Abstract
Nowadays, because of the current air pollution problems from automobiles in large cities, it is expected that low emission vehicles such as electric vehicles or hybrid electric vehicles (abbreviated as HEV) will be popularized.
This paper focuses on designing the controller of a parallel HEV. When designing the HEV it is important to consider how to use both the electric motor and Internal Combustion (abbreviated as IC) engine effectively from viewpoints of quick transient response as well as low exhausted gas. Some hybrid control systems have been developed and used in commercial use from such viewpoints. But it is not theoretically clear how optimally the control system can be designed to improve the transient response against arbitrary pedal input.
In this paper, from the control engineering aspect, the control system of the parallel HEV is mathematically formulated and a new design method of the hybrid control system is proposed. By using model matching control theory, the new controller is designed to realize a desired transient response model explicitly by feedback and feed-forward compensation. The model matching control method is actually utilized by an assisting electric motor by which the desired traction torque can be obtained with the combination of IC engine torque and electric motor torque. Since the time delay of electric motor is much shorter than that of IC engine, the electric motor assists the IC engine to achieve the desired traction torque response during the initial moment of acceleration especially when a high power is required. To make the combination of output torque follow the desired torque, a feedforward compensator with respect to the pedal input and a feedback compensator depending on state deviation are introduced. Consequently, the control input of electric motor consists of the input signals from each compensator. For designing the low cost hybrid control system, it is necessary to estimate the output torque instead of measuring it directly. According to the observer theory, the out put torque can be estimated by measuring the rotational speed, the pedal input, and the control input. As the IC engine has a speed-dependent torque characteristic, the output torque observer with the variable gain depending on rotational speed is proposed in this paper.
Computer simulation and the proposed Hardware -In-the-Loop simulator based on full vehicle model are carried out to verify the control effect on driving performance as well as the effectiveness of the proposed hybrid control system.