It’s becoming common to refer to the devices used for dimensional and coordinate measurement—vision/optics, lasers, touch probes, and others—as sensors. Multisensor measuring systems use two or more of these devices. It can be confusing because none of these “sensors” is actually a sensor. The sensing is done elsewhere.
Categories of sensors
Sensors used for dimensional measurement are usually categorized as contact or noncontact. Touch probes do just that—they touch (contact) the part to register a data point. Noncontact sensors use a variety of technologies that typically involve optical imaging. New sensor types are actually variations of the two categories. For example, a newer type of microsensing probe gets close to contacting the surface like a touch probe, but registers a point with virtually no surface contact.
Yes, there are other sensor technologies used for measurement such as eddy current, capacitance, ultrasonics, and x-rays that are noncontact. However, they’re typically not used for dimensional measurement, measuring other characteristics of parts instead.
Touch triggering
Touch probes trigger from stylus contact with a surface. The probe approaches the surface, contacts it, and moves further toward the surface until a sensor in the probe head triggers, registering a point. It’s necessary for the stylus to deflect to register a point. It’s also necessary for the stylus to back away from the surface to reset the trigger mechanism before it can probe another point. Typical touch probing involves moving the probe to a location, approaching and contacting the surface, registering the point, backing off, moving to the next location, and repeating.
This is quite obviously a contact measuring technique. However, the touch probe itself isn’t the sensor. Surface contact is sensed by the trigger mechanism in the probe head. It’s easier to simply refer to a touch probe as one of the sensors used for dimensional measurement.
Lasers
Calling a laser a sensor is off the mark, although it’s common to see laser sensors mentioned in sales material. Lasers are used as light sources for certain measurements. With the prevalence, low cost, and compact size of diode lasers, they’re used in numerous devices, including dimensional measurement systems. In any case, the laser itself isn’t a sensor. Some type of detector senses the reflected, direct, or scattered laser light. The output of that detector represents some characteristic of the part. Actually the differences in the sensor output under the varying part or environmental conditions represent the physical differences that the laser light encounters.
The laser scanner at the supermarket is a good example. Its sensor reads bar codes by sensing the reflection amplitude and spacing across the dark to light, or light to dark bars in the bar code. Those values output from the corresponding sensor represent specific items in a database.
The sensor output of a laser for surface measurements represents variations in the surface. A simple example is scanning a focused point of laser light across an undulating surface with ripples. A possible implementation is to scan the laser beam across the part at a fixed distance. Changes in distance from the laser to the surface cause the sensor output to vary in direct relationship to changes in the surface. For example, the peaks of the ripples closest to the sensor may cause a higher-voltage output than the signal from the valleys farthest from the laser. That varying sensor output directly represents the change in the position of the laser light on the surface. If the output of the laser sensor is sampled at intervals of time, each sample represents a point of data with its own coordinates.
No matter what type of implementation, the laser isn’t the sensor. The laser relies on a light sensor to provide the output used for a measurement.
A variation of the laser sensor is the white-light sensor. Scanning white-light devices can offer greater height resolution than other measuring techniques. The source white light provides a spectrum of colors that allow for spectral analysis with resolutions to a fraction of the wavelength of specific colors. Like lasers, scanning white-light devices rely on separate sensors to output signals, which represent some characteristic of the part being measured.
Microsensing
A variation on the touch trigger probe concept is a microprobe device with a very different sensing scheme. In this case, a miniature stylus is driven so that it’s in constant micromotion. As the stylus is brought near a surface the rate of micromotion is damped by its proximity to the surface. That change in the rate of micromotion represents a measurement data point. The technique is so sensitive that it’s possible to get a measurement reading from the surface of a drop of water without deflecting its surface. Because of its small scale, it can access intricate details either not accessible to traditional touch probes or not able to accommodate their required approach and back-off distances.
As with the touch probe, the microprobe itself isn’t the sensor. The sensor is the unit electronics that detect the change in micromotion and output a corresponding signal to register a surface point within the coordinate system.
Video/vision
Video and vision measurement systems perform measurements by analyzing images collected by an optical system and presented to a camera. In this case, the camera is the sensor. It isn’t that common to refer to a “video sensor” except in the context of multisensor measurements when other kinds of sensors are included. Like the other “sensors,” video is simply a convenient word for describing a measuring technique that relies on the output of a sensor to provide its value.
Like the sensor that monitors a reflected laser signal, a video camera outputs a signal that represents the variations in contrast and intensity of an image. If that image includes an edge, software algorithms can use that signal output to accurately determine the edge location because the camera’s pixels are of known size and in a known location relative to the part. Individual points along the edge each have coordinates in the measurement space. Video measuring is the technique. The camera is the sensor.
Sensing where the sensor is
The sensors described in this piece determine data-point coordinates from a part. However, a single point is rarely of much value alone. Because it’s necessary to know the relationship between points to get meaningful measurements, some type of encoder or scale is needed to know the coordinates of all the points collected across the part. These additional sensors work in conjunction with the measuring sensors to determine the position of the measured points from any of the sensors in the system. The data from all the sensors are used by the metrology software to derive distances and angles for measurement verification.
Semantics
Yes, this is an exercise in semantics. Once you understand that lasers and touch probes are referred to as sensors for convenience, you can deal with what each measuring device actually is.
Knowing how each of these devices actually sense to perform measurements seems sensible to me. Hopefully, all of this sensor talk makes sense to you too.
Until next time, yes, measurement matters.
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