiments demonstrate that strain feedback may indeed replace the actuator length feedback. It is shown that a signal may be produced which is proportional to the actuator extension, this may then replace the length sensor.