Phase compensation of rate limiters in unstable aircraft

摘要

For a modern aerodynamically unstable fighter, like the JAS 39Gripen, the flight control system typically provides 45° phasemargin. Therefore rate limiting of control surfaces, which may causelarge phase shifts, is an important issue. Software rate limiters areplaced on the control servo commands in order to prevent the hydraulicservos from rate limiting. When a rate limiter is saturated, the phaseshift drastically reduces the stability margins of the closed loop andincreases the risk for pilot-induced oscillation (PIO). This paperdescribes a novel method for compensating the phase shift of a ratelimiter. In contrast to earlier phase compensation methods, this methoduses feedback instead of logic or feedforward. Open loop and closed loopproperties of the method are discussed. The method gives a drasticimprovement on stability margins and reduces PIO tendencies