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Features

Contact Sensor Systems. For some dimensional inspection applications, the best way to obtain measurements is by using a sensor that comes into contact with the object. Though the probing forces can be extremely light, contact sensors work best when the object is rigid and not pliable or fragile. They are also often used when the surface of the object does not lend itself to optical sensors, such as structured light scanners, because it is reflective or too dark. The main categories of dimensional inspection contact sensor systems include the following:

  • Coordinate measuring machines (CMMs) are mechanical systems that use a contact measuring probe and transducer technology to convert physical measurements of a surface into electrical signals that can be processed and analyzed by metrology software. CMMs range from basic XYZ readouts utilizing hard probes to fully automated systems with articulating continuous contact probing that can perform CAD model-based inspections.

  • Articulating arms are another type of CMM that use rotary encoders on multiple axes of rotation instead of linear scales to determine the position in space of the hard probe or touch probe (laser line probes are also a common accessory). Such systems are manual in nature but are portable and able to reach around or into geometry in a way that cannot be accomplished with a conventional CMM.

  • Form and contour tracers are purpose-specific devices that use high-accuracy continuous contact sensors with varied styli to obtain small part geometry such as fillet radii and chamfers or that measure roundness, cylinidricity, and other geometric tolerances.

  • Optical CMMs are a lesser known but increasingly common hybrid of non-contact and contact technology used for gathering measurement data for areas that are difficult to reach. The handheld device transmits data wirelessly and allows the operator to move both the part and the scanner during the measuring process. Using stereoptics to scan an object, the optical CMM uses 2-3 cameras to track either passive retroreflective or active targets through space. This process allows objects to be rebuilt in 3D via the device's reflectors.

Noncontact Optical Sensor Systems. In many cases, the object being measured cannot or should not be touched by a contact probe or any other part of the measurement device. In cases where the object is either soft, elastic, very small, or fragile, optical inspection equipment may be most appropriate.

Optical Comparators. Optical comparators use light projected upon a screen to obtain a magnified silhouette or shadow of the object of interest. Measurements can be made by articulating the table axes to obtain width and height. Physical overlays are commonly used for complex shapes and quick-checks. Modern systems can employ fiber optic edge detection, automated movement and measurement, and even digital overlays.

Vision Systems. Vision systems are similar to optical comparators, but instead effectively project images directly onto a screen while a camera with interchangeable objective lenses and/or zoom optics relays images to the display. Most systems have edge detection and other sophisticated capabilities. Automation via a computer and controller is common, and some systems even come with multiple sensors including laser and touch probes.

3D Scanners. 3D scanners use lasers or structured light to capture 3D information about the given object's geometry. In addition to measuring objects and converting them to digital images, 3D scanners can also be used for reverse engineering as well as for other applications.

  • 3D laser scanners — A laser, either as a single point, line, or an entire field of view, is projected onto the surface of an object and a camera captures the reflection. Each surface point is triangulated, measured, and recorded to produce a 3D rendering of the shape and surface measurements of the object.

  • Structured light scanners — Sometimes also called white light or blue light scanners, these devices use light from halogen or LED lights to project a pattern of pixels onto the object. The pixels are distorted by the surface of the object when viewed by one or more cameras that are placed at an angle relative to the light projector, and measurements of the light pattern can be used to reconstruct a 3D image.

  • Range scanners — Range scanners use a time-of-flight laser rangefinder based on LiDAR technology to measure the distance between the laser and the object's surface. The laser rangefinder sends a pulse of light to the object and measures the amount of time it takes for the reflection to return in order to calculate the distance of each point on the surface. Point measurements are taken by aiming the device at the object and using a series of mirrors to redirect the light from the laser to different areas on the object. Although the process may seem cumbersome, typical time-of-flight 3D laser scanners can collect between 10,000 and 100,000 points per second, which is much faster (though less accurate) than contact sensors.

  • 3D noncontact surface profilometers (a.k.a. nanoscanners) — With the advent of nanotechnologies and growing demand for micromanufacturing, the need for the ability to accurately measure very small objects and geometry has increased the need for microanalysis. This technology utilizes ranges from confocal laser microscopy to white light interferometric optical profilometry. Typically, surface finish has required the highest precision in dimensional metrology. In addition to surface finish in 2D and 3D, now very small geometry can be dimensionally characterized and inspected. Relevant characteristics include flatness, wear, texture, sharpness, and other conditions that can affect functionality, but might otherwise not show up in a dataset of conventional metrology measurements.

Summary

Dimensional inspection is useful for much more than just production line setup and quality control. Manufacturing (and much more) can greatly benefit from dimensional measurement in all phases of product development ranging from research and prototypes, to first article inspections and capability studies, production inspection, to final inspection of the finished product. The manufacturing process is full of potential pitfalls, especially when you are trying to develop a new product on a tight timeline.

Understanding what it takes to integrate dimensional inspection into your quality process, whether in-house or outsourcing to an accredited measurement lab, will provide value and help your manufacturing process operate more efficiently. An objective that can be measured and that gets properly measured is an objective with the best chance of being brought to fruition.

This article was written by Mike Knicker, President of Q-PLUS Labs, Irvine, CA. For more information, Click Here.

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