When Will High-DOF Robots Matter?

Neil Tardella
Aug 19, 2019 7:42:58 AM

High-DOF robots (robots with more than six degrees of freedom, or axes) hold the promise for a more automated future. However, the real value of high-DOF robots will only be realized when integrators move away from the “one task, one robot” mindset. As robots become more general-purpose, the need for more axes becomes evident. Switching tasks (e.g., going from a pick-and-place task to a screwdriving task, and then to an inspection task) requires a system designed with inherent flexibility—flexibility that can only be achieved with high-DOF robots.

The Challenges of High-DOF Adoption

The majority of deployed industrial robots in the world have six or fewer axes. There have been attempts to popularize high-DOF industrial robots, with more than six axes, but these attempts have not been very successful. Why is that?

hi dof robots

From Left to Right - ABB YuMi, Motoman SDA-5F, Rethink Baxter.

In my opinion, there are two interrelated reasons why high-DOF robots have not caught on: cost and programming difficulties.

Every axis costs money for hardware and engineering, as each additional axis adversely affects repeatability and payload. With the “one robot, one task” mindset, integrators have historically chosen a robotic solution with the minimum number of axes needed to solve the one task at hand efficiently. The belief was that using a 6-DOF robot for a 5-DOF problem added unnecessary cost and complexity to the work cell. True enough in traditional automation.

However, what if one robot could be used for more than one task? What would it take for integrators to reconsider the “one robot, one task” paradigm? To consider this they would need access to high-DOF robot hardware that provides flexibility in movement, and a means to program and control the robot that is no more complex than the programming required for a low-DOF one.
Today, software to control high-DOF robots exists but has been mostly found in academia and R&D institutions like NASA and DARPA, but commercial solutions that can be applied to traditional automation are becoming increasingly available.

Kinematic Redundancy and Why it Matters

Without specialized software, it’s extremely difficult to program high-DOF robots. The more joints a robot has more difficult it is to control. The complexity of control grows non-linearly with the number of joints. For lower-axis robots (with 6-DOF or less) it’s usually possible to find a closed-form solution for how the joints must move to place the end effector. Robots with more than six axes are considered kinematically redundant because they can achieve a particular end effector pose from multiple joint states. The video below shows kinematic redundancy with a 7-axis KUKA iiwa robot.

 

It is incredibly important for high-DOF robots to be able to deal meaningfully with kinematic redundancy. Humans are experts at this. Think of all the different ways a human can reach down and pick up a ball. Our brains automatically determine how to optimally move our “axes” (a human has over 200 joints) to achieve the necessary hand motion—all while avoiding collisions with ourselves and the environment, preventing joints from locking up, and making micro-corrections based on visual and tactile feedback. All of this redundancy makes us humans highly generalized. We can move seamlessly from one task to the next. It’s really quite amazing. Virtually all evolved forms of life have musculoskeletal systems that allow for the exploitation of redundancy.

 

Most industrial robots, on the other hand, have not been equipped with software that can deal with kinematic redundancy at all, let alone use it to its advantage. The lack of sufficient software is why the world of industrial automation is filled to the brim with simple, low-DOF robots that have been purchased, programmed, and deployed to perform one task, and one task only. If you would like a traditional industrial robot to perform a new task, it’s possible. But only after a very costly integration effort, and then that robot will be able to only perform that new task. It’s completely lacking in flexibility, one of the key growth drivers for robotic systems deployed in industrial automation settings.

The Future of High-DOF Robots

With the advent of software that can exploit kinematic redundancy and simplify programming of high-DOF robots, the cost of additional axes can easily be justified by amortizing it over a broader set of tasks the robot can perform. The real benefit will be seen when high-DOF robots are used to replace a series of steps in a given manufacturing process—being much closer to a direct replacement for a high-DOF human. When this happens, the change will not be incremental but rather a step function.

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