Variable force solenoid

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A variable force solenoid (VFS) is an electro-hydraulic device that controls pressure proportionally or inversely proportionally to a signal (voltage or current) obtained from the powertrain's on board controller. A low-flow VFS is used as a signal level devices for transmission line pressure control or application of clutches. A high-flow VFS controls line pressure directly or are used for direct shift clutch control. A VFS is a type of Transmission Solenoid. On or more VFS can be used in an automatic transmission or installed in an automatic transmission valve body

[edit] Theory of Operation

A variable force solenoid includes:

• Actuator shaft that is aligned with the central axis of the valve and solenoid
• A electrical conducting wire core
• A cylindrical core through which the wire is wound that causes an induced magnetic flux flow
• An actuator that actuates a shaft spaced by a non-functional (fixed) air gap from the case and moves axially from the edge of the core
• Fixed Bearings at the opposite ends of the actuator shaft and compressed spring
• A valve spool that controls output flow and pressure

There are two types of forces applied to the valve. One is a spring (compressed) force on the spool valve. The other is a magnetic force developed by applying a current through the wound coil. When the magnetic force is applied causing an actuation that compresses the spring on the opposite end of the spool valve. This change in spool valve position cause an alignment or mis-alignment which translated to a change in pressure. A normally open valve reduces in output pressure as current is applied (increasing) and closes the valve. A normally closed valve increased in pressure as the current applied increases. Depending on the design of the magnet a residual magnetism may exist after current is removed causing a "sticking" phenomenon.