This article will take an in-depth look at optical comparators.
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Optical comparators are precision measurement tools that provide highly accurate and consistent data. They utilize video imaging, geometric dimensioning and tolerancing (GD&T), along with light, lenses, mirrors, and cameras to assess the tolerances of parts, assemblies, and components. These comparators are essential for quality control teams to evaluate production processes and ensure the quality of manufactured products.
During the inspection of a workpiece, an optical comparator projects its image onto a screen where it is matched against standard parameters. This process allows for the comparison of intricate components such as stampings, gears, cams, and threads to their models. Optical comparators are crucial in manufacturing precision machinery for industries including aviation, aerospace, horology, electronics, instrumentation, and various research and metering applications.
The comparator quickly identifies defects such as flaws, scratches, indentations, and dimensional inaccuracies by comparing the workpiece against its standard parameters. It is precisely calibrated to ensure accurate detection of any deviations.
Since its debut in the 1920s, the fundamental design of the comparator has remained largely unchanged. However, advancements in technology, calibration techniques, digital tools, and magnification have significantly enhanced its accuracy. Traditionally, the workpiece is positioned on a stage lit from below, with its shadow projected onto a screen via lenses and mirrors.
A telecentric optical system magnifies the projected image to ensure precise magnification of the workpiece. This lens system maintains accurate scaling without distorting the image. The projected image’s dimensional precision and surface imperfections are then compared to the standard parameters of the workpiece.
Optical comparators typically come in two main configurations: horizontal and vertical. A horizontal comparator provides a side view of the workpiece, whereas a vertical comparator offers a top-down view. Manufacturers have relied on both horizontal and vertical optical comparators for many years to assess the quality of products and parts.
Despite their long history of use, traditional optical comparators have limitations, particularly when addressing the complexities of contemporary manufacturing processes.
Optical comparators employ four fundamental types of optical systems: simple optics, corrected optics, fully corrected optics, and telecentric optical systems.
Using a telecentric lens, the workpiece is aligned for inspection with a grid overlay on the screen, allowing precise measurement of distances between points on the projection. Additionally, the profile projector may feature episcopic lighting, which illuminates internal areas by shining light from above.
Traditional manual methods for optical comparators are quickly being overtaken by sophisticated computer software. Manufacturers commonly use three advanced methods, including:
Traditional optical comparators rely on the outdated silhouette method, inspecting one part at a time, which remains popular. In contrast, digital video comparators handle the entire process electronically, offering faster operation, more precise data, and the ability to evaluate large quantities of parts efficiently.
In traditional comparators, the size and magnification of the projected image vary depending on the optics and screen dimensions, impacting measurement accuracy. Optical comparators generally feature screens starting at 12 inches. To accommodate larger images without distortion, larger screens require bigger enclosures due to the increased distance needed.
Digital video comparators display the examined part's image on a computer monitor, allowing for easy manipulation and analysis. Automated digital comparators use a camera to capture images of parts as they progress along the production line, comparing them electronically to CAD models or reference images.
For basic measurements such as locations and lengths, a simple XY digital readout will suffice. If your application requires measuring circles, angles, or parametric distances, choose a readout with geometric capabilities. For repetitive measurement and inspection tasks, consider CNC-capable readouts. Additionally, the optical edge detection method is crucial, as it reduces operator subjectivity and improves the accuracy and repeatability of measurements.
In traditional optical comparators, it's crucial to secure the workpiece firmly on the comparator table to ensure consistent and accurate measurements. Explore the various fixture options available to match the specific needs of your application. Modern fixtures include rotary fixtures, internal lens turrets, digital protractors, helix stages, corrected images, and LED lighting, each designed to enhance the accuracy and versatility of the inspection process.
Traditional optical comparators feature screen sizes ranging from 12" to 32". Before choosing a screen size, consider the specific dimensions of the components you need to measure. It may not be necessary to view the entire component at once. To calculate the visible area, divide the screen diameter by the lens magnification. For example, a 16" optical comparator with a 10X lens would display 1.6" of the component (16"/10 = 1.6"). Engineers often recommend staying within one inch of the screen's edge when using an overlay. Also, ensure the stage's dimensions, weight capacity, and capacity are suitable for the components being measured or inspected.
An attentive and focused operator can reliably distinguish 0.004" on a comparator screen. It’s important to consider the resolution provided by the lens magnification when selecting the appropriate lens for your needs. The magnification of the projection lens remains constant, but it can be adjusted to view different parts of the component. While a standard projector usually comes with a single lens, additional lenses can be added if required to meet specific measurement tolerances.
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.
Digital video comparators incorporate various types of cameras that blend optical comparators, digital microscopes, and non-contact measurement systems with automatic edge detection. These cameras feature high-resolution capabilities, multiple frame rates, user-friendly interfaces, and diverse spectral sensitivities. They are designed to capture even the smallest details of an image, allowing for precise manipulation and magnification with exceptional clarity.
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 reliable manufacturer should provide online technical support to assist customers globally. Support may involve guiding customers through issues over the phone or accessing the system remotely to diagnose and resolve problems. Opt for a comparator from a company that is committed to addressing any concerns promptly, ensuring minimal downtime.
Digital video comparators utilize CAD drawings for accurate comparisons, incorporating laser measurement tools and specialized comparison software. The vision system is linked to a computer, which is used to calibrate and adjust the video comparator's settings for precise measurements.
The workpiece is positioned on a glass plate with a light source underneath. This light illuminates the workpiece and is captured by a video camera positioned directly above the glass plate, providing a clear image of the item being measured.
The camera lens is encircled by LED lights, with the quantity varying by manufacturer and model of the comparator system.
Digital video comparators utilize various types of cameras, with the most common being charge-coupled device (CCD), complementary metal-oxide semiconductor (CMOS), and gigabit ethernet (GigE) cameras.
The image captured by the camera is displayed on a computer monitor, where the activation of LED lights is controlled. The display format of the captured image varies by the comparator manufacturer. Typically, the computer screen is segmented into sections that can be managed either manually or automatically.
During the initial setup, the lens and glass table knobs are adjusted to achieve proper image focus. A visual slide is used to calibrate and fine-tune the system. The video comparator features X, Y, and Z axes, with each axis's position shown on the computer screen. The software package includes options for adjusting measurements related to angles, temperature, and dimensions.
In assembly operations, the positions of the items to be inspected are programmed into the computer system. As the items progress along the assembly line, the camera captures their images and compares them to the CAD renderings.
Computer software provides various light controls, segmented into rings and sections of the lighting circle. The specific controls depend on the system and its software. Additionally, the software allows for controlling the type of view needed for examining the workpiece, including side, top, front, or back perspectives.
A condenser lens is an essential component in optical devices, responsible for converting divergent light rays from the light source into parallel rays. As a result, condenser lenses are also referred to as "objective lenses."
The condenser lens directs parallel light beams towards the reflective mirror, while the projection lens, positioned next to the condenser lens, facilitates this process.
The screen displays the image of the workpiece being measured.
The base supports the entire setup, including the table.
Plungers are metal components used as sensing elements to detect dimensional changes in the workpiece being measured. They operate by reciprocating a pivoting lever based on the workpiece's inconsistencies. The plunger and a mirror are connected via a lever fixed at a pivot point, with the plunger positioned close to the pivot. This pivoting lever mechanism enhances the plunger's movemen
A mirror in an optical comparator acts as a reflective surface, redirecting light rays from the light source. It is mounted at one end of the pivot lever and rotates around its center.
The workpiece to be inspected is placed on a level surface where it is contacted by the plunger. Key considerations include the workpiece's volume, X and Y travel, and weight capacity. To facilitate easier handling, a precise rotary table, part holder, and other accessories are often used. The comparator should also offer a wide working distance and a versatile, reliable focusing mechanism. With most modern optical measuring projectors now digitized, selecting appropriate data processing options is essential, so consider the system's data-processing capabilities as well.
The object is secured in the correct orientation for measurement using fixtures designed for optical comparators. For example, a spherical object can be clamped horizontally, while an object with an uneven bottom surface can be positioned to facilitate accurate measurement. Fixtures are available in various types, including clips, clamps, and magnets.
To use the overlay chart, it must be compared with the projected measurement image on the screen. Various types of charts are available, such as concentric scales or grid patterns. By overlaying the chart on the projected image, you can visualize how the design value contours compare to the actual measurement target, ensuring both are magnified at the same scale.
An optical comparator offers two illumination options: epi-illumination from above (the lens side) for projecting outlines, and transmission illumination from below to create shadows. While measuring the target using only the backlit image can be challenging, epi-illumination can still be effectively utilized.
Use blackout curtains to block external light and enhance the accuracy of shape depiction by eliminating ambient light interference.
Since their introduction in the 1920s, visual measurement systems have been integral to industrial and manufacturing processes. Initially, traditional platform systems used silhouettes as reference points for comparisons. With advancements in technology, digital video systems have been developed, offering more accurate and precise measurement data.
Over nearly a century of use, optical comparators have evolved significantly. They can measure small parts along the X, Y, and Z axes using magnification. However, each measurement is taken manually, which can be labor-intensive for operators. Despite lacking computer software and being less advanced, optical comparators are user-friendly and more cost-effective compared to newer technologies.
Digital video comparators utilize computer technology to inspect, measure, and evaluate components, assemblies, and parts individually, in batches, or on a conveyor. Their accuracy, speed, and efficiency make them well-suited for fast-paced modern manufacturing environments. Equipped with zoom optics and precision lighting, these comparators deliver highly accurate measurements and data.
Both optical comparators and machine vision systems are used to compare, inspect, measure, and identify objects, whether stationary or in motion. Optical comparators focus on visually measuring small two-dimensional parts along the X, Y, and Z axes using magnification, without relying on computer software for the inspection process.
Machine vision systems are automated inspection machines that leverage PC software to conduct inspections swiftly and accurately, with precision up to 0.0002 inch (0.00635 mm). They perform three-dimensional analysis by visualizing and measuring the features of an object.
Optical comparators are limited to 2D and 2½D measurements and do not integrate with computer software or CAD systems. They offer lower optical resolution, reduced throughput, and have limited lighting options, including 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 continually advancing to enhance accuracy in data and measurements. Unlike traditional comparators, which are limited to 2D imaging, contemporary 21st-century models can analyze 3D images with detailed complexity and various attachments. Modern comparators demand higher precision and accuracy than what traditional 2D optical comparators can offer.
Despite significant advancements in comparator technology, traditional models remain popular for measuring less complex parts and components. All comparators, including digital video types, come in either vertical or horizontal orientations.
Digital video comparators represent the most advanced and efficient type of comparators, utilizing CAD images for their evaluations. They operate quickly, can assess parts on a production line, deliver immediate and accurate data, and are computer-controlled, offering versatile use across various production environments.
Digital video comparators offer the thrilling advantage of magnifying captured images to the finest details. These images are transmitted to a computer monitor, where they can be expanded and examined with various software tools. This capability allows for detailed analysis, precise measurements, and thorough inspections, ensuring accurate data on tolerances.
Digital video comparators excel in speed and clarity, capable of evaluating a wide range of components. They display CAD images for real-time adjustments, allowing immediate comparison when design changes occur. Additionally, these comparators automatically calibrate visual images to the required magnification levels.
Video edge detection (VED) facilitates seamless integration between CAD files and video images. This technology enhances accuracy, boosts productivity, and accelerates the comparison process.
Digital video comparators enable operators to assess multiple parts simultaneously or in groups, making them a more efficient solution for modern manufacturing environments where inspecting numerous parts is essential.
GD&T (Geometric Dimensioning and Tolerancing) is a system used to communicate engineering tolerances and relationships through a set of symbols on engineering drawings and computer models. It describes the geometry of a part and its allowable variations, helping workers understand the required accuracy and precision while enhancing metrology practices.
The GD&T (Geometric Dimensioning and Tolerancing) system enables developers and engineers to enhance part functionality without raising costs. It provides a geometric representation of an object as a reference, focusing on the part's geometry rather than its linear dimensions. By implementing GD&T, rejection rates, assembly failures, and quality control expenses are significantly reduced.
Size refers to the physical dimensions of a part, controlled by ± tolerancing. Location describes the position of a part in relation to other parts, with positioning being the primary aspect of this control.
Orientation refers to how a part is angled in space relative to other parts and is a refinement of location. It is controlled by parallelism, perpendicularity, and angularity. Form encompasses the overall shape of a part, including its straightness, flatness, circularity, and cylindricity.
The main axis of the optical comparator is parallel to the plane of the projection screen. As a result, screens are often produced in medium and large sizes, which are ideal for inspecting heavy workpieces with large profiles or shaft parts. However, smaller machines with silhouette lighting may benefit from a horizontal table below the screen without a light transmission hole. In horizontal models, light travels horizontally, allowing the operator to view a silhouette of the part from the side. This model excels in applications where components are held in a fixed position, such as castings secured in a vice or screws 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 use a mylar overlay as a reference for manually comparing a single part. The operator aligns the overlay with the part and checks for inconsistencies. If discrepancies are found, the operator assesses whether the component remains usable.
Simple to use but labor-intensive, traditional comparators are less precise and accurate compared to modern systems. They measure one part at a time and cannot keep up with the demands of contemporary production. While additional lenses are available, these comparators typically come with only one standard magnification.
Constant handling of Mylar overlays can damage them, and storing the overlays requires significant space due to the number needed for each product. Additionally, producing Mylar overlays is costly, and expenses quickly escalate with product adjustments and changes.
The diagram illustrates the following components: A represents the screen where the image is projected, B is the lens used for projection, C denotes the adjustable stage, and D shows the controls for moving the stage along the X and Y axes.
A mechanical optical comparator enhances the slight movement of a plunger through both mechanical and optical systems. This device assesses the geometric specifications of a workpiece against a reference model. When light is directed at the mirror, it reflects at the same angle as the incident ray. The reflected light is then projected onto a calibrated scale, converting the mirror's angular movement into linear measurements. A plunger attached to the mirror facilitates its tilting action.
Initially, a datum and a permissible range are set on the scale before positioning the plunger over a reference specimen. Once the reference specimen is removed, the plunger is then placed in contact with the surface of the workpiece for evaluation. As the plunger traverses the irregular surface, it moves vertically, and this movement is significantly amplified by a pivoting lever. This lever causes the mirror to tilt. The mirror’s rotation around its pivot further enhances the mechanical amplification provided by the plunger. Light from the source is directed onto the mirror after passing through condensing and projector lenses. The tilted mirror then reflects the light rays onto the graduated scale's inner surface, which is visible through the eyepiece.
Mechanical optical comparators are known for their high precision due to their minimal number of moving parts. They eliminate parallax errors and, being lighter than other comparators with more components, are easier to handle. Their ability to achieve significant magnification makes them ideal for precise measurements. However, they do have some drawbacks, such as the need for an external power source and their unsuitability for extended use due to the need to view the scale through an eyepiece. Additionally, they are best used in darkroom environments.
An electrical optical comparator utilizes both electrical and optical elements in its design and function. Key components include the light source, detector, electronic amplifier, and optical lenses.
The light emitter in an electrical optical comparator generates a steady beam of light for magnification purposes. The receiver captures this light beam and converts it into an electrical signal. These electrical signals are then amplified by an electronic amplifier to enhance their strength.
In this system, electrical signals are processed to produce measurement data. The comparator supports various comparison methods, such as light intensity analysis, shadow projection, laser scanning gauges, and laser diffraction. Electrical optical comparators are commonly employed for component inspection without the need for retooling.
Optical comparators are utilized across various industries for a wide range of applications. Here is a list of common uses and applications for optical comparators:
While optical comparators are versatile tools for various measurements, they do come with some limitations that users might face.
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