Optical Technology Improves Accuracy of Visual Measurements

Precision manufacturing relies on accurate and precise dimensional metrology for process monitoring and control. Simply put, it is impossible to sustain a manufacturing process that is both accurate and repeatable without an equally accurate and precise method of verifying that the product is within specifications. The higher the production volume, and the more complex the parts, the greater the need for fast and reliable process metrology to ensure process control.

Years ago, machined, cast and molded 3D parts were measured with contact-probe coordinate measuring machines (CMM). While traditional CMMs provide accurate measurements, current advances in multisensor measuring systems allow manufacturers to combine the speed and accuracy of optical measurement with laser, white light, tactile touch and scanning probes to perform fully automated, high-precision measurement on a wide variety of parts.

A prime example of an advanced multisensor metrology tool is the SmartScope^® Quest from OGP^®.  SmartScope Quest systems are equipped with OGP’s patented TeleStar^® telecentric zoom lens system, along with a range of optional sensors to handle the most demanding 3D measurement applications.

Superior Imaging Optics for Accurate Metrology

Optical measuring systems are used to measure parts with many features, especially small features that are difficult to measure accurately with contact probes, or which are made from materials that could be deformed by probing. Advances in camera and image processing techniques through the years have raised the accuracy of optical measurement to be equal or better than other sensor technologies, while being considerably faster than contact probes.

But while virtually all optical measurement systems are fast, not all offer the same accuracy and precision. High accuracy lens systems have attributes that ensure accurate part imaging in real-world conditions. After all, it is the part image, rather than the part itself, which is measured, so the image must faithfully represent the part.

Complex, three-dimensional parts present a far greater challenge to image accurately than simple 2D parts. Part geometry, material and feature size all play a role in dictating the optical system best suited for measurement. In addition to the Quest, OGP offers six different optical arrangements across its family of products to suit the variety of applications SmartScope systems can handle. These solutions focus on the two most important factors in image accuracy: low distortion and high telecentricity.

“The TeleStar lens is able to produce images with zero distortion, which means it can accurately measure magnified part features quickly.”

Optical distortion causes changes in the geometry or form of the image of a part, which leads to measurement errors. Advanced optical systems such as OGP’s Quest TeleStar Zoom significantly minimizes optical distortions through careful design, manufacturing, centering, coating and assembly. Once the lens system is assembled, each unit is characterized and compensated for any residual distortion before installing the optics on the CMM.

The second key characteristic of high-performance optics is telecentricity, which ensures that the image size remains accurate even when the image is not perfectly focused. A common problem in optical measurement comes from round machined or turned parts that cause light to scatter or wrap around when measured. This “wall effect” causes the edges of the part to be fuzzy or indistinct, leading to measurement errors

OGP’s TeleStar optics are designed to be fully telecentric at all zoom positions, eliminating both size change due to focus variations and optical distortion across the field of view. This enables the system to maintain optical resolution of < 0.1 micron and field of view accuracy of < 1.0 micron.

The TeleStar’s long depth of focus and generous working distance to allow it to effectively measure 3D parts with deeper pockets and more complex features across the entire field of view. Moreover, it can do this with greater accuracy than has previously been possible, according to OGP Technical Sales Manager Tim Calby. “These capabilities let the SmartScope Quest tackle a wide range of applications with high accuracy,” he says.;

Of course, no measuring solution is perfect for all applications. While highly accurate vision systems can improve the speed of measuring, some part features cannot be measured optically, and need probes or laser sensors to make accurate measurements. For these situations, OGP pioneered multisensor systems that can deploy an array of different tools to quickly and accurately make measurements without operator intervention.

Using Multisensors to Improve Speed, Precision

Multisensor systems enable measurement of entire parts on a single measurement system, even when those parts require several different types of tools to measure all features. Multisensor capability not only saves time, it also significantly reduces the measurement uncertainty inherent in moving a given part to each of several different measurement systems.

Just as not all optical systems offer the same performance, not all multisensor systems are equally capable.  True multisensor systems allow the user to choose the fastest and best sensor or combination of sensors for each measurement, without sacrificing accuracy.

Let’s look at some of the different tools that are available on a SmartScope multisensor system:

• Touch Trigger Probe (TP20/TP/200) – When features are to optics, such as the inside or a bore, the touch-trigger probe is the go-to solution. OGP’s unique probe-to-optics offset calibration ensures that all sensors are referenced to the same coordinate system, so measurements can use points from any sensor.
• Scanning Probe (SP25) – Similar to a touch trigger probe, this tool makes continuous contact with a part surface to gather high-density point data for accurate profile measurements.
• Feather Probe^TM – Designed to measure minute features too small or too sensitive for conventional touch trigger probing, this miniature probe uses a stylus as small as 0.125 mm, triggering with forces as low as 1 mg. On a multisensor system, the optical sensor can precisely identify the location of these tiny features, while the Feather Probe measures inside them to ensure the internal feature dimensions are in tolerance
• Rainbow Probe – This compact device projects white light onto a part’s surface, separating and analyzing the reflected light wavelengths to find a Z height with sub-micron accuracy. The Rainbow Probe can measure translucent or specular materials that are not easily measured using triangular laser light.
• DRS^TM Laser – This side-mounted, deployable laser offers very high resolution even on specular surfaces that scatter the laser light resulting in noisy data. The DRS-500 includes the option of a blue laser emitter for measuring troublesome surfaces.
• TeleStar Plus Through-the-Lens (TTL) Laser – The TeleStar interferometric sensor – with built-in reference source – measures a wide range of materials and surface textures with extremely high accuracy. It boasts a working distance of 68 mm while optional objectives – in particular the 0.5× and 0.45× objective – enable an even greater working distance of 130 mm and 200 mm, respectively, with minimal degradation in the laser’s accuracy. Through-the-lens operation means there is no offset between the optical and laser sensors. The small spot-size and very long working distance enable the TeleStar Plus to measure deep inside recessed features that are not accessible by other means. The on-axis, non-triangulation sensor is perfect for scanning surfaces that are adjacent to other features, such as the bottoms of slots or trenches.
• Rotary Devices (4th and 5th Axis) – There is a variety of rotary capabilities from indexers to true 4th and 5th axis rotary options. Symmetric parts and parts with important features on many sides can be fully characterized on a SmartScope system with a rotary indexer. Available with different capacities and resolutions, rotaries allow complete measurement in a single setup. Dual-axis rotaries, comprised of two rotaries mounted together, provide full five-axis capabilities. Several combinations of rotaries are configurable, depending on the application and the machine’s available measurement envelope. Rotary indexers allow automatic part rotation to present features to available sensors at set angles for subsequent measurements.

Consider the machined casting shown here. The multisensor set up with video, TTL laser and star probe enables the measurement of feature size, location, and profile on two different planes of the part in a single fixture set-up, dramatically lowering the need for operator intervention, tooling change-over, measurement uncertainty and cycle time.

SmartScope multisensor systems are highly configurable. The user can choose virtually any combination of sensor tools to accompany the standard optical sensor and calibrate them as a set.

OGP’s SmartScope multisensor measurement systems designed to provide the highest accuracy in 3D measurement. The flexible, highly accurate precision multisensor instruments can handle complex measurement challenges. SmartScope Quest systems are the ultimate in multisensor performance, as the combination of sensor tools offered by the Quest allows complete measurement of complex parts in a single setup to handle the toughest measurement challenges. Learn why SmartScope Quest Systems are the choice of leading manufacturers worldwide.

Learn more about the SmartScope Quest.