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Defining Accuracy, Resolution & Repeatability
ACCURACY IS NOT a simple subject. It's not enough to say that a particular scale is "extremely accurate" or "more accurate than another brand. You need to understand all the different terms that help define accuracy and how they relate to one another. Once you achieve this, you will be able to help others understand the concept.
For a DRO system consisting of a readout and encoder(s), it is customary to explain both, the accuracy (of the encoder) and, the resolution (of the display unit). It is essential that the two are not confused.
Accuracy is the relationship of the physical movement of the encoder components to a known measurable standard. It is usually quoted as +/- an amount or TIR (Total Indicator Reading). The effect of both is to produce an "envelope" which is stated by the manufacturer to be the total deviation from an absolute which can be measured by means of length bars, slip gauges, etc. Accuracy is usually related to length (either an 'amount per unit of length' or 'an initial amount plus a further amount per unit of length') and is measured at a particular calibration temperature.
This is normally a function of the DRO and relates to the smallest digit which is shown on the display. It is possible, but unlikely, to have a high accuracy encoder reading through a low resolution DRO where the benefits of the high accuracy are eliminated by the limitations of the DRO’s low resolution. The exact opposite, which is also more likely, would be that high resolution in a DRO does not guarantee the encoder is also of high accuracy.
It is probably more helpful to speak of system accuracy, which is conventionally calculated on the basis of encoder accuracy plus one-half the resolution of the DRO. The modern tendency to quote very high resolutions often ignores the fact that a encoder cannot be accurate to the same limits and there is, therefore, a danger that the machine tool user is being "hoodwinked" into believing that he can obtain high degrees of precision when, in fact, he cannot.
Just as it sounds, repeatability is the ability of a system to produce the same reading every time the same dimension is measured. However, it is important to remember that this term is governed by factors outside the ambit of the DRO and encoder (i.e. condition of the machine, gib adjustment, hysteresis).
When choosing a DRO system, you must also take into account the machine to which it is being fitted and, the fact that you have a system which has a high accuracy does not necessarily mean that the degree of accuracy is either required or needed. A highly accurate DRO system will not correct the inherent inaccuracies of the machine tool.
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