Is your robot as accurate and repeatable as it claims?  The answer is yes,,,and no.
Here are the three biggest reasons for "robotic" inaccuracy.

Every year someone publishes a video of a six-axis robot articulating though a seemingly random series of moves and distances.   Eventually the robot will return to a digital indicator and show an amazing level of repeatability.

The reason these videos exist are often because of objections and concerns from customers who have been disappointed when their robot didn’t seem to work as well as it should over time. 

Robot manufacturers are typically very good at meeting their specifications.  So where is the disconnect?

It is much more common an outside influence is skewing the robot’s apparent results, and these conditions are typically the root cause of a robot’s apparent inability.  Understanding the cause and effect of these unique conditions will allow for countermeasures that will insure a customers desired results over time.

 Top Causes of Robotic Accuracy Error.

1.    Failure to consider an entire system in three-dimensional space.

This most often occurs when the robot performs numerous program changes due to a variety of fixtures and parts.  The larger the fixture the more potential for issue.  A verification program should be run at each changeover to insure the fixture is in the same location.  It should check at a minimum of three locations and associate those locations to points in three-dimensional space.  The increased use of technologies like cameras and lasers integrated with the robot have a tendency to mitigate the above issues when they are available.

2.    Thermal Expansion and Contraction.

Consider the changes to the robot’s environment and its working temperature from start up to maximum duty cycle.  The size of the robot and the necessary precision will influence the degree to which this is a concern.  Compensation algorithms can be used to mitigate such concerns when necessary.

3.    Ignoring changes to the tool center point.

When the robot uses components on its end effector that need replacement be mindful of the potential the Tool Center Point may have changed during the maintenance activity.  Again, use a verification program at the completion of the maintenance to insure everything is correct.

4.    Axis/Joint Wear (The one we didn’t mention)

This is often the first item considered, however it is rarely the cause unless you have a robot that has seen a tremendous amount of service over a long period of time.  If part to part repeatability is poor then more investigation may be required.