1. Field of the Invention
The invention relates generally to an electric pump system, and more specifically to an electric pump system that is suitable for use in a vehicle that has an idling stop function.
2. Description of Related Art
There is a vehicle that has a so-called idling stop function of automatically stopping an engine when the vehicle temporarily stops for parking or temporarily stops at a red light, and automatically starting the engine in response to a driver's vehicle starting operation. In the vehicle that has such an idling stop function, an idling time is reduced and therefore the fuel efficiency is improved.
A mechanical pump that is driven by the engine is usually mounted in the vehicle, and hydraulic fluid is supplied from the mechanical pump through an oil passage to hydraulically-actuated devices such as a transmission mechanism. Therefore, when the engine is automatically stopped by the above-described idling stop function, the mechanical pump is also stopped. In this case, when a driver attempts to start the vehicle after a restart of the engine, the hydraulic pressure that is required to actuate the hydraulically-actuated device may not be ensured and a shock may occur in the vehicle.
In a vehicle described in Japanese Patent Application Publication No. 2002-206634 (JP 2002-206634 A), a hydraulic pressure sensor that detects the hydraulic pressure in an oil passage and an electric pump system that uses a motor as a drive source are provided. While an engine is automatically stopped by an idling stop function, if the hydraulic pressure that is detected by the hydraulic pressure sensor becomes lower than or equal to a predetermined value, the hydraulic pressure that is required to actuate a hydraulically-actuated device is ensured by supplying hydraulic fluid from the electric pump system to the hydraulically-actuated device. In this way, it is possible to reliably ensure the hydraulic pressure that is required to actuate the hydraulically-actuated device at the restart of the engine. As a result, it is possible to suppress the above-described shock.
In order to further reliably suppress the shock, it is an effective way to control the hydraulic pressure of the hydraulic fluid, which is supplied from the electric pump system to the hydraulically-actuated device, to a value within a predetermined range of hydraulic pressures at which no shock occurs. Therefore, drive control on the electric pump system may be executed such that the hydraulic pressure that is detected by the hydraulic pressure sensor falls within the predetermined range.
On the other hand, the hydraulic pressure of the hydraulic fluid that is supplied from the mechanical pump to the hydraulically-actuated device is generally higher by several tens of times than the hydraulic pressure of the hydraulic fluid that is supplied from the electric pump system to the hydraulically-actuated device. Therefore, if the resolution of the hydraulic pressure sensor is set to a resolution corresponding to the hydraulic pressure of the mechanical pump, it is difficult to highly accurately detect the hydraulic pressure of the hydraulic fluid with the use of the hydraulic pressure sensor for the mechanical pump. Therefore, when drive control on the electric pump system is executed on the basis of the hydraulic pressure that is detected by the hydraulic pressure sensor for the mechanical pump, the hydraulic pressure of the hydraulic fluid may increase more than necessary and electricity may be wastefully consumed by the electric pump system.