# Advancements in Semi-Active Automotive Suspension Systems with Magnetroheological Dampers: A Review
## 2 Suspension Systems
### Types of the Suspension Systems  
Currently, the development of suspensions has advanced to effectively adapt to various road conditions. Based on existing research and the usage of vehicles in the market, suspensions are primarily categorized into three types: passive suspension, semi-active suspension, and active suspension, depending on whether the dampers can adjust their damping autonomously and whether external energy is required during operation, as shown in Figure 2. Additionally, suspensions can be classified into traditional passive suspensions and intelligent control suspensions based on the presence of a control unit. Obviously, active and semi-active suspensions belong to the category of intelligent control suspensions. As a conventional mechanical suspension system, passive suspension is characterized by its simple structure, reliable performance, and low cost. Undoubtedly, it is currently the most mature technology and the most widely used suspension system in the market, with the longest development history. It operates without requiring external energy input, but its stiffness and damping coefficients remain fixed and cannot adjust according to road conditions, thereby limiting its effectiveness in vibration reduction. Due to its inability to adapt to varying road conditions, vehicles equipped with passive suspension systems struggle to simultaneously optimize handling stability and ride comfort, thus failing to achieve optimal performance [21; 22]. Throughout vehicle operations, it primarily serves the fundamental role of providing basic damping to facilitate smooth vehicle travel. Regarding the shortcomings of passive suspension's fixed damping forces, the concept of \"active suspension systems\" was first proposed by [PERSON] in 1955 [23] and initially piloted on Citroen cars [24]. Active suspension systems replace the springs and damping elements of passive suspensions with actuators capable of adjusting output forces in real time based on vehicle and road conditions, thereby optimizing vehicle performances across all road surfaces [1; 2]. This advancement significantly enhances both vehicle comfort and safety, representing the pinnacle of suspension development and offering superior ride quality among the suspension types currently available. As scholars delved deeper into research, various types of active suspensions emerged. However, they are characterized by intricate structures, high energy consumption, elevated costs, and reduced reliability. In cases where control system signals are lost or actuator failures occur, the suspension may completely lose its damping function and even potentially exacerbate vehicle vibrations. Currently, active suspension systems are predominantly installed in a limited number of high-end vehicle models, posing challenges for widespread application. Figure 2: Types of vehicle suspensions: (**a**) passive suspensions; (**b**) semi-active suspensions; (**c**) active suspensions. The semi-active suspension system bridges the gap between passive and active suspensions, offering enhanced damping performance compared to passive suspensions while consuming less energy than active counterparts. During vehicle operation, the controller of a semi-active suspension adjusts the stiffness of elastic components or the damping coefficient of dampers in response to variations in road conditions [3,4], thereby suppressing vibrations in the suspension system. In the event of control algorithm failure, it can function as a passive suspension. Depending on the adjusted mechanism, semi-active suspensions can be categorized as stiffness-adjustable or damping-adjustable suspensions. Semi-active suspensions retain the inherent characteristics of passive suspensions without the addition of external actuators, often replacing traditional passive dampers with units that feature adjustable damping coefficients.