Boeing 747 Autopilot Pitch Modes Logic (Path Integrator)

Question

I am hoping this will be my final question(s) concerning the 747 autopilot, given that the roll modes are now complete and I am finally on to the pitch modes...

All information on how this autopilot functions is from the following document: https://ntrs.nasa.gov/api/citations/19730001300/downloads/19730001300.pdf

Okay, so: The Path Integrator. Integral gains are shown on page 567, and the block diagram showing the integrator implementation is on page 571.

I am struggling to understand just how the path integrator works. Based on the block diagram, I am led to believe that the integrator takes whatever output the pitch autopilot has calculated up to that point, multiplies it by the appropriate integral gain based on mode, integrates, then feeds this into the summing point with the originally calculated output. As an example:

The vertical speed mode proportioner gain is 0.09 degrees pitch command/ft per second of error. Assuming for example a 3 ft/sec error in vertical speed, the commanded pitch change would be 0.27 degrees. Is 0.27 the initial value that the integrator would begin working off of or am I misinterpreting the diagrams? The integral units shown on page 567 are simply listed as "integral/displacement" which seems to me it is integrating based on whatever displacement command the autopilot is currently giving.

Thanks!

Answer 1

Typically the integrator attempts to incrementally adjust the steady-state output so that when the error is zero, the resulting output will be what is required to hold the error at zero.
Think of it this way:
If trying to hold level-flight by controlling the elevator, a non-zero error signal is generated when the flight-path angle is non-zero. This will cause the elevator to deflect proportinally in an attempt to bring the FPA back to zero- the bigger the error, the bigger the correction. But! as soon as it is brought back to zero, the error becomes zero again and the elevator input goes back to what it was before. And it was that original elevator position that allowed the FPA to diverge from zero to start with. This is why an integrator is added. With the integrator, while the error is non-zero, a small percentage of that error is accumulated (integrated) and added to the output- this way when the error goes back to zero, the accumulated error term is still there to get added to the output. By tuning the percentage added, you can get the accumulated value to come out to just exactly what is needed as a correction to hold your desired FPA.