Am 21.12.2019 um 17:39 schrieb Craig Fink:
Hey Uwe, I've found FPGA interesting since their beginning. When I was
school, the University had an Analog Computer for control systems
development of Flight Control systems, have these Analog Algorithms made
it into the FPGA? Or has it remain more digital where the FPGA are more
of the interface, and if a lot is still digital, are multiple systems
ever implemented where an analog mode is included?
Speaking of Control Systems, fresh out of College, with my first real
job working GN&C on Space Shuttle I was assigned to help with the
On-Orbit post flight report for STS-1 and STS-2. One of the problems
these first few flights was in attitude control, the gains were wrong
resulting in excessive fuel usage. Essentially, banging back an forth
from positive limit to negative limit, hosing out propellant. Things
haven't changed much in 40 years with Starliner as the NASA
administrator is talking about the problem.
https://youtu.be/NpQlxN4xbKM?t=85
Here he's saying the attitude tried to maintain "to precise" an attitude
window, which isn't the problem, it's the gains were wrong. The precise
tolerance just means it takes much less time to get from the +side to
the -side, so it hosed fuel out faster. But even with a very loose
attitude window, if the gains are wrong, it's going to be banging back
and forth (+-+-+-+-+-+ firings) from one limit to the other. Forty years
and no progress, haven't these people heard of auto gains? It's such a
simple thing to implement, especially for a new vehicle it's flight
characteristics are fuzzy. From a single axis prospective, if the
control went from + to - or - to +, gain = 0.99 * gain, reducing the
next thruster firing on the other side of the window. It's the simplest
of iterative algorithms that quickly would stop the control system from
banging back and forth in it's window. Quickly, the gain is reduced so
that only thruster on one side of the attitude window will fire. On the
flip side, if the gain is to small, only thrusters on one side of the
attitude window will fire, either the + side (+++++++ firings) or the -
side (--------- firings) and the attitude never crosses the center of
the window (zero) then gain = 1.01 * gain.