ABSTRACT
Pilot-induced oscillation (PIO) has been noted in airplanes since the Wright Brothers. MIL-STD1797A defines PIO as sustained or uncontrollable oscillation resulting from efforts of the pilot to control the aircraft [1]. PIOs range in severity from annoying to catastrophic, which often occur suddenly and unexpectedly and often take place during high-gain events requiring tight control by the pilot, such as landing, air refueling and target tracking. This paper focuses on rate limiting-induced PIO, for rate limiting has appeared in almost all modern recorded severe PIOs. Rate limiting has been identified with PIO for three primary reasons. First, it introduces additional phase lag, or delay, between the commanded and actual control surface actuator position, which, in most control systems, tends to destabilize the closed loop system. Second, rate limiting has effective reduction in gain, which is seen by the pilot as a reduction in control effectiveness and so may compensate with larger input making the problem even worse. The two rate limiting concepts are shown in Figure 1. Third, the rate saturation may result in effective changes in the vehicle rigid-body dynamics (the worst case being changed from stable to unstable).
