Bernd Finkemeyer

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.

Executing assembly tasks specified by manipulation primitive nets

Numerous scientific publications in the open literature show approaches for automatic assembly planning, automated robot programming, notations for the task frame formalism, robot control architectures for hybrid control methods, and respective experimental results in these areas. But there are still significant gaps between these individual fields. Considering the whole chain, from assembly planning via autonomous robot programming to the execution of complex robot tasks, the latter part of it is discussed in this paper: manipulation of primitive nets as output of task planning systems are decomposed into single manipulation primitives, which are subsequently used to generate parameters for hybrid control. A hybrid controller computes set-points for a joint position controller. Our aim is to define versatile interfaces between the mentioned disciplines, in order to close the gaps between them. Derived from the task-frame formalism, manipulation primitives constitute the base interface in this sense. After its description, the composition of manipulation primitive nets is described. Regarding the control architecture, the interpretation of manipulation primitives as atomic commands and the setting of unambiguous low level control parameters is discussed. Subsequently, the software architecture necessary to realize the complex control structure for compliant motion, is introduced. To highlight the meaning for practical implementations, several experimental results of sample assembly tasks are shown.

Manipulation Primitives — A Universal Interface between Sensor-Based Motion Control and Robot Programming

This paper introduces a generic framework for sensor-based robot motion control. The key contribution is the introduction of an adaptive selection matrix for sensor-based hybrid switched-system control. The overall control system consists of multiple sensors and open- and closed-loop controllers, in-between which the adaptive selection matrix can switch discretely in order to supply command variables for low-level controllers of robotic manipulators. How control signals are chosen, is specified by Manipulation Primitives, which constitute the interface to higher-level applications. This programming paradigm is formally specified in order to establish the possibility of executing sensor-guided and sensor-guarded motion commands simultaneously and in a very open way, such that any kind and any number of sensors can be addressed. A further key feature of this generic approach is, that the control structure can be directly mapped to a corresponding software architecture. The resulting control system is freely scalable depending on the performance requirements of the desired system.

Adaptive implicit hybrid force/pose control of industrial manipulators: compliant motion experiments

The major purpose of this paper is to combine results of current robot force control research with scientific approaches in compliant motion, which are based on Masons task frame formalism. The embedding of adaptive implicit hybrid force/pose control in a robot control architecture for compliant motion control is described. By the usage of adaptive force control, the practicability of compliant motion applications is improved. The applied control concept is constituted in a theoretical as well as in a practical manner. To highlight the meaning for practical implementations, experimental results with industrial manipulators under adaptive force control in three degrees of freedom are finally shown.

A manipulator plays Jenga

This article describes an overview of a prototypical manipulation control system, which is able to play Jenga. The implementation of the Jenga game has been chosen to verify the integration of existing concepts such as force/torque control, distance control, real-time behavior of distributed control systems, sensor data fusion, online trajectory computation, and visual servoing in one exhibit. This exhibit has no direct industrial use, but it clearly shows the potential of multisensor integration and opens new possibilities for industrial manipulation. The scope of these kinds of systems is certainly not limited to industrial manipulation applications, e.g., the potential in the field of medical robotics is also very high and has to be investigated. A key part of this work is the online trajectory generator. Within this work, a second-order generator (rectangular acceleration signals) has been applied (Kroger et al., 2007), but, for the aim of bringing the mentioned concepts into industrial practice, a jerk-limited generator will be necessary. This and further research on sensor fusion methods will be a major focus of our future work. Besides, the Jenga game could be used as an international benchmark for manipulation control concepts (including force/torque control, distance control, and visual servoing). It is well-known and, since the manufacturer (Hasbro, 2007) has only one place of production, the game is exactly the same all over the world.

A task frame formalism for practical implementations

Masons Task Frame Formalism (TFF) is supposed to deliver robot application programmers an intuitive and powerful programming interface. The open literature provides many theoretic approaches, but almost none of them is practicable. To bring these research results into practice is the major aim of this paper. Sets of manipulation primitives specify compliant motion commands, which let us execute complex robot tasks. We introduce an appropriate notation and focus on all significant TFF values, which have to be applied to the transformations, which are required on the control level. All essential calculations are derived; to highlight the meaning for practical implementations, an example, how to embed this knowledge in control architectures, is given.

A Middleware for High-Speed Distributed Real-Time Robotic Applications

The development of modular and distributed real-time software systems — as they are common in the field of research and development of robotic manipulation control systems — can be greatly simplified by appropriate middleware concepts. This paper generically introduces the basic concepts of the middleware solution MiRPA (Middleware for Robotic and Process Control Applications). MiRPA’s employment allows the implementation of complex distributed real-time software architectures. It handles publisher/subscriber as well as client/server communication between local and distributed software modules with very small worst-case latencies (≪ 1 ms). Besides introducing basics on inter-module and inter-node communication for these two communication models, deadlock avoiding strategies, and methods of redundancy handling, we finally show results of real-world measurements and applications.

The adaptive selection matrix—A key component for sensor-based control of robotic manipulators

This contribution introduces a generic framework for sensor-based robot motion control. The key contribution is the introduction of an adaptive selection matrix for sensor-based hybrid switched-system control. The overall control system consists of multiple sensors and open- and closed-loop controllers, in-between which the adaptive selection matrix can switch discretely in order to supply command variables for low-level controllers of robotic manipulators. How control signals are chosen, is specified by Manipulation Primitives, which constitute the interface to higher-level programming. This programming paradigm is briefly specified in order to be able to define and execute sensor-guided and sensor-guarded motion commands simultaneously. The resulting control system is freely adaptable depending on the sensor and control requirements of the desired system and/or application.

Forward-model-based control system for robot manipulators

Although numerous sophisticated nonlinear control algorithms exist in literature, it is still state of the art to use simple linear joint controllers in industrial robotic systems. Most nonlinear concepts are based on a more or less accurate inverse model of the robot. In this paper a forward-model-based control system, the so-called Model Following Control (MFC), for robot manipulators is presented. Its theoretical basics and its concept are explained. The quality and the applicability of the MFC control concept has been analyzed in many experiments. The MFC system is compared with classical linear controllers and nonlinear feedforward controllers with respect to robustness. Qualitative as well as quantitative results are presented and discussed.

Simple two degree of freedom structures and their properties

A two-degree of freedom control system that is most frequently encountered in practice is the so-called Internal Model Control (IMC) structure. However, the design procedure of such a structure does not present an easy task, which implies a limited utility of IMC. In this paper two alternative solutions are proposed that may be lumped together as Model-Following Control (MFC). These are two-loop control systems being easy to implement and offering interesting properties. Theoretical assumptions have been verified experimentally on a two-joint robot manipulator. Both qualitative and quantitative results yielded by experiments are presented and discussed.