Tuned support structure for structure-borne noise reduction of inertial navigator with dithered ring laser gyros (RLG)

Litton Marine Systems is manufacturing the third generation of the marine inertial navigators at the Sperry Marine Division using a new generation of dithered ring laser gyros (RLG) by Honeywell. To satisfy the US and foreign navies, stringent military requirements for shock (MIL-STD-901D), vibration (MIL-STD-167) and structure-borne noise (MIL-STD-740) demanded that a new approach in vibration control be employed. A combined structure supporting the inertial sensor assembly (ISA) is designed with nominal vibration frequencies outside the dithered ring laser gyro's forcing frequencies. The ISA can then be rigidly supported without causing transmission of structure-borne noise. Structure-borne noise levels are then attenuated while easily maintaining alignment of the sensor axes. This paper addresses a passive vibration control solution implemented on the inertial measurement unit (IMU) of the MK39 Mod3A inertial navigator to meet the structure-borne noise levels of MIL-STD-740-2. By using a compact auxiliary mass, a tuned support structure housing a Honeywell DIG-20 ISA is achieved. The robustness of this tuned support structure is demonstrated as Litton Marine Systems expands the product line with the enhanced system performance and high shock single axis indexed MK39 Mod3C inertial navigator system (INS). Presented analytical and experimental results demonstrate the effectiveness of the MK39 Mod3A auxiliary mass basic design. This design required no further tuning for the MK39 Mod3C application. The shock and vibration advantage of using this approach on an inertial navigator is highlighted. The impact of the indexer assembly on system performance and wobble for the shock-isolated system is discussed.