A valve that can be positioned anywhere between fully closed and fully open by means of an incorporated drive motor.
A typical application is temperature control in a furnace using gas burners. In most cases a controller directly energizes the ‘open’ and ‘close’ motor windings using three point stepping control via two relay outputs, but some modulating valves have positioning circuitry that will accept a linear (mA or VDC) control signal. This circuitry energizes the motor until the valve is positioned proportionally to the signal. In either case, the controller would normally use PI control.