# Design of Constant Speed Controller for Hydraulic Retarder Based on Robust Control
## 5 Conclusions  
For the constant-speed braking control of the hydraulic retarder when the vehicle is travelling downhill for a long period of time, this research proposes a constant speed control strategy based on sliding mode control theory, establishes the vehicle dynamics model and the constant speed control model of the hydraulic retarder under the MATLAB/Simulink environment, and carries out simulation tests, which verify the control effect of the controller. At the same time, the controller designed in this research is simulated and compared with the fuzzy controller. The subsequent conclusions are drawn from the simulation research conducted:  
1. When the heavy-duty vehicle with constant mass is driving on a ramp with a fixed gradient of 6%, the fuzzy controller demonstrates high speed control accuracy with a maximum speed error of 0.0842 km/h. However, when the system is subject to parameter uncertainty, especially when the vehicle mass and the road gradient vary at the same time, the fuzzy controller has a long response time, and the maximum speed error grows to 0.37 km/h, with speed control accuracy that is low. At the same time, the retarder filling rate of the control output also swings to varying degrees. This is owing to the fact that the fuzzy controller relies on empirical and heuristic principles, has weak adaptability to non-matching disturbances, and has a considerable difference in anti-interference characteristics compared with the other two controllers under dynamic settings. 2. The sliding mode controller with saturation function shows good speed tracking performance under three simulation conditions, but its control output retarder filling rate signal produces obvious vibration when the vehicle speed reaches near the target speed or the system parameter changes, which is not conducive to the regulation of the hydraulic retarder filling rate in reality. Through simulation analysis, we find that although the saturation function can suppress part of the vibration in traditional sliding mode control, the selection of the boundary layer thickness is more dependent on the system model, and if the value of the boundary layer thickness is too large, it reduces the robustness of the sliding mode control with the saturation function, and if the value of the boundary layer thickness is too small, it increases to increase the degree of vibration in the control output. 3. The high-frequency controller has good robustness in the presence of dynamic disturbances in the system. When the vehicle mass and road gradient change, the high-frequency controller speed fluctuation is small; the root mean square error of speed is 0.0683 km/h, compared with the fuzzy controller's root mean square error of speed of as high as 0.184 km/h. The high-frequency controller under dynamic conditions of interference resistance has better performance. In terms of control output, the high-frequency controller, compared with the sliding mode controller with saturation function, has less fluctuation of the output filling rate when the system parametersare changing and is able to suppress chattering while maintaining strong robustness against perturbation and parameter uncertainty. Overall, the high-frequency controller is able to achieve fast reaction to changes in system parameters and has strong anti-interference performance under dynamic settings and is suitable for solving the constant-speed braking problem of heavy vehicles travelling downhill for long periods of time. In future work, the joint braking of the service brake, hydraulic retarder, and ABS (Anti-lock Braking System) under various braking conditions will be studied to further improve the control strategy of the vehicle braking system so as to improve the braking safety of the vehicle, and more control methods and experiments of constant-speed braking of the retarder will be considered. [PERSON] and [PERSON] conceived and designed the study. [PERSON] performed the controller design, carried out simulations, and wrote the original draft.