9.5.1 Sky Writing

If we extend a MARK vector linearly by a suitable amount before its original start point and after its original end point, the scanner can use these extensions to accelerate and decelerate while still outside the limits of the original vector.

The scanner then moves between the original start point and the original end point of each vector at constant velocity.

This is the fundamental concept behind Sky Writing as implemented on the SP-ICE-3 Card.

In the general case of multiple successive MARK vectors, where the end of a given vector and the start of the subsequent vector are both extended, it may be necessary to introduce an additional JUMP vector between the extended end point of the preceding vector and the extended start point of the succeeding vector.

The Sky Writing algorithm implemented on the SP-ICE-3 Card inserts this JUMP automatically as necessary.

The currently defined SKYWRITING_DECELERATION_DELAY will take effect after the additional Sky Writing deceleration vector. The current MARK_DELAY is ignored.

The currently defined SKYWRITING_ACCELERATION_DELAY will take effect before the additional Sky Writing acceleration vector. The current JUMP_DELAY is ignored.

It may be appropriate to use application-specific values for either or both of these delays while performing Sky Writing.

• If the calculated Extension Distance is enough to allow the scanners to settle completely after the change of heading, the delays may be reduced to 0.

• On the other hand, if the scanners cannot settle sufficiently within the available Extension Distance, the delays can be increased to allow more settling time.

In the interest of marking efficiency, and taking into consideration the level of marking quality deemed acceptable, it may be desirable to suppress the generation of extensions and jump vectors over part(s) of a vector sequence where the vectors are co-linear, or where the Change of Heading between successive mark vectors does not exceed a user-definable upper limit.

The Sky Writing mode ForceAtStartFinish can be configured to selectively merge extension vectors when a direct path doesn't cause a huge change of heading.

To enable this feature the parameter MergedExtensionsMaxCoH must be set to a value between 0 and π. The parameter can be set using the AppendSkyWritingMergedExtensionsMaxCoH method.

Normally, when there is a JUMP between two MARK vectors the Sky Writing mode ForceAtStartFinish creates extension vectors that the scan head must follow along. In some cases this causes a very inefficient and jittery path.

Setting MergedExtensionsMaxCoH tells the Sky Writing algorithm to replace this inefficient path with a single extension vector whenever the changes of heading are smaller than MergedExtensionsMaxCoH. The changes of heading refer to the angles between first MARK vector and JUMP as well as JUMP and second MARK vector.

The merged extension vector should not be confused with the original JUMP that would be executed with Sky Writing off. The extension vector is performed at MarkSpeed and without applying any mark delay or jump delay. This allows the scan head to move at a constant speed between the two MARK vectors without the need for acceleration or deceleration.

Due to this design a merged extension vector aside from reducing the path length also fulfills the purpose of the original extension vectors which is ensuring a constant speed and thus a consistent marking quality at the adjacent ends of the neighboring MARK vectors.

Note that a merged extension vector is only considered if the JUMP vector between the two MARK vectors is shorter than double the length of the first MARK's extension vector.

Length_JUMP < 2 · ExtensionTime₁ · MarkSpeed₁

Especially when marking collinear lines this feature can significantly speed up the marking process and also improve the marking quality.

All settings affecting the operation of Sky Writing can be found in ProcessVariables.

The values can be set via the APIs and by appropriate List Commands.