Why do current manipulators tend to use 7 DoF instead of 6 DoF?

In theory, controlling a 6D pose (3D position + 3D orientation) would require 6 joints, and - assuming the robot is carefully designed - the inverse kinematics should be solvable for that case.

Adding a 7th joint, and still targeting a 6D pose, would result in a null space of at least one dimension anywhere in the workspace. Why is this desirable?

My intuition is that it allows controlling the direction of the velocity and that the desired target changes; however, I didn't verify or investigate this so it is at most a best guess.

Adding a 7th joint, and still targeting a 6D pose, would result in a null space of at least one dimension anywhere in the workspace.

That is basically one the reasons people may wish to use a 7-DOF Robot for a 6-DOF End-Effector. It adds what is called Kinematic Redundancy. Your robot's end effector can reach a certain pose with different joint position sets.

Have a look at the following video for example:

As you can see, even though robot moves with changing some of the joint positions, end effector is not moving. You may wish to have multiple joint positions for a single end-effector pose if you have obstacles in the environment, or there is a possibility of your robot to have a self-collision.

Sometimes in industrial environments, these robot arms have limited free-space. Even though a certain configuration can be achieved with multiple joint positions, one of them may cause robot to collide with its surroundings (another robot arms, a vehicle, and etc.).

• So it is mainly used to be able to avoid obstacles in the workspace? Commented Oct 8, 2021 at 13:29
• It might be one of the reasons, self collisions are one to look out for as well. I believe some torque related concerns can go into that as well. The lower joints may require higher torques than the ones above, since they carry them as well, which requires higher energy and therefore battery usage. So that kind of concern may go into that as well. Commented Oct 8, 2021 at 13:42

Kinematic redundancy is certainly a good thing to avoid objects in the workplace etc., but we could simply see this as we want an extra degree of freedom for its own sake.

More fundamentally important is that 7DOF avoids gimbal lock situations. That's when some of the degrees of freedom in joint space happen to map to the same movement in object space. Because you still only have the same total number of dimensions in joint space, there's then suddenly one too few to cover all possible movements in object space. If you have one “spare” degree of freedom, this issue can be avoided – the extra DOF can jump in for the one of those that have become identical, and it gives you the choice-freedom to avoid the points where multiple joints might become locked.

Gimbal lock famously was an issue on the Apollo missions, here's a fascinating video about it: