A Curved Compliant Differential Mechanism with Neutral Stability

Abstract

People in healthcare, warehousing, and the agriculture sector all have one thing in common. They require labour which can be demanding on the human body, this could lead to problems in the long term. A way to alleviate these problems is to use a passive exoskeleton. While passive exoskeletons can have a variety of different use cases and types of support, this thesis will focus on a wearable passive back support. A company specialising in these passive wearable back supports is Laevo. They have their own version of such a mechanism using torsional springs which are attached to the upper torso and legs. This ensures that when bending the mechanism provides support. However when walking, these springs are also activated and make walking more difficult and require more energy. A way to solve this problem is to use a differential mechanism. Such a mechanism can be quite complex and bulky and comprised of a lot of parts. A solution could be found in the world of compliant mechanisms.

The goal of this project is to create and analyse a compliant differential mechanism for use in passive exoskeleton design. As a basis for a design, the proposed mechanism by Maurice Valentijn was used. The two main challenges were the location of the rotational axis and a relatively low stiffness ratio between the bending and walking scenario. While this mechanism showed potential as a compliant differential mechanism, it did have some problems which needed to be overcome before the use in a passive exoskeleton would be feasible.

The proposed design to solve these challenges consists of a thin-walled beam, with an H-shaped cross section, which has two curves forming a U-shape. By applying constraints on the sides the rotational axis of the mechanism could be changed to align with the rotational axis of the hip joint. This mechanism in combination with reintroduction of potential energy using springs to lower the stiffness of the mechanism when walking. This allowed for a design which could be used as a compliant differential mechanism in the use of exoskeleton design. To investigate the proposed design for a compliant differential mechanism, the mechanism first needs to be modelled and optimised to meet the requirements needed for the use in an exoskeleton. For the optimisation, a framework is proposed which integrates the use of a simulated Ansys model in combination with a Matlab optimisation problem. With this optimised model the behaviour of the mechanism can be analysed. The behaviour of the mechanism is investigated in the paper in Chapter 4. In this paper the simulated model is validated using an experimental setup. This paper is the main contribution of this thesis.

Findings of the paper show that the stiffness of the mechanism can be significantly reduced by reintroducing potential energy into the system to compensate the stored potential elastic energy in the material during walking. This caused the mechanism to have different types of behaviour: positive stiffness, zero stiffness, and negative stiffness. These stiffnesses depend on the initial preload of the springs, more initial preload means more energy is stored in the springs and releases more energy for the same displacement. This changes the overall potential energy to have these three aforementioned stiffness states. Zero stiffness is the most interesting for exoskeleton design, this minimises the amount of work required while walking without having bistable behaviour in the mechanism.

Finally, As a proof of concept for the mechanism a wearable exoskeleton prototype has been created to get a practical understanding of the mechanism and to find problems with the implementation of the mechanism for future works. The wearable exoskeleton prototype showed a lot of potential, the behaviour of the mechanism in the experimental setup was transferred to the prototype and low stiffness while walking was achieved without effecting the bending support.