In May 1997, Dr. Stephen Canfield submitted his PhD thesis entitled “Development of the Carpal Wrist; a Symmetric, Parallel-Architecture Robotic Wrist.” A patent application was also filed on this novel joint, better known in aerospace circles as the Canfield Joint, resulting in US patent number 5,699,695, issued December 23, 1997. This awesome little joint allows complete hemispherical movement for attached devices. Constellation’s CEV might have used Canfield joint-based solar panels and RCS thrusters, had it not been cancelled. Real-world prototypes of Canfield joint-based RCS thrusters have even been built!
The Technology Area
This patent describes a mechanical, multiple-degree-of-freedom, robotic wrist. Specifically, it discloses how to construct and use a mechanical wrist with a base plate and a distal plate, where objects mounted to the distal plate enjoy a wide range of motion in the hemisphere above the base plate.
Specifics of the Patent
The Canfield joint acts as a carpal wrist. That is, it mimics the behavior of a human wrist. Your wrist allows you to turn your hand in a nearly full hemisphere of motion relative to its interface with your radius and ulna. A Canfield joint allows an object, such as a thruster, mounted on a distal plate to rotate to virtually any orientation within a hemisphere above a connected base plate. Click here to see a Canfield joint-mounted RCS thruster. This device is broadly characterized in Claim 1:
A device for the movement and positioning of an element in space, comprising:
a [base] plate;
a distal plate;
six linking members, each having two ends and being of substantially equal length;
a first three of said linking members each being connected at one end to said [base] plate by a [base] joint allowing a single degree-of-freedom, and at an opposite end to one of a second three of said linking members by a mid-joint allowing three degrees-of-freedom, said mid-joint comprising a plurality of revolute joints; and
each of said second three of said linking members being connected at one end to said distal plate by a distal joint allowing a single degree-of-freedom.
Among other reasons, the Canfield joint is interesting because it fits into different spaces compared to traditional joints, allows precise control over the orientation of attached devices, and allows attached devices to move in a manner similar to the motion of a human wrist. It is capable of producing pitch and yam motions in excess of 180 degrees while moving up and down relative to the base plate. The Canfield joint can be configured to be controlled remotely by three actuators, eliminating parts and complexity in certain applications. The structure of the Canfield joint allows cables, wires, and other conduits to interconnect between devices mounted on the base and distal plates. This arrangement, similar to the way veins and nerves pass through a human wrist’s carpal tunnel, is essential for power, fluid, and wired data transmission between devices.
Many people have experimented with using the Canfield joint on future spacecraft. Constellation’s CEV would have replaced Apollo-style quad block RCS thrusters with single, Canfield-joint mounted thrusters, greatly reducing complexity. Kirk Sorensen has a lot of cool ideas on how to use the Canfield joint to solve solar panel orientation issues on a tether spacecraft and address issues with artificial gravity/nuclear-electric propulsion vehicles. Canfield joints can potentially replace other pivot points on spacecraft, as well. Because the Canfield joint allows a conduit to pass between the two plates, it could even be used as the basis for more flexible and forgiving docking systems. A flexible, pressurized tube could be mounted between the plates with openings on both ends, with one plate mounted to a spacecraft carrying astronauts and the other plate holding a docking port.
The invention described in the Canfield joint patent (#5,699,695) expired on December 26, 2001. The disclosed invention is currently in the public domain, although other inventions based utilizing the Canfield joint are not necessarily in the public domain.