Danfoss MCO 305 Design Manual page 50

MCO 305 Design Guide
The user can control the "gentleness" of the acceleration ramp using 4 parameters:
Parameter Limited Jerk
JERKMIN: Acceleration ramp-up constant. This specifies the number of milliseconds required to ramp the
acceleration up from 0 to maximum acceleration.
JERKMIN2: Acceleration ramp-down constant. This specifies the number of milliseconds required to ramp
the acceleration down from maximum acceleration to 0 (i.e. normally to constant maximum
velocity). If set to 0, this will default to the same value as JERKMIN.
JERKMIN3: Deceleration ramp-up constant. This specifies the number of milliseconds required to ramp the
deceleration up from 0 to maximum deceleration. If set to 0, this will default to the same value
as JERKMIN.
JERKMIN4: Deceleration ramp-down constant. This specifies the number of milliseconds required to ramp
the deceleration down from maximum deceleration to 0 (i.e. normally to 0 velocity). If set to 0,
this will default to the same value as JERMIN.
These constants correspond to the "slopes" of the different portions of the acceleration curve. This is shown
in the chart below. When set to larger and larger numbers, the acceleration and/or deceleration will become
gentler and gentler as the ramps become longer and longer. Note that the acceleration slope determined by
the JERKMIN acceleration ramp-up constant will be used whenever the acceleration is being ramped-up. It
is not used only to ramp-up from 0 to maximum acceleration. Similarly, JERKMIN2 is used whenever the
acceleration is being ramped-down, JERKMIN3 is used whenever the deceleration is being ramped-up, and
JERKMIN4 is used whenever the deceleration is being ramped-down.
Limited-jerk movements will not normally exceed the velocity and acceleration limits set for the controller
(e.g. limits set by the VEL, ACC, DEC, etc., commands). In the above chart, these limits can be recognized
by the "plateaus" in the acceleration curve. If the current velocity and/or acceleration are outside these
limits when the Limited-jerk movement is started, then the movement is accelerated or decelerated as
necessary to bring the movement within the set limits.
It is important to understand that for Limited-jerk movements, "acceleration" is defined as "speeding up" in
either direction (i.e. either forwards or backwards) and "deceleration" is defined as "slowing down" in either
direction. A result of this is that maximum velocity, maximum acceleration, maximum deceleration, and the
four JERKMIN values are all independent of the direction of movement. This can have important consequen-
ces when a Limited-jerk movement must "reverse" the direction of the motor, particularly when maximum
deceleration differs from maximum acceleration. In this case, the Limited-jerk movement will ensure that
the deceleration ramp flows smoothly into an acceleration ramp at exactly 0 velocity when the direction
changes and without exceeding either the deceleration or acceleration limits.
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__ Functions and Examples __
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