The most significant game-changing process in aerospace manufacturing is carbon fiber layup. In this process, carbon fibers are combined with a resin or epoxy material to create a lightweight, but strong composite. This material is highly suited for the aerospace industry because it can reduce the weight of the airplane in order to achieve better fuel economy without sacrificing strength or durability. Robotic accuracy is important in this process because the placement of the carbon fiber strands relative to each other is critical to the structural integrity of the component.
One important feature used to achieve high accuracy in robots is the use of secondary encoders. This reduces omnidirectional repeatability to nearly zero, and has been validated via laser tracker while exploiting the combined effects from moving all axes. Secondary encoders connected directly to the controller are installed on the output side of each axis drive train to measure and control the true position of each axis. This allows the robot to control position, eliminating errors due to backlash and essentially improve the ability of the robot to achieve a commanded position. This is ideal for applications that require high precision or need to compensate for external forces.
Deflection compensation and advanced motion planning tools are also critical in the manufacture of large aerospace parts where large tools are mounted to the robot, the robot tooling is required to contact the part, and where offline programming is critical. Some additional applications that have benefited from high-accuracy robots are aerospace engine components manufacturing and airframe painting/depainting.
Several factors contribute to robot accuracy and must be considered: robot foundation, mounting, and environmental considerations; end of arm tooling; software; maintenance and repair; and validation.
Foundation, Mounting, and Environmental Considerations
One of the first considerations when designing a highly accurate system is how the robot is mounted or anchored to the floor; this is a critical consideration and cannot be underestimated. If the robot is not securely anchored to the floor, the robot can pitch or yaw in different directions or shift from the inertia of the robot’s movement and payload. The amount of effort spent on making the robot accurate will not matter if the very foundation to which it is mounted is not secure. This includes the floor thickness, how it is anchored, if it is isolated, the number of anchors, floor flatness, and the thickness of the base plate. The riser construction needs to be as ridged as possible to ensure only minor deflections. Some options to consider are to create the riser as a hollow tube and fill the tube with concrete. Be sure to use ample gussets and the proper thickness for the base plates and robot mounting plate.
The effects of thermal expansion should be considered for the applications that require the highest level of accuracy. There are two factors that directly influence thermal expansion. The first is thermal expansion due to ambient temperature — the temperature in the atmosphere surrounding the robot. The second is thermal expansion due to self-heating; reducers and motors will heat up during robot operation causing the robot castings to expand, which could alter robot accuracy. The ambient temperature cannot control thermal expansion due to self-heating. There is one factor that will directly affect the magnitude of the thermal expansion in both cases. The coefficient of thermal expansion of the material used to construct the robot is a major factor when allowing for thermal expansion. The coefficient of linear thermal expansion of aluminum is 23, where the coefficient of linear thermal expansion of steel is 11 to 13, depending on composition. This means that the effects of thermal expansion for a robot constructed of aluminum will be approximately double that of a robot constructed out of steel.
End of Arm Tooling (EOAT)
The next consideration is the design of the End of Arm Tooling (EOAT), payload, and dress out. The mass properties of the payload must be accurately defined. This is also true for any valve packages that are mounted on the robot’s arm. The better the payload is defined, the better the robot software will be able to correct the robot’s position due to gravitational effects. For calibration proposes, it is best to do the calibration with the actual EOAT and dress out.
When designing the EOAT, considerable attention needs to be paid to minimize EOAT deflection. The design needs to eliminate the possibility of the EOAT shifting or moving either from the robot’s own inertia, or from any process forces that will transfer back to the robot. This is done by designing an EOAT that uses dowel pins to eliminate the possibility of the EOAT moving or shifting. Also, the effects of the dress out cannot be ignored. Large and improperly designed dress outs affect robot accuracy by pulling on the EOAT.
Software defines the relationship between the part or fixture and the robot base frame. It is important that it is established accurately to account for any variance in the location of either the part or the fixture. This variance will be important when trying to maintain an accurate system. Depending on the application, several software packages can be used to set up this relationship accurately.
Maintenance and Repair
A maintenance and recovery plan is an essential part of an accurate robotic system. Just like there are three components that contribute to the overall accuracy of a robotic system, the maintenance and recovery plan must address all three of these components.
Not all options and not all applications will require accuracy validation, but accuracy validation is achieved using metrology such as a laser tracker to measure the robot’s actual position, and compare it to the commanded position. A laser tracker, tooling ball reflectors, and a knowledgeable user will be required for most validations.
This article was contributed by FANUC Robotics America Corp., Rochester Hills, MI. For more information, visit http://info.hotims.com/45602-321.