**7. Conclusions**

According to neuroscience, a distributed control system is proposed for the locomotion of a legged robot, since the CPG, located in the spinal cord and in charge of the rhythmic motion, is an autonomous device, almost requiring neither the peripheral sensor feedback nor the regulation command from the brain-stem. A hexapod robot with biomimetic legs was built to realize such a distributed control system, where a mechanism is proposed to serve as the CPG and a computer to act as the brain-stem, wirelessly sending commands to the autonomous device. The proposed mechanism consists of two modules, i.e., the tripod gait generator and the Theo Jansen Linkage. The tripod gait generator is a device that uses a single motor to generate a tripod gait, while the TJL rhythmically executes the legged motion. The interesting point is that the complex mathematical function of the foot motion can be realized by the ensemble of links of the TJL. If the same function is executed by an electronic computer, it will require a grea<sup>t</sup> amount of the computational time and memory resources. Nevertheless, dimensioning the TJL appropriately is nontrivial, since the patterns of orbits generated by a TJL may vary according to its overall dimensions and can be cast into four groups: bell curves, ovals, sharp-pointed ovals, and lemniscates. In general, the ovals or bell orbits are legitimate, the sharp-pointed ovals are partly legitimate, while the lemniscates are illegitimate. Once the dimension of a TJL has been determined, it can generate an ellipse-like orbit with a transversal axis longer than the lateral axis, so as to achieve energy e fficient walking.

Admittedly, the mechanical computer is inferior to the electronic computer in regards to flexibility. Therefore, in the future, e fforts should be taken introduce the adjustable mechanical structure to increase the adaptability of this line of approach. Although the proposed method is not suitable for the case of a legged robot, adapted to diversified terrain, it suits the case of a legged robot operating in a specific environment with relative unevenness, such as construction sites, where the grounds are somewhat muddy and rugged, or factories, where the floors are scattered with debris. Our aim is to build a hexapod robot for material handling, since using a legged robot as a mobile platform is still more e ffective than using a wheeled robot with regards to crossing obstacles and avoiding skidding.

Robots are, in fact, complex and expensive machines, consisting of many actuators, sensors, transmissions, and hardware. Designing a robot capable of especially excessive function can further increase its cost. The cost of building a robot is generally proportional to the number of actuators it uses. For instance, most hexapod robots, designed with collocated actuators, require 18 servos. Based on the design of non-collocated actuators, the proposed hexapod robot uses merely three motors. The value of this approach lies on the fact that it gives a way of building a hexapod robot with much low cost. The significance of this research is in employing a mechanical mechanism, rather than electronic circuits or computers, to act as the CPG in achieving locomotion.

**Author Contributions:** M.-C.H. designed the mechanism and F.L. built the mechatronic control system. J.Y. analyzed the kinematics of the Theo Jansen linkage and Y.L. developed the programs. M.-C.H. and F.L. contributed to realization and revision of the manuscript.

**Funding:** This research was funded by the Jiangxi University of Science and Technology, People's Republic of China (Grant No. jxxjbs18018).

**Conflicts of Interest:** The authors declare no conflicts of interest.
