11. Gripper handling

11.1. Compaction force

One goal during manufacturing is to ensure that the gripper applies the patches correctly: compaction pressure should be applied on the full patch surface. The behavior is influenced by two parameters: the patch geometry derived gripper depth and the manufacturing parameter gripper push-in.

  • The gripper depth is automatically calculated per patch and should not be modified by the user.

  • The gripper depth simplifies the application of a user defined push-in, by making the push-in basically a constant for slightly curved surfaces.

Both parameters are described in detail in the following sections.

11.1.1. Gripper depth

The reference point for the application of a patch is always the patch center point. No additional data is needed for concave surfaces, since the patch center point will in this case always be touched last by the gripper. For convex shaped surfaces an algorithm must calculate an additional offset distance. In Patch Artist the surface underneath each patch is automatically analyzed. The following figure shows how this so-called gripper depth is defined.

../_images/gripper_depth_definition.svg

Fig. 11.1 Definition of gripper depth

A reference plane through the patch center point and orthogonal to the patch normal is constructed. For multiple sampling points on the patch surface the distance to this plane is calculated. The highest distance between a sampling point and the reference plane determines the required push-in that is needed to get the whole patch in contact with the surface.

The resulting value can be checked and modified in the patch property editor:

../_images/patchPropertyEditor_gripperDepth.png

Note

Since the orientation of the reference plane depends on the vector along which the gripper is applied, an update of the Push-in orientation will cause an update of the gripper depth. Don’t forget to manually update your corresponding machine data patch afterwards, in case you’ve already created a machine data set.

11.1.2. Gripper push-in

The above mentioned gripper depth is not to be confused with the additional push-in on top of the gripper depth that should be specified for manufacturing. The left side of the next figure shows the application of a patch with a push-in of 0.0 mm - i.e., only the automatically calculated gripper depth is applied. The depicted rectangular gripper bounding box only slightly touches the deepest point of the patch. Usually additional force is necessary to consolidate the patch - as shown on the right. The consolidation pressure can be influenced using the push-in parameter.

../_images/gripper_push-in_definition.svg

Fig. 11.2 Definition of gripper push-in

The settings for the push-in can either be modified globally in the machine config settings or per patch using the Modify patch dialog.

../_images/modifyPatchDialog_pushIn.png

This dialog is accessible via the context menu of the machine data patch.

Warning

The settings mentioned can be used to roughly adjust compaction pressure. They do not take gripper or patch deformation into account. Special care should be taken in case of (locally) high compression. When in doubt, start the step-through process with a low or even negative gripper push-in.

11.2. Push-in orientation

While working in Motion Artist you might realize that certain patches can’t be applied with the default settings, e.g. because this would cause the gripper to collide with the tooling. In such a case it is often sufficient to modify the axis along which the patch is applied by a few degrees.

You can change the orientation of the normal of an individual patch or a group of patches. This determines gripper’s push-in direction in cases when rolling-motion steps are not defined (see Rolling motion).

Right-click on a patch (or select a group of patches and right-click on one of them) and select Change orientation from the pop-up menu. The user can choose to either apply a direct movement or a rolling movement by selecting the appropriate radio button.

../_images/change_orientation.png

Change values in the Rotation X and Rotation Y fields to change the orientation of the normal by rotation around an axis parallel to the patch orientation (X) or around the axis perpendicular to the patch orientation (Y).

Note

Make sure that Show orientation is selected in the ribbon bar, to see blue arrows showing the adjusted orientation.

Click on Make prediction to get automatically calculated values. Note that these values depend on the gripper size.

Note

We allow interaction with the 3d view to allow changing viewing direction, but we advise not to change selection. The dialog modifies patches for which it was initially opened, irrespective of later change of selection. On the first tab (Selected items) you can see elements (patches, chains, etc.) for which the dialog is modifying the orientation (i.e. elements which were selected when the dialog was opened).

If the dialog was opened for more than one patch, and any of these patches have rolling motion, adjusting orientation is disabled. This is to avoid accidental removal of rolling-motion information. However, in the first tab (Selected items) you can click on Reset all button: it removes both orientation adjustments and rolling motion from all selected patches. After that, if you open Change orientation dialog again for the same set of patches, you will be able to adjust orientation of these patches.

If you make modifications to patches for which a Motion Artist dataset was already created, you have to either recreate the complete dataset or select the affected patches in the dataset update them.

11.3. Rolling motion

Note

New in 1.3

For patches on a more complex surface, a gripper approaching from a single direction and just pushing-in may not be enough. For example, edges of the patch might be not pressed onto the surface. The figure below depicts this issue.

../_images/rolling-vs-direct.png

By defining rolling-motion steps you may add additional points to the gripper movement, allowing pressing different zones of one patch in different directions.

../_images/rolling_animated.gif

Note

Not all production systems support rolling motion. Please check first whether your system supports it, before you apply this feature.

Modifying rolling motion is allowed only for exactly one selected patch. The patch has to be draped first (see Draping). Select a patch, right-click and choose Change orientation from the popup menu. The user can choose to either apply a direct movement or a rolling movement by selecting the appropriate radio button at the top of the dialog. Choose the Rolling movement radio button to start defining rolling-movement steps.

Initially the table showing rolling steps is empty. Choose discretization precision (default: 5 degrees) and click on Calculate button. Smaller values of discretization lead to larger number of rolling steps. Generated contact points between the gripper and the surface are always located on the line with the highest curvature. You can see the contact line in the preview: a red line of contact points on the gripper and an orange line of contact points on the surface:

../_images/rolling-contact_lines.png

For a selected contact point (rolling step) you can adjust the rotation of the gripper by clicking +/- in the table or clicking on the value in the table and typing.

  • X rotation rotates around axis parallel to gripper’s default orientation.

  • Y rotation rotates around axis perpendicular to gripper’s default orientation.

In the visualization you can see the initial orange coordinate system (with XY plane tangent to the surface) and the final red coordinate system which takes into account the manual adjustments. Origins of both these coordinate systems are at the selected contact point between the gripper and the surface. The gripper box is visualized in red, in the final adjusted orientation.

../_images/rolling-with_adjustments.png

Because of the order in which rotations are applied, we suggest that you adjust X rotation first, until the visible position of the Y axis is as you wish. Then adjust Y rotation, until the visible position of the X axis is as you wish. (You can also adjust X rotation after Y rotation has non-zero value, but in this case X rotation will change visible directions of both X and Y axis.)

You cannot change positions of the contact points for steps. However, you can create more steps (by choosing a small discretization value) and then delete those steps that are not needed. Select a step in the table and click on Delete button or press Delete key.

You can also duplicate a step and choose different rotation adjustments for each of the copies. This will create a rotating movement of the gripper around a fixed contact point. Select a step and click the Duplicate button or press the Insert key.

Also, you can reverse the order of all steps to create rolling movement in the opposite direction. To do this click on the Reverse button.

The maximum number of rolling steps is 10 per patch.

Switching to direct movement and accepting the dialog on direct-movement tab removes all rolling steps.

Press Alt to see keyboard shortcuts for all widgets.

Note

We allow interaction with the 3d view to allow changing viewing direction, but we advise not to change selection. The dialog modifies the patch for which it was initially opened, irrespective of later change of selection. On the first tab (Selected items) you can see the patch for which the dialog is modifying the rolling movement (i.e. the patch which was selected when the dialog was opened).