Why does the best robotic arm have 7 degrees of freedom instead of 6?

   How many degrees of freedom does the human arm (wrist joint to shoulder joint) have?I don’t think most people have thought about it, let alone understand how many degrees of freedom there are. Even those who study engineering may not be able to explain clearly.Unexpectedly, someone on Zhihu actually answered this question so professionally and connotatively, and at the same time so philosophical, I couldn't help but want to share this answer.

   The answer is as follows:

    Try to explain it in plain language.First, the answer to the question is:Just count it!

    7 degrees of freedom.

    Someone asked if 5 and 6 were the same.5 is the only joint that needs to be turned when turning the key. The power comes from the twisting of the two radiuses of the forearm;6 is the only joint that needs to be rotated when the mouse is placed on the desktop and turned by hand. The power comes from the rotation of the wrist.

    As for why human arms have 7 degrees of freedom instead of 8 or 6 degrees of freedom, it may be because God knows robot control very well, so let's try to briefly introduce it below.

First introduce a theorem:

    A manipulator with 6 degrees of freedom cannot change from one configuration to another without keeping the three-dimensional position of the end mechanism unchanged in space.

    This theorem is not easy to understand at first glance. Consider a simpler case:

    In this picture, the arm of a robot consists of a base, two joints, and two connectors (imagine opening a compass and pinching one end with your fingers).Can we turn the robot from the "lefty" state to the "righty" state while keeping the position of the upper end mechanism on the plane unchanged?

    The answer is no. No matter how you move the two joints, the position of the end mechanism must change during the movement.The judge can also try to move two pens on the table.

     Similarly, for a 6-DOF manipulator, even if the three-dimensional positions of the end mechanisms corresponding to two sets of configurations are the same, the manipulator cannot keep the end mechanisms still when moving from one configuration to another.

    If someone has watched industrial robot welding something on the TV, they will find that when it is welding in the same position, it will twist to this side for a while, and then twist it to that side for a while, it looks very cool.In fact, this is done only because, although welding only wants to change the orientation of the end mechanism without changing the position of the end mechanism, due to the limitation of the theorem, it must be moved back a little and then twisted in various ways to ensure that the end mechanism is moved. It will not hit anything during the orientation, because the three-dimensional position of the end mechanism will definitely move when it moves.If it can rotate a little bit at random, it can achieve its purpose, and it takes that effort to rotate the whole thing coolly...

    But with one more degree of freedom, it will be different.

    Think about the action of turning the key when opening the door. In this case, the three-dimensional position of the end mechanism (hand) of the human arm has not changed (always in front of the keyhole), but the three-dimensional rotation of the end mechanism (hand) has changed (the key is turned) .Humans can achieve this simple movement because our arms have 7 degrees of freedom.

    Having said that, the inspector may see it, hey, I understand, my end mechanism has 6 degrees of freedom (three-dimensional position, three-dimensional rotation), and the arm as a manipulator has 7 degrees of freedom, these two freedoms Degrees seem to be different, but the number is 7-6=1, so I can use this 1 degree of freedom to turn the key.

    If God designed our arms to have 6 degrees of freedom, the action of turning the key would be very exaggerated.You can feel it without turning your wrist when you turn the key.

    So why not give us more freedom?

     Because the more degrees of freedom, the worse the rigidity of the manipulator.If our arm has 8 degrees of freedom, the probability of injury will be much greater.Although there is no biological research to prove this (there is no biological body with 8 degrees of freedom in the world), the research of robots can prove this problem.

     So 7 is the best choice.I don’t know if 7 is the best and most sacred number in the Bible, and there are seven days in a week. It is related to the 7 joints of the human arm.