Where accuracy is concerned, robots have traditionally relied on repeatability. In the past, robotic accuracy has not been developed to a level of maturity acceptable to standard production processes. Critical aerospace manufacturing techniques such as fastening and drilling were historically not held to tight tolerances. Typical tolerances for airframe assembly fastening were in the +0.030" range. The standard is set by the positional requirement for drilling of fastener holes, which is a key target application for robotics in manufacturing.
Because there are many factors that influence robot accuracy, it is important to define the accuracy requirements for the system. Different levels of accuracy require different solutions; the higher the accuracy required, the more factors that must be considered, adding to the cost and complexity of the solution. The level of accuracy should be defined according to the process requirements. Some processes will only require positional accuracy while others require path accuracy, and some applications will require both.
Recently, for example, manufacturers have begun demanding that high-wear parts that require frequent maintenance and replacement be replaced seamlessly with identically manufactured parts. Inconsistent and inaccurately machined or assembled replacement parts might traditionally have meant time lost due to trimming, deburring, or other adjustments. Reducing fastener tolerances not only improves the reproducibility of an assembled component, but also allows for a reduction in overall structure weight due to reduced fastener size and weight. Eliminating these adjustments by machining or assembling precisely formed parts allows for predictable and timely part replacement, reducing costs and downtime, and allowing for parts to be interchanged repeatedly without any interruption in production. The introduction of robotic accuracy into the manufacturing process guarantees that this replacement is smooth, does not interrupt the manufacturing process, and is cost-effective and highly accurate.
High accuracy is also critical in data-driven applications — those developed using offline programming methods. For example, an advanced deburring process can begin offline with PC-based simulation software that allows users to import 3D representations of robots, parts, and system peripherals to create realistic “virtual” workcells. Part features can be selected from 3D CAD data of the part. Robot programs can then be generated automatically from these selected features. Even vision programming can be accomplished off-line. Until recently, it was not easy to offline-program a robot without having to perform extensive, manual touch-up of the programs on the production floor. Because of the large number of points required for airframe drilling, it is mandatory to offlineprogram without extensive program touch-up.
Robot Accuracy and Repeatability
Robot accuracy is a measure of how close a robot can attain a known position. It is required for systems where the paths are taught offline or if the process requires changing the robot position dynamically using vision or another means. Robot repeatability is a measure of the robot’s ability to return to a known position. High robot accuracy during manufacturing ensures that parts are precisely manufactured with predictable results, even after changes are made to the process. High-accuracy robots are becoming valuable tools for many processes in aerospace manufacturing such as drilling and fastening, deburring and trimming, and a variety of others such as non-destructive inspection, coatings, and composite layup. Manufacturers can enjoy significant cost savings as a result.
Systems that combine processes like drilling, routering, and material removal require both positional accuracy and path accuracy. Positional accuracy is a measure of how accurately the robot can achieve a commanded position. Positional accuracy is required for processes like drilling, where the robot moves to a position, stops, and holds that position while the process is completed. Path accuracy is a measure of how accurately the robot follows a line between two points. Path accuracy is required for processes like laser cutting where the process is taking place while the robot is moving between points.
Robot accuracy is improved when the work zone is defined as localized as possible. It is important to define where in the robot’s work envelope the process will take place. This is called the process work zone. A higher level of accuracy is achievable if the process work zone is defined and the calibration is restricted to this zone. When defining a process work zone, there are three considerations to follow. First, the process work zone needs to include all processes that require accuracy. Second, make the zone only as large as the process requires. Third, limit robot configuration (orientation) changes in the process work zone as much as possible.
Different levels of accuracy require different solutions. The required level of robot accuracy determines the number of options and calibration tools required to achieve that accuracy. The more calibration tools required, the more complex and expensive the solution will be.
Repeatable robot paths and tool execution means critical material cost savings in removal applications. An added benefit of using accurate robots for aerospace manufacturing is the inherent repeatability of robotic processes, allowing for better predictability and control of process parameters. This makes it easier to identify and refine process parameters that affect component quality. In addition, robots can execute complex or repetitive processes at very high speeds.