How does it work?

The working principle is as simple as it is effective. The AB-QM consists of two different parts: the control valve and the differential pressure controller (orange). The integrated membrane of the pressure controller maintains a constant differential pressure across the control valve. The flow through a valve is determined by the Cv value and the differential pressure across the valve. Because the differential pressure is now constant, excessive flows are prevented and the valve authority of the AB-QM has a linear characteristic.

Figures 1-3 (shown above)
In the illustrations you can see the membrane in action. If the differential pressure across the valve increases, the membrane will immediately be pushed down and close the pressure controller (2).
If the differential pressure decreases, the membrane will instantly move up again (3).

AB-QM conclusion:
Constant ∆p across the control valve, resulting in accurate flow limitation and 100% authority.