PLEASURE FLYING ON THE COMPANY’S NICKEL – THE “NAVVAN”


As part of the N73 system’s test equipment, we had bought a motorhome that was built to our NAA custom design and it was a very classy vehicle indeed. It was about thirty feet long and had seating for six people and a driver. The color of the interior was various shades of blue and it had a microwave oven and refrigerator. The purposes of the NAVVAN were twofold. The first being a mobile test test laboratory and the second was of a vehicle we could use to demonstrate the N73 system to potential customers. To support the operating requirements of the system, the NAVVAN was equipped with both 60  and 400 cycle gasoline powered generators. We spent many hours on the road testing the system and we were fortunate it had all the comforts of home, including a couch for an occasional nap. We were very impressed by the NAVVAN, but our first time out testing the system on the road was a disaster.

We had spent a considerable number of manhours determining that the NAVVAN was fully compatible with the N73 system and it was finally time the get the show on the road. We powered the system up and, with suitable fanfare, started on our first test trip, Our objective was Fort Tejon, located on the “grape vine” grade, north of Los Angeles. The test routine was simple and the objective was to create a map of our route, as measured by the N73 system, and compare the N73 system waypoints with the actual waypoints, determined from 15′ series USGS maps. The N73 measured waypoints were compared to the actual waypoints and the differences were used to compute the CEP (circular error probable) for that test. We turned around at Fort Tejon and started East, back to anaheim. We were moving at freeway speed and the N73 was navigating at the expected level of 0.15 NM/HR error rate. It was a common practice to monitor the eight suspension signals of the two ESG (electrostatically suspended gyroscopes) thru the use of multiple channel oscilloscopes. This was a convenient way to monitor the health of the system. The worst case event was a”ball drop”, in which case the four suspension signals of the afflicted gyroscope went to zero volts and stayed there. A “ball drop” was a catastrophic event – the gyroscope was destroyed.

It was common on freeways then to encounter tar-like patches on the freeway surface which had developed ridges which caused an automobile or a truck to vibrate substantially when they passed over these patches. It was the same for the NAVVAN, but not to worry as the system was isolated from the vibrations by carefully designed isolators and they had done the job well as we had encountered many soch rough patches on the way out to Fort Tejon. Until, that is, we passed over a particularly rough patch which caused all eyes to look over to the oscilloscopes where we saw eight flat lines. We had experienced a double “ball drop”. A disaster had occurred, but why? Had we not experienced this vibration many times on this our first road test? Why now?

We knew we had uncovered a fundamental issue, one that we must solve. Were the isolators too “soft” at that particular vibration frequency? Was there enough sway space to avoid bashing into the inner surface of the housing? We needed data! We instrumented the system with accelerometers and small cones made of soft modeling clay. The cones were placed such that they would reveal the actual sway space between the IMU (inertial measurement unit) and the inner surfaces of the system housing. The accelerometers were placed such as to yield some information about the attenuation characteristics of the isolators. Now we needed a vibration input to the system. What might we use? The NAVVAN of course! We drove around the parking lots near our lab until we found a bad spot that might serve to create a lot of vibration input to our instrumented system and away we went. We really rattled that system. When we arrived back at our laboratory area and opened up the system, we saw that several cones were flattened and we strongly suspected that we had sway problem to solve. we created more sway space and the composition of the isolator isomer was changed. We went back to now familiar parking lot rough spot for a final test of our fixes and we passed. We never had that problem again – even when we broke the NAVVAN’s rear undercarriage on the frost heave found on the pitifully rough freeways on the way back from Saint Louis, Mo.

 

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