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
closer to the valve timing performance observed in the single-stream tests than anything that had been attained with the original CGSE control circuit in the flight system. 6.4 Controller Implementation With the new CGSE control circuit, valve response times were fast enough to enable the proposed 5 Hz control cycle. However, commanded pulse widths had to be scaled down from the actual desired pulse width, and there was also an impulse centroid shift that had to be taken into account. These ideas are illustrated with the simplified example that follows. Assume that there is a thruster valve with Open Lag of 10 ms and Close Lag of 25 ms. Assume further that when the valve is opened, thrust rises linearly from 0% to 100% over 10 ms, and when the valve is closed, thrust falls linearly from 100% to 0% over 10 ms. Thus, these transient rise and fall periods together produce impulse equivalent to that produced by the thruster at 100% for 10 ms, and the overall shape of a thruster pulse is a trapezoid. If this imaginary simplified thruster were commanded on at 0 ms and off at 40 ms, it would actually produce a pulse longer than 40 ms. In fact, it would produce impulse equivalent to a 55 ms square pulse. Furthermore, while the ideal centroid for a 40 ms pulse started at 0 ms would be located at 20 ms, the centroid for the actual pulse produced by the thruster would be located at 42.5 ms. Figure 6-12 illustrates these differences between the commanded and actual thruster pulses. Figure 6-12. Simplified diagram of a commanded 40 ms thruster pulse and its actual results. 104
The imaginary simplified thruster can actually produce the amount of impulse equivalent to a 40 ms square pulse if it is given a commanded pulse of only 25 ms. However, as shown in Figure 6-13, the centroid of the actual pulse is still shifted, although now it is only located 15 ms later than the ideal centroid. It is not possible to remove this centroid shift, but with sufficient knowledge of the thruster timing characteristics, the location of the shifted centroid can be predicted. Figure 6-13. Simplified diagram of adjusted command to produce impulse of a 40 ms square pulse. Although the actual CGSE thrusters did not have linear rise and fall characteristics, and their timing metrics all varied slightly from those assumed for the simplified thruster, the basic ideas illustrated in Figure 6-12 and Figure 6-13 still applied. By scaling down commanded pulses and predicting centroid shifts, impulse bits equivalent to 40 to 160 ms square pulses could be delivered on a period of 200 ms, and the 5 Hz control cycle was considered to be feasible. However, this was not a very fast control cycle; at 5 pulses per second and an estimated flight time of 15 s, each thruster could only produce 75 pulses during a hop, which suggested that flight profiles would not be very smooth. Also, there were additional difficulties created by the decrease in thrust magnitude with gas usage. If this decrease could be predicted, pulse width commands could be gradually increased to keep the produced impulse bits constant. The decrease was only roughly characterized, though, and there was a chance that by the end of a hop, the thrust would fall so low that the controller would saturate. Thus, as flight testing and the validation of the CGSE system began, the possibility of having to perform further verification testing was also kept in mind. 105
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closer to <strong>the</strong> valve timing per<strong>for</strong>mance observed in <strong>the</strong> single-stream tests than anything that had been<br />
attained with <strong>the</strong> original CGSE control circuit in <strong>the</strong> flight system.<br />
6.4 Controller Implementation<br />
With <strong>the</strong> new CGSE control circuit, valve response times were fast enough to enable <strong>the</strong> proposed 5 Hz<br />
control cycle. However, commanded pulse widths had to be scaled down from <strong>the</strong> actual desired pulse<br />
width, and <strong>the</strong>re was also an impulse centroid shift that had to be taken into account. These ideas are<br />
illustrated with <strong>the</strong> simplified example that follows.<br />
Assume that <strong>the</strong>re is a thruster valve with Open Lag <strong>of</strong> 10 ms and Close Lag <strong>of</strong> 25 ms. Assume fur<strong>the</strong>r<br />
that when <strong>the</strong> valve is opened, thrust rises linearly from 0% to 100% over 10 ms, and when <strong>the</strong> valve is<br />
closed, thrust falls linearly from 100% to 0% over 10 ms. Thus, <strong>the</strong>se transient rise and fall periods<br />
toge<strong>the</strong>r produce impulse equivalent to that produced by <strong>the</strong> thruster at 100% <strong>for</strong> 10 ms, and <strong>the</strong><br />
overall shape <strong>of</strong> a thruster pulse is a trapezoid.<br />
If this imaginary simplified thruster were commanded on at 0 ms and <strong>of</strong>f at 40 ms, it would actually<br />
produce a pulse longer than 40 ms. In fact, it would produce impulse equivalent to a 55 ms square pulse.<br />
Fur<strong>the</strong>rmore, while <strong>the</strong> ideal centroid <strong>for</strong> a 40 ms pulse started at 0 ms would be located at 20 ms, <strong>the</strong><br />
centroid <strong>for</strong> <strong>the</strong> actual pulse produced by <strong>the</strong> thruster would be located at 42.5 ms. Figure 6-12<br />
illustrates <strong>the</strong>se differences between <strong>the</strong> commanded and actual thruster pulses.<br />
Figure 6-12. Simplified diagram <strong>of</strong> a commanded 40 ms thruster pulse and its actual results.<br />
104
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