Innovative Hand Exoskeleton Design for Extravehicular Activities in Space

This chapter deals with the identification of the requirements that drive the design of the hand exoskeleton to help crewmembers during Extra-Vehicular Activities (EVAs). The requirements were identified by means of literature review as well as interviews to users and stakeholders. After an introduction to EVAs, the method followed for the design is presented. The space environment and the main characteristics of spacesuits and gloves are reviewed, focusing in particular on the condition of the hands. The fatigue problem of arms and hands during EVAs is explained and some peculiarities of the training that astronauts undergo to prepare to this type of activities in space are stressed. Since one of the most important problems for materials and electronics in space is radiation, the total dose that has to be withstood in the typical International Space Station (ISS) environment is then estimated. Different requirements also come from the EVA spacesuit equipment, as well as from safety and cost considerations. A discussion on kinematics and dynamics follows, in which the main hand movements needed for EVAs and the different joints of the fingers are discussed. Finally, a table summarizes the identified requirements, which drive the design of the hand exoskeleton.

In this chapter we wrap up our literature investigation, pointing out the key focuses and requirements to be considered. We will go through our evaluations upon actuators, sensors, control systems and we will present some designs examples. Although literature is rich in studies on limbs exoskeletons aimed to several target applications, (such as rehabilitation, function restoring and virtual reality), the stringent requisites of EVA make most of the actuation and sensing solutions unsuitable for our device. Safety reasons and pressurization of the space suit force to avoid modern actuation technologies (i.e. air muscles), and focus on traditional motors such as piezoelectric ones. For what concerns sensors and control systems, most of the currently developed exoskeletons use physiological signals and are based on Electroencefalography (EEG) and on Electromyograpy (EMG). In the latter case, a big advantage is due to the fact that input signals are picked up directly from the motor units involved in the hand control. In the next pages we also present several examples of exoskeletons, including a pinching device for rehabilitation based on a cable mechanism, one finger device based on pulleys-lever structure, the four bar mechanism, a tendon system glove for function restoring and the EVA K-Glove from NASA. Finally, a couple of examples of feedback devices for medical applications are presented, integrating a variety of sensors, such as accelerometers, ultrasound, flow, pressure and vibration sensors, heat infrared camera, gyroscopes.

Starting from the user’s requirements previously defined, a new soft robotics approach was chosen and developed in order to overcome the criticalities arisen in the analysis of the state of the art. One of the key points of soft robotics is biomimicry: in place of heavy, rigid and noisy motors, artificial muscles are in charge of the movement of the soft structure, allowing a number of degrees of freedom unthinkable with traditional mechanics. The shift from hard to soft robotics brings the focus on materials: actuating and sensing devices are embedded in the material itself, which turns out to be smart. In particular, the attention was focused on Electroactive Polymers (EAPs): these polymeric materials work as transducers, converting electrical inputs into mechanical outputs, and vice versa. After an extensive material selection procedure among all the EAPs solutions currently available, Dielectric Elastomers (DEs) emerged as the most suitable materials for the intended application and a mathematical model of their electro-mechanical behavior is presented. The control of the hand exoskeleton is addressed in this section. Its objective is to help the astronaut accomplish the tasks he has to perform. The entire control system is composed of four phases: the recognition of the astronaut’s will (detect and distinguish the different movements), the control strategy, the enhancement of these movements and the measure of the actual position and force. The final measure system was implemented focusing on redundancy, safety control and to assure minimum performances also in off-nominal conditions.

The chapter presents the design of the device actuators and sensors in terms of concept developing, dimensioning, testing and prototyping. Inspired by soft robotics concept, the design of actuators is based on the characteristics of dielectric elastomers and their relationship between voltage applied and material elongation. Smart material properties are exploited in the multiple layers actuator proposed, capable of performing both linear elongation and bending, by means of the application of different voltage levels to different layers. Calculations of the torques required by the hand actions and, consequently, the maximum stress are presented. Then, the choice of the material leads to the identification of the correct dimensioning of the system, in terms of voltage to be applied and thick of the actuator layers. In order to control the actuation according to the human will, sensors based on physiological signals are required. EMG and MMG based sensors are chosen, specifying the sensors number and positioning required for an effective human will identification. Pressure sensors, realized with dielectric elastomers, give the feedback of the system. Calculations relating voltage measured and pressure applied on the sensors are presented. Finally, the prototyping and testing phase is described, in which two mockup hands are realized for a proof of concept while EMG and MMG sensors are tested. Experimental setup and qualitative results are extensively described.