Sorting Through Optical Options
February 1, 2008
Noncontact measurement and inspection is a fundamental practice in many manufacturing environments and quality control labs. Understanding the available options is a continual learning process as the power of video and the PC, as well as other advancements, constantly change and improve. As measuring experts continue to leverage these technology advances, more system choices have emerged.
Depending on the application, inspectors can choose from various noncontact inspection solutions to provide the results they require. In addition to defining the application requirements, companies must also consider budget, the current level of expertise and type of training that is required.
Scoping the BasicsAt the entry level, optical microscopes have long been a key component in manufacturing, education and research. There are many styles and types to choose from depending on the application. The most common used in manufacturing are stereo microscopes and toolmakers’ microscopes. These come in a variety of styles and in a range of prices. The zoom stereo scope combines two separate optical paths through individual zoom lenses and is intended mainly for visual inspection. It offers a clear, rich image with an extended depth of focus.
Toolmakers’ microscopes typically use a single optical path and a shallower depth of focus, which is better for measuring. They also use an XY stage for moving the workpiece into the field of view and making measurements to a reference line or reticle.
Many microscopes and video microscopes are designed in a modular fashion. Their functionality can be tailored to the application by adding the right lenses, illumination and workholding devices. In addition, software enhancements can add archiving, transmit images and generate work-related reports and documentation. Communication with a vendor is enhanced when a picture of the part or feature of a part in question can be identified visually. These enhancements also can streamline training.
Microscopes and other noncontact inspection systems continue to offer more capabilities. The simple LED evolved into a smart digital readout that offers complex computing capability in a simplified format. The power of this device is truly realized when measuring features with complex geometric shapes or when measuring multiples of the same part. The results reveal themselves in the way of enhanced throughput when an inspector is required to accomplish extensive measurements in a compressed time schedule.
Comparators and VisionOther noncontact options worth considering are optical comparators and video measuring systems. The optical comparator is a solution that inspectors rely on because of its ease-of-use and established track record. Comparators provide a combination of excellent image quality coupled with a full range of features including superb optics, readout devices, software interfaces, geometric measuring capabilities and screens that range in size from 12 to 40 inches. Light path options are offered in horizontal or vertical orientations, as determined by the application. In addition, in recent years many optical comparators are configured with optical edge detection.
Simple benchtop video measurement systems powered by a PC and independent motion control systems are another choice for many applications. These systems are small, powerful and affordable.
If an application demands brighter surface illumination and higher magnification, a manual vision system should be considered. Compact manual vision systems are available, with live video camera feed and automatic edge detection via digital readouts. This type of system is a video-based measurement solution for general purpose QA inspection, manufacturing and R&D noncontact measurement applications. It is a natural upgrade from a comparator, without the complexity of PC-based software.
Vision systems offer excellent surface illumination and the versatility of a zoom lens with magnification ranging from 10X to 240X. Inspectors that know how to use a comparator generally find a manual vision system easy to learn. Features that inspectors find particularly useful are image archival with the ability to add text and upload data to a flash memory device. Also, the digital readout adds an advantage by providing video edge detection that automatically detects and targets edge points. This is achieved by scanning the image area within a circle around the crosshairs on the display, eliminating operator subjectivity.
For high-volume applications that require significant throughput and repeatability, particularly when inspecting complicated parts, automated vision systems are invaluable. Automation removes operator subjectivity, which is essential when working within very tight tolerances.
Converging TechnologyThe evolution of noncontact inspection has created the vision metrology field that exists today. Systems are “precision coordinate positioning platforms” that use a variety of sensor technologies from zoom optics and video, to lasers and touch probes. These are more commonly known as multisensor systems. Multisensor systems are flexible, enabling companies to do a variety of measurement work with one system. They do, however, require a higher level of training.
For parts with complex contoured 3-D features, laser sensing can be a good choice. This method provides an extensive set of data points that more precisely define a complex shape. A touch probe can work well for larger parts with many recesses. However, when a touch probe will not reach critical areas, or when fine detail and magnification is essential, video is the best solution.
Size is an important variable. For small or delicate parts, the most efficient means of sensing is usually noncontact. With large parts such as features on an aircraft wing spar, a touch probe is a better candidate. These large-format, extended travel video measurement systems are best for working with large, flat or flexible parts.
If a company has a coordinate measuring machine (CMM) and requires performance enhancements, they could consider optimizing their current system with an optical-video adaptor. These adaptors can transform a CMM into a contact/noncontact measuring inspection system in minutes.
A good example of this is a manufacturer with a CMM that produces illuminated displays for aircrafts. Their challenge was to effectively measure and inspect parts that go into an instrument display assembly. Most of the components are large, flat, soft and flexible, making them difficult to handle and measure with traditional contact methods. The company also wanted to minimize the budget impact of purchasing a new piece of equipment. A solution was to add an optical-video head designed to be mounted on the Z-Ram of a CMM. By adding the optical-video head, the required functionality was available for this application and the transformed CMM became more fully utilized.
Taking the Long ViewVideo measurement applications will continue to evolve at a steady pace in the future so it always behooves users to stay abreast of the latest technological advances. In general, expect more intuitive systems that require minimal measurement programming, systems with higher resolution field of view measuring capabilities and an increase in automated inspection solutions.
These solutions are a clear outgrowth of the industry’s trend toward inspection of more complex miniature components, the need to remove operator error or subjectivity and to increase accuracy and throughput volume.
From microscopes to vision and multisensor systems, and even custom solutions using hybrid technologies, there are many ways to apply video/optical measurement and inspection. The key is to evaluate the options carefully and be sure to access appropriate sources that possess the technology, knowledge and expertise to properly guide one to the most effective solution. Q
Quality OnlineVisit www.qualitymag.com for more articles on noncontact inspection including:
- “Noncontact Inspection Keeps Pace with Production”
- “Seven Questions to Selecting a Measurement System”
- “Optical Scanning Solutions”