Siemens Simatic S7 Series System Manual page 526

Open loop motion control
12.7 Using the AXISx_ABSPOS subroutine to read the absolute position from a SINAMICS servo drive
To establish a common engineering unit definition between the CPU and the servo drive, you
must consider the following motion system variables when commissioning your system:
Electronic gearing
In the V90 servo drive, the "a" and "b" values determine the drive's electronic gear ratio, a
feature that allows a frequency conversion on the pulse train issued from the CPU. Since the
maximum pulse frequency issued from an Axis of Motion in the S7-200 SMART CPU is 100
kHz, while the encoder resolution of SIMOTICS S-1FL6 servo motors installed with absolute
encoders is 2^20 pulses per revolution, use of the drive's electronic gear feature is likely, in
many applications, to achieve higher motor speeds. For example, to achieve a 10x increase
in the setpoint pulse frequency within the servo drive compared to the frequency of the CPU
pulse train supplied to the drive, then you must set the electronic gear ratio to "10:1".
In the V90 servo drive, setting parameter "p29012[0]" establishes the numerator of the
electronic gearing ratio ("a"), while setting parameter "p29013" establishes the denominator
of the ratio ("b"). Also, when using electronic gearing, set the parameter "p29011" value to
"0". The valid range for the electronic gear ratio (a / b) in the V90 servo drive is between
"0.02" and "200".
Refer to the "Electronic Gear Ratio" section of the
Operating Instructions
Mechanical factors
The "m" and "n" values establish the mechanical relationship between a load revolution and
a motor revolution, applicable when a gearing mechanism is used. When the V90 servo drive
is in "PTI" control mode, its internal mechanical gearing ratio parameters are fixed at "1:1",
but the physical "m" and "n" values are important in establishing the correct engineering unit
conversion factors for the Axis of Motion, as shown below.
The "c' value establishes the relationship between load movement in the specified
engineering unit, and load revolutions. "20 cm of load movement per load revolution" and
"360 degrees of load movement per load revolution" are examples of this conversion factor.
Encoder resolution
The "r" value is the resolution of the absolute encoder in your servo motor. As stated above,
the encoder resolution of SIMOTICS S-1FL6 servo motors installed with absolute encoders
is 2^20 pulses per revolution or "1048576". When the V90 servo drive is paired with a motor
containing an absolute encoder, the drive automatically detects the encoder type and obtains
its resolution. However, in your program, you must specify this resolution value in the
AXISx_ABSPOS subroutine's "Res" input parameter and also in one of the engineering unit
conversion factor calculations shown below.
526
document for more information.
SINAMICS V90 / SIMOTICS S-1FL6
System Manual, 09/2015, A5E03822230-AC
S7-200 SMART