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9
Technical Principles to Aid Usage of MINISCALE PLUS
9.3
Analog Signal Frequency
9.4
Accuracy class
9.5
Resolution
9.6
Sampling Rate
9.7
Repeatability
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In order to achieve an appropriate resolution, the analog signal must be interpolat-
ed. The frequency of the electronics must be adjusted according to the travelling
speed.
v
f =
P
f = Frequency in Hz
v = Speed in m/s
P = Increment in m
Example calculation:
v Speed
P Signal period before interpolation
Therefore:
f Frequency
The accuracy class specifies the maximum expected measuring deviation of a
system under the specified operating conditions. A distance measuring system
with an accuracy class of 5 µm allows deviations of +/- 5 µm. For reasons of
comparability, the accuracy class is specified assuming a reference length of 1 m.
The resolution describes the smallest possible measurable positional change in the
measuring system. It is determined by the analog signal period, the interpolation
factor and the evaluation procedure (integration time or sampling rate). For
example, given a set interpolation factor of 250 and an input signal period of 100
µm you get an output signal period of 0.4 µm and according to four-edge
evaluation in the controller a resolution of 0.1 µm.
The sampling rate describes the frequency at which the analog signal is sampled
per time interval. Usually the time interval is one second, which is why the unit for
the sampling rate is Hz. In order to guarantee a complete reproduction of the
original signal, the sampling frequency should be at least twice that of the original
signal in accordance with the Nyquist–Shannon sampling theorem.
Unidirectional repeatability of a measuring system is generally understood to mean
the ability to repeat the results that a particular system returns under exactly the
same environmental conditions. In assessing this, the measuring deviation must be
known and be factored into the analysis.
The repeatability of an axis position can be determined for a specific travelling
speed using simple methods by calculating the arithmetic mean and the standard
deviation of many measurements.
2 m/s
100 µm (0.0001 m)
2 m/s
= 20'000 Hz
0.0001 m
29
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