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Hi, I'm Jason McKenna, director of global educational strategy for VEX Robotics. Welcome back to our moments in engineering series, where we talk about engineering concepts that can be applied to both classroom and competitive robotics with VEX engineers.

I'm Art Dutra, director of mechanical engineering at VEX Robotics. So Art, something used to happen to me all the time when I was teaching. The students would always want to free build the robot. They didn't want to follow the build instructions, whether it was with a VRC robot or an IQ robot, it didn't really matter. And when they would build their robot, invariably it would never drive straight. They would say, "Mr. McKenna, my robot would not go straight." It was because they really didn't understand this concept of center of mass. So explain to me center of mass, please.

The center of mass is a crucial concept in robotics. Every single part on your robot has mass. The center of mass is the point at which, if you can balance your robot on the tip of a pencil or pen, it's perfectly balanced. One of the big reasons why robots may not drive straight is that the center of mass is not near the center of the robot. For example, if you have a lot of stuff on one side of your robot, you have a lot of mass over these wheels and less mass over those wheels. That might cause an issue where the motors are actually working differently, and the heavier side of the robot may drift behind. This may be one of the reasons why, when you're trying to program a robot to drive straight, you have issues where it may drift.

The other issue with center of mass is if it's very much biased to one side, it can affect the turning point of your robot. If it's heavily biased, your turning point of all your wheels is no longer in between the traction wheels. It's going to be biased towards the center of mass. So instead of your robot turning in the middle, it may turn more off one side, and that may be unpredictable.

One of the big aha moments for the students is realizing that as they add stuff to their robot, the center of mass changes. For example, if I'm in a competition and I want to lift something up, I add this claw, and the center of mass on my robot could potentially change. Your center of mass is however the robot exists at this point in time. If you have linkages, as you move the linkage, the location of all the parts relative to each other is changing. So the center of mass, like the average point where it's going to balance, is going to change.

Typically, in most robots that are trying to lift things up, it's probably not going to shift left or right. It's most likely going to shift up or down. You can have issues, like if you build a robot with a claw or an arm that reaches up really high, and if you're trying to lift a really heavy object, your robot might be stable when it's down low. But then when you try driving high, you may realize, "Hey, my robot starts tipping or wants to fall over." The reason for that is the center of mass vertically on your robot. Again, it's going to be, if you can balance an imaginary pen tip on your robot.

When your robot is really high, it is stable if your center of mass is in between all of the wheels that are touching the ground. But if you build a robot really high, and the center of mass starts getting really close to one of the wheels, now you might have an issue. Or if you try to accelerate your robot quickly, or you go onto a ramp, now your center of mass is on the opposite side of the wheel. It's no longer in between the wheels that are touching the ground. That's when your robot is going to tip over.

Yeah, that's really interesting. And then I would imagine as you're grabbing objects that have a certain weight, that would also affect because you're adding mass to your robot, right? So that will also affect your center of mass. Yeah. It'd be thinking connected to your robot, all contributes to center of mass, whether it's gear mechanisms or game objects you're picking up or anything else your robot is, you know, grabbed onto.

So talk to me for a minute about how that would affect, so if I'm going into a competition, if I'm in a VRC competition, and I know that I have to pick objects up and put them in something that's high, so I'm going to need some type of linkage or a claw on my robot to be able to do that. Talk to me how that would affect your decision about how you would make your drivetrain for your robot. Knowing that as I lift certain things up, it's going to change the center of mass on my robot.

Yeah. So one of the things I would want to look at is, as you're trying to pick up objects, there are ways you can design a robot. The ideal way is to try to figure out a way of, hey, can I fit this mechanism in between my drivetrain? Where if I pick up a heavy object, my center of mass stays within the robot without adding weight. Sometimes that's not always possible. So sometimes teams will add weight to the robot. Like if I have a very heavy object here, I'm willing to try to add weight on the opposite side of the robot to keep trying to balance it out. 'Cause I have limitations on the size of my robot, right? Yeah. So I can't make my drivetrain this big to always keep the center of mass between my wheels.

Yeah. Yeah, so one way is like on VRCs is you could basically change where you mount the battery or things like that. That's a popular way. Another thing that's popular in VRC to kind of allude back to your point about changing your drivetrain, is even though you start in a certain size, sometimes teams have extra wheels that pop out of the robot as soon as the match starts. And those extra wheels pop out and rotate down. Now that adds a longer wheels on either side of your robot, wherever they pop out. And that increases your contact area with your ground. So that helps ensure that your center of mass vertically in your robot, if you're picking up heavy objects, ensures it doesn't go beyond your wheels and your robot doesn't tip over.

Now one of the things I've seen also in terms of center of mass, in terms of when you're talking about the robot tipping, because the motors are so powerful with this robot. If I go fast or if I accelerate that can also lead to the robot tipping, right? How's that all connected with center of mass?

Yeah, so on robots that are very quickly accelerating. If you have a very high center of mass, all, when you're accelerating your robot, the only way your robot accelerates the wheels, that's putting force against the ground. But if your center of mass is here, everything has inertia. You know, if an object is at rest, it wants to stay at rest. If an object is in motion, it wants to stay in motion. So when your center of mass is here and it has inertia and it doesn't, it wants to resist trying to be accelerated. This is actually going to cause it to kind of tip backwards a little bit.

Oh, okay. So if you accelerate slowly, you may not have an issue, but because you have force down here in the center of mass up here, your robot is going to want to tip. So that can be an issue, if you, you know, if you really want to accelerate quickly, you want to try to keep your center of mass low, that will reduce how much your robot is tippy, or you may have to reduce like, hey, I have to, I have to slowly ramp up or slowly ramp down. As I drive my robot to ensure that I don't have issues with the center of gravity and tipping.

So these are all different things that someone has to keep in mind, both from a building perspective and engineering perspective, but also from a driving perspective. As you're actually driving the robot in a competition, it is important to ensure you don't accelerate too quickly to get that action on there. And that's really good information about the center of mass and how you can apply it.

How would you introduce this concept? Whether I was in the classroom, with this robot, or at a competition, how would you introduce this to your team or to your students to get them thinking about that during the initial design of the robot? One of the easiest ways to visualize where this vertical center of mass is actually located is if you put your robot on ramps of various angles. If you assume that the floor is a ramp with a zero-degree angle, it's flat, and you could basically see that. But if you put it on a slope, you can see, as your robot tips up, your center of mass, relative to your robot, stays in the same spot. However, because gravity is still going down, your center of mass is getting closer and closer to the wheels and the end of your robot. Eventually, you get to a point where the slope is so steep that your center of mass is beyond your wheel. Now you can easily see, as you slowly tip the robot up, it gets more and more tippy. So that's an easy way to visualize where the center of mass is on a robot in a very isolated manner.

Excellent. Excellent. Well, thank you, Arthur. That was awesome. Talking about the center of mass and how it applies to the robot, we also touched on a little bit of physics with inertia and how all of these things tie together. We want to be able to create a robot that performs well, whether in a classroom challenge or a competition.

Thank you very much for enjoying this installment of Moments in Engineering. We talked about the center of mass and look forward to seeing you again in our next installment. Thank you.

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