An array of measuring options: when quality and speed are key concerns, then measuring equipment characteristics matter more than ever

Automotive Design & Production, May, 2004 by Gary S. Vasilash

Zeiss System: Speed Meets Certainty

Scanning has been a capability that Carl Zeiss IMT Corp. (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.

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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.

Tougher Sensors

When you think "vision systems" you might think something along the lines of "twitchy" or "delicate." Cognex Corp. (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 washdown 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."