LVDT Feedback
A linear variable differential transformer that measures valve spool position magnetically, without contact. It gives micrometer-level resolution without friction or wear; when this signal is returned to the driver, the valve runs closed loop rather than open loop.
In an LVDT a primary winding is excited, two secondary windings are connected in opposition, and the output voltage varies with the position of the movable core inside them. Because there is no contact, its life is far longer than a mechanical potentiometer's and it withstands vibrating, dirty environments.
- The output gives both the magnitude and the direction of position; at center the signal is zero.
- Because a closed loop valve compares measured position against the command and continuously corrects current, hysteresis typically falls below the one percent level.
- Temperature, oil viscosity, and supply variations can no longer shift spool position; the loop suppresses them all.
The cost of this is expense and complexity; that is why most mobile equipment settles for open loop current control. LVDT-equipped valves are found mainly in stationary industrial presses, test rigs, and machines where position accuracy is critical.
Where this term is used: Connect Plus and Minus, Run: Installation That Does Not Depend on Technical Staff