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Are two arms better than one?

30 November, 2018

Giving a robot a second arm offers many potential attractions in terms of flexibility, load-carrying and performing complex operations. But practical applications of dual-arm robots have so far have been limited. Contributing editor Tony Sacks reports on some of the latest developments.

Watching robots at work can be hypnotic – doubly so when they are they have two arms, making them appear that much more human-like. Perhaps not surprisingly, growing numbers of robot-makers are offering industrial robots with two arms.

Photos of the German chancellor Angela Merkel doing a “fist bump” with a two-armed robot at the Hannover Fair earlier this year, helped raise the profile of these machines. The robot she encountered was built by the German automation supplier IBG and was fitted with hand-like five-finger grippers developed by Schunk.

While dual-armed robots can certainly draw crowds at exhibitions, how practical are they for real-life industrial applications?

Robots with two arms are not new, although it is only in recent years that they have started to become widely available for general-purpose industrial applications. Some of the earliest two-armed machines were aimed at specialist, non-industrial uses such as bomb disposal, robotic surgery and agriculture. For example, Germany’s Fraunhofer Institute has developed a two-armed machine designed to harvest cucumbers.

One of the attractions of having two arms is that they can work together to handle objects and perform operations on them. For example, grippers on one arm can hold and orientate an object to the optimum angle for tools on the other arm to carry out tasks such as machining, inspection or inserting screws.

By carrying out operations simultaneously, the two arms can also save time compared to using a single arm which would need to perform tasks sequentially. In some applications, they can also save space compared to using two separate single-arm robots.

Another potential attraction of using two arms is that, in combination, they allow objects to be approached at angles that would not be possible using a single arm by itself. Similarly, they can re-orientate parts to achieve ideal approach angles.

In some applications, the two arms can work together to carry a load that might be too heavy or too large for one arm to handle by itself.

Most dual-arm robots are designed to operate safely alongside humans as collaborative robots (cobots), and are often portrayed in publicity material as working in harmony with humans on tasks such as small parts assembly, machine tending and loading/unloading. But the number of practical commercial applications of this type is unknown.

One of the pioneers of two-armed cobots was the US manufacturer Rethink Robotics which launched its Baxter machine in 2012. The “human-like” aspect the cobot was accentuated by incorporating a screen that can show a “face” display.

Three years later, Rethink gave Baxter a single-armed brother called Sawyer, which was smaller, faster and more precise. Judging by the number of applications described on Rethink’s Web site, the one-armed version has found more commercial applications than its older two-armed sibling.

Another early entrant to the two-arm scene was Epson which in 2013 announced a prototype of a movable cobot with built-in vision and force-sensing technologies. Its Worksense W-01 robot was designed to be wheeled from one location to another to perform assembly and other tasks.

The W-01 has six cameras – four on its head and two on its arms – designed to detect an object’s position and orientation in 3D space, as well as following it as it moves. The force sensors allow the robot to perform delicate tasks that need human-like control to avoid damaging objects. The two seven-axis arms can move independently to perform tasks that single-arm robots cannot, such as holding an object with a gripper on one arm while the second tightens a screw. The robot determines the path of each arm to avoid obstacles.

Given the complexity of Epson’s machine, it is perhaps not surprising that it was not until late 2017, that it reached the commercial stage. The company asserts that the robot will be able to automate production tasks “that previously defied automation”.

Epson has recently commissioned a new production line in Japan to manufacture dual-arm and other sophisticated robots, with the aim of more than tripling its robot production capacity by 2025.

Another two-armed contender, ABB’s YuMi, made its public debut at the Hannover Fair in 2015, although it wasn’t until the following year that the first practical applications were implemented. Like Baxter, YuMi has recently been joined by a one-armed version.

The Japanese robot-maker Kawasaki has adopted a slightly different approach with its duAro dual-arm robot, by opting for a pair of Scara-type arms, rather than conventional multi-axis arms. The company opted for this design after observing that most production line operations involve a combination of horizontal movements, with less demand for vertical movements. The two Scara arms share a common axis and occupy a similar amount of space to a human worker. The single-axis design makes the arms less likely to interfere with each other.

Visitors to the recent Automatica exhibition in Munich, Germany, could view a variety of dual-armed robots in action. Some were being demonstrated by companies that you might not think of as being robotics suppliers. Take, for example, the German machine safety specialist, Pilz. It has entered the robot market with a portfolio of modular products including a six-axis robot arm, a control module incorporating drive technology, and an operator module.

Pilz is targeting “service robotics” applications such as: small, semi-automated industrial cells; pick-and-place systems; mobile applications involving automatic guided vehicle (AGVs); as well as non-industrial duties.

As well as showing the single-arm device with a 6kg payload capacity, Pilz was also demonstrating a prototype two-arm robot, which was wrapping workpieces for another robot to transfer to an AGV, which carried them to a manual workstation, where they were handed to visitors.

One reason why we don’t see more practical applications of two-armed robots is that they need careful programming to ensure that their arms coordinate their movements and operations perfectly with each other. For example, you have to ensure that the arms – and any items they are carrying – will not clash with each other.

A related difficulty is that each robot manufacturer has its own method of programming its machines. There are no agreed standardised methods for setting up and implementing two-arm robot applications.

But this could be changing with an announcement at Automatica that ABB and Kawasaki are working together to develop a common operating interface for collaborative robots in general – and dual-armed robots in particular.

At the show, they demonstrated a pair of two-arm robots – ABB’s YuMi and Kawasaki’s Scara-type duAro machine – working in harmony with each other. The demonstration was the result of a collaboration that the two companies announced in November of 2017, designed to share knowledge and promote the benefits of collaborative automation, in particular dual-arm cobots.

ABB and Kawasaki are developing a simple human–robot interface with intuitive, smartphone-like navigation and icons. This interface will allow collaborative robots to be programmed and operated by people without specialised training, helping smaller businesses, in particular, to leapfrog traditionally lengthy robot learning curves.

One aim of the joint development is to help address the shortage of skilled workers in many industries – particularly in Japan where one person in five is within a decade of retiring.

“The new state-of-the-art, industry-standard operating interface will accelerate the already rapid growth we see in collaborative robots,” predicts Per Vegard Nerseth, managing director of ABB Robotics. “It will give manufacturers flexibility and scalability, while providing more interesting jobs for the world’s vital industrial workforce.”

ABB and Kawasaki say that they are open to working with other robot manufacturers who might also want to adopt the new operating interface.

As well as working on the interface, the two companies are also collaborating on other issues, such as safety. The aim is to develop common standards that will ensure worker safety, but also allow for new ways of working together without unduly restricting the many benefits of cobots.

If the techniques being developed by ABB and Kawasaki are accepted more widely by other manufacturers, then the two-armed robot may become a more common sight in factories around the world.

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