Compensatory Control - Siemens SINUMERIK 840C Installation Instructions Manual

12 Functional Descriptions 10.94
12.18.4 Link types with constant link factor
For normal operating conditions, it is advisable to operate only one leading axis with K4 link;
this will generally be the least well tuned axis(disturbances in measurement or closed-loop
control) or the axis with the slowest dynamic response (e.g. main spindle).
It must also be noted that a spindle in open-loop control mode which is operating as the
leading spindle will activate the position controller (link type K1) or deactivate it (K2 or K4)
depending on the link type used while a (leading) axis always operates with active position
controller. To ensure that the difference in dynamic response between activated and
deactivated position control is negligible, it is advisable to apply the dynamic feedforward
control function as a matter of principle.
When the dynamic feedforward control function is not used, the time response of the position
control (following error) has - in addition to the intrinsic dynamic response of the drive - a
smoothing action (e.g. KW = 1 corresponds to a smoothing time constant of approximately 60
ms) and must be taken into account when the setpoint filter is set.
The major advantage of the new GI link type is the "more steady" setpoint velocity control.
This GI link type should be applied when the leading and following drives do not have the
same dynamic response.
When the 611D system is used, an additional compensatory controller can be omitted. In
analog drive systems, the I-action component function should be activated to compensate drift
errors. It is still useful to implement dynamic response matching by means of the setpoint
filters which reduces the time required to optimize the controllers involved in the link during
start-up.

Compensatory control

In addition to the setpoint/actual value link, a compensatory controller can be connected into
the system. This controller checks the present actual values of the leading drives and following
drive, making allowance for the present link factors. The deviations (synchronism errors)
calculated in this way are used to generate an additive speed setpoint for the following drive
on which an inverted sign is superimposed.
The link can therefore be maintained in the event of a disturbance (e.g. load disturbance or
failure of a leading drive) and the dynamic response between the leading and following drives
(feedforward control effect) improved. The compensatory controller also causes an indirect
increase in the position control loop gain of the following drive. This generally has a positive
effect on the synchronism.
The compensatory controller can be activated and deactivated only for the whole GI grouping
from the PLC.
When the GI grouping is defined, the compensatory controller can be suppressed for a certain
leading drive by means of the setpoint link without compensatory control (K3). In this case, it is
not the actual values (which may be affected by load disturbances) which are evaluated, but
simulated actual values of the leading drive concerned which are derived from the setpoint.
Disturbances in the leading drive do not therefore have any effect on the compensatory
control.
12–134
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SINUMERIK 840C (IA)