Rockwell Automation 20P PowerFlex Digital DC Drive - Manual (Page 284)

284 Rockwell Automation Publication 20P-UM001I-EN-P - February 2013
Appendix C Application Notes
S-curve Configuration Setting Par 18 [Ramp Type Select] to 1 “S shaped,” enables S-shaped ramp
(S-curve) operation in the drive. When S-curve operation is enabled, it allows for
a smoother change in speed than a linear ramp.
Adding s-curve to the ramp will increase the total length of time to perform the
speed change. The S-curve will not exceed the maximum acceleration, set by
parameters 24 and 660 [Accel Time
x], when accelerating, or maximum
deceleration, set by parameters 32 and 662 [Decel Time
x], when decelerating.
When the S-curve times are much smaller (<20%) than the linear Accel times,
the speed profile is very similar to a linear ramp (just slightly delayed). As S-curve
times are increased, more of the speed profile is spent in the “s” (non-linear) and
less in the linear acceleration section. When s-curve and linear ramp times are
equal, there is no longer any linear portion of the ramp (although maximum
acceleration is reached at the mid-point of the total ramp). When S-curve time is
larger than the linear ramp time, again there is no linear portion, and maximum
acceleration is never reached. The same is true for deceleration ramps.
Note that the total ramp time is independent of the change in the speed
reference. So, it will take the same amount of time to make a 10 rpm change as it
will for a 1000 rpm change. This means that the reference should not be
“ramped” external to the drive while it is also enabled in the drive (it will make
the ramp time longer).
When the S-shaped ramp <= Linear ramp, the S-shaped ramp speed profile is
divided into three sections:
• Initial (positive) “S” (jerk)
• Linear (constant acceleration)
• Final (negative) “S” (jerk)
Time (s)
Speed (rpm)
Initial “S”
(T1)
Final “S”
(T1)
Linear
(T2)
Total Ramp (T3)