Linearity and repeatability of robots in optical scanning applications
The Problem – Visual quality control inspection of complex geometry parts
KTM Research recently investigated the linearity of several robotic arms and a linear stage for use in linear optical scanning of complex geometry parts. Reproduction of an accurate image (stability) and repeatability are both critically important to many modern optical scanning techniques used in manufacturing quality control processes. Automated optical inspection of parts on a manufacturing line saves time and money. We paired a line-scan camera end effector (designed and built in-house) with a UR5 robot, a Denso robot, and a Parker 404XR linear stage for comparison.
Results of the Study
The UR5’s overall accuracy and repeatability was not well suited for scanning parts as you can see below. The Denso showed strength in scanning parts with complex surface geometry, depending on the inspection requirements. The Denso scan is not completely stable although it is repeatable and consistent to within ~5 pixels even close to control singularities. Depending on use case, safety requirements may drive significant complexity and expense for overall system design. For high-accuracy applications, we found the Parker linear stage to be the best fit for the job. KTM Research systems using a Parker linear stage and the linear scan method are currently operating in factories in the United States, China, and Malaysia.
Analysis of the UR5 Robot
UR5 robots are “collaborative” robots that can work around people without separation from operators and be operated and maintained by people who are not robotics experts. The lack requirements for guarding/safety with the UR5 allows for a much smaller, simpler, and easy to use work cell for vision analysis as compared to other robotic systems. For work cells designed to be operated and maintained by personnel without a programming or robotics background, the UR5 can be a good choice to decrease required training and reduce safety risks.
With the KTM Research-designed linear optical scanner end-effector the UR5 was not able to move with enough linear accuracy to generate acceptable imagery for analysis with the line-scan camera even though the UR5’s specifications indicated that it should be possible to do so. To test the UR5 in a best-case scenario, a ruler was attached to the robot and moved in front of the line-scan camera to evaluate its linear accuracy without any payload.
We found that the UR5 is not able to manipulate the line-scan end-effector as designed, even though it was within the published specifications. With the line-scan end-effector mounted in a non-ideal setting, the UR5 shows extremely poor linear motion accuracy. Tested in a “best-case” scenario with almost no payload (a very light ruler), the UR5 still exhibits linear motion that is outside the acceptable limits for high precision linear optical scanning. For these reasons, we ruled out the UR5 as a possibility for a robotic vision platform used in high precision linear optical scanning.
Analysis of the Denso Robot
Denso robots are high-end motion system that has several key advantages over other systems that enable smooth and precise motion needed for imaging applications and other applications where stability and repeatability are important. There are advanced control algorithms available for Denso robots that take payload weight and moment of inertia into account when planning motion paths. Denso software has good singularity management during linear moves that reduce path variations. The Denso line of robots has the ability to manipulate heavy and awkward loads while maintaining movement path accuracy. However, Denso robots do require additional complexity, cost, and increased workcell size in the form of required machine guarding and safety systems. Denso robots are not currently rated to work directly with people.
We found that the Denso is able to easily manipulate the KTM Research-designed end-effector in any orientation or mounting configuration. The Denso robot velocity is able to exceed the camera’s imaging rate for the lighting configuration that we used. This means that with a different lighting configuration and a higher speed camera, the Denso could be used to take images even more rapidly. Our testing of several different lens/camera/robot combinations demonstrated that the optical analysis of a customer’s single three-faced assembly is possible within the motion space of an inverted VS-G Denso robot. The resulting test images demonstrate some linear motion scan issues that are surmountable in post-processing prior to image analysis. Linear path inconsistencies of the Denso robot are repeatable within several pixels.
We concluded that the Denso robot system is usable for vision analysis, but is not ideal. The linear motion inconsistencies are highly repeatable and thus could be addressed in the models used for vision analysis. However, this is not ideal because anything that causes the inconsistencies to shift will break models leading to high rate of false-rejects. Prior to deploying Denso robots in this application, we recommend more testing of the potential for inconsistency shift to ensure robust operation in a production environment to minimize false-rejects. One consideration that KTM Research customers should take into account is that a work cell for the Denso system requires machine guarding and safety systems that increase cost, complexity, and work cell size.
Analysis of the Parker 404XR Linear Stage
Parker linear stages are highly accurate and highly repeatable single axis actuators used in many manufacturing applications. A linear stage can be paired with a rotating table or with other linear stages to permit linear scanning of complex geometries. One downside of Parker linear stages is that they are not rated for direct contact with people. Appropriate safety systems and machine guarding are required for most manufacturing production applications.
We found that the Parker 404XR linear stage produces excellent results when coupled with the KTM Research-designed linear optical scanner. There are no stability issues and repeatability is extremely good. In all but the most complex and difficult scanning applications, a Parker linear stage is a good choice.
Through KTM Research’s laboratory testing, we determined that the UR5 robot is not a suitable solution for high precision optical scanning applications. The Denso robot is a possibility for scanning parts with complex geometries although the image scan is not completely stable and there is significant complexity and expense involved in deploying a Denso robot due to operator safety requirements. The Parker 404XR linear stage provided the best results in our tests and received our recommendation to our customers. There are now KTM Research systems deployed in manufacturing plants the USA and Asia using the Parker 404 XR linear stage.
About KTM Research:
KTM Research is an engineering firm that specializes in industrial machine vision systems for quality control and vision-guided robotics. Formed in 2009, we are located in Tualatin, Oregon. We serve industries in the fields of advanced manufacturing, consumer electronics, bio-tech, food and beverage, research, and logistics. Our systems have been successfully used by customers across North America and Asia.
Our goal at KTM Research is to be the first call you make when faced with a vision challenge. Our team of engineers view themselves as an extension of your organization and strive to be your trusted vision partner. Our success is our clients’ success. Our collaborative approach to projects with our conservative and robust design process allows KTM Research to successfully complete projects that many others cannot.
Contact KTM Research at firstname.lastname@example.org for more information on our vision solutions.