Siemens SIMATIC S7 Equipment Manual

2-4
SM 338 Equipment Manual
(4) J31069-D401-U1-A0-7618
2.2 Measuring Princi ple
A START pulse is sent by the SM 338 via the RS 422 interface. The sensor
electronics send an echo of this pulse back to the SM 338. The START pulse
generates a current pulse in the sensor electronics. This current pulse moves
from the sensor electronics toward the end of the measuring bar.
Figure 2-2 shows a diagram of the interaction between module and sensor.
µ ds
SM 338 RS 422
interface
Sensor
electron-
ics
Ultrasonic position sensor
Ultrasonic
pulse
Measuring bar
Measuring point
(magnet)
Current
pulse
START
(Echo)
STOP
START signal
ds
Attenuation
zone
Figure 2-2 Principle of ultrasonic position encoding
The moving magnet field with the current pulse and the permanent magnet
field of the measuring point begin to affect each other. An ultrasonic pulse is
generated at the position of the measuring point (i.e., magnetostrictive princi-
ple).
This ultrasonic pulse spreads out in the direc tion of the end of the bar and the
sensor electronics. The pulse is attenuated at the end of the bar. The sensor
electronics convert this pulse i nto an electrical signal (i.e., the STOP pulse).
The time between the echo of the START pulse and the STOP pulse is pro-
portionate to the distance. This time is acquired by the SM 338 and for-
warded.
Note
Conversion of the position into mm using an offset factor is performed by
the CPU.
The SM
3
3
8
Modu
l
e

Siemens SIMATIC S7 Specifications

General

Brand	Siemens
Model	SIMATIC S7
Category	Controller
Language	English