ThrustLevel:
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Computes the thrust level of a set of thrusters.
Since version 1.
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Form:
[thrustLevel, turnedOff] = ThrstLvl( time, pulsewidth, fallConst,...
thrustLevelOld, aFall, aRise, bRise )
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Inputs
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time (1,1) Time since pulse started
pulsewidth (n) Pulsewidth
fallConst (n) Thruster fall time constant
thrustLevelOld (n) Previous thrust level
aFall (n) exp(-timeStep/fallConst)
aRise (n) exp(-timeStep/riseConst)
bRise (n) 1 - aRise
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Outputs
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thrustLevel (n) Thrust level, between 0 and 1
turnedOff (x) Indices of any thrusters which were turned
off during this call. 0 <= x <=n
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