Experimental evaluation of a flexible joint driven by water pressure for underwater robots

We describe here the development a joint mechanism for underwater robotic manipulators. Arms of underwater robots require small-scale bodies and high waterproofing properties. In most of underwater robots, electric motors are used as actuators to drive the robotic arm/arms, but using electric motors for underwater manipulators may be problematic due to the size/weight of the robotic arm and need to waterproof the electric motors. We develop a joint mechanism composed of combinations of rigid and flexible members, which can be deformed by a prismatic actuator fixed onto two rigid parts. We utilize a leaf spring as the flexible joint and a McKibben actuator driven by water hydraulic pressure as the prismatic actuator. The number of members in this mechanism is smaller than that of a mechanism composed of a combination of one pulley and one coil spring. One advantage of this mechanism is the avoidance of gears, thus eliminating sliding parts from the joint.

I. Introduction

Many expert divers inspect the bottoms of ships and maintain immersed structures such as water pipes under water. Their working efficiencies are quite low, because the use of air bottles limits diving time and even expert divers cannot work easily at depths below about 20 m. These tasks may be better performed by robotic systems, both for efficiency and safety reasons. Lightweight underwater robots with manipulators may be used to perform the dexterous tasks currently performed by expert divers. Over the past few decades, there have been several studies of underwater robots with manipulators (Underwater Vehicle-Manipulator System; UVMS) in terms of the control schemes [1], [2], [3], [4], [5], [6]. Our group has also developed an underwater robot with two manipulators (Figure 1) instead of two human arms [7]. In most of these robots, electric motors are used as actuators to drive the robotic arm/arms, but using electric motors for underwater manipulators may be problematic due to the size/weight of the robotic arm and need to waterproof the electric motors. Actuators under water are often attached to magnetic couplings, which are highly waterproof, but the addition of a magnetic coupling increases the size of the joint because of the mechanism itself. We have utilized a leaf spring as the flexible joint and a McKibben actuator driven by water hydraulic pressure as the prismatic actuator. Fig. 1. Underwater robot with two arms driven by electric motors Fig. 2. Concept of flexible joint mechanism driven by prismatic actuator