###### Abstract
In order to increase a patient's active participation in exoskeleton-assisted rehabilitation training, it is necessary to control the exoskeleton robot to track the movement of the lower limb as per the patient's active motion intention [22; 23]. Impedance controller or admittance controller have been widely used in exoskeleton control due to their characteristics of human and robot physical interaction [24; 25]. The impedance controller calculates the force exerted by the robot to achieve the specified displacement. Admittance control is based on the impedance control and uses an external force loop and an internal position loop to form the entire controller [26]. [PERSON] et al. [27] adopted an admittance controller to convert the interaction force between the human body and the exoskeleton into the expected trajectory of the exoskeleton, so as to recognize and track the operator's intention. Aiming at the safety of robot-assisted interactive motion of the affected limb, [PERSON] et al. [28; 29] proposed an exoskeleton impedance control method based on safety monitoring, but the active muscle force of the patient's limbs was not considered in human-robot interaction. [PERSON] et al. [30] analyzed the stability conditions of admittance control of a single-degree-of-freedom exoskeleton swinging leg, and believed that admittance control can improve the compliance of the exoskeleton leg [31]. [PERSON] et al. [32] applied admittance control to the active training stage of patient rehabilitation, and adjusted the admittance parameters to meet the different needs for strength and speed during training. [PERSON] et al. [33] proposed that non-linear admittance control can promote a safer human-computer interaction in haptic devices. The above research results show that admittance control can effectively recognize the movement intention of exoskeleton wearers, and the control system using admittance control has a simple structure and is easy to implement. However, the above studies on exoskeleton admittance control have adopted fixed admittance parameters to control exoskeleton, which has limitations in the application of LLREs. On the one hand, when the patient has a negative intention, that is, the direction of the active torque of the lower limb joint is opposite to the preset motion trajectory speed, if the admittance parameters of the admittance controller is too large, that is, the exoskeleton stiffness is too large, the physical interaction between the patient's lower limbs and exoskeleton will increase,bringing discomfort and safety risks to the patient. On the other hand, when the patient has a positive intention, that is, the direction of the active torque of the lower limb joint is the same as the preset movement trajectory speed, if the admittance parameters of the admittance controller is too small, that is, the stiffness of the exoskeleton is too small, the intensity of the patient's rehabilitation training will decrease, leading to a decrease in active participation and thus a reduction in the rehabilitation effect. For the rehabilitation training of patients with dyskinesia that have residual muscle strength in the affected limb, the passive position control scheme for LLRE ignores the active participation of patients in rehabilitation training and cannot fully meet the rehabilitation training needs of such kind of patients. Moreover, admittance control with fixed admittance parameters for LLRE ignores the patients' positive and negative intentions in rehabilitation training, and affects the comfort, safety and active participation of patients in rehabilitation training.