Most industrial robots used today are six-axis, but the recently introduced collaborative robotic arm has seven degrees of freedom, which I've always wondered why. It wasn't until recently, when I saw a question on Zhihu: "How many degrees of freedom does a human arm (from wrist to shoulder) have?", that I realized that seven degrees of freedom is a realistic representation of a human arm.
How many degrees of freedom does the human arm (from the wrist to the shoulder) have? I think most people haven't thought about it, let alone tried to understand what those degrees of freedom are. Even engineering students might not be able to explain it clearly. I didn't expect someone on Zhihu to answer this question so professionally, insightfully, and philosophically. I couldn't help but share this answer.
The answer to this question was given by Zhihu user Yang Shuo, and the answer is as follows:
To be honest, I'm quite familiar with robot manipulation, but I couldn't understand the answers above at first glance. It's not that I'm being sarcastic, but rather that they're difficult for non-professionals to grasp.
Let me try to explain it in simple terms.
First of all, the answer to the question is: just count them!
7 degrees of freedom.
Some people ask if joints 5 and 6 are the same. Joint 5 is the only joint that needs to rotate when turning a key, and the power comes from the twisting of the two radius bones in the forearm; joint 6 is the only joint that needs to rotate when you place a mouse on the desktop and turn it by hand, and the power comes from the rotation of the wrist.
As for why the human arm has 7 degrees of freedom, instead of 8 or 6, it's probably because God was very knowledgeable about robot control. I'll try to explain it simply below.
First, let's introduce a theorem:
A six-DOF manipulator cannot transform from one configuration to another in space while maintaining the three-dimensional position of the end effector.
This theorem may seem difficult to understand at first glance, but let's consider a simpler case:
In this picture, a robot's arm consists of a base, two joints, and two connectors (imagine opening a compass and then pinching one end with your fingers).
Is it possible to rotate the robot from the "lefty" state to the "righty" state while keeping the position of the upper end effector on the plane unchanged?
The answer is no. No matter how you move the two joints, the position of the end effector will inevitably change during the movement. You can try moving two pens on a table to see what happens.
Similarly, even if the three-dimensional positions of the end effector corresponding to two sets of configurations are the same, a 6-DOF manipulator cannot keep the end effector stationary when moving from one configuration to another.
If anyone has ever seen an industrial robot welding something on TV, they will notice that when it's welding in the same spot, it twists and turns from one side to the other, which looks very cool.
The reason for this is that, although the welding only aims to change the orientation of the end effector without altering its position, due to the limitations of the theorem, it must be moved backward and twisted in various ways to ensure it doesn't collide with anything during the repositioning process. This is because the three-dimensional position of the end effector will inevitably shift erratically during movement. If it could achieve the same result with just a slight turn, why bother with the elaborate, fancy whole-body rotation?
But things change once you add another degree of freedom.
Think about the action of turning a key when opening a door. In this case, the three-dimensional position of the end effector (hand) of the arm does not change (it is always in front of the keyhole), but the three-dimensional rotation of the end effector (hand) changes (the key is turned). Humans can perform this simple action because our arms have 7 degrees of freedom.
At this point, you might have realized, "Oh, I get it now." My end effector has 6 degrees of freedom (3D position and 3D rotation), while the arm, as a robotic hand, has 7 degrees of freedom. These two degrees of freedom seem to be different things, but in terms of quantity, 7-6=1, so I can use this 1 degree of freedom to turn a key.
If God had designed our arms to have six degrees of freedom, the way a person turns a key would be incredibly exaggerated. Try turning a key without twisting your wrist and see how it feels.
So why not give us more freedom?
The more degrees of freedom a robotic arm has, the less rigid it becomes. If our arms had eight degrees of freedom, the probability of injury would be much higher. While there's no biological research to prove this (no living organism has eight degrees of freedom), robotics research can demonstrate this.
For more information, please visit the Industrial Robots channel.
