Dual flexure plate angular accelerometer

A dual capacitance accelerometer system includes two flexure plates coupled to the housing and defining a respective parallel flex axes. A fixed plate is adjacent to and in substantially parallel relation to the two flexure plates and is also coupled to the housing. The fixed plate and one of the flexure plates define a first distance and the fixed plate and the other flexure plate define a second distance. The first and second distances vary in response to acceleration forces acting upon the flexure plates.

BACKGROUND OF INVENTION

The present invention relates generally to capacitive accelerometers, and more particularly, to a flexure plate angular accelerometer.

It is well known that capacitive accelerometers measure the acceleration, vibration and the inclination of objects to which they are attached. These objects typically include missiles, spacecraft, airplanes and automobiles.

In general, capacitive accelerometers change electrical capacitance in response to acceleration forces and vary the output of an energized circuit. Capacitive accelerometer systems generally include sensing elements, including capacitors, oscillators, and detection circuits.

The sensing elements include at least two parallel plate capacitors functioning in differential modes. The parallel plate capacitors generally operate in sensing circuits and alter the peak voltage generated by oscillators when the attached object undergoes acceleration.

When subject to a fixed or constant acceleration, the capacitance value is also a constant, resulting in a measurement signal proportional to uniform acceleration.

This type of accelerometer can be used in a missile or in a portion of aircraft or spacecraft navigation or guidance systems. Accordingly, the temperature in the operating environment of the accelerometer changes over a wide range. Consequently, acceleration must be measured with a high accuracy over a wide range of temperatures. This is often a difficult and inefficient process.

The disadvantages associated with current capacitive accelerometer systems have made it apparent that a new capacitive accelerometer is needed. The new accelerometer should substantially minimize temperature sensing requirements and should also improve acceleration detection accuracy. The present invention is directed to these ends.

SUMMARY OF INVENTION

A dual capacitance accelerometer including a housing further includes a first flexure plate defining a first flex axis and coupled to the housing. A second flexure plate, fixed within the housing, is spaced apart from the first flexure plate and defines a second flex axis in parallel relation to the first flex axis. A fixed plate adjacent to and in substantially parallel relation to the first and second flexure plates is also coupled to the housing. The fixed plate and the first flexure plate define a first distance, and the fixed plate and the second flexure plate define a second distance. The first and second distances vary in response to acceleration forces acting upon the first flexure plate and the second flexure plate.

In accordance with another aspect of the present invention, a method for operating a dual flexure plate accelerometer system includes accelerating a first flexure plate and a second flexure plate in relation to a fixed plate, thereby causing a first distance between the fixed plate and the first flexure plate to change and thereby causing a second distance between the fixed plate and the second flexure plate to change. The method further includes generating a first frequency signal including a sum of a linear acceleration and an angular acceleration acting on the first flexure plate and generating a second frequency signal including a difference of a linear acceleration and an angular acceleration acting on the second flexure plate. An angular acceleration signal is generated from a difference of the first frequency signal and the second frequency signal.

One advantage of the present invention is that it generates a dynamic range and granularity sufficient for Inter-Continental Ballistic Missile (ICBM) usage. Additional advantages include that the accelerometer system consumes less power than prior accelerometer systems, while dramatically improving reliability and reduction in manufacturing costs.

Additional advantages and features of the present invention will become apparent from the description that follows, and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION

The present invention is illustrated with respect to a dual flexure plate angular accelerometer, particularly suited to the aerospace field. The present invention is, however, applicable to various other uses that may require accelerometers, such as any system requiring position and velocity measurements under extreme conditions, as will be understood by one skilled in the art.

Referring toFIG. 1, the missile or aerospace system10, including a dual flexure plate angular accelerometer system12(DFPAA) within an inertial measurement unit13, is illustrated. The aerospace system10is merely an illustrative example of an accelerating object and is not meant to be limiting. For example, the present dual flexure plate angular accelerometer12could be implemented in any accelerating object to sense acceleration forces, including any type of vehicle or missile system, such as a Minuteman III missile system or a Scud missile system.

The illustrated aerospace system10includes an inertial measurement unit13including three dual flexure plate angular accelerometers (first)12, (second)15, (third)17and a serial data bus18. The three accelerometers, the x-axis accelerometer12, the y-axis accelerometer15, and the z-axis accelerometer17, are coupled to gimbals and gimbal torque motors20(yaw, pitch and roll motors). The accelerometers12,15,17are also coupled to the serial bus18, which transfers information to a computer/processor14. The computer14is coupled to the missile steering nozzle (or vane actuators) unit16and the gimbal torque motors20.

The dual flexure plate angular accelerometer12is a single axis accelerometer that generates a robust wide dynamic range of performance. Important to note is that alternate embodiments of the present invention have one or more accelerometers, the three illustrated accelerometers12,15,17are only one example of a possible arrangement of accelerometers, and any number of accelerometers can be utilized.

Referring toFIGS. 1 and 4, an example of a possible configuration for the accelerometer12is included as an illustrative example of the three accelerometers12,15, and17. The accelerometer12is part of an inertial measurement unit13(IMU), as was previously discussed. The accelerometer12includes a shared capacitor sensor24, two oscillators25,26, a frequency subtraction device28, and a Linear Lookup Table (LLT) or linearizer29.

The shared capacitor sensor24includes two parallel flexure plates30,32, a single fixed plate34, and a metal housing structure36. The shared capacitor sensor24generates phase shift capacitance signals as a function of a periodic signal from the signal generator37and in response to acceleration of the aeronautical system10, as will be discussed later.

The flexure plates30,32are positioned substantially parallel to one side of the fixed plate34such that the first flexure plate30plate is a first distance (d1) from the side38of the fixed plate34, and the second flexure plate32is a second distance (d2) from the side38of the fixed plate34. The flexure plates30,32are affixed to the metal housing structure36through at least a portion of at least one edge of each of the flexure plates30,32. The fixed plate34is coupled to the housing structure36and to a ground40. The plates,30,32, and34are embodied herein as coupled to one side41of the housing36; however, numerous other attachment configurations could also be used.

One embodiment of the present invention includes the three accelerometers12,15, and17each having a fixed plate, two flexure plates, a pair of oscillators, a frequency subtraction device, and a linearizer. The orientation of the flex axis of the accelerometers12,15, and17are orthogonal, however numerous alternate orientations are embodied herein, as will be understood by one skilled in the art.

The flexure plates30,32are rigidly fixed to the metal housing structure36through almost any manner known in the art. Resultantly, all the system flexure is generated within the flexure plates30,32, which generally increases reliability and robustness of the system10. This, however, generates a non-linear output from the flexure plates30,32, which will be discussed regarding the linear lookup table linearizer29.

A gas or vacuum environment is enclosed within the sensor24through the metal housing structure36such that there is no interference with the movement of the flexure plates30,32other than the acceleration of the system10along a perpendicular axis. During acceleration, the flexure plate30flexes according to the reaction force of Newton's second law of motion, force=mass×acceleration (F=ma), causing the distance between the flexure plates30,32and the fixed plate34to vary, thus creating the two variable capacitors, one on each end of the fixed plate34.

The combination of the first flexure plate30and the fixed plate34forms a first parallel plate capacitor, and the combination of the second flexure plate32and the fixed plate34forms the second parallel plate capacitor. The equivalent capacitor for the first parallel plate capacitor is illustrated in broken lines as C1, and the equivalent capacitor for the second parallel plate capacitor is illustrated in broken lines as C2.

The capacitance of the parallel plate capacitors is illustrated by
C≅(ε0A)/d
where
ε0
is the permittivity constant, A is the area of a fixed plate34(if I is the length of one side and the cross section of the plate is square, then A=I2)and d is the effective distance between the fixed plate34and one of the flexure plates30,32. The first flexure plate30is coupled to the metal housing structure36and positioned a first distance (d1) from the flexure plate30. The first flexure plate30and the fixed plate34form a first capacitor whose operation is also governed by the equation
C≅(ε0A)/d.
The fixed plate34responds to movement of the first flexure plate30when d1either increases or decreases, thereby generating a first phase shift capacitance signal. The second flexure plate32is also coupled to the metal housing structure36and positioned a second distance (d2) from the fixed plate34. The second flexure plate32and the fixed plate34form a second capacitor whose operation is governed by the equation
C≅(ε0A)/d.

The fixed plate34responds to movement of the second flexure plate32when d2either increases or decreases, thereby generating a second phase shift capacitance signal.

The distances (d1and d2) between the fixed plate34and the flexure plates30,32are a function of acceleration and are proportional or equal when the system10is at rest. Each flexure plate30,32is connected to a respective oscillator25,26, which generates the phase shift capacitance necessary for predictable oscillation.

The distance, d, is the acceleration variable (F=ma), which determines oscillator frequency, f, and f=kd. As the flexure plates30,32sense acceleration, either linear (F=ma) or angular-tangential, each flexure plate will deflect in response to the sum of the forces acting thereon. Because the control mechanism keeps the flexure plates in the xy-plane, the total acceleration seen by each flexure plate is the sum of the linear acceleration and the tangential acceleration (al+at). This generates output frequency f1=(al+at)k and f2=(al+at)k. For equal distances of r1and r2, the expression f1=k1al+k2atand f2=k3al−k4at), where k1and k3are equal if r1=r2. Otherwise they are calculated or modeled for the exact expression. In this simplified case, however, f1−f2=(k2at)−(−k4at) and therefore at(f1−f2)/2*k.

The first flexure plate30is coupled to the first oscillator25, and the second flexure plate32is coupled to the second oscillator24. The two oscillators25,26are coupled to a frequency subtraction device26, and the frequency subtraction device26is coupled to the LLT28, which is coupled to a processor14(missile operations processor). The processor14is coupled to an actuator16, which is coupled to various system components, such as thrusters and attitude control devices.

The oscillators25,26are ideally precision designs utilizing GaAs or similar material. The oscillators25,26are also mounted on the metal housing structure36in the present embodiment.

The embodied first oscillator25includes components well known in the art. Although the embodied oscillator is a common oscillator type, one skilled in the art will realize that numerous other types of oscillators will also be adaptable for the present invention. The various components include, but are not limited to, two buffers,50, an inverter52, and at least one resistor54. The first oscillator25receives the phase shift capacitance signal from the first flexure plate30and generates therefrom a frequency signal (f1), which is proportional to d1.

The second oscillator26receives the phase shift capacitance signal from the second flexure plate capacitor and generates therefrom a second frequency signal (f2), which is proportional to d2. The embodied oscillator26is similar to the first oscillator25and also includes a set of buffers56, an inverter58, and at least one resistor60.

The frequencies (f1and f2) are functions of the distances (d1and d2) respectively. As the flexure plates30,32flex, one capacitor increases and the other decreases, thereby causing one oscillator25to increase output frequency and the other oscillator26to decrease output frequency.

The frequency subtraction device28receives the oscillator signals (f1and f2) and generates the difference thereof, i.e. f1−f2. Important to note is that the polarities of both f1and f2are determined before this difference is calculated. An overall frequency signal is generated from the frequency subtraction device28. In other words, the polarity of f1is positive if d1is increasing and negative if d1is decreasing. Likewise, the polarity of f2is positive if d2is increasing and negative when d2is decreasing. Determinations of increasing or decreasing d1and d2may generate from, for example, by the processor14.

A linearizer29or LLT receives the overall frequency signal. The linearizer29compensates for both the nonlinear function generated from the frequency subtraction device28and any manufacturing anomalies, as will be understood by one skilled in the art. The linearizer29value is established in manufacturing through taking large samples of performance curves, as will be understood by one skilled in the art. The linearizer29output is a digital word whose magnitude is proportional to the acceleration of the system10in either direction along an axis perpendicular to the flexure plates30,32.

Numerous alternate linearizers are also included in the present embodiment whereby a substantially linear function can be generated by compensating for nonlinear functions, for example, in the digital domain, a digital linearizer is included. The output of the linearizer29is an acceleration signal multiplied by a constant (k).

Statistical filtering of the linearized data somewhere significantly above the maximum flexure frequency also occurs in either the linearizer29or the processor14to reduce the overall noise impact on the system10. Herein, the filter31is included in the linearizer29.

The processor14receives the output signals and generates a processor signal and response thereto. The processor14is embodied as a typical missile or airplane processor, as is familiar in the art.

The actuator, here embodied as missile steering nozzle or vane actuators16receives the processor signal and activates system components (e.g. object control devices) in response thereto. System components include for example, thrusters or attitude control devices.

Referring toFIGS. 3 and 4, the angular accelerometer12ofFIG. 2is further illustrated. The angular accelerometer12includes the two flexure plates30,32and the fixed metal plate34.

The first flexure plate30is positioned a distance r1from a central y-axis or the z spin axis, and the second flexure plate32is positioned a distance r2, from the central y-axis or the z spin axis. Both flexure plates30,32are represented as plates having flex axes64,66parallel to the y-axis. Embodiments include multiple fixed plates or a single plate. The present embodiment, for simplicity, illustrates a single common fixed plate34.

One embodiment of the present invention includes the faces of the plates in the xy-plane, perpendicular to the z-axis at distances r1and r2from the coordinate origin. Numerous other arrangements are also included herein, such as the faces of the plates in the yz or xz planes for alternate configurations.

For the present invention, r1=r2. This is merely one embodiment, and in fact, they may be both on either side of the origin, as long as they are separated by a known distance, and at a known distance from the origin.

The accelerometers12,15, and17are herein included on an inertial platform. The platform may be a gimbal20or alternate inertial platform design known in the art. The system10utilizes the generated signals from the accelerometers15,17to control the platform position to maintain a near zero rotation.

Referring toFIG. 5, a logic flow diagram100illustrating a method for acceleration control is illustrated. Logic starts in operation block102where power is applied to the system, the missile platform is aligned and the capacitive accelerometer is activated.

In operation block104, strategic alert biasing occurs and sensor data is compared to a known reference.

In operation block106, the missile system10is launched.

In operation block108, the missile system10accelerates and the flexure plate flexes to either increase or decrease d1or d2for any of the three accelerometers12,15, or17. The oscillator activates and receives signals from the fixed plate capacitors which are generated in response to a change in either d1or d2. Notably, a change in d1will resultantly cause a change in d2. The oscillators25,26then generate frequency signals in response to the fixed plate capacitor signals. The frequency from the first oscillator25is subtracted from the frequency from the second oscillator26to generate a nonlinear overall frequency signal.

In operation block108, the overall frequency signal, i.e. the results of the acceleration, are linearized. This linearization is achieved through a linear lookup table (linearizer29), or other linearization methods known in the art. Data from the accelerometer(s) is processed by the missile computer or attitude controller.

In operation block110, aeronautical systems respond to the acceleration. In other words, the controller receives a signal indicating that acceleration of the system10has changed. In response to this change, for example, thrusters are activated to compensate for the acceleration change. In other words, the missile computer/controller/processor14controls the flight profile through the missile nozzle or steering vane actuators16.

In operation, a method for operating a dual flexure plate accelerometer system12includes accelerating a first flexure plate30and a second flexure plate32in relation to a fixed plate34, thereby causing a first distance between the fixed plate34and the first flexure plate30to change and thereby causing a second distance between the fixed plate34and the second flexure plate32to change. The method further includes generating a first frequency signal including a sum of a linear acceleration and an angular acceleration acting on the first flexure plate30and generating a second frequency signal including a difference of a linear acceleration and an angular acceleration acting on the second flexure plate32. An angular acceleration signal is generated from a difference of the first frequency signal and the second frequency signal.

From the foregoing, it can be seen that there has been brought to the art a new and improved accelerometer system12. It is to be understood that the preceding description of the preferred embodiment is merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. For example, a vehicle, such as an airplane, spacecraft, or automobile could include the present invention for acceleration detection and control. Numerous and other arrangements would be evident to those skilled in the art without departing from the scope of the invention as defined by the following claims.