A fast rotary soft actuator driven by piezoelectric vibration

Abstract

<p indent="0mm">The soft actuator is a soft driving device exhibiting large-deformation ability, which is significantly different from the traditional rigid actuator usually composed of rigid components such as motors, bearings, gears, hinges and so on. Soft actuators are usually made of soft materials; theoretically, they have the advantages of infinite motion freedom, continuous deformation, and compliant contact. Therefore, the soft actuator shows remarkable advantages in grasping and handling of fragile objects or motion in complex unstructured environments. However, most of the previous soft actuators are usually made of the forms of bending, stretching, and twisting deformation, for which it is hard to achieve continuous rotation. Some limbless creatures, such as snakes and worms, have bristles arranged periodically at a certain angle in nature. When sliding between the bristles and ground caused by vibration of the bristles or deformation of the soft body, the asymmetrical friction force is generated which can drive their forward motion. Based on this idea, an initiative rotating soft actuator, comprised of a piezoelectric bimorph and a soft rotating structure, which is driven by piezoelectric vibration is proposed in this paper. This actuator is composed of a piezoelectric bimorph and a soft rotary structure. The soft rotary structure is an array of inclined bristles arranged uniformly along the circumferential direction, fabricated by 3D printing and injection molding technology. By analyzing the friction force of a single bristle during vibration, the formula of the rotation moment is given. In addition, through finite element analysis of the deformation of the bristles, the contact force and friction force of the actuator in one vibration period are obtained. It shows that the static friction force generated during the downward motion of the bristles is larger than the sliding friction generated during the upward motion of the bristles, so that a net driving rotation moment is gotten. Besides, the effects of the actuation amplitude and frequency of the applied actuation force on the rotation speed of the actuator are studied by finite element analysis, based on which an optimized bristle geometry of <italic>θ</italic>=60°, <italic>l</italic><sc>=7 mm,</sc> and <italic>d</italic><sc>=1.6 mm</sc> is selected for subsequent experiments. The effects of voltage and frequency on the rotation performance of the rotary soft actuator are analyzed through experiments, which show that the rotation speed increases first and then decreases with the increasing of actuation frequency when the applied frequency exceeds <sc>700 Hz.</sc> Furthermore, when the applied voltage <italic>V</italic>_P-P is <sc>400 V</sc> and frequency is <sc>1400 Hz,</sc> the rotation speed can reach <sc>118.3 r/min,</sc> which is larger than the rotation speed of the soft actuator reported so far.