1. Introduction

Different kinds of multi-legged robots are extensively investigated in the literature, since they have great importance in engineering applications [1]. In general, each legged robot can be considered as a mechatronic system with limbs connected to the main body (i.e., the robot's trunk). In such a construction of the walking machine, the robot's legs are responsible for acting as a support frame and play a key role when it comes to the locomotion process. Namely, when the limbs of the robot are controlled with a degree of autonomy, it can move within its environment and perform the planned tasks [2]. For instance, the legged machines are able to reproduce animal or human movements and substitute humans in various activities [3]. An interesting state of the art regarding the most popular hexapod robots can be found in numerous review papers. Among numerous multi-legged robots met in the literature, we can distinguish especially robots with leg structure inspired based on the anatomy of insects or mammals. Large numbers of legs with different kinematic structures are useful to overcome complex obstacles met in nature without losing stability and increase stability of the robot.

About 50 percent of the Earth's land surface is not adapted for wheeled machines. However, it can be accessed by humans and animals that can walk in those difficult and irregular terrains. Therefore, nowadays, there is a lot of interest in different kinds of robots that use legged motion inspired by walking animals found in nature. This type of motion allows to overcome obstacles, move in an omnidirectional manner, and access uneven environments, and it is fault tolerant, in comparison to crawler or wheeled vehicles. Unfortunately, legged robots are challenging in terms of controlling their locomotion. However, on the other hand, they can be used in terrains, where wheeled machines cannot perform their tasks. It is because legged robots can overcome obstacles of heights equal to the height of their limbs, while the wheeled robots can overcome obstacles of heights up to the half of the wheel radius. On the contrary to wheeled robots, where the contact between the robot's wheels and the ground usually has a continuous character, in the case of legged robots, the contact between the feet and the ground usually takes a form of contact points. It is known that a large amplitude of the ground reaction force acting on the robot's feet has a negative impact on the dynamics of the whole robot. Therefore, these forces should be minimized, if possible. This is why one of the considered issues is also related to the problem of minimization of the ground reaction forces acting on the robot's legs.

A literature review summarized in our previous papers [4–11] indicate that investigations of different types of walking machines are still challenging and focus the attention of numerous researchers. The control possibilities can be especially helpful in a natural environment of the robot, when it comes to both the navigation and obstacle avoidance. That is why, in this chapter we developed a general full parametric simulation model of a hybrid walking robot, i.e., the robot which has different numbers of the legs inspired biologically by insects, reptiles, or mammals. To drive the legs of the abovementioned robot, we employed a novel gait generator, firstly introduced in our previous paper [11]. The mentioned approach can be treated as neural networks that generate rhythmic outputs in the absence of rhythmic input. Such kinds of rhythmic motions have been found by biologists in different biological periodic processes, including swimming, running, breathing, flying, chewing, or walking. Moreover, we also used own algorithm, which is suitable for a smooth transition between different gait phases, i.e., initial, rhythmic, and terminal phases [10]. The main goal of the abovementioned studies was to obtain better both kinematic and dynamical parameters of the investigated machines during the locomotion process, which finally lead to an increase in the stability of the robot and its control possibilities. Eventually, we considered the problem of controlling the direction of the movement of the robot and control all six spatial degrees of freedom of the robot's body, i.e., three deviations and three rotations along and around three different axes, respectively. It can be useful for the walking robot and to control all robot's legs on a planar, unstable, and vibrating ground, since both the navigation and obstacle avoidance matters are particularly important for the locomotion of legged robots in the natural environment.
