EP0328611A1 - Appareil de traitement de signaux - Google Patents

Appareil de traitement de signaux

Info

Publication number
EP0328611A1
EP0328611A1 EP19880907353 EP88907353A EP0328611A1 EP 0328611 A1 EP0328611 A1 EP 0328611A1 EP 19880907353 EP19880907353 EP 19880907353 EP 88907353 A EP88907353 A EP 88907353A EP 0328611 A1 EP0328611 A1 EP 0328611A1
Authority
EP
European Patent Office
Prior art keywords
signals
look
processing apparatus
tables
signal processing
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP19880907353
Other languages
German (de)
English (en)
Inventor
Mark Adrian Vincent Chapman
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Renishaw PLC
Original Assignee
Renishaw PLC
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Renishaw PLC filed Critical Renishaw PLC
Publication of EP0328611A1 publication Critical patent/EP0328611A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D3/00—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups
    • G01D3/02—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups with provision for altering or correcting the law of variation
    • G01D3/022—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups with provision for altering or correcting the law of variation having an ideal characteristic, map or correction data stored in a digital memory
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02055—Reduction or prevention of errors; Testing; Calibration
    • G01B9/02075—Reduction or prevention of errors; Testing; Calibration of particular errors
    • G01B9/02078—Caused by ambiguity
    • G01B9/02079—Quadrature detection, i.e. detecting relatively phase-shifted signals
    • G01B9/02081—Quadrature detection, i.e. detecting relatively phase-shifted signals simultaneous quadrature detection, e.g. by spatial phase shifting
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02083—Interferometers characterised by particular signal processing and presentation

Definitions

  • the present invention relates to signal processing apparatus.
  • the invention has particular but not exclusive relevance to apparatus which can be used for extracting information from a plurality of a.c. outputs of a detector system.
  • a problem which occurs in a laser interferometer system is that if the laser beam is broken for any reason while the machine on which a measuring operation is being performed is moving, the interferometer counters will miss one or more counts, so that when the beam is restored, all subsequent readings will be in error. Therefore it is essential to be able to determine, effectively on a continuous basis, the signal strength coming from the detector system for the interferometer.
  • the detector signals may be in the form of a plurality of a.c. signals in phase quadrature.
  • circuits can only monitor relatively slowly changing amplitudes depending upon the response time of the circuit. These circuits therefore are not suitable for high speed monitoring of signal strength such as to provide effectively continuous monitoring of signal strength as required in a laser interferometer. Another disadvantage of such known circuits is that they do not work at all if the frequency of the signal being monitored drops to zero.
  • Another requirement of a laser interferometer is to be able to interpolate between the fringe counts provided by the counters in order to increase the resolution of the in erferometer.
  • the present invention as defined in any one or more of the appended claims, in one aspect allows relatively high speed monitoring of the strength of the a.c. outputs of a detector sytem, and in another aspect allows high speed interpolation of such output signals.
  • the term "relatively high speed” as used throughout this specification should be understood to mean capable of processing several million readings per second so as to provide effectively continuous processing.
  • the invention as defined in one or more of the appended claims provides means for the high speed determination of variations of signal strength, and simultaneously for high speed interpolation between fringe counts provided by the counters linked to the detector system of a laser interferometer. By this means it becomes possible not only to know that the laser beam has been broken, so that all subsequent readings will be erroneous, but also at precisely which reading the beam was broken.
  • the invention may however, be used with a variety of detector systems which produce electrical signals in response to variations of a parameter being measured, and is not limited to a laser interferometer as described above.
  • the invention may be used in other optical or magnetic measuring systems.
  • the invention limited to the type of information to be extracted from the detector signals, (eg interpolation, or signal strength monitoring) and may for example, depending on the nature of the detector outputs, provide means for determining other parameters of the signals.
  • Fig. 1 is a block diagram of the main components of the signal processing apparatus of the present invention.
  • Fig. 2 is an input/output diagram for the converter.
  • Fig. 3 is a more detailed block diagram of the signal processing apparatus shown in Fig. 1 including some additonal components specifically for use in a laser interferometer embodiment.
  • the invention as illustrated in Figs. 1 and 2 is described with reference to the requirements of a laser interferometer but it is to be understood that the invention in its broadest aspects is not limited as to the origin of the signals being processed or the information to be obtained therefrom.
  • FIG. 1 three photo-diode detectors A,B and C are shown which form part of a detector system for a laser interferometer which may be of known type and the constructional details of which are not described in detail.
  • the three detectors in known manner, produce a.c. voltage signals in quadrature from the interference fringes in the light beams falling on them.
  • the a.c. signals will include a d.c. level, and this is removed by subtraction devices SI and S2, known per se, to produce two quadrature a.c. voltage signals (referred to as the sine and cosine signals) of amplitude A with no d.c. offset.
  • the two a.c. signals are respectively of the form A Sin ⁇ and A Cos ⁇ , where A is the amplitude of the signal and i a measure of the beam strength.
  • the invention provides two flash analogue to digital converters AD1 and AD2 which respectively receive the sine and cosine voltage signals and convert them to digital form, to produce binary numbers representing the instantaneous voltages of the signals.
  • the range of the converter outputs depends on the resolution of the converter used, so that, in the present example where a 7-bit conversion is used, the converter output is an integer in the range 0 to 127. As can be seen in Fig. 2 the arrangement is such that for an input to the converter of -V volts the output is 0, and for an input of +V the output is 127.
  • ⁇ or k can be determined from the values of x and y which are the outputs of the converters, k giving a measure of beam strength and ⁇ being the phase angle of the signals from the detectors.
  • One problem which can be solved using the invention is that of high speed monitoring of the amplitude of the interference signal received by the detectors in an interferometer.
  • Detector signal strength monitoring is also useful for example, in setting up the laser interferometer when the interferometer is stationary prior to taking measurements, where the correct alignment of the laser is indicated by -6- detection of the maximum signal strength on the detector system.
  • the conventional circuits for measuring signal amplitude mentioned earlier are not fast enough for high speed monitoring and thus cannot detect very short duration breaks of the laser beam, such as occur for example if a chip from an operating machine cuts the laser beam.
  • the conventional methods cannot determine changes in signal strength when the interferometer parts are stationary, since the signal frequency falls to zero and each detector outputs a constant, but not necessarily equal voltage at that time.
  • the detectors are receiving parts of a combined beam in which certain levels of constructive and destructive interference are present, there will still be a " constant phase difference between the detector outputs.
  • the invention makes use of this fact and provides an EPROM look-up table LUT into which the values of k have previously been programmed using the expression;
  • the look-up table consists of an array of data which has previously been calculated, and can be visualised as a two di ensional table having rows and columns for values of x and y respectively, with the value of k for any combination of values of x and y being shown in the square at the intersection of the respective row and column for x and y.
  • the output of the table is the information which has been previously programmed into each square of the table.
  • the x and y outputs of the two converters AD1 and AD2 are passed to the address lines of the look-up table from which the value of k is read at sufficiently high speed to enable effectively continuous monitoring of the signal strength to take place.
  • the look-up table includes a standard memory which produces an 8-bit output which is passed to a micro-processor MP which is programmed to produce information in the required form from the output of the look-up table.
  • 8 bits need to be encoded with the signal strength, some bits can be programmed to flag fault conditions, such as signal strength low, high or lost altogether. These flags are included in the information contained in the 8-bit information code as the look up table locations are programmed and are set according to the value of k at that location. For example, if the signal strength falls to zero then both k sin ⁇ and k cos ⁇ become zero so that both x and y are 63.5. The appropriate squares in the look-up table are therefore programmed so that five of the bits show a signal strength code of zero, the signal low, and signal lost bits are coded with a 1 or zero whichever is chosen to indicate a fault condition, and the signal high is coded oppositely to indicate a normal condition.
  • the value of k can be determined independently of changes in the value of ⁇ in the two signals k sin ⁇ and k cos ⁇ , and therefore enables any changes in the value of k to be determined even when the interferometer is stationary and the value of ⁇ is constant, i.e. when the frequency of the two signals drops to 0.
  • the value of ⁇ derived from the signals of the photo-diode detectors is a measure of the parameter being measured by the interferometer, e.g. distance moved by a moving element of a machine.
  • ⁇ changing from 0° to 360° usually represents a movement of the moving element of the machine of one half of the wavelength of the light used in the interferometer. This assumes that the light beam from the laser of the interferometer traverses the distance to the moving element of the machine twice, i.e. there and back, so that the optical path length change is twice the movement of the machine element.
  • the values of ⁇ are pre-programmed into the look-up table and by using the x and y outputs of the converters to address the rows and columns of the look-up table, the corresponding values of ⁇ can be read from the table at high speed to provide the required interpolation of the distance moved by the machine part.
  • This has the advantage of operating at much higher speed than a micro-processor and thus allows the apparatus to operate, for example, in a high speed, very high resolution servo loop position control sytem.
  • the resolution of the converter and the size of the look-up table need not be same for signal strength monitoring as it is for interpolation.
  • the choice of converter resolution and the size of the look-up table will depend on the amount of information required.
  • the speed with which the apparatus of the present invention provides information can be used beneficially in connection with the beam strength monitoring embodiment to provide automatic gain control for the signals at the subtraction devices SI and S2 to boost or reduce the signal level as appropriate.
  • Fig. 3 shows a more detailed embodiment of the invention in which additional refinements are included. Those parts of this embodiment which are identical to the parts shown in Fig. 1 are given the same reference numerals.
  • the detectors A, B and C are photo-diode detectors which are arranged to receive differently polarised parts of the interfering combined beam entering the detector system.
  • the arrangment may, for example, be as disclosed and claimed in our co-pending UK Patent Application No. 8718803.
  • each detector A, B and C is exposed only to light passing through a polaroid immediately in front of it, the polarisation states of which are respectively at 0°, 45° and 90° relative to one another.
  • the signals produced by the detectors A, B and C are sine waves in quadrature having the characteristics:
  • the subtractors SI and S2 From these three signals the subtractors SI and S2 produce the two sine and cosine signals A Sin ⁇ and A Cos ⁇ which form the inputs to the converters AD1 and AD2.
  • Table LUT1 is a composite having two parts, one of which PI, contains values of the a.c. signal strength of the detectors A, B and C and the other one of which P2, contains values of the d.c. levels in detectors A and C.
  • the output from table LUT1 is switched as described later to provide either of these values.
  • the BUS interfaces perform the function of simultaneously latching the instantaneous readings of the two look-up tables so that when addressed sequentially by the micro-processor, the values of the signal strength and the interpolated distance reading can be provided for the same instant in time.
  • This combined use of two look-up tables addressed from the same two digitised sine and cosine signals enables the interpolation angle ⁇ and the signal strength, to be simultaneously evaluated.
  • Figure 3 also shows electronic monitoring of one of the output bits from the signal strength look-up table as previously described. This bit has been programmed to flag signal loss. When momentary signal loss occurs a pulse is generated on signal line LI which triggers a flip flop device F, which then changes state to indicate that a momentary loss of signal has occured.
  • the output from flip flop F is periodically read by the micro-processor unit via bus interface Bl, thus allowing the detection of beam breakages of much shorter duration than would be possible using the MPU alone.
  • the flip flop F can be reset by the MPU using line L2. Gating circuitry (not shown) ensures that the flip flop is not triggered during the transition between successive data valid output states from the look-up table.
  • any variation in signal strength can be determined even when the interferometer is stationary, which assists in setting up the interferometer components.
  • the component being aligned does not involve the interferometer but may be a simple reflector which reflects the laser beam onto the detector.
  • there will be no a.c. interference signal to provide a phase difference in the detector signals so that subtracting signal B from signals A and C in subtractors SI and S2 respectively, could eliminate the d.c. level as well and leave no signal.
  • Another complication arises where the laser beam sends out polarised light, in that different optical components have different effects on the polarisation.
  • looking at the beam strength hitting a single detector behind a polaroid would provide a variety of signal strengths, even down to zero, if orientation of the polaroid did not match that of the returning light beam.
  • the values programmed into the appropriate look-up table may then be the sum of the two detector signals divided by two, or the square root of the sum of the squares of the signals.
  • the apparatus is modified by the introduction of a switch Tl in the line from the detector B, which removes the signal from detector B from the inputs to the subtracters SI and S2.
  • the converters receive signals relating to the d.c. levels of detectors A and C produced from the laser beam.
  • a second switch T2 is operated by the micro-processor unit to cause the address lines from the converter AD1 to address the second part P2 of the look-up table LUT1, in which is stored pre-programmed beam strength data.
  • the invention also provides the capability to monitor beam strength of a non-interfering beam, which capability is of value in setting up certain parts of the apparatus without requiring the use of the interferometer component.
  • a further feature which enhances the usefulness of the interferometer is the use of one of the bits from each of the converters AD1 and AD2 to drive an up/down direction decoder and fringe counting circuit (not shown) .
  • the top bits of the two converter outputs change state as the sine and cosine signals pass through zero, and thus they can be used as two logic input signals in quadrature for the decoder and counter.
  • these two logic signals in quadrature from the converters can be passed to a machine interface in place of the signals from the machine scale readers thus allowing the laser interferometer to be used as a measurement scale for the machine giving a basic resolution of ⁇ 8.
  • the resolution can be improved by re-programming the look-up table to include the codes needed to generate higher resolution quadrature signals and passing these to the machine controller.
  • the look-up table could be programmed to include grey codes if required by a particular machine controller.
  • the above-described method of improving resolution by programming codes into the look-up table can be adopted in apparatus other than a laser interferometer system.
  • the figures stored in the look-up tables may be corrected for errors whereby corrected figures can be output directly.
  • Errors may arise, for example, from the characteristics of the optical components not being exact, non-linearity of the photo-diode detectors, or the detector signals not being exactly in quadrature.
  • the errors will however be constant for any given combination of detectors and optical devices so that once measured they can be allowed for.
  • the errors can be determined either by physical calibration of the system or by analysis of the Lissajous figure distortion observed by plotting k sin ⁇ against k cos ⁇ .
  • the data to be stored in the look-up table is then calculated using corrected values provided from the error analysis.
  • the error data derived by analysis of the lissajous figure can be used by the micro-processor to dynamically correct values read from the look-up table using an algorithm which is updated according to the distortion of the lissajous figure at any given time.
  • the invention provides a very rapid and highly flexible system for processing signals which is significantly faster than a micro-processor alone.
  • the system can deal with massive flows of information giving what is effectively continuous monitoring of the information.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Technology Law (AREA)
  • Instruments For Measurement Of Length By Optical Means (AREA)
  • Length Measuring Devices By Optical Means (AREA)

Abstract

Appareil de traitement de signaux permettant le traitement d'une pluralité de signaux analogues provenant des détecteurs, par exemple, d'un système d'interféromètre à laser. Comme on peut le voir sur la Fig. 3, l'appareil utilise un ou plusieurs convertisseurs analogiques/numériques (AD1, AD2) pour numériser les signaux, ainsi qu'une ou plusieurs table(s) à consulter (LUT1, LUT2) dans lesquelles sont programmées des informations telles que des données d'intensité de signaux et des données de positions interpolées. On fait passer les signaux numérisés provenant des convertisseurs, aux adresses d'entrée des deux tables à consulter, qui produisent l'intensité des signaux et les données de positions correspondants extrêmement rapidement. Les sorties des tables à consulter sont verrouillées et ensuite lues par un microprocesseur qui fournit les informations requises. Le système est très souple et extrêmement rapide, permettant ainsi une commande en continu effective des signaux des détecteurs.
EP19880907353 1987-08-22 1988-08-19 Appareil de traitement de signaux Withdrawn EP0328611A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB8719878A GB8719878D0 (en) 1987-08-22 1987-08-22 Signal processing apparatus
GB8719878 1987-08-22

Publications (1)

Publication Number Publication Date
EP0328611A1 true EP0328611A1 (fr) 1989-08-23

Family

ID=10622674

Family Applications (1)

Application Number Title Priority Date Filing Date
EP19880907353 Withdrawn EP0328611A1 (fr) 1987-08-22 1988-08-19 Appareil de traitement de signaux

Country Status (4)

Country Link
EP (1) EP0328611A1 (fr)
JP (1) JPH02500218A (fr)
GB (1) GB8719878D0 (fr)
WO (1) WO1989002062A1 (fr)

Families Citing this family (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0455984A1 (fr) * 1990-05-08 1991-11-13 THE WARNER & SWASEY COMPANY Machine de mesure de coordonnÀ©es avec circuit d'interpolation amélioré
EP0936446A1 (fr) * 1998-02-10 1999-08-18 Primax Electronics Ltd Système pour déterminer le déplacement
US7990542B2 (en) * 2007-10-25 2011-08-02 Mitsubishi Electric Research Laboratories, Inc. Memory-based high-speed interferometer
JP2014013209A (ja) * 2012-07-05 2014-01-23 Keihin Corp 角度検出装置
US10151962B2 (en) 2016-09-29 2018-12-11 Mitutoyo Corporation Variable focal length lens system with focus monitoring and control

Family Cites Families (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2824472C3 (de) * 1978-06-03 1981-08-06 Volkswagenwerk Ag, 3180 Wolfsburg Verfahren und Anordnung zum Betrieb einer Brennkraftmaschine mit Fremdzündung
JPS5751921A (en) * 1980-09-16 1982-03-27 Honda Motor Co Ltd Fuel controller for internal combustion engine
EP0147481B1 (fr) * 1983-12-27 1987-09-02 Ibm Deutschland Gmbh Interféromètre à lumière blanche
JPS61104220A (ja) * 1984-10-26 1986-05-22 Fujitsu Ltd 物体移動検出器の故障検出回路
GB2174510B (en) * 1985-04-03 1987-07-29 Paul Telco Heating system controller

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO8902062A1 *

Also Published As

Publication number Publication date
GB8719878D0 (en) 1987-09-30
JPH02500218A (ja) 1990-01-25
WO1989002062A1 (fr) 1989-03-09

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