# Preliminary Biomechanical Evaluation of a Novel Exoskeleton Robotic System to Assist Stair Climbing
## 2 Materials and Methods  
### Exoskeleton  
The novel exoskeleton robotic system to assist stair climbing, shown in Figure 1, is according to ISO 13482 a restraint type physical assistant robot, since the system is fastened to the human during use. The intended purpose of the exoskeletal movement assisting system (**eBUS**; German abbreviation for **exoskeletales **B**ewegungsunterstutzungssystem) is to assist people with a natural lack of strength, such as elderly people. In previous biomechanical studies, it was found that an exoskeleton for elderly people should assist them in knee extension during ascent and descent as well [3]. Therefore, the active exoskeleton eBUS was designed that assists only the knee joint actively in extension in order to maintain minimal actuation. Furthermore, the eBUS is characterized by a unilateral and rigid structure and is symmetrically designed for the left and right leg. The system works independently for each leg and weighs 6.1 kg per leg. The components that are attached to the thigh have a total mass of 4.4 kg (72%), those on the shank 1.5 kg (24%), and on the foot 0.2 kg (4%). Thus, it was possible to consider the fact that the low mass at the distal segment (foot) is advantageous for the acceleration of the foot. The eBUS consists of seven parts per leg with two rotational joints corresponding to flexion/extension of the knee and dorsiflexion/plantarflexion of the ankle (Figure 1). Three of the seven segments are attached to the user's thigh, shank, and foot and represent the interface. These three segments are connected to each other through four sliding rails. The four rails create the two rotational joints with one degree of freedom, that should ideally be coaxial with the human knee and ankle joints, respectively. The sliding rails enable the eBUS to be adjustable for different anthropometries of users, as shown in Figure 2.  
The foot segment of the exoskeleton surrounds the human foot and consists of a compliant material to allow the foot to roll off. This design is intended to take into account the second DOF in the ankle joint so that lateral movements in the frontal plane are possible to increase the user comfort. All the three interface attachment components are adjustable to consider individual anthropometry to increase comfort. The active force to assist stair climbing is implemented through a Bowden cable, which is tensioned between the thigh segment and the shank segment of the eBUS and spans the human knee anteriorly. The drive system from a Maxon motor ag (Sachseln, Switzerland) consists of a brushless DC motor (EC 60 flat, 150 W) with integrated encoder (Encoder MILE 4096 steps per turn, two channels with Line Driver) and a gearhead (GP 52 C) that is fixed to the exoskeleton thigh. The energy storage unit is also attached to the thigh segment and consists of seven lithium-ion cells (Samsung INR18650-29E) of type 7S1P (FRIWO Geratebau GmbH, Ostbevern, Germany). The driver system pulls the cable and creates a pulling force. The motor is controlled by a servo controller (ESCON Module 50/5, Maxon motor ag, Sachseln, Switzerland) in combination with an Arduino(r) Mega 2560 (Arduino SA, Lugano, Switzerland). The force in the bowden cable is measured at any time by a ZAD 500-T tension force sensor (HAEHNE Elektronische Messgerate GmbH, Erkrath, Germany). The force vector direction is independent of the knee angle  
Figure 1: Novel exoskeleton robotic system to assist stair climbing called **eBUS**[4]: **(a)** attached to both legs in the frontal plane and **(b)** in the sagittal plane; **(c)** attached to the left leg in posterior view; (1) thigh segment, (2) thigh rail, (3) shank segment, (4) shank upper rail, (5) shank lower rail, (6) foot rail, and (7) foot segment. as the cable has a fixed distance with 0.1 m from the patella of the knee. The pulling force results in an external moment through the rigid structure of the exoskeleton to assist knee extension.