The chief obstacle to faster inspection on coordinate measuring machines is
dynamic error. At different speeds and accelerations, the machine's inertia
gives the CMM different levels of error in different regions of the work zone.
When the machine is scanning, this dynamic error affects the measurement
results. The error is so complex and unpredictable that the typical strategy is
simply to run the CMM at such a slow speed that inertial effects don't matter.
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| Here, the same circle is measured once
at a slow speed and once at a fast speed. The differences between the two
measurements show the CMM controller where the CMM's dynamic error lies. |
But it may be possible to scan faster than this, even on an existing machine.
The dynamic error, though unpredictable at first, remains consistent from one
workpiece to the next.
Renishaw (maker of CMM probes and CMM controllers) has taken advantage of
this fact to develop an algorithm for improved measurement efficiency.
The company's "Renscan DC" adjusts for dynamic error by inspecting the same
workpiece twice—once at standard speed and once at a faster speed. The
differences between the two measurement results show the CMM controller
precisely where the dynamic error lies. As a result, the next piece—and all
pieces thereafter—can be inspected accurately using only the faster speed.
There are caveats. The algorithm only applies to applications in which the
same part number is inspected multiple times. The map of dynamic error for one
part does not apply to another part, nor does it apply if the same part is set
up in a different location on the CMM.
But in those cases where multiple pieces of the same part number are all
inspected the same way, significant time savings become possible. In a test case
involving a V8 engine block, the faster inspection routine using dynamic error
compensation took just 17 percent of the time that the standard inspection cycle
had required.
The algorithm is available on Renishaw's "UCC" line of universal CMM
controllers. The company points to Renscan DC as a principal reason why it
developed its own CMM control in the first place. Adapting an algorithm such as
this for the various proprietary CMM controllers would have made the
functionality more expensive.
Article courtesy of Gardner Publications,
Inc..
