4. Snapshot Position

Prior to teaching object with the Camera Locate URCaps node (refer to the Programming with the Camera Locate Node section), the operator must define a Snapshot Position using the Snapshot Position wizard. The following section and sub-sections will guide you through this process.

Requirements:

• You must have completed the installation steps of the Installation section.
• Robot must set up to reach the desired workspace.
• You must have the correct calibration board in hand.

Reminders:

• You can teach as many Snapshot Positions as you want.
• Snapshot Position is where the robot is positioned to search for objects.
• One Snapshot Position can be used to search many objects.
• When the object teaching process of the Object Teaching section is done, it is linked to the Snapshot Position and you cannot change that position.
• Each Snapshot Position along with the workplane will define a workspace according to the field of view of the Camera. Refer to theSpecifications section for details on the field of view.

Calibration Boards:

• For UR5 and UR10

Along with your kit you will have the calibration board used for UR5 and UR10 on one side of the board, part number
ACC-CALIB-BOARD.
If you lose or damage your board, you can print the following file:

• For UR3

Along with your kit you will have the calibration board used for UR3 robots on one side of the board, part number
ACC-CALIB-BOARD. The color balance circles are not used yet with the vision system and are for future use. If you loose or damage your board, you can print the following file:

4.1. Guidelines on Snapshot Position

Snapshot position used for teaching the object may be different than the one used in production. However, camera angles and distances should be similar.

For example, conveyor areas are often hard to reach, thus causing object scanning issues. You can use a convenient area for object model teaching that will allow easy scanning. It is always possible to modify the final light and threshold adjustments at the runtime area using ignore_snapshot_position = True variable. Please refer toProgramming with the Camera Locate Node section

The following must be considered when choosing your Snapshot Position:

• Choosing the workspace:
  • Mostly uniform color;
  • Planar;
  • Provide contrast to your part; refer to theVision System Specifications section for details.
• Distance to the workplane:
  • Your Snapshot Position will determine the field of view of the camera and thus the workspace used afterwards.
    • See specifications in the Vision System Specifications section for details on the field of view & part size;
    • Getting closer to the workplane, will reduce your workspace but allow you to locate smaller objects;
    • Getting away from the workplace will increase the workspace but will increase the minimum size of objects to locate.

      Tip

      During the Snapshot Position define step, use the "Show Grid" button, a grid will appear. Your object should be larger than one grid cell.

• Angle of the camera:
  • Snapshot position should consider the Camera angle:
    • Robotiq greatly recommends to have the Camera perpendicular to the surface:
      • Be as close as possible to perpendicular, avoid reflections.
    • Object should be seen from the top view, not from the side.

    Tip

    The Align to axis button can be use to obtain a perpendicular-to-the-surface angle. This button will also align the camera to the base axis of the robot.

Fig. 4-1: Align to axis feature.

• Calibration board:
  • Snapshot position does not need to see the full calibration board, calibration step does.

    Tip

    The Snapshot position and Calibration pose are decoupled, they do not need to be the same.

  • Use the appropriate calibration board.
  • Board must be fully visible by the Camera.
  • Refer to theVision System Specifications section for Calibration board position specifications.

  Tip

  The Align over button can be used to move the camera over the center of the screen, while keeping it’s height position. The camera also becomes coplanar to the surface but keeps the same orientation.

(a)	(b)

Fig. 4-2: Align over feature; (a) Whitout grid shown (b) With grid shown.

4.2. Snapshot Position Wizard

Feature Point:

Prior to defining the Snapshot Position, the operator must define a feature point. To do so, from the Universal Robots Polyscope interface:

• Start a new program or open yours, then go to the Installation tab.
• Go to Features.
• Tap Point to insert a new point feature.
• Define that point position anywhere by tapping Set this waypoint, then tap OK, actual position does not mater.
• Tap Rename to give a significant name to your feature, that name will be used as a reference frame later on in your program.

Snapshot Position Wizard:

Snapshot Position is define using the Snapshot Position Wizard, from the Universal Robots Polyscope interface, still within the Installation tab:

• Go to Camera.
• Go to the Snapshot Positions tab.
• In the drop-down list, select the Feature you wish to use.
• Tap Define to launch the wizard.

Wizard step 1: Define Snapshot Position

• Move the robot arm using the Freedrive to place the Wrist Camera.
  • Remember the guidelines from the Guidelines on Snapshot Position section:
    • Distance will determine field of view and size of objects to be located.
    • Use the Show Grid to help you choose distance (object at least one cell).
    • Camera view should be quasi perpendicular to surface.
• Once the robot arm is in a proper position, tap Save position. Wizard will go to next step.

Wizard step 2: Calibrate

• Place the appropriate calibration board in the field of view of the camera
  • Remember to use the appropriate board, according to your robot model
  • The board orientation should match the screen - landscape orientation
  • Make sure the Camera see the whole board

    Tip

    Snapshot Position will determine the field of view, notice that the calibration step position does not have to be the same as Snapshot position. Thus, you can have a small field of view, then move back for calibration step.

  • Tap Calibrate to begin calibration

    Warning

    Calibration is an automatic process, the robot will move in order to perform calibration. Make sure the robot workspace is clear. You can tap Cancel to stop the process. Operator should watch the robot at all time and have emergency stop button at hand.

  • The Vision System will center on the board and take 27 poses of the board.
  • After the 27 poses, it will take 9 more photos for validating.
  • When the process is done, you will be asked to Accept or Re-Calibrate.
    • Accept if the calibration board grid is match on all pose (first 27 poses)
    • Re-Calibrate if the grid is not matched
  • The wizard will show the 9 validating poses.
  • Verify that the accuracy on the validation poses according to the color chart.
    • Dark blue: local accuracy of +/-0mm.
    • Dark red: local accuracy of +/-4mm and over.

Fig. 4-3: Validating poses.

• If the accuracy is larger than +/- 4mm, an message will inform you that you should perform the calibration again.
• When you tap Accept, you will exit the wizard and the process is completed.

Once the calibration has been accepted, the Snapshot Position will appear in the Snapshot Positions tab with the name of the Feature Point previously created. You can define other Snapshot positions, as long as you define new Feature Points. To delete a Snapshot Position, tap the bin.

Features

Icon	Feature	Description
	Define	Launches the Snapshot Position definition wizard.
	Show grid	Displays a grid on the camera's field of view. Meant to test the camera’s height relative to the workplane : a part should be at least as large as a square of the grid.
	Hide grid	Hides the grid of the camera's field of view.
	Save position	Saves the robot position for the Snapshot Position. This is the position from which the robot will do the part detection.
	Calibrate	Starts the calibration procedure. The robot will move automatically to defined positions for the calibration.
	Cancel	Cancels the calibration procedure while it is running.
	Re-calibrate	Restarts the calibration procedure.
	Accept	Accepts the calibration process.
	Align to axis	Aligns the camera to the base axis of the robot.
	Align over	Aligns the camera over the center point
	Delete	Deletes a defined Snapshot position.

4.3. Copy a Calibration

When defining a snapshot position, it is possible to copy the calibration from another snapshot position. This allows for a faster snapshot position modification when using the same work plane.

To copy a calibration, you need to have a snapshot position already defined from which you want to copy the calibration.

• Create a new Snapshot feature point on the same plane as the existing Snapshot point you want to copy.
• Move the robot to this position.
• Go to the Installation tab, select Camera from the menu on the left and tap the Snapshot tab
• Choose the new Snapshop position.
• Select the existing Snapshot position you want to copy and select Copy.
• In the Wizard, select Save This Position.
• The calibration from the previously selected snapshot position will be used for this new one.