# The Exoskeleton Balance Assistance Control Strategy Based on Single Step Balance Assessment
## 1 Introduction  
Falls are one of the main causes of disability and death of elderly people. The balance ability declines with age and the chance of falls is greatly increased [1]. Traditional walking aids, such as canes or crutches, play an important role in the life of elderly people. Although canes and crutches can prevent falls to some degree, they are unable to offer active assist to the user. Essentially, canes and crutches can only enlarge the stability region of walking, but the key for balance recovery still relies on the cognitive and physical status of the user. Hence, as the muscle strength and reaction speed of elderly people decline with age [2], it is still a big challenge for elderly people using canes and crutches to regain balance when they suffer from an unexpected perturbation. In fact, elderly people can still generate a strategy for balance recovery, but, unfortunately, their limbs are not strong enough and cannot be moved fast and to prevent falls [3]. Therefore, a new kind of walking aid that can provide active assist for human limbs is required. Exoskeleton technology is a promising solution for active balance assistance. For the last two decades, exoskeletons have been widely studied and have been successfully applied in load carrying [4,5], rehabilitation of paralyzed patients [6,7], and walking assistance [8]. Recently some exoskeleton research has shown the potential of active balance assistance [1,9,10]. [PERSON] et al design a light-weight active orthosis for hip movement assistance [11]. They test the performance of the device on a healthy subject. The results showed that the subject could walk with the device without being hindered and while he received a smooth assistive flexion-extension torque profile on both hip articulations. This research shows the potential of the exoskeleton in balance assistance for humans. Currently, the man-machine interaction performance is one of the key points for the application of exoskeletons. For the user who still has autonomous walking ability, the exoskeleton should follow the motion intention of the user, and \"assist-as-need\" have turned out to be a more effective method for balance assistance [12]. \"Assist-as-need\" means, for balance assistance purposes, an exoskeleton provides assistance only when the upcoming balance loss is detected. Therefore, a real-time balance assessment method is needed to evaluate the degree of balance and detect the moment of balance loss. When balance loss is detected, the exoskeleton will provide appropriate assistance power according to the degree of balance and the current motion state of the user. Hence, organic fusion of the human balance assessment method and the exoskeleton balance recovery control are the assurance of balance recovery. Many studies have focused on fall detection and have some achievements in the detection method based on visual detection and wearable sensors [13]. However, fall detection is different from balance loss detection, the detection purpose and detection time are different from each other. Fall detection aims at judging whether the human is falling down and when it happens a warning signal is sent out for rescue. Conversely, balance loss detection aims at judging whether a human has a trend of falls and when a trend is detected a signal will be sent out to the assistance devices to help the user prevent falls. Some balance loss detection methods based on zero moment point (ZMP) [14], capture point (CP) [15], and adaptive oscillators [16] were proposed. There are some limitations in these studies. ZMP method is not suitable for walking on the uneven ground. CP method is a powerful tool to assess balance degree during standing, but is not enough to assess balance degree during dynamic walking. Adaptive oscillators are only suitable for periodic gait. Current balance assessment methods ignore the position and velocity of the swing foot. Because the swing foot is the future support foot, the position and velocity of the swing foot are important for balance recovery.