Try to make the robot rotate its arms in a circular motion. Think of a windmill. How many pattern positions does it take to create a rotation? Why can't you do it with only two positions? Did you notice that the robot controller will take the path of the shortest angular distance from one pattern position to the next?
There are many ways to make a robot move across the floor other than walking. This is called locomotion and it relies on creating a repetitive pattern, also called periodic motion. Try working with the robot on the floor to achieve some sort of locomotion. What would you need to add to the robot to have it move in a certain direction?
Falling over is easy to achieve, but what is it that causes the robot to lose its balance? Xemo has a checkered ball that represents the center of mass of the robot. An image of the ball is projected onto the floor - we call that the center of gravity. Observe what happens when the center of gravity moves out from under the feet.
Lying Down & Rolling Over
There are two common positions for lying down: on your belly (prone position) and on your back (supine position). You can easily start a crawl move on your belly, but not so easy starting on your back. Try creating motions where the robot can roll over from prone to supine position and back again. This will help when you link moves together in a competition.
After the robot falls over, you have two choices: reset the robot or come up with a motion to get up from the floor. During game competition, players lose points and time if they reset the robot. Try to come up with ways to get the robot to stand up from a lying position on the floor.
Walking is something that is easy for most humans, but it's harder that it looks. Robot walking is a complex dynamic motion that shifts the center of mass from one foot onto another. Walking is really like falling forward each step and catching your self to regain balance. That might explain why it's difficult to get the robot to walk without having an automated balance control to recover from slight changes.
Walking can be difficult for people suffering for neurological diseases such as Parkinson's. With Parkinson's, people may have trouble starting a walking motion and their step (stride length) is often shortened. Once they are walking they can continue to walk but then may have trouble stopping. Why is this so? Think about how you start walking; the way you shift your weight onto one foot first. And when you want to stop, how does your motion change?
Robustness and Perturbation
In engineering, you'll often hear people talk about robustness. But what does that mean? Being robust means that the system continues to operate as expected even when unexpected changes happen. One example is a walking robot that can continue to walk even after being pushed from the front, side or back. It's robust when it can quickly recover from the push and continue walking. The push is what is known as a perturbation.
If you've watched competitive gymnastics, you've probably heard the pros talk about sticking the landing. This means that when landing after a jump, the gymnast does not fall or take an extra step. It's quite hard to do since it means that the body has to stop all of the motion smoothly. If you adjust the joint spring constants, you can make the robot jump quite high. But can you make it land smoothly? Do you have to make changes to the joint springs to absorb the impact of the landing? How would you combine balance and change in spring stiffness to land and remain upright?
Automated Balance Control
When standing upright or when walking, the robot needs to maintain balance to stop it from falling over. One way to do this is for the robot to keep the center of gravity somewhere between the robots feet. This is done using a feedback control circuit for balance. Xemo has just such a controller built in, but do you know how it works? Think about what your muscles have to do if you lean over too far. Some of the control comes from the ankles, and other control from the hips and knees. Which muscles are stronger and when do you use them to keep in balance? Is it different when you stand on one foot than when you stand on two feet?