The success to date of the human race can arguably be attributed to the evolution of the opposable thumb, which allows man to lightly hold fragile objects, pinch tiny objects, or tightly hold and manipulate large or heavy objects. The translation of the abilities of the human hand to that of a robotic analogue has met with significant challenges, and so far, no equivalent has been developed.
There are two primary applications for mechanical hands: robotics applications and human prosthetics. A larger amount of research has gone into developing robotics for industrial applications because of the revenue potential. However, casualties due to the current conflicts in Iraq and Afghanistan have resulted in an increase in prosthetics research.
Challenges in Mimicking the Human Hand
The human hand is extremely complex and is difficult to replicate mechanically. A human hand contains five digits, 27 bones (digits and wrist), and several muscles and tendons connecting the bones. The challenge to engineers is not only recreating the geometry of the hand but also recreating the sensors and control that the skin and the brain, respectively, provide.
Some of the most basic robotic grippers called end effectors, consist solely of two opposing fingers that can close on an object with a specific amount of pressure or a vacuum grip that can adhere to an object to lift it. When the same type of object is manipulated consistently, the gripper can be programmed to apply a specific amount of pressure. However, the human hand adapts to varying amounts of pressure and object size and weight, and it is difficult to package this variance of functionality into a mechanical package.
New Advances in Robotic Hand Design
The Shadow Robot Company has developed the most complex and anatomically accurate mechanical hand in the form of the Shadow C6M Smart Motor Hand. The C6M uses 40 “air muscles” to manipulate 24 joints to fully replicate the movements of the human hand. The hand is equipped with an array of sensors to provide tactile feedback to the processor. The current model has a list price of approximately $100,000 USD and is designed to be used for applications including telepresence, where an operator can control the hand remotely through the motion of his own hand, ergonomics research, and repetitive handling of fragile objects, such as fruits or eggs.
Researchers at the University of Chicago in Illinois are taking a different approach when it comes to robotic hand design. The team, led by Dr. Eric Brown, has developed a robotic manipulator that consists of a thin rubber sack filled with coffee grains or small spheres. When the sack touches an object, the air is sucked out of the sack, and the rubber skin forms to the shape of the object being touched.
The sack approach works well for some objects but not so well for porous objects. However, the hand concept is significantly simplified form a hand with multiple, independently controlled digits.