Influence on the look-ahead functionality (G115/G116/G117)

Overview of the functionality of the look-ahead function

Various single functions are implemented in the look-ahead function. They limit motion velocities to maximum values in order to always maintain the permissible axis velocities and axis accelerations.

The following single functions influence the velocity profile:

• (1) Calculation of maximum path velocities based on the maximum axis velocities of the axes participating in the motion as specified in the channel parameter list P-CHAN-00071. The velocity on the programmed path is limited so that each of the axes participating in the motion does not exceed its maximum axis velocity.
• (2) Calculation of maximum path velocities on curved paths based on maximum chord error. In this case, a curve radius can be determined on each curve at the point of greatest curvature. A circular section can then be defined on this circle and its starting and end points correspond to a point generated on the path. Provided that the real mechanical system is moved along the associated chord, the height of the associated circular section can be included as an error criterion.
• (3) Calculation of maximum path velocities on curved paths based on the maximum axis acceleration specified in the channel parameter list P-CHAN-00071. The maximum path velocity calculated based on the secant error can still be too high, particularly with a short tracing time and smaller circular movements. The resultant centripetal accelerations would then exceed the permissible axis accelerations.
• (4) Calculation of path transition velocity based on the block transition geometry, taking into account the axes participating in the motion and the limit values specified in the channel parameter list P-CHAN-00071. As a rule, the relevant portions of the single axes alternate in the total motion with a block transition. However, this change in axis portions only occurs in the first tracing cycle on the new path (for linear motion). If it is assumed that the transition should take place without the generated acceleration (e.g. by the slope function), the resultant axis accelerations can be estimated. The maximum path velocity at block transition can then be calculated so that the maximum axis accelerations are not exceeded at the transition.

General case:

Normal mode with look-ahead

After program start all functionalities of the look-ahead function are switched on (G117). This ensures that the parameters specified in the channel parameter list P-CHAN-00071, e.g. maximum axis velocity and maximum axis acceleration, are always maintained.

Special case:

Switch off single functions of the look-ahead function by G115

Programming Example

Influence the look-ahead functionality (G115/G116/G117)

The table below explains the possible combinations of the single functions.

The corresponding selected or deselected functions always refer to all axes.

Overview of permissible identification numbers (ID) related to G115:

Programming Example

Special case:

Influence the look-ahead functionality with G116, G117

The G116 function switches off the calculation of the block transition velocity (single function (4)) for single programmable axes. No reduction in block transition velocity (path velocity) takes place because of "corners" in the path.

In this case, the dynamic axis data (axis accelerations) are not normally maintained.

Programming Example

In this example, block transition N20/N30 is traversed without any drop in set velocity. The command values change at the block transition in jumps according to the axis portions on the path.

The G117 function switches all single function of the look-ahead function back to active (default setting after program start).

Programming Example