The guiding principle of SLAMMD follows that of Sir Isaac Newton's Second Law of Motion, F=ma, i.e., force is equal to mass times acceleration. Therefore, to find mass, the acceleration is divided into the force: m=F/a. For the HRF SLAMMD, the force is generated by two springs inside the SLAMMD 4 PU drawer. The acceleration used in calculating the mass is actually a calculated average acceleration with regression analysis. The acceleration is measured through the use of a precise optical instrument which detects the position versus time trajectory of the SLAMMD guide arm and a micro controller which collects the raw data and provides the precise timing. The final computation is done via portable laptop computer with SLAMMD unique software.
As mentioned earlier, the force is generated by two springs inside the 4 PU SLAMMD drawer. Each spring has Vectran attached to one end that is fed around a separate pulley and back towards a central cam. The cam is attached to a centrally located shaft which also has a flywheel and an encoder disk attached to it. The cam is designed and constructed so that throughout the distance the springs stretch a constant force will be applied the central shaft. This is an important design so that the mass can be calculated using the m=F/a equation. The large flywheel has a Lanyard attached to it and is fed through a small slit on the SLAMMD front panel outside and attached to a connector. The latch connector during SLAMMD operation is connected to a latch assembly on the SLAMMD guiding arm. With the cam design and presently selected springs, the force "pulling" the guide arm assembly inward is therefore constant at 5.25 lbs (23.3 N).
For human mass calculations, the astronaut wraps the legs around the leg support assembly like one would for a leg curl machine, align the stomach against the belly pad and either rest the head or chin on the head rest. For calibration and control calculations, a calibration arm assembly is attached to the SLAMMD guide arm. Using an 18 pound calibration mass at different lengths from the pivot point, different mass values can be simulated. The range will be from 90 pounds to 240 pounds.
The HRF SLAMMD is capable of:
- Operating as a rack-mounted device
- In-flight calibration.
- Displaying mass values in pounds and kilograms
- Being powered by the Human research Facility Rack (28 Vdc)
- Storing and downloading data to a computer
- Maintaining accuracy over the range of body masses from 90 to 240 lbs. An accurate averaged measurement should be obtained from the device after no more than five runs
- Determining the mass of a crewmember to within 0.5 pounds accuracy
- Accommodating a minimum memory capacity of 250 KB or enough memory to store at least 145 runs