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Professorship Machine Elements and Product Development
Compliant Mechanisms

For many applications, compliant mechanisms are an advantageous alternative to conventional mechanisms. While conventional mechanisms owe their deformability to the sliding or rolling interfaces in the joints, flexible mechanisms fulfill their function through elastic stretches in places that are deliberately designed to be flexible during design. In contrast to conventional mechanisms, the material closure does not have to be interrupted. Therefore, flexible mechanisms tend to be monolithic systems.
The advantages of flexible mechanisms include the reduction of individual parts, freedom from backlash, wear and friction, reduced maintenance requirements, increased precision, a more favourable ratio between load capacity and mass, low sensitivity to dirt and increased cleanliness of operation.
In addition to the substitution of classic joints, flexible mechanisms are particularly suitable for use in form adaptive systems. These systems do not have a limited number of discrete inputs and outputs, as is the case with conventional gearboxes, but are characterized by complex surface changes due to an imprinted input variable.

In many cases, and especially in form adaptive structures, flexible mechanisms are complex interacting beam structures in which a classical, intuitive or experience-based manual design according to the approach of classical engineering is hardly possible. The aim of the professorship is to automate the synthesis with the aid of computers. Methods of formal optimization and machine learning are used for this purpose. The structures designed in this way are used not only for shape adaptation, but also in softrobotics, for example. Of course, the methods developed can also be applied to classic problems of flexible transmission synthesis.

The schematic illustrated in the figure shows the process of structural optimization of a flexible, shape-adaptive structure. In the concrete case this is a shape-adaptive wing, which is also referred to in English as "Morphing Wing". Structure optimization of a flexible mechanism for shape adaptation The mechanism is generated by the optimization-based allocation of higher and lower values of the local stiffnesses in a specified, parameterized installation space. Particular attention has been paid to the formulation of the optimization problem, which in its current form requires only the specification of the target kinematics.

A Posterior Adaptation of the Stiffness of Compliant Mechanisms

Funding agency: DFG
Research association:
Duration:24 Months
Start time:May 2019
Editor:Kristian Mauser
Further Information:DFG-Project
logo:DFG

Compliant mechanisms use the deformability of their material to achieve a predefined motion rather than using sliding and rolling components as it is the case with conventional mechanisms. The functional principle requires that strain energy is stored when a compliant mechanism is deflected. With a defined amplitude of movement, the value of the strain energy is determined by the structural stiffness of the mechanism. From the application point of view, the latter is in a field of tension: On the one hand, there are applications in which the stiffness is desired, e.g. with tweezers to generate a restoring force; On the other hand, the stiffness reduces the efficiency of the mechanism during an energy transfer from the mechanism input to the output.Once a compliant mechanism is designed and manufactured, it usually has a defined stiffness. If the mechanism is to be used in different applications with different stiffness requirements, the result is a suboptimal system behavior. There are also applications in which the stiffness is completely undesirable, but this cannot be avoided due to the aforementioned functional principle of compliant mechanisms. It would therefore be advantageous to be able to adjust the stiffness of an existing compliant mechanism a posteriori. The approaches described in the literature in the field of compliant mechanisms partially rely on the targeted introduction of prestresses through force loading of the mechanism. However, the determination of the load is usually based on experience and intuition. With this project we aim to develop an optimization-based method to determine preload forces for the defined adaptation of the stiffness of existing compliant mechanisms.

Continuum-based design of selectively compliant mechanisms with smooth kinematics

Funding agency: DFG
Research association:
Duration:24 Months
Start time:Apr 2019
Editor:Stephanie Kirmse
Further Information:DFG-Project
logo:DFG

Compliant mechanisms are mechanical systems, which depict the function of a conventional mechanism, but without movable components (bearings, guide and joints). Instead, their function is based on the deformation of elastic areas. Design procedures for compliant mechanisms can be sharply divided into two categories: In the first (pseudo-static body approach), the procedure starts from a multi-body system, as with conventional mechanisms; in the other (continuum-based approach), a design space is defined in which a fixed or varying quantity of material is distributed in a defined manner. A fundamental difference between the two approaches concerns the mechanism’s topology. In the first case the topology is to be assigned a-priori, while in the second case it is part of the result. This makes the design by multibody methods relatively easy, however at the price of a substantial reduction of the choice of possible layouts. Continuum-based approaches are normally based on formal optimization methods due to their inherent complexity. A further important difference is related to the kind of deformation patterns which can be achieved: multibody methods tend to generate deformation patterns with elastic strain concentrated in particular areas, while continuum based methods allow arbitrary deformation patterns including smooth shape changes. Known continuum based methods are mainly focused on the synthesis of mechanisms with a pseudo-mobility of one. The pseudo-mobility of a compliant mechanism corresponds to the number of independent kinematic quantities which are to be prescribed in order to control the static deformation of the mechanism with sufficient accuracy. General-purpose procedures able to explicitly generate mechanisms with multiple pseudo-mobility are not available. The methodology to be developed in this project is intended to remove this deficiency. In addition, the new design approach shall be able to generate mechanisms with smooth deformation patterns.The methodology to be developed in this project is of practical interest for any kinds of shape-adaptable structures, like morphing wings, adjustable seats and bed surfaces, as well as for classical transmission mechanisms for robotics and in general for automation tasks.

Vibration reduction by energy transfer using shape adaption

Funding agency: DFG
Research association: SPP 1897
Duration:36 Months
Start time:Jan 2017
Editor:Alexander Nowak
Further Information:DFG-Project
Logo:Spp 1897logo:DFG

Lightweight design is one of the most important issues in engineering design. The objective is to reduce the mass of structural components for the purpose of saving costs, energy and resources in manufacturing and operation processes. However, the lighter the structure is, the more it is prone to unwanted vibrations. Such vibrations should be minimized in order to prevent the environment, products and human beings from being harmed and to maximize the lifetime of the products. Vibration reduction can be achieved by passive, semi-active or active measures, where passive means that no external energy is needed, while semi-active and active measures employ external energy to either control dissipation or directly counteract the vibrational motion, respectively. Since active measures usually do not rely on dissipation, they do not fall in the scope of the call for proposals and will not regarded in this project. In the realm of passive and semi-active measures, two general approaches can be used to reduce vibration in structures, namely that of damping, which is the dissipation of kinetic energy into another form of energy, or that of absorption, which is the transfer of kinetic energy from a critical mode into an uncritical mode. The envisioned approach will combine the concepts of damping and absorption in a novel way by integrating the functionality of a damped, tuned mass absorber into a shape adaptive structure. By dynamically adapting the stiffness of a slender, beam-like structure using shape adaption of the cross-section, kinetic energy will be transferred from the critical low-frequency bending modes into a specifically designed, higher frequency absorber mode, which can then be damped in an optimal way. Optimal design of the shape adaption mechanism and of the absorber mode will be pursued using compliant mechanisms. The dissipation will be optimized by a specifically designed friction damper.

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