Flow-Signal Curve
The characteristic curve showing the flow passing through the valve for each value of the control signal. The horizontal axis is the signal as a percentage and the vertical axis is flow in liters/minute; the flat section at the start of the curve makes the deadband visible, and its slope makes the valve gain visible.
This curve is the first graph to look at in a valve's catalog, because it contains everything the operator will feel. The three regions of the curve are read separately:
- Starting flat: the deadband. The wider it is, the more lever travel is wasted.
- Middle region: the actual working area. If the slope here is low you get precise control, if it is high you get fast control.
- Upper end: saturation. Flow is now limited by pump capacity or valve cross section; increasing the signal changes nothing.
Catalog curves are measured at a specific pressure differential, mostly around 10 bar per valve. On a real machine the pressure differential is variable; that is why the flow you measure at the same signal can deviate from the catalog value. On pressure compensated valves this deviation is largely eliminated. Knowing the curve explains why a motion sometimes "speeds up suddenly in the middle of the lever."
Where this term is used: The Importance of Hydraulic Control: Why Should It Be Commanded by an Electronic Signal?