# Towards a More Robust Non-Rigid Robotic Joint
## 1 Introduction  
Robots are becoming increasingly present in daily tasks, both from a personal and an industrial point of view. The use of robots to assist in the execution of freight transport tasks in a factory plant and their use for the disinfection of common areas have increased significantly. To perform certain tasks, robots usually rely on manipulators that act like human arms and hands. In some cases, it is necessary for the traction system of a mobile robot to be more adaptable, and in these cases, locomotion systems similar to legs are used, such as those in [1]. In the above examples, movement or manipulation are common in unstructured environments. This means that these robot systems must be prepared for unexpected disturbances, such as rough terrain, external forces, collisions, etc. Collisions with obstacles or people and the risk of the robot breaking itself are particularly important, as the integrity of people and equipment are at risk. Considering that conventional robots are usually designed with rigid elements, a higher degree of sensor technology and better control are required to cope with external forces. In these applications, the use of flexible joints helps to improve safety for the people working in the vicinity of the robot system and also for the robots themselves, as the probability of breaking something decreases. Despite the evolutionary effort for using flexible and more sophisticated solutions for the market [2; 3; 4; 5; 6; 7], it is not yet very easy to find solutions in mature states with which to explore and perform experimental work, as [PERSON] et al. [8] report. Even so, there are some works that should be mentioned, since they present varied solutions. The work from [PERSON] et al. [9] is one of them, which resorts to a six degrees of freedom (DOF) fully parallel manipulator with elastic joints. The elastic component is used as the last link of an industrial manipulator and consists of three links connected by an elastic ball joint and a Hooke's joint on each side. This joint is an example of a purely mechanicalsolution, but it does not have a damping effect on the system, which is very useful in the event of an impact, given its important characteristic. A different solution was proposed by [PERSON] and [PERSON] [10], which presents a flexible translational joint mechanism. The authors consider an adaptable adaptive joint with adjustable elasticity that can slide in multiple directions. The joint is built with servos that control the flexibility of a rubber band that locks into a ball bearing bed. Such joints are used as lower leg parts for a humanoid robot. Despite adding an elasticity component to the joint, this solution also does not have a damping component. Another characteristic of this joint is its variable elasticity, however it consumes energy for its control. [PERSON] and [PERSON] [11] have designed a fully passive elastic joint to be used on an inspection robotic vehicle. Each of the three links uses a spring, which can be different from each other, to introduce a torsional elasticity capability that allows the joints to be flexible even when the wheel is stopped. This joint also has no damping component, which could be an advantage for the use intended for this system. Regarding the control of an elastic joint, there are some works that address it, from the 1980s to the present day [12; 13; 14; 15; 16; 17; 18], indicating that this issue is still an open line of research. Since the non-rigid joint increases the flexibility of the robot, the control methods must be adequate for the movement and permissiveness of such specific elastic joints. The work from [PERSON] et al. [19] also stands out, since a compliance control for an anthropomorphic robot with elastic joints is used. It resorts to a PD plus gravity controller and the addition of a gravity-biased motor position variable. One thing that is anticipated by the control method is that, if the manipulator comes into contact with the environment, there are laws that achieve compliant behavior during this contact.