The apparatus of claim 15 wherein the setting the dynamic reference speed sets the dynamic reference speed to a value of ???MAX{min(W.sub.L, a.sub.2.times.W-a.sub.3), Ws}???, where W.sub.L is a maximum value of the dynamic preference speed, and a.sub.2 and a.sub.3 are predetermined coefficients that satisfy ???0<a.sub.2<1??? and ???0<a.sub.3???.