I did another flight of the stabilization system last weekend. The main objective of the flight was to test the derivative terms that we added to the yaw/pitch control scheme. Basically, the proportional term sets th canard angle in proportion to the deviation between the actual tilt and the set point tilt (although this is done separately for yaw and pitch). If the rocket tilt is moving towards the set point tilt, the derivative term (the rate of change of the tilt) moderates the canard angle to reduce the rate of change in tilt. Similarly, if the rocket is moving away from the set point, the canard angle is increased by the derivative term. The idea of this was to reduce oscillations around the set point.
Based on previous flight results, one thing we think we've learned is that air frame flex contributes to the oscillations we see during the flight. We don't want to try to control that, so I tried to minimize this by covering the upper air frame with some carbon. I used a carbon sock, and I can guarantee it is the first and last sock I will ever use. Nuff said on that.
The current flight was done on an L1050 Blue motor, The tilt program was to fly vertically for 4 seconds, tilt to 10°, and then go vertical again at 10 seconds. The "baseline" flight from last month was on an L1000, and the tilt graph from that flight was in Post 747. The current flight, with the L1050, had quite a bit more velocity when the tilt program engaged at 4 seconds. The tilt graph is attached, and there is less oscillation. Unfortunately, it's not possible to say whether to attribute this to less air frame flex (from the carbon) or the derivative term. The tilt graph shows that some air frame flex still occurred, which is indicated by an oscillation occuring before the tilt reached the 10° set point.
One other interesting thing that happened during the flight is that the rocket appeared to be unstable for about the first second of flight (and then went vertical once the velocity was high enough for the canards to kick in). The rocket is stable by 1.1 to 1.5 calibers, on paper, but is apparently not stable in actual flight. I haven't looked into this in any detail yet, but I suspect there is an issue associated with modeling the canards such that the normal rules for establishing the stability margin don't quite apply. Thoughts on that? If you look back in this thread to prior flights, you can see this has likely happened on other flights. Good thing I have a stabilization system on board to "rescue" the rocket! A short video snip showing this instability is linked and other pics and info from the flight are attached.
I should add that the "heading hold" feature added to the control logic resulted in no net roll through this flight, even with all of the events that occurred.
Jim