**4. Conclusions**

The considerable interest that is shifted to hybrid kinematic structures to exploit the advantageous features of the serial and parallel kinematic structures and avoiding their drawbacks has brought about some interest in overconstrained hybrid mechanisms. A study on the mobility of the three classes of mechanisms is presented and focuses on the mobility of overconstrained structures in view of their application in parallel and hybrid structures to reduce the number of passive joints. The mobility of the Exechon mechanism is analyzed and discussed as an example of a successful machine tools mechanism. The study of this mechanism reveals that its 3-DOF parallel kinematic part is a revolving 2-DOF planner mechanism. Strategies for kinematic designs of planner parallel mechanisms were developed and discussed based on the axiomatic design methodology. Optimum configurations for planner mechanisms were presented for 2-DOF planner mechanisms and were shown to be extendable to 3-DOF planner and spatial mechanisms by proper choice of joints and constraints. An alternative optimum parallel and hybrid mechanism is discussed and analyzed.

Fig. 10. Main Function Requirements of a hybrid design
