The SLS is the second version of a prototype subject loading system, developed by the Université catholique de Louvain (UCL) and Arsalis (Belgium) in cooperation with the support of Verhaert Space (Belgium), the European Space Agency (ESA) and Prodex, specifically for the second generation treadmill (T2) on the International Space Station (ISS).
Microgravity is known to cause loss of bone and muscle mass. Current evidences indicate that impact loading,bone stress and muscular work, as implied by exercises such as treadmill running, are important countermeasures for maintaining bone and muscular mass. In microgravity, such an exercise requires a Subject Loading System (SLS) in space, i.e. a means to hold the subject down onto the surface of a treadmill while he/she runs.
Over 15 candidate concepts, including different actuators (EMA’s, springs, pistons, bellows) and transmission systems (cams, gear-boxes, fusees), have been studied by Verhaert Space.
These concepts were rated against 15 trade-off criteria such as expected performance and accuracy, proof of concept, engineering budgets (mass, power, dimensions), safety, reliability, maintainability.
The retained concept for the SLS is the one proposed by Arsalis:
The pneumatic pressure system:
- Cable pulling the subject down from each side.
- Low pressure, buffered, cylinder pre-tensioning cable.
- High pressure buffer refilling both low pressure buffers.
- Compressor refilling the high pressure buffer.
- Total of 5 valves controlling the flow of air.
- Sensors: 3 pressures, 2 pull-down forces, 2 positions.
The SLS concept is designed to meet the following functional requirements established by the ISS International Partners (CSA, ESA,
JAXA, NASA and RSA):
- Subject size: 5th % Japanese ♀ – 95th % American ♂.
- Static load range: 178 – 979 N.
- Load adjusted in 22.3 N steps.
- Static load accuracy: ±5 %
- Dynamic load accuracy: ±15 %
- Total dynamic load accuracy ±20 % (left plus right).
- Load rate: 89.2 – 133.5 N/s (initialization).
- Load rate: 22.3 – 44.5 N/s (stage transition/session end).
- Vertical displacement: ±10.2 cm.
- Pull-down cords origin: < 55 cm apart, within ±2.5 cm of
the fore-aft centerline of the tread surface, within ±5.1 cm
of the running surface plane.
- Subject displacements: over the entire tread surface.
- Attach/detach: < 2 minutes.
- Emergency egress: < 30 s.
- Tension measured at ≥250 Hz and stored periodically.
- Engineering data measured every 10 s and stored > 2
- Operating lifetime of 10 years.
- Maintenance/repair specifications based upon 4 subjects
running 1 h/day and 4 days/week.
- Annual on-orbit maintenance: < 12 h for a 3-person crew.
- T2 system noise < 65 dBA at a distance of 60 cm.
The SLS is designed to fit the allocated volume within the T2 ISPR rack
The T2 SLS prototype is currently mounted in a simple test rack that allows the SLS to be driven with approximately sine wave displacements ranging in amplitude from ±5.0 cm to ±12.5 cm in 2.5 cm increments.
The ‘step frequency’ can be varied from 0.1 Hz to 3 Hz.
The pull-down force can be varied continuously between 0 and more than 500 N.
A dedicated software allows to:
- control manually or automatically the SLS (pressure, force, speed, start, stop, acquisition, motor speed, pressure and force level)
- Monitor all parameters of the SLS
- Record data for further analysis
- test the SLS performances under all possible conditions of loads, displacements and speeds.