The position equations for the fourbar pin-jointed linkage were derived in Section 4.5 (p. 152). The linkage was shown in Figure 4-7 (p. 155) and is shown again in Figure 6-20 on which we also show an input angular velocity ω_2 applied to link 2. This ω_2 can be a time-varying input velocity. The velocity loop equation is shown in equation 4.5a and 4.5c, repeated here for your convenience.