GB2072840A

GB2072840A - Process and Device for the Contact-free Measurement of a Dimension

Info

Publication number: GB2072840A
Application number: GB8108947A
Authority: GB
Original assignee: Zumbach Electronic AG
Current assignee: Zumbach Electronic AG
Priority date: 1980-03-25
Filing date: 1981-03-23
Publication date: 1981-10-07
Also published as: GB2072840B; CH645462A5; IT1135714B; AU6858581A; FR2479447A1; AU538202B2; FR2479447B1; ES500695A0; CA1168437A; ZA811935B; DE3111356A1; ES8203150A1; JPS56150302A; IT8120686A0; BE888013A; DE3111356C2

Abstract

In a process and apparatus for measuring the dimension of at least one object (8) without physical contact therewith, a fine light beam, for example a laser beam (2), is deflected across the object in the direction of the dimension to be measured and the instants in time at which the light beam impinges upon and moves off said object are determined and stored as measurement data. These data are then evaluated, together with reference data relating to the speed of deflection of the light beam, to provide a measure of the dimension. In one example a computerised system operates in a continuous cycle wherein the measurement data are recorded and stored in a first memory during a first cycle, the data is transferred to a second memory in the direct memory access mode, and then during the next cycle the data in the second memory is processed whilst new data is being read into the first memory. A chopper circuit linked to the means for deflection of the light beam, for example a rotary mirror (3), serves both to control the operating cycle and to provide the reference data. <IMAGE>

Description

SPECIFICATION Process and Device for the Contact-free Measurement of a Dimension The present invention concerns a process for the contact-free measurement of a dimension of gt least one object, wherein a fine light beam, more especially a laser beam, is deflected across the object, within a measuring field, in the direction of the dimension to be measured, and the period of interruption of the light beam by the object is compared with a reference period, and wherein the dimension of the object is ascertained from these periods.

In a known measuring process of this kind the period of interruption of the light beam by the object is compared with the period of the transition of the light beam across a window defining the measuring field (US-PS 4,082,463). The dimension of the object to be determined thus has the same relationship to the dimension of the window opening as the period of the interruption of the light beam by the object has to the period of the transition of the light beam across the measuring field, that is to say from window edge to window edge. Errors which can occur in this method of measurement due to a non-constant velocity of transition of the light beam across the measuring field are compensated by providing a coarse grid in the measuring field during the manufacture of the measuring apparatus, by means of which .calibration values can be stored.An interpolation is necessary to obtain measurement values between the calibration values. This known measuring process offers only limited possibilities for the determination and processing of measurement values, because data can be collected and processed only at each transition of the light beam through predetermined points.

The time available for the processing of data is correspondingly short. As stated only a relatively coarse calibration is possible for which a special program unit is necessary, which is such that a recalibration of an installed apparatus during servicing is very difficult. The comparison of the dimension to be measured with the dimension of a window opening requires that this window opening be very accurately measured and does not alter, a condition which is difficult to maintain, for example, with variations in temperature.

It is the object of the present invention to provide a versatile measuring process which has a minimum of sources of error.

The invention accordingly provides a process for measuring a dimension of at least one object without physically contacting said object, wherein a fine light beam is deflected across said object in the direction of the dimension to be measured, the instants at which the light beam is blocked and unblocked by said object are recorded and stored as measurement data, and then said measurement data are evaluated to provide a measure of the said dimension by comparison of the period of time defined between said recorded instants with a predetermined reference period of time related to the speed of deflection of said light beam. Preferably the said light beam is a laser beam. The evaluation of the recorded data may be effected by a microprocessor with appropriate software.The possibility of a versatile evaluation can thus be achieved, wherein one or more periodic transitions of the light beam across the object can be utilised as a basis for each measurement cycle, as desired. If a rotating mirror is used for the deflection of the light beam, the time span between consecutive transitions of the light beam across a predetermined point, or the period of rotation of the mirror, can be used as a basis for the reference period. The mechanical definition of a reference dimension, for example by means of a window opening, is then unnecessary.

The invention is illustrated by way of example in the accompanying drawings, in which: Figure 1 is a diagrammatic view of a measuring device, Figure 2 is a circuit diagram of the device shown in Figure 1, Figure 3 is a diagram illustrating waveforms occurring in the circuit of Figure 2, and Figure 4 is a diagram explaining the measurement of a transparent object.

The device according to Figure 1 comprises an optical system which partly consists of known elements. A fine laser beam 2 is directed from a diagrammatically illustrated source 1 onto an octagonal mirror 3 driven at a constant rotary speed. The reflected laser beam passes through an objective comprising a lens 5 and thence passes across a measuring field which is defined by a shutter or a window with boundaries 6. The optical system is so arranged that, beyond the lens 5, the light beam constantly extends parallel to the optical axis, and is moved from the upper to the lower edge of the window 6-6 as the mirror 3 rotates in the clockwise direction indicated by the arrow in Figure 1. Each facet of the mirror 3 causes one transition of the light beam across the window to effect a corresponding measurement.

In the illustrated example an object 8 to be measured is arranged in the measuring region, and may, for example, be a cable, a wire, a pipe or the like, which runs transversely to the optical axis of the measuring device and of which the outer diameter is to be determined. Beyond the measuring position there is arranged a condenser lens 9 which focuses the laser beam onto a photocell 10. The photocell produces an output signal E as shown in Figure 3, which is O or low when the beam is screened by the window elements 6 or by the object 8 and which is I or high when the laser beam is unobstructed. As shown in Figure 3, two impulses occur periodically, the impulses beginning with the entry of the light beam into the window 6-6 and ending with its exit therefrom, and the gap between the two impulses corresponding to the screening of the beam by the object 8.The embodiment of Figures 1 and 2 comprises a digital evaluation circuit with a microprocessor. A single photocell 22 is arranged on one side of the window 6-6, namely the side from which the light beam enters the window.

As shown in Figure 2 the microprocessor 23 is connected by way of a data bus 24 and an address bus 25 with further circuit parts.

The input signal E from the photocell 10 is applied to a logic circuit 36 incorporating a differentiator which responds to the rising and falling flanks of impulses of the input signal, an address counter control arrangement and further control circuits. A signal from a transmitter 3" is also fed to this logic circuit, said signal being produced by a chopper rotating with the mirror 3 and consisting of a toothed disc 3' with 8 teeth corresponding to the 8 faces of the mirror, The output of an oscillator 30 is continuously applied to a counter 29 which is thus constantly incremented, and the output of the counter is applied to a memory 27 by way of a buffer latch 28. The oscillator operates, for example, at a frequency of 18 MHz and the counter 29 has a high counting capacity of, for example, 24 bits.

The transfer of the output from the counter to the memory 27 is controlled by the logic circuit 36 in a manner to be described below. The circuit further comprises an address counter 35 which is connected to the memory 27 and, by way of circuits 37 and 38, with the data bus 24 and the address bus 25.

A permanent read-only memory 31 defines the program routine and also serves for the storage of correcting data at defined addresses. A dynamic memory 32 serves the processor as a working memory for all data to be processed. An inputoutput unit 33 applies the measurement values to a display 34.

The control of the memory 32 is effected by way of an OR gate 39 either in the direct memory access mode by way of the logic circuit 36 or during the processing of measurement values by the microprocessor (CPU) 23.

Figure 3 shows the input signals, namely the measurement signal E, which, at the instant No corresponding to the entry of the light beam into the window 6, changes from 0 to I, then, during a first transition of the light beam across the object 8 corresponding to the period P1, returns to 0, and again changes to I, when the light beam illuminates the photocell 10 in its transition between the object 8 and the exit from the window 6. A plurality of measurement periods are repeated in this manner, eight such measurement periods producing the measurement impulses P1 to P8 and corresponding to one measurement cycle or one revolution of the mirror 3. Figure 3 also shows the signal C of the chopper as well as a signal C' transmitted from the logic circuit 36 to the microprocessor (CPU) 23 and formed by dividing the chopper signal by the factor of eight.

At each occurrence of an impulse flank in the signal E, corresponding to each blocking or unblocking of the light beam, the instantaneous count of the counter 29 is stored in the memory 27 and remains available there as a measuring value. Shortly after the occurrence of the flank C't after each cycle of eight measurements, the microprocessor is interrupted and the circuit continues to operate in the DMA (direct memory access) mode. All of the values in the memory 27 are thus transferred to the memory 32. When this transfer is complete, the logic circuit is switched to the measurement mode and the microprocessor 23 is caused to run the program.

Since the DMA-logic has direct access to both memories 27 and 32, the transfer of the values can be effected in a matter of milliseconds, and in the present embodiment it can be effected in the time space between two consecutive measurement cycles, as illustrated in Figure 3. As the new measuring cycle begins, and new data is red into the memory 23, the evaluation of the previously recorded data, in accordance with the program contained in the memory 31, is effected as a separate operation, and the result is then indicated on the display by way of the output circuit 33.

The program can be changed, and can thus be adapted to desired requirements, for example to enable the measurement of a plurality of objects located in the measuring field, in which case the evaluation can easily be so programmed that one or more time spans or periods can be ascertained, during which the light beam is screened by a corresponding object, and the dimension of the or each of these objects is ascertained in the previousiy described manner by comparison of each period, or of a sum of the periods, with the perod of the measurement cycle.

Figure 4 shows a possibility for the measurement of cylindrical, transparent objects, for example plastics tubes for surgical purposes.

The signal E comprises the rising flank No upon entry of the light beam into the window 6, as already described. When the light beam impinges upon the transparent object total reflection of the light beam occurs so that light no longer reaches the photocell 10 and a first falling flank N1 occurs in the signal. Especiaily with hollow objects, several phases of transmission and reflection of light then occur, which result in the occurrence of a corresponding plurality of signal flanks. At the instant Nx the light beam moves off the object. A special circuit in the logic element 36 is effective to ensure that the data for the instants No and N1 alike are transferred to the memory 27. The data for the following instants N2 to N(x-1) ) only reach the buffer latch 28 and are constantly updated in the buffer. That is to say that only the data from the last occurring instant is stored. Upon the occurrence of the falling flank of the chopper signal the last value which has been read into the buffer 28, which corresponds to the instant Nx, is transferred to the store 27 and remains available there for further processing.

In the preceding case it is assumed that one measurement is effected during each measurement cycle, that is to say that the dimension of the object to be ascertained can be derived from the following periods: P1+P2+...P8 D=K.

No'-No in other words that desired dimension D of the object can be derived from the relationship between the sum of the periods of transition of the light beam across the object and the period of one rotation of the mirror 3. A mean value of eight individual measurements can also be established. Thus the measurement is independent both of the rotary speed of the mirror and of any variation in its rotary speed, and any lack of geometrical accuracy of the mirror is of no effect, since a plurality of individual measurements are averaged. The continuous determination of individual measurement values can be achieved, however, as well as any other evaluation or additional mode of operation, by means of the microprocessor program. Single measurements can be ascertained and evaluated individually.It is thus possible to ascertain the difference between the maximum and a minimum measurement value and to eliminate a measurement when this increases the maximum value or reduces the minimum value by, for example, more than 10%. Measurements which have been determined during a cycle of measurement can furthermore be eliminated when the number of flanks occurring in the signal E during the cycle is not a whole multiple of eight.

A counter may be provided to register each measurement rejected as invalid as well as each valid measurement and thereby to allow a diagnosis of the measuring device. It is further possible to effect a linearisation in a simple manner. If the lens 5 is not corrected in such a manner that with a constant rotary speed of the mirror 3 a constant velocity of transition of the light beam through the measurement field between the edges of the window 6 is effected, then a correction or calibration is necessary. This calibration can be effected in a manner such that there are coordinated to all measurement values which exist in the memory 27 in the form of counted impulses of the oscillator 30, corresponding corrected values which are contained in a special memory. The provision of these coordinated counts or calibration values can be effected in various ways.The impulse counts corresponding to various positions within the measuring field can be determined experimentaily and stored. Preferably however a calibration or correction is effected on the basis of a known mathematical correspondence between the angle of incidence of the light beam on the lens 5 and the distance of the light beam within the measuring field from the optical axis. For a given optical system and a given deflection mirror there exists a given mathematical function which can be taken into a consideration. For similar lenses, to which the same function basically corresponds, only one valuation is necessary through the input of characteristic values or constants K to take into consideration the curvature of the function or the variation from a linear course. According to this mathematical relationship a calibration memory can be set up, or a calculating program can be effected by the microprocessor which continuously converts the determined measurement value in accordance with the mathematical relationship. This kind of calibration or correction on the basis of mathematical relationship can also be matched to existing apparatus in a simple manner during servicing. If, for example, an optical system has to be changed, the corresponding characteristic values can be fed in. In the case of the initially described, known arrangement with a calibration grid, a suitable calibration logic is necessary which is not integrated into the measuring apparatus and makes very much more difficult a calibration outside the factory.

Claims

1. A process for measuring a dimension of at least one object without physically contacting said object, wherein a fine light beam is deflected across said object in the direction of the dimension to be measured, the instants at which the light beam is blocked and unblocked by said object are recorded and stored as measurement data, and then said measurement data are evaluated to provide a measure of the said dimension by comparison of the period of time defined between said recorded instants with a predetermined reference period of time related to the speed of deflection of said light beam.

2. A process as claimed in Claim 1, wherein said light beam is a laser beam.

3. A process according to Claim 1 or 2, wherein the said instances recorded during a plurality of consecutive deflections of the light beam across the object to be measured are stored as measurement data and the evaluation of the corresponding measurement data is effected collectively.

4. A process as claimed in any one of Claims 1-3, wherein the deflection of the light beam is effected by means of a polygonal rotating mirror, and the time period between two consecutive transitions of the light beam through a defined position, or the period of rotation of the mirror, is used as a basis for the reference period.

5. A process according to any one of Claims 1 to 4 wherein the said evaluation is effected by means of a microprocessor.

6. A process according to any one of Claims 1 to 5, wherein individual measurement results which differ substantially from an ascertained mean value are rejected.

7. A process according to Claim 4 or Claim 5 or 6 as appended thereto, wherein those measuring results are rejected which yield a number of measurement data which is not a whole multiple of the number of mirror faces of the polygonal mirror.

8. A process according to Claim 6 or 7, wherein the number of rejected measurements is registered and utilised to check the operation of the apparatus.

9. A process according to any one of Claims 1 to 8, wherein a calibration or correction of the measurement is carried out on the basis of the mathematical correlation between the angle of incidence of the light beam upon an optical system for directing the light beam onto said object and the distance of the light beam from the optical axis of said system when emitted from said system towards the object.

10. A process according to Claim 9, wherein the said calibration or correction is effected continuously by means of a computer.

11. A process according to Claim 9, wherein calibration or correction data corresponding to the measurement data are stored and are read out of the store during the measurement.

12. A device for the contact free measurement of a dimension of at least one object, comprising a light source for providing a fine light beam, means for deflecting said light beam across an object to be measured in the direction of the said dimension which is to be measured, means for determining the instants of time at which said light beam respectively impinges upon and moves off said object, means for dynamically storing measurement data corresponding respectively to said instants of time, means for calculating from said measurement data and from a reference value corresponding to the rate of deflection of said light beam the value of said dimension, determined in accordance with the time difference between said instants of time, and means for displaying the resulting measurement value.

13. A device according to Claim 12, wherein the said means for deflecting said light beam comprises a rotary polygonal mirror, and said device further comprises means for producing an output signal corresponding to the periodic deflection of the light beam by said mirror, the said means for calculating the value of said dimension being arranged to respond to said output signal to provide the said reference value.

14. A device according to Claim 13, comprising a computer controlled system including a central processor unit, a first memory for receiving said measurement data during each of a plurality of consecutive measurement cycles, and a second memory for receiving data from said first memory, the arrangement being such that between each two consecutive measurement cycles the system operates in the DMA (direct memory access) mode to transfer data from the first memory to the second memory, and that during each measurement cycle the data contained in the second memory is processed by said central processor unit whilst the data in the first memory is being updated.

15. A device as claimed in Claim 14, including a logic circuit arranged to respond to the said output signal corresponding to the periodic deflection of the light beam and to control in accordance with said signal the cycle of storage and processing of measurement data.

16. A device as claimed in Claim 15, wherein said means for producing said output signal comprises a chopper device arranged to cooperate with said rotary mirror.

1 7. A process for the contact free measurement of a dimension of at least one object substantially as described herein with reference to the accompanying drawings.

18. A device for the contact free measurement of a dimension of at least one object substantially as described herein with reference to the accompanying drawings.

1 9. The features as herein described, or their equivalents, in any novel selection.