A solenoid valve is either on or off, open or closed. It can work with different media, operate on many different voltages, have different configurations, different diameters, be normally open or normally closed, but it’s always either on or off.
A Proportional Solenoid Valve opens in steps, depending on the voltage applied to the solenoid, which works against a spring to position the spool in different degrees of “openness”. Because flow, pressure, temperature, and fluid cleanliness can change constantly, a given input voltage may not always produce the same spool position. To resolve spool position accuracy, a linear variable differential transformer (LVDT) may be used. An LVDT electronically compares the input signal with spool position and modifies voltage to give the same spool position regardless of system changes. An LVDT adds cost to the valve and the electronics, but is usually necessary in all but the most simple applications.