# Understanding Balance Control in the Context of Riding a Personal Mobility Device
## 1 Introduction  
A personal mobility device (PMD) is a compact, motorized vehicle that has no internal combustion engine and uses electricity as its primary and secondary power sources for transporting an individual. It is also known as personal mobility, personal mobility vehicle, and smart mobility depending on the country, media, and researcher. The PMD originated in the late 1990s, initially dominated by electric scooters/mopeds and electric bicycles, with the addition of the Segway PT [1]. It has some unique characteristics compared to other types of vehicles. It is eco-friendly because it uses electricity as a power source and it does not emit pollutants when driving on the road. If PMDs can replace existing vehicles for some short-distance driving or improve accessibility to public transport, they can greatly reduce congestion and pollutant emissions [2, 3]. Additionally, their portability is superior because of their small size and light weight compared to other types of vehicles. The majority of PMDs are designed to be small or foldable, making them convenient to carry in other vehicles or in public transport (without the concern of parking shortages) and easy to store. Owing to these features, the PMD is emerging as an alternative means of transportation for short distances in the urban environment and is gaining the spotlight as the first/last-mile connection with other means of transportation [4, 5, 6]. Among the various types of PMDs, self-balancing PMDs (SPMDs), which are controlled by weight shifting, are continuously being developed and attracting increasing attention. Consequently, they have become a representative type of the PMD. There are many types of SPMDs such as Segway PT, Segway mini, hoverboards, electric unicycles, and electric one-wheel stateboards. By leaning anteriorly or posteriorly, the rider can move the SPMD forward or backward. It is also applied by leaning to the lateral directions when riding the electric one-wheel stateboard. The more the rider tilts their body, the faster thevehicle moves. To maintain a constant speed, the rider should maintain their balance with the posture. The method of changing the direction depends on the type of vehicle and the weight-shifting direction. The Segway PT and Segway mini use a handle and a knee control bar to change directions, respectively. The hoverboard is operated similarly to the Segway (but without the handle) using the position and pressure of the rider's foot [7]. The electric unicycle redirects the vehicle by moving the center of gravity left or right as it moves forward. Commonly, they require diagonal weight shifting to change the moving direction of the devices. As an example, the electric unicycle is required to make a right turn by shifting the rider's weights to the right diagonal direction. It is also applied to other devices as well when controlling the moving direction. In summary, weight shifting and maintaining equilibrium are required to move, accelerate, decelerate, and turn the SPMD, and the rider's balance control is very important. As postural control strategies for maintaining balance vary depending on the type of human activity and the environmental condition [8], they are also applied differently depending on the SPMD riding context. Thus, it is necessary to consider the balance control of humans when designing SPMDs. There are several benefits of considering the human balance control in the design of SPMDs. First, it can be easier for riders to learn how to ride and operate the SPMD. The difficulty of boarding and maneuvering depends on the type of SPMD, and beginners need sufficient practice to gain familiarity with the vehicle and balance control. There are inherent abilities and specific patterns to control human balance, which need to be understood to increase learnability and ease of SPMD operation. Second, the SPMD rider's safety can be improved. The rider controls the direction and speed of the SPMD through weight shifting.