Measured Value Determination With Ssi Absolute Encoder - Siemens SIMATIC ST-1500 Function Manual

2.2 Basics of counting, measuring and position input (TM Count, TM PosInput, Compact CPU)
2.2.9.3

Measured value determination with SSI absolute encoder

Measuring range SSI absolute encoder
The measuring functions have the following measuring limits:
Measurement type
Frequency measurement
Period measurement
Velocity measurement
All measured values are returned as signed values. The sign indicates whether the position
value increased or decreased during the relevant time period.
Measuring principle
The technology module assigns a time value to each SSI frame. The measuring interval is
defined as the time between the last SSI frame with a change of position value before and
during the previous update time. The measuring interval and the total change in position
value in the measuring interval are evaluated to calculate a measured variable. The total
change in position value in a measuring interval corresponds to the number of encoder
increments in the same measuring interval.
If there is no change in position value within an update time, the measuring interval is
dynamically adjusted. In this case, a change in position value is assumed at the end of the
update time and the measuring interval is calculated as the time between that point and the
last SSI frame with a change in position value. The change in position value is then 1.
The feedback bit STS_M_INTERVAL indicates whether a change in position value occurred
in the previous measuring interval. This allows for a differentiation between an assumed and
an actual change in position value. If the technology module cannot calculate measured
values because the measuring range limit has been violated, the feedback bit
STS_M_INTERVAL is not set.
Counting, measurement and position detection
Function Manual, 12/2017, A5E32009889-AG
The basics of counting, measurement and position detection
Low measuring range limit
0,04 Hz
0,25 μs
Depending on the configured number of "increments per unit" and the
"time base for velocity measurement"
High measuring range limit
4 MHz
25 s
59