Patent Publication Number: US-2012024062-A1

Title: Low Cost Optical Accelerometer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     In general, the present invention relates to accelerometers that are used to detect and quantify changes in acceleration and/or orientation. More particularly, the present invention relates to accelerometers that detect changes in acceleration and/or orientation by placing an object in a path of light between a light source and an optical detector and measuring how changes in acceleration and orientation cause the object to interfere with the path of light. 
     2. Prior Art Description 
     There are many electronic and electro-mechanical devices that utilize small accelerometers. Accelerometers are devices that convert a change in acceleration into a corresponding electrical signal. As such, accelerometers are used in objects like video game controllers and smart phones to produce control signals when such objects are shaken or otherwise manually manipulated. 
     Gravity is an acceleration force that draws objects toward the earth. Consequently, any object that undergoes a change in position with respect to the earth also undergoes a change in relative acceleration forces. As such, accelerometers are also used in electronic devices to detect changes in orientation. 
     In the prior art record, there are many designs for accelerometers. Many simple accelerometers, called linear accelerometers, detect changes in acceleration in a single direction, that is in the X-axis, Y-axis or Z-axis. Accordingly, if a device requires that acceleration forces be accurately detected in more than one direction, then more than one linear accelerometer must be used. Although accelerometers do exist that can detect acceleration forces in multiple directions, such accelerometers tend to be much more complicated and expensive than linear accelerometers. 
     With growing advances in microelectronics, devices are becoming both smaller and more powerful. To service such electronic devices, accelerometers are being manufactured in smaller sizes. However, due to the functional nature of accelerometers, accelerometers typically have moving parts. Making a miniature accelerometer with moving parts requires a sophisticated manufacturing process and a large amount of expensive capital equipment. Consequently, although the size of accelerometers have been decreasing, the price of accelerometers has not. 
     The high price of accelerometers has excluded the use of accelerometers in many applications. Although an accelerometer may be no trouble to add to an expensive smart phone, accelerometers are difficult to add to inexpensive items that have small profit margins, such as toys. Toys typically do not use expensive electronics due to the cost and sophistication required to manufacture such components. The problem is compounded by the fact that many applications for accelerometers in toys require more than one accelerometer so that acceleration forces can be detected in more than one direction. 
     A need therefore exists for an accelerometer, that is small, inexpensive, and simple to manufacture. A need also exists for a simple accelerometer design that can detect acceleration forces in more than one direction, yet is small and easy to integrate into unsophisticated circuitry. These needs are met by the present invention as described and claimed below. 
     SUMMARY OF THE INVENTION 
     The present invention is an accelerometer assembly and its method of operation. The accelerometer assembly is designed to be very low cost and robust. This enables the accelerometer assembly to be used in traditionally inexpensive consumer products, such as toys and novelties. 
     The accelerometer assembly includes one or more molded elastomeric structures. Each of the elastomeric structures has a flexible neck section and a head section that is supported by the flexible neck section. The head section of the elastomeric structure is placed between a light source and a photodetector. The head section partially obscures the photodetector from the light source. As the elastomeric structure experiences acceleration forces, the neck section flexes and the head section moves. This varies the degree in which the head section obscures the photodetector. The amount of light detected by the photodetector, therefore, becomes a measure of changing acceleration forces. 
     Since the elastomeric structures are molded, they can be produced in large quantities at very low cost. Consequently, the complete accelerometer assembly can be manufactured using only a few inexpensive components. Yet the accelerometer assembly is capable of great accuracy in detecting changes in acceleration and/or orientation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, reference is made to the following description of an exemplary embodiment thereof, considered in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of an exemplary embodiment of an acceleration assembly; 
         FIG. 2  is a cross-sectional view of the embodiment of  FIG. 1 , viewed along section line  2 - 2 ; 
         FIG. 3  is a schematic showing no flex in the neck sections of the elastomeric structures; and 
         FIG. 4  is a schematic showing flex in the neck sections of the elastomeric structures in response to an applied acceleration force. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The present invention accelerometer assembly can be used to detect changes in acceleration and changes in orientation in a wide variety of circuits. In the exemplary embodiment of the accelerometer assembly being illustrated, the accelerometer assembly is embodied as a through-hole package for use on a printed circuit board. This embodiment is selected in order to set forth one of the best modes contemplated for the invention. The illustrated embodiment, however, is merely exemplary and should not be considered a limitation when interpreting the scope of the appended claims. The present invention accelerometer assembly can also be configured as a surface mounted component package, or as an isolated plug-in component. 
     Referring to  FIG. 1  and  FIG. 2 , an accelerometer assembly  10  is illustrated for used in a through-hole application upon a printed circuit board  12 . The accelerometer assembly  10  includes a light source  14  that is mounted at an elevated position. The light source  14  is preferably a light emitting diode  16 , however, other light sources can also be used. The light source  14  emits a beam of light  18  that is directed toward the circuit board  12 . 
     Two photodetectors  20 ,  22  are mounted to the circuit board  12 . The photodetectors  20 ,  22  generate an electrical signal or modify a preexisting electrical signal as a function of the intensity of the detected beam of light  18 . Two elastomeric structures  24  are provided. The two elastomeric structures  24  are identical in form and are therefore referenced with the same numbers. Each of the elastomeric structures  24  mounts to the circuit board  12  adjacent one of the photodetectors  20 ,  22 . The elastomeric structures  24  each have a head  26  that is supported upon a flexible neck  28 . The head  26  of each elastomeric structure  24  is positioned to partially block the beam of light  18  as it travels between the light source  14  and the photodetectors  22 ,  24 . The heads  26  move in response to changes in acceleration and/or orientation. The movement of the heads  26  causes the amount of the beam of light  18  being blocked to change. This either increases or decreases the intensity of the beam of light  18  impinging upon the photodetectors  20 ,  22 . The change in detected intensity causes corresponding changes in the electrical signals created or modified by the photodetectors  20 ,  22 . The changes in electrical signal therefore correspond to changes in acceleration of orientation that are required to produce such a signal. Consequently, the accelerometer assembly  10  is capable of detecting and quantifying changes in acceleration and/or orientation. 
     From  FIG. 1  and  FIG. 2 , it can be seen that each of the elastomeric structures  24  have a base  30  that sits in contact with the circuit board  12 . One or more attachment fingers  32  are molded onto the bottom of the base  30 . The attachment fingers  32  have enlarged tips  34 . The attachment fingers  32  are shaped and sized to pass through mounting holes  36  on a printed circuit board  12 , thereby mechanically connecting the base  30  of the elastomeric structures  24  to the circuit board  12 . 
     The head  26  of each elastomeric structure  24  is large enough to have a mass of at least one gram. Each head  26  illustrated is generally wedge-shaped, having a salient point  38 . The salient point  38  is the part of the head  26  that extends into the beam of light  18  when the accelerometer assembly  10  is at rest. Although a wedge shape is shown for each head  26 , other shapes, such as triangle shapes, diamond shapes and teardrop shapes that also have salient points can be used. 
     The heads  26  of each of the elastomeric structures  24  is supported at an elevated position by a flexible neck  28 . The flexible neck  28  is preferably thin and wide so that it is more prone to bend in one plane rather than another. The direction in which the flexible neck  28  is prone to bending is the same direction in which the salient point  38  of the supported head  26  points. 
     The flexible necks  28  support the heads  26  under their centers of gravity. In this manner, when the accelerometer assembly  10  is at rest and the flexible necks  28  are in a vertical orientation, the heads  26  do not bend the flexible necks  28  in any one particular direction. 
     Each head  26  and neck  28  are part of a molded elastomeric structure  24 . Each elastomeric structure  24  is preferably molded as a single piece in an injection mold. It is preferred that each elastomeric structure  24  be molded from a thermoplastic material such as thermoplastic rubber (TPR) or thermoplastic polyurethane (TPU) so that the flexibility of the elastomeric structures  24  does not vary much with changes in ambient temperature. The durometer of the thermoplastic material is preferably between shore A 10 to shore A 90, the preferred durometer being near shore A 60. The sensitivity of each of the elastomeric structures  24  can be customized for different applications, without changing the dimensions of the molded elastomeric structure, by changing the durometer of the thermoplastic material. Consequently, different elastomeric structures  24  that are adapted for different uses can be manufactured from a single injection molding tool. This greatly decreases the capital costs involved in manufacturing a variety of accelerometers. 
     In the shown accelerometer assembly  10 , two elastomeric structures  24  are mounted to the circuit board  12  below the light source  14 . Each elastomeric structure  24  has identical dimensions, being produced by the same injection mold. Both elastomeric structures  24  have flexible necks  28  that extend upwardly at a perpendicular to the plane of the circuit board  12 . However, the two elastomeric structures  24  are offset from each other by 90 degrees. Consequently, one elastomeric structure  24  is prone to bending in the east/west direction, as indicated by double-headed arrow  40 , while the other is prone to bending in the perpendicular direction in and out of the plane of the paper. 
     Referring to  FIG. 3 , it can be seen that when the accelerometer assembly  10  is at rest and the plane of the circuit board  12  is parallel to the ground, the flexible necks  28  do not bend. Rather, the necks  28  extend straight in the vertical. In this position, the two heads  26  block predetermined areas of the underlying photodetectors  20 ,  22 . The photodetectors  20 ,  22  create or alter an electrical signal that is unique for this orientation. 
     Referring to  FIG. 4 , it can be seen that if the accelerometer assembly  10  is accelerated in the direction of arrow  44 , or if the accelerometer assembly  10  were reoriented so that the arrow  44  were pointing upwardly in the vertical, then both elastomeric structures  24   a ,  24   b  respond. The elastomeric structure  24   a  prone to bending in the acting direction of acceleration force will have its flexible neck  28   a  bend backward in the direction opposite the direction of the acceleration. The degree of bending is directly proportional to the acceleration force experienced. The other elastomeric structure  24   b  will have its flexible neck  28   b  twist. This causes the two heads  26   a ,  26   b  to block light from the photodetectors  20 ,  22  to different degrees. 
     For any significant acceleration force encountered in any direction, the heads  26   a ,  26   b  of the two elastomeric structures  24   a ,  24   b  will move. This causes the heads  26   a ,  26   b  to block light from the photodetectors  20 ,  22  in different unique amounts. The combined degree of blockage created by the two heads  26   a ,  26   b  is unique for most all acceleration forces encountered. Consequently, by monitoring the signals produced from the photodetectors  20 ,  22 , an accurate determination can be made regarding the direction and severity of acceleration forces. 
     In the shown embodiment, the accelerometer assembly  10  uses two elastomeric structures  24  that are offset by ninety degrees. Each elastomeric structure  24  blocks light from a single photodetector  20   22 . It should be understood that in order to increase the accuracy of the accelerometer device  10 , more than two elastomeric structures  24  can be used. For example, a third elastomeric structure can be used that is offset from both the illustrated elastomeric structures. Furthermore, accuracy can be increased by providing more than one photodetector for each of the elastomeric structures. If more than one photodetector is used, the direction of deflection can be more accurately determined. 
     In the shown embodiment, the accelerometer assembly  10  is shown mounted directly to a circuit board. It will be understood that the accelerometer assembly  10  can be encased within a protective housing (not shown). A protective housing can help protect the various elastomeric structures  24  from contamination from dirt, dust, and debris that may adversely affect the moving components. 
     It will be understood that the embodiment of the present invention that is illustrated and described is merely exemplary and that a person skilled in the art can make many variations to that embodiment. For instance, more than one light source can be provided. The head of the elastomeric structure can be varied, as can the shape and size of the flexible neck and base. All such embodiments are intended to be included within the scope of the present invention as defined by the claims.