Siemens SIMATIC S7-300 Manual page 235

9.6 Encoders
Overview
One of the following encoders must be connected to the measuring-system
interface of the FM 354 (see Fig. 1-4).
Paths and positions are displayed in 10
selected with machine data item MD7.
The path resolution of the machine axis obtained by the encoder is calculated
within the FM from machine data MD11 to MD13.
Encoder selection The prerequisite for achieving a given positioning accuracy is an n-fold im-
provement in path resolution by the encoder.
For that reason, when configuring a given specific application, select an en-
coder that meets the desired positioning accuracy requirements.
The known design data for the machine axis and the desired measurement
resolution A:
A =
yield a calculation of the necessary pulse number per encoder revolution ac-
cording to the following relationship (taking a metric measuring system as an
example):
FM 354 Servo Drive Positioning Module
Incremental encoder
Absolute encoder (SSI)
Recommended values for n
Minimum
2
1
Positioning accuracy
n
Incremental encoder
S (mm)
I =
G
4 i A (mm)
GS
Description of Functions
–3 –4
mm, 10 inches or 10
Optimum Maximum
4
(mm), (inches), (degrees)
Absolute encoder (SSI)
S (mm)
S =
G
i A (mm)
GS
–4
degrees, as
10
9-59