ISS Risk Mitigation

Active Rack Isolation Systems (ARIS)


This risk mitigation experiment was designed to validate the design concept of the ARIS while it operated in a space environment. Analysis and studies have indicated that the planned ISS acceleration environment will not provide the low level of acceleration that are specified to support certain microgravity experiment activities. Microgravity disturbances would be caused by major component mechanical disturbers such as the solar and thermal radiator rotary joint mechanisms and control moment gyros, crew motion, and other equipment, such as pumps and fans. To meet the microgravity requirements, it was determined that an active experiment rack isolation design approach should be developed and tested.

Shuttle-Mir Missions

The rack was set up for use by the crew during STS-79. Data collection and execution tests were performed by ground control. When unlocked for microgravity operation, the ARIS rack will float within a half-inch clearance, connected to the Space Station module by eight actuators and a set of utility umbilicals. The umbilicals provide power, data, fluids, gases and vacuum to science payloads in the rack. The umbilicals also allow some of the module disturbances to enter the rack. The control system will use accelerometers to sense rack vibration and generate response signals to the rack actuators. Then the actuators in the ARIS rack will counter those vibrations by pushing between the rack and the Space Station module.

A series of 10 sets were performed while the ARIS operated during different phases of the Shuttle flight. These tests involved an initial performance checkout, utility umbilical reconfiguration, Shuttle Vernier Reaction Control System (VRCS) induced acceleration tests, and reduced payload mass tests. Tests were performed prior to docking, while docked, and after separation form the Mir. Also, due to actuator anomalies, the crew performed inflight maintenance procedures to correct and replace several actuator pushrods and pivots damaged while certain tests were being performed.

The results from the ARIS experiment show that the basic 6 degree-of-freedom control algorithms were successfully proven and do not require any significant modification for rack vibration isolation control on ISS. Isolated acceleration levels were at or below accuracy requirements and motion in rack sway space did not exceed 0.05 inches. This showed that the rack acceleration sensor noise and drift properties, actuator control resolution, and control configuration were microgravity compatible. Umbilical stiffness tests were accomplished and characterized the zero-gravity performance of two different supporting services umbilical configurations. The stiffness data gathered, particularly hysteresis, will be used to update the ISS ARIS simulation model.

Findings from failure investigation of the actuator/pushrod anomalies were incorporated into the Critical Design Review activities for the ISS ARIS currently underway. These include the need to increase the tolerance between the actuator control arm and its housing, strengthening of certain pushrod and actuator parts, and the incorporation of software for the detection of limit cycling oscillations.

Lofton R, Conley C. International Space Station Phase 1 Risk Mitigation and Technology Demonstration Experiments. 48Th International Astronautical Congress; 1997 Oct 6-10; Turin, Italy; International Astronautical Federation.

Principal Investigators
John Larson
Boeing Defense & Space Group

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Page last updated: 07/16/1999

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