Orbita: A 3D joint for robotic head motion realism
By: Augustin Crampette
Tags: Mechanics, Robotics, Innovation, Open Source, Humanoid Robot

Since we introduced Reachy about a year ago (beginning of 2020), its design has always been the center of all the attention. People find it expressive, cute and full of emotions. They are eager to interact with it. They instantaneously personalise the robot. They don’t need any user instructions to understand what the robot is doing, what is the interaction about.

At Pollen Robotics, design is at the heart of our approach. When we create a robot, we take special care of the interaction, the emotion and expressiveness. To us, this is key to leverage a natural and fruitful human-robot interaction and collaboration as it makes the whole process natural, easy to apprehend, efficient, but also better for acceptance. No-one will use social robots if the interaction feels awkward or is hard to understand.

This is obviously a very broad and complex topic, however we have always followed one guideline: to get inspiration from living creatures. By this I mean to study how they interact with their environment and to extract the essence of what makes it work so well. Note that we’re not trying to directly imitate — it would be an impossible task as it is too complex and we would probably miss what’s really important.

This above statement most likely feels really abstract to you right now. Let me illustrate it with a very tangible example: how we designed Reachy’s neck based on the Orbita actuator.

The need for a specifically designed robotic neck

The classical Pan-Tilt system

The first solution we developed for Reachy’s neck was a pan-tilt system. This is a very common solution for robotic necks as it is easy to integrate and to use.

The first head of reachy, based on a pan-tilt system.

Pan-tilt systems have 2 rotations that roboticists call degrees of freedom (usually noted 2-DOFs). Each of this rotation is actuated by a single motor. This allows the head to rotate from left to right and from top to bottom. Combining these rotations gives the robot the ability to look around itself. The two motors are typically placed one after the other as a serial system, which is quite simple to control and to integrate in a robot.

But while being simple to design, produce and control, this system has some significant limitations. To understand them, as I promised in the introduction, we will go back to studying the neck of a living creature.

Keep the head straight

First, this may not be obvious if you never thought about this, but the main role of the neck is to always maintain our head straight. Having a stable head is actually a critical feature, selected by years and years of evolution. This plays a key role, in locomotion, visual flow and tracking (how predators assess preys for instance). The commercial below illustrates how amazingly good some animals are at stabilizing their head:

Head stabilization that makes any roboticist wonder.

A pan-tilt is ok as long as your body doesn’t really move. But when it does, you need that third degrees of freedom to make sure your head stays horizontal!

For the moment, our robot Reachy doesn’t have locomotion capacities, however we plan to add such feature in the near future. On top of that, it is important to note the purpose of the neck is not only for maintaining the head horizontal…

Communication and conveying emotions 🤣

As you can now gather, species evolution has designed us humans with a 3D-actuated neck for vision purposes. On top of that, we developed further critical uses for this neck. Our head and its orientation plays a central role in communicating and conveying emotions. Our face (eyes, mouth, eyebrows) already conveys most of it, but our head itself is also important. Sure, we would also understand the meaning of 🤣 if the head was straight, but the tilt makes it much more impactful.

Some of the expressions conveyed by our neck can be reproduced by a pan-tilt system. But they often fail looking natural, realistic and aesthetical. To go even further, there are moves that we, humans, often do and that are simply impossible to operate with a 2-dof neck.

The head bending left can mean empathy, sweetness and sometimes seduction; bending right, it can mean questioning or disagreement, etc…