With pressure to reduce cost while improving productivity, every automotive manufacturer has had to consider in-process quality control. To an extent, quality has become a given, but there still remains a huge variance in the sampling rate depending upon a particular manufacturing component and process. The bottom line is that the larger percentage of the samples, the more reliability you gain with the process, positively affecting both productivity and quality. The goal is to achieve as high a sampling as possible to feed back up the pipeline and stabilize the manufacturing process without adversely affecting production rates. If a company can achieve 100% part inspection, it can 1) eliminate the shipping of bad parts to the customer; 2) reduce scrapping of numerous production batch parts after a sample offline CMM post-production sample inspection; 3) achieve real time feedback and corrective actions to the production process and 4) avoid mechanical repeated set-up of part for rework after post-process inspection.
In the current world of automotive parts inspection, mechanical touch or scanning probes on coordinate measuring machines are the most common technology applied. The use of CMMs has helped bring the quality process closer to production by identifying defective parts more quickly. However, CMMs are best suited to capture simple prismatic features whereas automotive components are typically characterized by complicated freeform shapes. This is a challenge to mechanical measurement systems where the probe must remain in constant contact with the part surface. This requirement affects the speed of an inspection process as well as the amount of data it can collect. This process is less accurate if parts are actually deviating from their nominal computer aided design (CAD) model form or dimensions.
Also, time is critical. Fast and smart part alignment is important when there are geometry deviations as compared to the CAD model or non-accurate part positioning on the measuring machine. To properly align the part, the system needs to quickly find part position relative to the CMM and move the coordinate system from the CMM to the part itself. It is usually a tricky trial-and-error approach unless very high precision-and very expensive-mechanical jigs and fixtures are used. The bottom line is that the mechanical sensors employed in CMMs have limitations inclusive of scanning speed, measurement dynamic range and feature size limitation, or an inability to measure sharp edges or very small radii.
There has long been a need for an effective non-contact inspection system that is efficient, accurate and reliable. While various non-contact systems exist in the market for vision, calibration and digitizing, none have overcome the technology barriers and provided the performance necessary to bring an accurate, efficient, fast and reliable 3-D sensor to the CMM world. And, although non-contact vision systems are starting to penetrate various 3-D applications by leveraging their 2-D performance capabilities, when used for 3-D applications, the vision systems provide fast scanning, but have offered limited accuracy in the third dimension.
The traditional design of laser-based probes is built around the fundamental triangulation principle (figure 1). It involves the manipulation of a laser beam projected from the object through a dedicated set of optics in a fixed position and angle as well as an optical detector. This could be a position sensing device (PSD) or a charge-coupled device (CCD). These sensors are very sensitive to the optical parameters of the measured object including color, material, glare and reflection, surface finish and relative angle between the laser beam and the object. This sensitivity generates large deviations and unreliable output in the measurement results.
To address the new challenges imposed by the 3-D free form shapes, a 3-D laser-based scanning sensor, combining advanced laser and vision technologies, has been developed. It is comprises an adaptable laser source, a sophisticated set of optics, an advanced real time adaptive control and a two-dimensional CCD sensor. An extensive set of image processing algorithms analyzes and elaborates the high-quality data gathered (fig. 2)
This implementation provides single point precision of 12 microns (2 sigma) and a feature precision of 2 microns (100 points best fit, 2 sigma), which can meet very high precision requirements. The small laser spot size enables scanning of very fine geometry details down to 30 microns. And, this proprietary technology enables the generation of an infinite number of laser beam triangulations simultaneously, previously unattainable in earlier laser measurement systems.
Additionally, this approach allows for unique real-time adaptive control, which enables on-the-fly processing of the measuring data and close loop adjustment and calibration. If there is a deviation between the actual part and the CAD part, there is automatic adjustment of the approach process to offset accordingly. This combination of optics, unique adaptive control, and sophisticated image processing software allows the system to handle almost all material and surface finish types in various environments while inspecting detail down to 2 microns accuracy.
Using this 3-D laser technology solution has proven exceptionally helpful in measuring and inspecting cast cylinder heads in-line. The inspection process for a cylinder head includes the cylinder-head CAD model dimensions input, automatic alignment, transformation of coordinate system into cylinder-head orientation, measurement of the relevant pre-defined dimensions and actual deviations from the CAD model. Defective parts not complying with the specified tolerances are automatically rejected and pushed to a side NotOK conveyor.
A typical automatic alignment time is 30 seconds, and the full measurement of up to 85 dimensions takes less than 60 seconds per dual-line cell. During the scanning process, the system records the numerical data in real-time and displays measurement results on the screen. The data is automatically compared to the designed values and to the required tolerances. Thus, the measurement results and quality control reports are generated in real-time.
Manufacturers that have taken this quality control approach have been able to reduce the traditional CMM inspection process from about 15 minutes down to one minute per head. This has not only eliminated significant bottlenecks in the quality control state, but provided faster delivery to the customer while achieving 100% product inspection.
Implementation of a 3-D laser inspection system allows manufacturers to evaluate the stability of the process prior to consequential parts going out of tolerance and to isolate the source of production problems by correlating inspected part ID numbers to the manufacturing source. The reduction in inspection time enables higher sampling rates and improved production processes that contribute to higher efficiency, improved quality and lower cost, all helping with the bottom line.