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

 G115    General influence on the look-ahead functionality                                             (modal) G116    Influence on the calculation of block transition velocity                             (modal) G117    Reset the look-ahead functionality                                           (modal, initial state)

## 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.

(1) 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.

(2) 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.

(3) The maximum path velocity calculated based on the secant error can still be too high, particularly with a short tracing time and small 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.

(4) 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:

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

## Programing Example

 Nnn G115 = 0 (Switch off the single functions (2),(3) and (4)) (of the look-ahead function)

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

## Notice

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

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

 Single functionID (1) (2) (3) (4) 0 * 2 * * 4 * * 6 * * * 8 * * 10 * * * 12 * * * 14 * * * *
 * Single function active Single function not active

## Programing Example

Nnn G115 = 2   (Switch off single functions (2) and (3))

Nnn G115 = 12  (Switch off single functions and (4))

Nnn G115 = 14  (Switch off single functions after)

(program start, cf. G117)

Special case:

Influence the look-ahead functionality with G116, G117

The G116 function disables 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.

## Notice

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

## Programing Example

N10 G116 X1 Y2              (No reduction in block transition)

(rate of different axis)

(rates of the axes X or Y)

N20 G01 G91 X100 Y-100 F1000

N30 X-100

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.

## Attention

The coordinate value is only required for syntax reasons; otherwise it is irrelevant.

The G117 function re-enables all single functions of the look-ahead-function (default setting after program start).

## Programing Example

 Nnn G117 (Enable all single functions (1),(2),(3),(4)) (of the look-ahead function)