Why does deadband occur in a proportional valve and how is it compensated?
In spool-type proportional valves, overlap is left at the center position to prevent leakage. No port opens until the spool passes that overlap, so nothing moves for a while even though the joystick has left center. Compensation is done by setting a minimum current in the controller, jumping the spool straight to the threshold where movement begins.
Deadband is not a defect but a design choice. The overlap between spool and body prevents leakage from the cylinder ports to tank and the load creeping on its own while the valve is centered. The cost is an unresponsive region around center.
From the operator's point of view the symptom is clear: you push the lever, nothing happens for a while, then movement starts abruptly. This makes fine approach almost impossible.
The solution is the minimum current parameter in the controller (called I-min or deadband compensation on some cards). The moment the joystick leaves its deadband, the output starts not from zero but from the current value at which the spool is about to begin opening. Summary of the setting procedure:
- The oil is warmed and the system brought to normal operating temperature.
- While the joystick is moved very slowly, the first current at which the actuator stirs is found.
- The minimum current is set just below that value.
If you set it too high the effect is reversed: the actuator jumps as soon as the joystick leaves center. That is why compensation is left slightly below the threshold where movement begins.
Full coverage of this topic: The Importance of Hydraulic Control: Why Should It Be Commanded by an Electronic Signal?