Why use a three-way directional control valve in automatic machine control?
It is useful in many pull-behind scraper applications.
The three-way spool - circled in red in the above graphic - powers the cylinder only in the extend direction. Gravity retracts the cylinder rod.
Center position of the spool is "B" blocked/"A" open to tank. The spool will prevent the ground from retracting the rod, but if the ground level "falls away," the rod can "float out" or extend.
When a "lower" correction is called for by the controller, the cylinder raises the wheels, which makes the cutting edge go down. If the four-way contol valve remains shifted too long, the wheels raise off the ground. This is self-defeating because:
1. The blade is already on the ground so raising the wheels father cannot move
the blade down any farther.
2. When a "raise-the-blade" correction is sent from the controller, the wheels
start to move down, but it will take some time for them to get to the ground.
The cutting blade cannot begin to raise until the wheels touch the ground, so
the blade position correction is delayed because the wheels had been raised
farther than necessary.
What is the significanct of a three-way valve in this situation?
The valve is used to power the blade up - that is, to power the wheels down, which raises the blade - but gravity is used to push the bucket and cutting edge down. The wheels never leave the ground because they are not powered in the "wheels up" direction.
The cylinders in the photographs above will be powered in the "extend" direction, but not in the "retract" direction.
One valve port - "A" or "B" - is permanently open to tank. When the spool is centered, the cylinder will be held in its loaded direction, but it can "float" in the other direction.
Cylinder speed can be proportional in both directions of travel, if a proportional solenoid is used.