Zeiss System: Speed Meets Certainty
Scanning has been a capability that Carl Zeiss IMT Corp. (zeiss.com; Maple Grove, MN) has long offered on some of its coordinate measuring machines (CMMs). In fact, scanning was first made available in 1974. Kevin Legacy, manager, Engineering, says the rationale for scanning versus point-to-point measurements is quite simple: "The more data you have, the more certainty you have."* Because the probe, or stylus, on a scanning CMM is in constant contact with the surface of the object being measured, there are more points obtained. Which translates into a higher level of confidence in the results of what's been measured.
To be sure, in the last 30 years Zeiss personnel have been working on developments in this area. Legacy points out that unlike other CMM manufacturers, Zeiss makes all of the major elements of the CMM: the scanning probe head, scanning controller, and software. So these developments have been integrated. The first generation of scanning lasted until 1995, when the company brought out its Prismo VAST CMM. Because of materials technology developments and a smaller/faster VAST scanning head, the machine permitted scanning to be performed on the shop floor. But now there's the third generation, which allows things to go much faster—faster in terms of moving along the surface of the part and faster with regard to getting the system setup and ready to scan.
Called the VAST Navigator, the third generation system greatly increases the velocity of scanning without giving up anything in the way of accuracy. That is, Legacy says in a head-to-head comparison of a CMM with and without the VAST Navigator, the same feature could be measured with a one-micron uncertainty at a rate of 40 to 50 mm/sec. with the new system, and just 20 mm/sec. without it. The issue isn't just moving the stylus around fast. It's moving it while maintaining gage-level accuracy.
What's more, there is the issue of getting to the point of doing the measuring. "With VAST Navigator," he says, "you don't have to premeasure the part." Which saves time.
Essentially, Zeiss engineers have developed a dynamic bending model of the entire system—the machine and the probe head—for its scanning CMMs. Not only does the probe bend when it is being moved across a workpiece during scanning—and Legacy points out that when measuring things like the inside of automatic transmission cases the probes can be 600- to 800-mm long, and consequently liable to bend—but there is another aspect that needs to be taken into account: "You have to understand the dynamic bending in the machine itself. There is a certain amount of movement in the structure as it keeps the probe on the surface of the part. So what we've done with our scanning machines is determine their dynamic bending characteristics when scanning." While performing finite element analysis (FEA) of CMM structures has become pretty much the status quo for CMM builders, the dynamic mapping is, Legacy says, a differentiator. "People always make an error compensation map," he notes. "But that's static. You know where the machine is at rest. With Navigator, it's a dynamic map: determining what the machine is doing in real life and correcting for errors on the fly."
One of the consequences of that information about the system behavior is that programming is greatly simplified. Explains Legacy, "A good programmer knows how fast he can push a machine for best throughput without affecting the accuracy." Still, this is often a case of trial-and-error: running a program and seeing how well it does the job; making modifications as required. But because of the modeling of the system, and because they've put the necessary algorithms in the control software (which Legacy says are transparent to the user), what happens with a VAST Navigator-equipped machine is that based on the probe configuration and the features to be measured, the system will automatically recommend the fastest scanning velocity. "What would have taken 15 minutes or more of trial-and-error takes no time," Legacy says.
He points out, "We have found so far in testing that people can expect a minimum 30% throughput improvement." That's improvement in both the programming and scanning time.
The VAST Navigator is available on new Zeiss scanning CMMs (e.g., PRISMO, CenterMax). According to Legacy, it is also available as a retrofit package for existing Zeiss scanning machines, which consists of a new VAST Gold probe head, a module for the Calypso or UMESS programming software, and a new controller firmware. For older machines, a new controller may also be required.
*Which leads to a question about why not simply use a noncontact scanning system—such as a laser-based system—instead of a hard probe. The simple answer to that is: the challenge presented by the part. Legacy notes that when measuring, say, engine blocks, the features are difficult to reach (let alone see) and some tolerances are in the 10- to 20-micron range. Because of these issues, noncontact measurement is just not the right choice: "It's not because the measuring system isn't capable," he says, "but because the parts present problems." Zeiss does offer noncontact systems, which, he says, are typically used for less-demanding measuring tasks.
When you think "vision systems" you might think something along the lines of "twitchy" or "delicate." Cognex Corp. (www.cognex.com; Natick, MA) has launched two new vision sensors in its In-Sight lineup, the 5100 and the 5400, that are built to take it: they meet IEC specs for shock and vibration and, when used with their lens covers, achieve an IP67 (NEMA 6) rating for dust and wash-down protection. The units feature a die-cast aluminum housing and sealed industrial M12 connectors. According to Jim Hoffmaster, chief operating officer for Cognex, "These are true industrial-grade vision sensors that can be deployed virtually anywhere on the factory floor."
When One Is Not Enough
Single setups are a good thing for part production. Which explains one of the advantages of the SmartScope Quest 650 system from Optical Gaging Products (www.ogpnet.com; Rochester, NY), which is a veritable sensor array, as it supports video, laser, touch probe, and microprobe devices for part measurement. With a measurement volume of 600 x 660 x 300 mm, the system can handle comparatively large workpieces. The bridge-style machine has a granite base, a Meehanite bridge, and high-speed, liquid-cooled linear-motor-driven stages. It provides 50-nanometer scale resolution. One notable feature is the patented TeleStar zoom lens, which is said to provide the accuracy associated with fixed-lens systems, but with a wide magnification range.
Enhancing the Legend
The vision systems from DVT Corp. (www.cognex.com; Duluth, GA), including the Legend family, have always focused on ease-of-use, functionality, and economy. In that regard, the company has brought out three ancillary products because, as company chairman and CEO Bob Steinke explains, "As more manufacturers adopt machine vision, they want a vision system that is easy to integrate and constitutes a total vision solution from one vendor." The three in question are:
CON-IOE. This is an Ethernet I/O assembly that provides eight outputs and eight inputs. It is packaged as an I/O rail assembly with removable wiring harnesses, which facilitates replacing modules (i.e., no rewiring is required). List price: $650 ACC-24l. This industrial power supply provides two amps of continuous current. Its power boost feature permits the unit to handle up to 4 amps for brief periods. List price: $125 CON-IBOB. An Intelligent Breakout Board. Facilitates optical isolation of I/O. It also provides status LEDs for each of the I/O lines. List price: $300.
Industrial-strength Digital Camera
Photogrammetry is a technique that is pretty much what it sounds like: a photograph-based 3D coordinate measuring technique. Geodetic Systems (www.geodetic.com; Melbourne, FL) has launched its third-generation digital photogrammetric camera, the INCA3, which employs a high-resolution CCD sensor and a compact industrial PC. The camera is said to be useful for in-place measurement and inspection of large objects. Geodetic Systems president John Brown claims that the unit is "the most precise photogrammetric camera ever made with accuracy of better than 0.001 in. at 160 in. The camera is fitted with an integrated strobe and provides WiFi connectivity.
Looking for a means to measure body assemblies or components on the shop floor? Objects that will fit within working envelopes from 3,300 x 2,000 x 1,500 mm to 4,000 x 2,000 x 1,800 mm? The answer could be one of the four models within the DEA GLOBAL eXtra coordinate measuring machine lineup from Brown & Sharpe (brownandsharpe.com; North Kingstown, RI). The systems feature an isostatic steel worktable, which means (1) it can be readily integrated with part transfer systems and (2) it doesn't need a dedicated foundation. The bridge-type machine features a highly rigid silicon carbide vertical ram; the material is beneficial from the standpoints of both rigidity and thermal behavior. Speaking of thermal behavior: an array of sensors is used throughout the machine structure and proprietary software is used to make real-time 3D volumetric performance compensation (it's called the "ACTIV Structural Thermal Compensation System"). Variable High Speed Scanning firmware facilitates scanning by controlling speed in real time based on the curvature of the path. For vehicle body features that are difficult to reach, the CMM can be equipped with a continuous wrist (DEA CW43L) that permits the use of extensions up to 570 mm.
Although the words "Renishaw" and "probes" may be thought by some people to be completely synonymous, there is more than that to the metrology offerings from Renishaw Inc. (www.renishaw.com; Hoffman Estates, IL). Case in point is a new patented technique, Renscan DC (short for "Dynamic Compensation"), a feature-based method that compensates for dynamic measurement errors introduced when scanning at high speeds with a coordinate measuring machine (CMM). It's worth noting that the technique even permits scanning with CMMs that would otherwise be incapable for performing the task. Renscan DC, which works within the company's UCC—or "Universal CMM Controller"—develops the compensation strategy by measuring features on a component at two speeds: first slow, then fast, right after the slow scan has been performed. While the dynamic errors during the slow scan are minimal, they are much greater during the dynamic scan. Consequently, the controller computes a dynamic compensation "map" for each of the features. Then all other parts with those features can be measured at high speed. It should be mentioned that this scanning is performed with probes—Renishaw probes. Such as the SP80 passive scanning probe. It can handle styli up to 500-mm long and 500-g mass.