Capacitance accelerometer

Abstract: A capacitive acceleration sensor using a common PWB as one of the plates of the capacitive transducer and the wiring connections thereto. An acceleration input moves a capacitor plate to produce a measurable change in capacitance. An electronic signal conditioner (14) accurately converts the capacitance output to a voltage signal to provide an acceleration sensor featuring accurate DC performance. The transducer and electronics are disposed on a PWB making up a complete sensor package. In a first embodiment, the capacitor comprises a fixed electrode (3) on the PWB (1) and a movable acceleration responsive electrode (9) secured to a support (7) by a conductive resilient member (8), the support (7) coupled to an electrically conductive region (2) on the PWB (1) spaced from the fixed electrode (3) by a guard ring (4) to minimize stray charge between the capacitor electrodes. In a second embodiment, the movable capacitor electrode (21) is supported at its center by a pair of electrically conductive members (22, 22'), one coupled to an electrically conductive region on a substrate (24) spaced from the movable electrode by a sealing spacer (25). The fixed capacitor electrode is disposed either on the same substrate or a separate substrate on the opposite side of the movable electrode spaced from the movable electrode by a sealing spacer and having the fixed capacitor electrode thereon. The movable capacitor electrode can be secured at its center or at an edge and can be produced by stamping and/or photoetching and provide a single element where plural elements were previously required.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to accelerometers for measuring acceleration by measuring the capacitance of a capacitor, the capacitance of which varies with change in acceleration. BRIEF DESCRIPTION OF THE PRIOR ART Prior art capacitive accelerometers generally utilize an inertial beam and air damping. Examples of typical prior art accelerometers are shown in Patents 3,339,419 of Wilcox, 3,709,042 of Lee, 3,089,343 of Rule, 3,132,521 of Krupick, 3,240,073 of Pitzer, 3,292,059 of Woods, 3,267,740 of Stedman, 4,333,029 of Kolm and 3,988,620 of McDavid. The prior art capacitive accelerometers have generally been complex, resulting in high cost of manufacture. It is therefore apparent that the art always seeks capacitive accelerometers which can be produced in a less costly manner In Figure 4 of US-A-4 434 665 and in Figure 4 of DE-3 201 198 A1, there is shown a capacitive transducer for measuring acceleration having a plate extending as a cantilever over a surface from a thickened part formed at one end of the plate by which it is attached to the surface. A mass is attached to the free end of the plate and conductive layers on the plate and the surface form the electrodes of the capacitor. In Figure 8 of US-A-4 744 248 and US-A-4 744 249 and in Figure 8 of DE-3 719 037 A1, there is shown an accelerometer having a sensing mass member that is formed as an integral part of a conductive sheet by anisotropic etching so that the sensing member is cantilevered from an inside edge of its support means. The sensing member and its support means are sandwiched between insulating plates having metallised areas adjacent to the sensing member to form capacitors responsive to the movement of the sensing member due to accelerometer of the device. US-A-4 694 687 discloses an accelerometer comprising two pairs of back to back capacitors. Each pair is made from five stacked layers of printed circuit board, the second and fourth layers of circuit board being in the form of rings for spacing the other layers apart. The third layer has a central portion which can move in response to acceleration. Connection to the various layers is made using conducting pins which pass through holes drilled in the stack. The two pairs are mounted by their pins at right angles to each other and the electronics for sensing them are connected to the pins. US-A-4 435 737 discloses an accelerometer comprising a pair of back to back capacitors formed from a conductive semiconductor component sandwiched between two glass substrates. The semiconductor component has a thick central portion which is in contact with conductive tracks on the glass substrate. Outside the central portion the semiconductor component has a thin disc portion and outside that a thicker mass portion separated from the substrates by small gaps. According to a first aspect of the present invention there is provided a capacitance accelerometer comprising, an insulating substantially planar substrate having an electrically conductive pattern thereon defining a capacitor plate, a conductor disposed around the capacitor plate and connections to the capacitor plate and the conductor, a spacer mounted around the capacitor plate on the conductor, and a support member that is disposed around a mass and is mounted on the spacer with a substantially planar face against the spacer, the mass being supported over the capacitor plate from the support member by a connecting beam of a spring material, and the mass having a conductive substantially planar face that is supported to be spaced from and parallel to the capacitor plate, thus forming a complementary capacitor plate, and that is substantially coplanar with the said face of support member, characterised in that the support member, the beam, the mass and the spacer are electrically conductive, and in that the accelerometer comprises electronic components that are mounted on the substrate and are connected to the connections, for operating the capacitor and producing from the capacitor a signal related to the acceleration of the accelerometer as represented by the variation in the spacing of the capacitor plates. According to a second aspect of the present invention there is provided a method of producing the capacitance accelerometer comprising the following steps: placing the mass and support member with their said faces on a flat surface and bonding the beam to the support member and the mass in that position using a low stress inducing technique. Briefly, there is provided a capacitive acceleration sensor which uses a common printed wiring board or substrate as both one of the plates of the capacitive transducer and the wiring connections thereto. The sensor utilizes a single variable capacitor and a fixed reference capacitor. The sensor is designed to measure low "g" accelerations, but can be easily modified to measure higher "g"s. An acceleration input moves a capacitor plate producing a change in capacitance. The sensor utilizes an electronic signal conditioner which accurately converts the capacitance output to a voltage signal. The transducer and electronics are laid out on a printed wiring board making up a complete sensor package. The result is a low "g" acceleration sensor featuring accurate DC performance. In a first embodiment of the invention, the capacitor comprises a fixed electrode on the PWB and a movable acceleration responsive electrode which is secured to a support by a conductive resilient member, the support being coupled to an electrically conductive region on the PWB which is spaced from the fixed electrode by a guard ring to minimize stray charge passing between the capacitor electrodes. In accordance with a second embodiment of the invention, the movable capacitor electrode is supported at its center region by a pair of electrically conductive members, one of which is coupled to an electrically conductive region on a substrate spaced from the movable electrode by a sealing spacer. The fixed capacitor electrode is disposed either on the same substrate or on a separate substrate on the opposite side of the movable electrode spaced from the movable electrode by a sealing spacer and having the fixed capacitor electrode thereon. The movable capacitor electrode can be secured at its center or at an edge. The movable electrode or seismic disk can be produced by stamping and/or photoetching and provide a single element where plural elements were previously required. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is an exploded view of an acceleration transducer with printed wiring board in accordance with the present invention; FIGURE 2 is a cross sectional view of the assembled transducer of FIGURE 1; FIGURE 3 is an exploded view of an acceleration transducer in accordance with a second embodiment of the present invention; FIGURE 4 is a cross sectional view of the assembled transducer of FIGURE 4; FIGURES 5a and 5b are illustrations of other configurations for securing the seismic disc as well as the disk configurations; and FIGURES 6a, 6b and 6c are side, top and cross sectional views showing a method of beam/mass/support fabrication resulting in a flat mass/support and fabricated in a batch or array assembly procedure. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGURES 1 and 2, there is shown the capacitive transducer 13 on a metallized printed wiring board (PWB) 1 having electronics 14 thereon in accordance with the present invention. The PWB can be formed of ceramic, epoxy or other well known material from which PWBs are generally fabricated. Metallization on the printed wiring board 1 acts as the fixed capacitor plate of the transducer. The metallization on the PWB, preferably of copper, includes a source ring 2 which is generally connected to an oscillator, a detect region 3 within the source ring connected to the electronics 14 and a guard ring 4 surrounding the detect region. The detect region 3 picks up the AC signal from the source ring 2 connected to the beam mass support structure and the charge collected thereon is an indication of the instantaneous capacitance of the capacitor. The guard region 4 minimizes surface currents from the source ring 2, which is oscillating in voltage, to the detect region 3. A preform spacer 5 is placed on the source ring 2 and provides electrical contact to the spring/mass element 6. The spacer 5 can be an electrical conductor such as, for example, an epoxy or it can be an insulator with an electrical conductor disposed thereacross. The spring/mass element 6 is composed of a support ring 7 of electrically conductive material, an electrically conductive beam 8 which acts as the spring of the system and an electrically conductive mass 9 which deflects under a given acceleration, inducing a capacitance change relative to the detect region 3. The beam 8 is secured to the support ring 7 and conductive mass 9. The support ring 7 and conductive mass 9 are generally of the same thickness and lie generally in the same plane. The spring/mass element 6 is assembled as a unit on a flat surface with a low stress inducing bonding means (solder, brazed, epoxy or other adhesive, spot weld or laser weld). The flatness of the spring/mass element 6 is unaffected by beam variations. A second preform electrically conductive epoxy spacer 10 is placed on the support ring 7 and spaces a section of double sided metallized printed wiring board 11 therefrom. This section of PWB is optional and assists in shielding from stray capacitances by being part of the overall ground plane which contains the other side of the printed wiring board. The PWB 1 also contains electronic circuitry 14 thereon to which the capacitor is coupled. The above described PWB can be modified to have electronics on one surface thereof and the transducer on a different or opposing surface thereof. The system as described above displays essentially no temperature effect to the first order when the temperature coefficient of expansion of the printed wiring board 1, the spacers 5 and 10, the board 11 and the support ring 7 are matched. The fact that a single beam 8 is used permits the conductive mass 9 to move in all directions, thereby minimizing the requirement that the beam or mass be temperature matched to the remaining components. The device as described above also provides protection against overload by providing stops by the upper and lower circuit board sections 1 and 11 which prevent excess vertical movement by the electrically conductive mass 9. Squeeze film damping is provided by the circular flat mass moving relative to the circuit boards. The spacing between the mass and overload stops is preferably less than or equal to 0.005 inches to obtain significant initial capacitance changes in capacitance and damping. The beam and/or mass dimensions can be modified to change the level of accelerations sensed and the frequency range desired. A differential capacitance mode could be obtained with slight modifications to the sensor if more signal is desired. The sensor is contained on a single printed wiring board, can be assembled with automated machinery and provides for simple edge-card connection along the pads 12 on the PWB. The whole assembly can be overmolded for environmental protection. In operation, the source ring 2 is coupled through the spacer 5 and the support ring 7 to the conductive mass 9 as well as to the electronics 14. The detect region 3 is also coupled to the electronics 14. Accordingly, as the conductive mass 9 moves due to acceleration and changes thereof, the capacitive coupling between the mass and the detect region 5 changes, resulting in a change in the charge on the detect region, this change being detected by the electronics to provide a readout related to acceleration. Referring now to FIGURES 3 and 4, there is shown a second embodiment in accordance with the present invention. The device comprises a thin, supported diaphragm or seismic disk 21 which acts as the seismic mass and spring of the accelerometer system. Supports 22 and 22' of conductive epoxy adhesive with glass spacers are electrically coupled to the diaphragm 21 and are disposed on opposite sides of the diaphragm to control capacitor function. An upper ceramic plate 23 with metallization thereon (not shown) acts as a mechanical stop as well as a fixed capacitor plate. The lower ceramic plate 24 also acts as a mechanical stop. Both plates 23 and 24 combined with the flexible diaphragm 21 determine the damping ratio of the system. Damping is provided by viscous air flow between the moving diaphragm 21 and plates 23 and 24 (squeeze film damping). The sealing of the device is accomplished with a non-conductive epoxy adhesive 25 around the periphery of the plates 23 and 24. The electronics (not shown) are connected to source (capacitor plate), detect (diaphragm) and guard ring (to protect detect from stray surface currents). Connection to the capacitor is made by contact to the metallization on the plate 23 and to a conductor on the plate 23 or 24 which is coupled to one of the supports 22 or 22′. The device allows for little stray capacitance with this integrated electronic configuration and provides great overload protection with the plates acting as mechanical stops. The diaphragm dimensions can be changed to add flexibility to the level of accelerations sensed and the frequency range desired. It should be noted that the seismic disk 21 can be secured at the center as shown in FIGURES 3 and 4 or secured at an edge as shown in FIGURES 1 and 2. The seismic disk 21 can also replace the beam/mass/support in FIGURES 1 and 2. FIGURES 5a and 5b are illustrations of other configurations for securing the seismic disc as well as the disk configurations. Seismic disks of the type shown in FIGURES 3, 4, 5a and 5b can be produced by stamping and/or photoetching and provide a single element there plural elements were previously required. Referring now to FIGURES 6a, 6b and 6c, there is shown a process for fabrication of the beam, mass and support structure of the above described embodiment of FIGURES 1 and 2 as single units or as an array. The mass and support structure can be made as a sintered powder metal part with interconnecting tabs. This part can then be subsequently ground on both sides since the mass and support must be of substantially the same equal thickness and have substantially parallel or coplanar surfaces on both sides. A beam can then be attached via a low stress inducing process, e.g., laser welding, brazing or reflow soldering and then the interconnecting tabs can be separated by laser cutting.

Claims: A capacitance accelerometer comprising, an insulating substantially planar substrate (1) having an electrically conductive pattern thereon defining a capacitor plate (3), a conductor (2) disposed around the capacitor plate (3) and connections to the capacitor plate and the conductor, a spacer (5) mounted around the capacitor plate (3) on the conductor (2), and a support member (7) that is disposed around a mass (9) and is mounted on the spacer (5) with a substantially planar face against the spacer (5), the mass (9) being supported over the capacitor plate (3) from the support member (7) by a connecting beam (8) of a spring material, and the mass (9) having a conductive substantially planar face that is supported to be spaced from and parallel to the capacitor plate (3), thus forming a complementary capacitor plate, and that is substantially coplanar with the said face of support member (7), characterised in that the support member, the beam, the mass and the spacer are electrically conductive, and in that the accelerometer comprises electronic components (14) that are mounted on the substrate (1) and are connected to the connections, for operating the capacitor and producing from the capacitor a signal related to the acceleration of the accelerometer as; represented by the variation in the spacing of the capacitor plates. An accelerometer according to claim 1 further including a second substrate (11) spaced from said mass (9) on its side remote from the first-mentioned substrate (1) and means to seal the mass (9) between the substrates (1,11). An accelerometer according to claim 2 wherein the sealing means comprises a second spacer (10) separating the second-mentioned substrate from the support member. An accelerometer according to any one of claims 1 to 3 wherein the mass is supported by a plurality of beams, each connecting the mass and the support member. An accelerometer according to any one of claims 1 to 3 wherein the beam (8) is the only beam connecting the support member (7) and the mass (9). An accelerometer according to claim 5 wherein the beam (8) is connected to a point on the mass (9) overlying the centre of the capacitor plate (3) on the substrate. An accelerometer according to any one of claims 1 to 6, wherein the mass (9), the beam (8) and the support member (7) are separate parts bonded together or attached to each other. An accelerometer according to any one of claims 1 to 6, wherein the mass (9), the beam (8) and the support member (7) are integral with each other. A method of producing a capacitance accelerometer according to claim 7 when dependent on claim 5 or claim 6; or according to claim 5 or claim 6; wherein the mass (9) and the support member (7) are placed with their said faces on a flat surface, and the beam (8) is bonded to the support member (7) and the mass (9) in that position using a low stress inducing technique of bonding. A method according to claim 9 wherein a structure comprising the mass (9) and the support member (7) is formed with tabs interconnecting them and wherein the tabs are removed after the beam (8) has been bonded to them. A method according to claim 10 wherein the structure comprising the mass (9), the support member (7) and their interconnecting tabs is made of sintered metal powder and wherein that structure is so ground on both sides as to give each of the mass (9) and the support member (7) parallel sides which are coplanar with respective ones of those sides of the other. A method according to any one of claims 9 to 11 wherein the beam (8) is bonded to the mass (9) and the support member (7) by soldering of brazing, or by adhesive, or by spot or laser welding.

Inventors: KAWATE KEITH W, REIDEMEISTER ERIC P, KAWATE, KEITH W., REIDEMEISTER, ERIC P.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1990
Main classification: G01P15
IPC codes: G01P15