# Creating a Robo-Dog Army: Tribological Aspects

Recently Boston Dynamics’ released a video of a robo-dog army pulling a truck down the road. The future is here. For now it looks like the good old Terminator I movie. But I guess we will also see the other parts of the sequel. Soon these things will look like real dogs. Or men. The good thing about this is that Tribology plays a role here 🙂

What has tribology to do with this movie? Well, friction is necessary for the robo-dogs to move the truck. While truck experiences friction that resists robo-dogs. So we are looking at two different types of friction, static friction and rolling friction.

Static friction is the resistance to movement prior to the initiation of sliding. When you are leaning back against a wall, it is static friction that keeps you from sliding down. In this case, it is the paws of the robotic dogs that are experiencing static friction. As their paws are in contact with the pavement, the force of static friction is preventing them from sliding backwards.

The truck is experiencing rolling friction, which is the result of the contact between the wheels of the truck and the pavement. Wheels do not remain perfectly round as the weight of a vehicle rests of them and this change of shape in the tires combined with the friction in the wheel bearings works to keep the truck from moving.

Obviously, in order for these hell creatures to start moving the truck, the static friction at the contact between the paws and the asphalt pavement must be greater than the rolling friction of the truck. The simplest way to estimate the static friction in this case is the use of old good Coulomb law:

Where is the force pushing the dogs to the earth, so . The mass of one dog according to Boston Dynamics is 25 kg , so each dog experiences the force of gravity of 245N. Assuming the static coefficient of friction between the paws and asphalt of around 0.7, the friction force per robo-dog is 171.5N, thus for 10 robo-dogs – 1715N.

This force must be greater than the force keepin the truck still – the rolling friction. It is tough to get the value for rolling friction, since it is influenced by many things, but we know it is much lower than the static COF, say 0.02 (tire on asphalt, here). The mass of a truck is 11793 kilograms as can be seen in the video. This would give a rolling friction force of 2311 Newtons. This value is slightly higher than the force estimated for the mini-terminator-army, but we could have made errors in the coefficients of friction, but still quite close.

So, if you want to create your own army of robotic dogs with the ability to pull a heavy truck you need to figure out how to create a greater static frictional force between the dogs’ paws and the road than the combination of the rolling friction and gravitational forces experienced by the truck. So studying tribology brings the possibility of one day building your own army of robotic dogs to take over the world. Or to fight these beasts and save humanity.

The story is based on a post by Rhett Allain: “Estimate the Pulling Force of Boston Dynamics’ Robo-Dog Army” (https://www.wired.com/story/estimate-the-pulling-force-of-boston-dynamics-robo-dog-army ).