Method and optical scanning apparatus for the identification of a code consisting of sequential light and dark fields

• US 5,569,900 A
• Filed: 07/11/1994
• Issued: 10/29/1996
• Est. Priority Date: 07/12/1993
• Status: Expired due to Fees

• Current Assignee: Erwin SICK GmbH Optik-Elektronik
• Original Assignee: Erwin SICK GmbH Optik-Elektronik
• Inventors: Blohbaum, Frank
• Primary Examiner(s): PITTS, HAROLD I
• Application Number:
  US08/273,561
• Time in Patent Office: 841 Days
• Field of Search: 235/462, 235/463
• US Class Current: 235/462.28
• CPC Class Codes: G06K 7/10851 Circuits for pulse shaping, amplifying, eliminating noise signals, checking the function of the sensing device basic electronic circuitry H03
• CPC Class Codes: G06K 7/14 using light without selection of wavelength, e.g. sensing reflected white light G06K7/10831 - G06K7/1097 take precedence

Abstract

The invention relates to an optical scanning apparatus for the identification of a code (11) consisting of sequential light and dark fields which respectively form modules (16, 17, 18, 19, 20, 21, 22) put together from an upwardly restricted integral multiple number of base running lengths (B1), to a module width (M), with a predetermined number of sequential modules (16, 17, 18, 19, 20) and base running lengths (B1) in the code forming a mark (25). The optical scanning apparatus guides a light bead (13), the extent of which is less than the base running length (B1) and at most only fractionally larger than the base running length (B1) of the code (11), over the modules (16, 17, 18, 19, 20, 21, 22) in sequence. By means of its light receiver (15), the light scattered back from the regions of the code (11) met by the light bead (13) is received, and a corresponding electrical signal is transmitted at the output (24). A sequence of individual pulses is formed by differentiation and rectification. A differential stage (26) with rectification and a counting stage (27) for the clock pulses generated by a clock generator (28) between two predetermined heights of the flanks of sequential individual pulses is connected to the light receiver (15). The individual counting results are applied to a memory (29) and stored there.

Claims

1. A method for identifying a code comprising a base running length and a plurality of marks with sequential light and dark fields which form modules, each module having a module width upwardly restricted by an integral multiple number of base running lengths, each mark having a predetermined number of sequential modules and base running lengths, the method comprising:

guiding a light bead with a light transmitter of an optical scanning device in sequence over the modules of the plurality of marks, the light bead having an extent less than 1.5 times greater than the base running length of the code;

receiving scattered light from the code with a light receiver;

generating electrical signals corresponding to the scattered light;

differentiating and rectifying the electrical signals to generate a sequence of individual pulses of differing widths, each pulse being associated with a boundary between adjacent modules in the mark, wherein the time at which the pulses reach a predetermined level defines measured boundaries of the modules to form measured module widths;

combining the boundaries into groups corresponding to each mark;

normalizing each group with respect to a temporal length and a time spacing of adjacent boundaries to determine actual module widths in units of the base running length of the code;

computing a combination module width based on the measured module width of two sequential modules;

determining an appropriate correction value for said two sequential modules from a quantity of predetermined correction values, the predetermined correction values being based on reference module widths for a plurality of combinations of sequential modules for dioptic scanning devices; and

correcting the combination module width of the two sequential modules based on the appropriate correction value.

2. The method of claim 1 wherein the extent of the light bead is smaller than the base running length of the code.

3. The method of claim 1 wherein the light bead is periodically scanned over the modules.

4. The method of claim 1 wherein the predetermined levels of the pulses is determined by an average height of a plurality of reference pulses.

5. The method of claim 1 further comprising the steps of repeatedly scanning the code with the light beads and averaging values received during the repeated scanning steps.

6. The method of claim 1 further comprising iteratively correcting the combination measured widths.

7. The method of claim 1 further comprising measuring known marks and determining the predetermined correction values.

8. The method of claim 1 further comprising subdividing the measured module widths into fractions of the base running length of the code.

9. The method of claim 1 further comprising empirically forming a first membership function for obtaining the correction values, the first membership function designating, for each measured module width, fractions of the first membership function to predetermined module widths based on integral multiples of the base running length, a maximum integral multiple being equal to a longest module encountered in the code.

10. The method of claim 9 further comprising empirically forming a second membership function which determines correction values based on a deviation of the measured module from the next closest integral module width.

11. The method of claim 10 the correction values are proportional to the deviation of the measured module width from the next closest module width, the correction values including a constant maximum region in an area of maximum deviation.

12. The method of claim 11 wherein only the fractions of the first membership function are formed from the measured module widths of a first module of the code.

13. The method of claim 12 wherein both the fractions of the first membership function and the correction values from the second membership function are formed from the measured module widths of a second module of the code.

14. The method of claim 13 the fractions of the first membership function and the correction values from the second membership function from the second module of the code are linked with the fractions of the first membership function from the first module of the code to thereby form corrected module widths which lie closer to the actual module widths than the measured module widths.

15. The method of claim 9 wherein the first membership function comprises components that lie outside of a respective interval of finite length corresponding to the associated integral multiple of the base running length, the finite length being less than or equal to twice the base running length of the code, the portion of the first membership function increasing with the respective interval of finite length to a maximum value and subsequently decreasing to an end of the respective interval.

16. The method of claim 15 wherein the maximum value occurs at an integral multiple of the base running length of the code.

17. The method of claim 15 wherein at least one of the components of the first membership function reaches the maximum value at a single discrete multiple of the base running length of the code.

18. The method of claim 15 wherein at least one of the components of the first membership function reaches the maximum value over a region of the base running length of finite width.

19. The method of claim 15 wherein the first membership function comprises at least one of the components for module in the code.

20. The method of claim 15 wherein the first membership function comprises one of the first components for each measured module width in the code, the first membership function further comprising additional components with maxima above and below the largest and smallest module widths, respectively, wherein the maxima corresponding to the component below the smallest module width is greater than half of the smallest module width and wherein the maxima corresponding to the component above the largest module width is less than the largest arising module width plus an integral multiple of the base running length.

21. The method of claim 15 wherein each component of the first membership function which has a maximum at a module width arising in the code has a symmetrical shape within a finite interval of the base running length.

22. The method of claim 15 wherein each component of the first membership function which does not have a maximum at a module width arising in the code has an asymmetrical shape.

23. The method of claim 15 wherein at least one of the components of the first membership function has a linear gradient from a beginning of an interval to the maximum value and from the maximum value to an end of the interval.

24. The method of claim 15 wherein the components of the first membership function has the formula:

space="preserve" listing-type="equation">EKOMP(X)=A+B*[SIN(X)/X]

with A being a constant substantially close to 0.0, B being a constant substantially close to 1 and X being equal to C* (the module width).

25. The method of claim 15 wherein the first membership function equals zero outside of the interval of finite length of leas than or equal to twice the base running length which lies near the considered multiple of the base running lengths.

26. The method of claim 10 wherein the second membership function comprises components within respective intervals of the integral multiples of the base running lengths, said components of the second membership function increasing to a maximum value and then decrease from the maximum value, said components having small values outside of the respective intervals of the integral multiples of the base running lengths.

27. The method of claim 26 wherein the second membership function equals zero between the base running length and zero.

28. The method of claim 26 wherein the components of the second membership function equal zero outside of the intervals of the integral multiples of the base running lengths.

29. The method of claim 26 wherein the components of the second membership function begin to increase at the integral multiples of the base running length and commence decreasing before the next integral multiple.

30. The method of claim 26 wherein the components of the second membership function have a linear gradient from a beginning of an interval to the maximum value and from the maximum value to an end of the interval.

31. The method of claim 9 further comprising:

linking the correction values for each combination module width with the fractions of the combination module width of the preceding module to form a first linked value;

linking the linked value with the respective combination module width to form a second linked value; and

averaging the second linked value with an evaluation function to generate an averaged value that lies closer to the actual module width than the measured module width.

32. The method of claim 31 wherein the evaluation function comprises functions which extend symmetrically around each integral multiple of the base running length, the functions having maximum values equal to the first linked value.

33. An optical scanning apparatus for identifying a code comprising a base running length and a plurality of marks with sequential light and dark fields which form modules, each module having a module width upwardly restricted by an integral multiple number of base running lengths, each mark having a predetermined number of sequential modules and base running lengths, the apparatus comprising:

an optical scanning device having a light transmitter for guiding a light bead in sequence over the modules of the plurality of marks;

a light receiver for receiving scattered light from the code;

means, operably coupled to the light receiver, for generating electric signals corresponding to the scattered light;

a differentiating stage, operably coupled to the generating means, for differentiating and rectifying the scattered light;

a clock generator for generating a sequence of individual pulses of differing widths, each pulse being associated with a boundary between adjacent modules in the mark, wherein the time at which the pulses reach a predetermined level defines measured boundaries of the modules to form measured module widths;

a memory storage, the clock generator storing the pulses in the memory storage;

a two module selection stage for detecting a combination module width based on the measured module width of two sequential modules; and

a correction stage for receiving the combination module width and for determining a correction value for said two sequential modules from a quantity of predetermined correction values, the predetermined correction values being based on reference module widths for a plurality of combinations of sequential modules for dioptic scanning devices, the correction stage correcting the combination module width of the two sequential modules based on the appropriate correction value.

34. The apparatus of claim 33 wherein the extent of the light bead is smaller than the base running length of the code.

35. The apparatus of claim 33 wherein the optical scanning device periodically scans the light beads over the modules.

36. The apparatus of claim 33 wherein the correction stage comprises a look-up stage for storing a first membership function having components associated with an integral multiple of the base running length of the code, the first membership function designating, for each measured module width, fractions of the first membership function to predetermined module widths based on integral multiples of the base running length.

37. The apparatus of claim 36 wherein the correction stage further comprises a second look-up stage for storing a second membership function which determines correction values based on a deviation of the measured module from the next closest integral module width.

38. The apparatus of claim 37 further comprising a algebraic linking circuit for receiving outputs of the first and second look-up stages.

39. The apparatus of claim 38 wherein the algebraic linking circuit comprises:

first and second memories connected in parallel for temporarily storing the fractions of the first membership function determined in the first and second look-up stages for a first measured module width, the first and second memories storing the fractions of the first membership function and the correction values for a second measured module width;

a combining stage coupled to the first and second memories for linking the fractions of the first measured module width with the correction values of the second measured module width to form a first linked output;

a second combining stage coupled to the first combining stage and the second memory for linking the first linked output with the fractions of the second measured module width to form a second linked output; and

an evaluation stage for averaging the second linked output with an evaluation function to generate a corrected module width closer to the actual module width that the measured module width.

40. The apparatus of claim 39 wherein the evaluation function is generated from functions that extend symmetrically about each integral multiple of the base running length.

41. The apparatus of claim 40 further comprising a threshold member coupled between the differentiating stage and the clock generator for transmitting a resetting signal and a trigger signal to the clock generator at a time when the amplitude of the pulses do not equal a predetermined amplitude.