Patent Publication Number: US-2013233078-A1

Title: Electret-Based Accelerometer

Description:
REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of and claims priority of U.S. patent application Ser. No. 11/166,636, filed Jun. 24, 2005, the content of which is hereby incorporated by reference in its entirety. 
     This application claims the priority benefit of U.S. provisional application 60/669,754 filed on Apr. 8, 2005 and entitled ELECTRET-BASED ACCELEROMETER. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to accelerometers. In particular, the present invention relates to electret-based accelerometers. 
     An accelerometer is a sensor that provides an electrical signal that is indicative of the acceleration experienced by the sensor. Examples of accelerometers include potentiometric, capacitive, inductive, optical and piezoelectric. 
     A capacitive accelerometer measures acceleration based on a change in the capacitance of some element in the sensor. One type of capacitive accelerometer is known as an electret-based accelerometers. 
     An electret-based accelerometer contains an electret, which is a permanently charged material that is an analogue to a permanent magnet. This permanently charged material is placed between a base plate and a metalized flexible diaphragm to form a charged capacitor. Either the base plate or the diaphragm is connected to a JFET transistor. As the diaphragm moves relative to the rest of the accelerometer, the capacitance of the capacitor changes thereby changing the output voltage of the JFET. 
     Under the prior art, electret accelerometers have been prone to producing noisy accelerometer signals especially in acoustically noisy environments. Thus, it is desirable to reduce the level of noise in the accelerometer signals. 
     Accelerometers are also relatively expensive to produce compared to other sensors found in electronic devices. As a result, it is desirable to reduce the cost of manufacturing accelerometers. 
     SUMMARY OF THE INVENTION 
     An electret accelerometer is provided in which a diaphragm, an electret, a back plate and an electronic circuit are placed in a closed casing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a prior art electret microphone. 
         FIG. 2  is a top view of a prior art electret microphone. 
         FIG. 3  is a cross-sectional of an electret accelerometer under an embodiment of the present invention. 
         FIG. 4  is a top view of the electret accelerometer of  FIG. 3 . 
         FIG. 5  is a cross-sectional view of a second embodiment of an electret accelerometer under the present invention. 
         FIG. 6  is a cross-sectional view of a third embodiment of an electret accelerometer under the present invention. 
         FIG. 7  is a top view of one embodiment of a weighted diaphragm of the present invention. 
         FIG. 8  is a cross-sectional view of the weighted diaphragm of  FIG. 7 . 
         FIG. 9  is a cross-sectional view of an alternative weighted diaphragm. 
         FIG. 10  is a top view of another embodiment of a weighted diaphragm of the present invention. 
         FIG. 11  is a cross-sectional view of the weighted diaphragm of  FIG. 10 . 
         FIG. 12  is a cross-sectional view of a further embodiment of a weighted diaphragm under the present invention. 
         FIG. 13  is a top view of another embodiment of a weighted diaphragm of the present invention. 
         FIG. 14  is a cross-sectional view of the weighted diaphragm of  FIG. 13 . 
         FIG. 15  is a flow diaphragm of a method of constructing an electret accelerometer. 
         FIG. 16  is an alternative method of manufacturing an electret accelerometer under the present invention. 
         FIG. 17  is a third method of manufacturing an electret accelerometer under the present invention. 
         FIG. 18  is a perspective view of a mobile device in which an accelerometer of the present invention may be used. 
         FIG. 19  shows the phone of  FIG. 18  in position on the left side of a user&#39;s head. 
         FIG. 20  shows the phone of  FIG. 18  in position on the right side of a user&#39;s head. 
         FIG. 21  is a front view of a necklace embodiment of a mobile device in which an accelerometer of the present invention may be used. 
         FIG. 22  is a back view of the necklace of  FIG. 21 . 
         FIG. 23  shows the necklace of  FIG. 21  positioned on a user. 
         FIG. 24  provides a perspective view of a bracelet/watch and ear bud in which the accelerometer of the present invention may be used. 
         FIG. 25  is a perspective view of a second embodiment of a mobile phone in which an accelerometer of the present invention may be used. 
         FIG. 26  shows the phone of  FIG. 25  in position on the left side of a user&#39;s face. 
         FIG. 27  is a block diagram of a general speech processing system. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The present invention provides a low-cost accelerometer that is formed by modifying an electret microphone. Electret microphones have been the focus of a significant amount of engineering in an effort to reduce the costs of producing them. This considerable effort has reduced the price point for electret microphones. Accelerometers, on the other hand, continue to be relatively expensive compared to electret microphones. 
       FIG. 1  provides a cross section of a prior art electret microphone. The microphone includes a casing  100  that has an opening  102 .  FIG. 2  provides a top view of the electret microphone showing opening  102  in casing  100 . In typical electret microphones, casing  100  is around 6 mm in diameter and 5 mm high. 
     Within casing  100 , the outer periphery of a metalized diaphragm  104  is attached to a spacer  112  such that the center of diaphragm  104  can move relative to the periphery. Metalized diaphragm  104  is designed to have a weight-to-flexibility ratio such that movement of container  100  does not induce relative movement between diaphragm  104  and container  100 . The space above and below diaphragm  104  contains air. 
     Below spacer  112  is an electret  108 . Electret  108  extends across the interior of casing  100  and is a statically charged non-conducting material. On the bottom surface of electret  108  is a metalized back plate  106 . A number of holes, such as holes  114  and  116 , extend through electret  108  and back plate  106  to allow air to pass though the electret and back plate when diaphragm  104  moves relative to electret  108 . 
     Electret  108  and back plate  106  are supported by a spacer  118 , which creates a space for an electronics circuit  109 . Often, electronics circuit  109  is just a single Junction Field-Effect Transistor (JFET). Electronics circuit  109  is connected to metalized diaphragm  104  by a conductor  120  and is connected to metalized back plate  106  by a conductor  122 . Electronics circuit  109  is powered by and provides a signal along output conductors  124  and  126 , which are typically connected to an external bias circuit. Note that in some systems, three or more power/output conductors are provided to electronics circuit  109 . 
     During operation, acoustic waves pass through opening  102  and cause diaphragm  104  to move. This movement changes the capacitance of a capacitor formed by diaphragm  104  and backplate  106 . Because of electret  108 , this change in capacitance appears as a change in voltage. Electronics circuit  109  amplifies this voltage change along output conductors  124  and  126 . 
     The present invention forms an accelerometer out of the electret microphone of  FIG. 1  by changing the weight-to-flexibility ratio of the diaphragm such that relative motion is created between the diaphragm and the casing when the casing is moved. In this case, the added weight causes the diaphragm to be inertially grounded above DC. In addition, in some embodiments, the diaphragm is acoustically isolated so that it does not move in response to acoustic signals. 
       FIG. 3  provides a cross-sectional view of one embodiment of the present invention. In  FIG. 3 , a weight  330  has been added to a diaphragm from an electret microphone to form diaphragm  304 . In one embodiment, weight  330  is circular, is centered on diaphragm  304 , and has a mass of 7.5 mg. The extra mass of weight  330  changes the mass-to-flexibility ratio of the diaphragm thereby allowing it to be used as an accelerometer. Specifically, the extra mass causes the diaphragm to move relative to the casing when the casing is moved. 
     To prevent acoustic interference in the accelerometer, the accelerometer of  FIG. 3  has a solid casing  300 . Casing  300  does not include a hole above the diaphragm. Thus, hole  102  of the electret microphone of  FIG. 1  is removed in the accelerometer of  FIG. 3 . This can be seen more clearly in the top view of  FIG. 4 . The casing has a diameter of 6 mm and a height of 5 mm under one embodiment of the invention. 
     The other components of the accelerometer of  FIG. 3  are the same as the components found in an electret microphone. For example, electret  308 , spacers  312  and  318 , backplate  306 , electronics  309  and conductors  320 ,  322 ,  324 , and  326  are the same type of components that are found in the electret microphone of  FIG. 1  and are constructed in the same manner as the components of  FIG. 1 . The complete mass of entire accelerometer is approximately 226 mg. 
     The accelerometer of  FIG. 3  functions in a manner similar to the electret microphone of  FIG. 1  except that it detects acceleration of the casing normal to the diaphragm instead of acoustic waves. In particular, as casing  300  is moved, diaphragm  304  moves relative to the casing. This relative movement changes the capacitance of the capacitor formed by diaphragm  304  and back plate  306  thereby creating a change in the voltage on the output conductors. 
     By using many of the same components and construction techniques used to form electret microphones, the accelerometer of the present invention leverages the knowledge and efficiencies of scale that have been achieved for electret microphones to make a low-cost accelerometer. In addition, it results in an accelerometer that has no static (DC) response. This is different from many modern accelerometers that provide signals even when there is no acceleration. 
       FIG. 5  provides a cross-sectional view of an alternative embodiment of the accelerometer of the present invention. The accelerometer of  FIG. 5  is identical to the accelerometer of  FIG. 3 , except that a casing  500  is used that has a hole  502 . Thus, the accelerometer includes a diaphragm  504  with an added weight  530  as found in the accelerometer of  FIG. 3 . 
     Casing  500  is the same type of casing that is used in an electret microphone. To isolate diaphragm  504  from acoustic noise, a plug  540  is inserted in hole  502 . Plug  540  includes an outer seal  542  and an inner seal  544  that engage the casing to keep plug  540  in place. Although not labeled, the other components of the accelerometer of  FIG. 5  are constructed in the same manner and operate in the same manner as the embodiment of  FIG. 3 . 
       FIG. 6  provides a cross-sectional view of another embodiment of the present invention. In  FIG. 6 , the accelerometer is similar to the accelerometer of  FIG. 5  except that instead of using plug  540 , the entire casing  600  is encased in an acoustic insulating material  640 , which covers opening  602 . Under some embodiments, this acoustic insulating material is an acrylic. Those skilled in the art will recognize that the thickness of the acrylic can be selected based on the overall desired size for the electret accelerometer and the degree to which the electret accelerometer is to be isolated from acoustic signals. In one embodiment, the acoustic insulating material has a mass of 163 mg. 
     The acoustic insulating material may be formed in two parts with casing  600  being inserted within the two parts. Alternatively, the acoustic insulating material may be applied to the casing and then cured. For example, a thin covering may be placed over hole  602  and then casing  600  can be repeatedly dipped in a liquid form of the insulating material to build up a thick layer of insulating material on the casing. 
       FIGS. 7 and 8  provide a top view and a side cross-sectional view, respectively, of a diaphragm for an electret accelerometer under one embodiment of the present invention. The diaphragm of  FIG. 7  includes a flexible material  700  and a weight  702 . Flexible material  700  includes a metalized layer  704 . 
       FIG. 9  shows a side cross-sectional view of an alternative embodiment of a diaphragm for an electret accelerometer under the present invention. In  FIG. 9 , a flexible material  900  has two masses  902  and  904  placed on it. Each of the masses  902  and  904  are circular in nature and are substantially centered on the round flexible material  900 . In the embodiment of  FIG. 9 , mass  904  is preferably coated with a metallic layer or formed completely of a metallic material so as to form part of the capacitor required for the electret accelerometer. 
       FIGS. 10 and 11  provide a top view and a side cross-sectional view, respectively, of another embodiment of a diaphragm of an electret accelerometer under the present invention. In the diaphragm of  FIGS. 10 and 11 , concentric mass rings  1000  and  1002  are placed around a center mass  1004  on a flexible material  1006 . 
       FIG. 12  provides a side cross-sectional view of an additional embodiment of a diaphragm under the present invention. In  FIG. 12 , concentric mass rings  1200  and  1202  and center mass  1204  are placed on one side of a flexible diaphragm  1206  and concentric mass rings  1208  and  1210  and center mass  1212  are placed on the other side of the flexible material  1206 . 
       FIGS. 13 and 14  provide a top view and a side cross-sectional view of a diaphragm of an additional embodiment of the present invention. In  FIG. 13 , a circular rigid diaphragm  1300  with increased mass is attached to the spacers of the accelerometer by a compliant support  1302  around the periphery of diaphragm  1300 . Diaphragm  1300  is metalized and provides a large flat surface area to improve the output of the accelerometer. Compliant support  1302  can be constructed of any compliant material such as rubberized fabric such as in speaker cones or concentrically corrugated Mylar. 
     Although specific designs for the placement of added mass on the diaphragm have been show, those skilled in the art will recognize that other patterns are possible and within the scope of the present invention. In addition, instead of adding mass to the diaphragm, manufacturing steps could be taken to increase the flexibility of the diaphragm while keeping its mass the same. This would have the same effect of increasing the mass-to-flexibility ratio of the diaphragm thereby making it useable in an accelerometer. 
       FIG. 15  provides a flow diagram for constructing an electret accelerometer under the present invention. In step  1500 , the electret, the backplate and the electronics are manufactured in the same manner as they would be manufactured for an electret microphone. In step  1502  a diaphragm with a higher mass-to-flexibility ratio than a diaphragm for an electret microphone is formed. In step  1504 , the electret, backplate, electronics and diaphragm are placed in a container using the same techniques that are used in forming an electret microphone. At step  1506 , the container is optionally sealed with an acoustic insulation, for example, plug  540  or acoustic insulating material  640 . 
       FIG. 16  provides an alternative method for producing an electret accelerometer under the present invention. In step  1600 , the electret, backplate and electronics of the accelerometer are produced in the same manner that they would be produced for an electret microphone. At step  1602  a diaphragm is produced that has a greater weight-to-flexibility ratio than diaphragms found in electret microphones. At step  1604 , the electret, backplate, electronics, and diaphragm are inserted into the same canister used for an electret microphone using a different technique than is used to form an electret microphone so as to accommodate the different diaphragm produced in step  1602 . At step  1606 , the canister is sealed so that the diaphragm is not affected by acoustic waves. 
       FIG. 17  provides an alternative method for forming the electret accelerometer of the present invention. In step  1700 , the electret, the backplate and electronics are produced in the same manner as they would be produced for an electret microphone. At step  1702 , a diaphragm is produced that has a greater weight-to-flexibility ratio than the diaphragms used in electret microphones. At step  1704 , the electret, backplate, electronics, and diaphragm are inserted into a closed canister such as the canister of  FIG. 3 . 
     In further embodiments, the electret-based accelerometers described above are manufactured with a dampening material, such as a viscoelastic material or liquid, placed within the casing so that the dampening is in contact with both the casing and the diaphragm. For example, the dampening material may be placed in space  360  of  FIG. 3 , space  560  of  FIG. 5 , and space  660  of  FIG. 6 . This dampening material may be added after the diaphragm is placed in the casing but before the casing is sealed or may be placed in a closed canister before the diaphragm in embodiments such as shown in  FIG. 17 . The dampening material dampens the frequency response of the diaphragm and helps to reduce resonance at certain frequencies. 
     The accelerometer of the present invention may be used in any desired application. The present inventors have found that the accelerometer is especially useful as a bone-conduction microphone, which detects vibrations of a user&#39;s head or throat when the user speaks. 
       FIG. 18  provides an example of a mobile phone  1800  that could use the accelerometer of the present invention as a bone-conduction microphone. Mobile phone  1800  includes a key pad  1802 , a display  1804 , a cursor control  1806 , an air conduction microphone  1808 , a speaker  1810 , two bone-conduction microphones  1812  and  1814 , and optionally a proximity sensor  1816 . Bone-conduction microphones  1812  and  1814  consist of one of the accelerometer embodiments described above. Mobile phone  1800  also includes a power source such as a battery, a processor, a global positioning satellite signal detector and processor, which are not visible from the exterior of the phone. Optionally, mobile phone  1800  may also include a pulse sensor, an oximetry sensor, a temperature sensor, and a video camera. 
     Keypad  1802  allows the user to enter numbers and letters into the mobile phone. In other embodiments, keypad  1802  is combined with display  1804  in the form of a touch screen. Cursor control  1806  allows the user to highlight and select information on display  1804  and to scroll through images and pages that are larger than display  1804 . 
     As shown in  FIGS. 19 and 20 , when mobile phone  1800  is put in the standard position for conversing over the phone, speaker  1810  is positioned near the user&#39;s left ear  1900  or right ear  2000 , and air conduction microphone  1808  is positioned near the user&#39;s mouth  1902 . When the phone is positioned near the user&#39;s left ear, as in  FIG. 19 , bone conduction microphone  1814  contacts the user&#39;s skull or ear and produces an alternative sensor signal that provides information about speech that can be used to remove noise from the speech signal received by air conduction microphone  1808 . For example, the information provided in the alternative sensor signal can include whether the user is speaking as well as low frequency information related to the user&#39;s speech. When the phone is positioned near the user&#39;s right ear, as in  FIG. 20 , bone conduction microphone  1812  contacts the user&#39;s skull or ear and produces an alternative sensor signal that can be used to remove noise from the speech signal. 
       FIGS. 21 and 22  show a front view and a back view of another mobile device in which an accelerometer of the present invention may be used. In  FIGS. 21 and 22 , mobile device  2100  consists of a necklace or choker  2102  and an ear bud  2104 . Necklace  2102  includes a decorative/ornamental disk or pendant  2106  that is suspended from a neck engaging piece  2108 , such as a string or a wire. The neck engaging piece supports the mobile device on the user and is designed to be attached around a user&#39;s neck. Decorative disk  2106  includes a microphone opening  2109  and a video opening  2110 . 
     As shown from the back view of  FIG. 22 , mobile device  2100  includes a battery  2111 , which powers an air conduction microphone  2112 , an accelerometer of the present invention  2114 , a video camera  2116 , a processing chip set  2118 , and a global positioning satellite (GPS) receiver  2120 . Processing chip set  2118  is connected to air conduction microphone  2112 , accelerometer  2114 , video camera  2116 , and GPS receiver  2120 . Processing chip set  2118  includes a processor, memory storage, and input/output interface and a communication interface. The communication interface allows the processor to communicate with a processor within ear bud  2104 , allowing the processor in processing chip set  2118  to transmit electrical signals representing acoustic information to ear bud  2104 . The communication interface of processing chip set  2118  may also wirelessly communicate with a collection of other devices, including a video display, a personal computer, a router, and other mobile devices. The protocol used for these communications can include any known protocol, including any variations of the 802.11 protocol. 
     Ear bud  2104  includes outer portion  2130 , ear canal portion  2132 , and speaker opening  2134 . Ear bud  2104  receives a signal from processing chip set  2118  and converts that signal into an auditory signal through a speaker that is internal to ear bud  2104 . This auditory signal exits through speaker opening  2134  into the user&#39;s ear. Ear bud  2104  includes a battery (not shown) and a communication interface that allows it to communicate with the communication interface of processing chip set  2118 . 
     As shown in  FIG. 23 , neck engaging piece  2108  goes around a user&#39;s neck  2304  to place pendant  2106  in contact with the front of the user&#39;s neck slightly below the thyroid cartilage of the larynx, commonly referred to as the “Adam&#39;s Apple.” In this position, accelerometer  2114  detects movement of the user&#39;s neck caused by vibrations of the vocal cords. Ear bud  2104  is placed in the user&#39;s ear such that exterior portion  2130  extends between the tragus  2300  and the anti-tragus  2302  of the outer ear. 
       FIG. 24  provides a pictorial diagram of another embodiment of a mobile device in which an accelerometer of the present invention may be placed. In  FIG. 24 , the mobile device includes a watch or bracelet  2400  and an ear bud  2402 . Watch  2400  includes an enclosure  2401 , which is mounted on a wrist engaging piece  2406 , such as a band, designed to be secured around the user&#39;s wrist. Enclosure  2401  has an interior that holds a set of electronic device, which includes a battery, a interface, a Global Positioning Satellite receiver, a video camera, speaker, air conduction microphone, pulse sensor, oximetry sensor and temperature sensor. The communication interface allows the processor to communicate with a processor in ear bud  2402  and thereby transmit acoustic information to ear bud  2402  and receive data from an accelerometer  2418  in ear bud  2402 . In addition, the communication interface allows for wireless communication with one or more of a router, a personal computer, and other mobile devices. 
     Enclosure  2401  includes openings corresponding to some of the electronic devices in the enclosure including pulse and oximetry meter opening  2408 , air conduction microphone opening  2410 , and video camera opening  2412 . The pulse and oximetry meter measures the user&#39;s pulse when the user places their finger over the meter and also measures the oxygen content of the user&#39;s blood using a light source and a light sensor. The exterior of one side of enclosure  2401  also includes a display  2404 . 
     Ear bud  2402  includes an ear portion  2414  designed to be placed in a user&#39;s ear canal and a speaker opening  2416 . In addition, ear bud  2402  includes an accelerometer  2418 , which rests against the user&#39;s jaw when the ear bud  2402  is in the user&#39;s ear canal. 
       FIG. 25  provides another example of a mobile phone  2500  that can use the accelerometer of the present invention as a bone-conduction microphone. Mobile phone  2500  is similar to mobile phone  1800  of  FIG. 18  except that an accelerometer  2502  is provided on the exterior of mobile phone  2500  attached to a flexible stock  2504 . As shown in  FIG. 26 , during use, flexible stock  2504  positions accelerometer  2502  next to the user&#39;s skull, just behind the ear lobe. In some embodiments, a clip is provided on mobile phone  2500  to store flexible stock  2500  and accelerometer  2502  when not in use. In other embodiments, flexible stock  2500  is capable of being pushed into and pulled out of casing  2506  of phone  2500  through an opening  2508 . In other embodiments, channels are provided in casing  2506  to house flexible stock  2504  and accelerometer  2502  when not in use. 
     To allow the mobile phone of  FIG. 25  to be used by left and right handed people, flexible stock  2504  may be positioned on either side of mobile phone  2500 . In some embodiments of the mobile phone, two separate flexible stocks having two separate accelerometers are provided on a single phone. 
     Using the accelerometer of the present invention, it is possible to provide an enhanced speech signal.  FIG. 27  provides a basic block diagram of a speech processing system that provides an enhanced speech signal using an accelerometer of the present invention. 
     In  FIG. 27 , a speaker  2700  generates a speech signal  2702  that is detected by an air conduction microphone  2704  and an accelerometer of the present invention  2706 . 
     Air conduction microphone  2704  also receives noise  2708  generated by one or more noise sources  2710 . Depending on the level of the noise, noise  2708  may also be detected by accelerometer  2706 . However, under most embodiments of the present invention, accelerometer  2706  is typically less sensitive to ambient noise than air conduction microphone  2704 . Thus, the accelerometer signal  2712  generated by alternative sensor  2706  generally includes less noise than air conduction microphone signal  2714  generated by air conduction microphone  2704 . 
     Accelerometer signal  2712  and air conduction microphone signal  2714  are provided to a clean signal estimator  2716 , which estimates a clean speech signal  2718  from accelerometer signal  2712  and air conduction microphone signal  2714 . Clean signal estimate  2718  is provided to a speech process  2720 . Clean speech signal  2718  may either be a filtered time-domain signal or a feature domain vector. If clean signal estimate  2718  is a time-domain signal, speech process  2720  may take the form of a listener, a cellular phone transmitter, a speech coding system, or a speech recognition system. If clean speech signal  2718  is a feature domain vector, speech process  2720  will typically be a speech recognition system. 
     The present invention utilizes several methods and systems for estimating clean speech using air conduction microphone signal  2714  and accelerometer signal  2712 . One system uses stereo training data to train correction vectors for the accelerometer signal. When these correction vectors are later added to a test accelerometer vector, they provide an estimate of a clean signal vector. One further extension of this system is to first track time-varying distortions and then to incorporate this information into the computation of the correction vectors and into the estimation of the clean speech. 
     A second system provides an interpolation between the clean signal estimate generated by the correction vectors and an estimate formed by subtracting an estimate of the current noise in the air conduction test signal from the air conduction signal. A third system uses the accelerometer signal to estimate the pitch of the speech signal and then uses the estimated pitch to identify an estimate for the clean speech signal. A fourth system uses direct filtering, in which the accelerometer signal and the air conduction signal are used to determine one or more channel responses of the accelerometer. The channel response(s) are then used to estimate the clean speech signal. 
     Although specific applications for the accelerometer of the present invention have been described, the accelerometer is not limited to these uses and may be used in any application requiring an accelerometer. Another example of a use for the accelerometer is as an activity detector included in mobile devices such as cell phones, pocket computers, laptops and tablet PCs. The activity detector detects a user state such as walking, running, driving, or non-activity. Another application is as a vibration detector for machinery, where the processing of the signals from one or more of these accelerometers is used to determine if the machinery is about to fail. Still another application is real-time feedback for speakers where an accelerometer is mounted on a speaker and provides a feedback signal to the amplifier system. 
     Although the present invention has been described with reference to particular embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.