February 2012 1 – 643
The position controller receives its nominal value, e.g., from the NC program; the actual value is
normally provided by a linear encoder (scale). The actual position value can also be provided by a motor
encoder instead of a scale. The position of the machine table depending on the number of counting
pulses or revolutions of the motor encoder is set in the machine parameters (e.g., one revolution of
the encoder changes the table position by 10 mm).
The speed controller receives its nominal value from the position encoder. Thus the output quantity
of the position controller is the input quantity of the speed controller. This is why this interface is also
referred to as "nominal speed value interface". With analog axes, the control leads the nominal speed
command interface (±10 V) "to the outside" to the analog servo amplifier. With digital axes, this
interface is "inside" the control.
The actual value for the speed controller is supplied by the motor encoder.
The current controller receives its nominal value from the speed controller. The actual value is
provided by current sensors in the power module or in the output stage.
Cycle times There is a separate time interval for each control loop:
Position controller cycle time
Time interval during which the interpolation points on the path are calculated
Speed controller cycle time
Time interval in which the actual speed value is compared to the calculated nominal speed value
Current controller cycle time
Time interval in which the actual current value is compared to the calculated nominal current value
The cycle times that apply vary depending on the control unit (CC, UEC) and on its settings.
Figure: Cycle times
Look ahead In order to adapt the feed rate to the machining process, the iTNC 530 HSCI calculates the geometry
ahead of time. This way changes in directions (corners, curvatures, and changes in curvatures) are
detected in time, and the participating NC axes can be braked or accelerated accordingly.
The number of NC blocks calculated ahead of time can be set by the machine manufacturer (default
setting = 256 blocks).
Interpolator The interpolator operates at a prescribed clock rate, the position controller cycle time.
The interpolator calculates a velocity from the programmed feed rate. In the CC61xx it makes this
calculation every 3 ms. The value is also dependent on the acceleration curve and the end position.
If more than one axis is moved simultaneously, the path acceleration a
path
is formed from the
appropriate axis components. The same applies to rapid traverse in the path.
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