Open Loop Valve Control
A control method that drives the coil with a calculated current without measuring spool position. It is cheap, simple and has a narrow failure surface; in return the result is exposed to oil temperature, load and valve wear, because there is no feedback measuring what the system actually does.
In open loop the controller assumes: "if I supply 700 mA the spool opens this much, so this much flow passes." If the assumption holds, the system works properly. The cases where it breaks down are well known:
- When the load changes the pressure differential changes, and a different flow rate passes through the same opening.
- When the oil is cold the viscosity is high; motion starts slower than expected.
- As the spool and body wear, the characteristic drifts over the years.
Even so, the vast majority of mobile equipment is open loop, because the operator already looks and corrects with the lever, meaning a human closes the loop. With current controlled drive, dither and deadband compensation added, open loop behavior becomes more than sufficient for most work. The decision to move to closed loop is usually made when there is a requirement for automatic cycling, remote control or millimeter level repeatability.
Where this term is used: The Importance of Hydraulic Control: Why Should It Be Commanded by an Electronic Signal?