Practical Problems with Digital Systems

When digital stability-augmentation systems first appeared, their most alluring advantage, as compared with analog systems, was their ability to change system gains, shaping networks and even architecture by software changes, instead of requiring time­consuming hardware changes. This is especially attractive in a prototype flight testing program, as may be imagined. However, a drawback to this capability is that the ease of making changes by software modifications encourages a cut and try approach to fixing problems.

The same design freedom that makes for easy changes in a digital stability-augmentation system makes it easy to load the design with overly complex gain schedules and cross-feeds. In a recent classified program, practically all system gains are complex functions of altitude, Mach number, angle of attack, center of gravity, and other measurable parameters, with no real proof that this complexity is needed. One result of complex gain schedules is an inordinate amount of time required for checkout in simulation and flight testing.

On the hardware side, one can be faced with digital flight control systems that incorporate several sampling systems, operating at different rates and not in synchronization. This is the case on the Grumman X-29A digital flight control system. Again, careful simulation and bench testing is needed to be sure that no problems arise from this. Anti-aliasing filters are generally needed on the inputs of analog-to-digital converters, to screen out input frequencies that are multiples of the digital sampling frequency.