Vision systems the clear choice when micro parts can’t be touched

Part measurement methods include physically touching the part with a probe or a micrometer, or using a noncontact approach, such as an optical comparator or vision system. With macroscale parts, touching a part is often acceptable since the part is not affected by the measurement. But with microparts, touch is sometimes not an option, since the measurement process moves or otherwise affects the part.

Sometimes, even the lightest touch is too much for certain parts. For example, contact forces can be as little as 0.5 mN with the Zeiss F-25 Microsystem CMM, which employs both contact and optical measurement technologies, according to Mike Roterdam, technical sales engineer at Carl Zeiss Industrial Metrology LLC, Maple Grove, Minn. “That is much lighter than you could humanly touch a part with an instrument, such as tweezers,” he said. “But even with such a light touch, some parts will move, and when a part moves you cannot measure it accurately, especially when you are chasing nanometers. Those kinds of parts are strong candidates for a noncontact system.”

Still, Roterdam stated that if the part can be touched without affecting it, contact measurement is usually preferable. Vision systems gather data based on contrast between points in an X-Y (2-D) plane, he said, and pointed out that no part is truly two-dimensional. Characteristics of a part, symbolized by geometric dimensioning and tolerancing found on a blueprint, are 3-D in nature.

Part feature size also influences the choice of measurement methods. Holes may be too small to access with a tactile probe, and the only resort is to measure them in a noncontact manner with a camera, according to Roterdam. Also, in most cases, optical approaches are faster than tactile ones.

This is especially true with X- and Y-type (planar) measurements that involve detecting edges. “Those you can pick up very well with optics,” he said. “The camera sensor can look at a pattern of holes and quickly take snapshots of each one, collecting thousands of points per feature.”

One exception to this rule is that, when collecting information in the Z-axis, elements of an optical system may have to travel through a long focus range searching for the best contrast, and sometimes the time required to do this becomes excessive.

“That’s when you need to get into a touch system that can pick up that third axis,” Roterdam said. He added that when using a camera, the accuracy of the measurement can be sensitive to surface properties, surface finish and the numerous settings involving light.

Faster alternative

In other cases, however, optical systems are the faster alternative, agreed Jeff Bibee, vice president of sales and marketing for Werth Inc., Old Saybrook, Conn. Most of the time consumed within a measuring program stems from machine movement, he said. “With touch probes, you have to go to a position, approach, touch the part, retract and then move over.” With optical measurement, the machine moves the camera to the part—or moves the part to the camera—and records an image.

“That all happens pretty quickly, but it is still time-consuming,” Bibee said.

A depiction of the on-the-fly option for CMMs from Werth. The feature employs an instantaneous flash of light to illuminate a part and eliminate motion blur, reducing the time required to make a measurement by eliminating the need for the machine to stop to record each image. Photo courtesy Werth.

To speed the process, Werth offers a software-based “on the fly” option for its multisensor CMMs. The machine never stops moving; as it passes over the part feature to be measured, a strobe-like light illuminates the part. “It dramatically reduces measuring time,” Bibee said. “The instantaneous flash eliminates motion blur and you get a sharp, crisp image.”

Another device that enhances image quality is the HiCam light multiplier option for Werth CMMs. According to the company, the system enables accurate measurement in situations that typically pose difficulties for an optical system, including cases where contrast is low, such as when measuring white features on a white part.

According to Bibee, the HiCam camera records images as quickly as 50 frames per second while software superimposes the pictures on each other, adding the gray-scale values together and magnifying the contrast of the part. “That image pops right out in very sharp contrast,” Bibee said.

Tailored systems

Maximizing image quality requires tailoring the optical system to the measurement application, according to Terry Herbeck, senior vice president of technology, Quality Vision International Inc., Rochester, N.Y. “For precision measurement, the optical system’s resolution and the camera resolution must be well matched,” he said.

This sensor array found on Carl Zeiss’ F25 Microsystem CMM combines both optical and tactile measurement modes. Photo courtesy Carl Zeiss.

For applications where part feature sizes are large (measured in millimeters) and moderate magnification levels are required, the convenience and flexibility of a zoom optical system can be combined with a VGA-format camera with nominal pixel size of 6µm to 8µm. (A Video Graphics Array is a low-resolution graphics display.) The zoom optical system provides detail sufficient to measure such relatively large-scale parts, but not so much as to require the image quality produced by a high-resolution camera.

However, measuring truly microscale parts, such as MEMs devices, inkjet-print heads and disk-drive recording heads with feature sizes in the 1µm to 50µm range, may require a microscope-style, fixed-lens optical system with 20× to 100× magnification and high-numerical-aperture objective lenses. (Numerical aperture characterizes the range of angles over which a system can accept or emit light. A higher-NA lens can visualize finer detail.)

“This arrangement will present a sharply focused, high-resolution image, and will benefit from a high-resolution, megapixel format camera with a pixel size of around 2µm,” Herbeck said.

Prismatic part solution

Readily available microscope optics provide both high magnification and high NA, but they typically have very limited working clearance and a small field of view, and therefore are not convenient for industrial measurement. To provide high-resolution imaging for prismatic parts, QVI recently developed a lens system for ultrahigh-accuracy measurement that combines a large field of view and, compared to other systems, a long working distance.

Developed to provide high-resolution imaging for prismatic parts, this lens system from QVI consists of 12 discrete lenses, providing a nominal 1× magnification, 70mm working clearance and a field-of-view size of roughly 8mm. Photo courtesy QVI.

The new system consists of 12 discrete lenses, providing a nominal 1× magnification, 70mm working clearance and a field-of-view size of roughly 8mm.

“Combined with a 5-megapixel metrology camera and high-intensity LED lighting, the system provides speed and accuracy in addition to a relatively large viewing area, allowing many small features to be imaged and measured in a single video snapshot,” said Herbeck. µ

About the author: Bill Kennedy is a contributing editor for MICROmanufacturing. Telephone: (724) 537-6182. E-mail: billk@jwr.com.