Siemens SLI-5310 Instruções de Operação descarregar pdf (Página 29)

29

Measuring accuracy

The quantities inﬂ uencing the accuracy of

linear encoders are listed in the Linear

Encoders for Numerically Controlled

Machine Tools and Exposed Linear

Encoders catalogs.

The accuracy of angular measurement is

mainly determined by

• the quality of the graduation,

• the quality of the scanning process,

• the quality of the signal processing

electronics,

• the eccentricity of the graduation to the

bearing,

• the error of the bearing,

• the coupling to the measured shaft, and

• the elasticity of the stator coupling (ERN,

ECN, EQN) or shaft coupling (ROD, ROC,

ROQ, RIC, RIQ)

These factors of inﬂ uence are comprised

of encoder-speciﬁ c error and application-

dependent issues. All individual factors of

inﬂ uence must be considered in order to

assess the attainable total accuracy.

Error speciﬁ c to the measuring

device

For rotary encoders, the error that is

speciﬁ c to the measuring device is shown

in the Speciﬁ cations as the system

accuracy.

The extreme values of the total deviations

of a position are—referenced to their mean

value—within the system accuracy ± a.

The system accuracy reﬂ ects position

errors within one revolution as well as

those within one signal period and—for

rotary encoders with stator coupling—the

errors of the shaft coupling.

Position error within one signal period

Position errors within one signal period are

considered separately, since they already

have an effect even in very small angular

motions and in repeated measurements.

They especially lead to speed ripples in the

speed control loop.

The position error within one signal period

± u results from the quality of the scanning

and—for encoders with integrated pulse-

shaping or counter electronics—the quality

of the signal-processing electronics. For en-

coders with sinusoidal output signals, how-

ever, the errors of the signal processing

electronics are determined by the subse-

quent electronics.

The following individual factors inﬂ uence

the result:

• The size of the signal period

• The homogeneity and period deﬁ nition

of the graduation

• The quality of scanning ﬁ lter structures

• The characteristics of the sensors

• The stability and dynamics of further

processing of the analog signals

These errors are considered when

specifying the position error within one

signal period. For rotary encoders with

integral bearing and sinusoidal output

signals it is better than ± 1% of the signal

period or better than ± 3% for encoders

with square-wave output signals. These

signals are suitable for up to 100-fold PLL

subdivision.

The position error within one signal period

± u is indicated in the speciﬁ cations of the

angle encoders.

As the result of increased reproducibility of

a position, much smaller measuring steps

are still useful.

Position errors within one revolution

Position error within one signal period

Position

f

Position error within

one signal period

Position error

f

Position error

f

Signal level

f

Signal period

360° elec.

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