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10 Axis and Spindle Installation 09.01
10.4.1 Drive optimization
10.4.1.5 Jerk limitation (as from SW 6)
Definition of term:
By jerk we mean the change in acceleration per unit of time.
Previous behavior (up to SW 5)
In the velocity control function used until now, the acceleration changes over time in steps.
The aim of this velocity control is to accelerate to the programmed feedrate at the maximum
permissible acceleration at the beginning of a block and then to decelerate at maximum
permissible acceleration at the end of the block. Jerk-free approach and deceleration of the
axes is not possible with this discontinuous, stepped acceleration behavior but a contour is
followed faster than it would be with jerk limitation at the same acceleration.
New behavior (as from SW 6):
Unlike stepped acceleration, in jerk-limited acceleration the axis setpoint adopts a jerk-limited
course.
This is achieved by limiting the change in acceleration. However, a smoother acceleration
curve results in a longer traversing time for the same path, velocity and acceleration than
would result for a stepped acceleration. This loss in time can be partially compensated for by
increasing the acceleration of the axes.
Jerk-limited acceleration offers the following advantages:
• Full exploitation of the acceleration possibilities of the machine
• Reduction in the mechanical load on the machine and drive
• Reduction in vibration of the machine system
• Improvement of the movement of machine parts with a high breakdown torque
Description of function
With a jerk-controlled interpolator the velocity is increased and reduced by a constantly
changing acceleration. At the beginning of the acceleration and braking phase the acceleration
value is increased from zero to the maximum value by a constant amount (jerk) and then
reduced in the same way at the end of the phase. The axis is traversed at constant
acceleration in the intermediate time. A steady change in the acceleration and a smooth
velocity curve are the result.
User interface
The function ”Jerk-controlled interpolation” is a function activated via NC MD 5198 bit 2. The
jerk value is stored axis-specifically and parameter-set-specifically in machine data 3332* to
3360*. The default for the jerk values is 0, i.e. jerk limitation is not active and acceleration-
controlled interpolation is used.
A path jerk is calculated from the axial jerk and path data as for velocity and acceleration.
– Jerk limitation is active in all operating modes.
Exceptions:
– Jerk limitation is not active:
• In emergency retraction blocks:
• In thread blocks with G33 (with and without ram), G34 and G35;
– The function ”Look Ahead” is not implemented. It is therefore the responsibility of the
programmer to program the path feedrate at a knee in the contour in such a way that the
axes involved move with as little jerk as possible.
10–28
© Siemens AG 1992 All Rights Reserved 6FC5197- AA50
SINUMERIK 840C (IA)
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