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
  • G01B11/30—Measuring arrangements characterised by the use of optical techniques for measuring roughness or irregularity of surfaces
  • G01B11/303—Measuring arrangements characterised by the use of optical techniques for measuring roughness or irregularity of surfaces using photoelectric detection means

Definitions

• the invention relates to a measuring method for determining a surface profile of a workpiece, the type mentioned in the preamble of claim 1.
• test devices are used to determine the surface quality of workpieces in order to be able to control the manufacturing process.
• a device with scattered light measurement which evaluates the light reflected from the surface of an illuminated workpiece. Integral characteristic values are determined from the intensity values of the scattered light distribution, which are a measure of the surface quality.
• a particular advantage of this device is that the measurement is carried out without touching the workpiece.
• an optical measuring arrangement for recording the quality of a surface in which a portion of the surface is illuminated with coherent light. A beam is reflected directly from the surface and, with the aid of a diffraction plate, is superimposed on the light backscattered by the illuminated section as a reference beam. There is a between the diffraction plate and the surface Relative movement generated so that the reference beam undergoes a frequency shift. The reference beam and backscattered light are superimposed and received by a photo detector.
• the scattered light has the same frequency at every solid angle and the reference beam has a location-independent, different frequency, which is dependent on the angular velocity of the diffraction plate, the diffraction angle and the angle of incidence with respect to the diffraction plate.
• the frequency of the photocurrent is equal to the difference between the frequency of the backscattered or illuminating light and the frequency of the reference beam. In the case of the rotating diffraction plate, this frequency depends on the ratio of pitch circle, radius and grating constant multiplied by the angular velocity.
• the directly reflected light is scattered and forms the reference beam. Due to the movement of the matt glass plate, the reference beam experiences a location-dependent frequency shift.
• the photocurrent then has a frequency spectrum that reflects the angular distribution of the scattered light. This angular distribution is a function of the size and distribution of the irregularities on the surface to be measured. Since the matt glass plate is a diffusely scattering plate, the phase distribution of the reference beam is statistical and can be changed by the movement of the matt glass plate and is therefore not known locally.
• the frequency spectrum therefore has statistical phase relationships and can therefore only be evaluated with regard to its intensity distribution (power spectrum). It is not possible to determine a surface profile by evaluating this frequency spectrum.
• the surface of the workpiece is illuminated with monochromatic light, e.g. B. illuminated from a laser source.
• the incident light beam is scattered back from the surface. This process is comparable to diffraction on an amplitude and / or phase filter, e.g. B. on a lattice structure.
• the incident light beam experiences a phase and amplitude evaluation.
• the backscattered light leaves the surface structure at different solid angles, with a spatial frequency being assigned to each solid angle.
• a reference beam is superimposed on the backscattered light in order to measure the phase information which indicates the depth profile of the surface in the measuring direction, that is to say the surface profile.
• This wave field is received with a detector arrangement and converted into electrical signals.
• the signals provide scatter components a complex solid angle spectrum.
• the solid angle spectrum is transformed from the solid angle range or spatial frequency range into the spatial range.
• the reverse transformation is complex.
• Their location-dependent amplitude indicates the reflection properties of the workpiece in the measuring direction.
• Their phase profile over the location is proportional to the depth profile of the workpiece in the measuring direction and thus indicates the surface profile.
• the proportionality factor is equal to the wavelength of the illuminating light divided by 4JL.
• the advantageous further development of the measuring method according to the invention according to claim 2 specifies a detector arrangement which consists of individual elements arranged in a line. A solid angle and thus a spatial frequency is assigned to each element. Their output voltages form the stray components of the complex solid angle spectrum.
• An advantage of the method according to the invention according to claims 1 and 2 is that the surface profile is scanned in a contact-free manner by means of a light beam.
• the phase information of the scattered light or the solid angle spectrum is made accessible by measurement technology. So far, only the intensity values of the scattered light distribution have been evaluated to indicate integral parameters as a measure of the surface quality. A phase evaluation was not feasible. It is also advantageous that the surface profile is not detected point by point, but rather that the portion of the surface profile illuminated by the beam is evaluated in parallel.
• the resolution of the surface profile measurement can be adjusted by the wavelength of the illuminating light and adapted to the roughness of the surface profile.
• the measuring method is e.g. B. with vertical lighting can be easily realized optically by attaching an arranged at an angle, semitransparent mirror in the beam path between the illuminating laser source and reflecting the scattered light onto the detector arrangement.
• a scattering lens or pin-hole is provided on the optical axis between the mirror and the detector arrangement for expanding the directly reflected light beam, which is superimposed on the backscattered light as a reference beam.
• Pattern recognition are to be recorded automatically.
• Scattering components of the solid angle spectrum at the output of the individual elements of the detector arrangement are checked for certain features by filtering and / or separation or compared with existing, known patterns. This pattern recognition takes place either optically or electronically.
• the detector arrangement itself would be by corresponding geometrical ones
• the computer circuit which also carries out the reverse transformation, receives corresponding filter circuits.
• the photoelectric detector arrangement here consists of only two photodiodes, which are arranged symmetrically to that scattering component whose frequency is equal to the frequency of the reference beam, namely to the zero order, if the reference beam is derived from the directly reflected light beam or the illuminating light.
• a relative movement is generated between the surface and the illuminating beam, either the workpiece is moved further under the illuminating beam or the illuminating beam is pivoted over the workpiece. Since the reference beam and backscattered light are superimposed in front of the detector arrangement, a speed component in the measuring direction of the surface profile leads to different frequency shifts in the scattering components of the solid angle spectrum due to the Doppler effect.
• the received interference field is squared by the quadratic characteristic of the two elements of the detector arrangement, each element acting simultaneously as a low-pass filter.
• the output voltage of one element now shows
• the output voltage of the other element has frequency components that can be assigned to negative solid angles.
• Output voltages are fed to a frequency analysis, which is carried out, for example, by a computing stage for forming the
• the frequency components of the output voltages give the Scattering components of the complex solid angle spectrum and are transformed back into the local area in a downstream computer circuit.
• the phase profile of the back-transformed is determined and the surface profile is displayed to scale, taking into account the proportionality factor.
• An advantage of the measuring method according to claim 3 is in particular that a robust construction is possible by dividing the detector arrangement in two. It is particularly advantageous that the resolution of the surface profile is determined solely by beam width and frequency analysis and does not depend on the geometry of the detector arrangement.
• the photoelectric detector arrangement consists of a single element with a square characteristic, for example a photodiode.
• the reference beam superimposed on the backscattered light here has a frequency that is selected, for example, higher than the frequency of the illuminating light beam.
• the time-dependent output signal of the individual element has a center frequency or beat frequency which can be set with the frequency of the reference beam and / or the relative speed in the measuring direction between the workpiece and the illuminating light beam.
• Frequency components which can be assigned to positive and negative solid angles are obtained symmetrically to this center frequency or beat frequency.
• the frequency analysis of the output signal provides frequency components that directly indicate the scattering components of the solid angle spectrum.
• a solid angle or a spatial frequency is assigned to each frequency.
• the phase of the frequency components contains the phase of the scattering components, their amplitude is proportional to the scattering amplitude.
• a location-dependent profile of the material-related reflectivity of the surface profile is determined in a simple manner from the scattering components of the solid angle spectrum.
• Fig. 2 is a sketch of a measurement setup for a heterodyne measurement method
• FIG. 3 shows a sketch of a measurement setup for an alternative heterodyne measurement method.
• the surface of a workpiece 10 is illuminated according to FIG. 1 with a laser source 11 in the region b with monochromatic light of the wavelength ⁇ .
• a semitransparent mirror 12 which reflects light scattered back from the surface onto a photoelectric detector arrangement 13 located in the far field.
• a light beam 15 reflected directly from the surface which is also referred to as a zero-order light beam diffracted on the surface, is expanded via a scattering lens or a pinhole 16 to form the reference beam, which is superimposed on the backscattered light in region 17.
• the detector arrangement 13 consists of individual elements 130, 131,... 13 n arranged in close proximity on a line in the form of photodiodes. Their output voltages form stray components of a complex solid angle spectrum S (k) of the interference field created in area 17.
• the reverse transformation is carried out in a downstream computer circuit 20
• the reverse transform has one location-dependent amplitude A (x) and phase ⁇ (x). ""
• Amplitude and phase characteristics are determined in the computer circuit 20.
• the computer circuit 20 is followed by a display unit 21 for the surface profile h (x) and a display unit 22 for the amplitude profile, which displays material-related reflectivity of the surface profile.
• the workpiece 10 here has a speed component v in the X direction.
• the reference beam and the light backscattered from the surface are received by a photoelectric detector arrangement 23.
• the detector arrangement 23 consists of two photodiodes 24, 25, which are arranged symmetrically to the zero order 26. Their time-dependent output voltages are frequency-analyzed in a computing stage 30, which carries out a Fourier transformation.
• the frequency components are the scattering components of the solid angle spectrum S (k), which are evaluated in the computer circuit 20.
• the frequency components of the output voltages of the photodiode 24 indicate stray components with positive solid angles / *, the frequency components of the output voltage of the photodiode 25 indicate stray components with negative solid angles -J.
• the detector arrangement here consists of a single element 50 with a square one Curve.
• the reference beam widened with the optics 16 has a different frequency than the illuminating light. The directly reflected
• light beam 15 is frequency-shifted via mirror 12 in an acousto-optical modulator 51.
• Output signal of the single element 50 is in a
• Computation stage 52 frequency-analyzed to form the Fourier transformation. Frequency components with a positive deviation from one determined by the light frequencies and the speed component v
• Center frequency provide scattering components with positive ones
• Display unit 21 and the amplitude profile are displayed on the display unit 22.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Investigating Or Analysing Materials By Optical Means (AREA)
• Length Measuring Devices By Optical Means (AREA)

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ID=6327590

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Cited By (2)

Citations (2)

Family Cites Families (4)

• 1987
  • 1987-05-15 DE DE19873716241 patent/DE3716241A1/de active Granted
• 1988
  • 1988-11-10 WO PCT/EP1988/001013 patent/WO1990005279A1/en unknown

Patent Citations (2)

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