# Preliminary Analysis of a Lightweight and Deployable Soft Robot for Space Applications
## 1 Introduction  
Traditional robot manipulators are widely employed in industrial and structured environment. Their rigid structure allows great accuracy and, potentially, high force generation. Nevertheless, rigid manipulators are relatively heavy, and they require a relatively large stowing bay if they must be mounted on vehicles. Soft robots, i.e., robots built utilizing soft materials, represent a growing trend in recent years. They provide adaptability, compliance and low mass, and can be optimally designed for a large variety of applications [1]. The main disadvantages of these systems are a low force production and difficulty in their control. The design of soft manipulators can span along all the spectrum between totally soft and rigid structures, depending on the desired robot features: more degrees of freedom imply lower precision and the increase of the structural compliance reduces the force exertion [2]. In space applications, volume and weight are critical features, and the use of deployable structures is often a benefit [3]. Inflatable structures, made by soft materials, can be easily transported in compact and lightweight packages and can be inflated to be deployed, when required. Some of the most relevant projects in this field were the Mars Pathfinder inflatable airbag landing system [4], the Inflatable Antenna Experiment (IAE) [5] and an inflatable and rigidable solar array [6]. Space inflatable booms have high packaging ratio and are extremely lightweight compared to other deployable technologies, although they have low deployment accuracy and post-development stability [7]. For these observations, soft robotics can meet and resolve aerospace issues, developing inflatable deployable robotic manipulators, based on soft materials and variable stiffness bodies. The design of inflatable manipulators must concern their architecture, fabrication procedures, kind of actuation and the control of the entire robot. Validated modeling methodologies are paramount to carry out the design and control of such robots. Inflatable manipulators are typically divided into continuum [8] and articulated robots. Articulated manipulators can consist of inflatable links with rigid [9] or variable stiffness joints [10] and completely soft structures with soft joints [11], e.g., using pneumatic [12, 13], or tendon-driven actuators [14]. Nevertheless, the use of soft joints leads to more complexity in the design and stiffness control of the actuator [15]. Furthermore, the manipulators with inflatable links can be modeled considering traditional approaches developed for flexible link arms [16], pseudo rigid bodies [17] and simplified small deflection assumptions [18]. In [19] a recursive numerical algorithm to compute inverse dynamics of a manipulator with an arbitrary number of elastic joints is presented, moreover a variant of the algorithm is used to implement a feedback linearization control law for the accurate tracking of trajectories. In [13] a joint for inflatable robotic arms (IRAs) is actuated by two antagonist pneumatic artificial muscles (PAMs), using a pressure control to perform their characterization. In [20] a pneumatical actuated humanoid robot, with two IRAs, is controlled using model predictive control (MPC) and linear quadratic regulation (LQR) in the joint space. Then, in [21] the MPC is compared against sliding mode control (SMC). This work lays the basis for the development of a lightweight and large size manipulator for space application, proposing a basis structure with two inflatable links and three degrees of freedom (DOF) that should be used to place a 3-dof wrist in a desired position. All the joints are actuated by electric motors in order to avoid the need of air supply and valves for motion control This solution allows the robot, when deflated, to be easily contained in a small package with respect to its extended configuration. The robot can be deployed through air supply when it is required. The inflation phase could be activated by a valve that connects the links to a pressurized tank.