Proportional Valve Deadband
The central region where no flow leaves the valve although the command signal rises; it comes from spool overlap and feels like a late-starting movement.
The deadband is the central region in which no flow at all comes out of the valve even though the command signal has begun to increase. Its source is that the spool covers the port openings somewhat in neutral: until the overlap distance has been travelled no port opens and that part of the signal goes to waste. The operator feels this as the movement starting late after the lever has been pushed.
Why it is left there deliberately
The overlap is not a manufacturing defect but a design feature that has been chosen. The ports staying partly closed in the central position keeps the load from creeping on its own in neutral and lowers the leakage. The price of it is the deadband. The detail of this trade-off is covered in the entry on spool overlap, and the spool itself in the entry on the valve spool.
What changes with the type of overlap
| Overlap type | Deadband | Leakage in neutral | Typical use |
|---|---|---|---|
| Positive overlap | Wide | Low | Most mobile equipment |
| Zero overlap | Almost none | A constant small leakage | Servo type valves |
The deadband is measured on both sides; the forward and the reverse direction may not be equal. That is why the compensation is generally set separately per direction. Setting both directions at once with a single value most often over-compensates one direction and under-compensates the other, and it looks like the machine rising more quickly to one side.
How it is noticed in the field
When working with a wireless proportional remote control, the deadband shows itself as the machine staying silent when you push the lever half way. Do not take this for a fault in the remote control: the movement starting late is most often the normal characteristic of the vehicle's own valve. The practical way of telling them apart is to push the lever very slowly and note exactly where the movement starts. If the starting point is in the same place on every attempt you are talking about the deadband; if it shifts on every attempt, the trouble is more on the hysteresis side.
There is also a way of measuring it that can be applied in the field: advance the lever from the centre in very small steps and mark the first point at which the movement starts, then repeat the same operation in the reverse direction. The region left between the two points is the deadband of that valve. This region changes somewhat as the oil warms up; that is why you should check a setting made on a cold machine once more on a warm one.
What the compensation does and does not do
Deadband compensation skips the wasted region by adding a starting value to the drive signal as soon as the lever leaves the centre. In this way the first movement of the lever and the first movement of the load come closer together. What it cannot do is just as clear:
- It does not change the physical overlap of the valve; it only applies an offset on the signal side.
- Compensation given in excess makes the load fail to stop at once when the lever returns to the centre; while the compensation is being raised, the behaviour at the centre has to be watched too.
- On work where a deliberate dead zone is wanted at the centre, opening the compensation right up is not the right thing to do; it is that region which keeps the lever from producing a command on its own in a cabin full of vibration.
You can find the order of adjustment and the measurement step by step under the heading how the dead zone compensation is done.
What to do in practice
On a new machine determine the width of the deadband in both directions by your own observation first, then raise the compensation separately per direction and check the stopping of the load at every step. Make this observation before changing the remote control on a complaint that the movement starts late; the source is most often the valve itself.
The context in which this term is used: Why should it be commanded by an electronic signal?