Visual positioning and grasping method of parts

Can the part be identified?

Most parts are identifiable, but there are still some important limitations. A part that can be recognized by the SCAPE system needs to meet the following requirements:

Parts must be rigid(without hinges or other possible deformations)

Parts must be opaque(transparent glass or plastic cannot be recognized)

The parts in the same batch of parts must have a high degree of similarity(Fruit does not meet this condition)

Parts cannot have mirror surfaces(SCAPE system can deal with highly reflective surfaces, but not mirror surfaces)

Also, if the “open” area of ​​a part is very large and does not have any visible surface, it is generally impossible to identify it. See the image below for an example. Note that many of the parts below are actually quite reflective, but the SCAPE system can still identify them.

Visual positioning and grasping method of parts

Bin-Picking target parts, except for the two parts marked with red crosses on the upper right, all parts can use the SCAPE system to complete the Bin-Picking work. The reason these two parts can’t be handled is that they don’t have a distinct continuous surface, and there are a lot of other parts that can be seen below the top part.

Can the part be grabbed from all directions?

When using frame grabbing for out-of-order parts, it is important that the part can be grabbed from all directions. If the part cannot be grasped from a certain face or an angle, the following results may occur:

Inability to grab parts close to box walls or in corners in a specific direction;

·During the material frame grabbing process, the system may encounter a certain layer of parts in a posture that cannot be grabbed;

·If the customer requires that all the parts in the material box must be basically grasped, there is a great possibility that this requirement cannot be met, because many remaining parts are in a posture that cannot be grasped.

Due to the limitations of the 6-joint Robot and the risk of hitting unseen parts, the tool unit (including the gripper) is allowed to deviate up to 40 degrees from vertical during Bini-Picking.

All these changes can be easily defined by using SCAPE Part Training Studio. In addition, the system can also take symmetry into account, (a circular suction cup has full rotational symmetry, while a two-finger gripper has double symmetry (rotated 180 degrees)). By taking into account the symmetry, the system can rotate the tool unit to an angle that does not hit the bin.

There are many types of grippers, and Scape tends to choose the following grippers, with our preferred types listed first:

1. Round vacuum suction cups.

This gripper has great flexibility (can be offset by a large angle from the vertical to the surface) and has full rotational symmetry. The vacuum cups selected should have at least 2.5 layers of corrugation in order to provide as much flexibility as possible. In addition, the gripper is also the most cost-effective.

2. Round magnetic gripper.

It can provide stronger suction (about twice that of a vacuum cup); however, it is less flexible than a vacuum cup (the gripping angle must be very close to vertical). In order to make the magnetic chuck as flexible as possible, Scape has developed a special mounting mechanism, the disadvantage is that the cost performance is not as good as that of the vacuum chuck.

3. Simple straight finger.

By this kind of fingers we mean two parallel finger grippers without any bends, which can be used to hold certain edge positions of parts. The finger gripper can also be used to grab from the inside of the gap when there is a suitable gap. In fact, grabbing from the inside is a better option than grabbing from the edge of the part, as it is much less likely to be disturbed by surrounding parts. Compared with the first vacuum suction cup, the disadvantage of this finger gripper is that the linear finger requires the use of a linear gripper driver and an expensive collision sensor, at the same time it takes up more space and only has double symmetry (rotation 180 degrees) .

4. Customized fingers.

In some cases it is necessary for the system to use custom fingers. An example is a part that has many locations with different thicknesses that can be grabbed. At this time, the fingers may need to be designed as “two layers” to correspond to different thicknesses (linear gripper drives may not always have the right stroke). Of course, this would be more expensive, and still require the use of straight-line grippers and crash sensors.

5. Extended magnetic grip.

In some cases, a single round magnetic gripper may not be enough to provide enough grip, in which case Scape can provide an extended magnetic gripper pair (two round magnetic grippers next to each other). The advantage is that it can provide twice the grasping force, the disadvantage is that it takes up more space and only has two-fold symmetry (rotation 180 degrees).

6. Other grips.

Rarely, a new type of gripper has to be designed. For example, Scape has custom developed a “two-stage” gripper for cylindrical parts, consisting of a vacuum suction cup followed by a finger gripper (see illustration below).

The priorities listed above are not strictly adhered to. Some types of grippers don’t really have much difference in priority, such as 2 and 3. But one thing is for sure, if we can use the round vacuum suction cup to complete the frame grabbing work, then we will definitely choose it.

Below are illustrations of grippers for different frame grabbing.

Visual positioning and grasping method of parts

Example of a custom finger, the finger on the left is used to grab a circular portion of a part with a specific diameter. The fingers on the right are used to grab from the inside of the empty space of the part. The front part of the grabber is designed to be stepped (in the red circle), so it can adapt to two different diameter holes (need to count the travel of the actuator). However, one disadvantage of this design is that when using the back part of the gripper (that is, the part away from the fingertip), the outer part of the finger needs to pass through the target part more, which may cause collision obstacles.

Visual positioning and grasping method of parts

Example of a magnetic gripper that uses two round magnetic grippers to form an extended row of grippers for higher gripping power. The gripper is mounted on a spring system to allow the gripper to align the gripping position even when the approach pose is not perpendicular to the target surface. The ability to self-align is extremely important for magnetic grippers because the magnetic force decreases significantly with increasing distance of the magnet from the target surface.

Visual positioning and grasping method of parts

An example of a tool unit with three grippers installed (also with a structured light scanner). Since large shaking of the parts is not allowed in this project, the vacuum cup at the bottom of the tool unit is equipped with a backstop to ensure a more stable grip. The three grippers on the left are used to grab from the inside of the part void (which is only a few millimeters deep). The last gripper on the right is not used for bin picking, but for other tasks in the work cell.

The Links:   3HAC13389-2 ELECTRONIC

Published on 09/07/2022