## 1 Introduction  
With the aging population, the physiological changes of organs and its consequent loss of function lead to the increased physical disability, which limits human mobility of the elderly. This has been one of the main reasons for the loneliness, and it affects the mental health of the elderly [1; 2; 3]. A four-wheeled rollator is a tool that assists human with motor dysfunction to walk. As a life partner for the elderly, the four-wheeled rollator creates a relative old-age lifestyle by empowering and assisting natural human mobility [4] and improves the quality of life in later life, allowing older people with motion system limitation to go out with the assistance from the four-wheeled rollator, which is beneficial to the physical and mental health of users. In recent years, the research in the design and control of smart rollators has attracted many interests [5; 6; 7; 8]. However, most of the research in the literature adopt questionnaires, interviews, observations and other methods to obtain user needs [9], without analyzing the type and importance of the requirements. They directly designed the mechanical structure according to the original demand. Functions of the design and the requirement that needs to be solved are still not satisfied. The solving the problem invention theory (TRIZ) [10; 11; 12] has been successfully applied in several domains such as aeronautics, car industry, and electronics, etc. It is known as using the TRIZ problem model to describe the design problem and then using TRIZ tool to solve the described problem. TRIZ is capable of modeling and converting the simulation solution into the practical solution of the industry domain. Kano model is a non-linear relationship between customer satisfaction and product objective quality. By integrating the advantages of Kano Model Analysis and TRIZ, the Kano-TRIZ design theory [13; 14] presents an effective methodology to study and to provide design principles of the four-wheeled rollator, which take account of the physical and mental characteristics of the elderly. Quality Function Deployment (QFD) is a user-driven quality function configuration method. However, in the research of rollator design [15; 16; 17; 18; 19], the popularity of TRIZ to solve the problems of four-wheeled rollator design is still relatively limited, basically in the germination stage. Except for the mechanical design of the four-wheeled rollator, its mobility is also of vital of importance for its market feasibility. The lateral motion stability control of four-wheeled rollator is the main challenge in terms of safety, especially for the elderly lacking motion ability. Hence it is necessary to develop a stable controller to validate the feasibility of motion control of the rollator. Since the longitudinal movement of the four-wheeled rollator can be controlled by handbrake and human traction, this paper focuses on its lateral motion stability. The purpose of the lateral stability control of the four-wheel rollator is to design an advanced controller to achieve the smooth trajectory tracking control of the rollator. According to the literature, the lateral control approaches of the rollators mainly include proportional-integral-derivative (PID) control, fuzzy adaptive control, sliding mode variable structure control, neural network control, etc. [PERSON] et al. [20] proposed a parameter self-tuning fuzzy PID control method to effectively reduce the yaw rate and slip rate. Based on the rollator model and steering system model, [PERSON] et al. [21] designed a neural network PID controller with good real-time and robustness. In order to overcome the problems of nonlinearity, parameter uncertainty, and time-varying external disturbance, ref. [22] proposed an adaptive fuzzy sliding mode control strategy to achieve adaptive control. The performance in terms of smooth is considered in [23], a fuzzy control algorithm based on iterative genetic algorithm is proposed. Ref.