Sensor technologies can be grouped in two major categories-contact and noncontact. Contact sensors are those that apply any pressure to a part to obtain a measurement. Coordinate measuring machine (CMM) touch probes are the most common type of contact sensors.
Noncontact sensors do not require physical contact with the part. There is a broad range of noncontact sensor technologies that use cameras for optical imaging. Laser scanners use optics and detectors to capture laser light reflected from a surface. New microprobing technologies use resonance technology or spectral analysis of light related to the distance between the probe and the surface. Any combination of contact and noncontact sensors on a single measurement machine makes it a multisensor measurement machine.
The three sensors commonly referred to are touch probe, video and laser. Touch probe typically refers to a touch-trigger probe such as those commonly found on a CMM. Video refers to video measurement, as with dedicated measurement machines with lighting, optics and software for measurement of video images of a part. Laser means a light source to illuminate a specific part of the surface and the associated detector to collect the reflected or scattered light. But other sensor technologies are in use, especially those in the realm of microsensors.
It is possible to use a scanning probe on some multisensor metrology systems. To scan, the probe makes contact with the surface and maintains contact as it moves across the part. While scanning the surface, data points are continuously collected. Scanning probes acquire more data points more quickly than touch-trigger probing, but they have the added complexity of stage control requirements to maintain surface contact across surface deviations as the probe or part moves. In addition, system software must retain the spatial relationships of all the data points.
New versions of most sensors are capable of scanning. Laser scanners move a laser point or line across a part, continually collecting data points.
By scanning a part while collecting focus points in the optical field of view, a video measuring machine can develop a three-dimensional contour map of the surface. Edge scanning, or tracing, uses a video system's edge-detection capabilities and closed-loop stage control to scan the perimeter of a part, even if the perimeter extends outside the field of view. A video measuring machine can continually acquire focus points as the part or optics are moved. Zoom optics allow scans to vary in resolution, to accommodate changes in surface texture and feature sizes of a part.
Acquired data points are used to determine dimensions or angles between the points. It makes no difference whether those points were acquired by single-point probing or scanning-a data point is a data point. This means that a multisensor metrology system can use a sensor best suited for specific part attributes. It can produce measurements of complex dimensional forms and surface relationships simply not possible with single sensor systems.
The value of a multisensor metrology system depends on the complexity and mix of products to be measured. Complex products can benefit from the ability to use multiple sensors to more fully characterize parts than is possible on any traditional single-sensor measurement system. On the other hand, a mix of different parts, with varying feature sizes and dimensional tolerances, can benefit from the availability of sensors ready for every situation.
Another consideration in system selection is how well the system supports sensor changes during a measurement routine. Most people are familiar with a CMM using a change rack for switching probes during a measurement routine. The CMM drops off one probe and picks up another under program control. Similar mechanisms make it easy to switch between sensors on a multisensor metrology system without operator involvement. Some designs retract sensors into the body of the system, automatically deploying them only when called on by the measurement routine. Such designs speed access to other sensors, while reducing the potential for sensor damage because unused sensors are out of the way.
Multisensor measurement machines are similar to multifunction office machines. One measurement system takes up less space, uses fewer utilities, requires one training and service obligation, and minimizes part handling and fixturing. In this era of continual productivity improvements and cost reductions, a multisensor metrology system can be an important way to meet those objectives. Q