Siemens SLI-5310 Instruções de Operação descarregar pdf (Página 23)

Transmission of measuring signals

The information above on rotary and angle

encoder signal transmission essentially ap-

plies also to linear encoders. If, for exam-

ple, one wishes to traverse at a minimum

velocity of 0.01 m/min with a sampling

time of 250 μs, and if one assumes that

the measuring step should change by at

least one measuring step per sampling cy-

cle, then one needs a measuring step of

approx. 0.04 μm. To avoid the need for spe-

cial measures in the subsequent electron-

ics, input frequencies should be limited to

less than 1 MHz.

Linear encoders with sinusoidal output

signals or absolute position values accord-

ing to EnDat 2.2 are best suited for high

traversing speeds and small measuring

steps. Sinusoidal voltage signals with lev-

els of 1 V

attain a –3 dB cutoff frequency

of approx. 200 kHz and more at a permissi-

ble cable length of up to 150 m.

The ﬁ gure below illustrates the relationship

between output frequency, traversing

speeds, and signal periods of linear

encoders. Even at a signal period of 4 μm

and a traversing velocity of 70 m/min, the

frequency reaches only 300 kHz.

Bandwidth

On linear motors, a coupling lacking in

rigidity can limit the bandwidth of the

position control loop. The manner in which

the linear encoder is mounted on the

machine has a very signiﬁ cant inﬂ uence on

the rigidity of the coupling. (See Design

Types and Mounting.)

On sealed linear encoders, the scanning

unit is guided along the scale. A coupling

connects the scanning carriage with the

mounting block and compensates the

misalignment between the scale and the

machine guideways. This permits relatively

large mounting tolerances. The coupling is

very rigid in the measuring direction and is

ﬂ exible in the perpendicular direction. If the

coupling is insufﬁ ciently rigid in the

measuring direction, it could cause low

natural frequencies in the position and

velocity control loops and limit the

bandwidth of the drive.

The sealed linear encoders recommended

by HEIDENHAIN for linear motors generally

have a natural frequency of coupling

greater than 650 Hz or 2 kHz in the

measuring direction, which in most

applications exceeds the mechanical

natural frequency of the machine and the

bandwidth of the velocity control loop by

factors of at least 5 to 10. HEIDENHAIN

linear encoders for linear motors therefore

have practically no limiting effect on the

position and speed control loops.

Traversing speed and resulting output frequency as a function of the signal

period

Signal period

Traversing speed [m/min]

Output frequency [kHz]

For more information on linear encoders for linear drives, refer

to our catalogs Exposed Linear Encoders and Linear Encoders

for Numerically Controlled Machine Tools.

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