Improving industrial robot accuracy and precision

Industrial robots are in high demand in manufacturing to perform dull, dirty, and dangerous tasks. As robots become more sophisticated as far as applications are concerned, they’re also being asked to become more precise and accurate.

However, there is a difference between precise and accurate, said Stan Gleizer, Mecademic sales and technical support manager, in the presentation “Improving the Accuracy of Your Industrial Robot” at Automate 2019 on April 9 at McCormick Place in Chicago.

“You can have a precise robot that isn’t accurate, but you can’t have an accurate robot that isn’t precise,” he said.

Precision is needed for applications such as welding or laboratory environments or anything that requires a position error of less than 0.5 mm. Accuracy and repeatability are best served for applications involving material removal or inspection. It isn’t necessary to be exactly accurate so long as the robot can consistently perform the task.

Improving accuracy and precision

There are ways to make a robot better and to improve their accuracy and their performance to make them on the plant floor. It’s a multi-step process that requires the user to:

• Develop a mathematical model
• Take measurements of the end-effector’s pose
• Identify error parameters that minimize Cartesian errors in the measurements
• Update the robot’s signature in the robot controller
• Validate the accuracy through additional measurements.

Gleizer added a small note of caution saying, “The mathematical model will never be perfect. There’s no way to make it accurate in every single point.”

While the human element does play a role, sending the data to a programmable logic controller (PLC), which does the calculation before sending the data to the robot, can help improve the robot’s accuracy. There are commercial third-party solutions to help users. It’s expensive, but flexible, and allows the user to calibrate several different robots.

Another way to make the robot more accurate is using secondary absolute encoders. Gleizer noted they are more expensive and do require calibration up front, but they are extremely accurate. External metrology equipment is another option, which does not require the entire robot to be programmed. The only parts that need to be programmed are the robot parts being used for sensing.

Online path correction, which requires third-party software, has the added requirement of research and development, but Gleizer noted how its benefits compensate for the upfront costs.

“It’s accurate and specific to the motion the robot is doing and its trajectory,” Gleizer said.

No two applications or settings are alike, but the tools are there to make robots more accurate, which is a must in today’s demanding manufacturing environment.

Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com.

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