Electret-Based Accelerometer
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.
Latest Microsoft Patents:
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 INVENTIONThe 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 INVENTIONAn electret accelerometer is provided in which a diaphragm, an electret, a back plate and an electronic circuit are placed in a closed casing.
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.
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
To prevent acoustic interference in the accelerometer, the accelerometer of
The other components of the accelerometer of
The accelerometer of
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.
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
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.
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.
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
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's head or throat when the user speaks.
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
As shown from the back view of
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'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
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's pulse when the user places their finger over the meter and also measures the oxygen content of the user'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's ear canal and a speaker opening 2416. In addition, ear bud 2402 includes an accelerometer 2418, which rests against the user's jaw when the ear bud 2402 is in the user's ear canal.
To allow the mobile phone of
Using the accelerometer of the present invention, it is possible to provide an enhanced speech signal.
In
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.
Claims
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.
Type: Application
Filed: Apr 30, 2013
Publication Date: Sep 12, 2013
Applicant: Microsoft Corporation (Redmond, WA)
Inventor: Michael J. Sinclair (Kirkland, WA)
Application Number: 13/873,848
International Classification: G01P 15/125 (20060101);