Patent Publication Number: US-8989397-B2

Title: System and method for high reliability sound production

Description:
FIELD OF THE INVENTION 
     Embodiments relate to loudspeakers and in particular to a loudspeaker system and method for reproducing sound with a low failure rate. 
     BACKGROUND 
     Loudspeakers change an electrical signal into sound. Prior Art  FIG. 1  shows a cross-sectional view of the main components of a moving-coil type loudspeaker  100 . A rigid metal frame  101  attaches to a magnet  102 , which subjects the air gap  108  between the magnet and the frame to a strong magnetic field. A lightweight diaphragm  103  attaches to the frame  101  and to the cylindrical shaped bobbin  107 . A voice coil  104  is wound around the bobbin  107 . The voice coil attaches through two voice coil wires  105  to two contacts  106 . When a current is run through voice coil  104 , the magnetic field in air gap  108  will interact with the current in coil  104  to create a force that causes the bobbin to move up or down depending on the direction of the electrical current. This in turn moves the diaphragm, which produces air pressure waves that result in sound. 
     Loudspeakers are electromechanical devices subject to failures. Some failure conditions may result in no sound at all being produced. In certain applications, it is critical that a sound is produced, even if distorted, such as is the case in medical devices, such as patient monitors. For this and other applications, a loudspeaker should have low failure rate of sound production. 
     Failure mechanisms of loudspeakers include voice coil wire breakage, damage of the diaphragm, diaphragm separating from the frame, and other mechanical failures. Although many of these failure mechanisms result in distorted sound, damage to the voice coil leads to no sound at all, which is undesirable in alarm sound applications. 
     Voice coil breakage results from mechanical stress. The voice coil moves with the diaphragm, but the wires of the voice coil attach to stationary contacts typically mounted on the loudspeaker frame. Hence the wires move and change shape with every diaphragm movement, or with every sound made by the loudspeaker. 
     Numerous technologies may reduce the chance of voice coil wire breakage. For example, the wires may be given extra length for strain-relief; they may be pre-shaped to allow movement, etc. The loudspeaker may also be designed such that the wires move in free space, away from other surfaces that may cause friction damage. 
     Despite such technologies, the voice coil wires have some probability of breaking, because they are subject to constant mechanical deformation and stress. If they break, the resulting total absence of sound is problematic in alarm applications. There is, therefore, a need for addressing these and other issues associated with the prior art. 
     SUMMARY 
     A system and method are provided where a loudspeaker includes two or more voice coils. The system is configured such that the system provides a warning when one or more voice coil wires are broken, but continues to produce sound using one or more of the remaining voice coils. Assuming that the warning is observed, and timely repair or replacement of the loudspeaker is performed, the probability of complete failure and the loudspeaker being unable to generate sound is reduced to a minimum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Prior Art  FIG. 1  shows the construction of a loudspeaker with its main components 
         FIG. 2  shows the bobbin and coils of a loudspeaker, in accordance with one embodiment. 
         FIG. 3  shows a high reliability sound production system in accordance with one embodiment. 
         FIG. 4  shows a coil drive mechanism in accordance with one embodiment. 
         FIG. 5  shows a coil drive mechanism in accordance with one embodiment. 
         FIG. 6  shows one embodiment of the high reliability sound production system, including amplifiers and a micro-controller as failure detection mechanism, according to one embodiment. 
         FIG. 7  shows one embodiment of a high reliability sound production system, including switches and a micro-controller as failure detection mechanism, according to one embodiment. 
         FIG. 8  shows one embodiment of a high reliability sound production system, using two amplifiers and a micro-controller as failure detection mechanism, according to one embodiment. 
         FIG. 9  shows a flow chart of a method according to one embodiment. 
         FIG. 10  shows one embodiment of a high reliability sound production system, using two voice coils with shared contacts, according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows a diagram of one embodiment. The bobbin  201  may comprise two coils wound thereon. Voice coil  202  with voice coil wires  204  and contacts  205  constitute the coil used for initial operation. Voice coil  203  with voice coil wires  206  and contacts  207  constitute another coil that can be used if voice coil  202  or its voice coil wires  204  break. The mechanism by which the two coils are driven and the way that coil break is determined will be described below. 
     The two coils may be adjacent to each other as shown in  FIG. 2 , or the coils may be wound on top of each other or in other configurations. The coils may be wound in the same direction, or in opposite directions. There may be two coils, or more than two coils may be used to further reduce failure rate. 
     The coils may have completely separate contacts, as shown in  FIG. 2 , or the coils may use a common contact, for example a common ‘ground’ contact. The coils may also share both contacts as will be described with reference to  FIG. 10  below. 
       FIG. 3  shows one embodiment of a high reliability loudspeaker system according to one embodiment. The coils  202  and coils  203  from  FIG. 2  are now shown as coil  309  and coil  310  in  FIG. 3 . The coil drive mechanism  301  may be configured to drive either coil  309  or coil  310 , with an audio input signal  306  received from the outside environment. The outside environment may be, for example, a patient heart monitor, another medical instrument or any electronic device in need of a high reliability loudspeaker system. The failure detection mechanism  302  detects failure of coil  309  or coil  310  by using signals  305  from the coil drive mechanism and optionally using a microphone  303  placed close to the speaker. An optional memory  304  allows the failure detection mechanism  302  to persistently remember coil failure across loss of system power. The failure detection mechanism may be configured to inform the environment of a coil failure through signal  307 . The signal  307  may be carried over a bus such as the I2C Inter IC bus, SPI Serial Peripheral Interface bus, another type bus, or one or more logic signals that indicate the status of the loudspeaker system, or any other notification mechanism. 
     The system of  FIG. 3  may be configured to operate as follows. Initially, the drive electronics  301  drives coil  309 . The failure detection mechanism may be configured to observe the status of coil  309  and coil  310 . This failure detection may involve voltage and or current data  305  obtained from the coil drive mechanism. Optionally, the failure detection mechanism may comprise microphone  303  to check for actual sound present condition. The failure detection may be carried out once at each system power on, by running power on test(s), or the detection may be configured to occur continuously while the system is in normal operation. Alternately, the system may periodically interrupt normal operation briefly to test coil  309  and coil  310 . Once a failure of coil  309  or  310  is detected, the failure detection mechanism  302  may instruct the coil drive mechanism  301  through connection  308  to drive the remaining good coil. At the same time, the failure detection mechanism may inform the outside environment that one of the coils has failed using connection  307 . According to one embodiment, the failure detection mechanism may store the identity of the failed coil failed in memory  304 , so that the next time the system is powered on the remaining good coil will be used right away without delay to test first. 
     The environment, for example the heart rate monitor or other instrument, may be configured to use failure notification  307  to alert an operator that maintenance is needed, while the instrument remains operational and usable. According to one embodiment, the alert mechanism may comprise, for example, displaying a special warning message or flashing a warning screen on the LCD display of the instrument or the generation of a warning tone in addition to normal operation sounds, to alert the operator of the condition that requires maintenance. The alert mechanism may comprise rendering a special ‘warning’ screen that requires the operator to confirm the condition with a special action. For networked instruments, the alert mechanism may comprise a notification of the device manufacturer, of a service department or of a hospital administration system, for example. 
       FIG. 4  shows one embodiment  400  of the coil drive mechanism of  FIG. 3  using switches. In this embodiment, the environment provides a low impedance speaker drive signal  405 . This embodiment may be suitable when an existing instrument is upgraded with the new high reliability speaker subsystem. The existing instrument already has its own speaker drive electronics and using the coil drive mechanism of  FIG. 4  ensures that the instrument needs minimal modification to accept the new high reliability speaker system. As shown in  FIG. 4 , the drive signal  405  may be steered to either coil  401  or to coil  402  by the Dual Pole Double Throw (DPDT) switch  403 . The settings of switch  403  may be controlled by signal  404 , which is derived from the failure detection mechanism. Note that switch  403  may be implemented in many ways. For example, the high reliability switch  403  may be implemented with CMOS switches, such as a Texas Instruments TS3A24157 or similar device available from many vendors. Alternately, switch  403  may be implemented with an electromechanical switch such as a relay or reed switch. Those skilled in the art may appreciate that switches may be used in many configurations, and that there are many modifications, deletions and substitutions possible with respect to the example of  FIG. 4 . According to further embodiments, the coil drive mechanism of  FIG. 4  may also be adapted in case that more than 2 voice coils are used, by adding additional switches or switch positions. 
       FIG. 5  shows one embodiment  500  of a coil drive mechanism, using two amplifiers. As shown in  FIG. 5 , the environment may provide a signal input  501 . This embodiment is suited for new designs, where the environment need not provide its own speaker driver amplifier. The actual speaker driver amplifiers may be implemented inside the high reliability speaker system by amplifiers  502  and  504 . Amplifiers  502  and  504  may be compact high-efficiency class D audio amplifiers with a shutdown provision, such as Maxim MAX9830, On Semi NCP2820 or similar parts available from other vendors. Alternately, any amplifier with the capability to provide low impedance speaker drive and the ability to shut down into a high-impedance output state may be used instead. In initial use, signal  503  enables amplifier  502  to drive coil  506 . At the same time, signal  505  disables amplifier  504  and its output remains in high-impedance state so that coil  507  is not loaded and is hence not affecting bobbin movement. Once a coil  506  failure is detected, signal  503  shuts down amplifier  502  and signal  505  enables amplifier  504  so that coil  507  is now driven instead. 
       FIG. 6  shows one embodiment of the high reliability speaker system including a failure detection mechanism using a micro-controller (computer or computing device)  610 . This embodiment may be configured to use a coil drive mechanism that is similar to that of  FIG. 5  but includes a micro controller  610  and one additional amplifier  606  and resistor  609  to test coil  614 . The micro controller  610  may be a very low cost micro-controller with a few digital outputs and two or more analog inputs, such as the Texas Instruments MSP430G2001, Freescale MKL04Z8VFK4 or similar micro-controllers available from many vendors. In  FIG. 6 , the environment provides an input signal  601 , and the actual speaker driver amplifiers are  602  and  604 . On system power up, or periodically, the micro-controller  610  may disable amplifiers  602  and  604  through shutdown control signals  603  and  605 . The micro-controller may then enable amplifier  606  through control signal  607 , provide a DC or AC test signal through output  608  and sense current through coil  614  and hence resistor  609  by using the micro-controller internal Analog to Digital (A/D) converters on input signals  611  and  612  taken from both sides of resistor  609 . The voltage across resistor  609  may be used to measure the current through coil  614 , or alternately, the combination of the voltage on line  611  and the current may be used to measure the impedance of coil  614 . A failure of coil  614  may be identified by the current being below a threshold, or by the measured impedance higher than a threshold. The micro-controller may then finish the test, disable amplifier  606  and use the measured current or impedance to determine whether coil  614  is operational (e.g., intact or broken). According to one embodiment, if coil  614  is operational, it enables amplifier  602  through control signal  603 . If coil  614  is non-operational, it may send a failure notification to the environment through connection  613 , and may enable amplifier  604  through control signal  605  to drive coil  615  instead. According to one embodiment, the circuit of  FIG. 6  may be modified to also test coil  615 , using one more amplifiers and resistors. Another embodiment enables avoiding use of a third amplifier, by using a switch to connect the input to amplifier  602  or  604  to either the input signal  601  or test signal  608  provided by micro-controller  610 . 
     According to one embodiment, resistor  609  may be omitted and the power supply current of amplifier  606  may be measured to determine whether the coil is broken. This measurement may, for example, be carried out using a small current measurement resistor in the power supply connection of amplifier  606 . One embodiment omits the resistor  609  and instead uses a microphone placed close to the speaker to test for coil failure. In this embodiment, the microphone may be connected to an analog input of micro-controller  610 . If necessary, a microphone amplifier may be used to bring the signal to the desired level for the micro-controller input. Advantageously, using a microphone instead of a current sensing resistor enables other failure modes to be detected in this manner. 
     According to one embodiment, the switch based circuit of  FIG. 4  may be modified to comprise a failure detection mechanism using a micro-controller.  FIG. 7  shows one embodiment of a high reliability speaker system using switches and comprising a failure detection mechanism using a micro-controller  710 . The micro-controller may be configured to, upon power up, or periodically, test coil  714  and/or coil  715 . For example, the micro-controller  710  may be configured to start by connecting the input signal  709  to coil  715  using signal  711  to control switch  702 . While coil  714  is not connected to input  709 , the micro-controller may use switch  701  through signal  712  to connect coil  714  to amplifier  703  through resistor  704 . The micro-controller may be configured to send a DC or AC test signal using signal  705  and to then observe the current through coil  714  and hence resistor  704  by carrying out A/D conversions on inputs  706  and  707 . The micro-controller may then determine whether coil  714  is intact or broken by using the observed current, or the measured impedance of coil  714 . If coil  714  is determined to be intact, the micro-controller may use signal  713  to put amplifier  703  in a high-impedance state. The micro-controller may then use signal  711  to connect coil  714  to input signal  709 . If coil  714  is broken, the micro-controller may then notify the environment using signal  708 , and may use signal  711  to connect input  709  to coil  715  instead. The micro-controller may also be configured to test coil  715  in a similar manner, to thereby allow for more elaborate failure reporting on signal  708  such as, for example, warning that either coil is broken or generating an error message alerting that both coils may be broken. Note that the latter is an extremely unlikely condition, if timely maintenance is performed when one coil is broken. 
     The high reliability speaker system of  FIG. 6  or  FIG. 7  may be modified to carry out continuous monitoring of the coil, while the coil is being driven by the environment audio signal. For example, the micro-controller may be configured to observe both the input signal and either a microphone or a coil current sensing resistor. The micro-controller may be further configured to perform tests whenever the input audio signal is active. For example, a medical monitor may emit regular soft beeping sounds that are of sufficient amplitude to perform a coil test with every beep. This method may be preferable over testing at power up, or at periodic interval. This method of testing is configured to detect a failure as it occurs and to switch over to a spare coil without ever interrupting device functionality. 
       FIG. 8  shows one embodiment of a high reliability speaker system using continuous monitoring. Blocks  810 ,  811  and  818  may comprise electronic circuits that allow a low-cost micro-processor with slow analog inputs to measure the amplitude of small AC voltages accurately. Blocks  810 ,  811  and  818  may contain a simple amplifier and rectification circuit, a peak detector with reset capability or any other circuits well known in the art.  FIG. 9  is a flowchart of a method according to one embodiment. According to one embodiment, this method may be carried out through software executed by the micro-controller  808 . The micro-controller executes block  901  and enables amplifier  802  through signal  803 , and disables amplifier  804  through signal  805 . This enables the input signal  801  to drive voice coil  806 . The micro-controller may be configured to execute block  902  and measure the voltage on input signal  801 . This for example detects a beep or other signal of sufficient input strength to carry out a reliable measurement of the impedance of coil  806  as detailed below. Decision  903  compares the measured voltage on input signal  801  against a pre-determined threshold1. If the voltage does not exceed the treshold1, the signal is too weak to perform a reliable coil impedance measurement and the micro controller reverts to block  902 . If the measured voltage on input  801  exceeds treshold1, the micro-controller goes on to measure the coil impedance. The micro-controller may execute block  904  to measure the AC voltages on signal  813  and signal  814 . After measuring the voltages, the micro-controller may then execute block  905  and compute the coil impedance. This computation involves the value of resistor  809  and the AC voltages on signals  813  and  814 . The impedance Z of coil  806  may be computed by the formula Z=R 809 *V 814 /(V 813 −V 814 ). The micro controller may then proceed with block  906 . In block  906  the measured impedance Z of coil  806  may be compared against a pre-determined treshold2. If the impedance Z is less than treshold2, the coil is determined to be intact, and the micro-controller resumes operation  902 . If the impedance is greater than treshold2, coil  806  is determined to have failed, and the micro-controller proceeds to block  907 . In block  907 , the micro-controller notifies the environment that coil  806  has failed using signal or bus  817 . The micro-controller may then disable amplifier  802  through signal  803  and may enable amplifier  804  through signal  805 . The input signal  801  may now drive coil  807  and normal operation continues. 
     Many variations of the system in  FIG. 8  are possible. For example, a more powerful micro-controller with fast analog to digital inputs may be used. In that case, circuits  810 ,  811  and  818  may be comprise a single capacitor and two resistors that pass the AC signal on and add a DC bias, so that the signal is in range of the Analog to Digital converters of micro-controller  808 . According to another embodiment, a low cost micro-controller with slow A/D inputs may be used, and circuits  810 ,  811  and  818  may be more complex circuits that perform amplification of the AC signal, rectification and peak detection. According to one embodiment, a full impedance measurement may be omitted, in favor of a simple measurement of the current through resistor  809 . If the input signal exceeds a threshold and the current through resistor  809  does not, then coil  806  may be considered to have failed. One embodiment does not measure impedance or current, but instead uses a microphone connected to the micro-controller input to replace steps  904 ,  905  and  906  by a measurement that determines if a sound of sufficient amplitude is generated, and determine the coil to have failed if such a sound is not detected. Yet another embodiment replaces the micro-controller by a simple logic design that performs the functional blocks shown in  FIG. 9 . Those skilled in the art to which this application relates will appreciate that there are many modifications, deletions and substitutions possible with respect to  FIG. 8  and  FIG. 9 . 
     According to one embodiment for testing for coil failure in a system using amplifiers is to not use a current sensing resistor between the amplifier and speaker, but instead to measure the power supply current used by the amplifier using a current sensing resistor or other sensing mechanism. When the current is below a certain threshold for a given audio input level, the coil may be determined to have failed. 
     Yet another embodiment may comprise a signal provided by the amplifier itself. Some amplifiers provide a speaker fail output signal or a speaker fail status bit that may be accessed over a bus, which may be used to good advantage herein. 
       FIG. 10  shows yet another embodiment of a high reliability speaker system according to one embodiment. In this embodiment, coils  1001  and  1002  share a common set of contacts. Input signal  1005  always drives both coils through amplifier  1003  and resistor  1004 . Micro-controller  1006  may be configured to observe, for example, the input signal  1005 , the amplifier output  1007  and the voltage on the coils  1008 . Such observation may comprise direct sampling of the signals, or may comprise a small electronic circuit enabling the micro-controller to accurately measure small AC signals, such as circuits  810 ,  811  and  818  described above. Whenever there is an audio signal of sufficient amplitude, such as for example the periodic beep of a monitoring device, the micro controller measures the impedance of the speaker voice coils, by a computation involving voltage  1007 , voltage  1008  and the value of resistor  1004 . The micro-controller, for example, may be configured to measure the Root Mean Square (RMS) voltage on signal  1007  and signal  1008 . The impedance of the speaker, Z, may then computed as Z=R 1004 *V 1008 /(V 1007 −V 1008 ). The micro-controller may then determine that one of the coils has failed when the measured impedance Z exceeds a threshold. The micro-controller may be configured to notify the environment of coil failure through signal or bus  1007 . According to one embodiment, the micro-controller may be configured to change the gain of amplifier  1003  to ensure that the volume of sounds remains the same, even though only one coil is now active. 
     The impedance may be computed using one of many different measurement methods. For example, a simple measurement of the minimum and maximum voltage of a set of samples of the voltage on signal  1007  and  1008  may be used to estimate the amplitude of the AC voltage on each signal and estimate the impedance of the coils. As another alternative a full measurement of impedance may be omitted and replaced by a simple current measurement using resistor  1004 . If the input signal  1005  exceeds a threshold and the current through resistor  1004  does not exceed another threshold, one coil may be considered failed. Other methodologies are possible, as those of skill in this art may recognize. 
     Yet another embodiment of a high reliability speaker system may comprise two or more speakers instead of one speaker with two or more voice coils. In this embodiment, the two coils in  FIGS. 3 ,  4 ,  5 ,  6 ,  7 ,  8  or  FIG. 10  are instead replaced by two speakers. This creates a high reliability speaker system with similar capabilities. Advantageously, existing single coil speakers may be used, at the cost of a physically larger product.