The fourth and final test, pressure vs. time of the primary chamber system when subjected to a more slowly occurring series of volume changes with the prototype active pressure regulation system enabled, aimed to characterize the effect of stroke rate on overall system performance. A series of three compression/expansion cycles were conducted at a stroke rate of 210 cm3/s (73% slower than previous tests). Because the stroke rate was reduced, the simulation time was set to twice the length of previous tests (30 s vs. 15 s). Again, the HPR and LPR were charged to the same pressure differentials (±69 kPag), and were not recharged between compression/expansion cycles. Plots of the primary chamber response vs. time, as well as the responses of the HPR and LPR vs.
time, are included as Figure 64a-c.
(a) Primary Chamber Pressure vs. Time Figures 63a-c: Primary chamber, HPR and LPR pressure vs. time for 770 cm3/s
stroke rate compression cycles (test 3)
(a) Primary Chamber Pressure vs. Time Figures 64a-c: Primary chamber, HPR and LPR pressure vs. time for 210 cm3/s
stroke rate compression cycles (test 4)
3.6 – DISCUSSION
Tests 1 and 2 demonstrate the improvement in primary chamber pressure response to significant changes in operating volume provided by the active pressure regulation system prototype for identical compression/expansion cycles (34% reduction in initial pressure spike magnitude, and an immediate return to the initial pressure state that does not occur in the unregulated condition). These improvements suggest that the prototype pressure system is sufficiently powerful to maintain constant pressure (minus decaying transients) in a closed volume environment with unstable internal volume (i.e. a gas pressurized space suit). Additionally, during 4.5 consecutive compression and expansion cycles, the system was capable of consistently maintaining primary chamber pressure within the levels seen during a single cycle (±1.848 kPag). Qualitatively, the system response to each of the cycles closely resembled that of the single compression/expansion cycle. This demonstrates that the system can maintain its initial regulation performance throughout a series of significant changes in primary chamber volume.
When compared to repeated cycles at a fast stroke rate, the primary chamber pressure response to repeated cycles at a slower stroke rate was considerably different. Rather than causing one large spike and corresponding decay, each compression or expansion stroke caused several spikes/decays of much smaller magnitude in immediate succession.
This difference was an interesting outcome. For tests using the fast (770 cm3/s) stroke rate, the primary chamber pressure changed more quickly than the system could mitigate, meaning that even after system activation (the point where chamber pressure crosses the detection threshold, identifiable symptomatically by a change in slope of the transient pressure spike), primary chamber pressure continued to rise for the duration of each compression stroke (or fall for each expansion stroke). Once each compression/expansion stroke ended, with the system still activated, the pressure spike then decayed to zero. This behavior resulted in a single large spike (with a discontinuity in slope during the growth of the spike) and subsequent rapid decay upon completion of the stroke. This behavior can be seen graphically as follows in Figure 65:
(a) Primary Chamber Pressure vs. Time
Figure 65: Relationship of 770 cm3/s stroke to HPR and LPR system activation for one cycle, leading to single large spike behavior (test 3)
For the slower (210 cm3/s) stroke rate, however, the system acted more quickly than the rate of change of primary chamber pressure, meaning that once activated the system would drive the pressure back to zero very quickly and then deactivate (and this would happen in a fraction of the time required to complete one compression/expansion stroke).
As a result, this activation/deactivation behavior would occur multiple times per compression/expansion stroke. This resulted in a series of miniature pressure spikes.
This behavior can be seen graphically as follows in Figure 66:
(a) Primary Chamber Pressure vs. Time
Figure 66: Relationship of 210 cm3/s stroke to HPR and LPR system activation for one cycle, leading to multiple small spike behavior (test 4)
A comparison between Figures 63b-c and 64b-c demonstrates that the HPR and LPR qualitatively behaved similarly regardless of stroke rate. Most importantly, though, is the fact that the regulator system was able to maintain a tighter control on primary chamber pressure for the slower stroke rate test run (±0.55 kPag vs. ±1.86 kPag, representing a bandwidth reduction of 70.4%). And, compared to the case with no regulation, the slower stroke rate test run reduced pressure bandwidth by 80.5% while maintaining constant primary chamber pressure (minus transients). This demonstrates that the pressure bandwidth can be significantly reduced despite changes in operating volume, and this reduction can be magnified if the user is willing to slow the rate at which he or she induces the changes in volume.
The HPR and LPR plots provide a complete system understanding of the prototype’s behavior. Every time the regulator was activated (in response to both positive and negative pressure spikes), it was possible to map the corresponding primary chamber pressure decay to a specific activation of the LPR/HPR. These activations each resulted in a degradation of the initial pressure charge of the respective reservoir. Every time a positive pressure spike was mitigated, on average a 4.41 kPag reduction in LPR pressure differential was measured. Similarly, every time a negative pressure spike was mitigated, on average a 5.03 kPag reduction in HPR pressure differential was measured. These tests showed that, as expected, without intervention the regulating reservoirs will eventually lose their starting pressure differentials, leading to a degradation of system performance and ultimately to total loss of system capability. It is for this reason that the prototype design calls for an in-line pump between the HPR and LPR (though this pump was not implemented in the prototype for these tests).