Reference variable generation

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The generation of command values in NC controls means the generation of specified position, speed and acceleration values ​​for machining workpieces with CNC machine tools. The processing of geometric data and their preparation to control the machine axes play a key role in the NC control. The data should be processed as quickly as possible, i.e. in real time. In older systems, the calculation process was carried out as an independent process before processing. Due to the progress in the development of microprocessor technology , the representation of the result of the geometry data processing can nowadays take place in floating point numbers . In the past, this was only possible as a fixed point number due to the limited computing power .

The program instructions, i.e. the description of the individual processing steps, are converted by the NC interpreter into a control-internal data format and passed on to the geometry data processing.

This fulfills the following functions:

Speed ​​control and acceleration control

For the speed control with NC controls, the consideration of the physical parameters of the path as a function of time , as well as its 1st derivative speed , 2nd derivative acceleration and 3rd derivative jerk , are decisive.

If the current rise time is neglected (1–10 ms depending on the motor), a sudden increase in the acceleration of the drives can be assumed as a first approximation. As far as these are lower than the necessary limit acceleration. The parameters through which this can be determined in the control are the maximum possible acceleration and the maximum possible acceleration jump. These values ​​are fixed in the control and stored as machine parameters.

The mathematical derivation of the acceleration jump results in an infinitely large positive or negative jolt, depending on the direction. The advantage of this type of speed control is the rapid increase in speed (ramp-shaped). The disadvantage, however, is that the mechanical elements of the drive train and the machine can be excited to vibrate due to the sudden increase in acceleration and the service life of the dynamically stressed parts (ball screw drive, bearings) is also reduced.

An alternative to speed control is to limit the jerk to a fixed positive and negative maximum value. This makes starting the machine appear smoother and calmer. The acceleration is ramped up to the maximum. This also leads to a ramp-like increase in the forces and torques in the drive train and thus to a reduction in the excitation of vibrations.

See also