Force-Reflecting, Finger-Position-Sensing Mechanism

Lyndon B. Johnson Space Center, Houston, Texas

An electromechanical device called the "Dexterous Master" (DM) is an exoskeletal mechanism that is worn on a human operator's hand for (1) measuring the positions and orientations of the fingers relative to the palm and (2) applying, to the fingers, feedback forces from a remote robotic manipulator. The DM is part of a force-reflecting, hand-operated control apparatus through which the operator controls the manipulator. The DM is installed on, and operated in conjunction with, another mechanism that measures the position and orientation of the hand and reflects forces to the hand in the six degrees of freedom that determine the position and orientation of an end effector (robot hand) on the manipulator.

The DM has nine passive parallel degrees of freedom (DOFs) and six actuated parallel DOFs. It includes five finger assemblies, which are mounted on the dorsal surface of the hand to minimize the probability of collisions among fingers and mechanisms. The design of the finger assemblies is based on the proposition that it suffices to measure the position and orientation of each fingertip, with no need to measure the angles of the joints between the fingertip and the palm; this proposition makes it unnecessary to encumber each finger joint with hardware.

A Finger Assembly contains joints and joint transducers that allow natural finger motion and measure the position and orientation of the fingertip.

A control computer calculates real-time coordinate transformations between (1) the positions and orientations of the operator's fingertips and (2) the positions and orientations of fingertips or other corresponding parts on the robotic end effector. This control scheme renders master/slave kinematic similarity unnecessary, thereby enabling the slaving of a variety of multilink robotic arms and hands.

The six actuated DOFs (two for the thumb, one for each of the other four fingers) are implemented by use of six identical servo-controlled actuator units. The actuators are small electric brushless motors connected to synchronous belt-drive trains.

With the exception of the thumb, each finger assembly includes one actuated revolute joint (ARJ), one passive prismatic joint (PPJ), and a passive revolute joint (see figure). The thumb assembly is similar except that it includes two ARJs. The particular revolute/prismatic arrangement was chosen primarily because it offers an optimum solution to the problem of transducing the position of three DOFs per finger while actuating only one.

Each ARJ consists of the bull gear from one of the synchronous belt-drive trains and a radial ball-bearing set. Each PPJ consists of a length of a steel slider bar in a linear recirculating-ball-bearing set. Each PRJ consists of a radial ball-bearing set; it allows revolute motion of the slider bar about an axis perpendicular to both its axis of linear travel and the axis of the actuated revolute joint. The motion of an ARJ (other than the second thumb ARJ) corresponds approximately to the inclination/declination of the proximal joint of the affected finger. The motion of a PPJ corresponds approximately to extension/retraction of the tip of the affected finger with respect to the palm. The motion of a PRJ corresponds approximately to adduction/abduction of the affected finger.

The position of a slider rod along a PPJ is transduced by a linear variable-differential transformer (LVDT) in which the slider bar serves as the transformer core. The angle of a PRJ is measured by a shrouded light-emitting-diode/photoreceptor pair aimed at a rotary polytetrafluoroethylene target. The angle of an ARJ is measured by use of a shaft-angle encoder on its motor. The ARJ, PPJ, and PRJ, through their respective transducers, together provide sufficient data to define the location of the fingertip in spherical coordinates. At the same time, servoing the motor presents the required haptic feedback to the fingertip. Separate actuation of the sliding rod is unnecessary because when the gear is held in place by the application of torque, the sliding rod is positioned so that it cannot be slid freely along its path without a very unnatural motion of the finger.

The tip of each finger assembly is equipped with a mechanism designed specifically for clamping the fingertip. The finger clamp is sprung closed, and a cam action is used to ensure effective clamping while accommodating a large variance in operator finger sizes. The DM superstructure is held stationary, relative to the palm, by means of a fingerless glove with hook-and-loop straps for donning and dofing.

This work was done by Vikas K. Sinha, Eric W. Endsley, Alan J. Riggs, and Brian K. Millspaugh of Cybernet Systems Corp. for Johnson Space Center. No further documentation is available. MSC-22846