The term metrology refers to the branch of science dedicated to inspection. Within that branch, there are hundreds, if not thousands, of measurement tools and systems. Calipers, gages, micrometers, coordinate measurement machines, optical comparators and vision systems are just a few of the many different categories of metrology equipment. The options are numerous, but when it comes to 2-dimensional, nondestructive testing (NDT), optical comparators and vision systems are the go-to choices for many quality control applications.
The optical comparator is the original profile inspection system. Lights, mirrors, and lenses create a shadow of the part, projecting it onto a screen. The technology was established nearly a century ago, and has stood the test of time because of its simplicity in design, reliability, and cost-effectiveness. As PCs became more widely used, advancing the capabilities of metrology systems, vision systems came onto the scene.
Vision systems can take the same measurements as comparators in a fraction of the time due to video edge detection, pre-programmed automated inspection routines, and optional CNC control.
While optical comparators and vision systems are still the preferred choice and ideal systems for many quality control departments, there are some limitations to each of the two systems. Training to become proficient on an optical comparator can be time consuming, and inspections are slower than vision systems when taking multiple measurements on a part. Conversely, while vision systems increase speed and accuracy, with less time required in training, many systems are only available in a vertical orientation, with lower stage weight capacities compared to optical comparators.
Hybrid Emerges
Since necessity is the mother of invention, and there are continually opportunities to improve capabilities even further, a new class of measurement and inspection systems has recently been developed: “Hybrid.” Hybrid metrology systems come in many forms: benchtop, floor standing, vertical view and horizontal. They combine the best technology of both optical comparators and vision systems to create solutions that not only fill the gaps between the two but can offer advantages in certain applications.
One prime example of a hybrid system is a “digital comparator.” What makes a digital comparator unique is that it combines rugged and durable traditional comparator qualities with vision-based software platforms. In addition to the legacy and familiarity of a traditional optical comparator, there are many physical characteristics that are still as desirable as ever. The body of the comparator is a sealed, self-contained unit, ideal for manufacturing facilities with harsher environments such as having ambient dirt and debris present. The work stage of an optical comparator also tends to have a much higher weight capacity. Where most benchtop vision systems have a stage weight capacity of 50lbs or less, the digital comparator can hold up to 110lbs. Floor standing digital comparator models can support even more, up to 330lbs.
However, rather than relying on lights and mirrors projecting an image onto a screen which a traditional comparator does, the digital comparator utilizes a video camera to take advantage of digital video edge detection. This virtually removes operator-to-operator subjectivity, while also eliminating the need to store and maintain conventional mylar overlays of part drawings. Video-based software also enables the generation of pre-programmed, automated part programs. This drastically cuts down on the time it takes to get new users up to speed on using the hybrid system. Rather than the weeks or months it takes to hone the craft necessary to take accurate and repeatable measurements on a traditional comparator, vision software will take the same measurements defined in the part program every time. When optional CNC programming is added to mix, new digital comparator/ hybrid system users only need to place a part on the work stage, initiate the program, and the CNC system takes the measurements in a matter of seconds, creating a report ready for export.
Some manufacturing facilities still prefer an optical comparator for specific applications or because of their familiarity with the platform. Hybrid comparators offer multi-lens build options such as a two-lens slide on benchtop models and multi-lens turrets on floor standing models. These systems can be equipped with both traditional optical lenses, which can be quickly and easily interchanged or rotated in the turret, as well as an optional zoom or telecentric video camera. With this type of setup, operators can use the traditional optical lenses to take measurements on the comparator screen, and then switch over to the video camera and PC to take full advantage of the powerful features of vision-based software, including video edge detection and pattern recognition. The addition of the video camera to the comparator provides the option to also add an LED ring light, providing superior surface illumination to that of a traditional optical comparator. These types of hybrid comparators are ideal for facilities who are interested in transitioning to vision-based platforms for new users and applications, but might also have applications and users who are accustomed to the traditional optical setup.
Unique Hybrid System
Another type of hybrid system which has recently been developed is a video-based platform that does not run on a Windows-based PC – and offers unique capabilities -- this system utilizes a powerful 6.5:1 zoom lens in combination with a manually driven X-Y stage. An LED ring light provides exceptional surface illumination and clarity, which is displayed on a 14” 1080P monitor. The unique capability, however, is the use of a traditional optical comparator digital readout paired in a stack display with the monitor, controlled with a USB mouse.
Many manufacturing facilities are restricted from having Windows-based PCs on the manufacturing floors for IT or security reasons, so this new type of hybrid metrology system takes advantage of the capabilities of a video platform while adhering to non-PC restrictions. And since optical comparator readout technology has improved dramatically over the years, users of the new hybrid system can take many of the measurements required in NDT applications which are offered in PC-based vision systems.
This hybrid system supports industry standard feature measurement and popular construction types including intersections, farthest and shortest distance, perpendicular line, mid/center point, and more, as well as geometric tolerancing. Because the system is also equipped with a video-based zoom lens, it has several unique features that are not found on an optical comparator. The video camera itself has digital zoom capability on a monitor, as well as built-in color restoration and image acquisition. A built-in SD card enables the collection and storage of screenshots with exceptional detail and clarity to use with the built-in comparison tool. The system also supports horizontal and vertical image flip capability, and the digital crosshair can be custom-colorized, depending on the application, to achieve maximum contrast and accuracy.
Manufacturing is not slowing down. Lead times are tightening, complexity is increasing, and quality control departments need to be able to keep pace with it. While the standard options for NDT are suitable for most applications, there are situations arising more frequently where the status quo is not practical anymore. Hybrid optical comparators with both traditional lenses and video camera compatibility can provide a distinct advantage over standard optical comparators. For those seeking to transition to a full vision system, the digital comparator is a logical and powerful next step when evolving from a traditional comparator. Still others, previously limited by technology restrictions, might find that a video-based inspection system in a non-PC format, enables users to take fast and accurate measurements while adhering to company regulations. No two companies’ requirements are the same and the metrology industry is adapting to the latest measurement and inspection requirements, in real time.
