joints together mechanically rather than pow ering each gimbal independently. What is often  unappreciated by designers and engineers is the radical tr ansformation that a gimbal must go through to  achieve a perpendicular orientation of the output in any direction from t he mechanism centerline. While it  is common to make pointers with stock actuators and components they invariably have singularities. This  is a result of either their ki nematics, structure or both.  

Table 1.  Preliminary  Free-Space Communications  Sensor Pointer Performance Goals 

 Load capacity   4.5 kg (10 lb)   Output Speed   60 deg sec   Unit weight   9.1 kg (20 lb)   Accuracy   0.06 deg > 3.6 arc minutes >216 arc seconds   Range of motion  180 degrees Hemisphere (2 π steradians)   Dexterity   Singularity-Free   Construction   17-4 Stainless Steel, Engineering Plastic   Physical Envelope  Cylinder, 20.3 cm (8") Diameter X 45.7 cm (18") high 

Test Results 

This section presents a summary of the test result s for the Omni-Wrist III Sensor Mount. A detailed report  was included in a paper presented at the SPIE Conferenc e in Seattle, July 2002 [2]. Two test methods  were used to determine the pointing accuracy. A Coordinated Measuring Machine (CMM) was initially  used and a second test method utilized an attached laser pointer to determine the repeatability of the  pointing accuracy. In this last test, the Omni-Wri st III Sensor Mount was exercised through various  deflections.  

CMM Test  Result s  The CMM was capable of high precision measurements of any point on the Omni-Wrist III Sensor Mount.  The tip of the Sensor Mount was used for these meas urements. As the Sensor Mount was exercised the  position of the tip was precisely determined. These positions were then compared with expected values  and the various errors were calculated. The Sensor  Mount was placed within the CMM and exercised at  two azimuth settings, (0 and 45 degrees). At each of these positions the declination angle was changed  from 0 to 81 degrees in several steps. All told ther e were 58 separate positions. The pointing error was  less than 0.05 degree (180 arc seconds).  

Laser Pointer Test Results   Figure 5 shows the Omni-Wrist III Sensor Mount attac hed to its mount that was anchored to the wall. A  laser pointer was attached to the Sensor Mount at the center hub. The central hub also incorporated the  means to attach various weights. We used regular bar  bell weights for the testi ng. Weights of 2.1 and 2.8  (4.6 and 6.1 lb) were the primary values used. 

The Omni-Wrist III Sensor Mount was exercised through many different sequences. Two home positions,  (Dec = 0, Az = 0) and (Dec = 89, Az = 180) were  used. The test sequences consisted of large angle  deflections, small and medium angle deflections, and mult iple angle movements. A weight of 2.1 kg (4.6  lb). was attached to the Sensor Mount for most of the tests. For all the tests only one of the tests  produced an average repeatability error greater than t he goal of 216 seconds of arc. In general the  reproducibility error was less at the home position (0, 0)  than at (89, 180). At (0, 0) the actuators are  retracted and the Sensor Mount movements were more certain than when the actuat or positions were at  their extremes. Speed tests indi cated that an angular deflection greater than 160 degrees/second could  be accomplished - much faster than the goal of 60 degrees/second.  

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