An actuator arm of an actuator assembly includes a 4-bar linkage structure defined by a trapezoidal shaped opening formed between a front end of the actuator arm and a pivot shaft. A first rounded extension portion and a second rounded extension portion are formed at two corners adjacent to the pivot shaft of the opening, respectively, and a third rounded extension portion is formed at two corners adjacent to a front end of the opening, and hinges H.sub.1, H.sub.2, H.sub.3 and H.sub.4 are formed at the actuator arm by the first, second and third extension portions. Design factors determining a shape of the opening are optimized to meet the following relations for the structure of the 4-bar linkage: W(cos .beta.-1)+Z(cos .alpha.-1)=.delta..sub.x=0, W sin .beta.+Z sin .alpha.=.delta..sub.y=0.