Should My Robot be More or Less Squishy?
Ben Finio, Ph.D., Science Buddies
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AbstractIn our introductory VoxCAD project, we provided a template with four materials: two active materials that expand and contract in the physics sandbox mode, and two passive materials — one hard and one soft. The project mentioned material properties, but it did not go into much detail — you just used the default settings from the template. Changing the material properties can drastically change how your robots behave. Engineers must take material properties into account when designing robots and other machines. For example, they might want to use a strong metal material for the legs of a robot to support its weight, but a soft rubbery material for the feet to help absorb impact when it walks.
This document will explain what the different material properties are, what they mean, and how to change them. But because this is an abbreviated project idea, we will not provide specific directions for how to run new tests or simulations with your robot designs. Deciding exactly what to do will be up to you.
To start, click on the Palette tab in the bottom-right corner of the VoxCAD window. Within the Palette tab are three sub-tabs: Appearance, Model, and Physical. These are where you edit the material properties, as shown in Figure 1.
The palette tab has three sub-tabs which are appearance, model, and physical. The sub-tabs are used to edit the material properties.
Figure 1. The Palette tab is in the bottom-right corner of VoxCAD and has three sub-tabs above it: Appearance, Model, and Physical. Select the material you want to edit from the list in the upper right, then edit its properties using the settings in these three tabs.
Important: Be sure you have the material you want to edit selected from the materials listed on the right-hand side of the screen.
The Appearance tab enables you to change the color of a material. This only affects how a material looks and does not change its other physical properties.
The Model tab lets you select the mathematical model used to calculate how a material deforms (stretches, bends, and compresses). We recommend sticking with the "Linear" model, which means that your materials will always "bounce back" to their original shape and never "break."
The Physical tab lets you set additional physical properties of the material.
Here is a complete list of properties you can change, assuming you are using the "Linear" option in the Model tab. These are only brief explanations — you can do more research about each topic if you want to know more.
- Elastic modulus is the material's stiffness, or how hard it is to stretch, bend, or compress. A material with a low elastic modulus will be stretchable and rubbery. A material with a high elastic modulus will be hard, like metal. Elastic modulus is measured in megapascals (MPa).
- Poisson's ratio is a measure of how much a material expands in one direction when it is squished in another direction. Imagine holding a ball of dough in your hands — when you flatten the dough, it will expand outward. Poisson's ratio does not have units.
- Density is a material's weight per unit volume. It is measured in kilograms per cubic meter (kg/m3).
- Coefficient of thermal expansion (CTE for short) determines how much a material expands or contracts as temperature changes. Note that, with our default settings, the red material has a positive CTE and the green material has a negative CTE. This makes one expand while the other contracts, and vice versa. CTE has units of 1/ °C.
- Temperature phase sets the time lag between expansion and contraction of materials in radians (radians are units used to measure angles. One radian is approximately 52.3 degrees, and there are exactly 2π radians in a complete circle). If two materials are 180 degrees (or π radians) out of phase with each other, then one will be at its maximum expansion while the other is at its maximum contraction. If they are 0 degrees (or 0 radians) out of phase with each other, then they will expand and contract at the same time.
- Static coefficient of friction determines the friction between a material and the ground when it is not moving (static). Coefficient of friction does not have units.
- Dynamic coefficient of friction determines the friction between a material and the ground when it is sliding (dynamic). Like static coefficient of friction, this does not have units.
To test what these settings do, change them one at a time, and by small amounts. For example, the coefficients of thermal expansion are originally set to +0.01 and -0.01. But if you change them to +100 and -100, your simulation may change so dramatically that it will be hard to understand what happened. It is more realistic to change them to +0.02 and -0.02 to start.
If you want to use more than the four basic materials, you can also create new materials by clicking the Add Material button in the Palette tab, represented by a plus (+) symbol. Watch the tutorial video in the Bibliography to learn about some of the more advanced options in the Palette tab.
Remember, because this is an abbreviated project idea, exactly what you do now is up to you. Here are a few ideas:
- Import your simulations from the introductory VoxCAD project. What happens when you change different material properties? Can you make your robot move faster by changing the material properties, without changing its shape?
- Look up material properties for real-world materials and change your simulation accordingly. How do your robot designs behave after making the switch?
- Create two robots with identical designs, but different material properties, and race them. How do different properties affect speed?
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