## 1 Introduction  
Mobile robots are capable of moving in a predefined environment and can be autonomous (AMR--autonomous mobile robots) or nonautonomous. The latter deploy guiding devices allowing movement along a predefined navigation route. A significant characteristic of mobile robots is that their moving structure includes all the components that are required for achieving the proposed task, from the energy source to the effectors. Due to its operational independence mobile robotics lends themselves to a practically unlimited number of applications. In addition to utilization in industry, agriculture or road traffic safety (self-driving motor vehicles), mobile robots can also perform tasks that are potentially dangerous to humans (e.g., disaster control or military applications). Frequently, mobile robots can be found in everyday household applications, such as, for example, semi-autonomous vacuum cleares [1]. Rehabilitation medicine is another growing field of applicability, where mobile robotics is present in the form of wearable assistive devices. Wearable assistive devices are designed to facilitate the performance of tasks by persons who, for various reasons, have lost certain abilities. Such devices can be of passive type (not powered) or can be powered by electrical, pneumatic or hydraulic actuation systems. Orthotic and prosthetic devices fall into the passive category and are externally applied wearable devices: the former being used to modify the structural and functional characteristics of the neuromuscular and skeletal system, while the second is used to replace wholly, or in part, an absent or deficient limb segment [1]. According to the International Society for Prosthetics and Orthotics (ISPO), a prosthetic or orthotic device allows a person with motor deficiencies to remain active and independent, and to live a normal life without the need for formal support services [2]. Orthotic and prosthetic devices are complex technical systems that, depending on the patient's needs, can be custom-made to exact measurements. Powered exoskeletons are portable devices of higher complexity. They were developed as augmentative devices to improve the bearer's physical performance or as orthotic devices for the rehabilitation of walking or locomotory assistance. Compared to stationary systems, powered exoskeletons must be compact, lightweight, safe, reliable, intelligent and portable, and can be potentially used at home [3; 4]. The mechanical power needed for an exoskeleton is obtained by pneumatic, hydraulic, and electrical actuators. Pneumatic and hydraulic actuation systems, while benefitting from a good power-to-weight ratio, are unattractive from the viewpoint of positioning precision. Electric motors, on the other hand, can be controlled with high precision, while having an unfavorable power-to-weight ratio [5; 6]. In [7], [PERSON] et al. compare two wearable assistive devices with different actuation systems, pneumatic and electro-mechanical, respectively. The masses of the two systems are 7.1 kg for the pneumatic and 10.1 kg for the electro-mechanical one. The conclusion reached by this study was to choose the pneumatic actuation for wearable assistive suits. Actuator positioning poses a challenge to designers of powered exoskeletons. Placing the motors near the joints entails a simpler construction, but inertial forces are generated that render the entire system more difficult to control. If the actuators are placed at a distance from the joints, the weight of the mobile components is reduced, but power transmission mechanisms are required that complicate the mechanical structure of the entire assembly. The growing use of compressed air in the construction of powered exoskeletons is determined by its benefits, such as easy generation and storage, non-flammability, minimum risk of explosion, minimal maintenance requirements of the pneumatic systems, compliance, etc.