Posts by mr.magnuso

Quote from SkyeFire

Some do. In some circumstances, tuning the robot in the working location with the actual working payload is the only way to achieve sufficient accuracy. Also, by reducing the calibrated volume to the volume where it's needed, instead of the entire robot reach volume (which is what the factory calibration does).

It is true that the measurement is done in the full workspace using laser tracker equipment but it also performed in the full payload range. We have had the opportunity to evaluate the AbsAcc option on several ABB robots. They are very good 0.3-0.5 mm accuracy in the full workspace. We usually test them with minimum and maximum payload. With our methods, we usually could push the accuracy even further including also orthogonal elasticity (how bearings is affected by the load of robot itself and payload). ABB is what I Know never performing such measurements at customer site. So the big drawback is the risk of invalidated model is something happen to the robot (repair or other incidents). We use a method to be able to generate robot programs in the subset of the workspace as you describe, to re-create the model if needed. Since we are in the production cell, we can also in the same time probe robot to fixture or workpiece. As always, only an accurate robot is not enough, tool and workcell calibration have to be in place to be able to run off-line generated programs without touch up.

Quote from SkyeFire

Which usually does not work very well for payloads different from the default payload.

Frankly, I'm not sure I've ever seen factory Absolute Accuracy that was worth the money. I've had better luck with field calibrations or compensation tables, or local landmark guidance when that's possible.

The complexity of the system could easily increase if you need sensors. To be able to run "blind" as I describe will require also high accuracy on your products (to manipulate or process). The automotive industry have succeeded to utilize 99% off-line generated robot programs without touch up (at least company that I know about). This is not possible without AbsAcc robots. The question is if this is the way forward for other industries?

There is ways to reach accuracy almost as good as the repeatability.

Many robot suppliers provide an Absolute Accuracy option. These specific robot individuals have then been measured and identified at factory. Not an option you can turn on after shipment. Usually the robot supplier don't have any solution to perform it on-site. You have to de-comission the robot and send it back for recalibration. This is also the case when Absolute Accuracy robot have been repaired (switched servo or reducer) or have been involved in a collision affecting the mechanical integrity of the robot arm.

I am working on a company that provide system and services where these kind of measurement on-site and in production cell and even with robot missing the Absolute Accuracy option from the beginning. The calibrated model take both kinematics and elasticity into account to enable off-line generated program to work without touch up.

Late answer to this post.

Unfortunately are robots quite poor (not only Motoman) considering about absolute accuracy. The robot suppliers always state the repeatability of the robot and that is often very good, typically 20-60 micrometers. When you use tools such as RobotMaster this is of no help. You need the robot to reach a specific coordinate since the target has never been taught on the specific robot individual you apply the program on.

The first step is a good mastering. Then you typically can reach accuracy +/- 2 mm (sometimes worse than this). Manufacturing and assembly tolerances of the robot prevent better accuracy.

I am working in a company addressing this problem. By using external optical measurement system, it is possible to create a model of the robot describing the differences against the "perfect" robot (both distances and angles between joints a.k.a kinematics but also elastic characteristics to have better accuracy for different payloads). We can then reach typical accuracy of +/-0.2 mm for a mid sized robot. We can then perform a post-processing step, using the program generated from for instance RobotMaster and calculate "fake" targets in a new program that will make the robot move as intended.