Rockwell Automation 57C422B 2 Axis Servo Module - Manual (Page 53)

PROGRAMMING 4-25
decelerate to a stop. If the position is not beyond the
distance required to decelerate, the axis will overshoot.
The velocity control bit causes the axis to run at the velocity
defined by the command velocity input (registers 19, 20).
This mode is used to run the axis as a velocity regulator.
Velocity regulation is achieved by sending a constantly
variable position reference to the position loop. This means
the axis can still "trip" due to position error, i.e., the value in
register 80 is greater than the value in register 8.
The direct drive reference bit commands the module to
output the value in register 28 to the drive. This command
causes the axis to function like a D/A converter.
The direction bit specifies the relationship between the
gearing axis and feedback axis when gearing is used, or to
specify the sign of the velocity command when velocity
control is used. When gearing is active, a positive direction
indicates that the gearing and feedback axis will move in the
same direction. A negative direction indicates that they will
move in opposite directions. If you jog into the positive or
negative software travel limit, and then try to jog off the limit
by changing the state of the direction bit, the axis will remain
at the travel limit. To be able to jog off a software travel limit,
you must first turn off the velocity control bit (bit 2), and then
turn it back on with the direction bit changed. See sections
4.13 and 4.14 for more information on software overtravel
limits.
The incremental/absolute move bit specifies the type of
move to be made. If you specify an absolute move, the axis
will move to the position specified in register 17. If an
incremental move is specified, the module will add the
current value in register 17 to the position of the axis to
determine the next position.
The velocity profile bit specifies the type of profile the drive
will execute when accelerating or decelerating, whether
doing an absolute move (bit 5) or velocity control (bit 2).
Using an S-Curve profile doubles the time required to
accelerate and decelerate compared with the trapezoidal
profile. Note that If you select S-Curve, you cannot change
velocity while moving (register 66 bit 8) or an illegal
command fault will be generated. See the figure below for a
sample profile comparison.