A range of signal change that produces no movement at the valve at all.
Trend showing backlash: the process variable stays flat while the controller output reverses, then follows once the deadband has been taken up. Flat spots on the process variable exactly where the output changes direction. The width of the flat spot is the size of the deadband. This is lost motion in the linkage or the actuator coupling, not a tuning problem.
What it means
Deadband is the amount the controller output has to change before the valve responds. It is most visible on reversals, where lost motion in the linkage has to be taken up before the stem moves in the new direction.
On a trend it shows as a flat section on the process variable occurring exactly where the output changes direction. The width of that flat spot, as a percentage of output travel, is the size of the deadband — a number worth measuring, because it is what the mechanical trade will want.
Trend showing backlash: the process variable stays flat while the controller output reverses, then follows once the deadband has been taken up. Flat spots on the process variable exactly where the output changes direction. The width of the flat spot is the size of the deadband. This is lost motion in the linkage or the actuator coupling, not a tuning problem.
Also called
On a plant you will hear this called backlash, lost motion. They mean the same thing.
Two photos, thirty seconds, no signup. The pre-flight check finds the structural
problems that no amount of tuning will fix — before you touch the controller.