When you need someone from Canada to calibrate€¦ · been implemented in AV&R Aerospace for FANUC robots. Do you still think your robots are accurate? Ilian Bonev Ilian Bonev is

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August 20, 2015

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When you need someone from Canada to calibrateyour robot in New Zealand

by Ilian Bonev

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Many researchers think that robot calibration is an issue that has been successfullyresolved decades ago, but they are wrong. While the underlying theory is wellestablished, its practical application continues to be in its infancy. This is the first of aseries of posts that will provide evidence that it is still very hard for a user to get anaccurate industrial robot. So hard indeed that a company in New Zealand didn’t hesitateto ask one of my postdocs to fly in for help.

In the past decade, robot simulation and off-line programming software packages havebecome very popular. However, apparently, many users still ignore the fact that industrialrobots are repeatable but not accurate, at least not “by default.” When these users startmachining with their robots, they suddenly realize that the actual results are quite differentfrom the simulations.

Industrial robots have (unidirectional) position repeatability that is usually better than 0.1

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mm (sometimes as good as 0.01 mm). This is due to the use of zero-backlash gearheadsand high-resolution encoders. However, because it would be too expensive to machineand assemble robot parts precisely enough and because gearheads are not perfectlyrigid, there are important differences between the robot’s mathematical model, that isimplemented in its controller, and the real robot. Because of these differences, if youprogram a robot off-line to go to a certain position, you may observe errors of up to a fewmillimetres in the real robot.

The process of finding a new mathematical model that represents the real robot moreclosely, and leads to smaller position errors, is called robot calibration. In order to identifythe parameters of the new model, sophisticated coordinate measurement equipment isneeded, such as laser trackers, which are extremely expensive (more than $100,000).The cost of the required measurement equipment is probably one of the main reasonswhy robot calibration is still in its infancy, in practice.

Robot manufacturers (at least ABB, FANUC, KUKA and MOTOMAN) offer factory robotcalibration as a relatively inexpensive option. I’ll examine the performance of these optionsin a different post, so let’s focus now on those users who forget to purchase such anoption.

This happened to a company based in Auckland, New Zealand, a two-hour drive fromHobbiton (the famous village from Lord of the Rings movies). The company manufacturesmarina products through robot milling but observed important errors in the surface qualityof their machined parts. It uses an ABB IRB 6640-130/3.2 robot, which has 130 kgpayload, 3.2 m reach, 0.050 mm repeatability, and… was not factory calibrated. On top ofthat, the robot is placed on a linear track, which introduces yet another source of errors.

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After weighing all options (such as shipping the robot back to Sweden), the companycalled us and asked for help. Not only was local expertise in robot calibration unavailable,but even a laser tracker was nowhere to be found nearby. And so, last January, one of myPh.D. graduates, Albert Nubiola (pictured above in Hobbiton), armed with a pair of flip-flops and a C-Track, headed to New Zealand.

The C-Track is a photogrammetry-based optical CMM developed by Creaform. It is a bitless accurate than a laser tracker (volumetric accuracy is 0.065 mm) and has a smallermeasurement volume, but is way more compact and less than half the price of a lasertracker. Furthermore, it can measure the pose of the robot’s end-effector (not just theposition) and (if properly used) is not influenced by vibrations or air currents.

The robot calibration solution was delivered using RoboDK. The latter is a software toolfor robot simulation and off-line programming (OLP), launched recently as a spin-off fromour lab. This simulation and OLP platform is highly versatile and can deploy thecalibration methods developed in our lab. It currently supports more than 200 robots from10 different manufacturers.

The robot calibration can be performed in 4 steps:

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1. Reference frame calibration (6 measurements)

2. Tool calibration (7 measurements)

3. Robot calibration (50-80 measurements)

4. Validation (optional, as many measurements as desired)

The measurement acquisition is fully automated by RoboDK and the calibration can beperformed in less than an hour using the C-Track or a FARO laser tracker (as inthis demonstration, where MATLAB was used instead).

The ABB robot’s mean position error was improved from 3.443 mm to 0.765 mm, asvalidated in 315 arbitrary robot configurations. The robot position accuracy was alsovalidated by milling a series of slots equally spaced and machined at different toolorientations. After calibration, machined distance errors between slots were reduced byhalf or better.

Once the robot is calibrated, the parameters of the new mathematical model can be taken

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into account by RoboDK and the offline simulator platform directly generates accuraterobot programs for milling. Although filtering programs is also supported by RoboDK, therobot milling programs can be automatically generated, accurately, from a CAM file (suchas G-code or APT files). RoboDK’s path solver automatically manages 6-axis robots tomake them behave like a 5-axis CNC for milling. This methodology can also be used forwelding and painting applications. It is also possible to simulate and generate robotprograms through Python. RoboDK automatically handles brand-specific syntax.

Of course, our lab is not the only organization offering robot calibration services.Dynalog, for example, has been operating exclusively in this field for 25 years. However,our specialty is the development of robot calibration methods based on novel low-costhigh-accuracy devices. Perhaps our most innovative device and method is the one basedon the use of a single telescoping ballbar from Renishaw. This method has alreadybeen implemented in AV&R Aerospace for FANUC robots.

Do you still think your robots are accurate?

Ilian Bonev Ilian Bonev is professor at École de technologie supérieure (ÉTS)and holder of the Canada Research Chair in Precision Robotics. .. readmore

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