BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of switches and more particularly to those types of switches that are actuated by acceleration forces. 
     Various types of acceleration responsive switches have been described in the prior art. For instance, U.S. Pat. No. 5,828,138 by McIver et al. discloses an acceleration switch wherein an inertial mass member is held in a holding position by an electrostatic force until the acceleration forces exerted upon it causes the inertial mass member to deflect to an actuated position. U.S. Pat. No. 5,600,109 by Mizutani et al. discloses an acceleration switch wherein acceleration forces cause an inertia ball to bridge one or more contacts located radially around the ball. 
     SUMMARY OF THE INVENTION 
     The present invention is a switch that changes between a first condition and a second condition in response to acceleration forces exerted upon it. The switch includes a material that changes from a high viscosity (first state) to a low viscosity (second state) when subjected to acceleration forces. The change in states results in the switch changing between its first and second conditions. 
     One embodiment of the invention is a normally-open, electrical switch with an open and closed condition. The switch includes a movable contact that is movable from an open position to a closed position. The switch also includes a mechanism, such as a spring, for biasing this movable contact towards the closed position. Thixotropic material in the switch is positioned so that it prevents the movable contact from moving to the closed position while the material is in its first state, keeping the switch in its open condition. When the material is subjected to acceleration forces, the material changes to its second state and allows the movable contact to move to its closed position, where the movable contact provides a conductive path for the switch to change to its closed conductive condition. 
     Another embodiment of the invention is a normally-closed electrical switch with a movable contact that is movable from a closed position to an open position. While in the closed position, the movable contact provides a conductive path for the switch to remain in the closed conductive condition. The switch also includes a mechanism, such as a spring, for biasing the movable contact towards the open position. Thixotropic material prevents the movable contact from moving to its open position while the material is in its first state. When the material is subjected to acceleration forces the material changes to its second state and allows the movable contact to move to its open position, interrupting the conductive path between a stationary contact and the movable contact, causing the switch to change to its open non-conductive condition. 
     In another embodiment of the invention, a normally-open electrical switch includes electrically conductive thixotropic material. The switch also includes a reservoir for retaining this material away, and electrically isolating it from stationary contacts. While the material is in the reservoir, the switch remains in its open, non-conductive condition. When subjected to acceleration forces, the material changes to its second state and flows out of the reservoir and into electrical contact with the stationary contacts, where the material itself provides the conductive path for the switch to change to its closed conductive condition. 
     In yet, another embodiment of the invention a normally-closed electrical switch includes conductive thixotropic material. The material provides a conductive path between stationary contacts, keeping the switch in its closed, conductive condition while the material is in its first state. When subjected to acceleration force, the material changes to its second state and flows out of contact with the stationary contacts, interrupting the conductive path and causing the switch to change to its open, non-conductive condition. 
     Another embodiment of the invention is a normally closed fluidic switch that changes from a closed, non-fluid flowing condition to an open fluid flowing condition. The switch includes a thixotropic material which is positioned in a tube, such that a fluid is prevented from flowing through the tube while the material is in its first state, keeping the switch in its closed condition. When the material is subjected to acceleration forces, the material changes to its second state and flows out of the tube and into a reservoir, allowing the fluid to flow freely through the tube and causing the switch to change to its open condition. 
     Yet, another embodiment of the invention is a normally open, magnetic switch with a magnetic sensor and a movable magnet that is movable from a first position to a second position. The switch also includes a mechanism, such as a spring, for biasing the movable magnet towards the second position. Thixotropic material is also included in this switch and is positioned so that it prevents the movable magnet from moving to the second position while the material is in its first state, keeping the switch in its open condition. When the material is subjected to acceleration forces, the material changes to its second state and allows the movable magnet to move to its second position where it is detectable by the magnetic sensor, causing the switch to change to its second condition. 
     Still, another embodiment of the current invention is a normally closed magnetic switch with a magnetic sensor and a movable magnet that is movable from a first position to a second position. The switch also includes a mechanism, such as a spring, for biasing the movable magnet towards the second position. The switch further includes thixotropic material positioned so that it prevents the movable magnet from moving to the second position while the material is in its first state, keeping the switch in its second condition. When the material is subjected to acceleration forces the material changes to its second state and allows the movable magnet to move to its open position where the magnet is not detectable by the magnetic sensor, causing the switch to change to its open condition. 
     Another embodiment of the invention is a capacitive switch that has a first and second condition, where the capacitance of the switch is higher in the second condition than it is in the first. The switch includes first and second spaced conductive plates. The second conductive plate is movable from a first position to a second position where the second plate is spaced closer to the first plate in the second position than it is in the first position. The switch also includes a mechanism, such as a spring, for biasing the second conductive plate towards its second position. The switch also includes thixotropic material, disposed so that it prevents the second conductive plate from moving to its second position while the material is in its first state, keeping the switch in its first condition. When the material is subjected to acceleration forces, the material changes to its second state and allows the second conductive plate to move to its second position changing the switch to its second condition. 
     Another embodiment of the invention is a capacitive switch that has a first and second condition, where the capacitance of the switch is lower in the second condition than it is in the first. The switch includes first and second spaced, conductive plates. The second conductive plate is movable from a first position to a second position where the second plate is spaced farther from the first plate in the second position than it is in the first. The switch also includes a mechanism, such as a spring, for biasing the second conductive plate towards its second position. The switch further includes the previously described thixotropic material, disposed so that it prevents the second conductive plate from moving to its second position while the material is in its first state, keeping the switch in its first condition. When the material is subjected to acceleration forces, the material changes to its second state and allows the second conductive plate to move to its second position changing the switch to its second condition. 
     Another embodiment of the invention is a capacitive switch that has a first and second condition, where the capacitance of the switch is higher in the second condition than it is in the first. The switch includes first and second spaced, conductive plates. The switch includes thixotropic material that has the property of being substantially non-conductive. The material is disposed in a first location, between the plates, while the material is in its first state, keeping the switch in its first condition. When the material is subjected to acceleration forces the material changes to its second state and flows to a second location, outside of the conductive plates, changing the switch to its second condition. 
     Another embodiment of the invention is a capacitive switch that has a first and second condition, where the capacitance of the switch is lower in the second condition than it is in the first. The switch includes a first and second conductive plate facing and spaced apart. The switch also includes non-conductive thixotropic material. The switch further includes a reservoir for retaining the material outside of the conductive plates while the material is in its first state, keeping the switch in its first condition. When the material is subjected to acceleration forces the material changes to its second state and flows out of the reservoir and to a location between the conductive plates, changing the switch to its second condition. 
     An advantage of the switch is that it is non-reversible. That is, once the switch changes conditions, it would require significant effort to reset the switch. Therefore, the switch may be used in a fuse or anti-fuse application. 
     Another advantage is that the switch changes conditions without the use of electrical power, making it useful in applications deployed in remote or inaccessible locations. 
     Still, another advantage of the switch is that the switch may be made by micro fabrication techniques, significantly reducing size, weight, and cost over modern acceleration responsive switches. 
     The previously summarized features and advantages along with other aspects of the present invention will become clearer upon review of the following specification taken together with the included drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS.  1 ( a ) and  1 ( b ) are diagrams showing the open and closed conditions, respectively, of a first acceleration responsive switch. 
     FIGS.  2 ( a ) and  2 ( b ) are diagrams showing the closed and open conditions, respectively, of a second acceleration responsive switch. 
     FIGS.  3 ( a ) and  3 ( b ) are diagrams showing the open and closed conditions respectively, of a third acceleration responsive switch. 
     FIGS.  4 ( a ) and  4 ( b ) are diagrams showing the closed and open conditions, respectively, of a fourth acceleration responsive switch. 
     FIGS.  5 ( a ) and  5 ( b ) are diagrams showing the closed and open conditions, respectively, of a fifth acceleration responsive switch. 
     FIGS.  6 ( a ) and  6 ( b ) are diagrams showing the open and closed conditions, respectively, of a sixth acceleration responsive switch. 
     FIGS.  7 ( a ) and  7 ( b ) are diagrams showing the closed and open conditions, respectively, of a seventh acceleration responsive switch. 
     FIGS.  8 ( a ) and  8 ( b ) are diagrams showing the first and second conditions, respectively, of an eighth acceleration responsive switch. 
     FIGS.  9 ( a ) and  9 ( b ) are diagrams showing the first and second conditions, respectively, of a ninth acceleration responsive switch. 
     FIGS.  10 ( a ) and  10 ( b ) are diagrams showing the first and second conditions, respectively, of a tenth acceleration responsive switch. 
     FIGS.  11 ( a ) and  11 ( b ) are diagrams showing the first and second conditions, respectively, of an eleventh acceleration responsive switch. 
    
    
     DESCRIPTION OF THE INVENTION 
     The material mentioned previously is commonly referred to as “thixotropic” material. Thixotropic materials generally are materials that change from a solid state to a fluid state when exposed to acceleration forces. Typically, they are colloidal gels, which liquefy when agitated by shaking or by ultrasonic vibration and return to the gel state when at rest. Examples of commercially available thixotropic materials and additives to create thixotropic materials which can be used in an acceleration responsive switch include a thixotropic material sold under the trademark “Disparlon”, by King Industries and synthetic precipitated silica thixotropic materials, Hi-Sil® T-600 and Hi-Sil® T-700, sold by PPG Industries, Incorporated. Further included is the additive by Dow Corning, “Thixo,A-300-1”, which is added to silicone to make a thixotropic material. RBC Industries makes available electrically conductive thixotropic materials, “RBC-6200” and “RBC-6400”. Further information on thixotropic materials is provided in U.S. Pat. No. 5,503,777 by Itagaki et al., U.S. Pat. No. 5,334,630 by Francis et al., and U.S. Pat. No. 4,544,408 by Mosser et al. It is to be understood that the above examples are for illustrative purposes and are by no means intended to be limiting. 
     A first embodiment of an acceleration responsive switch is shown in FIGS.  1 ( a ) and  1 ( b ). FIG.  1 ( a ) shows an electrical switch  10  in its normally open, non-conductive condition and FIG.  1 ( b ) shows it in its actuated closed, conductive condition. The switch  10  includes two stationary contacts  12   a  and  12   b  and a movable contact  14 , that is movable from an open position, as shown in FIG.  1 ( a ), to a closed position, as in FIG.  1 ( b ). While movable contact  14  is in its closed position, as shown in FIG.  1 ( b ), it provides a conductive path with stationary contacts  12   a  and  12   b.    
     Still referring to FIGS.  1 ( a ) and  1 ( b ), the switch  10  includes a mechanism for biasing movable contact  14  towards its closed position. By way of example, a spring  16  is attached to movable contact  14  so that the movable contact is biased towards stationary contacts  12   a  and  12   b . Also included is a material  18  that has the characteristic of changing from a high viscosity (first state) to a low viscosity (second state) in response to acceleration forces. This material  18  is disposed so that it prevents the movable contact  14  from moving to its closed position. By way of example, FIG.  1 ( a ) shows material  18  disposed between movable contact  14  and stationary contact  12   b , preventing movable contact from closing while material  18  is in its first state, keeping switch  10  in its open non conductive condition. When the acceleration responsive switch  10  is subjected to acceleration forces, material  18  changes to its second state and movable contact  14  is allowed to move to its closed position, as shown in FIG.  1 ( b ), changing switch  10  to its closed conductive condition. 
     A second embodiment of the invention is shown in FIGS.  2 ( a ) and  2 ( b ). FIG.  2 ( a ) shows electrical switch  20  in its normally closed, conductive condition and FIG.  2 ( b ) shows it in its actuated open, non-conductive condition. The electrical switch  20  includes two stationary contacts  22   a  and  22   b  and a movable contact  24 , that is movable from a closed position, as shown in FIG.  2 ( a ), to an open position, as shown in FIG.  2 ( b ). While movable contact  24  is in its closed position, as shown in FIG.  2 ( a ), it provides a conductive path with stationary contacts  22   a  and  22   b.    
     The switch  20  further includes a mechanism for biasing movable contact  24  towards its open position. By way of example, FIGS.  2 ( a ) and  2 ( b ) show a spring  26  attached to movable contact  24 , biasing it towards its open position. Also included is material  28 , which like the previously described material, changes viscosity in response to acceleration forces. Material  28  is disposed so that it prevents movable contact  24  from moving to its open position, while material  28  is in its first state. By way of example, FIG.  2 ( a ) shows material  28  disposed between movable contact  24  and switch housing  29  in order to prevent movable contact from moving to its open position while material  28  is in its first state, keeping switch  20  in its closed conductive condition. When switch  20  is subjected to acceleration forces, material  28  changes to its second state and movable contact  24  is allowed to move to its open position, as shown in FIG.  2 ( b ), changing switch  20  to its open, non-conductive condition. 
     A third embodiment of an acceleration responsive switch is shown in FIGS.  3 ( a ) and  3 ( b ). FIG.  3 ( a ) shows electrical switch  30  in its normally open, non-conductive condition and FIG.  3 ( b ) shows it in its actuated closed, conductive condition. The switch  30  includes two stationary contacts  32   a  and  32   b . The switch  30  also includes a conductive material  34 , that changes from a high viscosity (first state) to a low viscosity (second state) in response to acceleration forces. FIG.  3 ( a ) shows conductive material  34  retained away and electrically isolated from stationary contacts  32   a  and  32   b  by reservoir  36 . Conductive material  34  remains in reservoir  36  while conductive material  34  is in its first state, keeping switch  30  in its open non-conductive condition. When subjected to acceleration forces, conductive material  34  changes to its second state and flows out of reservoir  36  to another reservoir  37 , so that the conductive material  34  creates a conductive path with stationary contacts  32   a  and  32   b , as shown in FIG.  3 ( b ), causing switch  30  to change to its closed conductive condition. By way of example, FIG.  3 ( b ) shows a non-conductive reservoir  37 , located below stationary contacts  32   a  and  32   b , for retaining conductive material  34  after it flows from reservoir  36 . 
     A fourth embodiment of an acceleration responsive switch is shown in FIGS.  4 ( a ) and  4 ( b ). FIG.  4 ( a ) shows electrical switch  40  in its normally closed, conductive condition and FIG.  4 ( b ) shows it in its actuated open, non-conductive condition. The switch  40  includes two stationary contacts  42   a  and  42   b . Also included is conductive material  44 , which like the previous material changes viscosity in response to acceleration forces. FIG.  4 ( a ) shows conductive material  44  disposed so that it forms a conductive path with stationary contacts  42   a  and  42   b , while conductive material  44  is in its first state, keeping switch  40  in its closed conductive condition. When switch  40  is subjected to acceleration forces, conductive material  44  changes to its second state and flows out of contact with stationary contacts  42   a  and  42   b . By way of example, conductive material  44  flows to switch housing  46  when subjected to acceleration forces, as shown in FIG.  4 ( b ), causing switch  40  to change to its open non-conductive condition. 
     A fifth embodiment of an acceleration responsive switch is shown in FIGS.  5 ( a ) and  5 ( b ). FIG.  5 ( a ) shows a fluidic switch  50  in its closed non fluid flowing condition and FIG.  1 ( b ) shows it in its actuated open fluid flowing condition. The fluidic switch  50  includes a tube  52  for conveying a fluid  54 . The fluidic switch  50  also includes a material  56  that has the characteristic of changing from a high viscosity (first state) to a low viscosity (second state) in response to acceleration forces. The material  56  is disposed in the tube  52  so that fluid  54  is prevented from flowing through tube  52  while material  56  is in its first state, keeping switch  50  in its closed condition. When the fluidic switch  50  is subjected to acceleration forces, material  56  changes to its second state and flows out of tube  52  and into a reservoir  58  so that fluid  54  may flow freely through tube  52 , changing switch  50  to its open condition. 
     A sixth embodiment of an acceleration responsive switch is shown in FIGS.  6 ( a ) and  6 ( b ). FIG.  6 ( a ) shows a magnetic switch  60  in its normally open condition and FIG.  6 ( b ) shows it in its actuated closed condition. Magnetic switch  60  includes a movable magnet  62  that is movable from a first position, as shown in FIG.  6 ( a ), to a second position, as shown in FIG.  6 ( b ). Magnetic switch  60  also includes a magnetic sensor  64  for detecting magnet  62  when it is in its second position. Magnetic switch  60  further includes a mechanism for biasing magnet  62  towards its second position. By way of example, a spring  66  is attached to magnet  62  so that magnet  62  is biased towards magnetic sensor  64 . Also included is a material  68  that has the characteristic of changing from a high viscosity (first state) to a low viscosity (second state) in response to acceleration forces. The material  68  is disposed so that it prevents the magnet  62  from moving to its second position, while material  68  is in its first state, keeping switch  60  in its open condition. When the magnetic switch  60  is subjected to acceleration forces, material  68  changes to its second state and allows magnet  62  to move to its second position, changing switch  60  to its closed condition. 
     A seventh embodiment of an acceleration responsive switch is shown in FIGS.  7 ( a ) and  7 ( b ). FIG.  7 ( a ) shows a magnetic switch  70  in its normally closed condition and FIG.  7 ( b ) shows it in its actuated open condition. Magnetic switch  70  includes a movable magnet  72  that is movable from a first position, as shown in FIG.  7 ( a ), to a second position, as shown in FIG.  7 ( b ). Magnetic switch  70  also includes a magnetic sensor  74  for detecting magnet  72  when it is in its second position. Magnetic switch  70  further includes a mechanism for biasing magnet  72  towards its second position. By way of example, a spring  76  is attached to magnet  72  so that magnet  72  is biased towards magnetic sensor  74 . Also included is a material  78  that has the characteristic of changing from a high viscosity (first state) to a low viscosity (second state) in response to acceleration forces. The material  78  is disposed, so that it prevents the magnet  72  from moving to its second position, while material  78  is in its first state, keeping switch  70  in its closed condition. By way of example, FIG.  7 ( a ) shows material  78  between magnet  72  and switch housing  79 . When the magnetic switch  70  is subjected to acceleration forces, material  78  changes to its second state and allows magnet  72  to move to its second position, changing switch  70  to its open condition. 
     An eighth embodiment of an acceleration responsive switch is shown in FIGS.  8 ( a ) and  8 ( b ). FIG.  8 ( a ) shows a capacitive switch  80  in its first condition and FIG.  8 ( b ) shows it in its second condition. The capacitance of the switch  80  in its second condition (FIG.  8 ( b )) is higher than capacitance of the switch  80  in its first condition (FIG.  8 ( a )). Capacitive switch  80  includes conductive plates  82   a  and  82   b , facing and spaced apart. Second conductive plate  82   b  is movable from a first position, as shown in FIG.  8 ( a ), to a second position, as shown in FIG.  8 ( b ), where the distance between the plates when in the second position is less than the distance between plates when in the first position. Capacitive switch  80  further includes a dielectric material  84  disposed between the conductive plates. By way of example, dielectric material  84  may be air or other electrical insulator. A mechanism for biasing the second conductive plate  84  towards its second position is also included in the switch. By way of example, FIGS.  8 ( a ) and  8 ( b ) show a spring  86  attached to second conductive plate  84 , biasing it towards its second position. 
     Still referring to FIGS.  8 ( a ) and  8 ( b ), capacitive switch  80  further includes a material  88 , that changes from a high viscosity (first state) to a low viscosity (second state) in response to acceleration forces. The material  88  is disposed so that it prevents the second conductive plate  82   b  from moving to its second position, while material  88  is in its first state. By way of example, FIG.  8 ( a ) shows material  88  disposed between the first conductive plate  82   a  and second conductive plate  82   b , keeping second conductive plate  82   b  in its first position and thus keeping the capacitive switch  80  in its first (low capacitance) condition. When the capacitive switch  80  is subjected to acceleration forces, material  88  changes to its second state and second conductive plate  82   b  is allowed to move to its closed position, as shown in FIG.  8 ( b ), so that switch  80  changes to its second (higher capacitance) condition. 
     A ninth embodiment of an acceleration responsive switch is shown in FIGS.  9 ( a ) and  9 ( b ). FIG.  9 ( a ) shows a M capacitive switch  90  in its first condition and FIG.  9 ( b ) shows it in its second condition. The capacitance of the switch  90  in its second condition (FIG.  9 ( b )) is lower than capacitance of the switch  90  in its first condition (FIG.  9 ( a )). Capacitive switch  90  includes conductive plates  92   a  and  92   b , facing and spaced apart. Second conductive plate  92   b  is movable from a first position, as shown in FIG.  9 ( a ), to a second position, as shown in FIG.  9 ( b ), where the distance between the plates when in the second position is greater than the distance between plates when in the first position. Capacitive switch  90  further includes a dielectric material  94  disposed between the conductive plates. A mechanism for biasing the second conductive plate  92   b  towards its second position is also included in the switch  90 . By way of example, FIGS.  9 ( a ) and  9 ( b ) show a spring  96  attached to second conductive plate  92   b , biasing it towards its second position. 
     Still referring to FIGS.  9 ( a ) and  9 ( b ), capacitive switch  90  further includes a material  98 , that changes from a high viscosity (first state) to a low viscosity (second state) in response to acceleration forces. The material  98  is disposed so that it prevents the second conductive plate  92   b  from moving to its second position, while material  98  is in its first state. By way of example, FIG.  9 ( a ) shows material  98  disposed between the first conductive plate  92   b  and a switch housing  99 , keeping second conductive plate  92   b  in its first position and thus keeping the capacitive switch  90  in its first (higher capacitance) condition. When the capacitive switch  90  is subjected to acceleration forces, material  98  changes to its second state and second conductive plate  92   b  is allowed to move to its closed position, as shown in FIG.  9 ( b ), so that switch  90  changes to its second (lower capacitance) condition. 
     A tenth embodiment of an acceleration responsive switch is shown in FIGS.  10 ( a ) and  10 ( b ). FIG.  10 ( a ) shows a capacitive switch  100  in its first condition and FIG.  10 ( b ) shows it in its second condition. The capacitance of the switch  100  in its second condition (FIG.  10 ( b )) is lower than capacitance of the switch  100  in its first condition (FIG.  10 ( a )). Capacitive switch  100  includes conductive plates  102 , facing and spaced apart. The switch  100  also includes a non-conductive material  104 , that changes from a high viscosity (first state) to a low viscosity (second state) in response to acceleration forces. The material  104  is disposed between conductive plates  102  while material  104  is in its first state, so that switch  100  stays in its first (higher capacitance) condition. When subjected to acceleration forces, non-conductive material  104  changes to its second state and flows to a location outside of conductive plates  102 , so that non-conductive material  104  is no longer between conductive plates  102  and switch  100  changes to its second (lower capacitance) condition. By way of example, non-conductive material  104  flows to switch housing  106  when subjected to acceleration forces, as shown in FIG.  10 ( b ). 
     An eleventh embodiment of an acceleration responsive switch is shown in FIGS.  11 ( a ) and  11 ( b ). FIG.  11 ( a ) shows a capacitive switch  110  in its first condition and FIG.  11 ( b ) shows it in its second condition. The capacitance of the switch  110  in its second condition (FIG.  11 ( b )) is higher than capacitance of the switch  110  in its first condition (FIG.  11 ( a )). Capacitive switch  110  includes conductive plates  112 , facing and spaced apart. Also included is non-conductive material  114 , which like the previous material changes viscosity in response to acceleration forces. FIG.  11 ( a ) shows non-conductive material  114  retained to a first location, outside of conductive plates  112 , by a first reservoir  116 . Non-conductive material  114  remains in reservoir  116  while non-conductive material  114  is in its first state, keeping switch  110  in its first (lower capacitance) condition. When subjected to acceleration forces, non-conductive material  114  changes to its second state and flows to a second location, between conductive plates  112 , so that switch  110  changes to its second (higher capacitance) condition. Switch  110  optionally includes a second reservoir  118  for retaining non-conductive material  114  to its second location.
An acceleration responsive switch includes a material that changes from a high viscosity to a low viscosity when subjected to acceleration forces. The material's change in viscosity causes the switch to change from one state to another.