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Ronchi F.
"Modeling and Control of Automotive servomechanisms"
 
Type PhD Thesis
Author(s) Ronchi F.
Title Modeling and Control of Automotive servomechanisms
Editor University of Bologna
Keywords automotive, servovalve control, hydraulics, camless engines
Abstract
Two main automotive systems are considered in this thesis. The first one is a servo-actuated gear-box, which is becoming a valid alternative to both conventional manual or automatic gear-boxes, since it offer the functionalities of an automatic solution at the cost of a manual gearbox, allowing also for a further system optimization and significant fuel saving. The second project is about the development of control strategies for electromechanical actuated engine valves. Today, it is possible to install on the vehicle the necessary computing power, and the adoption of the independently actuated valves, that can lead to enormous advantages in fuel saving and pollutant emission reduction, is becoming a real possibility. The first system consists in a manual gear shift device with added actuators, controlled via an Electronic Control Unit (ECU), for clutch and gear control. The main drawback associated with this system is the interruption of traction torque occurring during the shifting process, when the clutch has to be disengaged to exit the current gear and entering the new one. To achieve an acceptable comfort, this lack of traction should last as short as possible. The gear-shift operation is managed by a controller, which generates the torque request for the engine and controls both the servoactuated gear-box and the clutch, achieving the proper synchronization. Since the torque transmitted to the driveline during clutch opening and closing heavily depends on the clutch position, the latter must be accurately controlled to prevent unpleasant oscillation due to driveline elasticity. The first part of the thesis focuses on the clutch subsystem modeling, nonlinear control and achievable performances. The main issues to cope with are the strong nonlinearities of hydraulic servovalves and mechanical clutch actuator, which call for accurate modeling to perform good feedforward actions. Feedback control laws have to fight mainly with the several and various uncertainties and environment dependant parameters of hydraulic systems. Accurate models of the clutch system, both for ``simulation'' and ``control'' purposes, are derived and identified in the first two chapters. The following chapters of this part are about the robust control and the performance evaluation of the clutch system, mainly utilizing Lyapunov techniques such as backstepping, nonlinear damping and singular perturbations. The second part of the thesis is dedicated to electromechanical actuators for camless engines. Internal combustion engines traditionally use mechanically driven camshaft to actuate intake and exhaust valves. Their lift profiles are direct function of the engine crank angle and cannot be adjusted to optimize engine performances in different operating conditions. This results in a compromise design that impacts on achievable engine efficiency, maximum torque and power, pollutant emission. Growing needs to improve fuel economy and reduce exhaust emissions lead to the development of alternative valve operating methods, that aim to alleviate or completely avoid the limitations imposed by a fixed valve timing. To fully exploit the possibilities offered by a complete variable valve timing system, camless engine valve-trains, in which the valve motion is completely independent of the piston motion, are currently the topic of an intensive research activity, both from academia and several manufacturers. Electro-hydraulic or electromechanical valve actuators have been proposed for camless engines. The latter type of actuator is considered in this work. The biggest control difficulty comes from the valve seating velocity, i.e. the valve velocity when it comes against the closing position. It should be very low to avoid acoustical noise and wear and tear of mechanical components. Dynamical response of the system, especially the travel time required for a transition between the two terminal positions of the valve, is another key point because the time available for the intake and exhaust process becomes low at high engine speed, thus opening and closing of the valve should be very fast to keep engine performances. In order to become a real commercial alternative, system costs for electromechanical valve control should be kept at reasonable levels and therefore performances must be achieved without a valve position sensor, which has been identified as the most critical element from this point of view. The above factors make the closed loop control system design a very complex task, further complicated by the fact that the system itself is unstable near valve terminal positions. In the first chapters of the second part of the thesis, an accurate model is presented for the system and the flux sensors and position reconstructor are addressed. It is shown how the position reconstructor must be properly designed not to penalize the achievable dynamic performances. Since system constraints make the trajectory design a key point, it is deeply addressed. Finally, a hybrid control capable to deal with position reconstructor characteristics and suitable to be implemented with the trajectory generator is described.
Notes
Dottorato di Ricerca in Automatica e Ricerca Operativa, Universitą degli Studi di Bologna, XVI ciclo. Coordinatore: Prof. Alberto Tonielli. Tutor: Prof. Alberto Tonielli.
Year 2004


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