Design of a Reconfigurable Space Robot with Lockable Telescopic Joints Farhad Aghili and Kourosh Parsa Space Technologies, Canadian Space Agency Saint-Hubert, Canada Email: farhad.aghili/kourosh.parsa@space.gc.ca Abstract— This work presents a new paradigm and a con- ceptual design for reconfigurable robots. Unlike conventional reconfigurable robots, our design does not achieve reconfigura- bility by utilizing modular joints. Rather, the robot is equipped with passive joints, i.e., joints with no actuator or sensor, which permit changing the Denavit-Hartenberg parameters such as the link length and twist angle. The passive joints will become controllable when the robot forms a closed kinematic chain. Also, each passive joint is equipped with a built-in brake mechanism which is normally locked, but the lock can be released whenever the parameters are to be changed. Not only will such a manipulator have the versatility to perform different tasks but also it can be packed adequately within its designated space on the launch vehicle. Kinematics of such a robot is analyzed, and a stable control algorithm which can take the robot from one configuration to another is devised. I. I NTRODUCTION Robotic manipulators working in extreme environments often need to change their configuration in order to meet the demands of a specific task within the constraints of the environment [1]. Particularly in space applications [2], it is desirable and cost effective to employ a single versatile robot capable of performing different tasks such as inspection, contact operation, assembly (insertion and removal of objects), and carrying objects (pick and place). Optimal operation of each of these tasks demands a specific manipulator design. For instance, large robots maximizing the structural length index are typically suitable for inspection [3]; robots with maximum manipulability measure are well-conditioned for dextrous con- tact tasks; and configurations maximizing the distance of the robot limbs and extremities from the environment are suitable for payload handling. Generally speaking, space systems are designed for mini- mum weight in order to reduce launch cost. Another design constraint for a space system is that it should be compact enough to be accommodated within its designated space of the launch vehicle. Since the links of a space manipulator are usually long, they have to be folded before launch. For instance, the CanadarmII has two long booms, each of which has a hinge at the middle, which allowed the booms to be folded before launch and then unfolded manually by astronauts in orbit. For on-orbit servicing missions whereby no human operator is present, the robot has to be able to deploy itself. Except for reconfigurable manipulators, there are two op- tions: (i) not using long booms or (ii) using hyper-redundant manipulators such as snake-type robots. The former option may limit the types of operation possible—for instance, capturing a tumbling satellite requires a manipulator with long reach—while the latter increases the complexity of the manipulation system. From an economical point of view, on- orbit servicing will only make sense if a single servicing satellite can perform several different servicing missions. Each of these missions may demand a robotic arm with a particular size and configuration. A reconfigurable manipulator is well-suited to be a general-purpose manipulator, whereas a nonreconfigurable manipulator is typically a special purpose manipulator designed for particular tasks. The original reconfigurable robot was introduced in [4] to add versatility to the robotic manipulator. The concept was then developed further in [5]. Cellular robots based on hexagonal modules and based on the concept of robot molecules were described in [6], [7] and [8], [9], respectively. Reconfigurable robots for space exploration were proposed in [2]. The design of Conro modules to build deployable modular robots that can be reconfigured to take different shapes such as snakes or hexapods were presented in [1]; this work has some similarities with earlier works on Tetrobot, discussed in [10]. All these reconfigurable robots are modular, hence needing an effective docking system for connecting and releasing the modules [11]. Reconfigurable robots have been identified by many space agencies to play a major role in future space missions. Due to the complexity of their docking system, the state-of-the-art reconfigurable robots with modular joints may have limited practical space or industrial applications. The new design, however, can offer a simpler and more effective solution to the problem at hand. Additionally, with such a design, one does not need to detach any link or joint to achieve a required configuration change. Thus, the entire reconfiguration operation can be performed autonomously with a higher level of reliability. This makes this new type of reconfigurable robots particularly attractive for space applications. Further to the versatility that this design provides, it makes it possible to contain the manipulator in a small volume, which is suitable for launch. Notably, each of the booms of Canadarm2 had two passive hinges, which required deployment by astronauts. In this paper, we describe the conceptual design of a new generation of reconfigurable robots that, using passive cylindrical joints between two adjacent active joints with non-