Abstract:
An incremental encoder rotational position detecting apparatus for detecting the incremental rotational position of a rotating member includes a rotatable disk plate rotated with the rotating member, a stationary disk plate opposite to the rotatable plate, and a detector circuit for supplying a rectangular signal to the stationary plate. The stationary plate has first, second, third and fourth electrodes and an earth electrode grounded. The earth electrode is arranged between the first electrode and the third electrode to reduce a distribution capacitance produced between the first and third electrodes. The rectangular signals with opposite phase to each other are applied between the first and second electrodes or the third and fourth electrodes of the stationary plate.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a rotation detector or more in particular to an electrode construction of an incremental encoder rotational position detector of electrostatic capacity type, which is capable of detecting the rotational speed of a rotary object such as an engine crank shaft of the automobile without delay regardless of the speed of the rotary object. 
     A conventional rotational position detector of electrostatic capacitance type comprises: a first plate including a first electrode having a plurality of electrode pieces radially arranged at equal spatial intervals and electrode pieces arranged radially between the electrode pieces of the first electrode; a third electrode and a fourth electrode; a second plate including 5th, 6th, 7th and 8th electrodes arranged opposedly to the first, second, third and fourth electrodes of the first plate respectively, the 5th electrode of the second plate being electrically connected to the 7th electrode thereof, the 6th electrode of the second plate being electrically connected to the 8th electrode thereof; means of supplying signals of opposite phases to the first and second electrodes of the first plate or the third and fourth electrodes thereof; and means of detecting the rotation of the second plate by detecting the change of the electrostatic capacitance generated between the first and second plates when the second plate is rotated in relation with the first plate. 
     The above-mentioned conventional electrode structure has such a disadvantage that among the signals of opposite phases applied to the first and second electrodes or the third and fourth electrodes of the first plate, the distributed capacitance formed mainly by the first and third electrodes causes the signal applied to the first or third electrode to be transmitted undesirably to the third electrode or the first electrode respectively, thus making it impossible to detect the signal satisfactorily, that is, to detect the rotation satisfactorily. 
     SUMMARY OF THE INVENTION 
     The present invention is intended to obviate this shortcoming and an object thereof is to provide an incremental rotation detector of electrostatic capacitance type in which an earth electrode of earth potential is added between the first and third electrodes, and thus by recuding the distributed capacitance between the first and third electrodes, the signal waveform applied to the first or third electrode is prevented from being transmitted to the third or first electrode respectively, thereby producing a satisfactory rotation signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1a and 1b are a cross sectional view and a longitudinal sectional view respectively of the essential parts of an embodiment of the rotation detector containing plates having an electrode structure according to the present invention. 
     FIGS. 2a and 2b are front views of the electrode structures of the first and second plates respectively of the detector shown in FIG. 1. 
     FIG. 3 an electrical circuit diagram showing an embodiment of a detector circuit section of the detector shown in FIG. 1. 
     FIGS. 4a and 4b are electrical circuit diagrams showing the positional relations of the electrodes upon rotation of a rotary object for explaining the operation of the detector shown in FIG. 1. 
     FIG. 5 shows signal waveforms at various parts for explaining the operation of the embodiment shown in FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be explained below with reference to the drawings. In FIGS. 1a and 1b, reference numeral 100 designates a housing fixed on the outside of a bearing 110, numeral 120 a shaft fixed on the inside of the bearing 110, which shaft 120 is coupled to, say, the crank shaft of the engine so that the rotation of the shaft 120 causes the rotation of the rotary object 160 fixed to the shaft 120 byscrews 190. The rotary object 160 is fixed by screws 180 on the second plate 150 including a printed board formed with electrodes and adapted to rotate by the rotation of the rotaly object 160. The first plate 140, on the other hand, which also includes a printed board formed with electrodesis fixed on the housing 100 by screws 170, and is connected with signal lines 141, 142, 143 and 144 from a detector circuit section 101 mounted onthe printed board 130 fixed on the housing 100. 
     FIG. 2a is a diagram showing the side 140a of the first plate 140 facing the second plate 150. The signal lines 141, 142, 143 and 144 are connectedthrough connectors 31, 32, 33 and 34 respectively to the saw-toothed first electrode 145, the saw-toothed second electrode 146, and annular third electrode 147 and an annular fourth electrode 148 of the first plate 140. The electrode pieces of the first and second electrodes 145 and 146 are radially arranged along the circumference of the first plate in engaged and spaced relation to each other. The third and fourth electrodes 147 and148 are arranged on the inside of the first and second electrodes 145 and 146, and an annular earth electrode 149 arranged between the first and third electrodes 145 and 147 is connected through a connector 35 to the grounding terminal of the detector circuit section 101. 
     The side 150a of the second plate 150 which faces the first plate 140 is shown in FIG. 2b. The saw-toothed 5th electrode is connected through a lead wire 153 to the annular 7th electrode 156, and the saw-toothed 6th electrode 152 is connected through a lead wire 154 to the annular 8th electrode 155. The electrode pieces of the 5th and 6th electrodes 151 and 152 are arranged radially along the circumference in engaged and spaced relation with each other. The plates 140 and 150 are printed with the electrodes 146 and 151, the electrodes 145 and 152, the electrodes 147 and156, and the electrodes 148 and 155 in opposition to each other respectively. 
     An electrical circuit diagram of the detector circuit section 101 is shown in FIG. 3. In FIG. 3, the terminal 301 is a power terminal impressed with a constant voltage Vc. The terminal 302 is a grounding terminal. Numeral 310 designates a well-known CR oscillator circuit, numeral 320 a referencesignal generator, numeral 330 a detector section including the upper electrodes 151, 152, 156, 155, and lower electrodes 146, 145, 147, 148, numeral 340 a comparator circuit, numeral 350 a phase detector circuit, numeral 360 an output circuit, and numeral 303 an output terminal. 
     The operation of the apparatus having the above-mentioned construction willbe described. In FIG. 3, an oscillator circuit including the CR oscillator circuit 310 having inverter gates 311, 312, 313, resistors 314, 315 and a capacitor 316 produces an oscillation waveform as shown in FIG. 5(a). Thisoscillation waveform 10 is applied to the reference signal generator 320 for producing a signal of the same phase as the oscillation waveform 10 and a signal 20 of opposite phase shown in FIG. 5(b). 
     As shown in FIG. 5(a), when the shaft 120 (as illustrated in FIG. 1) rotates, the second plate 150 rotates along therewith so that its 5th electrode 151 opposes the first electrode 145 of the first plate 140 and that the 6th electrode 152 of the second plate 150 opposes the second electrode 146 of the first plate 140. At this time, when the signal in phase with the oscillation waveform 10 is applied through the signal line 141 to the first electrode 145 of the first plate 140, a signal in phase with the oscillation waveform 10 is produced at signal line 153 through a capacitor (as represented by 331 in FIG. 3) formed by the electrode 145 ofthe first plate 140 and the 5th electrode 151 of the second plate 150. Then, this signal is transmitted through a capacitor (represented by 332 in FIG. 5) formed by the 7th electrode 156 of the second plate 150 and thethird electrode 147 of the first plate 140, and through the connector 33 tothe comparator 340 as a signal 30 shown in FIG. 5(c). Similarly, the signal20 transmitted through the signal line 142 to the second electrode 146 of the first plate 140 is applied through a capacitor (shown at 333 in FIG. 3) formed by the second electrode 146 and the 6th electrode 152 of the second plate 150, to the 6th electrode 152 of the second plate 150 as a signal in phase with the signal 20. Then, this signal is fed through a capacitor (shown at 334 in FIG. 3) formed by the 8th electrode 155 of the second plate 150 and the fourth electrode 148 of the first plate 140, and through the connector 34 to the comparator 340 as a signal 40 shown by FIG. 5(d). If the earth electrode 149 is lacking, the distributed capacitance formed by the first electrode 145 and the third electrode 147 of the first plate 140 as shown by the dashed line in FIG. 4 causes the signal applied to the first electrode 145 (the signal in phase with the oscillation waveform 10) to be applied to the third electrode 147, which signal is interposed on the signal 30 (shown in FIG. 5) and appears at theterminal 33 (the signal 30&#39; in FIG. 5). At this time, the signal 10 is in phase with the signal 30, so that the same operation is effected regardless of the presence or absence of the earth electrode 149. 
     However, the signals 30, 31 and 40 take the waveform based on the voltage V a  shown by FIGS. 5(c) and 5(d) and which is determined by resistors 343 and 344 of the comparator 340. These signals 30 and 40 are amplified by a differential amplifier which is formed of a resistor 345 and an operational amplifier (hereinafter, referred to simply as OP amp) 346 in the comparator 340. Thus, this differential amplifier produces a signal 50which is, as shown by FIG. 5(e), delayed by time ΔT with respect to the oscillation waveform 10, where ΔT is the delay time in the capacitor response and the switching of the OP amp 346. The signal 50 is shaped in waveform by inverter gates 351 and 352 of the phase detector 350. Thus, to the clock terminal of a D-type flip-flop 354 is applied a signal in phase with the signal 50, and to the clock terminal of a D-type flip-flop 355 a signal opposite in phase to the signal 50. The signal opposite in phase to the oscillation waveform 10 is applied from the CR oscillator 310 through an inverter gate 353 to the data terminals of the D-type flip-flops 354 and 355. Consequently, the D-type flip-flop 354 takes &#34;1&#34; state at the output terminal Q 1  and the D-type flip-flop 355 takes &#34;0&#34; state at the output terminal Q 2 . Then, the stage of NAND gates 356, 357, 358 and 359 thus supplies &#34;0&#34; signal shown by FIG. 5(f) to a signal line 60. This signal is applied to the output circuit 360in which it is fed through a resistor 361 to a transistor 362, which is thus caused to turn off. As a result, at the output terminal 303 there appears a signal &#34;1&#34; (a signal 70 shown by FIG. 5(g)) indicating that the 5th electrode 151 of the seocnd plate 150 opposes the first electrode 145 of the first plate 140 (or the 6th electrode 152 of the second plate 150 opposes the second electrode 146 of the first plate 140). 
     Moreover, as shown in FIG. 4b, when the 5th electrode 151 of the second plate 150 comes to be opposite to the second electrode 146 of the first plate 140 and the 6th electrode 152 of the second plate 150 to the first electrode 145 of the first plate 140, the signal in phase with the signal 20 appears at the 5th electrode 151 of the second plate 150. Then, to the third electrode 147 of the first plate 140 is applied a signal 31 with thereference potential of V a  as shown by FIG. 5(c). In addition, the signal in phase with the oscillation waveform 10 appears at the 6th electrode 152 of the second plate 150, and then to the fourth electrode 148 of the first plate 140 is applied a signal 41 with the reference potential of V a  as shown by FIG. 5(d). If the earth electrode 149 is lacking, the existence of the distributed capacitance between the first electrode 145 and the third electrode 147 of the first plate 140 as mentioned above causes a waveform in phase with the oscillation waveform 10 to be applied to the connector 33. In an extreme case, the signal at the connector 33 takes a waveform opposite to the waveform 31 as shown by numeral 31&#39; in FIG. 5(c), resulting in an unstable operation. In order to solve this problem, the embodiment of FIG. 2 is such that as mentioned above, the earth electrode 149 is inserted between the first electrode 145and the third electrode 147 of the first plate 140 thereby to eliminate thesignal transmission which otherwise might occur by the distribution capacitance of the first electrode 145 and the third electrode 147. In this way, only a signal 31 shown in FIG. 5(c) is produced at the terminal 30. At this time, to the output terminal of the comparator 340 is applied a signal 51 as shown by FIG. 5(e) and thus at the output terminal of the phase detector 350 there appears a signal 61 as shown by FIG. 5(f). Consequently, to the output terminal 303 is applied a signal &#34;0&#34; (a signal71 as shown by FIG. 5(g)) indicating that the 5th electrode 151 of the second plate 150 is opposite to the second electrode 146 of the first plate 140. 
     Thus, when the 5th and 6th electrodes 151 and 152 provided on the second plate 150 are passed above the first and second electrodes 145 and 146 provided on the first plate 140 as the shaft 120 rotates, signals of &#34;1&#34; and &#34;0&#34; alternately appear at the output terminal 303, thereby eanabling incremental detection of the rotation of the second plate 150, or the shaft 120. Thus, the present invention provides a signal which is indicative of detected incremental rotation (i.e. angular position changes). 
     As described above, the annular electrode 149 is added between the first electrode 145 and the third electrode 147 of the first plate 140. This prevents the waveform 10 applied to the first electrode 145 from being applied to the third electrode 147 by the distributed capacitance formed by the first electrode 145 and the third electrode 147, so that the signalproduced at the 7th electrode 156 of the second plate 150 is transmitted tothe third electrode 147 of the first plate 140 satisfactorily. 
     In the aforementioned embodiment, the first and second electrodes 145 and 146 of the first plate 140 are impressed with signals of opposite phases to produce signals from the third and fourth electrodes 147 and 148 of thefirst plate 140. As an alternative to this, the third and fourth electrodes147 and 148 of the first plate 140 may be impressed with signals of opposite phases thereby to produce signals at the first and second electrodes 145 and 146 of the first plate 140 with equal effect.