GB2143700A

GB2143700A - Object location

Info

Publication number: GB2143700A
Application number: GB08418300A
Authority: GB
Inventors: Hans-Gunter Woschni; Ralf Christoph; Andreas Reinsch
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 1983-07-20
Filing date: 1984-07-18
Publication date: 1985-02-13
Also published as: DD217621A1; GB8418300D0; DE3422689A1; GB2143700B

Abstract

A body is located using an array of photo-receivers which allows the position of lines or edges of the body to be determined. Analog processing reduces the expense and measurement time of prior art digital devices. Individual signal amplitudes read from an array of photo-receivers 1 are processed using delay elements 6, integrators 5,7, and a timing circuit 9, and positional information can be obtained from the time measurement. The location bodies can be realized with an accuracy considerably better than that normally dictated by the spacing of the receivers of the array by means of interpolation. <IMAGE>

Description

SPECIFICATION Apparatus for the photometric detection of the location of a body with a photo-receiver array The present invention resides in an apparatus for the photometric detection of the location of a body with a photo-receiver array.

The objective determination of the positions of optically effective bodies is of constantly increasing importance in technology, particularly in precision measuring technology. The location of optically effective bodies is generally ascertained with the aid of travelling microscopes together with a linear measuring system. One thereby orientates oneself to the silhouette of the body produced by optical imaging. The interference line method (Jenaer Rundschau 1976/4, pages 201 to 203) can be used for objectification of the sensing of body edges by optical methods.

In addition to ever increasing demands on the accuracy of measurement, minimum measuring times are of increasing interest. This constitutes a field of application of very rapid, highly accurate measuring apparatus for the photometric detection of the location of a body by digital or analog methods.

In addition to optical-visual, subjective methods, that is, comparison of an optical mark with a standard scale, apparata are known which realize objectively operating, electro-optical principles, such as single-cell, twin-cell, chopper and scanning methods (Lit. Woschni, H.-G., Wiss. Zeitschrift of FSU Jena, Mat.-Nat. Series (28) 1979, Issue 1). If high accuracy is required, it can be achieved by zero indicators. A scale is additionally required in order to be able to perform linear measurement over large ranges. The aforesaid photo-electric microscopes can serve for interpolation within the graduations of the scale. Furthermore, it is known that photo receiver arrays, such as CCD scanning devices, are used as the interpolation system and at the same time serve as measuring means.The localization capacity (resolution) obtainable with such arrangements exceeds the limit determined by the centre-to-centre distances between the individual receivers of the array. Patent Application No. WP G 01 B 234313/6 of the German Democratic Republic proposes a method in which the photoreceiver signals, which are proportional to the image exposure intensities from the body, appear serially at the output of the array, are digitalized by an analog-to-digital converter (ADC), and are fed to a digital computer (microcomputer) for further processing, such as, correction of the sensitivity and calculation of the measured positional value.

The disadvantages of this method are the high electronic expense and the relatively long measuring time occasioned by the conversion time of the ADC and the computing time.

It is an object of the present invention to provide an apparatus which enables interpolation between the centre-to-centre distances between the individual receivers of an array of photo-receivers, such that the use of an ADC and a digital computer is avoided. Hence, the measuring time and the electronic expense can be reduced.

In accordance with the present invention there is provided an apparatus for the photometric measurement of the position of a body, having a photoreceiver array onto which the body is imaged, and delay elements and integrators for processing the individual signal amplitudes, output from the array, for use in determining the position of the image of the body on the array.

In order to avoid the use of an ADC and digital computers, the photometric position of the image of the body on the array is determined with the use of delay elements and integrators. The determination of the position of the line or edge of an optically effective body can be attributed to the integral definition of the photometric centre (the above mentioned WP G 01 B 234 313/6). Signals proportional to the surface integral of the exposure over the corresponding photo element are produced in the array of photo-receivers. During the serial output of the array, signals appear succes lively at the output and their total corresponds to the integral of the intensity of illumination of the image of the body, on the array.Since the individual elements of the array can have differing sensitivities, it is necessary to correct the signals when a high degree of accuracy is required.

In the case of line localization, those possibly corrected signal amplitudes from the individual receivers which are relevant to the body and which are output serially from the array are electrically integrated. Half of this total integral is compared with the output of a second integrator by means of a comparator. This second integrator receives the same signals, although delayed by at least the output time for the entire sensing device. This is realized by means of a delay element, such as a delay line or an analog shift register. The time which elapses from the appearance of the first receiver signals at the output of the delay element to the coincidence of half the total integral with the integral of the second integrator is indicative of the position of the line or edge of the body. The more accurately is the time measurement effected, the greater is the resolution.Instead of measuring the time, the integration with the associated individual signal amplitude can be concluded after the input signals to the comparator become equal, and, by forming the difference between the two values, the input signals to the comparator, and standardization by analog electronic methods, it is possible to ascertain that fraction of the pixel spacing by which the photometric centre lies from the edge of the last receiver of the array to be taken into account, by the second integrator. Hence, it is possible to achieve a sensitivity which falls considerably below the magnitude of the centrecentre distance between the individual receivers of the array used.

In order to minimise errors of measurement as a result of stochastic uncertainities of the individual signal amplitudes and any background brightness existing, only the signals associated with the body are evaluated. The body boundaries can be determined by differentiation of the step-shaped signal, formed by the individual signal amplitudes, by means of a differentiation element and comparison of the output signals obtained with an adjustable desired value, or by direct comparison of the threshold value of the output signal amplitudes by means of comparators. When using the interference line method, an interference line is produced which lies at a defined distance from an edge of the body. This interference line can be evaluated in an analogous manner by the present invention for the purpose of line localization.

In order to determine the position of an edge by the silhouette method, it is necessary to add up electrically the possibly corrected signal amplitudes which are associated with the body and which are output serially from the array of photoreceivers. Integration is effected by means of a second integrator by way of a stored maximum individual signal amplitude until this integral is equal to the total integral of the first integrator. Comparison is again effected by means of comparators.

The difference between the time which elapses between output of the first receiver signal and the termination of integration by the first integrator and the time which elapses between this termination of integration and the appearance of the coincidence signal of the comparator is indicative of the position of the edge of the body. Instead of time measurement, formation of the difference of summed signal amplitudes can be used for interpolation in the same manner as in the case of a line on the body. Here also, falsification of the measurement information is to be reduced by evaluating only those signal amplitudes of the individual receivers which are relevant to the body. This is possible in a similar manner to that in which the position of the line is detected.The accuracy of measurement is again dependent upon the accuracy of time measurement and can be increased by way of the resolution determined by the centre-tocentre distances between the individual receivers.

The invention will now be described further hereinafter, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a block circuit diagram of an apparatus for determining the position of a body in accordance with the invention., and Figure 2 shows a block circuit diagram of one embodiment of an apparatus for determining the location of an edge of a structure.

Referring to Figure 1, the signal amplitudes, output from the individual photo receivers of a CCD scanning device 1 and representing images sensed in the CCD scanning device 1 are corrected by means of a sensitivity correction circuit 2 and, are then fed to a circuit 3, for comparison with a threshold value, an integrator 5 and an analog shift register 6. The comparator 3 applies a signal to the integrator 5 and causes integration of only those receiver signals which are associated with the body under observation. For the same purpose, a second integrator 7 receives the threshold value signal, delayed with respect to time, by way of a control logic circuit 4. A time-measuring device 9 starts counting upon commencement of the output of the array and is interrupted by the presence of the leading edge of the body.The counter 9 starts counting again when the second integrator 7 commences with the integration. The time-measurement is terminated when the input signals to a comparator 8 become equal, the input from the integrator 5 being halved by a potential divider consisting of two resistors of equal value R before being applied to the comparator 8. The total time thus measured is indicative of the position of the body. All the sub-assemblies of the described apparatus are synchronized by the control logic circuit 4.

An embodiment for determining the location of the edge is shown in Figure 2. The signal amplitudes output serially by the CCD scanning device 1 are again applied to the threshold value comparison circuit 3 by way of the correction circuit 2. In conformity with the positional body limits thereby determined, the individual signal amplitudes, indicative of the body position, are integrated in the integrator 5. A maximum individual receiver signal is stored simultaneously by the shift register 6. After integration in the integrator 5 is terminated, the signal stored in the shift register is integrated in the second integrator 7 until the input signals to the comparator 8, from the two integrators, are of equal amplitude. Time measurement commences with the beginning of the output of the individual signal amplitudes from the CCD scanning device 1.

The counter, or time-measuring device 9, is thereby started. The direction of counting is reversed upon the commencement of integration by the second integrator 7. The counter counts backwards until the comparator signal appears. The reading of the counter corresponds to the time-difference which is sought and which is indicative of the position of the body. All the sub-assemblies are synchronized by the control logic circuit 4.

Claims

1. An apparatus for the photometric measurement of the position of a body, having a photo-receiver array onto which the body is imaged, and delay elements and integrators for processing the individual signal amplitudes, output from the array, for use in determining the position of the image of the body on the array.

2. An apparatus as claimed in claim 1, in which the signal amplitudes of the individual receivers of the array which pertain to the body, and which are output serially from the photo-receiver array and which are corrected if necessary, are integrated for detecting the position of a line mark, and half of the total integral signal from a first integrator is compared by means of a comparator with the output of a second integrator which receives the same signals with a time delay, until equality exists between the signals from the integrators, the measured time which elapses during the comparison being indicative of the position of the image of the body on the receiver array.

3. An apparatus as claimed in claim 1 for determining the position of an edge of the body by the silhouette method, in which the signal amplitudes of the individual receivers which pertain to the body, and which are output serially from the photo-receiver array and which are corrected if necessary, are integrated by a first integrator, and the output signal of this integrator is compared by means of a comparator with the output of a second integrator which integrates, by way of a stored maximum individual signal amplitude of the body, until equality exists between the signals from the integrators, the difference between the time required for integration of the first integral and the time required for realizing the comparison being indicative of the position of the structure.

4. An apparatus as claimed in any of claims 1 to 3, in which, instead of the measurement of time, integration with the associated individual signal amplitude is concluded after the input signals to the comparator become equal and, by difference formation of the two values of the said input signals, and standardization by analog electronic methods, that fraction of the pixel spacing is determined by which the photometric centre is spaced from the edge of the last receiver of the array taken into account.

5. An apparatus as claimed in any of claims 1 to 4, in which the boundaries of the optically effective body are determined by differentiation of stepshaped signals, formed by the individual signal amplitudes, by means of a differentiation element and comparing the output signals obtained with an adjustable desired value by means of a comparator.

6. An apparatus as claimed in any of claims 1 to 4, in which the position of the body is determined by threshold value comparison by means of a comparator from the individual signal amplitudes which are output from the array.

7. An apparatus as claimed in any of claims 1 to 6, in which the determination of the position of optically effective bodies can be realized with a sensitivity which falls considerably below the centrecentre distances between the individual receivers of the array used.

8. An apparatus as claimed in any of claims 1 to 7, in which measurement is effected in an analogous manner by the interference line method.

9. An apparatus as claimed in claims 1 to 8, in which transit time chains or analog shift registers are used as delay elements.

10. An apparatus for the photometric measurement of the location of a structure substantially as hereindescribed with reference to the accompanying drawings.