Abstract:
A capacitance accelerometer includes a housing, and a plate fixed within the housing. A moveable plate is disposed in substantially parallel relation to the fixed plate and is coupled to the housing along at least an edge. The moveable plate and the fixed plate define a distance. The distance varies in response to acceleration forces acting upon the moveable plate, and wherein the moveable plate and the fixed plate generate a charge displacement capacitance signal. A transimpedance amplifier receives the charge displacement capacitance signal and generates a scaled voltage signal therefrom. An acceleration signal is generated from the scaled voltage signal.

Description:
BACKGROUND OF INVENTION  
       [0001]     The present invention relates generally to accelerometer systems, and more particularly, to a single plate capacitive acceleration derivative detector.  
         [0002]     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.  
         [0003]     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.  
         [0004]     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.  
         [0005]     When subject to a fixed or constant acceleration, the capacitance value is also a constant, resulting in a measurement signal proportional to uniform acceleration.  
         [0006]     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 for current accelerometer systems.  
         [0007]     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  
       [0008]     In accordance with one aspect of the present invention, an accelerometer includes a housing, and a plate fixed within the housing. A moveable plate is disposed in substantially parallel relation to the fixed plate and is coupled to the housing along at least an edge. The moveable plate and the fixed plate define a distance. The distance varies in response to acceleration forces acting upon the moveable plate, and wherein the moveable plate and the fixed plate generate a charge displacement capacitance signal. A transimpedance amplifier receives the charge displacement capacitance signal and generates a scaled voltage signal therefrom. An acceleration signal is generated from the scaled voltage signal.  
         [0009]     In accordance with another aspect of the present invention, a method for operating a single plate capacitive acceleration derivative detector includes accelerating the moveable plate, thereby causing a distance between the moveable plate and a fixed plate to change; generating a variable capacitor signal; generating a scaled voltage signal in response to the variable capacitor signal; and generating an acceleration signal in response to the scaled voltage signal.  
         [0010]     One advantage of the present invention is that it generates a dynamic range of temperature and a 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.  
         [0011]     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. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]     In order that the invention may be well understood, there will now be described some embodiments thereof, given by way of example, reference being made to the accompanying drawings, in which:  
         [0013]      FIG. 1  illustrates an aeronautical system in accordance with one embodiment of the present invention;  
         [0014]      FIG. 2  illustrates a capacitance acceleration derivative detector system in accordance with  FIG. 1 ;  
         [0015]      FIG. 3  illustrates an equivalent diagram for the variable capacitance sensor from the capacitance acceleration derivative detector system of  FIG. 2 ; and  
         [0016]      FIG. 4  illustrates a logic flow diagram of the aeronautical system of  FIG. 1  in operation, in accordance with another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]     The present invention is illustrated with respect to single plate capacitive acceleration derivative detector, particularly suited to the aerospace field. The present invention is, however, applicable to various other uses that may require acceleration detection, such as any system requiring acceleration detection under extreme conditions, as will be understood by one skilled in the art.  
         [0018]     Referring to  FIG. 1 , the missile or aerospace system controlling acceleration  10 , including a capacitance acceleration derivative detector system  11  (CADD) within an inertial measurement unit  13 , is illustrated. The aerospace system  10  is merely an illustrative example of an accelerating object and not meant to be limiting. For example, the present single plate capacitive acceleration derivative detector system  11  could 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.  
         [0019]     The illustrated aerospace system  10  includes an inertial measurement unit  13  including three accelerometers (first)  12 , (second)  15 , (third)  17  and a serial data bus  18 .  
         [0020]     The aerospace system  10  further includes a computer/processor  14 , a missile steering unit  16 , and a platform  20 .  
         [0021]     The three accelerometers  12 ,  15 , and  17 , are coupled to the inertial platform  20  and the serial bus  18 , which transfers information to a computer/processor  14  from the accelerometers  12 ,  15 ,  17 .  
         [0022]     Important to note is that alternate embodiments of the present invention have two or more accelerometers, the three illustrated accelerometers  12 ,  15 ,  17  are only one example of a possible arrangement of accelerometers for the accelerometer system  11 , and any number of accelerometers can be utilized.  
         [0023]     In accordance with one embodiment of the present invention, each accelerometer  12 ,  15 ,  17  includes at least one transimpedance amplifier, i.e. first  22  for accelerometer  12 , second  26  for accelerometer  15 , and third  30  for accelerometer  17 . Each accelerometer  12 ,  15 ,  17  is a single axis accelerometer generating a robust wide dynamic range of performance. The accelerometers  12 ,  15 ,  17  will be discussed in further detail in reference to  FIGS. 2 and 3 .  
         [0024]     The platform  20 , whereon the accelerometers  12 ,  15 ,  17  are mounted, may be a single flat platform  20  or gimbals and gimbal torque motors (yaw, pitch and roll motors) or any other accelerometer or derivative detector mount known in the art.  
         [0025]     The processor  14  is coupled to the missile steering nozzle (or vane actuators) unit  16  and the platform  20  and receives signals from the accelerometers  12 ,  15 ,  17 . The processor  14  will be discussed in detail later.  
         [0026]     Referring to  FIGS. 2 and 3 , an example of a possible configuration for the accelerometer  12  is included as an illustrative example of the accelerometers  12 ,  15  and  17 .  
         [0027]     The accelerometer  12  is part of an inertial measurement unit  13  (IMU), as was previously discussed. The accelerometer  12  includes a variable capacitor sensor  52  and a housing  53  for the sensor  52 , one transimpedance amplifier  22 , a power supply  60 , an analog-to-digital converter  64 , a time integrator  66 , and a digital linearizer and filter  68 .  
         [0028]     The variable capacitor sensor  52  includes a single moveable plate  30 , a fixed plate  32 , and a metal housing structure  53 . The variable capacitor sensor  52  generates charge displacement capacitance signals in response to acceleration of the aeronautical system  10 , as will be discussed later.  
         [0029]     The moveable plate  30  may be a flexured diaphragm, a cantilevered beam, a flexible beam, or any object which moves under acceleration with respect to the fixed plate.  
         [0030]     The moveable plate  30  is positioned parallel to the fixed plate  32  such that the fixed plate  32  is a distance (d) from a side  31  of the moveable plate  30 . The moveable plate  30  is affixed to the metal housing structure  52  through at least a portion of at least one edge  37  of the moveable plate  30 .  
         [0031]     The moveable plate  30  is rigidly or hingably fixed to the metal housing structure  53  through at least the one plate edge  37  through almost any manner known in the art. Resultantly, all the system flexure or movement is generated within or by the moveable plate  30 . This generally increases reliability and robustness of the system  10 . This, however, generates a non-linear output from the moveable plate  30 , which will be discussed regarding the linear lookup table linearizer  68 .  
         [0032]     A gas or vacuum environment is enclosed within the sensor  52  through the metal housing structure  53  such that there is no interference with the movement of the moveable plate  30  other than the acceleration of the system  10  along a perpendicular axis. During acceleration, the moveable plate  30  moves or flexes according to the reaction force of Newton&#39;s second law of motion, force=mass×acceleration (F=ma), causing the distance between the moveable plate  30  and the fixed plate  32  to vary, thus creating the variable capacitor on one side of the moveable plate  30 .  
         [0033]     The combination of the fixed plate  32  and the moveable plate  30  forms a plate capacitor. In  FIG. 3 , the equivalent capacitor for the parallel plate capacitor is illustrated in broken lines as C.  
         [0034]     The capacitor is constructed from a single fixed plate and a single moveable plate. The capacitor is excited by a single power supply  60 , as indicated in  FIG. 2 . The return for the power supply  60  is provided by the virtual ground  41  of the transimpedance amplifier  22 . At rest, the distance between plates is d.  
         [0035]     The capacitance of the plate capacitor is determined by 
        C≅(ε 0 A)d,        
 
         [0037]     where 
        ε 0          
 
         [0039]     is the permittivity constant, A is the area of a fixed plate  32  (if l is the length of one side and the cross section of the plate is square, then A=l 2 )and d is the effective distance between the moveable plate  30  and the fixed plate  32 .  
         [0040]     The fixed plate  32  is coupled to the metal housing structure  53  and positioned a distance (d) from the moveable plate  30 . The capacitance of the fixed plate  32  responds to movement of the moveable plate  30  when deither increases or decreases, thereby generating a charge displacement capacitance signal.  
         [0041]     The embodied transimpedance amplifier  22  includes components well known in the art. The various components include, but are not limited to, an amplifier  40 , a ground  41 , and at least one resistor  42 . The transimpedance amplifier  22  receives the charge displacement capacitance signal from the fixed plate  32  and generates therefrom a scaled voltage, which is proportional to d.  
         [0042]     The transimpedance amplifier  22  is coupled to the fixed plate  32 . The transimpedance amplifier  22  is also coupled to A/D converter  64 , which is connected to the time integrator  66 , which is coupled to the LLT  68 , which is coupled to the processor  14  (missile operations processor). The processor  14  is coupled to an actuator  16 , and to various system components  11 , as well as thrusters and attitude control devices.  
         [0043]     The charge q on the capacitor is generated by the equation q=CE, where E is the excitation from source  60  and C=C 0 +ka, k being a scalar constant and a being the acceleration. As the system  10  accelerates along a sensitive axis (x for accelerometer  12 , y for accelerometer  15 , and z for accelerometer  17 ), the voltage on the capacitors is held constant. Under acceleration, the charge changes as the capacitor charges according to dq/dt=E dC/dt where dq/dt≡i g , and i g  is the capacitor current into the virtual ground  41  of the transimpedance amplifier  22 .  
         [0044]     The accelerometer  12  is excited with an DC source  60  at one end and grounded at the other. The ground  41  is a component of the transimpedance amplifier  22 .  
         [0045]     The accelerometer configuration reduces the temperature sensitivity and the DC excitation allowing narrow band analog filtering, both of which enhance the signal-to-noise ratio. The accelerometer  12  circuitry utilizes high speed CMOS, as the accuracy required for performance will require low propagation delays.  
         [0046]     The present configuration reduces the bias error since the instrument is now DC coupled. The circuitry will be a precision design utilizing high speed CMOS, as the accuracy required for performance will require low propagation delays.  
         [0047]     The A/D converter  64  receives the capacitor signals and generates therefrom digital values, which are then time integrated in the time integrator  66  to generate acceleration. This output is a digital word whose magnitude is proportional to the acceleration of the system  10  in either direction along the perpendicular axis.  
         [0048]     The time integrator  66  performs signal integration in the digital domain after initialization. The sensor output, which is gain adjusted and represents a signal proportional to the time rate of change of acceleration. The voltage polarity generates direct indication of the direction of acceleration.  
         [0049]     The compensation for the non-linearity of the flexure structure and overall transport error will be compensated for by a digital corrector within the processor  14  having a value established in manufacturing by taking samples of performance curves.  
         [0050]     In the digital linearizer and filter  68 , statistical filtering of the data somewhere significantly above the maximum flexure frequency followed by a time integration of the digital signal is generated. This reduces the overall noise impact and the exact performance of this filter  68  is determined during, for example, development. This final output represents the integral ∫da/dt of the acceleration of the moveable plate  30  from the initialization time.  
         [0051]     The digital word (time integrator signal) is filtered and linearized in the digital linearizer and filter  68  for manufacturing and flexure non-uniformities. The filter is embodied, for example, as a multi-pole filter reducing noise to the required time domain level. The filter output is a digital word having a magnitude proportional to the acceleration of the system  10  in either direction along the perpendicular axis. The output of the linearizer  68  is an acceleration signal multiplied by a constant (k).  
         [0052]     Statistical filtering of the linearized data above the maximum flexure frequency also occurs in either the digital linearizer and filter  68  or the processor  14  to reduce the overall noise impact on the system  10 . The compensation for the non-linearity of the flexure structure and overall transport error is compensated for by the linearizer and filter  68  whose values are established in manufacturing through sampling performance curves.  
         [0053]     The processor  14  receives the output signals from the accelerometers  12 ,  15 ,  17  and generates a derivative detection signal and response thereto. The processor  14  is embodied as a typical missile or airplane processor, as is familiar in the art. The processor  14  may include the analog-to-digital converter  64 , the time integrator  66 , and the linearizer  68  or any combination thereof. The processor  14  may also be a stand alone component receiving signals from the aforementioned components.  
         [0054]     The processor  14  also compensates for the non-linearity of the flexure structure and overall transport error by a digital corrector within the processor  14 , such as the linearizer  68 , having a value established in manufacturing by taking samples of performance curves.  
         [0055]     The actuator, here embodied as missile steering nozzle or vane actuators  16  receives the derivative detection signal and activates system components (e.g. object control devices) in response thereto. System components include for example, thrusters or attitude control devices.  
         [0056]     Referring to  FIG. 4 , a logic flow diagram  100  illustrating a method for acceleration control is illustrated. Logic starts in operation block  102  where power is applied to the system and the capacitive accelerometer  12 ,  15 , or  17  is activated.  
         [0057]     In operation block  104 , strategic alert biasing occurs and sensor data is compared to a known reference.  
         [0058]     In operation block  106 , the missile system  10  is launched.  
         [0059]     In operation block  108 , the missile system  10  accelerates and the moveable plate flexes to either increase or decrease d for any of the three accelerometers  12 ,  15 , or  17 . The transimpedance amplifier  22  activates and receives signals from the fixed plate capacitor, which are generated in response to a change in d. The transimpedance amplifier  22  then generates scaled voltage signals in response to the fixed plate capacitor signals.  
         [0060]     In operation block  108 , the overall frequency signal, i.e. the results of the acceleration, are time integrated in the time integrator  66 , thereby generating an initialized time integrated signal. The time integrated signal is then linearized. This linearization is achieved through a linear lookup table (linearizer  68 ), or other linearization methods known in the art. Data from the accelerometer(s) is processed by the missile processor  14  or attitude controller.  
         [0061]     In operation, a method for operating a moveable plate capacitance accelerometer system includes accelerating the moveable plate, thereby causing a distance between the moveable plate and a fixed plate to change; generating a variable capacitor signal; generating a scaled voltage signal in response to the variable capacitor signal; and generating an acceleration signal in response to the scaled voltage signal.  
         [0062]     This process is typically engaged when a missile is at rest, prior to launch, or in flight.  
         [0063]     From the foregoing, it can be seen that there has been brought to the art a new and improved accelerometer system. 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 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.