A mechanical approach to realize reflexive omnidirectional bending motion for pneumatic continuum robots

Abstract

A mechanism that allows a robotic arm to quickly grip various forms of objects at disaster sites will enhance the mobility of rescue robots by keeping their bodies stable and maintaining manipulability for target objects, such as debris. Such a mechanism requires the ability to quickly and omnidirectionally change arm postures toward the target and hold it in a stable manner. Continuum robots are expected to provide this functionality. Conventional continuum robots realize the function of changing arm postures and grasping objects by controlling pneumatic actuators with multiple air chambers arranged in parallel. However, conventional robots cannot be applied to potential disaster sites filled with flammable gases, gasoline, or high radiation levels because they require electronic components (e.g., solenoid valves, and sensors) to control air pressures. This study proposes a unique approach to realize reflexive omnidirectional bending motion using only mechanical components without any electrical devices. The proposed system realizes a reflexive motion to bend the arm in the target’s direction by detecting a contact location using a mechanical reactive system. The proposed simple mechanism has the advantages of high durability and easy implementation. This paper aims to confirm the proposed concept by prototyping a drive mechanism coupled with contact detection and bending motion using mechanical port valves. We report the design concept and development of this prototype. The fundamental characteristics and feasibility of the proposed mechanism are experimentally confirmed. First, a prototype is developed using a mathematical model. Its performance in the bending and omnidirectional motions is evaluated. The results show that the model has a margin of −4.9% error in the bending angle and −7.4% error in the central curvature compared with the experimental values. We also confirm that using a higher pressure could realize a smaller radius of curvature and reduce an unnecessary twisting motion. We also tested a second prototype to confirm the grasping motion and force by changing the applied pressures. The influence of the bending direction was then evaluated. We confirm that a higher pressure generated a larger grasping force. The prototype can omnidirectionally produce approximately the same forces although the generated forces depend on the number of air chambers excited by the contact pads. Subsequently, we experimentally confirm the influence of gravity. The test shows that the effect of own weight greatly influences the posture after the object is in contact. This effect should not be ignored. Furthermore, the curve became sufficiently large when its contact pad is pressed. This result experimentally proved that self-holding is possible. The experimental results show the potential of the proposed mechanism.