Siemens SINUMERIK 840D sl Function Manual page 185

Setpoint delay
From NCU-U's point of view, it is interpolating with the local X axis and the two link axes C
and Z when performing eccentric machining. To achieve a correct contour, it is necessary to
compensate for the time delay which occurs when the setpoints are transferred to the link
axes and for the differences in clock cycles by delaying the setpoints to the local X axis. This
can be done by setting machine data:
MD18720 $MN_MM_SERVO_FIFO_SIZE.
Use the following formula for assigning MM_SERVO_FIFO_SIZE:
MM_SERVO_FIFO_SIZE = 2 * link cycle / IPO cycle +1
The formula applies for NCU-U and NCU-A/NCU-B as illustrated in the diagram "Schematic
example of a configuration with several NCUs and eccentric machining unit" (Section
"Diagram of general solution").
Examples:
NCU-U:
Link cycle = 16 msec
IPO cycle n = 2 msec
The cycle ratio is therefore 8; the value of the formula 2 * 8 + 1 = 17
NCU-A/NCU-B:
Link cycle = 16 msec
IPO cycle m = 16 msec
The cycle ratio is therefore 1; the value of the formula 2 * 1 + 1 = 3
In some cases, incorrect settings cause an alarm to be output in machine data:
MD18720 $MN_MM_SERVO_FIFO_SIZE
Effect of the SERVO_FIFO_SIZE settings
● All functions that use actual values when interpolating for setpoint generation will be
● Alarm responses that are triggered directly in the servo are not affected by any additional
● Acceleration processes on the link axes are always output with the link cycle and are
● If high precision is required for the machining operation, the tool should not have contact
● It is only permissible to switch axis containers with rotating spindles on NCUs with IPO
Extended Functions
Function Manual, 01/2008, 6FC5397-1BP10-3BA0
Several Operator Panels on Several NCUs, Distributed Systems (B3)
affected by the delay of the slower link cycle instead of the fast IPO cycle. This also
applies to the response at faults (alarms that are issued to disable a mode group or for
interpolatory braking).
delay.
therefore not exactly synchronous with the axes physically connected to the fast NCU.
Therefore, it is only advisable to use this configuration with interpolating axes on NCUs
with different IPO cycles if you are machining with link axes that are only accelerated a
little or not at all. Thus the axes with high dynamic requirements must be physically
connected to the fast NCU.
with the workpiece during acceleration/deceleration of the axes. Otherwise contour
violations will result.
cycles that are equal to the link cycle. If it is possible to delay the container rotation,
switching is delayed until the spindle is motionless. Otherwise alarm 4019 is issued. Axis
container switching with motionless axes and spindles is also permissible on the faster
NCUs. (See "Axis container").
2.12 NCU link with different interpolation cycles
185