292 Rockwell Automation Publication 1756-RM006K-EN-P - November 2018
Description
The primary requirement of the SCRV instruction is to ensure that the rate never
changes by more than the specified jerk rate.
You can configure the SCRV instruction to produce an S-Curve profile or a Ramp
profile for a step input.
S-Curve Profile
To produce an S-Curve profile, set JerkRate such that (JerkRate * DeltaT) <
AccelRate and/or DecelRate.
In S-Curve profile mode, the SCRV instruction ensures that the rate never
changes more than the specified JerkRate. The algorithm used to produce the S-
Curve profile is designed to produce a smooth, symmetric S-Curve for a step
input. A trapezoidal integration of Out is incorporated to facilitate this. As a
result, changes in Rate will be less than JerkRate during portions of the profile.
When a step change occurs on the input, rate is increased to the programmed
AccelRate or DecelRate. The AccelRate or DecelRate is maintained until a point
at which rate must begin decreasing in order for the output to reach input when
rate reaches zero.
In some cases, depending on the values of acceleration, deceleration, and jerk, the
acceleration rate or deceleration rate might not be reached before the rate must
begin decreasing by jerk rate.
For very small step changes, the SCRV instruction will not attempt to produce an
'S' profile. In this mode the entire step will be output and Rate will reflect the
change in output. This behavior will occur if Out = In and the next step change to
In can be output with a rate less than or equal to the programmed JerkRate.
The SCRV instruction supports an algebraic ramp and an absolute value ramp.
For an algebraic ramp, the acceleration condition is defined by an input that is
becoming more positive, and the deceleration condition is defined by an input
that is becoming more negative. For an absolute value ramp, the acceleration
condition is defined by an input moving away from zero, and the deceleration
condition is defined by an input moving towards zero.
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