Setting up the gripper

This tutorial shows how to set up a gripper for the CollisionCheck component.

The gripper in the CollisionCheck component represents the collision geometry of the real gripper mounted to the robot. That means, the outer geometry of the gripper has to be modeled in a way that it completely contains all parts of the robot’s gripper, including cables, connectors etc.

In this tutorial we will show how to model the sample gripper shown in Fig. 55.

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Fig. 55 Sample vacuum gripper to model for collision checking

The easiest way to create a gripper is via the Web GUI’s CollisionCheck tab. The Grippers section lists all defined grippers and allows to edit and remove them. To create a new gripper, click on “+ Add a new gripper”.

A dialog window opens that allows creating the gripper. A 3D visualization on the left helps to check the created gripper geometry. Initially, it only shows the robot flange with a default radius and the flange coordinate frame.

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Fig. 56 Creator dialog for gripper creation

The gripper should be modeled in a way such that each element is completely contained in a cylinder or box.

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Fig. 57 Modeling of the gripper geometry with cylinders and boxes. R is the outer flange radius. The axes correspond to the flange coordinate frame.

Flange radius

The flange radius is the outer radius of the robot’s flange (see Fig. 57). It is used during a safety check if the check_flange parameter is enabled (see Check Flange). It is required to set the flange radius even if the check_flange functionality will not be used.

Gripper elements

The gripper is built in the direction from the flange to the Tool Center Point (TCP) and consists of a list of elements which can be cylinders and boxes. To add a new element, click on the + symbol in the Elements section (see Fig. 56).

The first element of our sample gripper is a vacuum generator. We model it as a cylinder with the same height as the vacuum generator and a radius corresponding to the maximal elongation of the vacuum generator from the z axis of the flange coordinate frame. Since the vacuum generator is mounted directly on the flange, the position of the cylinder will be (0, 0, 0). It is important that the modeled collision geometry completely includes the real gripper element to prevent undetected collisions.

The second element of the gripper is a cylindrical connector part which we will also model as a cylinder. We use the height of the connector as height of the cylinder for the collision geometry. The radius of the cylinder again is the maximal elongation from the flange z axis. The position of this cylinder is also (0,0,0), which means it is exactly below the previous element.

The last element of the gripper is the suction part. It will be modeled as a box because it is elongated in one direction. We choose the box dimensions such that the suction part is completely contained in the box. It is important to check that the orientation of the modeled box corresponds to the orientation of the real gripper element. In this case, the longer side of the box is in the x-direction of the flange coordinate frame. The position of the box element is also (0,0,0) because it is mounted directly in the center below the previous cylinder.

TCP position

Finally, we have to set the TCP position. By default, this position is in the center of the bottom plane of the last gripper element. The position of the TCP is used to decide whether a grasp is collision-free or not: Whenever any part of the gripper would be in collision with the load carrier while the TCP is at the grasp point, the grasp will be detected as collision. Since suction grippers have to slightly push against an object to grasp it, the TCP must be set to a position inside the suction part, e.g. 2 mm. Thus, we set a TCP offset of (0,0,-0.002), which means, that the TCP will be shifted along the z-axis by 2 mm in negative direction.

Checking and saving the gripper

Below the TCP offset, the gripper creator displays the position of the TCP in the flange coordinate frame.

Warning

The computed position of the TCP in the flange coordinate frame must be the same as the TCP position defined in the robot controller to ensure that the gripper geometry is modeled correctly with respect to the gripper height.

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Fig. 58 Final model of the gripper geometry

The last step of the gripper creation is to save the gripper. It will now be shown in the Grippers section of the Web GUI’s CollisionCheck tab and can be used in the ItemPick and BoxPick modules.