Laser Telemetric System (Metrology)
Laser telemetric system is a non-contact gauge that measures with a collimated laser beam (Refer Fig. 10.26). It measure at the rate of 150 scans per second. It basically consists of three components, a transmitter, a receiver and processor electronics. The transmitter module produces a collimated parallel scanning laser beam moving at a high, constant, linear speed. The scanning beam appears as a red line. The receiver module collects and photoelectrically senses the laser light transmitted past the object being measured. The processor electronics takes the received signals to convert them to a convenient form and displays the dimension being gauged.
The transmitter contains a low-power helium-neon gas laser and its power supply, a specially designed collimating lens, a hysteresis synchronous motor, a mutli-faceted reflector prism, a synchronous pulse photodetector and a protective replaceable window.
The high speed of scanning permits on-line gauging and thus it is possible to detect changes in dimensions when components are moving or a continuous product such as in rolling process moving at
very high speed. There is no need of waiting or product to cool for taking measurements. This system can also be applied on production machines and control them with closed feedback loops. Since the output of this system is available in digital form, it can run a process controller, limit alarms can be provided and output can be taken on digital printer. It is possible to write programs for the microprocessor to take care of smoke, dust and other airborne interference around the workpiece being measured.
Fig. 10.26. Schematic diagram of laser telematic system.
Laser and LED Based Distance Measuring Instruments (Metrology)
These can measure distances from 1 to 2 m with an accuracy of the order of 0.1 to 1% of the measuring range. When the light emitted by laser or LED hits an object, it scatters and some of this scattered light is seen by a position sensitive detector or diode array. If the distance between the measuring head and the object changes, the angle at which the light enters the detector will also change. The angle of deviation is calibrated in terms of distance and output is provided as 0—20 mA. Such instruments are very reliable because there are no moving parts. Their response time is in milliseconds. The measuring system uses two distance
Fig. 10.27. Distance meter.
meters placed at equal distance on either side of the object and a control unit to measure the thickness of an object. The distance meter is focused at the centre of the object.
Optical techniques are used to measure linear dimensions—like length, diameter, gap and displacement. These systems are also used for automatic inspection. Optical techniques for this purpose employ linear diode arrays, lasers, and diffraction phenomena. An optical measurement system consists of a light source, optical components, photodetectors, and an electronics unit which converts the light into an electrical signal for control and/or display. System accuracy depends on the way in which the dimensional information carried by the light beam is coded and decoded.
A linear diode array consists of photodetector diodes regularly spaced at constant intervals along a straight line. It is thus optical gauging’s analogy to the ruler. The object whose dimension is to be measured, is illuminated and its image formed on the linear diode array. The electronic devices are used to count the number of adjacent, illuminated diodes, the length being directly proportional to the number of diodes counted—of course, taking magnification of image into account. In such a system, the accuracy is dependent on the fidelity of the image and on the spacing of the diodes and the uniformity of their response.
Scanning laser beam. It employs a low-mass rotating mirror to sweep a single beam of laser light across a planar area. The laser beam is reflected from the rotating mirror at its axis of rotation which coincides with the focal point of a lens. The rays originating at the focal point of an ideal lens are parallel after emerging from the lens, thus the laser beam is continuously translated parallel to itself, generating a continuous plane of light.
Fig. 10.30. Scanning Laser beam.
A second lens focusses the parallel rays of laser light onto a photodetector located at the focal point of the second lens. In the absence of any object in the planar light beam, the photodetector provides a continuous output signal. If any object is placed between the two lenses, it partially blocks the plane of light and the length of the object is measured by timing the intervals over which no laser light reaches the photodetector.
Diffraction effects are used for gauging objects like small-diameter wire and narrow gaps. A narrow beam of light incident on a small-diameter wire is spread due to diffraction, with the spread inversely proportional to the wire diameter. The diffraction beam can be focused on a screen using a suitable lens, resulting in the typical diffraction pattern of alternate light and dark bands. The linear distance between these bands is measured using a linear diode array. .
At the focal plane of the lens, the angle of twist 9 is equal to the wavelength of the light divided by the diameter of wire (0 = k/d). The distance in the focal plane from the lens axis to the first minimum is equal to the angle 8 multiplied by the focal length of the lens (/). Thus the wire diameter can be calculated directly from the diffraction pattern measurement.
The displacement along a straight line can also be measured by the method Moire fringes produced by using two similar, transmission diffraction gratings placed nearly face to face and with the grating lines nearly parallel. The fringes are quite visible to the naked eye even though the grating-line spacing
is too fine for visual discrimination. The distance between adjacent dark fringes depends on the grating-line spacing and on the angle between the lines on the two gratings. If one grating
Fig. 10.31. Diffraction technique for gauging.
is moved relative to the other, the pattern of fringes moves perpendicular to the direction of relative motion. The ratio of fringes pattern displacement to grating displacement is a function of the angle between the two gratings (of the order of 1000).
Objects with rotational symmetry properties can be inspected by projecting its image onto a ring-shaped sensor made up of an appropriate number of multi-detector sensing array having same symmetry as object. Any deviation from symmetry, like a worn out gear tooth, causes an uneven illumination, which can be interpreted as detect by electronics unit as an unacceptable object.
Precision Instruments Based on Laser (Metrology)
Development of laser enabled production of clear coherent light. The biggest advantage of coherent light is that whole of the energy appears to be emanating from a very small point. Further by a system of lenses, such a beam can be focused easily into either a parallel beam or on to a very small point. A typical helium-neon laser source produces a 1 to 2 mm diameter beam of pure red light having power of 1 mW. When such a beam is focused at a point, it has very high intensity.
Upto a great distance beam has no divergence but then it begins to expand at a rate of about 1 mm/m. The laser beam is visible on any screen and on virtually any surface and it can be observed easily from a distance and it remains in position for reference. Its centre can be easily judged to an accuracy of 1 mm over 2 m. It can be used for very accurate measurements of the order of 0.1 ^m in 100 m.
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