This article will take an in-depth look at optical comparators.
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An optical comparator is a measurement system that offers extremely accurate and repeatable measurement data. Optical measuring tools include optical comparators. This gadget employs the principles of optics by utilizing light, lenses, and mirrors to display a magnified silhouette of a part on a screen. A silhouette is the dark shape and outline of someone or something that may be seen in dim light against a brighter background. Optical comparators are measurement tools frequently used for inspecting manufactured parts. A magnified silhouette of a part is projected onto a screen in a comparator, and its picture dimensions and geometry are compared to predetermined limits. The dimensions of manufactured items are examined, measured, and compared using comparators. It is a valuable tool for the quality control inspection team in a small parts machine workshop or production line.
Optical microscopes' measurement principles are comparable to optical comparators. On the stage, the target is positioned, and it is illuminated. As a result, the target's shadow or profile is projected onto the screen. Accurate measurements are made possible by a telecentric optical system. The initial purpose of optical comparators was to examine target outlines. Later, models with measuring features started to appear. The screen diameter of some large optical comparators is greater than 1 m. The optical comparator is frequently used to compare the measured contour model and complex-shape stampings, gears, cams, and threads. As a result, it is frequently employed in producing precision machinery for the aviation, aerospace, watch and clock industries, electronics, the instrumentation business, research facilities, and detection metering stations at all levels. In addition, optical comparators are used to check for surface flaws like scratches and indentations and dimensional accuracy. In other words, they provide non-contact measurement and observation, reducing handling while enabling close examination.
The notion of optical measurement, which refers to non-contact measuring using various light sources, underlies how optical comparators function. Lighting is typically done via diascopic illumination, which involves backlighting. When the specimen is translucent, and light may pass through it, this sort of lighting is also referred to as transmitted illumination. If the specimen is opaque, the light won't pass through it and instead creates its profile. A projection screen is provided with the shadow of the measuring object that is set up on the stage through the use of a projection lens and optical comparator, which is illuminated from below. It also goes by the names shadowgraph and profile projector because of this.
The silhouette of whatever is on the stage is projected onto a screen by a light that travels through the stage and through several lenses and mirrors. The same idea underlies optical comparators. A light source shines on a part attached to a stage, creating a shadow image of the part. The shadow is reflected off mirrors, enlarged by lenses, and projected onto a screen's back. For measuring purposes, this screen is mounted at a given distance.
The built-in projection screen shows the specimen's profile magnified by the projector. To study or measure a straight edge of the machined item, the X-Y axis of the screen can be aligned with a grid that can rotate 360 degrees on the screen. This magnified projection screen shows the specimen's profile and makes it easier to calculate linear measurements.
Simple optics, corrected optics, fully corrected optics, and telecentric optic systems are the four basic categories of optical systems utilized in optical comparators.
It's possible to align the edge of the specimen to be examined with the grid on the screen. Simple measurements for distances to other points can then be made from there. On a magnified profile of the specimen, this is being done. Measuring using a profile projector's enlarged projection screen might be easier and less prone to error. On the projection screen, the sample can be measured. Additionally, a profile projector might have episcopic lighting (light shining from above). This helps lighting show internal areas that need to be measured or bores.
There are three distinct methods of image measurement for comparators:
Traditional optical comparators use the first two techniques, also the most popular ones on the market. Digital optical comparators, which manage the entire process electronically, use the third.
The size and magnification of the projected picture can differ among optical comparators. The comparator's optics and screen size affect both of these measurements. Although variants with larger screens are available, 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.
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.
The 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 typically 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.
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.
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.
The horizontal and vertical layouts are the two main variations of optical comparators.
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.
With conventional comparators, a part must be lined up with a mylar overlay that acts as a reference for manual comparison. If there are inconsistencies after aligning the overlay with the part, the operator assesses if the component may still be used. Traditional comparators are generally quite simple to use, but the manual procedure is laborious and less precise. Other time-consuming manual labor is needed for conventional comparators. In addition, these comparators only measure one part at a time, which can take a long time. Although one can purchase extra lenses, they normally only come with one magnification.
Constant handling can quickly ruin conventional mylar overlays, which is a drawback. Storage for the overlays demands a sizable amount of room due to the enormous number of overlays required for each piece of gear. This effort also implies that discovering an overlay that has been misfiled may take some time, delaying the inspection procedure. Mylar overlays are expensive to produce, and the expense of replacements can mount up rapidly when product designs change. In the diagram below, A is the projection screen, B the projection lens, C the movable stage, and D shows the stage movement in the X and Y axis.
As technology advances, digital optical comparators are becoming more common and important. Likewise, digital comparators are much more effective because mylar overlays are not required.
A digital optical comparator displays the picture of a part using an internal projector and digital Computer-Aided Design (CAD) charts. The virtual charts automatically adjust to magnification changes without needing real overlay updates. Additionally, digital CAD charts can be instantly added to a part, compared to an overlay, and tracked and moved alongside the part to ensure alignment. When designs change, there is no need to make a new mylar overlay because the digital CAD charts are immediately available. Due to the ability for CAD files to be unrestrictedly loaded, openly available, and kept on a secure network, one can finish automatic parts comparisons much more quickly.
Since they enable operators to assess several parts simultaneously, digital comparators are frequently a more efficient solution for applications requiring routine inspection of numerous parts. The specifications of an industry will determine which optical comparator is ideal for it. There are applications for both categories of comparators.
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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