Hardware Description
Control Overview

Hardware Description

Dexter is a two-fingered robotic hand, with two degrees of freedom per finger. Each degree of freedom is powered by a low friction, low inertia DC servomotor. The motor is connected to the link through a cable/drum drive similar to those found in haptic feedback devices such as the PHANToM®. As a result, the hand has low friction and is backdrivable. The robot has four RE035 Maxon motors which are basket wound low inertia motors with rare earth magnets to provide a max continuous torque of about 80 mN-m. Attached to each motor is an HP encoder with 500 lines of resolution, giving us 2000 cpr using qudrature. The motors are driven by four linear current amps (APEX PA-05) which is in turn controlled by the servo card D/A channels. The servo card is an 8-axis ISA servo I/O card from Servo To Go (Model I).

The motors are fairly small, due to weight and space limitations, but are still capable of providing enough force at the fingertips to pick up a 250g object, such as a softball, which more than suffices for the purpose of these experiments. The links are 100 mm long, and each has over 120° of motion. The workspace of the hand is about 400 mm by 150 mm, with a positional resolution of 0.08 mm. This workspace is sized to best manipulate objects from about one to three inches in diameter. Manipulation is not limited to spherical objects; the telemanipulation tasks involve manipulation of square blocks and cylindrical objects.

Two-axis strain gage force sensors have been incorporated into the robot fingertips to read the forces applied by the robot to the object. The force sensors have good linearity and are accurate to ± 0.1 N up to 5 N. The fingertips have an dovetail mounting system so that different tip types can be easily attached to the robot and investigated.

The robot hand is placed on the end of an Adept 1, a 4 DOF SCARA industrial robot arm. The Adept has a positional resolution of 0.04 mm and 0.05° in the rotational axis. The workspace is approximately 1100 mm long by 350 mm wide by 175 mm high. The robot trajectory is controlled by the Adept controller, which requests new commanded positions as needed. The commands are sent from the computer that is tracking the wrist motion.

Control Overview

The robotic hand is controlled using a 1000 Hz servo control loop implemented on a Pentium II computer running the QNX operating system. Details of the control algorithm are covered on the Shared Autonomous Control page.