Passive shoulder supports show large potential for a wide range of applications, such as assisting activities of daily living and supporting work-related tasks. The rigid-link architecture used in currently available devices, however, may pose an obstacle to finding designs that offer low protrusion and close-to-body alignment. This study explores the use of mechanisms that employ a flexible element which connects the supported arm to an attachment at the back and acts as energy storage, transmission and part of the load bearing structure. Based on the synthesis method explained in this paper, a large scope investigation into possible flexure-based mechanism topologies is conducted. Thereby, many potential designs are discovered, which are presented, categorized and compared. Two promising designs are developed into prototypes, and are built and tested on a dedicated test bench. These two mechanisms reduce the necessary moment to lift the arm by more than 80 % throughout 85 % of the range of motion, while staying within 18 cm and 10 cm distance from the body, respectively. The study indicates that, due to its lower protrusion and interface loads, a design with a tapered flexure connecting the upper arm via a hinge to a spring-loaded slider at the back offers the most promising solution.

In this letter we demonstrate a pneumatic bending actuator for upper-limb assistive wearable robots which uses thin McKibben muscles in combination with a flexure strip. The actuator features both active soft actuation and passive gravity support, and in terms of force transmission bridges the gap between the classic rigid type actuators and the emerging soft actuator technologies. Its flexure strip leverages the high-force low-displacement properties of McKibben muscles towards a large rotational range of motion and reduces localized forces at the attachments. We explain the synthesis method by which these actuators can be obtained and optimized for high specific moment output. Physical specimens of three optimized actuator designs are built and tested on a dedicated experimental setup, verifying the computational models. Furthermore, a proof-of-concept upper-limb assistive wearable robot is presented to illustrate a practical application of this actuator and its potential for close-to-body alignment. We found that based on our currently available components actuators can be built which, given a width of 80 mm, are able to produce a moment exceeding 4 Nm at an arm elevation of 90 deg.

The use of overconstrained mechanisms is often avoided in precision mechanics. Misalignments in the mechanism can cause deteriorated system behaviour, such as buckling. Overconstrained designs do have several advantages, such as higher load bearing capacity and higher natural frequencies. However, these advantages are only present if the mechanism is aligned within certain tolerances. In this paper a method is introduced to identify the limits of these alignment tolerances. The method allows the calculation of the forces in the mechanism due to misalignment. The internal forces are compared to the buckling loads of the mechanism yielding the critical misalignments; the method is corroborated using a multibody simulations. Subsequently, both analyses are compared to an experimental setup; this setup measures the first three modal frequencies and identifies the buckling modes. The proposed method and multibody simulation match with each other and the experiment. However, the critical misalignments are about 20% larger in the experiment; this is mainly attributed to hardware imperfections. Due to misalignment and flatness limitations of the flexures, the undeflected stiffness in the experiment is lower than modelled. The deterioration of the support stiffness is smaller in the experiment. In the most serious case, it retains 80% of the modal frequency in the support directions. The proposed method can be used as a guideline to estimate the manufacturing and assembly tolerances of an overconstrained flexure-based mechanism.