Metrology Trends in Multisensor Measurement
The key to maximizing performance lies in understanding the available capabilities and choosing those that will deliver meaningful benefits in your specific applications.
Tactile and optical measurement techniques each have their strengths, and there are measurement tasks that certainly require only one of the two. But in many cases, because the two methods complement one another, the combination can yield better data than either alone. In some specialized cases a third technology, either laser or ‘white light’, can be added. And while separate machines are certainly an option, a multifunction machine eliminates the need to move test pieces, speeding up the process and reducing labor cost. A multifunction machine can cost less than multiple single-purpose machines and reduces ‘real estate’ costs. And, depending on the recurring demands of the operation, multisensor and single-function machines can be deployed side-by-side allowing you to choose the most efficient and cost-effective functionality for each task.
Optical measurement can collect a large amount of data very quickly and can address parts or features that are too small or too fragile for practical tactile measurement. But like a photograph, visual measurement flattens what it sees into two dimensions, so while it is very good at measuring two axes, typically X and Y, to identify outside edges or edges where two surfaces meet, it is not suited for measuring smooth contours along a third, usually the Z axis. And while it can see down into holes, it is limited to line-of-sight.
Tactile measurement can handle all three dimensions including smooth curves in the Z axis and is not limited to line-of-sight. It can reach down into holes and along walls and come at vertical surfaces sideways to gather data that visual methods cannot, but it may be limited by the physical reach of measuring tips. The amount of data collected depends on the contact method used. Touch-trigger systems make contact, withdraw, move, and repeat, collecting one data point at a time. Scanning systems slide across the surface collecting hundreds or thousands of data points as they move, and are both faster and more thorough.
Multisensor systems combine the strengths of both technologies. Examining a computer mouse, for example, a camera could quickly gather data regarding the edges while a scanning tactile sensor collects data on the curved upper surface.
Optimizing visual measurement
While many consider multisensor measurement a mature market, there are still details and capabilities that can significantly impact its effectiveness, its suitability for any particular application, and the user’s return on investment. For example, it is widely believed that ‘if you can't touch it you can’t really trust the data.’ While this has some basis in fact, there are visual technologies that are every bit as accurate as contact measurement (in addition to delivering visual’s particular advantages in certain applications). For example, magnification is a key factor contributing to the accuracy of visual measurements and can greatly improve an optical system’s accuracy. Unless you know exactly what you are going to measure, both now and in the future, you will want a wide range of readily adjustable magnifications. A wider field of view lets you quickly cover a larger area; while a higher magnification lets you address small parts or finer features with greater accuracy. Higher magnification can also narrow depth of field to eliminate irrelevant details.
Another key factor in the accuracy of visual measurement is the angle at which the system views the feature being measured. Ordinary lenses collect images from a conical area in front of the lens. This creates a true image of only what is in the center of the field of view; anything not in the center is viewed at an angle and, therefore, distorted. Telecentric lenses view everything in their field of view ‘head on’ and without distortion, greatly improving accuracy.
Features can tell you a lot about a system’s capabilities, but the best way to determine the real accuracy of a visual technology is GR&R (gage repeatability and reproducibility), the comparison, preferably in a live demonstration, of repeated measurements. That will tell you how accurate the system is at the demo site, but realizing that same accuracy in your own installation depends on one more step: the qualification of a system and certification of equipment during your installation. For maximum accuracy, a system should be qualified in true 3D using a step gauge. And at installation, the system must be calibrated to ensure that it is performing to its full potential. In this process, the accuracy of visual sensing can be adjusted to match that of the system’s contact sensing.