Hierarchical Force and Positioning Task Specification for Indirect Force Controlled Robots

Abstract

Abstract-Indirect force control (IFC) architectures are a common approach for dealing with unknown environments. What all IFC schemes have in common, is that the relation between the set-point and the actual configuration of the robot is determined by a mechanical relationship (e.g. a mass-spring-damper system). In this work, we propose a setpoint generation method for IFC schemes, enabling intuitive specification of mixed force and positioning tasks on joint and Cartesian level. In addition, the formulation of equality and inequality tasks is supported and a passivity based stability proof is formulated using the concept of virtual energy storage.The resulting task programming interface is demonstrated on a 7 degree of freedom robot, running a joint space impedance controller. One sample task demonstrates the application of the developed approach and highlights the basic features. I. MOTIVATIONCompliant control involving force and positioning tasks has been investigated elaborately in the last decades. A popular approach to realize compliance is provided by indirect force controllers, where the motion and interaction forces of the physical robot are indirectly controlled by moving a virtual robot, which is coupled to the physical robot via a virtual mechanical relationship. Fig. 1 depicts the basic idea. The most popular variation of an IFC is the well known impedance control paradigm, introduced by Hogan in his seminal paper [1], but also simpler variants, like stiffness control or an ordinary PD-controller with compensation of the gravitational torques and sufficiently low proportional gain. This control scheme has also nice stability properties which are basically independent from the environmental dynamics. Furthermore, series elastic actuator (SEA) robots, which are equipped with physical springs in their joints, can also be considered as IFC's.The set-point selection for the virtual robot is also referred to as virtual trajectory generation and while there is an extensive amount of work, having the purpose of improving the accuracy of the virtual mechanical relationship or proposing different extensions or variations for IFC's (for impedance control in particular), the literature covering pure virtual trajectory generation is very sparse.The present work is meant to fill the gap between the low level control design and high level application programming by introducing an additional control layer, which we call set-pointgenerator (SPG). The purpose of the SPG is to provide set-points for the virtual manipulator according to the specified tasks, using only information on the current joint state. These tasks are defined by the application programmer via a task specification interface, where a set of hierarchically ordered force and positioning subtasks on joint and Cartesian level is determined. To our best knowledge, there is no work treating virtual set-point generation for joint level IFC's in order to achieve mixed force and positioning tasks without additional sensory feedback, like force or vision.In [...

Interactive Force Control Based on Multimodal Robot Skin for Physical Human−Robot Collaboration

“…Improvements have been made by the introduction of set point generation, which adapts the impedance set point to enhance task execution. [22,23] Other strategies accurately apply desired forces by combining impedance control and direct force control. Such a generalization is, for example, proposed in the study by Magrini et al [24] and enables force control at any point on the robot.…”

A constraint-based programming approach for robotic assembly skills implementation

“…The adoption of force control policies when using collaborative robots can be found e.g. in [19,20,21]. In addition, the kinematic redundancy allows for a more dexterous manipulation but demands for redundancy resolution in order to plan a robot trajectory [22].…”

An Adaptive Robust Hybrid Force/Position Control for Robot Manipulators System Subject to Mismatched and Matched Disturbances

“…In paper [8], a new generalized proportional integral observer is designed to estimate velocity and force to achieve the robot manipulator's adaptive force/ position control. Ewald et al [9] proposed an indirect force controller to deal with unknown environments, providing intuitive control over hybrid force and positioning tasks in joint space and workspace. Wang et al [10] proposed an adaptive fuzzy computed torque controller based on the workspace of a robot manipulator to enhance the accuracy of force control in unknown environments.…”