This article will take an in-depth look at optical comparators.
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Optical comparators are measurement systems that offer extremely accurate and repeatable measurement data. They use video images, geometric dimensioning and tolerance (GD&T),light, lenses, mirrors, and cameras to examine the tolerances of parts, assemblies, and components. Optical comparators are a valuable tool used by quality control inspection teams to determine the effectiveness of production methods and the quality of manufactured items.
In the examination of a workpiece, an optical comparator projects the image of the workpiece onto a display screen where it is compared to the standard parameters of the workpiece. Complex stampings, gears, cams, threads, and other components are compared to their model. It is used for producing precision machinery for the aviation, aerospace, watch and clock industries, electronics, the instrumentation business, research facilities, and detection metering stations at all levels.
Flaws, scratches, indentations, and dimensional inaccuracies are immediately highlighted by the comparator, which is calibrated using the standard parameters of the workpiece.
The first comparator was introduced in the 1920s and has changed very little since its introduction. Technologies, calibration methods, digital and software enhancements, and magnification have been added to improve and increase accuracy. In the traditional method, the workpiece is placed on a stage that is illuminated by light that comes from below the stage. A shadow of the workpiece is projected, through a lens and mirrors, onto a screen.
The image that is projected is magnified using a telecentric optical system to project the workpiece with accurate magnification. The telecentric lens projects the workpiece at the correct magnification without warping the image. The dimensional accuracy and surface defects of the projected image are compared to the parameters for the workpiece.
The primary configurations of comparators are horizontal and vertical where a horizontal comparator has a side view of the workpiece while the vertical view looks down on the workpiece. Horizontal and vertical optical comparators have been used for years by manufacturers as a means for determining product and part quality.
Regardless of their many years of use, traditional optical comparators have disadvantages especially in regard to the complexities of modern manufacturing.
Simple optics, corrected optics, fully corrected optics, and telecentric optic systems are the four basic categories of optical systems utilized in optical comparators.
With a telecentric lens, the workpiece is aligned to be examined with a grid on the screen with measurements for distances to other points made from the projection. Additionally, the profile projector may have episcopic lighting (light shining from above), which illuminates internal areas.
The traditional manual method for comparators is rapidly being replaced by technologically advanced computer software. The three methods commonly used by manufacturers include:
Traditional optical comparators use the outdated silhouette method to observe one part at a time, which is still popular on the market. Digital/video comparators manage the entire process electronically in a fraction of the time, produce more accurate data, and can assess large numbers of parts..
The size and magnification of traditional comparator’s projected images differ with optics and screen size affecting measurements. Optical comparators typically have screens that start at 12 inches. As a result of the greater distance needed to produce a larger image without distortion, larger screen sizes are only feasible with larger enclosures.
Digital/video comparators have the image of the part being examined displayed on a computer monitor for easy manipulation and study. With automated digital comparators, a camera captures an image of a part as it moves along a production line and electronically compares it to a CAD rendering or part image.
Choose a basic XY digital readout if only fundamental measurements like locations and lengths are needed. However, having geometric capabilities is necessary if the application calls for measuring circles, angles, and parametric distances. If the measurement and inspection are repetitive, CNC-capable readouts should be considered. The optical edge detection method is another important factor. Edge detection removes operator subjectivity and enhances measurement accuracy and repeatability in general.
For traditional optical comparators, workpiece should be held firmly to the comparator table to guarantee repeated and precise measurements. To choose a fixture that shows the qualities of their component following the light path that best suits the application, one should be sure to investigate the range of fixtures offered. Today's choices on the market include rotary fixtures, internal lens turrets, digital protractors, helix stage, corrected images, and LED lighting.
Screen sizes for traditional optical comparators range from 12" to 32." One should identify the specific aspects of their program that need to be measured at once before selecting a screen size. When measuring, it might not be possible or required to see the full component. Calculations can be made by dividing the screen diameter by the lens magnification. For instance, a 16" optical comparator with a 10X lens would allow the user to see 1.6" of the component on the screen (16"/10 = 1.6"). When viewing a picture with an overlay, many engineers believe that staying within one inch of the screen margin is an excellent practice. Ensure that the stage's dimensions, weight limit, and capacity will accommodate the components to be measured or inspected.
An operator who is typically alert and focused on the task at hand can consistently discriminate 0.004" on a comparator screen. One should consider the resolution of the lens' magnification. One can choose the right lens based on the necessary tolerances. The magnification of the projection lens is constant. Magnifications are frequently adjusted for varied views of measured parts. Although a single lens is often included in a projector's standard configuration, additional lenses can be utilized if necessary.
One must be aware of the ideal light route for their application. For example, a horizontal light beam traverses a stage in instruments with horizontal light paths. Applications include thread form measurement, castings, transmission shafts, and machined components. Instruments with vertical light paths feature a light beam that moves vertically. On a glass plate, parts being examined and/or measured are put. The system's XY stage has a glass plate, through which the beam passes. Flat parts including stamped parts, gaskets, electronics, and O-rings are best suited for vertical systems.
There are several types of cameras that are part of a digital/video comparator. They combine an optical comparator, digital microscope, and a non-contact inspection measurement system with auto edge detection. Digital/video cameras are designed with large scope resolution, various frame rates, easy interfaces, spectral sensitivities, and sensors. They capture the most minute details of an image for easy manipulation and magnification with precision resolution.
When considering the implementation of a digital/video comparator, it is important to consider the types of parts to be assessed and how they will be presented. Digital/video cameras are capable of measuring parts as they move on a conveyor or individually like a traditional optical comparator. Additionally, the size, volume, and precision that is required has to be considered due to the high accuracy of digital/video comparators.
A trustworthy manufacturer will offer online technical assistance. Customers can contact the business if they need help with their goods thanks to support available from anywhere in the world. Customers may be asked to walk them through the issue over the phone, or they may even gain remote access to the system to investigate and fix the problem themselves. Choose a comparator from a company that will take the time to assist in resolving any issues with minimal downtime.
Digital/video comparators reference CAD drawings to make comparisons and use laser measurement tools and comparison software. Its vision system is connected to a computer that is used to adjust the systems of the video comparator unit.
The workpiece is placed on a glass plate that has a light source beneath it that shines up to the video camera that is directly above the glass plate and captures an image of the workpiece being measured.
LED lights surround the lens of the camera. The number of LED lights vary depending on the manufacturer and model of the comparative system.
There are several types of cameras that are used with digital/video comparators with the most popular being charge coupled device (CCD), complementary metal-oxide semiconductor (CMOS), and gigabit ethernet (GigE) cameras.
The image captured by the camera is projected on a computer monitor where the number of LED lights to be activated are controlled. How the captured image is displayed is dependent on the manufacturer of the comparator. In the majority of cases, the computer screen is divided into sections that are controlled manually or automatically.
During the initial setup, knobs on the lens and glass table are adjusted to properly focus the image using. A visual slide is used to setup the system and focus it. The video comparator has X, Y, and Z axes with the position of each axis displayed on the computer screen. Included in the software package are measurement adjustments for degrees of angle, degrees of temperature, and dimensions.
For use in assembly operations, the position of the items to be assessed is programmed into the computer system. As the items move down the assembly line, the camera records the image of the item and compares it to its CAD rendering.
Computer software has a variety of light controls that are divided into segments, rings, and sections of the circle of lights. The types of light controls vary according to the type of system and its software. Additionally, the software of the system allows control of the type of view required for an examination of the workpiece, which can be side, top, front or back.
A condenser lens is a component of every optical device. The divergent light rays from the light source are transformed into parallel light rays as their primary function. Therefore, the term "objective lenses" also applies to condenser lenses.
The parallel light beams from the condenser lens are projected to the reflective mirror by the projection lens, which is situated adjacent to the condenser lens.
The screen is the display of the workpiece being measured.
The base is mounted with the full arrangement, including the table.
Plungers are metal parts that serve as sensing elements to subtract dimensional changes from the workpiece under measurement. Depending on the inconsistencies in the workpiece, it reciprocates a pivoting lever. A plunger and a mirror are connected at both ends by a lever fixed at a pivot point. The plunger is situated not far from the pivot point. The pivoting lever mechanism improves the plunger's ability to move.
A mirror serves as a reflecting medium in an optical comparator, reflecting the arriving light rays from the light source. The mirror is pivoted at one end of the pivot lever and pivots at its center.
The workpiece that needs to be inspected is set down on a level surface, where the plunger will contact it. Critical factors include its volume, X, Y travel, and carrying capacity. A precise rotary table, a part holder, and other accessories are typically installed to make holding the workpiece more convenient. The comparator also needs a wide working distance and a flexible, reliable focusing mechanism. All current optical measuring projectors on the market have been digitized, and the user chooses suitable data processing options. Therefore, one should also take into account pertinent data-processing capabilities.
The object is fixed in situ to be measured in the proper orientation using fixtures for optical comparators. For instance, a spherical object can be clamped horizontally, or an object with a non-flat bottom surface can be fixed in a measurement-friendly position. Fixtures come in various forms, such as clips, clamps, and magnets.
Utilizing the overlay chart requires comparing it to the projected measurement image on the screen. Charts come in a variety of forms. For instance, concentric scale or grid patterns are frequently used. Additionally, it is possible to see how the contour of the design value differs from the real measurement target by superimposing the diagram chart on the projected image in which the design value of the measuring target is magnified at the same magnification.
The optical comparator has two options: illumination from above (lens side), which projects outlines, and illumination from below, which transmits light to cast shadows. Even though it is challenging to measure the target using only the transmission (backlit) picture, epi-illumination can be used.
To prevent light coming from outside, use blackout curtains. It is utilized to portray a shape more precisely by obstructing ambient light.
Since the 1920s, visual measurement systems have become a staple part of industrial operations and manufacturing. For many years, traditional platform systems presented a silhouette of a part as a reference point for comparisons. As technology has advanced and methods of comparison have improved, digital/video systems have been developed to provide more accurate and precise data.
Optical comparators have grown and advanced during the nearly 100 years of their use. They are able to produce measurements along the X, Y, and Z axis of small parts using magnification. Measurements are taken individually, which is labor intensive since it requires an operator. Optical comparators do not include computer software, are easy to use, and cost much less than more advanced technologies.
Digital/video comparators use a computer to inspect, measure, evaluate, and identify components, assemblies, and parts individually, in groups, or as they move along a conveyor. The accuracy, speed, and efficiency of digital/video comparators makes them ideal for modern fast paced manufacturing. They use zoom optics with precision lighting to provide exceptionally accurate measurements and data.
Optical comparators and machine vision systems compare, inspect, measure, evaluate, and identify moving or nonmoving objects. An optical comparator visually measures small two dimensional parts and components along the X, Y, and Z axes using magnification. They do not rely on computer software to complete the inspection process.
Machine vision systems are automated inspection machines that use PC software to quickly and efficiently complete the inspection process with an accuracy of 0.0002 inch (0.00635 mm). They are able to do a three dimensional analysis by seeing and measuring the features of an object.
Optical comparators are restricted to 2D and 2 ½ D dimensional capabilities, do not use computer software and are not compatible with computer aided design (CAD) software. They have lower optical resolution and low throughput with limited lighting options or contour illumination.
Machine vision systems are built on PC software that is easy to use. They are capable of contact measurements and compatible with CAD software. Machine vision systems have high throughput and are able to measure parts that are too small for CMM or parts that cannot be touched. They have larger working distances, fields of vision, and measuring envelopes.
The comparator industry is constantly developing and perfecting methods to produce accurate data and measurements. While traditional comparators are restricted to 2D images, modern 21st century comparators are capable of examining 3D images with intricate details, complex configurations, and attachments. The technologically advanced products of the modern era require greater precision and accuracy than that which can be provided by traditional 2D optical comparators.
Regardless of the many advancements in comparator technology, traditional versions are still widely used for less detailed parts and components. The orientation of all comparators is either vertical or horizontal including digital/video versions.
Digital/video comparators are the latest and most efficient form of comparators that use CAD images as the source of their comparisons. They work faster, are capable of assessing parts on a production line, provide instant and precise data, are computer controlled, and have the flexibility to be used in any production format.
One of the exciting benefits of digital/video comparators is their ability to magnify downloaded images to the most minute detail. Captured images are gathered and sent to a computer monitor where they can be expanded, magnified, and observed using a wide array of computing softwares. Measurements, close examinations, detailed images, and data are easily manipulated to provide precise data regarding tolerances
Digital/video comparators work faster, provide exceptionally clear imaging, and are capable of assessing any type of component. CAD images are displayed in order to make real time adjustments. When a design changes, the CAD rendering is immediately available for comparison. Digital/video comparators automatically adjust visual images for any magnification.
Additionally, video edge detection (VED) makes it possible to have interaction between the CAD file with the video image. Digital/video imaging increases accuracy, productivity, and the rate of comparison.
Since digital/video comparators allow operators to assess several parts simultaneously or in groups, they are a more efficient solution for the needs of modern manufacturing where inspection of numerous parts is a necessity.
GD&T is a system for communicating engineering tolerances and relationships using a set of symbols placed on engineering drawings and computer models that describe the geometry of an image and its allowable variations. The system lets workers know how accurate and precise a part needs to be and improves metrology.
The GD&T system allows developers and engineers to improve the function of a part without increasing costs. It uses a representation of an object as its reference and describes the geometry of a part and not its linear dimensions. The use of GD&T drastically reduces rejection rates, assembly failures, and the investment in quality control.
Size is the physical size that is controlled by ± tolerancing. Location refers to where a part is located in space relative to other parts with positioning being the main aspect of this control.
Orientation is in regard to how a part is angled in space in relation to other parts and is a refinement of location. It is controlled by parallelism, perpendicularity, and angularity. Form is an all inclusive term for the shape of a part and includes how straight, flat, circular, and cylindrical a part is.
The main axis is parallel to the projection screen's plane. Thus, screens are typically produced in medium and large sizes, which are best for inspecting heavy workpieces with large profiles or shaft parts. However, small machines with silhouette lighting may find it more convenient to have a horizontal table below the screen without a hole for light transmission. In a horizontal model, the light from the optical comparator travels horizontally, allowing the spectator to see a silhouette of a part as seen from the side. When holding components in a fixed position, this model performs well. Examples include castings that need to be held in a vice or screws that are fixed in place.
The observer is looking down on the component in a vertical model because the light from the optical comparator travels vertically. Smaller workpieces, such as gaskets, or flat components that can lay on the work surface function best for this. They also perform well when measuring flexible or soft objects that need to lie flat to be precise. Both optical comparator types are used in quality control labs and production facilities. The industrial fields of science, transportation, healthcare, aerospace, and defense enjoy the greatest popularity.
Traditional optical comparators line up a mylar overlay that acts as a reference for a manual single part comparison. If there are inconsistencies after aligning the overlay with the part, the operator determines if the component can still be used.
Simple to use and labor intensive, traditional comparators are less precise and inaccurate. They measure one part at a time and are incapable of keeping up with modern production. Although additional lenses are available, traditional comparators come with one magnification.
Constant handling of the mylar overlays damages them. Storage of the overlays requires significant room due to the number of overlays required for each piece produced. Mylar overlays are expensive to produce and expense that rapidly increases with product adjustments and changes.
In the diagram below, A is the projection screen, B the projection lens, C the movable stage, and D indicates the knobs for the X and Y axis movement of the stage.
The modest plunger movement is amplified in a mechanical optical comparator using both mechanical and optical components. This tool compares the workpiece's geometric requirements to those of the reference specimen. Every time a light source is made to incident onto the mirror, it is reflected at the same angle that the ray incident. The light ray is then made to incident onto the calibrated scale to transform the angular movement of the mirror into linear readings of the scale. Finally, a plunger is mounted to the mirror, allowing it to be tilted.
A datum and an allowed range are set on the scale before the plunger slides over the standard specimen. After that, the standard specimen is taken out and the plunger is brought into contact with the work piece's surface for comparison. The plunger vertically moves as it passes over the uneven surface, and a pivoting lever considerably amplifies this movement. The mirror tilts as a result of this lever. The rotation of the mirror around its pivot amplifies the mechanical amplification that the plunger already provides. After passing through the condensing and projector lenses, the light from the light source is produced to incident onto the mirror. The tilted mirror surface reflects light rays that strike it onto the inner surface of the graduated scale through the eyepiece which can be seen.
The fact that mechanical optical comparators have so few moving parts allows them to achieve high precision. Error with parallax is prevented. Due to having fewer pieces than other comparators, they weigh less. They are ideal for precision measurements because they can attain great magnification. However, they require a separate electricity source, which is a drawback. Due to the scale that must be seen through the eyepiece, they are not suited for prolonged usage. They are only appropriate for usage in a darkroom.
Electrical and optical components are used in the construction and operation of an electrical optical comparator. The light emitter, receiver, electronic amplifier, and optical lens are crucial parts of electrical optical comparators.
An electrical optical comparator's light emitter is a light source that provides a constant beam of light for magnification. The receiver takes in the light beam, transforming it into an electrical signal. Finally, an electronic amplifier is used to boost these electrical signals.
The electrical signals are handled here, and the result is measurements data. Numerous comparison techniques are available with this comparator, including the method of light intensity, shadow casting technique, a laser scanner gauge technique, and the laser diffraction technique. Without retooling, electrical optical comparators are frequently used for component inspection.
Optical comparators are used by businesses from a variety of industries to address a variety of applications. The following list of typical usage and applications for optical comparators:
Despite being a handy tool for performing different measures, the optical comparator has a few drawbacks that users may encounter.
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