Having recently wrapped up a webinar for the AIA on “Machine Vision Technology in High Precision Online Inspection and Measurement” (register and view here), I decided to expand upon the material here over the next 3 posts about precision measurement using machine vision.
Understanding Precision Measurement Concepts
Precision measurement using machine vision technology is a valuable application for online production inspection. A good place to start in addressing how to successfully perform machine vision metrology is to understand precision measurement concepts. The term metrology often is used interchangeably with measurement and indeed they are closely linked. Actually, metrology is the “science of measurement”. When we talk about metrology in an industrial process, it refers to “applied metrology” or “industrial metrology”; the application of the science of metrology for manufacturing. As we consider metrology in this sense, let’s review some terms and concepts used in measurement and metrology.
The terms precision and accuracy are used in the specification and qualification of a measurement system. Precision is the ability to repeat a certain measurement, while accuracy is how well a measurement agrees with a true or known value. It is very important to understand that accuracy does not provide a very useful metric in determining the quality of a metrology device in the context of the plant floor. In specifying and qualifying production machine vision solutions, then, precision is most important, and in specific, the uncertainty or bias of the measurement. The terms uncertainty and bias refer to the amount of variation that will be exhibited by a measurement system over repeated measurements (or the degree of precision). To be clear, the goal of the on-line metrology system will be to produce a specific measurement with a reliable and repeatable level of uncertainty that meets the application requirements.
For example, a machine vision system might report the diameter of a specific part to be 20.4mm, with an uncertainty over many repetitions of +/- 0.05mm (that is, no measurement of the same part deviated from 20.4mm by more than 0.05mm). Of course, the actual amount of uncertainty that is acceptable for any application is dependent upon the application specification.
Note: machining specifications are not the equivalent of inspection specifications. If a part must be manufactured to 24mm +/- 0.01mm in length, what is the acceptable uncertainty for inspecting that length? If the inspection uncertainty is specified as +/- 0.01, a good machined part could be 24.01mm in length, but that measurement system would fail it a statistically significant number of times since in this example, the 24.01mm measurement could report between 24.00 and 24.02mm. To be certain of the target measurement criteria, consult with the quality team and determine in advance the allowable inspection uncertainty based upon the needs of the application.
Next: Image and inspection resolution