Capacitive torque detection with strain dependent dielectric layer

Abstract: A dielectric layer 32 variable in dielectric constant in dependence upon the strain is formed on at least a part of an outer peripheral surface of a bar 31 having a circular cross section. Interdigital electrodes are faced to the dielectric layer to thereby form a single capacitor. The interdigital electrodes have a plurality of linear electrodes 34a and 34b inclined at a predetermined inclination angle with respect to a center axis of the bar. Preferably, the dielectric layer is provided with a cylindrical insulator 33 faced thereto with a small gap kept therebetween, and the linear electrodes are formed on the peripheral surface of the cylindrical insulator. The linear electrodes may be formed directly on the surface of the dielectric layer.

Description: Technical Field This invention relates to a torque sensor and a torque detecting method and, in particular, to a torque sensor and a torque detecting method which are suitable for an electric power steering apparatus in an automobile or a power-assisted bicycle. Background Art Conventionally, in order to supplement a force to manipulate a steering wheel (hereinbelow, referred to as a wheel), an electric power steering apparatus has been developed. The electric power steering apparatus is an apparatus which detects a torque applied to the steering wheel from a driver and, in response to the torque thus detected, makes an electric motor provided in a steering mechanism to generate a driving force for steering assistance. In comparison with a hydraulic power steering apparatus, the above-mentioned apparatus has such characteristics that no engine power is consumed to drive a hydraulic pump and that the controllability is excellent because the supplemental driving force is electrically controlled. The above-mentioned electric power steering apparatus is required to have a torque sensor simple in structure and capable of carrying out non-contact detection from the viewpoint of reliability and cost. Fig. 1 shows one example of the torque sensor which is applicable for such use. In the torque sensor of Fig. 1, a torsion bar 11 made of a material variable in magnetic permeability due to strain, that is, a magnetostrictive material is incorporated between an input shaft and an output shaft for input and output of the torque. On the outer periphery of the torsion bar 11, two coils 12 and 13 are placed with a given distance kept therebetween. On the surface of the torsion bar 11, elongated slits 14 inclined at ± 45° with respect to its center axis are formed in correspondence to the two coils 12 and 13. When the torque is produced between the input shaft and the output shaft, the torque is applied to the torsion bar 11 so that torsional strain is produced in the torsion bar 11. Generally, the torsional strain generated in a solid cylinder or a hollow cylinder is decomposed into elongation strain in a direction of 45° with respect to the longitudinal direction of the solid cylinder or the hollow cylinder and compression strain in a direction perpendicular thereto. Consequently, the elongation strain and the compression strain are respectively caused to occur in the directions of the slits 14 formed on the surface of the torsion bar 11. The magnetic permeabilities of the respective portions change in the manner such that one is increased while the other is decreased, and have levels substantially proportional to the torque applied. As a result, the inductances of the two coils 12 and 13 change in the manner such that one is increased while the other is decreased. Each of these two coils 12 and 13 is connected to a ground terminal through a fixed resistance as an impedance element, and an a.c. voltage having a predetermined frequency is applied between the fixed resistance and the ground terminal. At this time, a terminal voltage of each of the coils 12 and 13 is supplied to a differential amplifying circuit. In this event, the differential amplifying circuit produces an output voltage proportional to the magnitude of the torque applied. Therefore, it is possible to detect the magnitude of the torque according to the above-mentioned output voltage. Typically, an electric motor including a strong magnet is used in an automobile or a power-assisted bicycle. In order to avoid the influence of a magnetic field generated from the magnet, a robust magnetic shield is required. This brings about a drawback that the structure around the torque sensor becomes complicated. Further, the electric power steering apparatus requires a steering angle sensor for detecting a steering direction in order to control the steering supplementing force produced by the electric motor by detecting a steering speed and a steering angle simultaneously when the torque applied to the wheel is detected as described above. The above-mentioned steering angle sensorgenerally adopts a method of detecting the N-pole and the S-pole of a multipolar ring-shaped magnet by the use of a Hall device. In order to prevent a magnetic field leaking from the ring-shaped magnet from influencing the above-mentioned magnetostrictive material, a shield for the torque sensor portion is required also. This results in a drawback that the structure becomes complicated. Therefore, it is an object of the present invention to provide a capacitive torque sensor which does not require a magnetic shield and has a simple structure. It is another object of the present invention to provide a torque detecting method using the above-mentioned capacitive torque sensor. Disclosure of the Invention According to the present invention, there is provided a capacitive torque sensor including a bar having a circular cross-section, a dielectric layer which is formed on at least a part of an outer peripheral surface of the bar and which is variable in dielectric constant in dependence upon the strain, and a pair of interdigital electrodes facing the dielectric layer and forming a single capacitor, the interdigital electrodes comprising a plurality of linear electrodes inclined at a predetermined inclination angle with respect to a center axis of the bar. According to the present invention, there is also provided a torque detecting method using a capacitive torque sensor comprising a bar having a circular cross-section, a dielectric layer which is formed on at least a part of an outer peripheral surface of the bar and which is variable in dielectric constant in dependence upon the strain, and an interdigital electrode capacitor facing the dielectric layer and inclined at a predetermined inclination angle with respect to a center axis of the bar, the method comprising the steps of activating a modulating operation in response to a change in capacitance of the interdigital electrode capacitor to produce a modulation frequency signal, and detecting a torque from the modulation frequency signal. Brief Description of the Drawing Fig. 1 is a view for describing a conventional torque sensor. Fig. 2 is a perspective view of a capacitive torque sensor according to a first embodiment of the present invention. Fig. 3 is a perspective view of a capacitive torque sensor according to a second embodiment of the present invention. Fig. 4 is a circuit block diagram showing a torque detecting circuit using the capacitive torque sensor in Fig. 3. Fig. 5 is a perspective view of a capacitive torque sensor according to a third embodiment of the present invention. Fig. 6 is a circuit block diagram showing a torque detecting circuit using the capacitive torque sensor in Fig. 5. Best Mode for Embodying the Invention Referring to Fig. 2, description will be made about a capacitive torque sensor according to a first embodiment of the present invention. This capacitive torque sensor includes a circular bar 21 having a circular cross section and made of insulating plastic or insulating ceramics. The circular bar 21 is connected in series to and between an input shaft and an output shaft for input and output of a torque in a steering wheel. Accordingly, the circular bar 21 is rotated in response to manipulation of the wheel. The circular bar 21 has an outer peripheral surface provided with a thick-film or a thin-film dielectric layer 22 directly formed thereon and made of a material variable in dielectric constant in dependence upon the strain. On the surface of the dielectric layer 22, linear electrodes (digital electrodes) 23a and 23b are formed to be inclined at a predetermined inclination angle of about 45° with respect to a center axis (the longitudinal direction) of the circular bar 21. The linear electrodes 23a and 23b are alternately connected to common electrodes 24a and 24b, respectively, to thereby form so-called interdigital electrodes or an interdigital electrode capacitor. Thus, a single capacitor having capacitance is formed. A pair of terminals 25a and 25b are connected to the interdigital electrodes. A part of the circular bar 21 is provided with a first coil 26 wound along an outer periphery of the bar in a direction perpendicular to the axial direction of the bar to thereby form an inductor. This inductor is connected in parallel to the above-mentioned capacitor to thereby form a closed circuit. Specifically, the both ends of the first coil 26 are connected to the terminals 25a and 25b, respectively. In this case, the terminals 25a and 25b serve as connecting members which form a closed circuit. In addition, a second coil 27 is provided so as to face the first coil 26 with a small gap left therebetween. The second coil 27 is stationary. Next, description will be made about the principle of operation of the capacitive torque sensor in Fig. 2. When a steering wheel is manipulated, relative rotation force acts between the input shaft and the output shaft to thereby apply a torsional torque to the circular bar 21. Then, strain occurs in the dielectric layer 22 formed on the surface of the circular bar 21, resulting in a change in dielectric constant. In other words, the dielectric layer 22 has a strain to dielectric-constant-variation characteristic. The strain of the dielectric layer 22 can be decomposed into elongation strain in a direction of +45° and compression strain in a direction of - 45° with respect to the torsional axis direction. Thus, in dependence upon the direction of the torsional torque, the dielectric constant changes in the direction in which the linear electrodes 23a and 23b of the interdigital electrodes face each other. Following this, the value of the capacitance between the terminals 25a and 25b changes. With reference to the above-mentioned change in capacitance, the magnitude of the torque being produced can be detected. In order to detect the above-mentioned change in capacitance in a non-contact manner, the first coil 26 and the second coil 27 are electromagnetically coupled by means of the structure described above. With this structure, a change in capacitance formed by the interdigital electrodes is converted into a change in impedance of the coil 27, and the magnitude of torque can be detected from the latter change. According to the capacitive torque sensor in Fig. 2, it is possible to detect the torque in a non-contact manner without relying on a magnetic shield and without being influenced by an external magnetic field. Further, the structure is simple and the reliability of detection can readily be improved. The circular bar 21 may comprise a metal bar having conductivity and a plastic layer or a ceramic layer formed thereon and having a low dielectric constant. By using a magnetic substance at the portion where the first coil 26 is provided, it is possible to improve electromagnetic coupling with the second coil 27 and to obtain a capacitive torque sensor with higher sensitivity. Moreover, in case where a recessed portion for receiving the first coil 26 is formed along the circumference of the circular bar 21 and a recessed portion for receiving the second coil 27 is formed to face the above-mentioned recessed portion, the same effect is exhibited so that a capacitive torque sensor can easily be obtained. In Fig. 2, the LC resonance inductor and the electromagnetic coupling transformer inductor have a common structure. However, with a structure in which these inductors are separated, a capacitive torque sensor can be similarly obtained. Referring to Fig. 3, description will be made about a capacitive torque sensor according to a second embodiment of the present invention. This capacitive torque sensor includes a circular bar 31 made of insulating plastic or insulating ceramics and having a circular cross section. The circular bar 31 is connected in series to and between an input shaft and an output shaft for input and output of a torque in a steering wheel. Accordingly, the circular bar 31 is rotated in response to manipulation of the wheel. The circular bar 31 has a peripheral surface provided with a thick-film or a thin-film dielectric layer 32 formed at a part thereof and variable in dielectric constant in dependence upon the strain. A concentric cylindrical insulator 33 is fitted over the circular bar 31. The cylindrical insulator 33 faces the dielectric layer 32 with a small gap kept therebetween and is stationary. Preferably, a bearing is arranged in the small gap. On the outer peripheral surface of the cylindrical insulator 33, linear electrodes 34a and 34b are formed to be inclined at a predetermined inclination angle of about 45° with respect to a center axis of the circular bar 31. The linear electrodes 34a and 34b are alternately connected to common electrodes 35a and 35b, respectively, to thereby form so-called interdigital electrodes or an interdigital electrode capacitor. Thus, a single capacitor having capacitance is formed. A pair of terminals 36 and 37 are connected to the interdigital electrodes. Next, description will be made about the principle of operation of the capacitive torque sensor in Fig. 3. When a steering wheel is manipulated, relative rotation force acts between the input shaft and the output shaft to thereby apply a torsional torque to the circular bar 31. Then, strain occurs in the dielectric layer 32 formed on the surface of the circular bar 31, resulting in a change in dielectric constant. In other words, the dielectric layer 32 has a predetermined strain to dielectric-constant-variation characteristic. Since the linear electrodes 34a and 34b are capacitively coupled to the dielectric layer 32 with a small gap left therebetween, capacitance between the terminals 36 and 37 changes following a change in dielectric constant of the dielectric layer 32. With reference to the above-mentioned change in capacitance, the magnitude of the torque being produced can be detected. According to the capacitive torque sensor in Fig. 3, it is possible to detect the torque without relying on a magnetic shield and without being influenced by an external magnetic field. Further, the structure is simple and an LC oscillation circuit can readily be formed. The circular bar 31 may comprise a metal bar having conductivity and a plastic layer or a ceramic layer formed thereon and having a low dielectric constant. Further, the interdigital electrodes may be formed on an inner peripheral surface of the cylindrical insulator 33. Referring to Fig. 4 in addition, description will be made about a torque detecting circuit using the capacitive torque sensor in Fig. 3. An oscillating circuit 41 is an LC oscillating circuit or an RC oscillating circuit including capacitance of the capacitive torque sensor in Fig. 3 and supplies a f-V converter 42 with a frequency corresponding to the change in capacitance of the capacitive torque sensor in Fig. 3. The f-V converter 42 converts the frequency into a voltage and supplies the voltage to an operational amplifier 43. From an output terminal 44 of the operational amplifier 43, an output signal representative of a torsional torque being applied is produced. Therefore, it is possible to accurately detect a very small torque. Preferably, the frequency produced from the oscillating circuit 41 is supplied to a microcomputer which directly calculates the frequency, and the output signal is produced as a digital signal. Referring to Fig. 5, description will be made about a capacitive torque sensor according to a third embodiment of the present invention. Similar parts are designated by like reference numerals. In the capacitive torque sensor in Fig. 5 also, a circular bar 31 has a surface provided with a thick-film or a thin-film dielectric layer 32 formed at a part thereof and variable in dielectric constant in dependence upon torsional strain. Two cylindrical insulators 33a and 33b are fitted over the circular bar 31. These cylindrical insulators 33a and 33b face each other with a small gap left therebetween, are placed in different regions on an outer peripheral surface of a dielectric layer 32 which are different in the axial direction of the circular bar 31, and are stationary. Preferably, a bearing is arranged in the small gap. On the outer peripheral surface of each of the cylindrical insulators 33a and 33b, linear electrodes 34a and 34b are formed to be inclined at a predetermined inclination angle of about 45° with respect to a center axis of the circular bar 31. In one cylindrical insulator 33a and the other cylindrical insulator 33b, the inclining directions of the linear electrodes 34a and 34b are different from each other. The linear electrodes 34a and 34b are alternately connected to common electrodes 35a and 35b, respectively, to thereby form so-called interdigital electrodes or an interdigital electrode capacitor. Thus, around the circular bar 31, a reference capacitor part 52 is formed in addition to a detection capacitor part 51 as shown in Fig. 3. The reference capacitor part 52 has thermal characteristics equivalent to those of the detection capacitor part 51. Therefore, by correcting the change rate in capacitance of the detection capacitor part 51 with respect to the change in ambient temperature by the use of the change rate in capacitance of the reference capacitor part 52, it is possible to cancel the fluctuation in characteristics depending on the change in ambient temperature and to improve torque detecting sensitivity to a substantially twice level. Next referring to Fig. 6, description will be made about a torque detecting circuit using the capacitive torque sensor in Fig. 5. An oscillating circuit 61 is an LC oscillating circuit or an RC oscillating circuit including capacitance of the detection capacitor part 51. The oscillating circuit 61 supplies a f-V converter 62 with a frequency corresponding to the change in capacitance of the detecting capacitor part 51. The f-V converter 42 converts the frequency into a voltage and supplies the voltage to a comparator 63. An oscillating circuit 64 is an LC oscillating circuit or an RC oscillating circuit including capacitance of the reference capacitor part 52. The oscillating circuit 64 supplies a f-V converter 65 with a frequency corresponding to the change in capacitance of the reference capacitor part 52. The f-V converter 65 converts the frequency into a voltage and supplies the voltage to the comparator 63. The comparator 63 calculates a difference between the voltages thus supplied and produces, from an output terminal 66 thereof, an output signal representative of a torsional torque being applied. In this manner, temperature correction is performed so that a small torque can accurately be detected even if the thermal environment is changed. Preferably, the frequency produced from each of the oscillating circuits 61 and 64 is supplied to a microcomputer which directly calculates the frequency, and the output signal is produced as a digital signal. In any one of the embodiments described above, it is possible to use, as the dielectric layer, a dielectric ceramics film which is used in a ceramic capacitor or the like. In case where the reference capacitor and the detection capacitor are substantially same in dielectric material, film thickness, and size of the interdigital electrodes, the reference capacitor and the detection capacitor need not be formed on the same surface but may be separately arranged provided that the thermal environments of the detection capacitor and the reference capacitor are considered to be equivalent. Industrial Applicability The capacitive torque sensor of the present invention is suitable as a torque sensor for use in an electric power steering apparatus in an automobile or a power-assisted bicycle.

Claims: A capacitive torque sensor including a bar (21) having a circular cross-section, a dielectric layer (22) which is formed on at least a part of an outer peripheral surface of said bar and which is variable in dielectric constant in dependence upon the strain, and a pair of interdigital electrodes (23a,b) facing said dielectric layer and forming a single capacitor, said interdigital electrodes comprising a plurality of linear electrodes inclined at a predetermined inclination angle with respect to a center axis of said bar. A capacitive torque sensor as claimed in claim 1, wherein said dielectric layer has a thickness which is substantially uniform or wherein said predetermined inclination angle is about 45° or wherein said bar is made of an insulator or wherein said bar comprises a center conductor and a layer formed on the surface of said conductor and having a low dielectric constant. A capacitive torque sensor as claimed in claim 1, wherein said linear electrodes are directly formed on the surface of said dielectric layer. A capacitive torque sensor as claimed in claim 3 ,comprising a first coil (26) provided along the outer periphery of said bar and forming one inductor, a connecting part connecting said capacitor and said inductor with each other to form a closed circuit, and a second coil (27) electromagnetically coupled to said first coil with a small gap left therebetween. A capacitive torque sensor as claimed in claim 4, wherein said bar has a magnetic portion formed on at least a part thereof in a direction along its center axis, said first coil being provided in said magnetic portion. A capacitive torque sensor as claimed in claim 5, wherein said magnetic portion has an outside-facing recessed portion extending around the center axis of said bar, said first coil being received in said outside-facing recessed portion. A capacitive torque sensor as claimed in claim 6, comprising an inside-facing recessed portion facing said outside-facing recessed portion in the radial direction, said second coil being received in said inside-facing recessed portion. A capacitive torque sensor as claimed in claim 1, comprising a cylindrical insulator fitted to said bar and facing said dielectric layer with a small gap left therebetween, said linear electrodes being formed on the peripheral surface of said cylindrical insulator. A capacitive torque sensor as claimed in claim 8, wherein said interdigital electrodes are formed on the inner peripheral surface of said cylindrical insulator or wherein said interdigital electrodes are formed on the outer peripheral surface of said cylindrical insulator or comprising an additional cylindrical insulator fitted to said bar and facing said dielectric layer with a small gap left therebetween, and a pair of additional interdigital electrodes formed on the peripheral surface of said additional cylindrical insulator and forming one capacitor, said additional interdigital electrodes having a plurality of additional linear electrodes inclined at a predetermined inclination angle with respect to the center axis of said bar. A torque detecting method using a capacitive torque sensor comprising a bar (21) having a circular cross-section, a dielectric layer (22) which is formed on at least a part of an outer peripheral surface of said bar and which is variable in dielectric constant in dependence upon the strain, and an interdigital electrode capacitor facing said dielectric layer and inclined at a predetermined inclination angle with respect to a center axis of said bar, said method comprising the steps of activating a modulating operation in response to a change in capacitance of said interdigital electrode capacitor to produce a modulation frequency signal, and detecting a torque from said modulation frequency signal. A torque detecting method as claimed in claim 10, wherein said capacitive torque sensor comprises an additional interdigital electrode capacitor facing said dielectric layer and inclined at a predetermined inclination angle with respect to the center axis of said bar, said method comprising the steps of activating a modulating operation in response to a change in capacitance of said additional interdigital electrode capacitor to produce an additional modulation frequency signal, and carrying out temperature correction by the use of said additional modulation frequency signal.

Inventors: UENO TORU, WAKO NAOKI, YOSHIDA TETSUO, UENO, TORU, WAKO, NAOKI, YOSHIDA, TETSUO, UENO, Toru Tokin Corporation, WAKO, Naoki Tokin Corporation
Applicants: NEC TOKIN CORP, NEC TOKIN CORPORATION
Application year: 2000
Main classification: G01L3
IPC codes: G01M3, G01L3, B62D5, B62M6