Feedforward control
The spindle's following error can be minimized using speed and acceleration feedforward control. Feed forward control of the spindle is necessary above all when tapping threads with an endlessly rotating spindle because, as far as possible, both the contour axes and the spindle must follow the specified setpoints without a following error if a good machining result is to be achieved.
Conventional feedforward control
This kind of feedforward control is only possible for conventional drives and for drive simulation.
Here, the theoretical following error is calculated based on the current speed and acceleration and is added to the setpoint specified by the interpolator. The expected following errors can be calculated in accordance with the relationship.
for constant speed and
for constant acceleration. Ta is the drive time constant, which can be set in the axis parameter list P-AXIS-00225/ P-AXIS-00226.
Conformity with the permissible axis acceleration values is considered when the feedforward control variable is added to the specified setpoint.
Contour fidelity
The prerequisite for the described equations is the use of a P position controller, whose speed gain
for linear axes or
for rotation axes is essentially defined by the drive dynamics.
If all axes of a system have the same kv factor, after stabilization there are no deviations from the specified path, neither during straight ahead nor circular travel. Here, the following error merely sets in according to the equations described here.
Sampling time and pre-filtering Settings
When deriving the speed v and the acceleration via differentiation of the distance, attention must be paid to sufficiently long sampling times to minimize disturbing quantization noise. A better behavior is also achieved via pre-filtering of v and a. To this end, linear averaging of several values of v and a takes place.
Coupling in feedforward control
Feed forward control leads to an increase in dynamic acceleration stress and an increase in the control ranges. An improvement of contouring fidelity by feedforward control is only possible with position controller settings whose speed gain is less than the optimum values needed for the response to setpoint changes because there will otherwise be an extreme inclination to overshoot.
Activating or deactivating feed forward control during motion can lead to oscillations. This is why 'soft' coupling in of the feed forward control is necessary, paying attention to the maximum permitted axis acceleration values or the maximum jolt (e.g. in the case of a non-linear 'slope' function). This makes it possible to allow activation or deactivation of feed forward control as required at block boundaries without interrupting motion. Wear and tear on the drive, and on the mechanics of the machine tool, can be reduced by activating feed forward control only during machining of contours to be machined exactly.
 | Further information on the subject of feed forward control and parameterization can be found in the description of functions [Feedforward control]. |
Programming
S[G135] Spindle feed forward control on
S[G136] Weighting
S[G137] Spindle feed forward control off
| In the case of digital drive systems (e.g. SERCOS), feed forward control can be activated and parameterised in the drive. |