Physical human-robot collaboration is characterized by a suitable exchange of contact forces between human and robot, which can occur in general at any point along the robot structure. If the contact ...location and the exchanged forces were known in real time, a safe and controlled collaboration could be established. We present a novel approach that allows localizing the contact between a robot and human parts with a depth camera, while determining in parallel the joint torques generated by the physical interaction using the so-called residual method. The combination of such exteroceptive sensing and model-based techniques is sufficient, under suitable conditions, for a reliable estimation of the actual exchanged force at the contact, realizing thus a virtual force sensor. Multiple contacts can be handled as well. We validate quantitatively the proposed estimation method with a number of static experiments on a KUKA LWR. An illustration of the use of estimated contact forces in the realization of collaborative behaviors is given, reporting preliminary experiments on a generalized admittance control scheme at the contact point.
With reference to robots that are redundant for a given task, we present a novel and intuitive approach allowing to define a discrete-time joint velocity command that shares the same characteristics ...of a second-order inverse differential scheme, with specified properties in terms of joint acceleration or torque. Our main goal is to show how commands in the null space of the task can yield different locally optimal solutions, working only at the velocity control level. By following our general method, it is possible to obtain simple implementations of possibly complex robot control laws that (i) can be directly interfaced to the low-level servo loops of a robot, (ii) require less task information and on-line computations, (iii) are still provably good with respect to some target performance. The method is illustrated by considering the conversion into discrete-time velocity commands of control schemes for redundant robots that minimize the (weighted and/or biased) norm of joint acceleration or joint torque. The approach can be extended to auxiliary tasks, possibly organized with priority. Numerical simulations and experimental results are presented for the control of a 7R KUKA LWR IV robot.
•A method to convert in a discrete-time velocity law any second-order control scheme.•In redundant robots, preserves the original properties (minimum acceleration/torque).•Equivalence is obtained in all cases by a proper choice of a vector in the task null space.•Solutions are simple to implement and are interfaced directly to low-level controllers.•Validating simulations and experiments on a 7R KUKA LWR IV robot.
We present an efficient method for addressing online the inversion of differential task kinematics for redundant manipulators, in the presence of hard limits on joint space motion that can never be ...violated. The proposed Saturation in the Null Space (SNS) algorithm proceeds by successively discarding the use of joints that would exceed their motion bounds when using the minimum norm solution. When processing multiple tasks with priority, the SNS method realizes a preemptive strategy by preserving the correct order of priority in spite of the presence of saturations. In the single- and multitask case, the algorithm automatically integrates a least possible task-scaling procedure, when an original task is found to be unfeasible. The optimality properties of the SNS algorithm are analyzed by considering an associated quadratic programming problem. Its solution leads to a variant of the algorithm, which guarantees optimality even when the basic SNS algorithm does not. Numerically efficient versions of these algorithms are proposed. Their performance allows real-time control of robots executing many prioritized tasks with a large number of hard bounds. Experimental results are reported.
We present an efficient method to evaluate distances between dynamic obstacles and a number of points of interests (e.g., placed on the links of a robot) when using multiple depth cameras. A ...depth-space oriented discretization of the Cartesian space is introduced that represents at best the workspace monitored by a depth camera, including occluded points. A depth grid map can be initialized off line from the arrangement of the multiple depth cameras, and its peculiar search characteristics allows fusing on line the information given by the multiple sensors in a very simple and fast way. The real-time performance of the proposed approach is shown by means of collision avoidance experiments where two Kinect sensors monitor a human-robot coexistence task.
We present a novel approach to estimate the distance between a generic point in the Cartesian space and objects detected with a depth sensor. This information is crucial in many robotic applications, ...e.g., for collision avoidance, contact point identification, and augmented reality. The key idea is to perform all distance evaluations directly in the depth space. This allows distance estimation by considering also the frustum generated by the pixel on the depth image, which takes into account both the pixel size and the occluded points. Different techniques to aggregate distance data coming from multiple object points are proposed. We compare the Depth space approach with the commonly used Cartesian space or Configuration space approaches, showing that the presented method provides better results and faster execution times. An application to human-robot collision avoidance using a KUKA LWR IV robot and a Microsoft Kinect sensor illustrates the effectiveness of the approach.
During human-robot interaction tasks, a human may physically touch a robot and engage in a collaboration phase with exchange of contact forces and/or requiring coordinated motion of a common contact ...point. Under the premise of keeping the interaction safe, the robot controller should impose a desired motion/force behavior at the contact or explicitly regulate the contact forces. Since intentional contacts may occur anywhere along the robot structure, the ability of controlling generalized contact motion and force becomes an essential robot feature. In our recent work, we have shown how to estimate contact forces without an explicit force sensing device, relying on residual signals to detect contact and on the use of an external (depth) sensor to localize the contact point. Based on this result, we introduce two control schemes that generalize the impedance and direct force control paradigms to a generic contact location on the robot, making use of the estimated contact forces. The issue of human-robot task compatibility is pointed out in case of control of generalized contact forces. Experimental results are presented for a KUKA LWR robot using a Kinect sensor.
We present an integrated control framework for safe physical Human-Robot Interaction (pHRI) based on a hierarchy of consistent behaviors. Safe human robot coexistence is achieved with a layered ...approach for coping with undesired collisions and intended contacts. A collision avoidance algorithm based on depth information of the HRI scene is used in the first place. Since collision avoidance cannot be guaranteed, it is supported by a physical collision detection/reaction method based on a residual signal which needs only joint position measures. On top of this layer, safe human-robot collaboration tasks can be realized. Collaboration phases are activated and ended by human gestures or voice commands. Intentional physical interaction is enabled and exchanged forces are estimated by integrating the residual with an estimation of the contact point obtained from depth sensing. During the collaboration, only the human parts that are designated as collaborative are allowed to touch the robot while, consistently to the lower layers, all other contacts are considered undesired collisions. Preliminary experimental results with a KUKA LWR-IV and a Kinect sensor are presented.
We present a methodology for estimating joints torque due to external forces applied to a robot with large joints backlash and friction. This undesired non-linearity is common in personal robot, due ...to the use of low cost mechanical components and type of usage. Our method enables contact detection and human-robot physical interaction capabilities without using extra sensors. The effectiveness of our approach is shown with experiments on a Romeo robot arm from SoftBank Robotics.
A novel method to handle multiple robotic tasks with priorities is presented. The occurrence of singularities, both of the kinematic and algorithmic type, may affect the correct hierarchy in task ...execution. Existing methods deal with singularities either by using damped least squares solutions or by relaxing the enforcement of secondary tasks. Damped pseudo-inversion mitigates undesired effects near singularities, at the cost of non-negligible task errors and deformation even of the highest priority task. When secondary tasks are not enforced, hierarchy is preserved but these tasks are not executed accurately even when this would be possible. In our approach, joint motion contributions are added following the reverse order of task priorities and working with suitable projection operators. Higher priority tasks are processed at the end, avoiding possible deformations caused by singularities occurring in lower priority tasks. The proposed Reverse Priority (RP) method allows executing at best all tasks while still preserving the desired hierarchy. The effectiveness of the RP method is shown through numerical simulations and with experiments on a 7-dof KUKA LWR.
Variable stiffness actuators (VSAs) are currently explored as a new actuation approach to increase safety in physical human–robot interaction (pHRI) and improve dynamic performance of robots. For ...control purposes, accurate knowledge is needed of the varying stiffness at the robot joints, which is not directly measurable, nonlinearly depending on transmission deformation, and uncertain to be modeled. We address the online estimation of transmission stiffness in robots driven by VSAs in antagonistic or serial configuration, without the need for joint torque sensing. The two-stage approach combines (i) a residual-based estimator of the torque at the flexible transmission, and (ii) a recursive least squares stiffness estimator based on a parametric model. Further design refinements guarantee a robust behavior in the lack of velocity measures and in poor excitation conditions. The proposed stiffness estimation can be easily extended to multi-degree-of-freedom (multi-DOF) robots in a decentralized way, using only local motor and link position measurements. The method is tested through extensive simulations on the VSA-II device of the University of Pisa and on the Actuator with Adjustable Stiffness (AwAS) of IIT. Experiments on the AwAS platform validate the approach.
PDF
