The present invention relates to electrically operated valves and particularly valves operated by an electromagnetic actuator of the type employed for flow control proportional to the level of an electrical control signal. Electrically operated proportional flow control valves are used in emission control systems for automotive engines and, in particular, are employed for electrically controlling the purge flow of fuel vapor from a charcoal filled storage canister into the engine intake manifold. In such applications, it is necessary to control the purge flow of fuel vapor from the canister electrically because the flow of fuel vapor from the canister can drastically alter the fuel air ratio of the engine in certain operating regimes; and, in some cases the purge vapor flow can cause erratic engine operation. The problem is particularly acute at low flow rates encountered at engine idle where the vapor flow from the storage canister can be a substantial portion of the fuel supply to the engine, and if not carefully controlled can result in engine stalling.
Heretofore, known electrically operated control valves utilized for controlling fuel vapor purge flow from a storage canister to an engine have utilized linear solenoid operators for effecting movement of a valve member for controlling the vapor flow from the canister to the engine intake. An example of this type of canister purge flow control valve is that shown and described in U.S. Pat. No. 5,551,406 issued to Everingham, et al.
Valves of the aforesaid type are typically operated by a linear solenoid and have the disadvantage of being erratic in at low-flow rates as shown in FIG. 8 where the upper set of curves for each of three levels of engine manifold vacuum are plotted as the purge vapor flow rate versus duty cycle of the electrical control signal. Furthermore, the linear solenoid valves of the prior art have been found to be particularly susceptible to vibration along the critical axis, namely the axis of the solenoid coil for a linear solenoid; and, such known valves have been found to be quite erratic over the range of vibration frequencies encountered in typical automotive applications as shown in the graphs in FIG. 10.
Thus, it has long been desired to find a way of electrically controlling fuel vapor canister purge flow to an engine in a motor vehicle application in a manner which provides linear or proportional control of the flow with the electrical signal and which is accurate at low flow ranges and is resistant to vibration.