© 2009 Quality Digest Magazine. All Rights Reserved.
By: Walter Pastorius
Assemblers of large objects, such as automotive bodies, have strongly recognized the need to continually improve processes to enhance dimensional integrity of assemblies, maximize customer aesthetic perception, and optimize product performance. Â
Implementing laser sensors for dimensional monitoring has allowed assemblers to move from low-volume sampling to inspecting every production part, and more recently, implementing in-process monitoring and error-proofing for rapid response to dimensional variation.
In the past two decades, a number of technological advances have been made in the dimensional monitoring of assemblies. Until the late 1980s, dimensional monitoring was typically accomplished by removing a component from the assembly line and transporting it to a coordinate measuring machine (CMM) or off-line checking fixture. Time required for CMM measurement was several hours, resulting in data being available for only a few components per day, and not providing statistically significant results.
About 20 years ago, manufacturers began installing laser-sensing stations at the end of assembly lines. These stations used large mechanical frames to mount a number of 3-D laser sensors around the component for inspection, as seen in figure 1. Each laser sensor measures a critical feature for deviation from nominal for every production assembly.
 Figure 1: In-Line dedicated laser-gauging station |
The laser sensors used can measure a number of different parameters, depending on the measurement type required. Laser-line triangulation techniques measure surface, vertex, and edge positions. Grey scale imaging is used for feature locations such as hole and slot positions, and weld nut positions. Both capabilities are combined in a single sensor package, including laser and surface illumination.
As part of the sensor manufacturing process, each laser sensor is individually calibrated against a series of precisely positioned artifacts to ensure accuracy of measurement data. The sensors are fully solid state, with no moving parts, so recalibration in the field is not required unless physical damage occurs.
Because these dedicated in-line measurement stations are located on the plant floor, they are subject to significant seasonal changes in ambient temperature. As part of the sensor manufacturing sequence, each individual sensor undergoes a complex temperature compensation process to correct for any thermally induced variations.
The measurement speed of laser sensors in these fixed measurement stations gives the station the ability to measure every component, providing complete documentation on 100 percent of production—a dramatic improvement over the CMM or check fixture.
Systems are typically equipped with data collection, archiving, and automated statistical analysis software for trend analysis, and an alarm function to indicate when user-defined limits are exceeded. Such software also reports both raw data and trends in all common reporting formats, in simple to understand color-coded graphical formats, and measurement data is available on the plant network for remote viewing. These end-of-line stations have been broadly implemented in industry at major automotive manufacturers in the Americas and Europe.
To optimize production, while responding to changing customer demands, many assembly plants have converted from dedicated production of a single model family to flexible assembly lines. The fixed location sensor approach can't practically inspect a broad mixture of models due to cost and complexity.
For the flexible assembly line, a new approach to in-line inspection was needed. The solution was found in integrating a 3-D laser sensor on a robot end effector, and using the robot to position the sensor sequentially at each measurement point. Different production models are accommodated by multiple robot programs, automatically selected for each assembly. Introduction of a new vehicle model simply requires the creation of new program. With the speed of today’s robots, 100-percent inspection on flexible assembly lines can be achieved.
A number of challenges need to be overcome for successful implementation of these flexible measurement systems.
The 3-D vision sensor must have algorithms available for any type of measurement requirement. The sensor must be extremely rugged to survive the constant acceleration and deceleration of the robot. Cabling to the sensor must be extremely flexible to survive constant flexing. The sensor must be small enough to fit into areas of limited clearance. Communication between the sensor and robot must be closely coupled to minimize any latency. The robot must communicate to the sensor the type of measurement required for each measurement point. The sensor must report its data in sensor coordinates, which will be combined with the robot coordinate system to transform measurement data into a global coordinate system. These, in turn, will be compared to the tolerances allowed for each measurement point.
Flexible inspection of large structures often requires use of several vision equipped robots for reach requirements and cycle time. A car body robot inspection station typically uses four robots (see figure 2). Methods had to be developed to coordinate data from all robots, and to transform their data into a common coordinate frame, generally based on the use of known artifacts in the measuring volume.
 Figure 2: Flexible robot gauging station |
Commercial robots typically used in assembly have very good repeatability, but are subject to some thermal growth issues as the robot warms up from a cold start, and as the plant ambient temperature changes over time. While the amount of thermal growth is often insignificant for assembly operations, it can influence measurement accuracy, relative to the small tolerances needed to maintain dimensional integrity. To overcome these variations, methods have been developed to implement temperature compensation for the 3-D vision robot system
Like the fixed-sensor systems, the flexible robot vision measurement systems must be equipped with a comprehensive data archiving and statistical process control reporting package, and with ability to place data on the plant network for remote viewing.
Manufacturers have found these final inspection solutions, both fixed and dedicated, valuable for 100-percent final inspection, but have expressed a need to have in-process monitoring and error-proofing for rapid response to process variations much earlier in the production process. Final inspection ensures that only acceptable parts are passed to the next operation, but doesn't generally provide information for diagnosing where a problem occurred in a production line. Â
Implementing 3-D vision sensors in the production line for in-process monitoring has required development of rugged 3-D laser sensors, which are installed in individual assembly tools to provide in-process monitoring. Rugged 3-D laser sensors give essentially instantaneous feedback and allow rapid response to any process variations. Figure 3 shows an in-process gauging station checking the width between rails in a motor compartment.
 Figure 3: In-process gauging station |
These in-process sensors can be distributed through multiple stations on the assembly line to monitor the process as it progresses and provide information that helps manufacturers determine where in the assembly line corrective action is required.
The new generation of 3-D laser sensors for error proofing have the ability to provide "Go/No Go" decisions based on the 3-D dimensional information, as well as reporting variable data.
Multiple sensors can be connected in a daisy chain configuration, using quick-fit industrial connectors. Data output can be provided by an Ethernet connection to the host. Feature measurement setups are easily programmed for easy setup. Measurement cycles are automatically triggered from the line product life cycle.
In order to mount sensors throughout the assembly line, sensor designs and packaging must be sufficiently rugged to allow mounting them in assembly weld tools in close proximity to weld guns.
In-process laser sensors are packaged in rugged housings, making them effective in even the most demanding manufacturing conditions. Patented weld splatter shields protect the sensor windows from damage.
3-D laser vision sensors for process monitoring have been implemented first for fixed stations, and then integrated with robots to provide flexible final inspection stations for assembly monitoring. These implementations have provided capability for 100-percent dimensional monitoring at the end of the assembly line.
The latest generation of in-process 3-D laser vision sensors provides dimensional error proofing for assembly operations. Rugged packaging provides ability to mount the sensors anywhere in the assembly tools, even next to weld guns, for instant response to process variations.
Laser sensors can be implemented in a variety of ways to respond to specific needs in dimensional monitoring in assembly operations.