How does a four legged robot work?

You might have seen a traditional wheeled robot. The traditional wheeled robot is best for flat surfaces but, it is struggling hard on difficult surfaces (like moon surface). Here comes the use of the latest robot technology- A four legged robot and wheeled robot as shown in fig 1. These robots can move individual legs to specific spots and can also tilt their bodies.

Now, let’s get into the most interesting part of this article ― the movement of the four legged robot.

Fig 1 : Four legged robot and wheeled robot

Leg movement of four legged Robot

In order to move a leg, it should be lifted, extended, and then brought forward. This is accomplished with two servo motors. Firstly, motor 2 is rotated in an anticlockwise direction, which causes the leg to be raised off the ground (refer fig 2a). Afterwards, motor 1 is rotated in the anticlockwise direction to extend the leg (refer fig 2b).

Fig 2a: Rotate motor 2 to lift limb 1 of leg
Fig 2b: Rotate motor 1 to lift limb 2 of leg

Lastly, motor 2 is turned clockwise to place the leg on the ground as shown in fig 2c. It is clear here that the contact point of this leg has moved forward (from point A to B). This is the leg movement of a robot from one place to another place.

Fig 2c : Robot’s leg has moved forward from point A to point B

In this condition if the robot body also moves forward, we are done with the robot's forward motion.

Body movement of four legged robot

Let’s see how the robot body can be moved forward. It is clear from fig 3a, that to achieve this forward motion, we need to move all these four points of the robot body in a straight line.

Before proceeding further, we should clearly understand how these yellow points are affected by the rotation of the two motors. Let’s focus on the first yellow point (refer fig 3b). Let's rotate motor 1 only. When the motor 1 rotates, the bottom limb rotates clockwise and the other limb rotates anticlockwise.

Fig 3a : Robot body moves in forward motion
Fig 3b : Path traced by Motor 1

This is due to the reaction torque of the rotor against the stator. The path traced by the yellow point is shown in figure 3b. Now, let’s rotate only motor 2 and find out the path traced by the yellow point (refer fig 3c).

Fig 3c : Path traced by Motor 2

The trick of the robot body’s forward movement is that you mix the movement of these two motors. I can operate motor 1 for a small movement then motor 2 for another small movement. The net movement of the yellow point is interesting. I have drawn it in fig 4a. It has moved forward. However, if you observe carefully, from top the yellow point has moved at an angle, not straight (refer fig 4b).

Fig 4a : Movement of yellow point
Fig 4b : Yellow point moves in angle

To fix the issue of the body moving at an angle not in a straight position, we need to introduce one more motor in the robot. This motor can turn the robot’s body or leg in a vertical axis. So, the solution is simple. Just activate the third motor (refer fig 5a) after the second motor. The yellow point movement is perfectly straight now. We can get the beautiful forward motion shown in Fig 5b, created by the combined motion of three motors. It is brilliant, right? The same operation is happening at the other three legs as well simultaneously to make all the four yellow points move forward. The smoothness of this straight line depends on the minimum angle at which the motors can be rotated.

Fig 5a : Path traced by Motor 3
Fig 5b: Combined motion of three motors

If you remember, we had stopped the robotic movement after the first leg was put forward. Now, with this body forward movement the robot has reached this position. After moving the first leg and the body, the next leg it should move will depend on maximum stability. Moving leg 3 will provide the maximum stability margin(refer fig 6). A stability margin is always preferred to remain stable even if an unexpected force or high momentum is applied. I will explain the details of the stability margin in the next article. After that we move leg 2 and leg 4 to get a complete walking cycle.

How does the robot turn?

Now, I will explain how the robot turns (in fig 6). The first leg is raised during a turn, and instead of placing it forward, it is rotated and then placed on the side. When I repeated the same operation on the other legs the robot turned.

Fig 6 : Robot turning movement

I hope you have learned a lot of things about four legged robots. In the next article. I will explain the impressive features of robot stability and its sensor system.

Thanks for reading!


Mrunal Shinde

Mrunal Shinde is an experienced product developer with extensive knowledge of engineering principles, theories, specifications, and standards. Mrunal has worked on projects such as piezoelectric material and legged rovers. For more details of author check this link.