Pramath Raj Sinha

A contact stress model for multifingered grasps of rough objects

A model that utilizes a contact-stress analysis of an arbitrarily shaped object in a multifingered grasp is developed. The fingers and the object are all treated as elastic bodies, and the region of contact is modeled as a deformable surface patch. The relationship between the friction and normal forces is now nonlocal and nonlinear in nature and departs from the Coulomb approximation. The nature of the constraints arising out of conditions for compatibility and static equilibrium motivated the formulation of the model as a nonlinear constrained minimization problem. The model is able to predict the magnitude of the inwardly directed normal forces and both the magnitude and direction of the tangential (friction) forces at each finger/object interface for grasped objects in static equilibrium. Examples in two and three dimensions are presented along with an application of the model to the grasp transfer maneuver. >

Robotic Exploration of Surfaces With a Compliant Wrist Sensor

This article presents some results of an ongoing research project to investigate the components and modules that are necessary to equip a robot with exploratory capabilities. Of particular interest is the recovery of certain material properties from a surface, given minimal a priori information, with the intent to use this information to enable a robot to stand and walk stably on a surface that is unknown and unconstrained. To this end, exploratory procedures (EPs) have been designed and implemented to recover penetrability, compliance, and surface roughness by exploring the surface using a compliant wrist sensor. A six-degree-of-freedom compliant wrist sensor that combines passive compliance and active sensing has been developed to provide the necessary flexibility for force and con tact control, as well as accurate position control. This article describes the compliant wrist and sensing mechanism design along with a hybrid control algorithm that utilizes the sensed information from the wrist to adjust the apparent stiffness of the end effector as desired.

Robotic exploration of surfaces and its application to legged locomotion

It is noted that material properties like penetrability, compliance, and surface roughness are important in the characterization of the environment. The authors have designed and implemented exploratory procedures on a robotic system to actively explore a surface to extract its material properties. The exploratory procedures for exploration are integrated into an active perceptual scheme for legged locomotion. The perceptual scheme is designed around creating the ability for the robot to sense variations in terrain properties while it is walking, so that it may be able to avoid sinking, slipping, and falling due to unexpected changes in the terrain properties, and make suitable changes in its foot forces to continue locomotion. The active perceptual scheme is implemented by simulating a leg-ankle-foot system with a PUMA arm-compliant wrist-foot system and an accelerometer mounted on the foot to detect slip.<<ETX>>

How does a robot know where to step? Measuring the hardness and roughness of surfaces

Presents an overview of ongoing research on surface exploration at the GRASP Lab. The authors investigate the necessary components and modules that mst be embedded into a robot for it to have the exploratory capabilities required to recover mechanical properties from a surface given minimal a priori information. Eventually, this information will be used to enable a robot stand and walk stably in an environment that is unknown and unconstrained. The laboratory setup involves a compliant wrist with six degrees of freedom, mounted on a robot arm, and a prototype foot mounted on the wrist. They have successfully designed and implemented exploratory procedures to recover penetrability, material hardness and frictional characteristics by exploring the surface.<<ETX>>

An instrumented compliant wrist for robotics applications

The design and implementation of an instrumented compliant wrist is presented. The compliance helps reduce the impact effects of robot/environment interaction and improves force control performance. However, position accuracy of the end-effector degrades with increased compliance. Instrumentation of the compliant wrist enables it to serve as a compliant force/torque sensor which can be used to achieve both responsive force control and accurate position control. The wrist device uses rubber elements for compliance and damping, and a serial linkage, with potentiometers at each joint, is used for sensing deflections produced in the wrist. The wrist is connected in series between the end of the robot and the tool, and is designed to partially surround the tool, thus reducing the distance between the end-effector and the tool. A hybrid control scheme is implemented to enable the wrist to be used in a variety of applications. Example applications, including tool usage, are presented.<<ETX>>

Implementation of an active perceptual scheme for legged locomotion of robots

For robots to traverse rugged terrain successfully using legged locomotion, they need not only constantly to maintain structural stability but also, and perhaps more importantly, to detect and adapt to changes in the terrain properties. The authors address the issue of exploration to extract material properties from a given surface for the specific purpose of aiding in and improving the quality of legged locomotion. While it is important to evaluate terrain properties prior to the start of locomotion, it is even more important to evaluate these properties actively during locomotion so that the robot does not sink, slip or fall. It is proposed that the legs of a robot be used not only for stepping and walking but also as probes to examine those properties of the surface that would contribute to the efficiency of locomotion, one way or another. The proposed framework for active perception for legged locomotion suggests that for stable stepping and walking in an unknown environment, it is necessary actively to recover the material properties of penetrability, compliance and surface traction from the supporting surface. These attributes must be recovered by exploratory procedures that are built into the mobile robotic system. This paper focusses on the implementation of the perceptual scheme so that feedback from the measurement of material properties is used to control robot foot forces during legged locomotion.<<ETX>>