Linearity Error
It is the error expressing how far the actual flow-signal curve departs at most from the ideal straight line. It is given as a percentage of full scale; on mobile proportional valves it is typically on the order of a few percent and shows how reliable the mapping between command and result is.
Linearity measures something different from hysteresis. Hysteresis is the difference between approaching the same point from two directions; linearity is the deviation of the curve from the ideal straight line even in a single direction. A valve can have low hysteresis and poor linearity: it behaves the same way every time, but its behavior is not a straight line.
- The deviation usually grows right after the deadband and as saturation is approached; the middle region is the smoothest part.
- Measurement is made at a constant pressure difference; under variable load the actual deviation is larger.
- The deviation can be corrected to a large extent by defining a multi-point curve table in the controller.
In practice the cost of linearity error is that the operator cannot build a single mental mapping between lever angle and speed. Accepting this and deliberately choosing a curve such as parabolic is better than putting up with an irregular deviation; at least the behavior becomes predictable.
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