Patent Publication Number: US-2002007677-A1

Title: Printed circuit board inclinometer/accelerometer

Description:
FIELD OF THE INVENTION  
       [0001] The field of the present invention is that of condition responsive sensor units such as accelerometers and inclinometers and the like, and the present invention relates more particularly to a deflectable sensor plate included within a multi-layered printed circuit board.  
       BACKGROUND OF THE INVENTION  
       [0002] Condition responsive sensors, such as accelerometers, inclinometers and the like, find a wide range of applications in industry, and are commonly used in aircraft, automobile, and boating applications, for example, where sensors are likely to be subjected to shock, vibration, contamination and severe temperature and environmental changes. There are many examples of seismic mass sensors which function as deflectable sensors, for example, the center plate of a balanced loop differential capacitor.  
       [0003] With advancing technology, improved condition responsive sensors are desirable. Lighter, more compact sensors which can be manufactured easily with minimal cost, are especially desirable. It has become increasingly important to manufacture sensors which are of solid construction, and therefore resistant to contamination and other environmental effects, temperature effects, electromagnetic interference, and the effects of vibration and shock. As such, sensors formed on or within brittle substrates such as semiconductor materials, are less desirable and of limited utility. Sensors which are separately micro-machined must later be attached to a sturdy substrate such as a printed circuit board. Sensors which are not solidly encased are subject to contamination and other environmental effects.  
       [0004] As the drive to produce more lightweight and compact sensors continues, it becomes increasingly difficult to fabricate sensors having very small microelectromechanical parts. It has therefore become a challenge in today&#39;s field of electronics, and condition responsive sensors more particularly, to produce a versatile condition responsive sensor which is lightweight, compact, inexpensive to manufacture, and which finds a wide range of applications due to the solid construction of a deflectable sensor member which provides a sturdy unit which is resistant to shock, vibration, contamination, electromagnetic interference, and severe temperature and environmental effects.  
       SUMMARY OF THE INVENTION  
       [0005] It is an object of the present invention to provide a condition responsive sensor which is versatile in nature and can be adapted for use as an accelerometer, inclinometer, tilt switch, G-switch, pressure switch, or pressure transducer, for example. Briefly described, the novel and improved condition responsive sensor unit of the present invention comprises a condition responsive sensor unit of printed circuit board (PCB) construction. The condition responsive sensor unit includes a deflectable sensor plate functioning as a seismic mass and formed between electrically insulating substrates which are coupled substantially in parallel to form a multi-layered printed circuit board. The electrically insulating substrates may be printed circuit boards or they may be formed of ceramic or other materials commonly used in the PCB industry. The deflectable sensor plate which is disposed and encapsulated between the coupled substrates and within the PCB, is suspended and movable within a cavity formed between the substrates, by means of at least one flexure arm which connects the deflectable sensor to a peripheral frame. The deflectable sensor is formed of a conductive material.  
       [0006] The condition responsive sensor unit is of durable construction as the deflectable sensor plate is shielded from environmental effects, and resistant to contamination, shock, vibration, and other mechanical disturbances. The sensing means for sensing movement of the deflectable plate may include a differential, parallel-plate capacitor, it may utilize contact plates, or it may comprise a proximity sensor using optical means or means for sensing the change in a magnetic or other electrical field. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0007] The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may be best understood by reference to the following description taken in connection with the accompanying drawing.  
     [0008] It is to be understood that the features and objects of the various figures of the drawing are not drawn to scale. Rather, many of the features have been reduced or expanded and/or exaggerated to more clearly point out aspects of the invention, according to common practice.  
     [0009]FIG. 1 is a plan view of an exemplary embodiment of the central plate including the deflectable sensor plate, flexure arms, and a peripheral frame;  
     [0010]FIG. 2 is an exploded, perspective view of the various components of a condition responsive sensor unit according to an exemplary embodiment;  
     [0011]FIGS. 3A and 3B are plan views of electrically insulating substrates having electrical circuits formed thereon;  
     [0012]FIG. 4 is a cross-sectional view of an exemplary embodiment of the condition responsive sensor unit;  
     [0013]FIG. 5 is a cross-sectional view of another exemplary embodiment of to the condition responsive sensor unit using optical sensing means;  
     [0014]FIG. 6 is a cross-sectional view of a condition responsive sensor unit adapted for use as a pressure transducer;  
     [0015]FIG. 7 is a plan view of an exemplary embodiment of the central plate used in the pressure transducer shown in FIG. 6;  
     [0016]FIG. 8 is another exemplary embodiment of the condition responsive sensor unit including multi-layered substrates; and  
     [0017]FIG. 9 is a circuit diagram of an exemplary circuit for providing an output signal from an exemplary embodiment of the condition responsive sensor unit.  
    
    
     DETAILED DESCRIPTION  
     [0018] The novel and improved condition responsive unit of the present invention provides a conductive deflectable sensor plate integral within a multi-layered printed circuit board. The versatile sensor unit of the present invention can be used for sensing various conditions. Various exemplary embodiments of the particular arrangement of the features of the present invention are shown in the following figures.  
     [0019]FIG. 1 is a plan view of an exemplary embodiment of the central plate  14  including a deflectable plate  2  or seismic mass of the present invention. A central plate  14  is formed integrally between two substantially parallel electrically insulating substrates (FIG. 2) such as printed circuit boards, which are coupled to each other. Deflectable sensor plate  2  is coupled to peripheral frame  4  by means of a plurality of flexure arms  6 A- 6 D. It can be seen that there is a space  13  between peripheral edge  12  of deflectable sensor plate  2  and inner peripheral edge  7  of peripheral frame  4 . Inner peripheral edges  7  define opening  11  within which deflectable plate  2  is disposed. This aspect of the invention, combined with the arrangement of flexure arms  6 A- 6 D provide for a movement of deflectable sensor plate  2  generally perpendicular to the plane of peripheral frame  4  of central plate  14 . Flexure arm  6 A, for example, is connected to deflectable sensor plate  2  at a first location  8  which is peripherally spaced from a second location  10  wherein flexure arm  6 A is coupled to peripheral frame  4  of central plate  14 . When formed integrally between generally parallel electrically insulating substrates coupled to each other, deflectable sensor plate  2  is therefore free to move within a cavity formed between the substrates which combine to form a multi-layered printed circuit (PC) board. The construction of the unit, most particularly the arrangement of flexure arms  6 A- 6 D, provides for free movement of deflectable sensor plate  2  with respect to peripheral frame  4  and within the PC board (not shown), in response to conditions such as acceleration, inclination and pressure changes. When in rest position, generally flat deflectable sensor plate  2  is suspended within the plane formed by peripheral frame  4 .  
     [0020] Although shown and described in conjunction with four flexure arms each having the same orientation, it should be understood that other exemplary embodiments may include a fewer or greater number of flexure arms than shown in FIG. 1. The flexure arm or arms are designed and arranged so that, in combination, they provide for unencumbered lateral movement of deflectable sensor plate  4  with respect to the plane formed by peripheral frame  4  (and the PC board within which the deflectable sensor plate is included). Deflectable sensor plate  2  of central plate  14  is formed of a conductive material. In an exemplary embodiment, deflectable sensor plate  2  may be formed of beryllium copper. In other exemplary embodiments, other metals such as nickel may be used to form central plate  14 , which includes deflectable sensor plate  2 .  
     [0021] Now turning to FIG. 2, an exploded perspective view of the various components of an exemplary embodiment of a condition responsive sensor unit, are shown. Central plate  14  is centrally located, and includes deflectable sensor plate  2  disposed within opening  11 , shown and described previously in conjunction with FIG. 1. The condition responsive sensor unit of the present invention includes a pair of electrically insulating substrates  18  which are coupled together in assembled form and therefore form the external components of the unit. In the preferred embodiment, each electrically insulating substrate is a printed circuit (PC) board. In an exemplary embodiment, electrically insulating substrates  18  may be formed of a fire retardant epoxy-bonded fiberglass. In another exemplary embodiment, electrically insulating substrates  18  may be formed of other ceramic materials. In yet another exemplary embodiment, electrically insulating substrates  18  may be formed of various other laminated epoxy materials. Although shown as a single electrically insulating substrate, electrically insulating substrate  18  may be a multi-layered substrate structure. According to the preferred embodiment in which electrically insulating substrate  18  is a PC board, each PC board may be a multi-layered PC board. In the embodiments including substrate  18  having multiple layers, electrical circuits and other features may be formed integrally between the layers which combine to form electrically insulating substrate  18 , as well as on the outer surface of electrically insulating substrate  18 , which forms the outer sections of the condition responsive sensor unit of the present invention.  
     [0022] In various exemplary embodiments, inner surfaces  20  of each electrically insulating substrate  18 , may include a conductive plate  26 . Conductive plate  26  may be formed of copper or other suitable metals. Outer surfaces  30  of substrates  18  will generally include electrical circuitry  22 . The condition responsive sensor unit may also include through-holes  24  extending through electrically insulating substrate  18  to provide electrical connection between the circuitry  22  formed on the surfaces, and the central plate, for example. According to various exemplary embodiments which include each substrate  18  being formed of multiple layers having electrical circuitry on various surfaces, through-holes  24  may provide electrical connection between circuitry formed on various surfaces, and the central plate.  
     [0023] Spacers  16  are disposed between each of substrates  18  and central plate  14 . Spacers  16  may be formed of copper or other suitable metals which can be manufactured to tight tolerances. Each spacer  16  includes an opening  17  that extends through spacer  16 , which, together with opening  11  in central plate  14 , forms a cavity within the assembled condition responsive sensor unit, enabling deflectable sensor plate  2  to move within the cavity and substantially perpendicular to the plane formed by central plate  14 . In another exemplary embodiment, more than one spacer  16  may be used between central plate  14  and either or both of substrates  18 . Electrically insulating substrates  18 , spacers  16 , and central plate  14  are laminated together in the arrangement shown by use of “pre-preg” adhesive sheets  28 , each of which includes opening  29 . By “pre-preg”, it is meant that sheet  28  is formed of a material pre-impregnated with an A-stage epoxy which has been processed and partially cured to form a B-stage epoxy fabric sheet, for example. The semi-cured B-stage epoxy is resistant to reflow during the laminating process used to join the components and further cure adhesive sheets  28 . In other exemplary embodiments, other epoxies may be used to form “pre-preg” adhesive sheets  28 .  
     [0024] In this manner, the present invention offers the advantage that an adhesive material is not reflowed into a cavity (shown in FIG. 4) formed between substrates  18  during the laminating process which utilizes elevated temperatures. This insures that deflectable sensor plate  2  is free to move within the cavity in response to conditions such as acceleration, inclination and pressure changes. In an exemplary embodiment, the pre-preg material may be a commercially known material grade B11 prepreg “re-flow ” bonding plies.  
     [0025] The components shown and described in conjunction with FIG. 2 are assembled to produce the assembled unit (shown in FIG. 4), using a laminating press. Any suitable laminating press such as available in the art, may be used. The components are positioned as shown for the exemplary embodiment shown in FIG. 2, namely in the following order: first electrically insulating substrate  18 , first adhesive sheet  28 , first spacer  16 , second adhesive sheet  28 , central plate  14 , third adhesive sheet  28 , second spacer  16 , fourth adhesive sheet  28 , and second electrically insulating substrate  18  (going from top-to-bottom or bottom-to-top). The components are aligned so that respective openings are aligned with one another and with conductive plates  26  of electrically insulating substrates  18 . It can be seen that the respective openings formed within the spacers  16 , the adhesive sheets  28 , and the central plate  14 , are essentially the same size and shape according to the exemplary embodiment.  
     [0026] After the components have been positioned and aligned as such, they are laminated together by the laminating press which uses a force directed along opposed directors  90  and  91  and heats the unit being joined at an elevated temperature, typically within the range of 300-400° F. The force directed along opposed directors  90  and  91  and used to join the components together may be on the order of 75-150 psi, but other temperatures and force values may be used alternatively. According to an exemplary embodiment the components may be joined together using a vacuum lamination process as commonly available in the art. Using the preferred laminating temperature of 350° F., the B-stage epoxy used to form adhesive sheet  28  does not reflow during the curing process, offering the advantages as above.  
     [0027] According to another exemplary embodiment (not shown), the spacers may not be needed and each inner surface of the electrically insulating substrates may include a recessed portion formed within the surface. In an exemplary embodiment, the conductive plates (feature  26  shown in FIG. 2) may be included within the recessed portions formed within the inner surfaces of the electrically insulating substrates. During the lamination process as described above, these respective recessed portions are aligned with the openings formed in the adhesive sheets and the central plate, and the components laminated together according to the various exemplary laminating processes described above. The joined components include a central cavity formed of the respective recessed portions formed on the confronting inner surfaces and the central opening of the central plate. The deflectable sensor plate of the central plate is suspended and free to move within the formed cavity.  
     [0028] Now turning to FIGS. 3A and 3B, plan views of outer surfaces  30  of electrically insulating substrates  18  are shown. Outer surfaces  30  form the outer surface of the assembled condition responsive sensor unit. FIGS. 3A and 3B show exemplary embodiments of electrical circuit  22  which is formed on surface  30 . Electrical circuit  22  may be used to sense and analyze the movement of deflectable sensor plate  2  (not shown) within the condition responsive sensor unit. In the preferred embodiment, each of the electrically insulating substrates  18  shown in FIGS. 3A and 3B may be multi-layered PCB&#39;s.  
     [0029] Now turning to FIG. 4, a cross-sectional view of the assembled condition responsive sensor unit is shown. Condition responsive sensor unit (hereinafter, “sensor unit”)  77  is of PC board construction, and represents a multi-layer PC board. Sensor unit  77  includes a pair of electrically insulating substrates  18  which form the outer components of the unit. Although shown and described as a pair of individual substrates, electrically insulating substrate  18  may represent a plurality of individual electrically insulating layers, joined together to form an integral unit as described in conjunction with FIG. 2. Electrically insulating substrates  18  each include an inner surface  20  and an outer surface  30 . Disposed on outer surface  30  is electrical circuit  22 . Although electrical circuit  22  is shown as being formed on each of the outer surfaces  30 , it should be understood that, in other embodiments, electrical circuit  22  may be disposed upon only one of outer surfaces  30 .  
     [0030] Sensor unit  77  includes central plate  14  centrally arranged within the unit. Spacers  16  are included between central plate  14  and the pair of electrically insulating substrates  18 . Pre-preg adhesive sheets  28  provide for the unit to be laminated together. Cavity  39  is formed integrally within sensor unit  77  because of the centrally located openings included within spacers  16 , pre-preg adhesive sheets  28  and central plate  14 . (Opening  17  of spacer  16  and opening  29  of sheet  28  are shown in FIG. 2. Opening  11  of central plate  14  is shown in FIG. 1.) Deflectable sensor plate  2  is thereby suspended centrally within cavity  39  when in a rest position, but is free to move along direction  44  or  42  when sensor unit  77  is exposed to acceleration, inclination, or the like. Flexure arms  6  couple deflectable sensor plate  2  to peripheral frame  4  of deflectable sensor plate  14  and thereby centrally suspend deflectable sensor plate  2  within cavity  39 . In an exemplary embodiment, thickness  34  of conductive sensing plate  2  may be on the order of 0.004 inches, but may vary in other embodiments. Also in the exemplary embodiment, spacing  36  between deflectable sensor plate  2  and conductive plates  26 L,  26 R formed on the inner surfaces  20  of the electrically insulating substrates  18 , may be on the order of 0.005 to 0.015 inches, but may vary in other exemplary embodiments.  
     [0031] In an exemplary embodiment, each of the pair of inner surfaces  20  includes conductive plates  26 L and  26 R. When sensor unit  77  is subject to conditions such as acceleration, or inclination, deflectable sensor plate  2  may move along the direction  44  towards conductive plate  26 L, or it may move along direction  42  towards conductive plate  26 R. In another exemplary embodiment, only one of conductive plate  26 L and  26 R may be needed. Flexure arm  6  allows for central mass (deflectable sensor plate  2 ) to move freely and along the indicated directions and substantially perpendicularly with respect to inner surfaces  20 .  
     [0032] Various means may be used to form the sensing mechanism. In one exemplary embodiment, conductive plate  26 L, conductive deflectable sensor plate  2 , and conductive plate  26 R may form the parallel plates of a differential capacitor having conductive deflectable sensor plate  2  as a central electrode. Electrical connection is provided between the conductive plates of the parallel plate capacitor through holes  24  and  32  formed through electrically insulating substrates  18  thereby connecting the features to electrical circuits  22  disposed on outer surfaces  30  of electrically insulating substrates  18 . Although only two through-holes  24  and  32  are shown, it is understood that any number of holes may be provided to allow for electrically coupling the components of the capacitor to electrical circuitry used in sensing changes in capacitance. Electrical circuits  22  are used as part of the capacitance sensing circuitry in an exemplary embodiment. It should be understood, however, that other external electrical circuitry (not shown) may additionally or alternatively be used. In another exemplary embodiment, wherein substrate  18  is formed of a plurality of insulating layers coupled together, electrical sensing circuitry may be provided along the surface of the multiple layers (not shown) and the components of the capacitor may be coupled to the circuitry through various through-holes. For example, in the preferred embodiment in which substrate  18  is a multi-layered printed circuit board, electrical circuitry may be provided on one or more surfaces of the printed circuit boards which combine to form the multi-layered printed circuit board. Each of the above-described embodiments may be used as an accelerometer, inclinometer, tilt switch, or G-switch.  
     [0033] In another exemplary embodiment, sensor unit  77  may be designed so that the flexure arms allows for deflectable sensor plate  2  to move greater than or equal to distance  36  in either direction  44  or  42 . In this embodiment. the range of motion of deflectable sensor plate  2  allows sensor plate  2  to contact either or both of conductive plates  26 L and  26 R. In this embodiment, conductive plates  26 L and  26 R serve as contact plates, and the electrical sensing circuitry coupled to the various components forming the contact sensor, are designed to sense contact between conductive, deflectable sensor plate  2  and either or both of conductive plates  26 L and  26 R.  
     [0034] In another exemplary embodiment, the condition responsive sensor unit  77  of the present invention may use a proximity sensor based on Hall effect, or optical sensing. Still referring to FIG. 4, acceleration or inclination of sensor unit  77  urges motion of deflectable sensor plate  2  along either or both of directions  44  and  42 . In this embodiment, however, the motion of deflectable sensor plate  2  is restricted so that it cannot contact either of conductive plates  26 L and  26 R. Electrical sensing means may be used to detect the proximity of deflectable sensor plate  2  to either of conductive plates  26 L and  26 R. In another exemplary embodiment (not shown) using a proximity sensor, conductive plates  26 L and  26 R may not be needed on respective inner surfaces  20 .  
     [0035] In one exemplary embodiment of a proximity sensor, magnetic field producing means may be used to produce a magnetic field between deflectable sensor plate  2  and conductive plates  26 L and  26 R. When deflectable sensor plate  2  moves in response to acceleration or inclination or the like, and its position within cavity  39  changes, electrical circuitry is provided for sensing the change in the magnetic field caused by the movement of deflectable sensor plate  2 . In one exemplary embodiment, a Hall effect sensor may be included in the electrical circuit used to sense the change in magnetic field.  
     [0036] In another exemplary embodiment of a proximity sensor as shown in FIG. 5, reflective optical sensing means may be used. In FIG. 5, optical source  38  provides optical beam  40  through opening  41  and which is directed onto surface  48  of deflectable sensor plate  2 . In an exemplary embodiment, optical source  38  may be a photodiode. As the location of deflectable sensor plate  2  within cavity  39  changes responsive to external conditions, and deflectable sensor plate  2  travels along direction  42  or  44 , the change in location alters the reflected optical beam  49  and is sensed by optical sensor  46 . Optical sensor  46  is coupled to electrical circuitry (not shown) capable of analyzing the changed position. In an exemplary embodiment, optical sensor  46  may be a phototransistor.  
     [0037] In another exemplary embodiment as shown in FIG. 6, sensor unit  77  of the present invention may form a pressure transducer. In this exemplary embodiment, vent hole  54  allows for vented cavity  39 A to be exposed to, and have the same pressure as, external environment  55 . Internal cavity  39 B is a sealed cavity and therefore maintained at a constant pressure. Changes in the pressure level of external environment  55 , and therefore vented cavity  39 A, provide for the movement of deflectable sensor plate  2  along direction  44  towards conductive plate  26 L, or along direction  42  towards conductive plate  26 R. The movement of conductive sensor plate  2  may be sensed using the parallel plate capacitor method, the contact plate method, or the proximity sensing means previously described.  
     [0038] In the exemplary embodiment including a pressure transducer, vented cavity  39 A is isolated from internal cavity  39 B by means of flexure membrane  6 ′ which connects deflectable sensor plate  2 , to peripheral frame  4  of deflectable sensor plate  14 . In this exemplary embodiment, flexure member  6 ′ is a continuous diaphragm member and does not include holes therethrough, as does the flexure arm arrangement shown in FIG. 1. Rather, the material used to form diaphragm member  6 ′ is chosen to be gas-impermeable, but bendable in response to pressure changes. Flexure member  6 ′ therefore maintains a pressure differential across cavity  39 B and cavity  39 A. Movement of conductive sensor plate  2  along the direction  44  or  42  may be sensed using the various means previously described.  
     [0039]FIG. 7 shows a plan view of the diaphragm unit of the pressure transducer embodiment of the present invention shown in FIG. 6. Flexure member  6 ′ may be any separating wall or membrane, having an elastic quality, and which is air-tight. In an exemplary embodiment, flexure member  6 ′ may be a bendable metal, a gas-impermeable fabric, or another elastic, gas-impermeable material.  
     [0040] Returning to FIG. 6, it can be seen that electrical circuit  22  is disposed on only one of the outer surfaces  30  of electrically insulating substrate  18 . As described in conjunction with the previous embodiments, it should be understood that the sensor unit  77  forming the pressure sensor embodiment of the present invention, may alternatively include an electrical circuit  22  on each of outer surfaces  30 , in order to sense and analyze the movement of deflectable sensor plate  2  as it moves within the cavity formed by cavity sections  39 A and  39 B.  
     [0041]FIG. 8 is an expanded, cross-sectional view of an exemplary embodiment of the condition responsive sensor unit of the present invention including a pair of multi-layered substrates. On each side of the central cavity  39  portion of the sensor unit of the present invention, electrically insulating substrates  18  are shown. Each electrically insulating substrate  18  is formed of two insulating layers. Inner, electrically insulating layer  18   a  and outer electrically insulating layer  18   b  combine to form multi-layered electrically insulating substrate  18 . Although two layers combine to form multi-layered substrate  18  shown on each side of the embodiment shown in FIG. 8, it should be understood that the multi-layered substrate may contain three or more individual electrically insulating layers coupled together.  
     [0042] In the preferred embodiment, each electrically insulating substrate  18  will be a multi-layered printed circuit board including a number of layers of individual printed circuit boards such as the two ( 18   a ,  18   b ) shown in FIG. 8. Electrical circuit  56  is shown formed between the two layers  18   a  and  18   b  on the right side, and along outer surface  30  on the left side, of the exemplary embodiment shown. It should be understood that each individual layer may contain circuitry on either or both surfaces. As such, it should be further understood that each of the various exemplary embodiments of the present invention may include a pair of multi-layered substrates containing two or more individual electrically insulating layers, and each layer may include electrical circuitry on the surface used to sense and analyze the position and the movement of the deflectable sensor plate as it moves within the cavity formed between the substrates.  
     [0043] In the preferred embodiment, the pair of multi-layered printed circuit boards  18 , the central plate  14 , and the spacers  16  combine to form an integral multi-layer printed circuit board. In the preferred embodiment, the multi-layered printed circuit board includes an internal cavity  39 , within which the deflectable sensor plate  2  is suspended and free to move within, generally along directions  42  and  44 . It should be further understood that holes (not shown in FIG. 8) may be provided through the substrates as necessary to provide electrically connection between the circuitry disposed on the surfaces of the substrate and the components of the internal parts of the sensing unit.  
     [0044] To protect the assembled, completed sensor unit  77  from the environment the unit may be potted after it is assembled. An epoxy, or other encapsulating material  60  may be formed to cover the outer surfaces of the condition responsive sensor unit.  
     [0045] Now turning to FIG. 9, a circuit diagram showing the circuitry for sensing and analyzing the movement of the deflectable sensor plate of the present invention, is presented. Sensor detection circuit  100  is responsive to the deflectable sensor plate which moves within the cavity in response to acceleration, inclination, pressure changes and the like. Sensor detection circuit  110  senses the magnitude of the movement of the deflectable sensor plate and converts this information into a representative measure of the condition such as acceleration, angle or degree of inclination, and external pressure, to which the unit is being subjected. In the exemplary embodiment shown in FIG. 9, a differential capacitor is used as the sensor unit. In other exemplary embodiments, various other sensing means may be formed using the deflectable sensor plate, such as the contact sensor arrangement and the proximity sensor previously described.  
     [0046] The differential capacitor includes a pair of parallel plate variable capacitors represented as features  113  in the circuit diagram. In an exemplary embodiment using the differential capacitor as a sensor unit, sensor detection circuit  100  may comprise a capacitance sensor detection circuit. Power is provided to sensor detection circuit  100 , and regulated by means of power regulation unit  102 . Offset adjustment  104  is provided to adjust the offset of signals processed in the sensor detection circuit  100 . Output signal  112  of sensor detection circuit  100  is processed by scaling amplifier  106  and filtering amplifiers  108  and is provided to output means  110 . Output means  110  may include any of various conventional means available in the art. For example, output means may include an audible alarm, a visual alarm, a digital readout, or any other electrical or visual means which indicates the acceleration value, angle or degree of inclination, or pressure in the environment, for example.  
     [0047] It should be understood that the foregoing exemplary embodiments are intended to be just that—exemplary. The present invention is not intended to be limited to the embodiments shown. Rather, various combinations of the elements described above may be used. For example, the condition responsive sensor unit may be used to sense acceleration, inclination, pressure changes or other conditions. The sensor unit may include a capacitor, a differential capacitor, a contact sensor, and proximity sensors which may use optical, magnetic, or other means to detect proximity. For each of the embodiments, multi-layered electrically insulating substrates such as PCB&#39;s may be used on each side of the deflectable sensor plate, and the cavity in which it is free to move. The electrical circuitry used to sense and analyze the motion of the movable sensing plate, is not intended to be limited to a particular type of electrical circuitry. Electrical circuitry may be included on either or both sides of each of the layers which combine to form the electrically insulating substrates. The electrical circuitry is responsive to the particular sensing means used, and combines with other electrical circuitry features to provide an output means indicative of the external condition, as shown in FIG. 9.  
     [0048] The structure of the condition responsive sensor unit is also not intended to be limited to the specific examples shown. For example, each of the various sensors may include a fewer or greater number of flexure arms than the four flexure arms ( 6 A- 6 D) shown in FIG. 1. The exact configuration and relative placement of the flexure arms may also be varied within the scope of the present invention. Any combination, configuration, and arrangement of flexure arms which provides for free movement of the deflectable sensor plate within the cavity, may be used. The range of motion of the sensor plate is to be determined by the specific application and dimensions used. Spacers of various thicknesses may be used to provide a cavity having the desired dimensions. In alternative embodiments, more than one individual spacer may be used on either or both sides of the centrally disposed deflectable sensor plate. In yet another exemplary embodiment, spacers may not be needed and the inner surfaces of each of the electrically insulating substrates may include recessed portions which are aligned and assembled in confronting relation to form an inner cavity in which the deflectable sensor plate is suspended and is free to move within.  
     [0049] The condition responsive sensor unit of the present invention is a lightweight, compact unit. The materials used in forming the various components of the present invention, are not intended to be limited to the exemplary embodiments described. Any suitable materials having the required functional characteristics, may be used.  
     [0050] The present invention is also not intended to be limited to specific physical dimensions. In the exemplary embodiments shown, the deflectable sensor plate is formed of round construction. In other exemplary embodiments, the deflectable sensor plate may be oblong, elliptical, rectangular, square, or any other suitable shape adapted for of movement within the cavity formed between the pair of substrates. In an exemplary embodiment, the round deflectable sensor plate may have a diameter of 0.5″, but in other exemplary embodiments, sensor plates having diameters ranging from 0.2″ to 3 to 4 inches may be used depending on the specific application. In the exemplary embodiment having a round, 0.5″ deflectable sensor plate, attached to a peripheral frame by a plurality of flexure arms, each spacer may include a central hole having an inner diameter of 0.8″ to accommodate the bending of the flexure arms and the movement of the sensor plate within the cavity. The hole formed in each spacer however, is formed to accommodate the motion of the associated deflectable sensor plate within the cavity. As such, the hole within the spacer may take on various shapes and dimension to accommodate the movement of deflectable sensor plates of different configurations.  
     [0051] In an exemplary embodiment, the thickness of the individual spacers may be 0.010″, but other suitable thicknesses may be used. Also, while the thickness of the central plate including the deflectable sensor plate may be 0.004″ in an exemplary embodiment, other thicknesses ranging from 0.002″ to 0.020″ may be used in other exemplary embodiments, depending on the application and the thickness and number of spacers used which determine the thickness of the cavity within which the deflectable sensor plate is free to move. Also in the exemplary embodiment, the total thickness of the condition responsive sensor unit including the pair of multi-layered electrically insulating substrate, may be within a range of 0.12 to 0.14 inches. It should be understood that other suitable thicknesses may be used in various other exemplary embodiments.  
     [0052] Although several particular exemplary embodiments of the condition responsive sensor unit of the present invention have been described to illustrate the present invention, the present invention includes all modifications and equivalents of the disclosed embodiments falling within the scope of the appended claims.