What’s the difference between a 6 DoF and a 7 DoF robotic arm?
First off, DoF or DOF, meaning degrees of freedom or 'degrés de liberté' in French 🇫🇷, is just a sophisticated word for an axis or joint. From experience the term DoF or DOF is more used within the research and educational world while the term 'axis' is preferred inside the industrial automation and manufacturing world. That being said here follows the answer to the question asked above.
A 7 DOF robot will allow you to get to an end effector position with several joint configurations. It will help to avoid singularities and potential objects in the workspace by placing the arm in a different orientation than what a 6 DOF robot is limited to. However, a 6 DOF robot may be sufficient to cover the workspace of most applications and may offer a better ROI as it should be less expensive than a 7 DOF of the same model. If needed, a 6 DOF robot can be mounted on a linear guide or timing belt to provide a 7th axis movement, thus increasing the reach of the robot inside the work area.
A 6 DOF robot is also usually more repeatable, in addition to the following features:
Great answer Mark!
Just to add to this, most seven axis robots only come in a light-weight type. Light-weight robots generally have a low power consumption and are used for mobile or collaborative applications. There are not many seven axis traditional robots out there. These are generally more robust and can handle higher payloads.
Not to get too technical, but is also matters how the 7th axis is actually implemented (just like all other axes). The robot's Mark is discussing are more like the Kassow, Kinova, and Productive Robotics style of 7 axis arms where the extra axis is integrated to make the robot arm similar to a human arm (generally the extra axis is integrated into the forearm). There are other designs out there such that serve other purposes. For instance, Fanuc has a seven axis system that is specifically designed for narrow workspaces. The seventh axis is integrated into what could be considered the "bicep" of the robot. This doesn't produce the results previously described, but allows the robot to pick items much closer to its base than would normally be possible by folding the "bicep" area up. https://www.fanuc.eu/de/en/who-we-are/news/de-press-release-new-robot-r-1000ia
Thank you to both Mark and Nick for the great insight!
Let me tie everything together. There are multiple types of robotic arms (cartesian, cylindrical, SCARA, Delta, articulated, etc) but as Mark mentioned, a 6 degrees of freedom (DoF) robotic arm is synonymously used when addressing the 6 axis articulated robot. When we identify a 7 DoF robotic arm, it usually refers to a 7 axis articulated robot with a joint integrated into the forearm, like Nick mentioned. This kind of robotic arm is used for very specific applications, usually within a narrow workspace, because of the difficulty it creates in calculating the inverse kinematic (IK). For this reason, a joint position is often locked in order to compute the IK. It can also be used for humanoid robots, as the “robot equivalent” of a human leg.
Check out the DARwIn-OP for great practical examples: https://www.youtube.com/watch?v=1WEgNQjL66g
Another way to think about a 7 DoF robotic arm is to mount a 6 DoF on a timing belt, in order to create that 7th axis. By adding this prismatic joint, you are increasing the robot’s reach, as Mark explained. A tactile example of when to apply this type of solution is when palletizing with a 6 DoF articulated cobot. Vention offers this design out of the box with the UR10e and the Liftkit Telescopic Column from Ewellix for instance: