Conventionally, as disclosed in a patent document, Japanese Patent Laid-Open No. JP H10-19156 A (Patent document 1) listed below, a control unit performs a feedback control for controlling an electric current flowing in a solenoid of an electromagnetic valve.
The control unit described above includes: a switching section (i.e., a Pulse Width Modulation (PWM) circuit) on a power supply path toward the solenoid for flowing an electric current to the solenoid when being turned ON; a detector (i.e., an electric current detection circuit) for detecting an actual electric current value flowing in the solenoid; and a feedback controller (i.e., a microcomputer) for setting a duty ratio, which allows the detected actual electric current value to follow a target electric current value, and generating a PWM signal having the set duty ratio in a preset cycle for supplying the signal to the switching section.
The oil pressure valve, which is operated by the solenoid, is provided in the hydraulic circuit of the automatic transmission. Therefore, the control unit disclosed in the patent document 1 is used for the control of the automatic transmission. In recent years, the hydraulic circuit has a complicated structure, and a configuration of such structure includes two or more hydraulic valves in the oil circulation portion of the circuit, among which one or more hydraulic valves may be operated by the solenoid.
In such a configuration, a coupled oscillation may occur, which results from an oil pressure effect bouncing around between the multiple hydraulic valves. The coupled oscillation in such a configuration/structure is confirmed by the inventor of the present application. The mechanism of how coupled oscillation occurs in the hydraulic circuit is understood as follows.
The propagation rate of oil pressure affects the characteristic of the coupled oscillation such as frequency, amplitude and the like. The propagation rate is determined by the viscosity of the oil, and the viscosity of the oil changes according to the oxidization of the oil and the environmental temperature of the oil in which the oil is used. Therefore, when the viscosity of the oil changes according to the change of the environmental temperature of the oil, for example, the oscillation of the oil in the circuit may become noticeable (i.e., the oscillation exceeding an allowable level has occurred), thereby coupling the oscillation of many parts of the oil and the circuit to result in the coupled oscillation.
For example, when the hydraulic circuit has three hydraulic valves in the circulation portion, an influence of the operation of the first oil pressure valve is transmitted to the second oil pressure valve through the oil. Therefore, an input pressure of the second oil pressure valve is not stabilized, and a valve position of the second oil pressure valve is not converged (i.e., is not stabilized). Further, an influence of the operation of the second oil pressure valve is transmitted to the third oil pressure valve through the oil. Therefore, an input pressure of the third oil pressure valve is not stabilized, and a valve position of the third oil pressure valve is also not converged. Furthermore, an influence of the operation of the third oil pressure valve is transmitted to the first oil pressure valve through the oil. Therefore, an input pressure of the first oil pressure valve is not stabilized, and a valve position of the first oil pressure valve is not converged.
Thus, the coupled oscillation occurs from the coupling of the effects from each of the hydraulic valves, which is understood as causing a continuous operation of the same valve. That is, as the convergence of the valve position of each of the hydraulic valves stays unachieved for a long time (i.e., the continuous operation of the valve lingers on), and the oil pressure does not really attenuate, causing a continuation of the oscillation of the oil. In such a situation, the controllability of the automatic transmission may deteriorate.