Gunnar Borchert

Design of a holonomic ball drive for mobile robots

Omnidirectional mobile robots offer interesting features for industrial and service applications, in particular, when operating in tight spaces. Compared to car-like nonholonomic vehicles, they provide a higher degree of maneuverability, and often require less complex path planning and control schemes. Three different types of holonomic wheels that enable omnidirectional motion have been proposed in literature: universal, Mecanum, and ball wheel mechanisms. A problem commonly associated with the first two wheel types is that they induce vibrations in the system due to the discontinuous contact points. In this article, a ball wheel mechanism with superior features including slip detection, free-wheel modus and attrition sensing is presented. The first prototype was built using additive manufacturing of polypropylene. The requirements for such a design are discussed. Based on the ball wheel drive presented in this article, a design for an omnidirectional mobile robot platform driven by three redundant ball wheel units is proposed. The velocity kinematic model of this mobile base is also addressed.

Design and validation of the binary actuated parallel manipulator BAPAMAN2

This paper describes a procedure for designing a novel mini-scaled parallel manipulator having three binary-actuated degrees of freedom, whose name is BAPAMAN2 (Binary Actuated PArallel MANipulator version 2). This is a low-cost, easy-operation manipulator that has been designed as an evolution of previous designs in the BAPAMAN series (BAPAMAN and BAPAMAN1). Main novel desired design features have been a smaller size and higher stiffness and repeatability performances. For this purpose, proper models have been developed and main design characteristics have been validated by means of numerical simulations. Experimental tests have been also carried out for validating the operation performances of a built prototype also in terms of stiffness and repeatability features.

Design and Testing of a 2-DOF Ball Drive

Omnidirectional mobile robots offer interesting features for industrial and service applications, in particular, when operating in tight spaces. Compared to car-like nonholonomic vehicles, they provide a higher degree of maneuverability, and often require less complex path planning and control schemes. Three different types of holonomic wheels that enable omnidirectional motion have been proposed in literature: universal, Mecanum, and ball wheel mechanisms. A problem commonly associated with the first two wheel types is that they induce vibrations in the system due to the discontinuous contact points. In this article, a ball wheel mechanism with superior features including slip measurement, free-wheel modus and attrition sensing is presented. The first prototype was built using additive manufacturing. The requirements for the design and possible improvements for future versions are discussed. Based on the presented ball wheel drive, a design for an omnidirectional mobile robot platform driven by three redundant ball wheel units is proposed. The velocity kinematic model of this mobile base is also addressed. Moreover, motion planning for an individual ball drive is demonstrated by means of an online trajectory generation scheme. The pseudocode of the trajectory planning algorithm implemented in LabVIEW is then presented. Finally, the motion characteristics of the ball drive mechanism are tested and its functionality is evaluated in detail. Measurements obtained from these tests show that the slip between the ball wheel and the ground can be estimated quite accurately. Hence, it is expected that these improved dead-reckoning estimates will result in a higher positioning accuracy of the final base.

Normal Operation Input Signals for Parameter Estimation in Underactuated Structures

This paper addresses challenges of parameter estimation of an arbitrary object which is manipulated by an underactuated handling system. In the present scenario, a robot is extended with a passive orientation device. Since the passive joints are steered by energy control, knowledge of the inertial parameters of the gripped object must be obtained. For this purpose, an evaluation process is shown to find excitation inputs that are based on normal operation motion profiles. The general applicability of the excitation is then demonstrated along with an optimization to improve the excitation of the passive joints which yields a better estimation. Since it is difficult to obtain acceleration signals, the influence of their accuracy on the estimates is additionally illustrated. The article closes with the identification of future developments.

Opportunities and Challenges for the Design of Inherently Safe Robots

An approach for solving the challenges that arise from the increased complexity of modern assembly tasks is believed to be human robot co-operation. In these hybrid workplaces humans and robots do not only work on the same task or interact during certain assembly steps, but also have overlapping workspaces. Therefore, ‘safe robots’ should be developed that do not harm workers in case of a collision. In this chapter, an overview of methods for designing a hardware based soft robot that is inherently safe in human-machine interaction is given. Recent projects show that robots could be soft enough for interaction but they are not able to resist forces that occur in the assembly process. Current solutions show that the designer of such robots must face a trade-off between softness and dexterity on the one hand and rigidity and load carrying capabilities on the other hand. A promising approach is to integrate variable stiffness elements in the robotic system. The chapter classifies two main design rules to achieve stiffness variability, the tuning of material properties and geometric parameters. Existing solutions are described and four concepts are presented to show how different mechanisms and materials could be combined to design safe assembly robots with a variable stiffness structure.

Design Methodology for a Compliant Binary Actuated Parallel Mechanism with Flexure Hinges

This paper discusses the further development of a binary parallel manipulator named BaPaMan1 (Binary Actuated PArallel MANipulator), which is aimed at the improvement of the structural stiffness and allows task-adaptation. BaPaMan1 is a three DOF spatial parallel robot which comprises flexure hinges and Shape Memory Alloy (SMA) actuators to achieve a low-cost design, well suited for easy operation applications. Measurements have shown that this comes at the cost of poor structural stiffness and end effector accuracy. To counter these issues BaPaMan2 and BaPaMan3 have been developed and are elaborated within this work. During the design phase, an empirical FEA was used to improve the flexure hinge performance, in which relations between several design parameters and the stiffness of the entire system were analyzed. Finally, task-adaptation was achieved using a developed design methodology and parametric CAD model for BaPaMan3, which take advantage of deduced stiffness influencing equations.

An Offline-Signal Processing Algorithm for Combining Superior Excitation Methods for an Ultrasonic Piezo Motor

Ultrasonic piezomotors have been applied as a drive concept in miniaturized robot systems for many years. They have various beneficial characteristics, such as low costs, high accuracy and velocity. This has been demonstrated, with instance, with APIS, a miniaturized robot developed at the Technische Universitat Braunschweig. With the example of APIS, this contribution discusses the challenge of an offline signal processing techniques in order to combine different excitation methods for the piezomotors used in this robot. Since each excitation method influences the motor characteristic in a variety of ways, this research is motivated by desire for an optimal adaptation of certain features to a given trajectory, such as velocity, acceleration, moving direction or step resolution. Thus, our approach is to develop different ways to split a given trajectory into segments with regard to the requirements of the motor. So these segments can be linked to an excitation method which brings about the desired features. To this end, model trajectories are first generated and reasonable excitation methods for the piezomotor are identified and characterized. Key features are velocity, acceleration, moving direction and step resolution. Apart from defining key features, an assessment algorithm has been developed along with suitable weighting criteria. Offline signal algorithm means that the trajectory will be analyzed and that the arrangement of suitable excitation methods will be defined before the motion process starts.Finally the results, that is, each model signal together with the associated excitation method, are shown and validated. To achieve better results, possibilities for future developments are identified.Copyright

System Identification of a Grinding Machine Excited by an Active Tool Holding Device

Reducing the cost of manufacturing products can be realized by high performance, precise and productive machine tools. This goal is often obtained by the user increasing the adjustable machine parameters, which include the feed rate or the cut depth. Both will increase the grinding force during the grinding process, which can result in an excitation of the machine structure. If the excitation is too high, the machine structure can show an undesired dynamic behavior, which can cause the machine to vibrate. These vibrations can result in chatter marks on the work piece surface and if large enough can result in a production stop. One possibility to minimize the chance of excessive vibration is a conservative choice of machine parameters which are well below the optimal set for a particular machine. This paper presents an approach which shows the first results of an active tool holding device aimed at reducing unwanted vibrations resulting from an aggressive parameter choice.Copyright