Coordination of Two-Arm Pushing

Coordination of two manipulators performing the task of transporting objects is studied in this paper. Each manipulator is equipped with end effector - a flat surface palm. Grasping is achieved by the two palms pushing an object from two ends. The task requires simultaneous control of the object motion and the interaction force. The control of the interaction force is needed to ensure that the object is not dropped and to avoid excessive pressing. The motion and force control problem is further complicated by the presence of unilateral constraints since the manipulators can only push the object. This paper describes a control method which utilizes a state feedback to decouple position control and force control loops. A force control planning algorithm is also proposed which ensures the satisfaction of unilateral constraints. The effectiveness of the control method is verified by simulations

Coordinating Locomotion and Manipulation of a Mobile Manipulator

A mobile manipulator in this study is a manipulator mounted on a mobile platform. Assuming the end point of the manipulator is guided, e.g., by a human operator to follow an arbitrary trajectory, it is desirable that the mobile platform is able to move as to position the manipulator in certain preferred configurations. Since the motion of the manipulator is unknown a priori, the platform has to use the measured joint position information of the manipulator for motion planning. This paper presents a planning and control algorithm for the platform so that the manipulator is always positioned at the preferred configurations measured by its manipulability. Simulation results are presented to illustrate the efficacy of the algorithm. The use of the resulting algorithm in a number of applications is also discussed

A New Range Finding Method Using a Varifocal Mirror

A new range finding method is proposed in this paper which makes use of a varifocal mirror. The three-dimensional object space is first discretized into a sequence of spherical shells with a specially designed nonlinear vibrating varifocal mirror. These discrete spherical shell images are then recorded by a video camera. A deblurring algorithm is introduced in this paper which is used to remove the blurred components in the images. Different depth ranges can be obtained by controlling the vibration amplitude and the direct current component of the driving wave for the varifocal mirror. The depth accuracy is adjusted by varying the vibration period of the varifocal mirror. This range finding technique can be made real time by increasing the frame frequency of the camera

Robotics Research at the GRASP Laboratory

The General Robotics and Active Sensory Perception (GRASP) Laboratory of the University of Pennsylvania does research in various areas of robotics including coordinated control of multiple robot manipulators, strategics for robotic sensing, multi-sensor integration, distributed real-time operating systems, telerobotics with communication delays, image understanding, and range image analysis

On Feedback Linearization of Mobile Robots

A wheeled mobile robot is subject to both holonomic and nonholonomic constraints. Representing the motion and constraint equations in the state space, this paper studies the feedback linearization of the dynamic system of a wheeled mobile robot. The main results of the paper are: (1) It is shown that the system is not input-state linearizable. (2) If the coordinates of a point on the wheel axis are taken as the output equation, the system is not input-output linearizable by using a static state feedback; (3) but is input-output linearizable by using a dynamic state feedback. (4) If the coordinates of a reference point in front of the mobile robot are chosen as the output equation, the system is input-output linearizable by using a static state feedback. (5) The internal motion of the mobile robot when the reference point moves forward is asymptotically stable whereas the internal motion when the reference point moves backward is unstable. A nonlinear feedback is derived for each case where the feedback linearization is possible

Increased brain activation and functional connectivity after working memory training in patients with ischemic stroke: an fMRI study

ObjectiveWorking memory (WM) impairment is common in patients after a stroke. WM training (WMT) has been suggested as a way to improve cognitive function. However, the neural effects following WMT in stroke patients remain largely unclear. This study aimed to explore the behavioral changes and neural effects of WMT on patients with chronic ischemic stroke.MethodsFifty first-ever ischemic stroke patients with WM deficits in the chronic stage were randomly assigned to either a 4-week WMT group or a control group. Verbal n-back, digital and spatial memory-span, Raven's standard progressive matrices, and the Stroop color-word test, as well as task-state and resting-state fMRI were assessed for all patients at baseline and after the intervention.ResultsThe WMT group showed improvements in WM, fluid intelligence, and attention after training. Additionally, the WMT group exhibited increased activation in the left middle frontal gyrus (MFG) and middle occipital gyrus after training. At baseline, all patients were impaired in their abilities to elevate activation in their WM network as a response to increasing WM load. However, in the WMT group, increased activation was observed in the left cerebellum anterior lobe, right cerebellum posterior lobe (CPL), and MFG in the 2-back vs. 1-back contrast after WMT. We also found increased functional connectivity between the left MFG and the left inferior parietal lobule (IPL), and between the bilateral IPL and the right CPL after training in the WMT group.ConclusionOur study supported that WMT potentially improved WM capacity in ischemic stroke patients during the chronic stage, and that the training effects might transfer to fluid intelligence and attention ability. Our results also demonstrated that repeated WMT potentially increased brain activation and resting-state functional connectivity within the WM network in patients with ischemic stroke. These findings provided robust evidence to support WMT as an effective intervention to enhance cognitive rehabilitation and shed light on the functional neuroplasticity mechanism of WMT on cognitive recovery after ischemic stroke

An Approach to Simultaneous Control of Trajectory and Interaction Forces in Dual-Arm Configurations

Multiple arm systems, multifingered grippers, and walking vehicles all have two common features. In each case, more than one actively coordinated articulation interacts with a passive object, thus forming one or more closed chains. For example, when two arms grasp an object simultaneously, the arms together with the object and the ground (base) form a closed chain. This induces kinematic and dynamic constraints and the resulting equations of motion are extremely nonlinear and coupled. Furthermore, the number of actuators exceeds the kinematic mobility of the chain in a typical case, which results in an underdetermined system of equations. An approach to control such constrained dynamic systems is described in this short paper. The basic philosophy is to utilize a minimal set of inputs to control the trajectory and the surplus inputs to control the constraint or interaction forces and moments in the closed chain. A dynamic control model is derived for the closed chain that is suitable for designing a controller, in which the trajectory as well as the interaction forces and moments are explicitly controlled. Nonlinear feedback techniques derived from differential geometry are then applied to linearize and decouple the nonlinear model. In this paper, these ideas are illustrated through a planar example in which two arms are used for cooperative manipulation. Results from a simulation are used to illustrate the efficacy of the method

Force-Closure Grasps With Two Palms

This paper studies force-closure grasps of rigid objects by using two palms. The two palms are instrumented with tactile sensors capable of detecting the presence of contacts, and are assumed to be respectively installed on two robotic manipulators capable of motion and force control. Established in this paper is an existence condition under which the two palms form a force-closure grasp. The salient feature of this condition is that it does not require the information on the shape of the object and the contact locations. A configuration of the two palms in contact with the object satisfying this condition is called a force-closure grasp configuration (FCGC). Further an algorithm is developed to check the condition for FCGC in terms of the position and orientation of the palms