Electret-Based Accelerometer

Abstract

An electret accelerometer is provided in which a diaphragm, an electret, a back plate and an electronic circuit are placed in a sealed casing to prevent external acoustic signals from reaching the diaphragm.

What is claimed is: 
     
         1 . An accelerometer comprising:
 a back plate;   a flexible diaphragm;   a dampening material in contact with the flexible diaphragm;   an electret positioned between the back plate and the flexible diaphragm; and   an electronic circuit coupled to the back plate and the flexible diaphragm and providing an accelerometer output based on changes in the distance between the flexible diaphragm and the back plate.   
     
     
         2 . The accelerometer of  claim 1  further comprising a mass placed on one side of the diaphragm. 
     
     
         3 . The accelerometer of  claim 1  further comprising masses placed on two sides of the diaphragm. 
     
     
         4 . The accelerometer of  claim 2  wherein the mass is placed around a center of the diaphragm. 
     
     
         5 . The accelerometer of  claim 4  wherein the mass comprises concentric rings on the diaphragm. 
     
     
         6 . The accelerometer of  claim 1  wherein the diaphragm comprises a rigid mass surrounded by a compliant support. 
     
     
         7 . The accelerometer of  claim 1  further comprising a sealed casing that surrounds the backplate, the flexible diaphragm and the electret to isolate the flexible diaphragm from external acoustic signals. 
     
     
         8 . The accelerometer of  claim 7  wherein the sealed casing comprises a plug in a hole. 
     
     
         9 . An accelerometer comprising:
 a back plate;   a flexible diaphragm;   an electret positioned between the back plate and the flexible diaphragm;   an electronic circuit coupled to the back plate and the flexible diaphragm and providing an accelerometer output based on changes in the distance between the flexible diaphragm and the back plate; and   a sealed casing surrounding the back plate, diaphragm, electret and electronic circuit to isolate the diaphragm from external acoustic signals.   
     
     
         10 . The accelerometer of  claim 9  wherein the flexible diaphragm comprises a diaphragm for an electret microphone that has been altered to add mass to the diaphragm. 
     
     
         11 . The accelerometer of  claim 10  wherein the diaphragm comprises a flexible material with a pattern of material bonded to the flexible material. 
     
     
         12 . The accelerometer of  claim 11  wherein a pattern of material is bonded to two sides of the flexible material. 
     
     
         13 . The accelerometer of  claim 12  further comprising a dampening material located between the diaphragm and the sealed casing and in contact with the diaphragm. 
     
     
         14 . The accelerometer of  claim 9  wherein the sealed casing comprises a plug in a hole of the casing. 
     
     
         15 . An accelerometer comprising:
 a casing having a hole;   material covering the hole to prevent external acoustic signals from passing through the hole;   a back plate in the casing;   a diaphragm in the casing;   an electret positioned between the back plate and the flexible diaphragm in the casing; and   an electronic circuit coupled to the back plate and the diaphragm and providing an accelerometer output based on changes in the distance between the diaphragm and the back plate.   
     
     
         16 . The accelerometer of  claim 15  wherein the diaphragm comprises a diaphragm for an electret microphone that has been altered to add mass to the diaphragm. 
     
     
         17 . The accelerometer of  claim 16  wherein the diaphragm comprises a flexible material with a pattern of material bonded to the flexible material. 
     
     
         18 . The accelerometer of  claim 17  wherein a pattern of material is bonded to two sides of the flexible material. 
     
     
         19 . The accelerometer of  claim 18  further comprising a dampening material located between the diaphragm and the casing and in contact with the diaphragm. 
     
     
         20 . The accelerometer of  claim 15  wherein the material covering the hole comprises a plug in the hole of the casing.