The first level involves modelling the actuators. Lower limb rehabilitation robots often use electric motors and hydraulics. For the motor drive, the servo system is a three-closed-loop control including a position control loop, speed control loop and current control loop, which is generally simplified to a second-order differential link [
40]. For hydraulic systems, the corresponding drive system model is usually set up according to the hydraulic components adopted, such as the general valve-controlled asymmetrical cylinder system [
41]. Both these drives are rigid drives. To improve human–robot collaboration, studies have been conducted on the design of the drive system. On the one hand, pneumatic [
42] drives with stronger flexibility are adopted, and on the other hand, serial elastic actuators (SEAs) [
43] and cable-driven actuators [
12] are adopted. At the level of the drive system, the elastic actuator is modelled. The drive is connected to the load via a compliant element. The drive dynamics are represented by the inertia and motor torque. According to the structure designed by SEA, the corresponding actuator modelling can be obtained by considering friction and other links [
44]. The distribution of driven cables is various, potentially satisfying different requirements of the robot and obtaining better performance. More emphasis should be put on the unidirectional characteristics and the coupling relationship of cables [
5]. In the process of driving modelling, the most important problem is the identification of system parameters and the accuracy of the model that affects the control effect. Another aspect is the modelling of the robot system level. Lower limb exoskeleton rehabilitation robots generally adopt serial structures after simplifying the joints of the human lower limbs [
2]. In recent years, parallel structures have also been reported based on further studies of real human movement [
8,
45,
46]. The series and parallel structures are rigid structures. The Lagrange method [
47] and Newton‒Euler method [
48] can be used to establish dynamic models for rigid-body dynamics modelling, which is widely used in industrial robots. For the lower limb exoskeleton robot in a series configuration, the robot is simplified into a multi-link model, and dynamic modelling of the lower limb exoskeleton robot is carried out [
49]. For robots with parallel configurations, similar ideas as in industrial robots can be used for dynamic modelling [
9].