Servo control design for electronic throttle valve with nonlinear spring effect

The electronic throttle control (ETC) is a key component in modern automotive, which affects the performance of traction control and engine torque control. An ETC control system consists of a DC servo motor driving a circular-shaped valve plate, which is connected to a preloaded spring. The opening angle of the valve plate controls the air flow into the engine. When the control system fails, the spring can keep the valve at a pre-defined angle, i.e. the limp-home position, in order to provide the necessary air flow to keep the engine running. The ETC is commanded by the engine control unit (ECU) to achieve economic fuel consumption and the required performance. The spring in the ETC unit exhibits a piecewise linear torque-displacement relationship due to the preload. The stiffness around the limp-home position is much higher than that in other area in order to withstand the vacuum pressure in the engine cylinder. There are two major nonlinear effects in an ETC system: one is the nonlinear torque-displacement relation resulted from the preloaded spring; and the other is the friction force between the valve plate and the manifold. In order to improve the performance of the current ETC system, we develop a mathematic model for the spring-loaded ETC system and adopt a closed-loop identification approach to obtain the nonlinear displacement-torque function in the complete working range. To design the servo control system for the ETC, we analyze the nonlinear spring effect on the ETC system robustness, and found that a control design based on the low spring constant area is robust even when the spring constant becomes ten times larger around the limp-home position. We design and implement a two-degree-of-freedom control system, the experiment results show very good command response regardless the preload spring effect and excellent disturbance rejection performance against the friction force.