composed of two harmonic drives powered by DC mo tors with redundant winding and electronics. Wiring  for electronics, motors and antenna is integrated within  the structure. The simple mechanical design is  very stiff and rugged. Although compact and precise, with in limits, this design has a singularity (it jams)  when the antenna is pointed straight up. Yaw bec omes roll and the resultant singularity degrades  precision and complicates control. This is particula rly evident when tracking high speed objects or in the  case of ship pointing systems where the ship is pitching and yawing due to rough seas. Another popular  packaging variation is manufactured by Wescam, Inc,  Toronto, Canada this spherical sensor platform  appears to be based directly on the ball turret from World War II bomber aircraft (Figure 4). 

The second common antenna pointing system is r epresented by the “APS” manufactured by the  Honeywell Space Systems Group, Phoenix, Arizona [1]. Two perpendicularly mounted actuators produce  ±110 degrees each. Singularity is mitigated when the antenna is pointing straight up because of the  perpendicular orientation of the two actuators. However, singularity or gimbal lock occurs when the  pointing system attempts to move in circumducti on (a combination of pitch and yaw motion) at the  extremes of its range of motion.  Example: when the pointer is tracking along the horizon the upper  actuators must toggle over each ot her in a saddle shaped profile making precision pointing impossible.  The Honeywell design lacks stiffness bec ause the orientation of the joints  creates longer lever arms that  could cause deflection under high inertia loading. As in the Schaeffer system two harmonic drives with  redundant motor windings and electronic power the syst em. Complex flexible cables (a major design  problem) are found in both designs.  

KVH Industries has received a license for its sales of  certain of its stabilized satellite communications  antenna products that include robotic wrist actuator  technology under U.S. Patent No’s. 4,723,460,  4,729,253 and 4,686,866 originated by Mark E. Rosheim. 

From the above one may conclude that aside from  improvements to motor speed reducers, i.e., Harmonic Drive  devices, the fundamental kinem atics and structures for  sensor pointers has not been advanced since W.W.II. They  are still predominantly simple azimuth/elevation devices or  variations on the simple gim bal. Singularity-ridden, they are  unable to maintain a lock within the full hemisphere which is  required for horizon-to-horizon tracking.  

Figure 3.  Schaeffer Kinematics  

To answer the growing need for improved singularit y-free pointers that complement the growing  sophistication in optics and contro l, Ross-Hime Designs, Inc. sens or pointer was developed under Phase  I and Phase II SBIR contracts for the Ballistic Missile De fense Organization. By way of contrast it uses a  new, innovative, unique, patented, double universal  joint that reduces response time by enabling a  single calculation to drive the motion of the joint.  RHD has designed and built a number of gimbal  system for the robotic industry since 1987. Over  6,000 have been used in the RF antenna pointing  industry over the last eight years. The main  advantage is singularity-free motion and  ruggedness. Flexible cables are eliminated as the  motors are stationary. Pure circumduction without  gimbal lock about its extreme range of motion is  possible through its unique double universal joint  design making horizon-to-horizon as well as zenith  tracking practical. This singularity-free motion is  unique to Omni-Wrist III creating a hemispherical  range due to the unique patented structure and  kinematics [3-5]. The double universal joint has  orthogonal axes like the compass gimbal. The  range of motion problem is solved by adding a second joint. Simplicity is maintained by phasing the two 

Figure 4.  W.W.II Ball Turret 174