Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B11/00—Measuring arrangements characterised by the use of optical techniques
  • G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
  • G01B11/026—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness by measuring distance between sensor and object

Definitions

• the invention relates to a process for measuring absolute distance and distance variations between a reference plane and the surface of a body, in the course of which process light emitted from a source of light is reflected from the surface of the body and a part of the reflected light is captured in an optical system, said light being converted, with the aid of a light-sensitive device, into an electric signal which is a measure of said distance or distance variation.
• the invention also relates to an apparatus for carrying out the process.
• Examples of such distances and distance variations are dimensions of mechanically machined objects, roughness values of these objects and vibrations in mechanical systems.
• Optical measuring methods are frequently contactless so that they do not affect the object to be measured, which is a great advantage in many measuring situations, especially when measuring soft objects and generally wherever high measuring speeds are required.
• a source of light illuminates the object through a collimation lens and a focussing lens. These lenses form an image of the reflection point on a 4-segment detector with the aid of a beam splitter and two prisms in such a way that the 4 detector segments are uniformly illuminated only if the reflection point is in the focal plane of the focussing lens. It is possible, therefore, to adjust the position of the focussing lens to a uniform illumination of the 4 segments.
• the adjustment is effected electrically with the aid of a servo-circuit, a movable coil and a permanent magnet.
• the electric signal to the coil is a measure of that distance.
• a major problem with this method consists in the fact that a possible hysteresis of the coil causes some error in the electric signal.
• Another problem consists in the inertia of said coil, inasmuch as it imposes an upper limit on the frequency of the distance variations which can be detected. In the given case the latter amounts to 600 Hz.
• a third problem consists in the fact that the servo-circuit makes it difficult to exchange, if this should be required, the focussing lens for other lenses since the mechanical system is modified thereby.
• the object of this present invention consists in bringing about a process enabling absolute determination of distances and distance variations, but which does not necessitate moving components in that part of the optical system which gives rise to the electric signal which is a measure of that distance or distance variation.
• the surface of the body is illuminated by a source of light and a part of the reflected light is received, via a focussing device and a trans ⁇ mission device, on a light-sensitive device and in that the distance between the focussing device and the transmission device is equal to the focal length of the focussing device and in that the distance between the transmission device and the light-sensitive device is designed so as to be equal to twice the focal length of the trans- mission device.
• the source of light can be modulated with a fixed frequency.
• the distance of the source of light from (the fixed) collimation device can, if required, be adjusted, subject to control by an electric signal derived from the optical enlargement in that part of the optical system which consists of a collimation lens and a focussing lens, together with the electric signal, which is a measure for the distance between said reference distance and the reflected object.
• the process according to the invention makes it possible to measure very rapid distance variations.
• the process according to the invention causes a symmetric connection between the electric measuring signal formed and the measured distance in respect of a reference distance corresponding to the focal length of the focussing lens.
• the process according to the invention also makes it possible to improve the electrical signal-noise ratio during the detection process, by modulating the source of light and an electronic circuit for processing the detected signal adjusted to the modulation selected.
• the process according to the invention also makes it possible to ensure as small a lightspot diameter on the object as possible by adjusting the distance between the source of light and the fixed collimation lens, thus ensuring that any possible hysteresis in the mechanical adjusting system causes uncertainty only as regards the diameter of the spot and not the distance between the object and the reference plane.
• the process according to the invention also makes it possible to modify, by simple change of the focussing lens to another focal length, the optical enlargement in that part of the optical system which consists of focussing lens and cylindrical lens, and thus to modify the sensitivity and range of measuring distances and distance variations in relation to a reference distance corresponding to the focal length of the focussing lens.
• the invention also relates to an apparatus for measuring absolute distance and distance variations between a reference distance and the surface of a body by carrying out the process according to claim 1.
• the apparatus comprises a source of light, the divergent light beam of which is transmitted through a device for collimating the trans ⁇ mitted light; a device for focussing the transmitted beam of light, devices for receiving the light reflected from the surface of the body, devices for branching off a light beam derived from the reflected light beam, devices for transmitting the derived light beam and for transmitting the light beam to a number of light-sensitive devices, each of which is especially equipped to produce a signal depending on the intensity of the light, as well as signal processing devices, comprising devices for continuous recording of the intensity of light of each of the light-sensitive devices and devices for converting this recording to an output signal which is a measure of that absolute distance or distance variation.
• the apparatus according to the invention is special in that the device for receiving the light reflected from the surface of the body and the device for transmitting the derived light beam are firmly located at a mutual optical distance corresponding to the focal length of the device for receiving the light reflected from the surface of the body, and in that the source of light is located on a device for varying the optical distance from the device for collimating the transmitted light.
• the apparatus does not require any movable components within the signal-generating part of the optical system and is therefore capable of carrying out measurements at considerable speed and can, in simple manner, be caused to cover a wide measuring range by exchanging the device equipped for receiving the light reflected from the surface of the body.
• the apparatus By varying the position of the source of light it is possible to ensure, with e. g. especially exacting measurements, the least possible spot of light on the surface of the body, and, owing to the optical enlargement, it is nevertheless possible to achieve considerable measuring speeds.
• the source of light is located on the surface of a piezo-electric crystal for varying its position in relation to a collimation lens.
• the transmission device is designed as a cylindrical lens, which forms an image of the light reflected from the surface of the body on a light detector consisting of four individual detector elements. The electric signal from these detector elements contains information about the required measuring distance.
• Figure 1 shows a diagram of an optical system suitable for use in the apparatus according to the invention.
• Figure 2 shows the curve illustrating the relationship between the distance from the surface of the body to a reference distance and the electric signal emanating from the light- sensitive devices in an apparatus according to the invention.
• Figure 3 shows a diagram of a preferred embodiment of the light-sensitive devices according to the invention.
• Figure 4 shows a diagram of an embodiment of signal processing devices in an apparatus according to the invention.
• the process according to the invention is based on the use of an optical system, in which a divergent light beam emanating from a source of light passes through a collimation lens, a beam splitter and a focussing lens, said light beam being transformed thereby to a thin light beam reflected from the surface of the body.
• the reflected light is caused to be transmitted through the focussing lens, the beam splitter and through the cylindrical lens located at a distance from the focussing lens corresponding to the focal length of the focussing lens to the detector element, which is located at a distance from the cylindrical lens corresponding to twice the focal length of the cylindrical lens.
• the distance signal is derived from the signal emanating from the various detector elements.
• the optical system comprises a light source 1, emitting a divergent light beam 2 which is transmitted through a collimation lens 3 and further on through a partly reflecting mirror 4.
• a part of light beam 7 is branched off and transmitted through a focussing lens 5, which focusses the branched-off light beam 8 on surface 6 of the body.
• a part of the light beam is reflected back through focussing lens 5 and the partly reflecting mirror 4.
• a part of the reflected light beam is transmitted without refraction through the partly reflecting mirror and further through a cylindrical lens 9 and is then directed towards a light-sensitive device 11.
• the optical distance between cylindrical lens 9 and the light-sensitive device 11 is exactly equal to twice the focal length of the cylindrical lens.
• the optical distance between focussing lens 5 and cylindrical lens 9 is exactly equal to the focal length of the focussing lens.
• the optical distance between source of light 1 and collimation lens 3 can be varied by means of a piezo-electric crystal 10, on which the source of light is mounted.
• the signal for changing the dimension of the piezo-electric crystal and, as a result, the distance between the source of light 1 and collimation lens 3 can, if required, be derived from the electric signal processing devices 12.
• the light-sensitive device 11 consists of not fewer than four light-sensitive partial- devices 11a, lib, lie, lid, which are separate from one another.
• the electric signal from each of these light-sensitive partial devices is passed on to the electric signal processing devices 12, from which a signal is derived, which contains information about the absolute distance between reflecting body surface 6 and the focal plane of focussing lens 5.
• d is the distance between the reflecting body surface and the focal plane of the focussing lens
• x and y are constants.
• the signal processing devices shown are largely based on conventional signal processing technology, and the diagram is intended solely by way of a specimen embodiment showing how the electric signals from the light-sensitive partial devices can be caused to indicate the distance between the reflecting body surface and the focal plane of the focussing lens.
• the signals from the four light- sensitive partial devices are passed on to four current supply devices 13a, 13b, 13c and 13d.
• the output signal from 13a and 13c is conducted to a summation device 14a and the output signal from 13b and 13d is similarly conducted to an identical summation device 14b.
• the output signals from these summation devices are passed on to a differentiation device 15 so that the entire signal processing operation, analogue or digital, can be written
• I » , I to , I,_, I, a are the signals from the four partial devices 11a, lib, lie and lid.
• the above signal processing can, if required, be developed by standardisation in respect of the total sum of the signals Iregistering, I e , I c and I rf .

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measurement Of Optical Distance (AREA)
• Length Measuring Devices By Optical Means (AREA)

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Citations (2)

• 1987
  • 1987-11-02 DK DK574487A patent/DK574487A/da not_active Application Discontinuation
• 1988
  • 1988-11-02 WO PCT/DK1988/000179 patent/WO1989004459A1/en unknown

Patent Citations (2)

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