Introduction

Have you ever wondered how something walks? A majority of us do not have memories of learning to walk, so walking is more or less instinctual. Animals more or less know how to walk when they are born, but a robot has no idea what it is or even can do. One has to tell the robot what to do. That’s why for MicroSpot, we researched how four-legged animals walk so that we could implement a similar technique on a robot.

Two (2) Leg Sync

The first video I looked at was Boston Dynamics’ Spot. Through Youtube, I was able to slow down the video so I could better analyze the way Boston Dynamics had their Spot walk. They had the Spot move with two legs on the ground at all times. For a better mental image, it would have the right front leg and the back left leg move forward while the other legs are on the ground. It was almost like it was marching with the two legs to walk. One interesting thing to look in that video are it’s feet, which are rounded.

Figure 1 Boston Dynamic’s Spot

Another robot “dog” that implemented a similar walking style was SONY’s Aibo. Even though Sony’s Aibo implents the same walking method as Boston Dynamics. In the video it’s apparent that it does not walk as cleanly as Boston Dynamic’s Spot. Another noticable thing for Aibo was that it swayed a bit more than Spot.

Sequential Walking

By watching videos of dogs, I saw similar walking styles. However, I noticed some of them had more of a sequential walking pattern. In the sequential walking pattern, the dog would start off moving forward with one of the front legs. For this example let start off with the front right leg. When the front leg was back on the ground the back left leg would start to come off the ground and when that leg was back on the ground the front left leg would lift off. Then when the front left leg is back on the ground the back right leg will lift and that cycle would be repeated for more steps.

Figure 2 Sequential Walking Diagram- Showing how Spot would take 1 step

Pros and Cons to Each Style

The main benefit of using the sequential walking style is that there’s more stability. Since there are always three legs on the ground it should not topple over as easily. The drawbacks of this way it that it would be a lot more work to code and get the timing on when each leg should be starting the walk cycle. For the two legs at a time walking style, it forfeits stability as seen in “How To Make A Very Simple 4 Legged Robot” by Amr MCI. In the video, the robot can be seen swaying side to side and it sort of leans on the back leg that’s supposed to be in the air. The benefit of this style is the ease of programming it. One would only need to experiment with the phase between the 2 sets of legs.

Flowchart

These are flowcharts of how I thought the two walking algorithms could be implemented for MicroSpot.

Figure 3. Sequential Walking Flow Chart- Using Photo Interrupters

Figure 4. Two(2) Leg Sync Flowchart- Using Photo Interrupters

For both of these flowcharts, I was trying to achieve one step cycle. The Sequential Walking flowchart and the Two (2) Leg Sync flowchart went with the assumption that one leg walk cycle would be 50% air time and 50% ground time. I have photo interrupters to sense the position of the crank. Ideally they should be set at rest position. Instead of the photo interrupters you can use Hall Effect sensors to know the position.

Conclusion

Time is probably one of the biggest constraints for this project, so I think the best way to approach this is to implement the 2 legs at a time method first. Once that’s done if stability becomes a problem then moving to sequential walking should be implemented. I’m sure there are plenty of other ways that you could get a four-legged robot to walk. These were just two that I could blatently see through videos. Hopefully this will be useful for future generations of MicroSpot.

References/Resources

How to make a very simple 4 legged robot- https://www.youtube.com/watch?v=wY08f3MIDnc&t=30s
Boston Dynamic’s Spot- https://www.youtube.com/watch?v=aFuA50H9uek
Sony Aibo-https://www.youtube.com/watch?v=AFNmKF_pgL8&t=17s
Dogs walking- https://www.youtube.com/watch?v=BqtqEbOilfs
Feature Image-https://www.dogingtonpost.com/the-benefits-of-walking-your-dog/

MicroSpot: Walking on 4 Legs