Interpreting Accuracy Specifications

In OGP measuring machines, the error of indication for size measurement, E_x, is related to MPE E, which is from ISO 10360-2 where it is referred to as the maximum permissible error of indication for size measurement. However, due to the unidirectional nature of some sensors i.e. video and laser, multisensing systems require an axis-dependent specification. OGP uses subscript 1, 2, and 3 as such designators. Thus the convention OGP uses is derived from ISO, but more specifically takes into account the number of axes relevant to the sensor.

OGP accuracy specifications are expressed in the following form:

E_x = [k + (multiplier * L) / 1000] µm

E = the maximum measuring error, in microns, under the given conditions
x = 1, 2, or 3 referring to a linear accuracy, planar accuracy, or volumetric accuracy, respectively
k = systemic or inherent machine error that is not length dependent
multiplier = a constant that defines the travel-dependent error
L = the length of travel over which the accuracy specification is desired, in millimeters
µm = microns, the accuracy unit of measurement

In all cases, the result is +/-

E₁ is the linear accuracy in a particular axis - X, Y, or Z

NOTE: A specification shown as "E₁ X,Y" with a comma delimiter between axes, means a linear accuracy specification that is the same in the X and Y axes

E₂ is the accuracy within a plane, typically the XY plane.

E₃ is the accuracy within an XYZ volume; sometimes referred to as volumetric accuracy.

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