Getting Robotic Calibration on Track

(Image courtesy of Hexagon MI.)

It seems like we’ve been hearing about the coming wave of industrial automation for decades, but it’s hard to deny that the robotics market has seen significant growth in recent years, including those designed for broader applications. The consensus has generally been that robots are well-suited to applications requiring high precision and less so for those requiring high accuracy. Improvements to robotic accuracy would go a long way to accelerating their adoption, but how do we do that?

Six Degrees of Freedom

“As robots move from pick and place processes to in-process manufacturing, we need to be able to identify if our robots are accurate,” said Joel Martin, laser tracker product manager at Hexagon Manufacturing Intelligence. “Obviously, the manufacturer who sells us the robot gives us a specification, but how do we verify that?”

It’s a problem more and more manufacturers will face as the proliferation of industrial robots continues. Martin discussed Hexagon’s solution at this year’s HxGN LIVE.

“We wanted to take it from a 3D technology—an SMR measuring X,Y and Z—and add I, J and K or pitch, roll and yaw,” he said. “So, we developed a technology called 6DoF, for six degrees of freedom.”

The technology uses a camera mounted in the laser tracker and LEDs attached to the robot’s end effort to achieve the extra axes. Martin explained that, “The tracker gives us X, Y and Z, and the camera gives us pitch, roll and yaw, so between the two we get a full X-Y-Z-I-J-K rotation of any object in space.”

“6DoF technology has given us the ability to put something on a machine and not use the machine’s scales to determine where an epifactor is but use the laser tracker itself,” Martin added.

The conventional wisdom is that if a machine isn’t making chips, then it isn’t making money. Martin pointed to particularly large parts that require high accuracy measurements, such as a carbon composite component for an aircraft.

“If you have to move that part from the machining center, balance it on a CMM to get a measurement and then move it back, rebalance it and recut it, the time lost there is huge,” he said.

In contrast, using a robotic arm and 6DoF technology, quality engineers can inspect parts on the machine and verify them in-process from a secondary source.

Robot Calibration with a Laser Tracker

As 6DoF technology can be used to improve robotic accuracy, it can also be used for robot calibration. This is enabled via Hexagon’s RoboDyn calibration and inspection system, which was designed with the ISO 9283 standard in mind. The system enables engineers to use simulation in conjunction with a laser tracker to calibrate a robot and ensure its accuracy.

“So, I have a 6-axis robot and a laser tracker,” Martin explained. “I bring those into a model environment and can tell the robot that I only care about a particular area, so make yourself as accurate as you can within that space. Then, in the simulation, we can calculate all the joint space movements for what will make that robot as accurate as possible within this work envelope without having to go back and reverse engineer them to get us the most articulation of the arm possible.”

(Image courtesy of Hexagon MI.)

Despite the benefits of using this system to calibrate a robot, it also has a significant limitation.

“The problem with robot calibration is that even though I have the ability to calculate the TCPs and the DH parameters, very few robot manufacturers will allow me to write that to their controller,” Martin said. “If I have my own controller—like a Siemens N(40)D running a separate robot controller—I have that ability, but if you’re running an off-the-shelf KUKA or FANUC, most of them won’t allow us to write those parameters into the robot. So, just because we have the ability to calculate them, doesn’t mean we can apply them back into the system. It varies by robot manufacturer, but it is a limitation.”

Robot Calibration on the Fly

Another notable limitation in the RoboDyn system comes when dealing with robots that aren’t mounted to a fixed based.

“[A]ll of this works well, until I put the robot on a translation slide or a gantry—something that allow us to move that robot in the 7^th degree,” Martin said.

There are several possible solutions to this issue, but Martin expressed doubts about some of the more obvious ones. For example, you could try to ensure that the rails on which your robot moves are as straight as possible, but, as Martin pointed out, there are limits to how straight you can make the rails, especially as they get longer. Alternatively, you could set it to calibrate whenever the robot stops on the rail. The problem in this case, according to Martin, is that a robot’s movements on a rail are more dynamic. It only takes a slight movement at its base to have a large impact on the end effector.

Hexagon’s solution in this case is 7DoF or “7^th degree of freedom,” which refers to time.

“We focused on low-latency, real-time communication back to the robot,” Martin said. “It’s running at 1000Hz with a 5μs latency, which allows the laser tracker to measure the end effector of the robot and translate that data back to the robot so that it’s correcting, pose by pose.”

The most impressive example of 7DoF technology in action comes from the KRAKEN project, a collaboration between various European companies and organizations focusing on large-scale additive manufacturing. The KRAKEN uses a six-axis robot, mounted upside down on a gantry and fitted with various additive and subtractive capabilities. The system employs 7DoF to ensure its accuracy, which is particularly vital in using additive manufacturing at large scales.

“This is the catch-22 in additive manufacturing: the faster you lay down material, the more process variation you generate,” Martin said.

(Image courtesy of KRAKEN Project.)

Using 7DoF in conjunction with a 3D scanner mounted to the robot’s end effector offers a solution to this conundrum.

“We can change the way that we manufacture the part, to throw down as much material as possible, scan that layer to understand what’s there and modify the robot for the next layer so we don’t have this common stack up in additive manufacturing, where you can start to get this Leaning-Tower-of-Pisa effect when things start to get out of control,” Martin said.

Seven Degrees of Freedom

Martin’s characterization of the 7^th degree of freedom as time is fitting for industrial robots. Though he was speaking within the context of low-latency robotic calibration, when it comes to time more generally, robots offer a degree of freedom impossible for human beings to match. The best employee in the world can’t work 24 hours a day, and everyone needs a vacation or sick day now and then. Of course, robots aren’t perfect, and accuracy is one area where they still need to improve. With solutions like 7DoF, we could very well see a new wave of adoption in industrial robotics.

For more information on laser trackers, check out our white paper How to Avoid 3 Common Mistakes When Using Laser Trackers.