In robotics, industrial machinery, and other electro-mechanical systems, it is often required that electrical signals be transmitted across a rotating mechanical coupling. For example, a television camera mounted on a robot arm receives power and control signals from a control platform and returns data signals representative of the images being captured back to the control platform. The robot arm is coupled to the control platform via a rotating coupling which allows rotation of the robot arm about an axis. A flexible cable is used as the pathway for the electrical signals across the rotating coupling. This cable, which forms a rotatable electrical interconnection, must be specially configured so as not to restrict the freedom of motion of the rotating coupling.
A first commonly used rotatable interconnection, floppy interconnection 100, is shown in FIGS. 1(a) and (b). A first member 102 is coupled to a second member 104 via a hinge or coupling pin (not shown) to allow relative rotation around an axis 106. Floppy interconnection 100 provides a signal pathway between members 102,104.
Floppy interconnection 100 includes a flexible cable 108 and two strain-relief cable mounts (hereinafter, "strain-relief") 110. Flexible cable 108 is secured to first member 102 with a first strain-relief cable mount 110. Flexible cable 108 is secured to second member 104 with a second strain-relief 110. Flexible cable 108 forms a large loop portion 114 between the two strain-reliefs 110. Loop portion 114 includes enough "slack" or "slop" to allow rotation of member 104 with respect to member 102.
Floppy interconnection 100 has several problems associated with it. First, loop 114 is susceptible to becoming tangled or snagged on nearby objects. Second, strain-reliefs 110 tend to concentrate stresses on the portions of the cable directly beneath the strain-relief points such that these areas are common points of failure. Third, when the system including members 102,104 is subjected to rapid accelerations or decelerations, loop 114 tends to flop freely about, magnifying the stresses on the cable at the strain-relief points. Finally, because of its inexact nature, it is very difficult to predict the reliability of floppy interconnection 100.
A second commonly used rotatable interconnection, coil interconnection 200, is shown in FIGS. 2(a) and 2(b). Coil interconnection 200 is shown in the environment of first member 102 and second member 104 as described above. First member 102 is rotatably coupled to second member 104 via a coupling or hinge pin 202. Coupling pin 202 allows rotation about axis 106. Coil interconnection 200 includes a flexible coil 204 of cable 206. Coil 204 is centered about axis 106. A first end of cable 206 is secured to member 102 via a strain-relief 110. A second end of cable 206 is secured to member 104 via a second strain-relief 110. Coil 204 is hung from a third member 208 using a third strain-relief 110.
Coil interconnection 200 has several problems associated with it. First, like loop 114 of floppy interconnection 100, coil 204 is susceptible to becoming tangled or snagged on nearby objects. Second, coil 204 must be centered on axis 106. This is not always possible. Third, coil 204 tends to be bulky, requiring a lot of space. Finally, it is difficult to predict the reliability of coil interconnection 200.
A third possible rotatable interconnection, wiper-type interconnection 300, is shown in FIG. 3. Wiper-type interconnection 300 works on the same principle as the brushes of a DC motor which provide power to the rotor windings. Wiper-type interconnection 300 is shown in the environment of first member 102 and second member 104 as described above. First member 102 is rotatably coupled to second member 104 via a hinge or coupling pin 302. Pin 302 is fixedly attached to member 104 and is adapted to rotatably mate with opening 304 in member 102. Wiper-type interconnection 300 includes a wiper or brush assembly 306 and a plurality of conductive tracks 308. Tracks 308 are disposed on pin 302. Wiper assembly 306 includes a plurality of conductive brushes 310 which are adapted to make ohmic contact with conductive tracks 308. Each conductive track 308 and brush 310 pair forms a single conductor rotating interconnection. A plurality of wires 312 provides electrical connection to wiper assembly 306. A plurality of wires 314 provides electrical connection to conductive tracks 308. Wires 312,314 are attached to members 102,104, respectively, by strain-reliefs 110.
Wiper-type interconnection 300 has several problems associated with it. First, the interconnection can become quite large and complex for a large number of conductors. Second, the brushes are relatively delicate and are susceptible to frictional wear.
What is needed is a compact rotatable interconnection which is not susceptible to tangling, is reliable, and allows straight-forward reliability predictions.