Sliding Mode Control of an Electro-Pneumatic Clutch Actuator
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This report investigates proportional-derivative (PD) controller and different versions of sliding mode controllers, including a 2-sliding mode controller invented by Arie Levant, applied to a pneumatic actuator on a truck clutch, The purpose of the clutch system is to develop a transmission system consisting of a normal clutch and transmission controlled automatically as an automatic gearbox, called automated manual transmission. The goal is to increase driver comfort and performance, as well as reduce fuel consumption. It is put an effort in implementing an accurate simulation model of the clutch system in Matlab Simulink. The model output includes clutch position, velocity and acceleration, actuator chamber pressure and temperature. The accuracy of the model developed is assumed to be accurate enough for control design. The only measurement available is position measurement, because more sensors increase cost. The measurement noise is low, which enables direct use of the position measurement for control. For the controllers and other parts of the control system that is dependent on velocity, acceleration or pressure estimates, the measurement has to be differentiated. Differentiation of noisy signals is problematic, therefore filters have to be used. In this report a first order low pass filter differentiator is compared to a robust differentiator, which is inspired by higher order sliding modes and developed by Arie Levant. The reason for comparing it with a very simple filter is the simplicity of the first order filter. It is easy to understand and tune. The performance of the robust differentiator is in this application not better than the first order filter. Therefore the first order filter is used in the tests. A simplified version of the simulation model is used in the design of the controllers. A PD controller with limited derivative action is tuned on the basis of a linearized version of the control model. PD controllers have turned out to perform well and is suitable for comparison, because they are independent of measurement filters, well known and have well established design methods. The PD controller is compared to different sliding mode controllers. The most promising sliding mode controller, which is a boundary layer controller with variable boundary layer width, is tested thoroughly on the simulation model. Different tests where the simulation model parameters are altered, are performed to investigate the robustness and performance properties of the controllers. The most promising sliding mode controller were supposed to be tested against the PD controller on a test truck at Kongsberg Automotive. New and faster valves were supposed to be tested in the truck. Unfortunately they did not arrived in time for the test and in addition, the driver circuit of the older and slower valves broke down under the test startup. Therefore the real tests could not be accomplished. A brief overview of the planned field tests is given. The PD controller and sliding mode controller are compared in a view focusing on robustness. It is found that the ideal sliding mode controller is highly robust, but not usable in practise for this application. The developed variable boundary layer sliding mode controller performs better than the PD controller on the simulation model. The reason may be that it is tailored to the reference trajectory used, as opposed to the PD controller which is tuned using Bode diagrams, gain and phase margins. Both controllers possess approximately the same robustness properties.