Publications

In the field of soft robotics there is still a great need for a versatile, simple, and affordable gripper with a high level of adaptability to unknown objects of different sizes, shapes, and stiffness. Most of the existing soft robotic grippers are complex solutions realized with fluid-mechanically driven actuators, active smart materials, cable-driven actuation, and different form-closure principles. However, soft grippers based on compliant mechanisms are rarely introduced and explored so far. Therefore, we present a novel compliant two-finger gripper mechanism for adaptive and gentle gripping, especially of soft and easily squeezable objects like fruits, vegetables, sweets, and sushi. The structurally inherent adaptability is achieved using an optimally synthesized compliant mechanism in combination with a conventional linear actuator. Furthermore, the two-finger gripper passively realizes pinch (parallel) or/and encompass (power) grasping. It is shown by FEM simulations and confirmed by prototype tests that the developed gripper realizes both pinch and encompass grasping with high adaptability. A special advantage of the gripper is the possibility to achieve gentle food-handling of objects with comparable weight independent of the object shape, size, and position without the need for sensors. Moreover, the precise, safe, and fast manipulation of very delicate objects is exemplarily demonstrated for different sushi pieces using the gripper mechanism with an industrial robotic arm.

Compliant mechanisms are state of the art in micropositioning stages due to their many beneficial features. However, their design usually compromises between motion range, motion accuracy and design space, while mechanisms with distributed compliance are mostly applied. The further use of flexure hinges with common notch shapes strongly limits the stroke in existing high-precision motion systems. Therefore, this paper presents a high-precision compliant XY micropositioning stage with flexure hinges capable of realizing a motion range of ± 10 mm along both axes. The stage is based on a novel plane-guidance mechanism, which is optimized to realize a precise rectilinear motion of the coupler link while keeping the rotation angles of all hinges below 5˚. The XY motion is then achieved by coupling two of these mechanisms in a serial arrangement. Next, the synthesis of the monolithic XY stage is realized by replacing all revolute joints of the rigid-body model with flexure hinges using optimized power function notch shapes. Emphasis is also placed on the embodiment design of the stage and the actuator integration to minimize possible error sources. Finally, the quasi-static behavior of the compliant stage is characterized by a simulation with a 3D FEM model and by an experimental investigation of a prototype. According to the results, the developed compliant XY micropositioning stage achieves a maximum positioning deviation of less than 10 [my]m in both axes and a yaw error of less than 100 [my]rad over a working range of 20 mm × 20 mm with a comparably compact design of the compliant mechanism of 224 mm × 254 mm.

Soft Robotics ist ein aktuelles Forschungsfeld. Die Anwendung mechanisch nachgiebiger Materialien in technischen Applikationen wird aktuell intensiv vorangetrieben. Intelligente nachgiebige Materialien, beispielsweise Materialien, die ihre mechanischen Eigenschaften reversibel ändern können, werden in immer mehr technischen Anwendungen eingesetzt. Vordergründig geschieht dies mit den Zielen der Funktionserweiterung und der individuellen reversiblen Anpassbarkeit an veränderte Umgebungsbedingungen. Magnetorheologische Elastomere sind intelligente nachgiebige Hybridmaterialien, bestehend aus einer Elastomermatrix, die mit ferromagnetischen Partikeln gefüllt ist. In Abhängigkeit von der beabsichtigten Verwendung dieser Hybridmaterialien werden weich- oder hartmagnetische Partikel eingesetzt. Diese Materialien besitzen die besondere Fähigkeit, ihre mechanischen Eigenschaften zu ändern, wenn auf sie Magnetfelder einwirken. Diese vorteilhaft nutzbare Eigenschaft macht ihren Einsatz in zahlreichen technischen Anwendungen dort effizient, in welchen Strukturen mit reversibel veränderbarer mechanischer Nachgiebigkeit eingesetzt werden sollen. Bedingt durch ihre vorteilhaften Eigenschaften werden diese Materialien aktuell intensiv erforscht. Im Fokus der meisten Forschungsarbeiten auf diesem Gebiet stehen dabei die mechanische und magnetische Charakterisierung der Materialeigenschaften, sowie die Erarbeitung von Materialmodellen, die eine Beschreibung des magneto-mechanischen Verhaltens dieser Materialien mit hoher Genauigkeit erlauben. Viele Effekte im Verhalten dieser Materialien sind aktuell noch nicht tiefgründig untersucht bzw. verstanden worden. In dieser Arbeit werden diese Hybridmaterialien aus ingenieurtechnischer Sicht betrachtet. Die Untersuchungen an diesen Materialien werden auf potentielle technische Applikationen fokussiert. Dabei wird der Schwerpunkt auf den magnetfeldinduzierten elasto-plastischen Effekt dieser Materialien, sowie auf die Nutzung dieses Effektes in einer exemplarischen technischen Anwendung, in End-Effektoren für die Greifertechnik, gelegt. In der Arbeit werden magnetorheologische Elastomere untersucht, die weichmagnetische Partikel in verschiedenen Volumenkonzentrationen enthalten. Im ersten Teil der Arbeit werden schwerpunktmäßig experimentelle Untersuchungen durchgeführt, um geeignete Material- und Zusammensetzungsdesigns für das Hybridmaterial im Hinblick auf die gewünschte Anwendung definieren zu können. Mit Hilfe der anschließenden theoretischen Untersuchungen wird die Quantifizierung der magnetischen und mechanischen Eigenschaften dieser Materialien auf der Makro-Skala erreicht. Aufbauend auf diese Untersuchungen wird eine Methodik erarbeitet, die die Beschreibung des magneto-mechanischen Verhaltens dieser Materialien mittels Anwendung der Finite-Elemente-Methode ermöglicht. Die exemplarische Verwendung dieser Materialien als End-Effektor beim Greifen von empfindlichen Objekten impliziert die formschlüssige Umhüllung der zu greifenden Gegenstände. Mit Magnetfeldern werden die Effekte der Nachgiebigkeitsänderung und der magnetfeldinduzierten Plastizität in diesen Hybridmaterialien stimuliert. Dadurch wird eine Formanpassung und damit formschlüssiges Greifen ermöglicht. In der Arbeit wird ein End-Effektor-Prototyp hergestellt und getestet, um die vorteilhafte Anwendbarkeit magnetorheologischer Elastomere in der Soft Robotics aufzuzeigen.

All known locomotion principles are limited respective to environmental conditions. Often, the occurrence of obstacles or gaps means the break-off for the operating motion systems. For such circumstances, a controllable jumping locomotion is required to cross these barriers. However, this locomotion demands sophisticated requirements to the actuation. The abrupt actuation is commonly realized by high dynamic actuators or complex mechanisms. In this work, a simple solution utilizing the multistability of a compliant tensegrity structure is described. Therefore, a two-dimensional tensegrity structure featuring four stable equilibria is considered. Based on bifurcation analyses a feasible actuation to control the current equilibrium configuration is derived. Changing between selected equilibrium states enables a great difference in potential energy, which yields a jumping motion of the structure. Based on numerical simulations a suitable actuation strategy is chosen to overcome obstacle and steps by jumping forward or backward, respectively. The theoretical approach is examined experimentally with a prototype of the multistable tensegrity structure.

The use of non-conventional materials is nowadays of much interest in scientific community. Magneto-rheological elastomers are hybrid materials, which in presence of magnetic fields state a change in their mechanical properties. They are composed by an elastomeric matrix with embedded magnetic particles. One of the most attractive features of these materials is that as soon as the magnetic field is removed from the material, the original mechanical properties are completely recovered, with negligible differences in comparison to the original state. This paper focuses on the study of magnetic characteristics of these smart materials, such as relative permeability and demagnetizing factors, for samples with different volume concentration of ferromagnetic particles.

This chapter contributes to the modeling, analysis and control of terrestrial artificial locomotion systems. Inspired by previous models, we set up an unconventional model for a snake-like locomotion systems in form of a chain of visco-elastically interconnected mass points in a plane with passive joints, but - in contrast to literature - active links (time-varying link-length) and rotatable skids to change the movement direction and to avoid obstacles. We investigate this model in a dynamical way and focus on controlling these link lengths to achieve a global movement, steered by the skids. From dynamics, the actuator forces have to adjust the prescribed link length for the locomotion. Since it is impossible to determine the necessary actuator forces a-priori, we apply an adaptive lambda-tracking controller to enable the system to adjust these force outputs on-line on its own. Prescribed motion patterns, i.e. specific gaits, are required to guarantee a controlled movement that differ in the number of resting mass points, the load of actuators and spikes, and the lateral forces of the skids. In contrast to literature, the investigated system of n = 10 mass points exhibit a large variety of possible gaits. To determine the most advantageous gaits, numerical investigations are performed and a weighting function offers a decision of best possible gaits. Using these gaits, a gait transition algorithm, which autonomously changes velocity and number of resting mass points depending on the spike, actuator and lateral skid force load, is presented and tested in numerical simulations.

Animal vibrissae are used as natural inspiration for artificial tactile sensors, e.g., the mystacial vibrissae enable rodents to perform several tasks in using these tactile hairs: object shape determination and surface texture discrimination. Referring to the literature, the Kinetic Signature Hypothesis states that the surface texture detection is a highly dynamic process. It is assumed that the animals gather information about the surface texture out of a spatial, temporal pattern of kinetic events. This process has to be analyzed in detail to develop an artificial tactile sensor with similar functionalities. Hence, we set up a mechanical model for theoretical investigations of the process. This model is analyzed in two different directions using numerical simulations: at first a quasi-static and then a fully dynamic description.