Can the part be recognized? Most parts can be recognized, but there are some important limitations. A part that can be recognized by the SCAPE system needs to meet the following requirements: • The part must be rigid (no hinges or other possible deformations) • The part must be opaque (transparent glass or plastic cannot be recognized) • The parts in the same batch must be very similar (fruit does not meet this condition) • The part cannot have a mirror surface (SCAPE can handle highly reflective surfaces, but not mirror surfaces)
In addition, if the part has a large "open" area and does not have any obvious surfaces, it will generally not be recognized. See the following image for an example. Notice that many of the parts below are actually quite reflective, but SCAPE can still recognize them.
Bin-Picking target parts, all parts can be bin-picked by the SCAPE system except for the two parts marked with red crosses on the upper right. The reason why these two parts cannot be processed is that they have no obvious continuous surfaces and a lot of other parts can be seen under the top part.
Can the part be picked from all directions? When using bin picking for out-of-order parts, it is important that the parts can be picked from all directions. If the parts cannot be picked from one side or one angle, the following results may occur: • Parts close to the bin wall or in the corner cannot be picked from a certain direction; • During bin picking, the system may encounter a layer of parts that are in a posture that cannot be picked; • If the customer requires that almost all parts in the bin must be picked, there is a high possibility that this requirement cannot be met because many remaining parts are in a posture that cannot be picked.
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 from the vertical direction by up to 40 degrees during Bini-Picking.
All these variations can be easily defined by using SCAPE Part Training Studio. In addition, the system can take symmetry into account (a round suction cup has complete rotational symmetry, while a two-finger gripper has two-fold symmetry (rotated 180 degrees)). By taking symmetry into account, the system can rotate the tool unit to an angle that will not collide with the bin. There are many types of grippers. Scape tends to choose the following grippers. The first one listed is our preferred type:
1. Round vacuum cup. This gripper has great flexibility (can be offset at a large angle from the surface vertically) and has complete rotational symmetry. The selected vacuum cup should have at least 2.5 corrugations to provide the greatest flexibility possible. In addition, this gripper is the most cost-effective.
2. Round magnetic gripper. It can provide stronger suction (about twice as much as vacuum cup); but it is less flexible than vacuum cup (the gripping angle must be very close to the vertical direction). In order to make the magnetic cup as flexible as possible, Scape has developed a special mounting mechanism, but the disadvantage is that it is not as cost-effective as vacuum cup.
3. Simple straight fingers. This finger gripper refers to two parallel fingers without any bends, which can be used to clamp certain edge positions of parts. When there is a suitable gap, the finger gripper can also be used to grasp from the inside of the gap. In fact, grasping from the inside is a better choice than grasping from the edge of the part, because this method is much less likely to be interfered with by surrounding parts. The disadvantage of this finger gripper compared to the first type of vacuum cup is that the linear finger requires a linear gripper drive and expensive collision sensors, and it takes up more space and has only two-fold symmetry (rotated 180 degrees).
4. Custom fingers. In some cases, it is necessary to use a custom finger for the system. An example is that the part has many different thickness positions that can be used for gripping. In this case, the finger may need to be designed as "two layers" to correspond to the different thicknesses (the linear gripper drive may not always have the appropriate stroke). Of course, this will be more expensive and still require the use of a linear gripper to pick up things and a collision sensor.
5. Extended magnetic gripper. Sometimes, a single circular magnetic gripper may not be enough to provide sufficient suction force. In this case, Scape can provide an extended magnetic gripper combination (two circular magnetic grippers next to each other). The advantage is that it can provide twice the gripping force, but the disadvantage is that it takes up more space and has only two-fold symmetry (rotated 180 degrees).
6. Other grippers. In very rare cases, a new type of gripper must be designed. For example, Scape has custom developed a "two-stage" gripper for cylindrical parts, consisting of a vacuum cup followed by a finger gripper (see the following illustration).
The priorities listed above are not strictly required. Some types of grippers actually have similar priorities, such as 2 and 3. But one thing is certain, if we can use a circular vacuum cup to complete the material frame grasping work, we will definitely choose it. Below are illustrations of different grippers for material frame grasping.
Example of a custom finger, the finger on the left is used to grasp the circular part of the part with a specific diameter. The finger on the right is used to grasp from the inside of the part vacancy. The front section of the gripper is designed to be stepped (red circle), so it can accommodate two different diameter holes (need to take into account the travel of the actuator). However, one disadvantage of this design is that when using the back section of the gripper (that is, the part away from the finger tip), the outside of the finger needs to pass through the target part more, which may cause collision obstacles.
Example of a magnetic gripper that uses two circular magnetic grippers to form an extended row gripper for higher gripping forces. The gripper is mounted on a spring system to allow the gripper to align itself even when the approach position is not perpendicular to the target surface. Self-alignment is extremely important for magnetic grippers because the magnetic force drops significantly with increasing distance from the target surface.
Example of a tool cell with three grippers installed (also equipped with a structured light scanner). Since large part movements are not allowed in this project, the vacuum suction cups at the bottom of the tool cell are equipped with backstops to ensure more stable gripping. The three grippers on the left are used to grip from the inside of the part cavity (which is only a few millimeters deep). Finally, the gripper on the right is not used for bin picking work, but is used to complete other tasks in the work cell.
