Jürgen Hesselbach

How to reach the dynamic limits of parallel robots? An autonomous control approach

Based on closed kinematic chains, parallel robots obtain favorable dynamic properties as well as high stiffness. Hence, their application can significantly enlarge the productivity of automated production processes. A control concept for tapping the high potential concerning low cycle times and high path-tracking accuracy is presented. The proposed approach adapts autonomously to changing dynamic parameters as varying payload. The autonomous behavior is achieved by combining an adaptive control approach with an adaptive, time-optimal trajectory planning concept and an online-trajectory adaption mechanism. Extensive experimental results prove the performance of the proposed approach. Note to Practitioners -Many applications in the field of production automation (material handling, assembly, etc.) require high operating speeds and accelerations. During the past years, parallel robots proved to be an efficient and suitable supplement to serial robots. Unfortunately, the promising possibilities of parallel robots often cannot yield profit because their dynamic potential is still not fully exploited. The payload/robot mass ratio of parallel structures is even higher compared to serial robots, where the influence of the payload on the impedance of the robot is negligible. By use of direct drives the influence of a variable payload cannot be ignored. A modified adaptive control concept, which adapts autonomously to changing dynamic parameters-as varying payload due to diversity of assembly processes-guarantees high tracking accuracy and therefore better process quality as well as accurate estimates of changing dynamic parameters and therefore better process quality. In addition the productivity of the process can be enlarged, if the full drive power can be used at each point on the path. Thus, a new adaptive time-optimal trajectory planning algorithm is used to exploit the dynamic potential of the direct drives and consequently to shorten the cycle times. The aim of time-optimal trajectory planning, as it is commonly understood, is the determination of the maximum velocity profile along a given path that complies with all given dynamic and kinematic robot constraints like limited drive forces/torques, limited path and/or drive velocities and limited path jerk. Combining the adaptive control scheme and the adaptive, time-optimal trajectory planning algorithm with an online trajectory adaption mechanism, a control concept is realized, which autonomously adapts to changing dynamic robot behavior. Using this new approach, the advantages of parallel robots-as well as serial robots with direct drives-can better be utilized. This is a necessary prerequisite for a larger extension of PKMs for industrial applications.

Aspects on design of high precision parallel robots

This paper presents a concept for a micro‐assembly station and shows different possibilities for increasing the positioning accuracy. The main part of the station consists of a spatial parallel structure with three translational degrees of freedom. An additional rotational axis is integrated into the working platform. This structure is constructed with low friction joints, which are nearly free of backlash. The construction of these high precision joints is presented and the characteristics of the robot such as workspace and resolution are discussed. After this an approach for increasing the accuracy of parallel robots by integrating flexure hinges into the structure is described.

How Does Torsional Deformity of the Radial Shaft Influence the Rotation of the Forearm?: A Biomechanical Study

Objective The aim of this experimental study was to measure the exact influence of isolated torsional deformities at the middle third of the radial shaft on the rotation of the forearm. Design Biomechanical study in cadavers. Setting Trauma Surgery Research Laboratories at the Medical School of Hannover, Hannover, Germany. Intervention Fourteen intact and fresh cadaver specimens were fixed in a newly developed apparatus that allowed free pronation and supination. A ring fixator was applied to the radial shaft with K-wires that allowed us to stabilize torsional deformities in steps of 10°. The middle of the radial shaft was osteotomized via a small soft tissue window, leaving the other soft tissues, including the interosseous membrane, intact. Main Outcome Measurement Supination and pronation were measured using a goniometer in a standardized fashion. Results The mean (standard deviation) supination value before osteotomy of the radius was 71.6° (15.2°), and the mean (standard deviation) pronation value was 64.5° (12.4°). Radial osteotomy caused no significant difference in the range of motion before creation of torsional deformities. Supination torsional deformities >30° showed a significant loss of pronation. In turn, pronation torsional deformities >30° resulted in a significant loss of supination. The amount of mean rotational loss was approximately the same in the respective pronation and supination torsional deformities. Conclusion An axial torsional deformity of the radius of >30° causes a statistically significant loss of forearm rotation in fresh cadavers.

A new kinematic model of pro- and supination of the human forearm

We introduce a new kinematic model describing the motion of the human forearm bones, ulna and radius, during forearm rotation. During this motion between the two forearm extrem-positions, referred to as supination (palm up) and pronation (palm down), effects occur, that cannot be explained by the the established kinematic model of R. Fick from 1904. Especially, the motion of the ulna is not properly reproduced by Ficks model. During forearm rotation an evasive motion of the ulna is observed by various authors, using magnetic resonance imaging MRI) technology. Our new kinematic model also simulates this evasive motion. Furthermore, the model is enlarged to include angulations of the forearm bones. Using these results the influence of forearm fractures on the range of forearm motion can be predicted. This knowledge can be used by surgeons to choose the optimal therapy in re-establishing free forearm mobility.

PARVUS – miniaturised robot for improved flexibility in micro production

Purpose – Until now, the size range of most machines for precision assembly was much larger than the size of the pieces to be handled or the necessary workspace. Flexibly scalable miniaturised production machines can help to develop much more flexible micro production systems. The paper aims to describe the development of a micro‐parallel‐SCARA robot adapted in size to MEMS products.Design/methodology/approach – The robot consists of a miniaturised parallel structure, which provides a high level of accuracy in a workspace of 60 × 45 × 20 mm3. It has a base area of 130 × 170 mm2 and offers four degrees of freedom.Findings – Based on simulations, the degree of miniaturisation in terms of a smaller structure and a high level of accuracy is determined. The results show that a miniaturised hybrid robot with a plane parallel structure driven by miniaturised zero‐backlash gears and electric motors can reach a theoretical repeatability better than 1 μm.Research limitations/implications – The first prototype provi...

Elastodynamic optimization of parallel kinematics

Parallel kinematic machines have inherent advantages for many applications in the fields of robotics and machine tools. They obtain high dynamic capabilities combined with high accuracy and stiffness. But choosing the optimal mechanism dimensions for the best performance is still a challenging task. Furthermore there are a lot of performance criteria which have to be taken into account and which are pose dependent. The main idea of this paper is to present the fundamentals for a multi-criteria optimization approach for parallel kinematic machines according to given application requirements. Therefore we discuss a large number of performance criteria dealing with workspace, velocity transmission, inertia and stiffness. Finally the main idea of an optimization approach using evolutionary algorithms is shown.

A New Parallel Mechanism to Use for Cutting Convex Glass Panels

This paper presents a new four-degree-of-freedom mechanism, which was developed for cutting convex glass-panels. Proceeding from a concrete industrial task several serial and parallel structures were examined, but none could fulfill the given requirements. Therefore it was necessary to develop a mechanism which had to combine the advantages of both the serial and the parallel principle. With this motivation a parallel mechanism was designed which has two guiding chains between the base and the tool head. On the one hand this mechanism is characterised by compact design and high mobility for the rotational movements and on the other hand it gives the possibility to arrange all drives in the fixed frame, which results in reduced inertia and therefore in very good dynamic features.

Open Modular Robot Control Architecture for Assembly Using the Task Frame Formalism

The task frame formalism allows the programmer to overcome the drawbacks of the traditional robot oriented assembly programming, moving the programmers focus on the robot task. Additionally skill primitives contribute to a more natural programming paradigm. In this paper a robot control architecture is presented that implements both of these concepts providing a framework to easily implement new control features. Focus is put on a novel modular trajectory generator and the applied three-layered scheduling design. This architecture is based on the communication middleware MIRPA-X and has been experimentally validated on the HEXA parallel manipulator. The future use of distributed computing and runtime scheduling optimization are discussed.

Self-calibration of the HEXA-parallel-structure

In order to enhance absolute accuracy of the HEXA-parallel-robot this paper presents a technique to calibrate the structure by means of redundant angular sensors added to its passive joints. Compared to traditional calibration strategies which are based on pose-measurement by external measurement devices the so-called self calibration approach possesses several advantages. Besides a derivation of the kinematic transformation equations of the system under consideration it is shown how to formulate an appropriate residual function that has to be minimized in order to identify the geometric parameters of the robot manipulator. An important difference to previous work on the topic of calibration is the fact, that only angular measurements are available from both the actuator encoders as well as passive joint sensors, making HEXA-self-calibration more elaborate. Simulation studies finally indicate efficiency of the proposed strategy. In order to consider the effect of measurement inaccuracies, noise has been taken into account.

RCA562: Control Architecture for Parallel Kinematic Robots

The design of powerful control that suits multiple types of parallel kinematic robots is extraordinarily challenging. The diversity of parallel kinematics and their optimization that customizes them to specific tasks require highly individualized control functionalities. This work intends to provide principles, methods and tools for the development of such control software. It aims at the time-efficient realization of custom robot controllers that suit particular application domains and use hardware resources as sophisticated as possible. A task-frame formalism for trajectory generation is defined exploiting the full potential of parallel robots. This formalism can be used as a generic programming interface for parallel robots. Design patterns for so-called active connectors, modular motion planning, sensor integration and restricted state machines in token-passing context are discussed with regard to control of parallel robots. RCA562, a control application for parallel robots incorporating this knowledge, serves as an illustrative validation example.