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44
Manual – XFE24A EtherCAT Fieldbus Interface
7
EtherCAT introduction
Motion Control via EtherCAT
7.1.3 Position setpoint interface (Position mode)
In Position mode, the controller transmits a position setpoint cyclically to the servo in-
verter. The position actual value is read back by the servo inverter or a separate encod-
er.
In Position mode, the servo inverter follows the constantly changing position setpoint
and generates the required speed setpoint for the speed controller [2] from the position
actual value (from [5] or [6]). The time slices of the controller, the bus transmission and
the internal processing cycles of the servo inverter and of external encoders, if applica-
ble, must be synchronized to one another.
After the position in the controller has been referenced to the position in the servo invert-
er, the monitoring of permitted travel ranges or limit switches can be performed in the
servo inverter. Whether the settings of a load-dependent ramp specification and lag er-
ror monitoring in the servo inverter make sense must then be checked in detail.
To prevent unwanted, high acceleration values for larger control intervals (>1 ms), the
speed setpoint is not directly adopted by MOVIAXIS
®
, but interpolated linearly. This
means for a setpoint cycle of 5 ms that the controller sets the required position change
in the MOVIAXIS
®
not every 5ms as one large step, but rather as 5 small steps of 0.5ms.
61479AXX
Figure 21: Position mode – Cascade with bus interface
M
V
X X
X
ref
v
ref
t
ref
X
act
v
act
[C]
[A]
[3]
[2] [1]
[4]
[5]
[5] [6]
[B]
[A] Control [B] Fieldbus interface [C] Servo
inverter
x
ref
Position setpoint [1] Position controller
x
act
Position actual value [2] Speed controller
v
ref
Speed setpoint [3] Output stage of the servo inverter
v
act
Actual speed value [4] Driven machine
t
ref
Torque setpoint [5] Encoder (V = speed; X = position)
[6] Optional encoder
0 0
I