Keeping a steady hand is vitally important for many professions where the use of a static or purely mechanical handrest just isn’t practical or possible. A new computer-controlled, motorized hand and arm support will let doctors, artists, machinists and others precisely control scalpels, brushes and tools over a wider area than otherwise possible, and with less fatigue.
A person using the Active Handrest puts their wrist on a support that can slide horizontally in any direction. The user's wrist sits on a round wrist pad on an arm that is attached to a motorized base that can move from side to side and back and forth to re-center the hand. Under the wrist rest is a force sensor similar to a bathroom weight scale. The base also is attached to an elbow rest.
The system includes a personal computer to control the handrest. The computer detects the position or force of the user’s wrist and decides how the armrest should move. Meanwhile, with position control the device monitors the tool motion and repositions the handrest to follow the tool’s motion.
"For force control, the device senses how you are leaning on the wrist rest. If you push forward a little bit, the handrest will move forward in response," says William Provancher, an assistant professor of mechanical engineering at the University of Utah who developed the device with his students.
In this way the handrest allows a person to maintain a steady hand while it senses the position of a hand-grasped tool or the force exerted by the hand - or both. Then, the device's computer software moves the handrest so it constantly re-centers the user’s fingertips in the center of their dexterous workspace – that is the range over which users can move their fingers and be very precise.
For example, if a person places their arm on a desk to write, their hand is able to move the pen about 4 inches in any direction, but precise writing is practical only within a 1-inch wide "dexterous workspace," says Provancher.
The existing prototype of the Active Handrest lets a user move their hand precisely within a workspace of about 10 by 10 inches, re-centering the hand as the arm moves to reach a larger area. But future devices could allow for three-dimensional motion and even larger work areas with the same precision, Provancher says.
People often try to increase their precision when doing manual tasks by bracing their wrist or arm against a fixed object. Over the years, there have been many devices to help: wrist rests used when typing or operating a computer mouse, repositionable braces for artists painting fine detail on a large canvas, and repositionable supports for typists.
More recently, research devices have been developed in which a robot and a human user simultaneously hold a tool to increase precision in using it.
"In all of these devices, control of the tool is shared between the human and the robot," Provancher and colleagues wrote. "In contrast, the Active Handrest provides ergonomic support and increased precision, but allows the user to maintain complete control of the tool. Furthermore, the Active Handrest can be used with any tool."
As part of a series of experiments conducted using the prototype handrest sixteen participants traced circles or arcs displayed on a computer screen. They did each of three sizes of circle or arc four times, for a total of 12 tracings, while using the Active Handrest. For three alternative forms of support, they also did 12 tracings each, for a total of 48.
The experiment found that that there was no significant difference when drawing the smallest circle, among the four types of support. That was expected because people could draw such a small circle without moving the entire hand.
However, when participants drew the medium-sized circle and the large arc, drawing error was significantly lower with the Active Armrest than with the fixed handrest, fixed elbow support or no support. "Use of the Active Handrest leads to a 36.6 percent reduction in drawing error over the unsupported condition," the study says. "The Active Handrest also provides a 26 percent reduction in drawing error over static hand support."
A patent on the device is pending, and Provancher says he may form a spin-off company to commercialize it, or may license it to companies that produce touch-feedback devices, make robotic surgery equipment, produce art or refurbish electronics.
He will discuss development and testing of the Active Handrest on March 25, during the Institute of Electrical and Electronics Engineers' Haptics Symposium in Waltham, Mass. Haptics is to the sense of touch as optics is to the sense of sight.