# Lateral Stability of a Mobile Robot Utilizing an Active Adjustable Suspension
## 1 Introduction  
Mobile robots are widely used for diverse applications, such as terrestrial and planetary exploration, forestry, agriculture, mining industries, and reconnaissance [1; 2]. They are normally used to enter unstructured, severe, and hostile environments while equipped with various tools [3]. In these scenarios, mobile robots must conduct high-performance locomotion tasks while ensuring system safety. Such tasks can result in a loss of wheel traction, entrapment, loss of stability, and tip over. Clearly, tip-over instability can result in robot damage and total mission failure [4]. Therefore, mobility (viz. stability) on a rugged terrain is critical for mobile robot design. Enhancing the locomotion performance in such an environment requires the design of innovative locomotion systems. [PERSON] and [PERSON] reviewed the locomotion systems of ground mobile robots, which are classified as wheeled robots, tracked robots, legged robots, wheel-leg robots, wheel-track robots [5], wheel-legged-tracked robots [6], and hybrid mobile robots [1]. Usually, wheeled robots can be constructed in the simplest configuration so that fast movement and good energy efficiency can be guaranteed without any complicated control strategy [7]. Thus, a passively articulated frame is thetraditionally adopted design for wheeled mobile robots to ensure that all wheels remain in contact with the ground. Mobile robots with active suspensions can alter their structures and configurations to change their center of mass to avoid tipping over while traversing complicated unstructured terrains [1, 2]. The Sample Return Rover [8], Scarab [9], ATHETE [10], Tri-star [11], Sherpa [12], Workpartner [13], Azimut [14], TRREx [15, 16], and Hylos [17] are examples. Additionally, [PERSON] et al. proposed a wheel-step rover with a modified active rocker-bogie suspension that could use both wheeled and legged motions [18]. Considering stability, [PERSON] et al. proposed a method for stability-based articulated suspension control and demonstrated it experimentally on the Sample Return Rover developed by the Jet Propulsion Laboratory [4]. [PERSON] et al. developed a wheeled-legged hybrid mobile robot and proposed a control method that optimized both traction and stability [17, 19]. [PERSON] and [PERSON] investigated a posture-control method to increase rollover stability and adhesion for an actively articulated rover with a rocker-bogie suspension [20]. However, these studies primarily focused on controlling the robot to increase their stability. Thus, the relationship of the attitude of the mobile robot and stability was neither considered explicitly, nor was it evaluated. [PERSON] and [PERSON] developed a dynamic model for wheeled planetary rovers with active suspensions; when the attitude of the rover is controlled, strong lateral wheel slips are prevented [21]. [PERSON] et al. proposed a reconfigurable rover and modeled the relationship between the attitude of a rover and its slip, showing that a rover could traverse slopes covered with loose soil with less downhill slippage by properly adjusting its configuration [22, 23]. [PERSON] et al. developed a hillside vehicle power chassis with a balanced rocker suspension mechanism. This platform could obtain variable ground clearance and variable wheel tracking. They also studied the vehicle's attitudinal effect on stability [24, 25]. [PERSON] et al. developed a small remote control hillside tractor. They made the tractor level on different slopes by adjusting its lateral tilt [26]. All these studies indicate that stability during locomotion can be enhanced by adjusting the robots' configurations. However, the studies rarely addressed the specific active suspension, which can be used to adjust the attitude of the locomotion. Consequently, it is important to understand the complex interactions occurring between the specific active suspension and the stability of the mobile robot.