Photodetectors that measure the brightness of light sources find use all around us. They are contained within such everyday items as cameras, bar-code readers and laser printers. They are a mainstay in most scientific instruments used to measure light intensity or color.
But there is another class of optical detectors, known as position sensors, which are unrelated to light source brightness measurements, per se. And though these devices find use in a wide range of applications, many technologists are unfamiliar with them.
Remote Angle Sensing
One of the primary uses of a position sensing detector is for measuring angles, usually of mirrors, but sometimes of relatively diffuse surfaces. UDT Instruments manufactures a number of electronic autocollimators that connect directly to the Models 531 and 431.
However, under special situations where the distance to the mirror is large, or where the mirror itself is small, the configuration shown here works well.
The 2mW helium-neon laser is positioned approximately two feet from the mirror under test. The receiving lens and the working distance are chosen to provide the desired mirror angular coverage. In this example, a 2-inch diameter 135 mm focal length lens would cover a mirror tilt of 38 milliradians.
The Model 531 should be set to bias the detector to operate in the photoconductive mode when a relatively high power light source such as HeNe laser is used. Otherwise, the detector will saturate and become nonlinear.
Varying the lens focal distance varies the angular sensitivity. A bare detector may also be used; however, it will limit the angular coverage.
Sensing the Position of Moving Objects
Here, two cases are considered, both using the same detector configuration. Real-world applications include sensing the position of machine tools and robot arms, or monitoring movement in bioengineering.
Case I: The light source is a 20 mW helium-neon laser shining onto a white, diffuse, Lambertian surface. Using a 28 mm focal length f/2.8 camera lens, the field of view of the lens system at 6 inches is 2.1 inches (given a detector with a 0.4 x 0.4 inch active area). A narrow-band 633 nm filter will reduce detector sensitivity to ambient light. A faster lens and shorter working distance will utilize the Model 531’s accuracy to an even greater extent.
Case II: Using a 1000 W tungsten lamp, working distances up to or greater than 200 feet are possible. For instance, for the Model 531 set on low gain, at 8 feet with an f/1.4 lens, the signal level is 8% of full scale. Using a higher gain on the Model 531 adds another factor of 1000 to the signal level. Therefore, on high gain the same performance can be achieved.
Sensing Displacement of a Wire
Position sensing detectors are often used in measurement situations where the sample is too delicate, small, or unsuited to contact measurements. In this example, the position of a 0.010-inch diameter wire is determined in one dimension. The location information may be useful to servo-control equipment such as wire take-up reels.
The light source is a 2 mW helium-neon laser, and its beam is expanded into a fan of light by a 25-mm focal length cylindrical lens. This fan is 5/16-inch wide by the time it arrives 15 inches away at the sample. A higher intensity light source can be used or the light source area coverage can be reduced to increase the signal level and the resolution.
Though the sample in this case is a wire, anything similar in shape and size will produce about the same signal. For instance, the sample may be glass fiber from a fiber furnace, a thread, liquid stream, or a stream of solid particles.
The collection system consists of two short focal length lenses, and a 633-nm narrowband filter, which decreases interference from ambient light.
This is essentially a variation on the surface displacement measurement scheme discussed earlier.
A helium-neon laser or collimated laser diode is focused onto the test surface with a microscope objective. This provides the requisite small measurement spot. Then, another microscope objective images the reflected spot onto the position sensing detector.
The working distance, lens magnification, and test surface reflectivity all influence the system’s overall profiling sensitivity.
Optical Position Sensors and Position Sensing Instruments
Our extensive selection of optical position sensors, position amplifiers/indicators and accessories allows our applications engineers to assemble and configure a tremendous variety of position-sensing systems.
- UDT Instruments Position Sensors
- Position Sensing Amplifiers
- Position Sensing Indicators
- Position Sensor Configuration Guide
- Download the Free UDT Instruments Guide to Position Sensing (PDF)