Siemens SIMATIC S7 System Manual page 372

102BTechnology instructions
9.3 Basic motion control
● Mode 2 - Passive Referencing: When the axis is moving and passes the Reference Point
● Mode 3 - Active Referencing: This mode is the most precise method of Homing the Axis.
Modes 0 and 1 do not require that the axis be moved at all. They are typically used in setup
and calibration. Modes 2 and 3 require that the axis move and pass a sensor that is
configured in the "Axis" technology object as the Reference Point Switch. The reference
point which can be placed in the work area of the axis or outside of the normal work area but
within movement range.
Configuration of homing parameters
Configure the parameters for active and passive homing in the "Homing" configuration
window. The homing method is set using the "Mode" input parameter of the motion control
instruction. Here, Mode = 2 means passive homing and Mode = 3 means active homing.
NOTICE
Use one of the following measures to ensure that the machine does not travel to a
mechanical endstop in the event of a direction reversal:
 Keep the approach velocity low
 Increase the configured acceleration/deceleration
 Increase the distance between hardware limit switch and mechanical stop
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Switch the current position is set as the home position. This feature will help account for
normal machine wear and gear backlash and prevent the need for manual compensation
for wear. The Position input on the Homing instruction, as before, adds to the location
indicated by the Reference Point Switch allowing easy offset of the Home position.
During passive homing, the MC_Home instruction does not carry out any homing motion.
The traversing motion required for this step must be implemented by the user via other
motion control instructions. When the reference point switch is detected, the axis is
homed according to the configuration. Active traversing motions are not aborted upon
start of passive homing.
The initial direction and velocity of movement is configured in the Technology Object
Configuration Extended Parameters-Homing. This is dependent upon machine
configuration. There is also the ability to determine if the leading edge or falling edge of
the Reference Point Switch signal is the Home position. Virtually all sensors have an
active range and if the Steady State On position was used as the Home signal then there
would be a possibility for error in the Homing position since the On signal active range
would cover a range of distance. By using either the leading or falling edge of that signal
a much more precise Home position results. As with all other modes the value of the
Position input on the Homing instruction is added to the Hardware referenced position.
In active homing mode, the MC_Home instruction performs the required reference point
approach. When the reference point switch is detected, the axis is homed according to
the configuration. Active traversing motions are aborted.
S7-1200 Programmable controller
System Manual, 11/2011, A5E02486680-05