Speed detection apparatus having a rotation phase detector

Abstract: A speed detection apparatus provided with a rotor magnet (11,12,13) that has many magnetic coil portions formed in the direction of its circumference, and a coil pattern for signal detection that rotationally displaces relative to an opposing said rotor magnet (11,12,13). The rotor magnet (11,12,13) has a first magnetized portion (5) for detection of a rotation phase of the rotor and formed in one portion of a specific position and a second magnetized portion (8,9,10) for detection of a rotation speed of the rotor magnet (11,12,13) and provided at a portion other than the first magnetized portion (14,15,16) and a non-magnetized portion having a crescent shape and provided at a specific position of the rotor magnet (11,12,13) magnet and that is symmetrical from a center of rotation of the rotor magnet (11,12,13), with respect to the first magnetized portion (5).

Description: BACKGROUND OF THE INVENTION The present invention relates to an improved speed detection apparatus having a rotation phase detector. In general, in rotational drive apparatus such as rotating magnetic heads or the like, there are provided a speed detector (speed control circuit) and a phase detector that detect the speed of rotation and the phase of rotation of a rotating body, so that the speed of rotation of the rotating body such as a magnetic head drum or the like can be made a constant speed rotation of a required rotational phase, and each of these detection outputs is used in a drive motor having phase servo and speed servo. In this manner, a detection signal generator portion is formed by the combination of a FG (frequency generator) coil pattern, PU coil pattern and a rotor magnet. The structure and principle of operation of a conventional speed detection apparatus having a rotation phase detector and having such a configuration will be described, with reference to FIG. 1 and later. FIG. 1 is a plan view of a rotor magnet 3 (one portion of a rotor) used in a conventional speed detection apparatus having a rotation phase detector, and 4 is a magnetized portion used for speed detection and formed in the direction of the circumference of the rotor magnet 3 for the greater portion (and hereinafter termed an "FG magnetized portion"), and 5 is a magnetized portion for detection of the phase and formed at portions other than those of the FG magnet 4, of those rotor magnets 3 (and hereinafter termed "PU magnetized portion"). The following is a description of the principle of operation in the conventional apparatus, with reference to FIG. 2. FIG. 2(a) is a view describing the principle of operation, when the portion in the vicinity of the PU magnetized portion 5 of the rotor magnets 3, is extended in a line, and as shown in this figure, the PU magnetized portion 5 has a portion corresponding to two poles of the FG magnetized portion 4 divided equally into four, so as to form a pattern of a two-cycle portion. The FG magnetized portion 4 comprises 38 N or S poles. FIG. 2(b) shows a coil pattern 2 provided so as to oppose this rotor magnet 3, where 6 is an FG coil pattern (hereinafter termed an "FG pattern") for speed detection, and 7 is a PU coil pattern (hereinafter termed a "PU pattern") for phase detection. In such a configuration, when the position relationship is as shown in FIG. 2(a), that is, when the FG pattern 6 is opposed to the FG magnetized portion 4, FG signal is generated and is obtained across the terminal α and the terminal β, as shown in FIG. 2(c) . In addition, the PU signal is generated only when the PU pattern is opposed to the PU magnetized portion 5, as shown in FIG. 2(d), and is obtained across the terminals γ and δ. In this case, the place where the pitch of the PU magnetized portion 5 and the pitch of the PU pattern 7 have a one-to-one correspondence is one per rotation, the place is provided so as to correspond to the reference position necessary for detection. As has been described above, in a conventional speed detection apparatus having a rotation phase detector, the PU magnetized portion 5 does not contribute to the generation of FG signal when it is opposite the FG pattern 6. More specifically, when seen from the FG pattern 6, the status is the same as if the rotor magnet 3 had one non-magnetized portion, an encoder error occurrs when the center point of the FG pattern 6 is displaced from the rotational center of the rotor magnet 3. The cause of this (principle of occurrence) is described with reference to FIG. 3. In FIG. 3, 6c is a circle of average radius R of the FG pattern 6, O is the center point of the FG pattern 6, and O′ is a center of rotation of the rotor magnet 3. Here, the angular speed of rotation of the rotor magnet 3 is ω and the distance between both centers O and O′ is r. Now, when r ≠ 0, that is, when the two centers O and O′ are displaced each other, assuming that the FG magnetized portion 4 is around the entire circumference of the rotor magnet 3, speeds V₁ and V₂ shown in FIG. 3 are combined, and the average speed is V, according to the following equation. (V₁ + V₂) 2 = {(R+r)ω + (R-r)ω} 2 = Rω (=V) However, in reality, the FG magnetized portion 4 is in the status where there is a one pulse portion missing on the circumference of the rotor magnet 3 and so V₁ and V₂ do not combine. Here, the FG magnet can be thought of as having an encoder error in the status where there exists only a one-pulse portion. the other conditions can be the ideal status. More specifically, (1) The FG pattern is of a type of integration for the entire circumference, and a true circle, and the line elements are equal to each other, with no surface deviation. (2) The FG magnet is divided equally with respect to the center of rotation, and the amount of magnetic flux generated by each pole is equal, and there is no surface deviation. (3) The speed of rotation of the rotor (magnet) is constant. For these conditions, when the encoder error (fundamental speed of rotation component or the component per rotation per time) is made E e , then: E e = V₁-V V × 100 [%o-p] = (R+r)ω-Rω Rω × 100 [%o-p] = r R × 100 [%o-p] = r R × 100 2 [%rms] In reality, since there are FG magnets in other portions, if the number of FG pulses is made n, then the influence of one pulse of the FG magnet becomes l n . Furthermore, if the average diameter of the FG pattern D = 2R, then the FG pattern radial deviation d = 2r and substituting this into the equation above gives the encoder error E e as follows. E e = d (nD) · 50 2 FIG. 4 shows the theoretical values (broken line) calculated by substituting the actual numerical value for the rotation speed detector into this equation, and the values (dots) actually measured. As can be seen from this figure, there is a positive correlation of significance level 1% between these two sets of values, and this can be understood as resulting from the encoder error as been theoretically described above. One attempt to solve this problem involves providing a portion where there is either no magnetism or a weak magnetism at a position that balances the PU magnetized portion 5, so as to reduce the encoder error but since this causes places where there is a large difference in the amount of magnetic field generated between adjacent pairs of poles of the FG magnetic portion 4, the PU noise (noise component included in the PU signals) becomes large and there is the disadvantage that this is a cause of malfunction. EP-A-0149360 discloses a rotation sensor comprising: a rotary component including a magnetic element having a plurality of first segments arranged about the periphery of said rotary component and generating a magnetic field between each adjacent pair of first segments and a plurality of second segments disposed near said first segments and representing a reference angular position of a rotating object, said magnetic element being coupled with said rotating object, the angular motion of which is to be monitored for rotation therewith; and a stationary component on which a first signal generator is cooperative with said first segments of said magnetic element to output a first signal representative of angular motion speed of said rotating object, and a second signal generator is cooperative with said second segments of said magnetic element to output a second signal indicative of the presence of said rotating component in said reference position, the ends of said first signal generator being connected to output terminals via communications lines lying on the same surface on which said first signal generator is arranged, the rotation sensor being characterised in that said communication lines extend across the region in which the magnetic field formed by said second segments of said magnetic element exists and are arranged so that the tangent to said communication lines at each point on said communication lines is substantially parallel to said magnetic flux formed by the nearest pair of adjacent second segments. SUMMARY OF THE INVENTION The present invention provides a speed detection apparatus having a rotation phase detector, comprising: a first disk having a rotor magnet mounted thereon, said rotor magnet comprising a plurality of segments of alternating magnetic polarity arranged in the direction of the circumference thereof, wherein said rotor magnet comprises a first magnetic pattern extending around a majority of the circumference of said rotor magnet and consisting of a plurality of said segments each having a first angular pitch, and a second magnetic pattern extending around the remainder of the circumference of said rotor magnet and consisting of a plurality of said segments each having a second angular pitch equal to half the first angular pitch; and a second disk, rotatable in relation to said first disk, carrying a first coil pattern associated with said first magnetic pattern for producing a signal (FG) indicative of rotation speed of said rotor magnet, and a second coil pattern associated with said second magnetic pattern for producing a signal (PU) indicative of rotation phase of said rotor magnet; characterised in that said rotor magnet further comprises a non-magnetised portion having a crescent shape and positioned adjacent to or within said first magnetic pattern, said crescent shaped portion being symmetrical about a diameter of the disk, said diameter passing through the centre of said second magnetic pattern. A preferred embodiment of speed detection apparatus according to the present invention has the advantage that the noise component included in the PU signals can be prevented from becoming larger even if a portion with either no magnetism or a weak magnetism is provided, and therefore remove a cause of misoperation, and which can also reduce encoder error. Accordingly, when the speed detection apparatus having a rotational phase detector is applied to a rotational drive apparatus of a rotating magnetic head of a VCR, there is the excellent feature of being able to obtain high accuracy for the control signals for the speed servo, while incorporating signals for phase servo. BRIEF DESCRIPTION OF THE DRAWINGS In the appended drawings, FIG. 1 is a plan view that shows a model of the rotor magnet used in a conventional speed detection apparatus; FIG. 2(a) through (d) are respectively, a model view that extends in a linear shape a PU magnetized portion of a rotor magnet in a conventional apparatus, a model view showing a coil pattern corresponding to this, an FG signal waveform diagram obtained by this pattern, and a PU signal waveform diagram; FIG. 3 is a view showing the principle for describing the causes of the generation of encoder error; FIG. 4 is a correlation diagram for the measured values and the calculated values (theoretical values) for the generation of encoder error; FIG. 5 is a plan view showing the magnetism status of a rotor magnet of a speed detection apparatus according to a first embodiment of the present invention; FIG. 6 is a view showing the principle for the analysis of the shape of a non-magnetized portion according to a first embodiment of the present invention; FIG. 7 and FIG. 8 are respectively, a plan view of a magnetization status of a rotor magnet of a speed detection apparatus according to a second embodiment of the present invention, and a view showing the principle for shape analysis of the non-magnetized portion; FIG. 9 is a plan view of the magnetization status of a speed detection apparatus according to a third embodiment of the present invention; FIG. 10 is a view showing measured values for encoder error in a speed detection apparatus according to a first through third embodiment of the present invention, and a view of characteristics showing a comparison with a conventional apparatus; FIG. 11 is a view of characteristics showing a comparison of measured values for the FG signals for first through third embodiment of the present invention, and a view of characteristics showing a comparison with a conventional apparatus; and FIG. 12 and FIG. 13 is a view of characteristics showing a comparison of measured values for the PU noise and the PU signal output level for first through third embodiment of the present invention, and a view of characteristics showing a comparison with a conventional apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 5 is a plan view showing the magnetism status of a rotor magnet and FIG. 6 is a view showing the principle for the analysis of the shape of a non-magnetized portion according to a first embodiment of the present invention. In FIG. 5, those portions that are the same as or similar to corresponding portions in the conventional apparatus are indicated with the same numerals and the corresponding descriptions of them are omitted. As is shown in the figure, inside the magnetized portion (FG magnetized portion) for speed detection formed in the rotor magnet 11 is provided a non-magnetized portion 14 of a crescent shape. And, the central portion 14c that has the widest width of the non-magnetized portion 14 is positioned so as to be on the opposite side to the PU magnetized portion 5 described above. By having suitable values for the dimensions and the shape of this non-magnetized portion 14, it is possible to balance the PU magnetized portion 5 and to reduce the encoder error. Also, when this is done, the magnetic field generated between pairs of adjacent S and N poles of the FG magnetized portion 8 for speed detection gradually increases and decreases and so it is possible to control the amount of generation of PU noise and to eliminate malfunction. The following is a description of the optimum conditions for the shape of the non-magnetized portion in the first embodiment described above, with reference to FIG. 6 describing the principle of operation. As is shown in this figure, r is the distance between the center O of the FG magnetized portion (rotor) and the center O′ of the arc of the non-magnetized portion that is to be formed, R₁ is the distance (radius) from the center O′ to the external edge of the non-magnetized portion, R₂ is an inner circumference radius of FG pattern 6, R₃ is an external radius of the FG pattern 6, and ϑ c is the angle of opening from the center of the PU magnetized portion 5 to the distal end of the non-magnetized portion 14. In the case of the first embodiment (inner-side non-magnetized portion) shown in FIG. 5, the equation applied to FIG. 6 becomes as follows: (R₃²-R₂²)sin(π/n)-(R₁²-R₂²+r²)·sinϑ c -rR₁{π-(ϑ c + (sin2ϑ c ) 2 )} + 2r² 3 sin³ϑ c = 0 Where, ϑ c = tan⁻¹ 4r²R₂²-(R₁²-R₂²-r²)² R₁²-R₂²-r² In the case of the apparatus of the present invention, the PU magnetized portion 5 does not contribute to the generation of the FG signal and so the PU magnetized portion 5 is in the same non-magnetized status as FG. Because of this, there is an FG imbalance on the side of the rotor and this combines the imbalance on the stator side and generates the encoder error E e (one rotation per one time) component. When the magnetic flux density is expressed as |B| and the speed of rotation of the rotor as v, then the non-magnetized status can be expressed as follows. ∬ s v × |B| dS = 0 Here, the magnetic flux density |B| is practically constant. As is shown in FIG. 6, the center O of the rotor is made the origin, and the center portion of the PU magnetized portion 5 is in agreement with the + side of the x axis, so that the components in the direction of the x axis of the velocity v cancel each other in the upper portion and the lower portion with respect to the x axis, and so the required conditions are ∬ s v·cosϑdS = 0 Properly speaking, equation (5) is established when there is FG (magnetic flux and coil pattern) formed around the complete periphery and so when the PU magnetized surface as seen from the FG pattern is rewritten in terms of equation (6) as S₁ and the non-magnetized surface is rewritten as S₂, then: ∬ s1 v·cosϑdS₁ + ∬ s2 v·cosϑdS₂ = 0 The following determines these conditions by calculation. First, in the case of the non-magnetized side on the inner side, the symbols of the preceding item shown in FIG. 6 can be used and the following equations established. (x+r)² + y² = R₁² (magnetized inner periphery of broken line) x² + y² = R₂² (FG pattern inner periphery) x² + y² = R₃² (FG pattern outer periphery) tanϑ = y/x (inclination of line of angle ϑ) When these are solved, A R = -r·cosϑ + R₁² - r²sin²ϑ (distance from the origin to the point A) B R = R₂ (distance from the origin to the point B) ϑ c = tan⁻¹ 4r²R₂²-(R₁²-R₂²-r²)² R₁²-R₂²-r² (angle from the x axis to the point C) The determining conditions are as follows: Moreover, the first item in the left side of equation (8) is the condition relating to the PU magnetized portion 5 and the second item of the same is the condition relating to the non-magnetized portion 14. When ϑ = π and in the vicinity of this, R₁² » r²sin²ϑ and so solving the above equation as R₁² - r²sin²ϑ ≐ R₁² determines the following equation. (R₃²-R₂²)sin(π/n)-(R₁²-R₂²+r²)sinϑ c -rR₁{π-(ϑ c + sin2ϑ c 2 )} + 2r² 3 sin³ϑ c = 0 Here, the invention is applied to a specific apparatus (FG pulse number n = 20) that is actually used and values suitable for r and R are successively substituted so that values for r and R so that the left side becomes zero are determined as follows. The non-magnetized inner side: R₁ = 13.3 + r, R₂ = 13.85, R₃ = 15.05 From the above, r = 0.417 Accordingly, r ≐ 0.42 Moreover, in the first embodiment, the non-magnetized portion 14 is as shown in FIG. 5, provided adjacent to the inner side of the FG magnetized portion 8 and the present invention is not limited to this, as it can be provided adjacent to the outer side of the FG magnetized portion 9 as in the case of the second embodiment shown in FIG. 7. In addition, in the case of the second embodiment (outer side non-magnetized) shown in FIG. 7, the required condition is expressed by the following equation used in FIG. 8. (R₃²-R₂²)sin(π/n)-(R₃²-R₁²-r²)sinϑ c -rR₁{π-(ϑ c + sin2ϑ c 2 )} - 2r² 3 sin³ϑ c = 0 Where, ϑ c = tan⁻¹ 4r²R₃²-(r²-R₁²+R₃²)² r²-R₁²+R₃² (x-r)² + y² = R₁² (magnetized outer periphery of broken line) x² + y² = R₂² (FG pattern inner periphery) x² + y² = R₃² (FG pattern outer periphery) tanϑ = y/x (inclination of line of angle ϑ) When these are solved in consideration of the conditions the same as for the first embodiment as has been described, the following equation is determined. (R₃²-R₂²)sin(π/n)-(R₃²-R₁²-r²)sinϑ c -rR₁{π-(ϑ c + sin2ϑ c 2 )} - 2r² 3 sin³ϑ c = 0 Here, when each of the values are calculated for the case when the second embodiment for the outer side magnetism is applied to a specific apparatus (FG pulse number n = 20) that is actually used, then for The non-magnetized outer side: R₁ = 15.65 - r, R₂ = 13.85, R₃ = 15.05 From the above, r = 0.435 Accordingly, both the inner side and the outer side non-magnetism (the first and second embodiments) have r ≐ 0.42 (mm) and so becomes the optimum condition. In this case, the dimension (the width of the position symmetrical with respect to the central portion of the PU magnetized portion 5) of the maximum width of the non-magnetized portions 14, 15 is approximately 0.6 mm. Moreover, the configuration for the rotor inner-side of the magnetized portion provided to the rotor magnet is not limited to that of the first and the second embodiments, as, for example, the configuration can also be as shown for the third embodiment in FIG. 9. As shown in FIG. 9, it can also be provided between the FG magnetized portion 10, that is, in the central portion on the normal line, and if the above conditions are satisfied that the central portions 15c, 16c for which the width of the non-magnetized portions 15, 16 is widest are positioned so as to be precisely opposite with respect to the PU magnetized portion 5, then the same effect can be obtained and so the present invention is not limited to a specific position. Here, when a third embodiment having an intermediate magnetism is applied to a specific embodiment, then from equation (9) and equation (11), r and R₁ through R₃ become intermediate numerical values for the inner side magnetism and the outer side magnetism. Accordingly, the maximum dimension and the shape of the non-magnetized portion 16 shown in the third embodiment in FIG. 9 can also be formed so as to have a similar shape. FIG. 10 is a view showing the values actually measured for the fundamental speed of rotation component E e (45 Hz component) of the encoder error, for a total of seven types of rotor magnet having maximum widths of the non-magnetized portion 14, 15 of 0.2, 0.6, 1.0 for each of the embodiments (inner side, outer side non-magnetized) of the present invention and for a conventional apparatus. In the figure,  is a degree of pattern coaxially. As is clear from this diagram, when compared to the conventional example (where the dimension of the maximum width of the central non-magnetized portion = 0), there is a significant reduction in the embodiment of the present invention (with the same dimension = 0.6 mm). In each of the embodiments of the present invention, the width of the FG magnetized portion 8-10 becomes narrower by the amount where the non-magnetized portion 14-16 are formed and so the lowering of the FG output and the increase of the PU noise could be anticipated, but as a result of measurements, as shown in FIG. 11, the FG signal output dropped at about one tenth when compared to the conventional embodiment and there was practically no increase in the PU noise (refer to FIG. 12). Moreover, the output level of the PU signals showed practically no change, as seen in FIG. 13.

Claims: A speed detection apparatus having a rotation phase detector, comprising: a first disk having a rotor magnet (11,12,13) mounted thereon, said rotor magnet comprising a plurality of segments of alternating magnetic polarity arranged in the direction of the circumference thereof, wherein said rotor magnet comprises a first magnetic pattern (4,8,9,10) extending around a majority of the circumference of said rotor magnet and consisting of a plurality of said segments each having a first angular pitch, and a second magnetic pattern (5) extending around the remainder of the circumference of said rotor magnet and consisting of a plurality of said segments each having a second angular pitch equal to half the first angular pitch; and a second disk, rotatable in relation to said first disk, carrying a first coil pattern (6) associated with said first magnetic pattern (4,8,9,10) for producing a signal (FG) indicative of rotation speed of said rotor magnet, and a second coil pattern (7) associated with said second magnetic pattern for producing a signal (PU) indicative of rotation phase of said rotor magnet; characterised in that said rotor magnet further comprises a non-magnetised portion (14,15,16) having a crescent shape and positioned within said first magnetic pattern (8,9,10), said crescent shaped portion being symmetrical about a diameter of the disk, said diameter passing through the centre of said second magnetic pattern (5). A speed detection apparatus according to claim 1, wherein said crescent shaped portion (14) is located adjacent to the inner side of the first magnetic pattern (8,9,10) of said rotor magnet (11). A speed detection apparatus according to claim 2, wherein said crescent shaped portion (14) has an inner side which is an arc of a first circle whose centre of rotation (0) is the center of rotation of said first magnetic pattern (8,9,10) and whose radius is the radius of the inner side of said first magnetic pattern (8,9,10); and an outer side which is an arc of a second circle whose center of rotation (0′) is displaced from the center of rotation (0) of said first circle by a predetermined distance (r), in a direction away from the center of said second magnetic pattern (5). A speed detection apparatus according to claim 1, wherein said crescent shaped portion (15) is located adjacent to the outer side of the first magnetic pattern (8,9,10) of said rotor magnet (12). A speed detection apparatus according to claim 4, wherein said crescent shaped portion (15) has an outer side which is an arc of a third circle whose center of rotation (0) is the center of rotation of said first magnetic pattern (8,9,10) and whose radius is the radius of the outer side of said first magnetic pattern (8,9,10); and an inner side which is an arc of a second circle whose center of rotation (0′) is displaced from the center of rotation (0) of said first circle by a predetermined distance (r) in a direction towards the center of said second magnetic pattern (5). A speed detection apparatus according to claim 1, wherein said crescent shaped portion (16) is located between the inner and outer sides of said first magnetic pattern (10), and wherein the thickest part of said crescent shaped portion (16) is diametrically opposed to said second magnetic pattern (5). A speed detection apparatus according to any of the preceding claims, wherein said second magnetic pattern (5) comprises four segments of alternating magnetic polarity.

Inventors: KUMAGAI YOSHIKI, KUMAGAI, YOSHIKI
Applicants: VICTOR COMPANY OF JAPAN, VICTOR COMPANY OF JAPAN, LTD.
Application year: 1991
Main classification: G01P3
IPC codes: A61F13, G01P3, A61F5, G01D5