The controllability of very small flow rates in continuously adjustable valves requires special measures on the control edges. In servo valves straight control edges with high precision are used (zero cut, (Figure F 7)) that provide a linear opening stroke characteristic curve.
However, a progressive characteristic is better for fine control of flow, as is achieved approximately with a chamfered edge. Triangular notches are even better; these provide a significantly better resolution at low cross-sections.
Figure F 7: Different control edge forms with characteristic curve