Ulrike Thomas

Error-tolerant execution of complex robot tasks based on skill primitives

This paper presents a general approach to specify and execute complex robot tasks considering uncertain environments. Robot tasks are defined by a precise definition of so-called skill primitive nets, which are based on Masons hybrid force/velocity and position control concept, but it is not limited to force/velocity and position control. Two examples are given to illustrate the formally defined skill primitive nets. We evaluated the controller and the trajectory planner by several experiments. Skill primitives suite very well as interface to robot control systems. The presented hybrid control approach provides a modular, flexible, and robust system; stability is guaranteed, particularly at transitions of two skill primitives. With the interface explained here, the results of compliance motion planning become possible to be examined in real work cells. We have implemented an algorithm to search for mating directions in up to three-dimensional configuration-spaces. Thereby, on one hand we have released compliant motion control concepts and on the other hand we can provide solutions for fine motion and assembly planning. This paper shows, how these two fields can be combined by the general concept of skill primitive nets introduced here, in order to establish a powerful system, which is able to automatically execute prior calculated assembly plans based on CAD-data in uncertain environments.

A new skill based robot programming language using UML/P Statecharts

This paper introduces the new robot programming language LightRocks(Light Weight Robot Coding for Skills), a domain specific language (DSL) for robot programming. The language offers three different level of abstraction for robot programming. On lowest level skills are coded by domain experts. On a more abstract level these skills are supposed to be combined by shop floor workers or technicians to define tasks. The language is designed to allow as much flexibility as necessary on the lowest level of abstraction and is kept as simple as possible with the more abstract layers. A Statechart like model is used to describe the different levels of detail. For this we apply the UML/P and the language workbench MontiCore. To this end we are able to generate code while hiding controller specific implementation details. In addition the development in LightRocks is supported by a generic graphical editor implemented as an Eclipse plugin.

Multi Sensor Fusion in Robot Assembly Using Particle Filters

In this paper, we present a new method for sensor fusion in robot assembly. In our approach, model information can be derived automatically from CAD-data. We introduce force torque maps, which are either computed automatically exploiting modern graphical processors or are measured by scanning forces and torques during contact motions. Subsequently, force torque maps are applied as model information during execution of real assembly tasks. Also, computer vision is included by comparing relative poses of features in virtual images with their real relative poses given from measured images. For fusion of these two (or more) different sensors we suggest to use particle filters. Experiments with variations of peg in hole tasks in a real work cell demonstrate our new approach to be very useful for the whole process chain from planning to execution.

Towards a new concept of robot programming in high speed assembly applications

In this paper, we present work about robot control architecture, assembly planning and task planning for manufacturing robots. The interface between an offline planning unit and control systems is handled with skill primitives. Thus, skill primitives and skill primitive nets are explained in detail. Our long term aim is to combine robot control with task and assembly planning, so that with less human interaction manufacturing costs can be reduced. Even parallel kinematic machines provide enormous opportunities to reduce cycle times and thus the benefit should not be wasted by expensive specialized robot programming. Thus, we give an overview of our system and focus on some aspects to implement such a sophisticated system.

A system for automatic planning, evaluation and execution of assembly sequences for industrial robots

This paper describes a new system to automatically generate, evaluate and execute assembly sequences, With the commands generated by the system, robots are enabled to assemble complex products without explicit robot programming. Our approach uses CAD-models, symbolic spatial relations and a robot work cell description as input. The system provides a user-friendly interface to define the goal state of the parts to be assembled. The automatically generated and decomposed assembly plans can be executed by robots by means of a set of predefined skill primitives. For this purpose we classify the robot tasks by analyzing the symbolic spatial relations between the objects to be assembled, the depart-spaces and the necessary tools. The tasks are decomposed into suitable elementary robot operations, the skills, automatically. For guiding the robot during assembly we employ internal and external sensors.

A unified notation for serial, parallel, and hybrid kinematic structures

This paper proposes a new notation for kinematic structures which allows a unified description of serial, parallel, and hybrid robots or articulated machine tools. During the past decades, the Denavit-Hartenberg (DH) parameters have been used widely to describe serial kinematics of robots in science and industry. Till now, such a common notation for parallel manipulators has not yet been accepted. This paper tries to fill this gap by presenting a new notation, which is based on the graph representation known from gear trains. In parallel manipulators, spherical and cardan joints are widely used. In order to describe these kinds of joints, the DH-parameter notation has been extended, so that, to each joint as many joint variables can be assigned as degrees of freedom exist. The notation is not only very useful for design, programming, and simulation of parallel robots; it also can be applied as a convention to refer to parallel or hybrid kinematic structures elsewhere.

Flexible assembly through integrated assembly sequence planning and grasp planning

This paper describes an assembly sequence planner able to generate feasible sequences for building a desired assembly. The assembly planner takes geometrical, physical and mechanical constraints into account. Moreover, the planner considers the feasibility of grasps during the planning process and takes into account work-cell specific constraints. The approach uses AND/OR-graphs for planning. The generation of such graphs is implemented by using a specialized graph cut algorithm that employs a dynamically changing priority queue. These graphs are further evaluated by considering the feasibility of grasping sub-assemblies and individual parts during the process. The grasp and the sequence planner are generic, hence the proposed solution can be applied to arbitrary assemblies of rigid parts. The system has been evaluated with different configurations obtained by the combination of standard item-profiles.

Planning sensor feedback for assembly skills by using sensor state space graphs

In this paper, it is shown how robust execution of assembly skills can be planned by using sensor state space graphs. The here proposed method is evaluated by some assembly skills in which force feedback is applied. Assembly skills are implemented by manipulation primitive nets which constitute an interface between planning and execution of robotic systems. The sensor state space graph is introduced, which is an extension of the contact formation graph in a more general way, when various sensors might be used simultaneously for assembly execution. It is shown, how contact formation graphs can be generated by simulation of rigid body motions. The known contact formation graphs are enhanced by the definition of contact types between higher order surfaces. Additionally, a more general view is given by introducing sensor state space graphs. It is shown how contact formation graphs can be mapped to manipulation primitive nets, which allow the robust execution of assembly skills, despite the appearance of uncertainties. The approach is demonstrated successfully on some assembly tasks. Here the task of plugging a power socket on a top hat rail is illustrated due to its complex sequence. The shown assembly task is characterized by small fitting tolerances, where the application of force feedback is indispensable.

Definition and execution of a generic assembly programming paradigm

Purpose – This paper aims to introduce a generic robot‐programming paradigm for assembly tasks that overcomes the strong coupling between the motion commands and underlying algorithms of programming languages currently on the market. Therefore, it allows an improved method of assembly task programming.Design/methodology/approach – A manipulation primitive (MP) is defined which decouples the programming concept from the algorithms. These primitives can be integrated into existing programming languages and are supported by an intuitive graph‐based language which is introduced in this paper. An open reference architecture to support those primitive‐based programming languages has been designed.Findings – It is possible to describe complex assembly tasks such as manipulation on conveyors or sensor‐integrated compliant motion without abandoning the generality of the programming paradigm. Execution on a reference control system has proven to be successful for several manipulation tasks on different machines.Ori...

An integrative approach for multi-sensor based robot task programming

In this paper, we suggest skill primitive nets for multi-sensor integration in robot task programming. Each skill primitive is either a hybrid motion or a command interpreted by an external vision system. For the integration of different sensors into one general and comprehensive approach, one has to consider different sensing modalities. With an extension of our previously outlined approach of skill primitive nets, multi-sensor integration is possible. Thereby, a large amount of applications can be realized. We also give a precise specification as XML-interface for skill primitive nets. In this paper advantages are outlined, when using this interface between programming and control. Our skill primitive net approach for multi-sensor integration has been evaluated with industrial applications.