Abstract

In our introductory VoxCAD project, we provided you with a template file that included default material properties and physics simulation settings. These settings include friction, gravity, and temperature — all of which can drastically change how fast your robots move. However, we did not explain what exactly all these physics simulation settings meant, or how to change them. You can access the physics settings through the Physics Settings tab after entering the Physics Sandbox mode, as shown in Figure 1:

We highly recommend watching this video tutorial as an introduction to the different physics settings:

For reference, we have listed the available settings here. Note that you can also hover your mouse cursor over the text describing each setting in the VoxCAD window to read a description. Note: this page was created in July 2013 using VoxCAD version 0.9.9. Future versions of VoxCAD may add or remove options, or change the exact locations of different buttons in the program. The general concept should remain the same, so this page should still serve as a good introduction to the physics settings.

These are always available toward the top of the Physics Settings tab:

• Stop condition determines whether your simulation will stop automatically, or wait for you to hit the Pause button. There are several different options for stop conditions.
• Time step changes the amount of time between each step of the simulation. A smaller time step will make the simulation slower but more accurate. In general, you want to keep the time step as small as possible.
• Ground damp ratio determines the "damping" (similar to friction) between objects and the virtual "world" surrounding them. A low ground damp ratio will enable objects to move around with relative ease, like moving through air. A high ground damp ratio will make it more difficult for objects to move, like moving through water or syrup.
• Enable self collision: This checkbox determines whether different objects will bounce off each other. If left unchecked, objects will pass through each other.
• Collision damp ratio determines the amount of energy lost in collisions. A low collision damp ratio will make the simulation more "bouncy" when objects collide with each other or the ground. You must check the enable self collision checkbox to use this control.

These settings are available under the Environment sub-tab:

• Enable temperature: This checkbox enables simulation of temperature, which will affect materials that have a non-zero coefficient of thermal expansion. When checked, you can set the model temperature in °C using the slider bar or the text box to the right.
• Vary temperature: This checkbox enables temperature oscillations, with an amplitude set by the Temp ( °C) slider (or textbox) and a period set by the Period (sec) slider (or textbox). Materials with nonzero coefficients of thermal expansion will alternately expand and contract if this option is turned on. A bigger period means slower oscillations, and a smaller period means faster oscillations.
• Enable gravity turns gravity on or off in the simulation, with a value in meters per second squared (m/s²) set by the slider bar or text box. Note that "down" requires a negative number for gravity — if the number is positive, your objects will fall "up," away from the virtual floor.
• Enable floor turns the floor in the virtual world on or off. If you disable this while gravity is on, objects will "fall" through space.

These settings are available under the Other sub-tab:

• Bulk damp ratio: This value adjusts the damping between connected voxels — a higher bulk damp ratio is like having more friction at each bond. A low bulk damp ratio will make your model more "jiggly".
• Enable velocity limit: This checkbox enables you to limit each voxel's maximum velocity.
• Maximum voxel velocity determines the maximum velocity at which voxels can travel. You must check the enable velocity limit checkbox to use this control.

Remember, this is an abbreviated project idea, so exactly what you do next is up to you. Here are a few suggestions:

• Import the robot simulations you used for the introductory VoxCAD project. What happens to your race as you change the period of the temperature oscillations? Does it change the speed of your robots? Does the race have a new winner, or does the order stay the same? Do some robots have a maximum speed that only occurs at a certain frequency? If so, look up the concept of resonance.
• Can you design a robot that completely changes the way it moves depending on the magnitude of the temperature oscillations? For example, can you design a robot that bounces with low temperature changes, but rolls with high temperature changes?
• How does changing the different damping coefficients affect the speed of your robots?

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