Phase compensation of rate limiters in JAS 39 Gripen

Abstract

For a modern aerodynamically unstable fighter, like the JAS 39 Gripen, the flight control system (PCS) typically provides 45° phase margin. Therefore rate limiting of control surfaces is an important issue. Software rate limiters are placed on the control servo commands in order to prevent the hydraulic servos from rate limiting. When a rate limiter is saturated, the phase shift drastically reduces the stability margins of the closed loop and increases the risk for pilot-induced oscillation (PIO). This paper describes a novel method for compensating the phase shift of a rate limiter. In contrast to earlier phase compensation methods, this method uses feedback instead of logic or feedforward. Open loop and closed loop properties of the method are discussed. The method gives a drastic improvement on stability margins and reduces PIO tendencies. This is demonstrated on a JAS 39 Gripen example. Phase compensated rate limiters are now used in the JAS 39 Gripen PCS production software. Their effects on closed loop stability and flying qualities are discussed in the paper.