Collisions of solids and solid state physics
Collisions of mechanical elements and solid state physics in Simultus.
Simultus enables modeling the behavior of 3D solids during mechanical collisions.
The following describes how to configure the program correctly and optimally, in regards to the CPU load.
Selection of device elements for which the collision mechanism will operate.
Solids imported into 3D space have no physical properties by default. They are only a visual representation of the mechanical components. They can interpenetrate and are not affected by the collision (reaction) mechanism.
This solution was applied because the calculations of the solid physics put a heavy load on the computer processor. In practice, when modeling devices, the effects of only a few of its components are important. For example, in the case of the conveyor with gripper shown in the graphic below, the impact of the orange part of the gripper on the transferred green box and the impact of the box and stands on which the box will be put down will be significant.
Therefore, it is enough to define the collision mechanism only for these few elements. The conveyor frame and positioning modules do not have to be physical. Thanks to this, the calculation of the model's behavior will be optimized and therefore faster.
Creation of collision meshes.
In order for the solids to interact with each other, a collision mesh must be created for each of them. It is a mesh that will represent its shape in the algorithm responsible for the collisions of mechanical elements. The mesh is created by Simultus algorithm, but the appropriate type must be selected.
VERY IMPORTANT NOTES FOR THE CREATION OF COLLISION GRIDS:
1. The mesh for a given solid is not an exact representation of its shape!
2. A more precise collision mesh of an element can be built by dividing it into several parts with separate simplified collision meshes.
The second point of the above remark will be explained in more detail below.
By selecting a figure in the menu on the left side of the screen, its properties window will appear. Among them, the physical properties will be shown below.
In addition to the mass, air resistance and material, in the 'Collision' window you can set the type of collision mesh that will be assigned to the solid.
The types of collision meshes available in the program are described below:
[none]
The collision mesh is not mapped to the solid
[simplified] THE BEST CHOICE!
Simple collision mesh. It is the fastest-performing mesh type that most accurately follows the shape of a solid, provided that the solid does not have dents. Nevertheless, if it is possible and the dents do not affect the simulation results, it is recommended to use this type of collision mesh.
Below are shown the collision meshes created for example solids. The green lines in the drawings show the edges of the collision mesh surrounding the gray bodies. It is worth analyzing them carefully in order to avoid errors and unexpected behavior of the simulation after selecting the collision mesh.
When analyzing the shapes of the above-mentioned exemplary collision meshes, it can be seen that they surround the solids, ignoring any dents and holes. In some cases, you can eliminate this problem by dividing the body into several parts.
Connecting collision meshes
If the solid has dents, then it can be divided into smaller component solids and separate simplified collision meshes for them can be created. This way, in many cases, we obtain a very good effect, combining the accuracy of mapping the shape of the solid and the speed of calculations. Consider the solid number 4 in the figure above. It consists of a base and side walls. The created simplified mesh fills the entire space between the side walls, so it very inaccurately reflects the shape of this simple solid. In such case, the solid can be divided into three components: the base and two separate side walls. Three separate simplified collision meshes should be created for them. This way, we will obtain the best representation of the solid’s shape, which also puts the least load on the processor during the simulation.
[sphere]
Sphere-shaped collision mesh.
This type of mesh also places a low CPU load during simulation. However, it perfectly reproduces only one shape - a sphere. Nevertheless, it can be used for solids which shape is close to it.
[collider A]
A collision mesh consisting of a large number of cuboids filling the interior of the solid. Compared to types B and C, their number is the smallest, so it works faster than them during the simulation. It may happen that the cuboids protrude slightly beyond the solid and they will 'cover' some holes and dents.
[collider B]
A bit more precise than type A. It contains more cuboids. It works slower than type A. It may happen that the cuboids protrude beyond the solid, or they cover some holes and dents.
[collider C]
A collision mesh that most densely fills the body with cuboids. Loads the processor during simulation more than type A and B. Cuboids may extend beyond the area of the solid or not fill it completely.
Comparison of collision meshes for selected solids:
Shown belowe are different types of collision meshes for selected solids. On the right, there is a solid with a simplified mesh, consisting of one cuboid, drawn on its view. On the left side there are meshes of types A, B and C. It can be seen that while the simplified mesh consists of only one cuboid, there are several dozen of them in the mesh of type B and C, and in the case of a more complicated shape of the solid there will be several hundred of them. This is what affects the speed of simulation computing. For this reason, we also suggest dividing the solid into parts and creating simplified meshes for them. The use of A, B and C meshes is justified by the fact that they can be created quite quickly (with one click) for more complex solids.
Collision mesh algorithm errors.
Above are shown the collision meshes generated for the nut. There are cases in which the A, B, or C type of collision mesh algorithm does not work properly. This is the case with solids of complex shapes and with relatively thin walls in relation to their dimensions. In this case, we suggest dividing the solid into several parts or, if possible, changing the shape / dimensions of the solid. Type B and C meshes can, in some cases, extend beyond the shape of the solid. We have not found the simplified and spherical mesh algorithm to work incorrectly. However, in the case of the nut above, the simplified algorithm, as previously described, did not take into account the dent on the bottom, which may be crucial in some cases.
Two ways to use solids with collision meshes.
Solids with collision meshes can be used in two ways:
- as a solid rigidly bound to the controlled 6DOF_AXIS coordinate system,
- as a solid that moves freely along with the 6DOF_SOLID system.
A solid rigidly bound to the controlled 6DOF_AXIS coordinate system:
We attach such a solid to the 6DOF_AXIS coordinate system. It has a collision mesh, so it can participate in collisions, but its movement is not a free movement. It is rigidly bound to the 6DOF_AXIS coordinate system and moves only with it. The movement of such a solid can only be forced by changing the position of the coordinate system. This can be done by using the function block 6DOF_AXIS in the diagram:
In the example of the conveyor above, solids of the gripper are moved in this way.
Such a solid can affect other solids, but other solids cannot move it. When it collides with another solid in the same way connected to another 6DOF_AXIS coordinate system, one of the two situations described below will occur. This will depend on the type of connection between AXIS and Solid (STL) set in the 'options' menu.
If the connection is permanent, the solids will pass through each other as if they do not have a collision mesh set.
If the connection is stringed, then both solids that collide will vibrate.
Vibrations of solids will also be visible when, in the case of a string connection, the solid will have a collision with the wall limiting the area.
In reality, such situations cannot happen, therefore in the event of any collision between such solids, these collisions should be eliminated and not allowed to occur.
A solid that moves freely with the 6DOF_SOLID system.
We attach such a solid to the 6DOF_SOLID coordinate system. From that moment on, this system will be rigidly bound to the solid and will move with it. The movement of the solid will be affected by collisions with other solids. In the example above, the green box is an example of such a connection. The positions of this block can be read in the function block diagram using the 6DOF_SOLID block:
NOTE: During simulation, the solid position available on the 6DOF_SOLID block outputs is updated no faster than 30 times per second. Which is much slower than the frequency of calculating the state of the function block diagram and has nothing to do with the frequency set for the simulation.
After connecting the solid to the coordinate system, we can change its position by changing the positions of the entire coordinate system. During the simulation, we cannot change the position, layout or solid from the function block diagram. The only possibilities to change the position of a free solid are:
- solid displacement through collisions with other solids or the boundaries of the physics area.
- using the GO HOME button
from the 6DOF_SOLID properties menu, which moves the selected solid to its base position.- using a
button from the top menu that moves all the shapes in the project to their base positions.The base position of the coordinate system can be set by pressing the SET HOME button
from the top menu.NOTE: If the center of the coordinate system moves beyond the boundaries of the design area, the solid will stop moving even though there is no collision with another solid. It should then be moved by shifting the coordinate system so that the center of the system is within the project area. For this reason, it is best to place the 6DOF_SOLID coordinate system and the solid in relation to each other so that the center of the system is inside the solid.
The boundary area of physics.
Physics only works in the project area. The boundaries of this area can be changed in the 'options' menu for each axis separately:
IMPORTAT! Simulation limitations (read more...)