1. Field of the Invention
The present invention relates generally to a solenoid valve. More specifically, the present invention relates to a method and apparatus for controlling the solenoid valve to achieve a desired flow rate.
2. Description of the Related Art
Solenoid valves are used in a multitude of various operations. One use is the automotive industry wherein a solenoid valve is used in conjunction with a vapor canister in a vehicle emission system. For instance, under normal operating conditions, fuel vapors from the vehicle's fuel tank are stored in the vapor canister. The canister is purged by drawing fresh air through the canister into the intake manifold of the engine. Purging the canister disrupts optimum air-fuel ratio and may result in inefficient operation of the engine. Thus, a solenoid valve is used to control the flow rate of fuel vapor being drawn from the vapor canister into the intake manifold.
Modern engines are tightly tuned for optimum operating performance. The amount of canister purge vapor entering the intake manifold is controlled by the solenoid. The solenoid valve is turned on and off or cycled based on various operating parameters. The duty cycle or percentage of time that the solenoid valve is open regulates the flow of fuel vapors being purged from the canister.
Various types of control systems to control or regulate the desired flow of fuel vapor from the vapor canister are known. One type of system controls operation of the solenoid valve through a duty cycle pulse width modulation. Duty cycle pulse width modulation control systems use from 5 to 100 percent of the duty cycle to vary the flow. Such systems fail to provide the flexibility necessary to control and, more specifically, regulate flow at both the low and high ends of the duty cycle. For instance, the slope of the flow rate versus percent duty cycle for optimum low end control may not be suitable for high end flow and vice versa.
Thus, it is advisable to have a control system which provides optimum flow control characteristics throughout the entire range of the duty cycle. For instance, the flow rate may be modified independently of the duty cycle wherein the overall slope of the flow rate with respect to the duty cycle may change for various percentages of duty cycle.
Another type of control system uses a current signal to actuate an armature so that flow is proportional to current. These control systems tend to have more hysteresis and are not useful for the initial 25 percent of the full scale signal.