# Design of Hybrid Phase Sliding Mode Control Scheme for Lower Extremity Exoskeleton
## 1 Introduction  
The lower extremity exoskeleton is a wearable device with an active actuator that provides active torque to the operator and helps the operator to accomplish subjective willing movements. Exoskeletons can be used in many fields, such as military [1; 2], construction [3; 4], service [5; 6], and medical [7; 8; 9], which have attracted scholars' extensive attention from all over the world [10]. In recent decades, different research institutions have adopted different control mechanisms for the exoskeleton [11; 12], the Hybrid Control Strategy [13] among them is applied in exoskeleton systems for Human-Performance and obtains improved performance successfully. In BLEEX, a hybrid control strategy [14] is put forward to control the lower limb exoskeleton in the swing phase or stance phase respectively, where the sensitivity amplification controller (SAC) is employed for the swing leg, the position control strategy is applied for the stance leg. Another Hybrid Control Strategy is presented to control a hydraulically actuated lower limb exoskeleton, where the \(H_{\infty}\) control method is designed to improve the performance under all kinds of uncertainties. Besides, the control algorithm for switching is used to reduce the delay of exoskeleton tracking [15]. For a Load-Carrying lower extremity exoskeleton, the position control approach is applied in the stance phase, and the following control method is used in the swing phase [16]. In [17], for different dynamic models of a gaitcycle, a hybrid phase control method is described to determine the phase by the Back F/T sensor and Foot F/T sensor. In [18], a control mechanism is proposed, where the stance phase adopts active control, the swing phase then switchs to follow-up control, and the foot sensor performs the phase discrimination. Most hybrid control strategies adopts a phase-dependent control strategy to control the corresponding phase model, which usually depends on the characteristics of the lower extremity exoskeleton. The lower extremity exoskeleton model has the nonlinear and strong coupling characteristics [19], in addition, the exoskeleton systems exist uncertain disturbances such as parameter changes and extern interference caused by the working environment. Hence, it is not suitable to achieve excellent control performance by applying traditional control methods [20]. In [21], the adaptive iterative learning control (AILC) method is proposed, which can effectively reduce such error without sacrificing the iteration efficiency. In [22], a model-free control method is presented to estimate the knee joint angles of the exoskeleton. Then, to improve the control precision and robustness of the system, the discrete sliding mode control law and prior torque are introduced. In [23], an improved single-input fuzzy sliding mode controller together with an adaptive switching controller is proposed to promote the tracking performance of the system. Because the system is affected by interference, the rejection control [24] is regarded to overcome the active interference and applied it to track the human gait trajectory, in that the extended state observer is designed to estimate the total uncertain disturbances. In [25], the sliding mode controller based on the extended state observer is proposed to solve the tracking control problem of the double joint dexterous hand and improve control performance when compares with a single sliding mode controller. With aforementioned, this article focus on the hybrid phase control and the phase switching problem. The block diagram of the whole control system is shown in Figure 1. The main innovations are as follows: first, assuming that the human body moves at a constant speed in the horizontal direction, according to that pneumatic lower extremity exoskeleton structure, the dynamic models are established in the swing phase and stance phase respectively using the Euler-Lagrange formulation. After that, the SMC based on ESO is designed to fit to the working environment and characteristics of lower extremity exoskeleton.