Manufactured parts need to be measured to ensure they meet the original design intent. Modern manufacturing techniques allow complex parts to be designed with numerous critical dimensions. A key element in precision metrology is having the right measurement tool for the job. Modern measurement machines use a variety of sensors to collect measurement data. Metrology software analyzes the measurement data, and through numerical and graphical reports, allows the user to make confident decisions about the part design and manufacturing processes. By Terry Herbeck, Vice-President of Asian Operations at OGP
There are a variety of sensors used for metrology, each with its own strengths for measurement of particular feature types. Some metrology machines offer one sensor type for data collection. Multisensor metrology systems offer a variety of sensor technologies to acquire data to provide virtually any dimension of interest.
Savvy users of metrology systems need to understand what these sensors can do, and what they do best.
Metrology Sensor Basics
Tactile measurement sensors include touch trigger probes which collect a point at a time, and scanning probes which collect points continuously. Tactile sensors physically touch the part during the measurement process. Touch probes are the most common sensors used on CMMs (Coordinate Measurement Machines).
Touch trigger probes approach the part, make contact, retract, move and repeat, collecting location information about each point, one point at a time. There are many combinations of styli length and tip sizes that allow access to most areas of a part being measured, but constraints include the physical reach of the probe tip, and enough space to approach, trigger, and back off.
Scanning probes are a variation of touch probes designed to maintain contact with surface contours, collecting measurement data points as they move. Metrology system software works with the probe controller to drive the probe along the part contour, collecting up to several hundred surface points each second.
Another variation of touch probing is resonating Micro-Probes. Resonating micro-probes trigger when they are brought into close proximity to a surface, but barely touch it. They allow measurement in tight spaces as back-off and approach distances are minimal, and probe tips can be smaller than a millimeter. With low trigger force micro-probe technology, the surface of a liquid can be probed without breaking the surface tension.
Optical measurement is non-contact. Numerous technologies are used for optical measurement including imaging with cameras (video measurement), lasers, interferometry, and chromatic confocal imaging.
Video measurement systems analyze magnified images of a part to determine the location and size of features within those images. By knowing the location of the part and the camera within its measuring volume, video measurement systems rapidly measure features with high resolution across distances exceeding a meter. Area array cameras and large field lenses allow for a high number of measurements within a field of view based on high-powered software analysis of the entire image. This is much faster than tactile probing. It is the speed and precision of this software-based edge detection process that makes video measurement widely used and very popular. Depending on the size of parts being measured and the imaging field of view, it can be possible to measure multiple small parts or numerous features simultaneously – particularly useful in a production environment.
Sensors that use lasers are popular and highly versatile for rapid and accurate measurement. A typical laser sensor emits light onto a surface which is reflected and imaged on a detector, identifying the point’s position in XYZ space. Laser point sensors measure a point at a time while laser line sensors acquire hundreds or thousands of points in a line simultaneously. Both types of sensors can usually be scanned across surface contours. The positioning electronics in the measuring system keep the sensors within their capture range as they follow surface contours across the part. Laser range sensors often use triangulation between the laser and detector to derive surface data points. When used in conjunction with a video measurement system, through-the-lens (TTL) laser sensors are completely integrated within the existing optical system of a video measuring machine. Unlike off-axis triangulation sensors, the steep imaging angle of some TTL laser sensors, coupled with a long working distance, provides access to surface features that are recessed or located adjacent to vertical surfaces.
Interferometric range sensors provide measured point resolution in the sub-micrometer (< 0.1 µm) range, with excellent performance on both specular and light-scattering diffuse surfaces. When used as a point sensor, they are fast, accurate, and well-suited for precise measurement of surface contours or measurement deep inside bores or blind holes.
Chromatic confocal point sensors are another non-contact optical measurement technology. This sensor uses a white light source and measures surfaces without contact by analyzing the optical spectrum of the light reflected from a surface. This extremely high resolution sensor is capable of measuring parallel surfaces of transparent parts.
The Multisensor Advantage
Different data acquisition technologies target different measurement tasks. Video measurement is the optimal technology for noncontact detection and measurement of edges. When used on a video measurement system, touch probes are able to measure areas of a part inaccessible to optical imaging. Laser and white light sensors quickly and accurately measure surface form and contours.
Above: Images of touch probes
A single sensor metrology system may be the right choice for a dedicated in-process measurement system if it is confirmed the chosen sensor can measure the desired part features with appropriate resolution, accuracy and speed. If this is the case, multiple sensors may not be necessary.
However, the versatility of a multisensor system allows an engineer to optimize the measurement process, and is an important attribute in the business case when making a multisensor vs single sensor decision. Even if a single sensor measurement system may be sufficient today, you can future-proof your organization with a multisensor system.
Metrology systems have proven to be long term contributors for companies to control and ensure quality with appropriate measurement capability, capacity, and productivity. Take the time to fully understand the company measurement philosophy, and current and future needs. Configure the measurement system thoughtfully, and the return on investment will show on the company’s financial bottom line.