## 1 Introduction
To enhance robustness against parameter variations and disturbances, two model-free control system structures that include sliding-mode control were proposed and verified by real-time experimental results related to the position control of laboratory equipment represented by the twin rotor aerodynamic system in Reference [18]. In Reference [19], adaptive second-order sliding-mode control strategies were presented for a class of uncertain systems with unknown uncertainty bound. The authors adjusted the amplitude of the control law to control the uncertainties, enhanced the realization of second-order sliding mode, and made the state variables of the auxiliary system converge near the origin in finite time. In Reference [20], time-varying and constant switching frequency-based sliding-mode control methods were proposed for three-phase transformerless dynamic voltage restorers. The authors of Reference [21] proposed a new adaptive fuzzy sliding-mode control (AFSMC) design strategy for the control of a special class of three-dimensional fractional order chaotic systems with uncertainties and external disturbance and showed the efficiency of the proposed fractional adaptive controllers. Although SMC has strong robustness to parameter perturbation and external disturbance, it is worth noting that the above control methods are all based on measuring the speed and acceleration of signals, and the accuracy of measurement is vital for control results. However, in practical engineering, the measurement process has a certain degree of difficulty and the economic cost of the process is higher; therefore, the control method still has some limitations and challenges in practical situations. To solve these problems, some scholars have proposed a high-gain observer (HGO) control strategy [23; 24; 25; 26; 27]. In Reference [23], an adaptive HGO was established to estimate the unmeasured states for the output-feedback control problem of a class of nonlinear large-scale system. In Reference [24], a decentralized control algorithm was developed based on HGO theory and fuzzy adaptive control algorithm for second-order uncertain nonlinear multiagent systems. In Reference [25], [PERSON] presented a sensorless control based on a HGO to reduce the effects of uncertainties on the electromagnetic valve actuator model and unmeasurable external disturbances. To solve the problem of single-rod electro-hydraulic servo having only one measured output state, a nonlinear backstepping control method based on a HGO was proposed to estimate the full state used by backsteppingcontrollers in Reference [26]. The authors of Reference [27] proved that the extended HGO can be used to reduce the effect of disturbances while estimating and canceling the disturbance in one link of an underactuated system. Some scholars have combined SMC with HGO to achieve a better control effect. In Reference [28], a method was proposed for a flexible joint robotic arm, which used a high-gain observer with multiple sliding modes to ensure the observability of fault/unknown input signals, and sliding modes were utilized for their reconstruction. Reference [29] focused on the design of a high-gain observer that estimated the unmeasured states to get the knowledge of all state variables for sliding-mode controller in order to control tracking position and pressure. In the particleboard glue mixing and dosing system, the technical requirements differ with the thickness of the particleboard. When the thickness of the particleboard changes, the flow of the particleboard and gluing also changes. Meanwhile, the frequency of the system input can be adjusted to change the speed of the motor, and the speed of the dosing can be controlled to accurately control the dosing of the glue.