The magnitude and frequency of disturbances to the microgravity environment observed on Space Shuttle flights and predicted for the International SpaceStation (ISS) are critical factors in determining mission success and defining engineering design requirements. The Dynamic Load Sensors (DLS) experiment provides an opportunity to attain this critical data during spaceflight. A stable, very low-gravity environment is necessary to assure that life science, material science, and astronomical investigations on board Shuttle, the Russian Mir space station or the International Space Station (ISS) yield the most accurate data.
The DLS sensors, flown in conjunction with the Middeck 0-gravity Dynamics Experiment (MODE) helped determine middeck crew-induced loads during the STS-62 Space Shuttle Mission which flew in March of 1994. In response to the need to ascertain crew disturbances of the microgravity specification, DLS incorporates a trio of instrumented load sensing interface devices that are designed to measure nominal crew. Two six degree-of-freedom sensors, the hand-hold and the foot restraint, measure the applied loads in three axes (x, y and z directions) as well as the moments about these directions. The third sensor, a three degree-of-freedom touch pad, measures the applied translational loads.

Objectives

The primary objective of the EDLS experiment is: To assess nominal crew induced reactions (forces and moments) during spaceflight. Crew motion force measurements are recorded by DLS for hand hold, foot restraint, and push-off and landing activities on orbit.

The Hardware

In order to design, fabricate, and flight qualify the DLS sensors a review of specifications of the microgravi ty environment as dictated by potential space station investigations was undertaken, then the sensor design was specified, and finally the sensors were fabricated and flight qualified. The geometry of the loadcells were iteratively determined by modeling the complete sensor with a finite element model (FEM). The FEM stresses were used to predict the sensor voltages to ensure that the sensor met the desired resolutions and that the loadcells would not fail under the specified maximum applied loads. The FEM was verified by calculating the deflection and stresses that occur when a central beam of the loadcell is subjected to a vertical force. There was a small difference between the FEM and hand calculation attributable to the other cross-beams in the full FEM, but the hand calculations support the use of the FEM results in the design procedure. The FEM was used to predict the dynamic characteristics of the sensors. The first predicted modal frequency of 120 Hz is well above the design specified minimum (60 Hz) (Newman and van Schoor, 1992).
Over 67 hours of data was collected on STS-62. The results show RMS values of less than 50 N and 10 Nm for force and moment measurements, respectively. Eight characteristic motions were identified for the hand hold and foot restraint motions, while four of the characteristics motions were seen during touch pad usage (See Table). Complete data analysis yields maximum, RMS, frequency content, and vector representation for the force and moment measurements. Fast Fourier transform analysis reveals that the frequency content of the signals is primarily less than 10 Hz.

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Funding

This risk mitigation experiment is funded by NASA Langley Research Center (LaRC) and the NASA JSC Space Station Office under NASA Contract NAS1-18690. The MICR0-G Effort is funded by NASA under researcj grant NAG9-1003.