Detector head

Abstract: A detector head for use in a measuring device includes a plurality of constituent measuring elements arranged in at least two blocks, each block producing an output signal of equal amplitude, and phase shifting means provided to shift the relative phases of the output signals of the blocks to have a predetermined relative phase-difference, whereby selected higher-order harmonic components are cancelled from the resultant signal formed by summing the signals produced by the separate blocks of constituent elements.

Description: This invention relates to a detector head for obtaining scale signals from a measuring device using a magnetic or optical scaling device and, more particularly, to improvements in removing distortion in the scaling detector signals and for ameliorating the precision of the measuring device. As is known, there are measurement systems of various types, such as magnetic, optical, magnetic induction, and capacitance. All of these systems detect variations in magnetic flux, optical intensity, induced voltage, or electrostatic capacitance, corresponding to some displacement and transform the variations into electric signals by using a reference scale having a periodically varying pattern and a pickup head that undergoes the displacement. In order to increase the resolving power, the detector output of these systems is often divided into a plurality of portions that are much smaller than the period of the pattern in the reference scale, so that precise positions of the pickup head can be obtained by the electrical division, that is, by interpolation. For this purpose, it is desirable that the waveform of the detector output be approximately sinusoidal, with distortion of the waveform being kept at a minimum. In the case where the waveform of the detector output is distorted, even if the electrical division (interpolation) possible, high precision measurements cannot be obtained. Moreover, in the optical or capacitance kinds of systems, because the waveform of the detector output, which corresponds to variations in optical intensity or capacitance, is trapezoidal or triangular with respect to the mechanical displacement, it contains harmonic components, principally third order harmonic components. This trapezoidal waveform can be represented by: f₁(x) = 4 π a b 1 1² (sin b . sin x + 1 3² sin3b . sin3x ...) and the triangular waveform by: f₂(x) = 2a π² ( sin x 1² - sin 3x 3² + sin 5x 5² ... ) The distortion factor can be calculated by using the following formula: Distorsion Factor = Σ (harmonic component)² fundamental component Thus, it can be seen that the major part of the distortion that is present can be attributed to third order harmonic components. The above-described situation exists for a magnetic head, for example, in a phase-modulation type scaling device, in which a magnetic grating on a magnetic scale is read out by a multiple-gap head responsive to magnetic flux, as disclosed in Japanese provisional publication No. 137812/82. In such scaling device, scaling signals detected by a conventional multiple-gap head responsive to magnetic flux contain third order harmonic components in addition to the fundamental components, and this gives rise to the above-described drawbacks because interpolation errors are provoked by distortions due to these third order harmonic components. The present invention is based upon a study of the causes of these errors. In order to ameliorate the resolving power required for a precision measuring system or a machine tool using the above-described scaling device, it is necessary to electrically interpolate for a distance equal to the pitch of the grating, however, for various reasons, in the course of this interpolation, interpolation errors are produced. The principal reasons are variations in the distance between the heads of the individual channels, direct current deviations in the reproduction output signal levels, variations in the amplitudes of the reproduced output signals, and distortions due to the third order harmonic components in the reproduction output. GB-A-2034053 discloses a magnetoresistive displacement sensor which employs a plurality of magnetic sensing elements to produce a signal in response to linear displacement with respect to a magnetic grating. The phases of the elements are relatively displaced by an amount which allows determination of the direction of displacement and also an increase in accuracy by interpolation. US-A-3427463 discloses an optical device for measuring relative displacement which comprises a stationary receiver screen having at least two screen elements with spaces and bars, the two being relatively displaced and also a reciprocable emitter screen system having bars and spaces in predetermined relationships to the bars and spaces in the stationary receiver system. Light from an illuminating system passes through the two screens and is detected by photo-detectors and the signal is suitable for interpolation. In one embodiment the two screens can be inclined to each other to eliminate additional harmonics. In order to realize a high precision measurement, the interpolation errors should be suppressed and all errors, except for those due to the third order harmonic components, can be alleviated by electrical regulation. Distortion due to third order harmonic components are provoked in the electromagnetic transformation system by various causes, such as noise in the electrical circuit and scale distortions, to name only a few, but the errors due to scale distortions are the most important. Consequently, interpolation errors can be reduced significantly by removing these scale distortions. The interpolation errors due to third-order harmonic components can be calculated as follows. Supposing that the third order harmonic wave is c₃ times as large as the fundamental wave, then the output voltage e of a head can be represented by: e₁ = sin ω₀t {cos 2π λ x +C₃. cos 2π λ 3x} e₂ = cos ω₀t {sin 2π λ x - c₃.sin 2π λ 3x} By assuming that ω₀t = T and 2π λ x = x, the following equation can be obtained: e = e₁ - e₂ = sin T {cos x + C₃.cos 3X} - cos T {sin X-C₃.sin 3X} Assuming that C₃ = O, equation (6) above can be simplified to: e = sin (T - X) Consequently, when T = X, e = 0, that is, X can be obtained by detecting the phase difference between the voltage represented by equation (7) and the reference wave e = sin T. Putting T = X + ▴X, equation (6) can be transformed to: e = sin(X + ▴X){cosX+C₃cos 3X} - cos(X+▴X) {sinX-C₃sin3X} Assuming that e = 0, ▴X remains an interpolation error and can be calculated as follows: ▴X = tan ⁻¹{- sinX-C₃.sin 3X cosX+C₃.cos 3X } - X Accordingly, it is an object of this invention to provide a detector head in which deleterious harmonic waves are removed from the output, specifically, harmonic waves of the third order, which can cause distortions in the scaling detector signals or the output waves with respect to displacement of a transducer. According to the present invention there is provided a detector head for use in detecting dimensions of an object being measured, comprising: a plurality of sensor means for use in conjunction with a scale having a predetermined pitch, each of said sensor means producing an electrical signal having a relative phase-difference therebetween determined in accordance with the pitch of the scale and together indicating the dimensions of the object with greater resolution than would be possible from any one of said sensor means; each of said sensor means comprising conversion means for producing one of said electrical signals and a plurality of constituent elements arranged in a pattern; said pattern being formed of at least two blocks of the constituent elements, each of said at least two blocks producing an intermediate signal of equal amplitude including higher harmonic components cancellable by combination of signals and characterised by having the blocks relatively positioned to produce a relative phase-difference corresponding to one sixth of the effective pitch of the scale, wherein the intermediate signals from said at least two blocks of constituent elements are combined within the respective sensor means to cancel the third-order harmonic components such that said one of said electrical signals produced by the respective conversion means is free from the third order harmonic components. The blocks may be so located that the phase difference between the signals coming from each of the blocks is equal to π / α (1 + 2n), where n is an integer. In this way, by combining the outputs of the blocks, an output having no harmonic components of αth order can be obtained. The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which:- FIG. 1 schematically illustrates the basic construction of an optical scaling device with which the present invention is useful; FIG.2 is a graphical illustration of waveforms useful in explaining the working principle of an embodiment of the present invention, as applied to an optical scaling device; FIG.3 is a block diagram illustrating the construction of a phase modulation type magnetic scaling device; FIG.4 is a graphical illustration showing interpolation errors present in the device of FIG.3; FIGS. 5(A), (B), and (C) schematically illustrate an embodiment of a magnetic detector head according to this invention; FIGS. 6(A) and (B) schematically illustrate another embodiment of a magnetic detector head according to this invention; FIG.7 schematically illustrates a block pattern of another embodiment of a magnetic detector head according to this invention; FIG.8 schematically illustrates a block pattern of another embodiment of a magnetic detector head according to this invention; FIGS. 9(A), (B), and (C) are a waveform diagram and schematic illustrations of a metal-film element lines in a ferromagnetic detector head, respectively, according to another embodiment of this invention; FIG. 10 schematically represents an embodiment of an optical index scale having a grating pattern according to this invention for use with an optical scaling device; FIG. 11 schematically represents another embodiment of an optical index scale having a grating pattern according to this invention for use with an optical scaling device; Fig. 12 schematically represents another embodiment of an optical index scale having a grating pattern according to this invention for use with an optical scaling device; Fig. 13 schematically illustrates another embodiment of an optical index scale having a grating pattern according to this invention for use with an optical scaling device; Figs. 14 (A) and (B) schematically illustrates the basic construction of an optical scaling device having an index scale with grating pattern and another embodiment of an index scale with optical grating, respectively, according to this invention; Fig. 15 schematically illustrates another embodiment of an optical index scale having a grating pattern according to this invention for use with an optical scaling device; Fig. 16 schematically illustrates another embodiment of an optical index scale having a grating pattern according to this invention for use in an optical scaling device; Figs. 17 (A) and (B) schematically illustrate another embodiment of this invention as applied to an optical scaling device; Figs. 18 (A) and (B) schematically illustrates intercepting plates useful with optical scales in an optical scaling device according to this invention; Fig. 19 schematically illustrates the relationship between an index scale and a main scale of an optical scaling device according to this invention; Fig. 20 is a graphical representation of the output of a detector head according to this invention and the output of a prior art head. Referring now to Fig. 1, an optical-type detector system is shown in which the waveform of the detector output signal, which corresponds to variations in optical intensity can be trapezoidal or triangular and will contain harmonic components, principally consisting of third order harmonics. The following discussion relative to this optical detector also applies to a capacitance-type detector head. The two waveforms f₁(x) and f₂(x) are represented in equations (1) and (2) above and are shown graphically in Fig. 2. This optical detector system will be described in more detail hereinbelow. In Fig. 3, a system device is represented using a known, phase-modulation-type magnetic scaling device and including magnetic scale 1 being scanned by first and second channel heads 2 and 3, respectively, which are multiple-gap heads responsive to magnetic flux. An output signal from oscillator 4 is divided down by frequency divider 5 and fed to magnetic exciter circuit 6 having an output excitation signal fed to multiple-gap heads 2 and 3. The output from head 2 is fed through phase-shift unit 7 as one input to mixer amplifier 8, and the output from head 3 is fed directly as the other input to mixer amplifier 8. The system output signal S is the output signal from mixer amplifier 8 after being fed through bandpass filter 9 and limiter amplifier 10. The output signal f₀ of oscillator 4 is the reference signal, ω₀ is the angular frequency of the oscillator output, f₀/2 is the output of the frequency dividing circuit, and (m ± 1/4) λ represents the distance between heads 2 and 3, based on these values the following relations are obtained. f₀ = sin ω₀t and f₀/2 = sin ω₀t/2, When λ represents the pitch of the grating in the index scale and x denotes the relative displacement between the index scale and the head, the output of the limiter-amplifier 10 (phase modulation signal) can be represented by: S = sin (ω₀t + 2π λ x) A scaling signal f(x) detected by a multigap head responding to magnetic flux is composed of two detector signals f(x₁) and f(x₂). f(x) = f(x₁) + f(x)₂ Supposing now that the detector signals f(x₁) and f(x₂) have an identical amplitude and a phase difference therebetween, they can be expressed by the following Fourier series. Consequently, the scaling signal f(x) can be expressed as follows: When n= α the following two identical equations are valid. a α {cosαx₁ + cos(αx₁ + π)} = 0 and b α {sinαx₁ + sin(αx₁ + π)} = 0 That is, in this case, f(x) is represented by a function containing no harmonic components of the α th order. In order to remove the harmonic components of the third order, it is sufficient to set α =3 and to combine the detector signals f(x₁) and f(x₂), which have a phase difference equal to π/3 therebetween, to obtain a scaling signal. This principle can be applied to a multiple-gap head responsive to magnetic flux, for example, the pattern of the constituting elements of each of the two heads are divided into two blocks so that they produce outputs of identical amplitude having a phase difference therebetween, which is selected to be equal to m λ±λ and the outputs f(x₁) and f(x₂) of the blocks are combined. In this way, distortions due to the harmonic components of the third order can be removed from the output of the head, which output is the desired scaling signal. It should be recalled that in prior art systems such distortions typically arose in the interpolation operation that was performed in order to obtain more accurate measurements. Fig. 4 shows a graph of a computer simulation of this interpolation error and it is seen that the error has a frequency four times as high as that of the fundamental wave. Figs. 5(A), (B), and (C) represent an embodiment of a coaxial-type, multiple-gap head responding to magnetic flux constructed according to the above described principle, in which N₁ ....., S₁ ..... and N₁′ ..... are laminate cores made of a high permeability material, such as permalloy, and between these laminate cores are inserted gap spacers GP₁....., made of non-magnetic material such as beryllium-copper. As indicated in these figures, the pattern of said cores is divided into two blocks a and b, with a spacer GA being inserted therebetween for phase regulation, so that they produce identical amplitude output signals having a relative phase difference equal to mλ± λ / 6 therebetween. Fig. 20 shows the relation between the wavelength λ and the relative output 0 of a head according to the above-described embodiment and the output of a conventional, prior art head. In Fig.20 , the solid line represents the characteristics of the head according to this invention and the broken line shows those of the conventional, prior art head. It is seen that distortions due to the third order harmonic components have been removed, as indicated for example at A. Figs. 6 (A) and (B) schematically illustrate a planar-type multiple-gap head of the kind responsive to magnetic flux according to this invention that is similar in construction to the embodiment described above. The division described above of the pattern of the constituent elements of each of the heads need not necessarily be into two, and the head can be divided into four blocks a₁, a₂, b₁ and b₂, or into six blocks a₁ ... a₃ and b₁ ... b₃, as indicated in Figs. 7 and 8, respectively. In general, they can be divided into an arbitrarily number of blocks. In Figs. 9 (A), (B), and (C) H₁ to H₈ represent magneto-resistance element lines, connected in series, made of ferromagnetic metal film having anisotropic effect in magneto-resistance, which element lines form the first channel head. They are disposed on an insulating substrate (not shown) with a distance between adjacent element lines so that predetermined phase relation with respect to the magnetic grating pattern of the magnetic scale is obtained. They are maintained to be parallel with a determined clearance to the magnetic grating pattern and a bias magnetic field applied in the direction of the element lines or magnetic anistropy given in the direction of the element lines produces the same effect as the application of the bias magnetic field. H₁′ to H₈′ form the second channel head having the same construction as the first channel head described above. The effective wavelength of the magnetic scale using said head is λ/2, λ being the registered wavelength of the magnetic scale. Putting 2π/ λ x=X, being the relative displacement of the head with respect to the scale, the distance between the first channel head CH₁ and the second channel head CH₂ is equal to (m±1/4)) λ/2. In each of the heads, element lines H₅ to H₈ and H₅′ to H₈′ are so disposed that they are displaced by λ/6 (1/6 of the effective wavelength) from the element lines H₁ to H₄ and H₁′ to H₄′, respectively. When driving voltage e₀sinT and e₀cosT are applied to the first and the second channel heads, respectively, signals detected by the respective element line e₁ to e₄ can be represented as follows. (i) X e₁ = e₀(α₁sinX + sin2X + α₃sin6x) cosT + e₀(α₁cosX + cos2X + α₃cos6X) sinT (ii) X→ X + π (iii) X→X + π / 6 (iv) X→X+π+ π / 6 Consequently, the resultant output voltage E of the two heads indicated below is obtained. Thus, it is possible to obtain phase modulation signals consisting only of the effective wavelength and that contain no harmonic components of the third order of the registered wavelength and the effective wavelength. The present invention can also be realized as an optical scaling device, as shown in Fig. 1, and referring thereto, in an optical scaling device if it is assumed that the duty cycle of main scale 11 and index scale 12 is 1:1, then scale output f(x) detected by light sensor 13 can be expressed by: f(x) = A 2 + 4A π² +{ cosx 1² + cos3x 3² + cos5x 5² } Referring still to Fig. 1, illumination is provided by light source 14 passing through lens system 15. Duty cycle or ratio 1:1 in an optical grating means that the width of the transparent part and that of the intercepting part are equal, and the sum of the two is the grating constant. The signal f(x) mentioned above has a triangular waveform, as indicated in Fig. 2. Consider now the resultant wave F(x) obtained by adding a second triangular wave f(x-ε) expressed by equation (24) below, which has a phase difference ε with respect to the first triangular wave. f(x+ε) = A 2 + 4A π² { cos(x+ε) 1² + cos3(x+ε) 3² + cos5(x+ε) 5² ...} The condition under which the harmonic components of third order in equation (25) are zero, is as follows. 3ε 2 = π 2 , ε = π 3 That is, when the second triangular wave f(x- π /3) having a phase difference of π/3 with respect to x is added to the first triangular f(x), the third order harmonic components compensate each other, and scale outputs having only small amounts of distortion are obtained. Thus, it is understood that following the teaching of the present invention the precision of the scaling device can be ameliorated. In Fig. 10 an embodiment of this invention based on the principle described above is shown in which n₁ to n₄ are arbitrary integers and in which index scale 12′ is suitable for use in an optical scaling device, such as that shown in Fig. 14. Each of the four patterns Q, R, S and T in the index scale of Fig. 10 is divided longitudinally into two parts, and the two parts are so disposed that they produce a spatial phase difference equal to λ/6. In this way, for the reason mentioned above, the harmonic components of the third order are compensated. In the embodiment of Fig. 11, each of the patterns of index scale 12′ is divided transversely into two parts. This division method in the main scale gives the same effect as described above. Fig. 12 shows another embodiment, in which each of the patterns of index scale 12′ is divided into four parts, instead of two parts, as in the embodiments of Figs. 10 and 11. By increasing the division number, the compensation of distortions due to the third order harmonic components is increased, because the difference in output due to different locations of the patterns is reduced. In the embodiment shown in Fig 13 each of the patterns of index scale 12′ is divided longitudinally, as well as transversely. As mentioned above, by dividing each of the patterns of the index scale longitudinally and/or transversely at least into two parts and by disposing them so that they produce a spatial phase difference of λ/6 (λ is the wavelength), it is possible to remove the harmonic components of the third order from the scale output. The same effect can be obtained by dividing main scale 11 transversely. Furthermore, not only distortions due to the harmonic components of the third order but also other periodical distortions can be removed by displacing a part of each of the patterns of the index scale with respect to the other, so that a spatial phase difference of π is induced in the periods of distortions, as described above in relation to the various embodiments of the present invention. It is also possible to remove distortions by the same method as that disclosed in the preceding embodiments in the case where the patterns of the index scale 12′ are arranged as shown in Figs. 15 and 16. Figs. 17(A) and (B) illustrate another embodiment, in which this invention is applied to an optical scaling device similar to that shown in Fig. 1, in which holder 16 retains index scale 12′, and which is secured to fixed frame 18 through fine regulation adjustment mechanism 17, shown here as micrometer adjusters. Main scale 11 is displaced horizontally left and right in this embodiment, while being maintained parallel to, and at a certain distance from, index scale 12′. The light-sensor (not identified here but shown at 13 in Figs. 1 and (14) is arranged parallel to main scale 11 and divided also into a plurality of parts, which correspond approximately to the parts of each of the patterns of the index scale. As indicated in Fig. 18B, each of the patterns Q and T of index scale 12′ are divided by first intercepting plate 19 into two parts, upper and lower. The first intercepting plate is wider at the central hatched portion than at the ends. By displacing this intercepting plate 19 left and right for fine regulation, it is possible to balance light intensities passing through the patterns R and S. A second intercepting plate 20 shown in Fig. 18A is superposed on the first intercepting plate 19 and divides the patterns R and s into two parts, upper and lower, and at the same time balances light intensities passing through the patterns Q and T. Index scale 12′ is so disposed that it is slightly inclined with respect to main scale 11, and this relation is indicated in Fig. 19, and they are so finely regulated that the upper and lower parts divided by intercepting plates (the central hatched parts) 19 and 20 produce a phase difference of λ/6. For this purpose, holder 16 of the index scale 12′ is so constructed that it touches fixed frame 18 at a point along a great circle (not shown) and can be fixed after having been slightly displaced horizontally. In this way, it is easy to regulate finely the angular direction of the index scale mentioned above by relatively large rotation of the fine regulation device or mechanism 17. By utilizing this construction described above, since each of the patterns of index scale 12′ is divided by the intercepting plates into two parts, upper and lower, and because respective light fluxes correspond to previously mentioned f(x) and f(x - π/3), when the two light fluxes are received by one light sensor, it is possible to obtain a resultant output F(x) from f(x) and f(x - π/3).

Claims: A detector head for use in detecting dimensions of an object being measured, comprising; a plurality of sensor means (2;3) for use in conjunction with a scale (1,11) having a predetermined pitch, each of said sensor means (2;3) producing an electrical signal having a relative phase-difference therebetween determined in accordance with the pitch of the scale (1,11) and together indicating the dimensions of the object with greater resolution than would be possible from any one of said sensor means (2;3); each of said sensor means (2;3) comprising conversion means (2;3;13) for producing one of said electrical signals and a plurality of constituent elements (N₁, S₁, N′₁; 21) arranged in a pattern (Q, R, S, T); said pattern (Q, R, S, T) being formed of at least two blocks (a,b,c,d) of the constituent elements (N₁, S₁, N′₁; 21), each of said at least two blocks (a,b,c,d) producing an intermediate signal of equal amplitude including higher harmonic components cancellable by combination of signals and characterised by having the blocks relatively positioned to produce a relative phase-difference corresponding to one sixth of the effective pitch of the scale (1,11), wherein the intermediate signals from said at least two blocks (a,b,c,d) of constituent elements (N₁, S₁, N′₁; 21) are combined within the respective sensor means (2;3) to cancel the third-order harmonic components such that said one of said electrical signals produced by the respective conversion means (2;3;13) is free from the third order harmonic components. A detector head according to claim 1, in which said constituent elements (N₁, S₁, N′₁) comprise a multiple-gap magnetic transducer head (2;3) responsive to magnetic flux. A detector head according to claim 2, in which said multiple-gap magnetic transducer head (2;3) comprises a core-type magnetic head, including an air gap (G) disposed having a length substantially equal to the wavelength of the third-order harmonic component being cancelled from said electrical signal. A detector head according to claim 1, in which said constituent elements (H₁ - H₈) comprise ferro-magnetic films arranged on a substrate and exhibiting anisotropy in magneto-resistance. A detector head according to claim 4, in which said constituent elements (H₁ - H₈) comprising said ferro-magnetic films include magnetic metal film element lines (H) arranged in a pattern with respective element lines (H₅ - H₈) of one block being displaced by one sixth of the effective pitch (π) from respective element lines (H₁ - H₄) of the other block, and the respective element lines (H₁ - H₈) of one sensor means (Fig. 9B) being displaced from the respective element lines (H₁′ - H₈′) of the other sensor means (Fig. 9C) by one quarter of the effective pitch (π/4) wherein the effective pitch of the scale is one half the actual pitch. A detector head according to claim 1, comprising an optical scaling device having a main scale (11), wherein the constituent elements are formed as an index scale (12′) arranged parallel to said main scale (11), and said conversion means comprises light receiving means (13) for detecting light passed by said main scale (11) and said index scale (12′), in which the intermediate signal in response to relative displacement of said main (11) and index (12′) scales corresponds to optical grating formed thereon, said main scale (11) and said index scale (12′) including a plurality of optical grating blocks (a,b,c,d) as said blocks. A detector head according to claim 6, comprising an optical device (19,20) for scaling the output signals produced by said constituent elements (21).

Inventors: HIMURO KIYOSHI, MIBU KATSUTOSHI, NARIMATSU AKIHISA, YOSHIOKA TAKAMOTO, HIMURO, KIYOSHI, MIBU, KATSUTOSHI, NARIMATSU, AKIHISA, YOSHIOKA, TAKAMOTO
Applicants: SONY MAGNESCALE INC, SONY MAGNESCALE INCORPORATION
Application year: 1984
Main classification: G01D5
IPC codes: G01D5