NASA has used two systems to train astronauts for weightlessness. The first is the Reduced Gravity Simulator that suspended the astronaut at an angle of 80.5 so that only 1/6th of his or her weight was supported by the ground, while the rest was supported by a pulley system. The other system, designed during the Apollo era, is the Partial Gravity Simulator (POGO), which uses an air-controlled piston along with air bearings and gimbals to simulate reduced gravity. This pneumatic system is attached to an air-bearing rail to allow for maneuverability in two dimensions. POGO was used to train astronauts on how to use tools and other hardware in microgravity.
Both of these systems have downsides. The Reduced Gravity Simulator only allowed motion in one dimension, and the pulleys that were used to support the astronauts were uncomfortable and were not able to support every part of the body. POGO, although adding another dimension for motion, induced huge inertial loads on the users. The friction within the system, along with its mass, led to less than perfect simulations. Also, these systems did not match the dynamics of the person moving, and limited their range of motion.
More recently, it was found that reduced gravity could be simulated on an airplane flying parabolic trajectories. Lunar, Martian, and microgravity can all be simulated using a C-9 aircraft. The downsides to this method are that there is limited space within the aircraft to move around, these simulated environments are only possible for 20- to 30-second intervals, and the accuracy of the simulated gravity is dependent upon the precision in which the parabolas are flown.
The Active Response Gravity Offload System (ARGOS) offers a simulated environment capable of performing reduced gravity testing with robotic systems and humans. ARGOS provides the ability to test in microgravity, lunar, and Martian environments with one system. The system works by providing a constant force offload through a motion-based platform. The Gen 1 system repurposed a COTS (commercial off-the-shelf) load management system as a proof of concept. Testing showed increased horizontal speed and lifting capacity were required. A new lifting hoist and horizontal system were designed into the Gen 2 ARGOS system to meet these requirements. The software used to operate the Gen 2 horizontal and vertical systems is custom and unique.
The ARGOS control software features a Graphical User Interface (GUI) for user interaction. The horizontal control software allows for the operation of the ARGOS horizontal drive system motion, and the vertical software allows for the operation of the ARGOS vertical axis hoist system. There are numerous safety controls in the software to ensure safe operation of the ARGOS system at all times and prevent accidental incorrect control input entry in the GUI.
There are many sources of noise in the load cell data that impact its resolution and the fine control capability of the system. Many of these sources of noise reside in the same frequency range that the system needs to control, rendering conventional filtering techniques useless. A novel, nonlinear tracking filter was developed in software to reduce this load cell noise. The purpose of the digital filter is to filter unwanted load cell noise and vibrations from the force data, while preserving all pertinent system data and minimizing phase lag from the filter. This allows for a highly responsive vertical control system in ARGOS, and increases the realism of the simulation.
The overall design and implementation of the ARGOS system is unique and is currently the only system of its kind. The system is human-rated and has been used for robotic and human testing.