Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 300)

Publication 1784-UM003A-EN-P – June 2003
292 Motion Object Attributes
When this sequence is performed, the axis moves in the specified Home
Direction at the specified Home Speed until the home limit switch is detected.
The axis then decelerates to a stop and then moves in the opposite direction at
the specified Home Return Speed until the home limit switch is cleared. When
the home limit switch is cleared, axis position is immediately redefined to be
equal to the Home Position and the axis decelerates to a stop. If Home Offset
is non-zero, then the Home Position will be offset from the point where the
home switch is cleared by this value. Once the axis decelerates to a stop, the
controller then moves the axis back to the Home Position at the Home Return
Speed using a trapezoidal move profile. If the axis is configured as a Rotary
Axis, the move back to the Home Position takes the shortest path (i.e., no
more than ½ revolution). The motions for this active homing sequence are
shown below.
Figure 13.3 Active Bi-Directional Homing Sequence
If the controller detects that the state of the home switch at the start of the
homing sequence is active, the controller immediately reverses the homing
direction and begins the return leg of the homing sequence.
Neglecting the mechanical uncertainty of the home limit switch, the accuracy
of this homing sequence depends on the time uncertainty in detecting the
home limit switch transitions. The position uncertainty of the home position is
the product of the maximum time for the control to detect the home limit
switch (~10 milliseconds) and the specified Home Return Speed. For this
reason, the Home Return Speed is often made significantly slower than the
Home Speed.
For example, if a Home Return Speed of 0.1 inches per second (6 IPM) is
specified, the uncertainty of the home position is calculated as shown below:
Uncertainty = 0.1 Inch/Sec * 0.01 Sec = 0.001 Inch.