Development of a Cold Gas Propulsion System for the ... - SSL - MIT
Development of a Cold Gas Propulsion System for the ... - SSL - MIT Development of a Cold Gas Propulsion System for the ... - SSL - MIT
Figure 6-10. Redesigned CGSE control circuit for a single thruster solenoid valve. The resistors and capacitor at the lower left of Figure 6-10 form a voltage divider; the MOSFET had a gate voltage input limit of 20 V, and the voltage divider cut the 24 V signal from the RIO down to 12 V so it would come in under this limit. The polarized capacitor at the upper right was put in the circuit to help smooth out transient effects in the power supply. A hardline dump variant was also made for the new CGSE control circuit, as it was considered important to retain the ability to vent gas in the event of RIO failure. However, in the new circuit, the hardline dump closed an alternate path to ground, rather than closing a circuit with an alternate power supply as it had in the first CGSE control circuit. This is illustrated in Figure 6-11 below. 102
Figure 6-11. Redesigned CGSE control circuit for thruster solenoid valve with hardline dump. The redesigned CGSE control circuit illustrated in Figure 6-10 and Figure 6-11 allowed for a great improvement in valve response time compared to the original circuit illustrated in Figure 6-4 and Figure 6-5. Table 6-4 demonstrates this with a comparison of the performance of several thruster valves controlled via the original and redesigned circuits. Table 6-4. Comparison of valve timing metrics for original and redesigned CGSE control circuits. Valve Timing Metrics [ms] Thruster 3 (old circuit) 3 (new circuit) 7 (old) 7 (new) 8 (old) 8 (new) Open Lag 13 11.2 12 12.5 22 11.2 Rise Time 16 8.4 15 9.3 19 9.6 Close Lag 72 26.0 62 25.2 39 26.0 Fall Time 16 10.8 16 9.9 17 14.3 Table 6-4 shows that not only did the new CGSE control circuit shorten nearly all of the valve timing metrics, especially Close Lag, it also made them more consistent for different thrusters. With the redesigned control circuit, all eight CGSE thrusters had an opening time (sum of Open Lag and Rise Time) of about 20 ms and a closing time (sum of Close Lag and Fall Time) of about 35 ms, which was much 103
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Figure 6-11. Redesigned CGSE control circuit <strong>for</strong> thruster solenoid valve with hardline dump.<br />
The redesigned CGSE control circuit illustrated in Figure 6-10 and Figure 6-11 allowed <strong>for</strong> a great<br />
improvement in valve response time compared to <strong>the</strong> original circuit illustrated in Figure 6-4 and Figure<br />
6-5. Table 6-4 demonstrates this with a comparison <strong>of</strong> <strong>the</strong> per<strong>for</strong>mance <strong>of</strong> several thruster valves<br />
controlled via <strong>the</strong> original and redesigned circuits.<br />
Table 6-4. Comparison <strong>of</strong> valve timing metrics <strong>for</strong> original and redesigned CGSE control circuits.<br />
Valve Timing Metrics [ms]<br />
Thruster 3 (old circuit) 3 (new circuit) 7 (old) 7 (new) 8 (old) 8 (new)<br />
Open Lag 13 11.2 12 12.5 22 11.2<br />
Rise Time 16 8.4 15 9.3 19 9.6<br />
Close Lag 72 26.0 62 25.2 39 26.0<br />
Fall Time 16 10.8 16 9.9 17 14.3<br />
Table 6-4 shows that not only did <strong>the</strong> new CGSE control circuit shorten nearly all <strong>of</strong> <strong>the</strong> valve timing<br />
metrics, especially Close Lag, it also made <strong>the</strong>m more consistent <strong>for</strong> different thrusters. With <strong>the</strong><br />
redesigned control circuit, all eight CGSE thrusters had an opening time (sum <strong>of</strong> Open Lag and Rise Time)<br />
<strong>of</strong> about 20 ms and a closing time (sum <strong>of</strong> Close Lag and Fall Time) <strong>of</strong> about 35 ms, which was much<br />
103
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