Metrology White Papers

Electroimpact has produced dozens of white papers focused on the use of laser trackers, lasers, and other instruments to make advances in quality and accuracy of our systems. Modern instruments present many opportunities to improve today’s assembly systems.

Electroimpact has produced a new in-process inspection system for use on drilling and fastening systems. The system uses a high-accuracy, non-contact, laser system to measure the flushness of installed fasteners. The system is also capable of measuring part normality and providing feedback to the machine for correction. One drawback to many automatic inspection systems is measurement error. Many sources of measurement error exist in a production environment, including drilling chips, lubrication, and fastener head markings. Electroimpact’s latest system can create a visualization of the measured fastener for the operator to interpret. This allows the operator to determine the cause of a failed measurement, thus reducing machine downtime due to false negatives.

Electroimpact created a custom C# WPF application that queries the point-cloud data and analyzes the raw data. A custom “circle Hough transform” scoring algorithm is used to find the center of the nosepiece (pressure foot). A best fit plane is calculated from the point cloud data to find the panel surface. This plane is then used to output panel normality in the A and B axes. Flushness is determined by computing the distance of each point in the fastener pointcloud to the best fit plane previously calculated. Finally, the point cloud is made into a surface and displayed on the screen using HelixToolkit open source 3D libraries. This allows the user to rotate, zoom, and center the 3D image on the PC.

With a mission to compensate and locate several hundred 2 or 3 axis machines with an evolving software process several significant data handling problems arose. How are all input data, scripts and other software versions kept under revision control when changes are being made on a weekly basis, and multiple teams are running the system across multiple shifts? How is it possible to process the avalanche of incoming data without being overwhelmed by it? If it is necessary to repeat metrology operations for a given machine how are the current measurements distinguished from old measurements? What can be done to prevent data loss due to operator error? How can all data be archived reliably and reports protected from accidental change? This case study describes an effort to grapple with these questions and a gradual evolution from primitive to more sophisticated solutions that raised the bar a little on our own best practices for repeated metrology operations.

Aerospace structures manufacturers find themselves frequently engaged in large-scale 3D metrology operations, conducting precision measurements over a volume expressed in meters or tens of meters. Such measurements are often done by metrologists or other measurement experts and may be done in a somewhat ad-hoc fashion, i.e., executed in the most appropriate method according to the lights of the individual conducting the measurement. This approach is certainly flexible but there are arguments for invoking a more rigorous process. Production processes, in particular, demand an automated process for all such “routine” measurements. Automated metrology offers a number of advantages including enabling data configuration management, de-skilling of operation, real time input data error checking, enforcement of standards, consistent process execution and automated data archiving. It also reduces training, setup time, data manipulation and analysis time and improves reporting. This paper draws on experiences from a recent automated metrology project and examines some of the challenges and benefits of successful measurement automation.

In AFP manufacturing systems, manually inspection of parts consumes a large portion of total production time and is susceptible to missing defects. The aerospace industry is responding to this inefficiency by focusing on the development of automated inspection systems. The first generation of automated inspection systems is now entering production. This paper reviews the performance of the first generation system and discusses reasonable expectations. Estimates of automated inspection time will be made, and it will be shown that the automated solution enables a detailed statistical analysis of manufactured part quality and provides the data necessary for statistical process control. Data collection allows for a reduction in rework because not all errors need to be corrected. Expectations will be set for the accuracy for both ply boundary and overlap/gap measurements. The time and resource cost of development and integration will also be discussed.

The customer’s assembly philosophy demanded a fully integrated flexible pulse line for their Final Assembly Line (FAL) to assemble their new business jets. Major challenges included devising a new material handling system, developing capable positioners and achieving accurate joins while accommodating two different aircraft variants (requiring a “flexible” system). An additional requirement was that the system be easily relocated to allow for future growth and reorganization.

Accurate and rapid joins required an advanced metrology solution. Integrating this automated metrology based positioning system posed a challenge. The accuracy requirement meant that the system had to measure and accommodate slight differences between the incoming parts i.e., be an “adaptive” system. A Human Machine Interface (HMI) was developed to enable de-skilled automated metrology and to communicate with the metrology and PLC systems. The HMI presents a virtual task checklist and restricts the user from deviating from the order of operations or omitting any tasks. Established tolerances must be achieved before proceeding to the next task. A robust architecture allows failed tasks to be re-attempted without restarting the join process, resulting in a forgiving and flexible process. Integrated supervisor-override privileges make it possible to execute alignment adjustments if dictated by engineering or circumstance.

Inspection of fasteners prior to installation is critical to the quality of aerospace parts. Fasteners must be inspected for length/grip and diameter at a minimum. Inspecting the fasteners mechanically just prior to insertion can cause additional cycle time loss if inspection cannot be performed at the same time as other operations. To decrease fastener inspection times and to ensure fastener cartridges contain the expected fastener a system was devised to measure the fastener as it travels down the fastener feed tube. This process could be adapted to inspection of fasteners being fed to the process head of a running machine eliminating the mechanical inspection requirement and thus decreasing cycle time.

This Powerpoint presents the use of adaptive tooling and a flexible HMI in conjunction with automated metrology to improve a Final Assembly Line process.

Accurate measurement of countersinks in curved parts has always been a challenge. The countersink reference is defined relative to the panel surface which includes some degree of curvature. This curvature thus makes accurate measurements very difficult using both contact and 2D non-contact measurements. By utilizing structured light 3D vision technologies, the ability to very accurately measure a countersink to small tolerances can be achieved. By knowing the pose of the camera and projector, triangulation can be used to calculate the distance to thousands of points on the panel and countersink surface. The plane of the panel is then calculated using Random Sample Consensus (RANSAC) method from the dataset of points which can be adjusted to account for panel curvatures. The countersink is then found using a similar RANSAC method. As the full geometric definition of the countersink and the plane are known, the radius and angle of the countersink can be calculated by intersecting of the two geometries to find the countersink diameter and depth. By inspecting the fit of each set of point to their respective geometric entities a confidence factor can be generated for the overall countersink measurement. Utilization of this technique would allow for more detailed measurement of countersink features.

In many existing AFP cells manual inspection of composite plies accounts for a large percentage of production time. Next generation AFP cells can require an even greater inspection burden. The industry is rapidly developing technologies to reduce inspection time and to replace manual inspection with automated solutions. Electroimpact is delivering a solution that integrates multiple technologies to combat inspection challenges. The approach integrates laser projectors, cameras, and laser profilometers in a comprehensive user interface that greatly reduces the burden on inspectors and decreases overall run time.

This paper discusses the implementation of each technology and the user interface that ties the data together and presents it to the inspector.

An aircraft final assembly line (FAL) offers many opportunities for improved assembly via metrology. This article describes an implementation of an FAL with automated positioners and a metrology system. The aircraft in question is a business jet with an approximate 105-ft length and 105-ft wingspan. Automated metrology solutions reduce assembly time, improve quality, increase repeatability, and deskill the operation so that those who are not engineers can carry out a more rapid and accurate assembly process. A novel human-machine interface (HMI) gives a common look and feel throughout all operations in the multiple work cells, provides user instructions at the taskby-task level, and places a list with task checkoff functionality on the screen. The HMI uses SpatialAnalyzer (SA) from New River Kinematics to control laser tracker operations, record data, and communicate with a programmable logic controller (PLC) to command machine actions, yet all functionality is programmable via a Microsoft Excel spreadsheet for easy modification of user instructions, graphics, and automation.

Starting in 2003 Electroimpact began development on a comprehensive kinematic and compensation software package for machines with large envelopes. The software was first implemented on Electroimpact’s Automatic Fiber Placement (AFP) equipment. Implementation became almost universal by 2005. By systematically collecting tracker measurements at various machine poses and then using this software to optimize the kinematic parameters of the machine, we are able to reliably achieve machine positional accuracy of approximately 2x the uncertainty of the measurements themselves.

The goal of this paper is to document some of the features of this system and show the results of compensation in the hope that this method of machine compensation or similar versions will become mainstream.

Spirit AeroSystems' process of producing carbon fiber nacelle panels requires heat and high force plus a high level of dynamic accuracy. Traditionally this would require large and expensive custom machines. A low cost robotic alternative was developed to perform the same operations utilizing an off-the-shelf 6-axis robot mated to a servo-controlled linear axis. Each of the 7 axes is enhanced with secondary position encoders and the entire system is controlled by a Siemens 840Dsl CNC. The CNC handles all process functions, robot motion, and executes software technologies developed for superior dynamic positional accuracy, including enhanced kinematics. The layout of the work cell allowed the robot to span two work zones so that parts can be loaded and unloaded while the robot continues working in the adjacent zone.

Automated countersink measurement methods which require contact with the workpiece are susceptible to a loss of accuracy due to cutting debris and lube build-up. This paper demonstrates a non-contact method for countersink diameter measurement on CFRP which eliminates the need for periodic cleaning. Holes are scanned in process using a laser profilometer. Coordinates for points along the countersink edge are processed with a unique filtering algorithm providing a highly repeatable estimate for major and minor diameter.

The versatility of the accurate robot has been increased by coupling it with a mobile platform with vertical axis. The automation can be presented to fixed aircraft components such as wings, fuselage sections, flaps, or other aircraft assemblies requiring accurate drilling, inspection, and fastening.

The platform accommodates a tool changer, ride along coupon stand, fastener feed system, and other systems critical for quality automated aircraft assembly. The accurate robot’s flexibility is increased by a floor resynchronization system. The indexing system is replaced by an automated two-camera onboard vision system and miniature targets embedded in the factory floor, with accuracy comparable to cup and cone alternatives. The accurate robot can be deployed by casters, curvilinear rail, or air bearings.

[Poster] An adaptive, flexible tool and a novel HMI enable rapid, accurate, deskilled laser tracker assisted aircraft joins.

In an attempt to be more flexible and cost effective, Aerospace Manufacturers have increasingly chosen to adapt a manufacturing style which borrows heavily from the Automotive industry. To facilitate this change in methodologies a system for locating robots has been developed which utilizes cameras for both locating and guidance of a mobile platform for a robot with drilling and fastening end effector.

Precision hole inspection is often required for automated aircraft assembly. Direct contact measurement has been proven reliable and accurate for over 20 years in production applications. At the core of the hole measurement process tool are high precision optical encoders for measurement of diameter and countersink depth. Mechanical contact within the hole is via standard 2-point split ball tips, and diametric data is collected rapidly and continuously enabling the system to profile the inner surface at 0 and 90 degrees. Hole profile, countersink depth, and grip length data are collected in 6 seconds. Parallel to the active process, auto-calibration is performed to minimize environmental factors such as thermal expansion. Tip assemblies are selected and changed automatically. Optional features include concave countersink and panel position measurement.

Wing and fuselage aircraft structures require large precise tools for assembly. These large jigs require periodic recertification to validate jig accuracy, yet metrology tasks involved may take the tool out of service for a week or more and typically require highly specialized personnel. Increasing the time between re-certifications adds the risk of making out-oftolerance assemblies. How can we reduce jig re-certification down time without increasing the risk of using out-oftolerance tooling? An alternative, successfully tested in a prototype tool, is to bring automated metrology tools to bear. Specifically, laser tracker measurements can be automated through a combination of off-the-shelf & custom software, careful line-of-sight planning, and permanent embedded targets. Retro-reflectors are placed at critical points throughout the jig. Inaccessible (out of reach) tool areas are addressed through the use of low cost, permanent, shielded repeatability targets. Simple locators enable adequate location of the tracker for each position, while automated tools within off-theshelf software such as Spatial Analyzer provide a vehicle for very rapid measurements. Custom software guides the nonexpert through the use of the metrology system so that the periodic "quick checks" are de-skilled, low cost, and fast

Incorporation of laser projectors in AFP machine cell controller reduces ply boundary inspection time, on-part course identification and part probing.

Machine compensation challenges increase with machine size. Increased production of large CNC machines led to the pursuit of improved methods for machine compensation. These methods included the use of volumetric compensation, implementation of a custom software solver tool, the use of API’s Active Target, the development of various laser tracker triggering tools, and eventually a custom software solution for communication between the CNC and tracker PC. The resulting process reduces station time for taking measurements, eliminates many blunder points, increases process flexibility, and reduces postmeasurement analysis, as well as decreases overall engineering time.

Very large multi-axis CNC machines offer a special challenge for efficient and accurate machine compensation. Aerospace applications demand tight tolerances, but conventional compensation methods become expensive for large machines. Volumetric compensation offers an approach for reducing costs and improving accuracies. A unique control architecture enabled by volumetric compensation enables the use of a single part program by multiple machines. Combining multiple technologies (a proprietary volumetric compensation solver program, Spatial Analyzer, API's Active Target, a laser tracker and bespoke CNCTracker communication software for measurement triggering) significantly reduces machine compensation time. Available analysis tools also enable the engineer to evaluate measurement uncertainties and determine the best locations for additional stations as well as quantify the accuracy benefits such stations would offer.

Electroimpact Automatic Fiber Placement (AFP) machines lay-up composite parts by accurately placing carbon fiber tow (strips of impregnated carbon fiber) on a mould. In order to achieve high accuracy at high speeds, the processes of feeding and cutting tows must be tuned. Historically, the tuning has been a time-consuming, manual process. This paper will present a methodology to replace manual measurements with an automated laser, improve measurement speed by an order of magnitude, improve accuracy from +/- 0.020" (manual) to +/- 0.015" (laser), and eliminate human error.

Accurately measuring the length of a pintail type fastener is limited by the process of forming the fastener. When the pintail is formed its overall length is not dimensionally controlled. To accurately measure the grip of the bolt a vision system is utilized that finds the notch between the tail and bolt shank. The grip, diameter and angle of the bolt prior to insertion are then measured. This method proves to be more accurate than measuring the bolt mechanically and provides a number of other advantages including; decreased measurement speed, accuracy, FOD detection and angle of the bolt in the fingers prior to insertion.

Once limited by insufficient accuracy, the off-the-shelf industrial robot has been enhanced via the integration of secondary encoders at the output of each of its axes. This in turn with a solid mechanical platform and enhanced kinematic model enable on-part accuracies of less than +/-0.25mm. Continued development of this enabling technology has been demonstrated on representative surfaces of an aircraft fuselage. Positional accuracy and process capability was validated in multiple orientations both in upper surface (spindle down) and lower surface (spindle up) configurations. A second opposing accurate robotic drilling system and full-scale fuselage mockup were integrated to simulate doubled throughput and to demonstrate the feasibility of maintaining high on-part accuracy with a dual spindle cell.

Metrology services Contact: 425-293-0767