To achieve posture control and ride comfort (vibration isolation performance) of a robot in unstructured terrain, a novel four-wheel-legged robot (FWLR) with an actively-passively suspension system is first designed. In the suspension system, the active parts are responsible for posture control and the passive parts are responsible for vibration isolation. Then, a closed-loop and decoupled posture control model with 11 DOF are proposed, with which we designed the posture controller with a second-order lowpass filter (SLPF). To test the posture control performance of FWLR in unstructured terrain, both simulation and experiment are carried out, the simulation and experimental results show that the posture angles in unstructured terrain are reduced by 54.65% and 59% on average, respectively. In addition, the frequency response shows that the posture angles are reduced by more than 50% in low-frequency unstructured terrain. Finally, to validate the ride comfort of FWLR, dynamic models with different degrees of freedom (DOF) are established and simulated, and the results present that the ride comfort can be improved with the posture angular acceleration is reduced by 15.83% and 46.7% on average. Generally, with the actively-passively suspension system proposed in this paper, FWLR can be equipped with excellent ride comfort and posture control in unstructured terrain. The research in this paper has potential reference value and practical value for enriching the posture control of robots and vehicles.

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This paper presents a novel solution for the posture control and ride comfort between the proposed wheel-legged robot (FWLR, Four Wheel-Legged Robot) and the unstructured terrain by means of a passively-actively transformable suspension system. Unlike most traditional robots, each leg of FWLR is independent from each other with a spring-damping system (passive system) is connected in series with an actuator (active system), so the posture control and ride comfort in complex terrain can be realized by the combination between active and passive systems. In order to verify the performance of posture control in complex terrain, a prototype and complex terrain are established firstly, then a posture control model, algorithm, and controller considering the suspension system are proposed and verified by the comparison between co-simulation and experiment, the results showed that the pitch angle and roll angles in complex terrain can be controlled. To show the impact of passively-actively transformable suspension system on ride comfort (vibration isolation performance), different dynamic models with different DOF (Degree of Freedom) are established, the co-simulation results showed that the passive system and active posture control system can also effectively improve the vibration isolation performance and ride comfort of FWLR in complex terrain. The research results of this paper have important reference significance and practical value for enriching and developing the mechanism design and theoretical research of wheel-legged robot and promoting the engineering application of all terrain robot.

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