Embedded Sensors and Actuators for Lightweight Structures

Abstract

Much effort has been invested in developing control methodologies that modify the joint torque profiles in a lightweight, high speed robot manipulator in order to suppress vibration in the flexible links and improve end-point positional accuracy [ 1-7]. Similar concerns arise regarding the interaction between structures and control methodologies for large space structures [8]. These techniques rely on a model of the flexural behavior of the link(s). Some of them operate in real-rime [1,2], while others are computed prior to motion and may depend on inverse dynamics [3-6]. The principal drawback to all of these is that while the vibration modes are properties of the flexible links, the attempted solutions rely on actuation at the joint motors. This condition of non-collocation of the vibrational coordinates and the actuators' degrees-of-freedom has hampered the satisfactory identification of a real-time controller. Furthermore, techniques for sensing have also relied on location of sensors remote from the actual vibrational coordinates [1-6,8]. The control of a high performance structures relies on sensory feedback. With the promised advent of lightweight high strength composite materials, the loss of weight to improve performance will result in the introduction of flexural and vibrational modes of significant amplitude, which have generally been ignored in structures. These "vibration coordinates" must now also be addressed. A desirable property of embedding sensors in lightweight structures is that they too contribute a minimum of weight and volume within the structure and not compromise the components' integrity. This paper describes a system for sensing strain in flexible structures. It consists of a piezoelectric ceramic bonded to the surface of a thin, flat, flexible beam fabricated from graphite/epoxy composite. The results for the piezo-ceramic sensor indicate that substantial signals can be obtained for vibrations above 1 Hz; these signals are proportional to strain and can be used with a simple controller developed to actively damp the beam vibration. The mass penalty imposed on the structure by addition of the sensor is negligible.