This article will take an in-depth look at optical comparators.
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Optical comparators are high-precision measurement instruments that deliver accurate and reliable data. These tools employ video imaging, geometric dimensioning and tolerancing (GD&T), in addition to light, lenses, mirrors, and cameras to analyze the tolerances of individual parts, assemblies, and components. For quality control teams, optical comparators are indispensable in evaluating production processes and ensuring the quality of manufactured goods.
When analyzing a workpiece, the optical comparator projects its image onto a screen, where it is compared to established standards. This technique permits the examination of intricate components, including stampings, gears, cams, and threads, against their models. Optical comparators play a vital role in producing precision machinery for various industries such as aviation, aerospace, horology, electronics, instrumentation, as well as a myriad of research and metering applications.
The comparator promptly detects flaws such as defects, scratches, indentations, and dimensional errors by matching the workpiece to its standard specifications. Its precise calibration ensures accurate identification of any deviations.
Since its introduction in the 1920s, the basic design of comparators has remained mostly unchanged. Nevertheless, technological advancements, improved calibration methods, digital technology, and enhanced magnification have significantly increased their accuracy. Historically, the workpiece is placed on a stage that is illuminated from below, casting its shadow onto a screen with the help of lenses and mirrors.
A telecentric optical system enlarges the projected image to ensure exact magnification of the workpiece. This lens setup maintains precise scaling without image distortion. The dimensional precision and surface details of the projected image are then measured against the workpiece's standard parameters.
Optical comparators are generally available in two primary styles: horizontal and vertical. Horizontal comparators deliver a side perspective of the workpiece, whereas vertical ones offer an overhead view. Over the years, manufacturers have trusted both horizontal and vertical optical comparators to evaluate the quality of products and components.
Though traditional optical comparators have been utilized effectively for decades, they face limitations in dealing with the sophisticated demands of modern manufacturing processes.
There are four primary types of optical systems utilized by optical comparators: simple optics, corrected optics, fully corrected optics, and telecentric optical systems.
Using a telecentric lens aligns the workpiece for inspection with a grid overlay on the screen, facilitating precise distance measurements between projection points. Additionally, the profile projector could utilize episcopic lighting for illuminating internal sections from above.
Manual methods are rapidly being replaced by advanced computer software. There are three principal modern approaches:
Conventional optical comparators still primarily depend on the silhouette method, inspecting one piece at a time, retaining popularity despite digital advancements. Video comparators expedite operations, furnish precise data, and examine large part volumes efficiently.
Traditional comparators experience variable image sizes due to optics and screen characteristics, impacting measurement comprehension. Comparators typically feature screens starting at 12 inches. To handle large images without distortion, larger screens necessitate roomier enclosures to accommodate this.
Digital video comparators project images onto a computer display for easy manipulation and evaluation. Automated digital comparators utilize cameras to capture part images as they pass along the production line, comparing them electronically to CAD displays or references.
For basic measurements like positions and lengths, an XY digital readout is enough. For tasks involving circles, angles, or parametric distances, opt for a more capable readout with geometric capabilities. For repetitive measurement tasks, CNC-capable readouts are beneficial. Optical edge detection is crucial for reducing operator subjectivity and enhancing measurement accuracy and repeatability.
Securely clamping the workpiece on the comparator table is essential for consistent measurements in traditional comparators. Consider available fixtures tailored to your application needs. Modern fixtures, like rotary fixtures, lens turrets, digital protractors, helix stages, and LED lighting, enhance inspection process versatility and accuracy.
Traditional comparators offer screen sizes from 12" to 32". Before deciding, assess the workpiece dimensions. Viewing the entire component may not be necessary. The visible area is determined by dividing the screen diameter by lens magnification. For example, a 16" comparator with a 10X lens shows 1.6" of the component (16"/10 = 1.6"). Engineers advise keeping within an inch of the screen's edge with overlays; ensure stage dimensions and weight capacity suit the components for measurement or inspection.
An attentive operator can discern 0.004" on a comparator screen with reliability. The lens magnification resolution is crucial when choosing a suitable lens. Although the projection lens magnification stays constant, it can be altered to view different component areas. A standard projector usually includes a single lens, though additional lenses may be introduced to meet tight measurement tolerances.
Understand your application's optimal light path. For example, in systems with horizontal light paths, a beam crosses a stage, suitable for measuring threads, castings, shafts, and machined parts. Systems with vertical light paths direct a beam upward, placing exam objects on a glass plate. The beam passes through the system's XY stage glass plate. Flat parts such as stamped components, gaskets, electronics, and O-rings suit vertical systems best.
Digital video comparators merge optical comparators, digital microscopes, and non-contact systems with automatic edge detection. They possess high-resolution capabilities, varied frame rates, user-friendly interfaces, and broad spectral sensitivities. Designed for detail capture, they offer magnification and image manipulation with superior clarity.
When considering digital video comparators, evaluate parts to be measured and presentation methods. Digital cameras measure parts on a conveyor or individually, like traditional optical comparators. Consider part size, number, and precision requirements, given the high accuracy of digital comparators.
A trustworthy manufacturer should offer global online technical support, assisting customers with issues via phone or remote system access for diagnosis and resolution. Opt for manufacturers committed to promptly addressing problems, minimizing 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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