positions. Note that the coupler, link 3, is merely a line from A to point P. Point P can be thought of as a coupler point which happens to coincide with the pin joint between links 3 and 4. As such it will have simple arc motion, pivoting about O4, rather than, for example, the higher-order path motion of the coupler point Pi in Figure 5-4. our function generator uses link 2 as the input link and rakes the output from link 4. The "function" generated is the relationship between the angles of link 2 and link for the specified three-position positions, PI, P2, and P3. These are located in the plane with respect to an arbitrary global coordinate system by position vectors R1.R2, and R3. The function is: