Design and Sensor-Based COntrol for Hyper-Redundant Mechanisms Page: 2 of 4
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Instead of going with the bevel gear design, we opted for a simpler actuated universal-
joint (U-joint) design was selected for its simplicity and ruggedness. In this design, U-
joint "crosses" are connected to one link with a pitch pivot joint, and to the next with a
yaw pivot joint. The pitch and yaw joints are always orthogonal, and intersect along the
link centerlines; this leads to simple kinematics. See Figure 1 The pitch and yaw joints
are actuated by linear actuators in the two links.
Figure 1 Two Views of Actuated Universal Joint with Orthogonal Degrees of Freedom
Figure 2 We Use a Ball Screw Design with Conventional Actuators
Links are configured such that the axes at each end of any link are parallel; thus, one link
will have pitch joints at both ends actuated by its two linear actuators; the next link will
have two yaw joints. This arrangement facilitates packaging of the two linear actuators
side-by-side in the link. Ball screws were chosen for the linear actuators because of their
high efficiency (compared to lead screws) and effective speed reduction. The screws are
fixed in bearings mounted to the links, while the nuts drive clevises connected to the
crosses of the U-joints. The screws are driven by brush-type, permanent-magnet, DC
motors which can be operated with simple, pulse-width-modulated (PWM) electronics.
For compactness, the gearmotor and ball screw are placed side-by-side with a small
toothed-belt drive connecting them. Each actuator is mounted to the link through a steel
flexure that accommodated the slight lateral movement of the screw as the joint angle
changes.. See Figure 2
A novel feature of this design is the overload mechanism or "snubber." It is designed to
absorb the kinetic energy of the links and motors when the mechanical stops are reached,
and to accommodate imposed loads on the snake without damage to the actuators or
structure. Belleville spring washers--4 series sets of 3 parallel-stacked washers--are
mounted in the "snubber housing" such that the ball screw can move axially by 1mm if
the preload value is exceeded. The thrust load of the screw is taken by a custom-made, 4-
point-contact bearing that is integrated into the snubber housing.
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Choset, Howie. Design and Sensor-Based COntrol for Hyper-Redundant Mechanisms, report, May 12, 2006; [Pittsburgh, Pennsylvania]. (digital.library.unt.edu/ark:/67531/metadc892629/m1/2/: accessed January 23, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.