Capacitive rotational position detector

A capacitive rotational position detecting apparatus includes a stationary disk plate and a rotatable disk plate in opposite relation. The stationary plate has a first and a second electrode on one surface, each of which is electrically connected through apertures by a metal film formed on the other surface of the stationary plate to reduce a distribution capacitance formed between metal films of the stationary and rotatable plates.

The present invention relates to an apparatus for detecting the rotational 
position of a crankshaft of an engine in an automobile or the like by 
utilizing static capacitance, and particularly to a rotation detecting 
electrode structure capable of easily and instantly detecting the rotation 
position by utilizing static capacitance. 
There has been proposed an apparatus for detecting rotation position by 
static capacitance, in which a rotor with the outer peripheral surface 
shaped like a gear is used to rotate with the rotation of a shaft and a 
stator with the inner peripheral surface shaped like a gear to oppose the 
outer peripheral surface of the rotor is used to form a static capacitance 
with the rotor, thus the capacitance to be detected being changed as the 
distance therebetween changes with the rotation. 
In this kind of apparatus, however, to increase the number of signals to be 
detected per revolution of the shaft, it is necessary to increase the 
number of gear-like electrodes provided on the rotor and stator. This is 
not only difficult but increases the distributed capacitance, making it 
difficult to detect the static capacitance change. 
There has also been proposed a rotation position detecting apparatus which 
obviates the above defects as disclosed in the Japanese Patent Publication 
No. 49421/1976. This apparatus includes stationary and rotating plates 
made of insulating material which have metal film electrodes arranged in 
the circumferential direction so that the rotation can be detected by the 
static capacitance change between the electrodes of the plates. Since the 
electrodes are interconnected by a metal film on the same plane of each 
plate, the distributed capacitance of the connecting metal film for the 
electrodes of each plate acts to reduce the static capacitance change 
occuring due to the rotation of the rotating plate and thus decrease the 
change of the signal appearing at the electrodes on the output side, so 
that satisfactory detection of the rotation can not be effected. 
Accordingly, the present invention proposes a rotation detecting apparatus 
so arranged that in order to solve the above problems, first and second 
plates are provided to oppose to each other, the first plate having first 
and second electrodes alternately arranged at constant intervals in the 
circumferential direction so as to oppose to the electrodes of the first 
plate which are formed at constant intervals in the circumferential 
direction, and the electrodes of either of the first and second plates are 
electrically connected together on the rear side of the plate through 
apertures provided therein, thereby decreasing the distributed capacitance 
between the electrodes of the first and second plates so as to increase 
the change of a signal appearing at the electrodes on the output side of 
the second plate.

An embodiment of this invention will hereinafter be described with 
reference to the drawings. 
Referring to FIG. 1, reference numeral 100 represents a housing which is 
fastened to the outside of a bearing 110, and 120 a shaft fastened on the 
inside of the bearing 110 and connected to, for example, the crankshaft of 
an engine. Thus, when the shaft 120 rotates, a rotating member 160 which 
is fastened to the shaft 120 with screws 190 is rotated together with the 
shaft 120. To the rotating member 160 is secured with screws 180 a second 
disk plate 150 which is formed of a printed board on which electrodes are 
formed by printing, which disk plate is rotated with the revolution of the 
rotating member 160. Moreover, a first disk plate 140 formed of a printed 
board on which electrodes are printed is fastened to the housing 100 with 
screws 170, and connected with signal lines 141, 142, 143 and 144 from a 
detector circuit 101 which is mounted on a printed board 130 secured to 
the housing 100. 
FIG. 2 shows a side 140a of the first plate 140, opposing to the second 
plate 150. The signal lines 141, 142, 143 and 144 are connected through 
junctions 21, 22, 23 and 24 each being of metal film, to first metal film 
electrodes 145, second metal film electrodes 146, a third circular 
electrode 147 of metal film and a fourth circular electrode 148 of metal 
film, of the first plate 140, respectively. The first and second 
electrodes 145 and 146 are alternately arranged in the circumferential 
direction to have a predetermined spacing therebetween. The inner ends of 
the first electrodes 145 and the outer ends of the second electrodes 146 
are provided with apertures 145a, 146a, respectively which are board 
through the first plate 140. As shown in FIG. 3, on a rear surface 140b 
opposite to the surface 140a, the electrodes 145 are electrically 
interconnected by a metal film connection 145b through the apertures 145a 
and similarly the electrodes 146 are electrically interconnected by a 
metal film connection 146b through the apertures 146a. 
FIG. 4 shows a side 150a of the second plate 150, opposing the first plate 
140. First toothed metal film electrodes 151 are connected to a third 
circular electrode 156 of metal film by a lead metal wire 153, and second 
toothed metal film electrodes 152 to a fourth circular electrode 155 by a 
lead metal wire 154. The first and second electrodes 151 and 152 are 
disposed so that their equispaced radial teeth are alternately 
interdigitated with each other in the circumferential direction. On the 
plates 140 and 150, the electrodes 146, 145, 147 and 148 are respectively 
opposed to the electrodes 151, 152, 156 and 155, in which case all the 
electrodes are formed by printing. 
FIG. 5 is an electrical wiring diagram of the detector circuit 101. 
Referring to FIG. 5, there are shown a power supply terminal 301 to which 
a constant voltage V.sub.c is applied, and a terminal 302 which is 
grounded. Shown at 310 is a known CR oscillator, 320 a reference signal 
generator, 330 a detector having the upper electrodes 151, 152, 156 and 
155 and the lower electrodes 146, 145, 147 and 148, 340 a comparator, 350 
a phase detector, 360 an output circuit, and 303 an output terminal. 
The operation of the above-mentioned arrangement will next be described. As 
shown in FIG. 5, the CR oscillator 310 consists of inverter gates 311, 312 
and 313, resistors 314 and 315 and a capacitor 316 and produces an 
oscillation waveform 10 as shown by FIG. 7(a). This oscillation waveform 
10 is transmitted to the reference signal generator 320 including inverter 
gates 321, 322 and 323, which then produces a signal equal in phase to the 
oscillation waveform 10 and a signal 20 opposite in phase thereto as shown 
by FIG. 7(b). 
As shown in FIG. 6A, when the shaft 120 (as illustrated in FIG. 1) rotates, 
the second plate 150 rotates along therewith so that its first electrode 
151 opposes the first electrode 145 of the first plate 140 and that the 
second 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, the oscillation waveform 10 is 
passed through a capacitor (as represented by 331 in FIG. 5) formed by the 
electrode 145 of the first plate 140 and the first electrode 151 of the 
second plate 150 and appears at the lead metal film 153. Then, this signal 
in phase with the signal 10 is transmitted through a capacitor 
(represented by 332 in FIG. 5) formed by the third electrode 156 of the 
second plate 150 and the third electrode 147 of the first plate 140, and 
through the junction 23 to the comparator 340 as a signal 30 shown by FIG. 
7(c). Similarly, the signal 20 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. 5) formed by the second electrode 146 and 
the second electrode 152 of the second plate 150, to the second 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. 5) formed by 
the fourth electrode 155 of the second plate 150 and the fourth electrode 
148 of the first plate 140, and through the junction 24 to the comparator 
340 as a signal 40 shown by FIG. 7(d). However, the signals 30 and 40 take 
the waveform with a reference potential V.sub.a shown by FIGS. 7(c) and 
7(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 50 which is, as shown 
by FIG. 7(e), delayed by time .DELTA.T with respect to the oscillation 
waveform 10, where .DELTA.T 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 "1" state at the 
output terminal Q.sub.1 and the D-type flip-flop 355 takes "0" state at 
the output terminal Q.sub.2. Then, the stage of NAND gates 356, 357, 358 
and 359 thus supplies "0" signal 60 shown by FIG. 7(f) to a signal line 
60. This signal is applied to the output circuit 360 in 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 "1" (a 
signal 70 shown by FIG. 7(g)) indicating that the first electrode 151 of 
the second plate 150 opposes the first electrode 145 of the first plate 
140 (or the second electrode 152 of the second plate 150 opposes the 
second electrode 146 of the first plate 140). 
Moreover, as shown in FIG. 6B, when the first electrode 151 of the second 
plate 150 comes to be opposite to the second electrode 146 of the first 
plate 140 and the second 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 first electrode 151 of the second plate 150. 
Then, to the third electrode 147 of the first plate 140 is applied a 
signal 31 with the reference potential of V.sub.a as shown by FIG. 7(c). 
In addition, the signal in phase with the oscillation with the oscillation 
waveform 10 appears at the second 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.sub.a as shown by FIG. 7(d). 
At this time, to the output terminal of the comparator 340 is applied a 
signal 51 as shown by FIG. 7(e) and thus at the output terminal of the 
phase detector 350 there appears a signal 61 as shown by FIG. 7(f). 
Consequently, to the output terminal 303 is applied a signal "0" (a 
signal 71 as shown by FIG. 7(g)) indicating that the first electrode of 
151 of the second plate 150 is opposite to the second electrode 146 of the 
first plate 140. 
Thus, when the first and second 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 
"1" and "0" alternately appear at the output terminal 303, thereby 
enabling detection of the rotation of the second plate 150, or the shaft 
120. 
In this embodiment, the signals appearing at the first and second 
electrodes 151 and 152 of the second plate 150 are compared by the 
comparator 340, and thus a large output is produced therefrom in 
accordance with the difference between the signals at the first and second 
electrodes 151 and 152, thereby assuring detection of the rotation. 
According to an experiment on the embodiment of the invention, the 
connection metal films 145b and 146b act to reduce the 
distributed-capacitance effect by an amount corresponding to the thickness 
of the first plate 140 as compared with the case when the first and second 
electrodes are respectively connected together on the surface 140a of the 
first plate 140. Thus, the signal difference appearing between the first 
and second electrodes 151 and 152 arranged on the second plate 150 becomes 
so large as to be detected easily thereby assuring the detection of 
rotation. 
While in the above embodiment the first electrodes 145 and the second 
electrodes 146 arranged on the first stationary plate 140 are respectively 
connected together on the rear surface, the first electrodes 151 and the 
second electrodes 152 arranged on the second rotating plate 150 may 
respectively be connected together or such connection may be made on the 
rear sides of both the first and second plates 145 and 146. 
Moreover, either of the first and second electrodes 151 and 152 arranged on 
the second plate 150 may be omitted and in this case either of the third 
and fourth electrodes 155 and 156 will be arranged on the second plate 
150. Then, if the signal appearing at either of the electrodes as 
described above is compared with the reference voltage determined by the 
resistors 343 and 344 shown in FIG. 5, the detection of rotation can be 
effected substantially in the same way. Furthermore, the first plate 140 
and the second plate 150 may be interchanged to serve as rotating and 
stationary plates, respectively. 
Thus, the rotation position detector according to the invention comprises a 
first plate having first and second electrodes alternately arranged 
thereon with a constant spacing in the circumferential direction, and a 
second plate having electrodes arranged thereon with the same spacing in 
the circumferential direction to oppose the electrodes on the first plate, 
at least electrodes arranged on one side of the first and second plates 
being connected together by connecting metal films on the rear side of the 
plates through apertures provided on the plates. Therefore, the influence 
of the distributed capacitance of the metal films is reduced by an amount 
corresponding to the thickness of plates, thereby enabling static 
capacitance change to be prevented from decreasing. In addition, these 
plates can be made of a printed board and thus contribute to reduction in 
cost.