In fact, assigning numerical values to accuracy is described by no less an authority than the British National Measurement Laboratory as strictly a marketing activity! The numerical values questioners are really seeking are called uncertainties.
To more clearly visualize these ideas, recall the familiar "bull's-eye" picture frequently used to illustrate the concepts of repeatability and bias. This drawing assumes that we know the actual true value. But since that is physically impossible, the more correct picture today recognizes a zone centered on our actual measurement results. The size of this zone is our uncertainty. We use our best knowledge of the measurement process and related conditions to estimate this uncertainty value. We assert with confidence that the true value is within this zone, even though we do not know exactly where.
However, we know from long experience that some instruments are consistently better than other instruments-or, that in most cases, one instrument will be better for certain measurements while a different instrument will be better for other measurements. Being "better" simply means having a smaller uncertainty for a particular measurement. Of course, other qualities such as speed and cost influence our assessment of "better" in addition to accuracy. Precisely because the actual uncertainty is associated with a particular measurement, one instrument may have a lower uncertainty for the measurement of quantity, diameter, while another instrument may have a lower uncertainty for measurement of another, roundness, when measuring the same object.
Because uncertainty really applies to a specific measurement result, manufacturers cannot just assign-or advertise-an uncertainty value for an instrument. What we can do is report uncertainties for particular measurements under known conditions, limiting the influence of external error sources such as the environment. This gives values for particular measurement tasks, or the "intrinsic uncertainty" of an instrument, a concept derived from the "intrinsic error of an instrument" defined in VIM. For commercial purposes, manufacturers typically use knowledge of their instruments' intrinsic uncertainties and publish a slightly larger value as the "maximum permissible error" (MPE) for acceptance testing of the instrument. Just to confuse matters more, these published values are usually called "uncertainties."
Currently the manufacturers of multisensor machines and plain optical CMMs without multisensor capability, have taken a variety of approaches to presenting the "accuracy" of their machines. These are mostly variations of ISO 10360-2, but they present some real problems for optical systems. Multisensor machines that include contact probes as well as optical probes can run the real standard tests using only their contact probes, which gives an excellent baseline number for the machine. Machines that do not include contact probes often run a private variation of ISO 10360-2, usually performing only the length measurements and ignoring the required probe error testing. Even length testing is problematic for optical-only machines, since typical optical length standards do not work very well in three dimensions.
Even worse is measuring the optical standard only in a single horizontal plane, which misses numerous errors because of the geometric imperfections of the horizontal motion axes. An especially pernicious version of this is the so-called "map and check" trick of a few unscrupulous machine calibrators. Many machines are error mapped to improve their accuracy. However, an incomplete or incorrect mapping procedure usually has the effect of making the machine more accurate in areas where the mapping data was collected and much less accurate elsewhere in the machine's working volume. The trick is to map in one area, such as at the optical table surface, and then only perform an acceptance test in the same area. The machine appears exceedingly accurate, but in fact at higher planes or along inclined directions, the errors can actually be greater than if no mapping had been performed.
Another common difficulty in using the 10360-2 tests arises from optical averaging. The 10360-2 tests were intentionally designed to be sensitive to the single point repeatability of contacting probes, as this characteristic is often a limiting factor in the uncertainty of a CMM measurement. Fortunately or unfortunately, video-based optical sensors capture an entire image and use algorithms to extract a large number of edge or surface points from a single image. From these numerous raw extracted points, the operator then can choose from among a multiplicity of methods to determine a single point of interest.
Some of these methods simply select one point, which nearly approximates the original single point repeatability of a contacting probe, but may not be appropriate for a system intended to use averaging for its lowest uncertainty work. However, other point determination methods use all of the appropriate points in an entire image to find a "best-fit" point instead of an actual point. This gives a more stable, and usually more accurate result, but depends on the actual number of raw points in the image and on the details of the way the fitting is done. Unfortunately, some manufacturers use one method and some use another when executing their private version of 10360-2. Without knowing the full details of how the single points are determined for computing results in the 10360-2 tests, you have little real knowledge of the system's performance capability.
1. Assess your real measurement needs for the system. Is 3-D measurement important in your application of the machine or will you primarily be using the system for two measurements in horizontal planes? Is the integration of contact probing and optical sensing important for your application?
2. Be sure to get complete specifications for the intended use of the system. If you have a 3-D application, be certain to obtain specifications that show the full 3-D measurement performance of the systems. Verify that these specifications include the errors because of vertical nonorthogonality to the horizontal measuring plane. Ask for probe (sensor) performance specifications, especially for optical probes.
3. Ask for details of the procedures for verifying the specifications. Length dependent uncertainty formulas-2.1 + L/250-generally are only standardized for contact probing. Ask for the full procedural details for assessing optical probing systems to be certain that all major error sources are rigorously evaluated. Pay careful attention to how "points" are determined and be certain that this approach is sensible for your applications. Inquire if software compensation techniques are used and whether they correctly account for all Z-axis errors.
4. Be a skeptic. In the end, understanding manufacturer specifications for multisensor measurements uncertainty depends on understanding precisely what the claimed numbers mean, and how they are relevant to your intended use of the multisensor system. Q
2. If everyone uses a different measurement to evaluate instruments' intrinsic uncertainty, the results cannot be compared.
3. If you have a 3-D application, be certain to obtain specifications that show the full 3-D measurement performance of the systems.