Patent Publication Number: US-6339368-B1

Title: Circuit for automatically driving mechanical device at its resonance frequency

Description:
FIELD OF THE INVENTION 
     The present invention relates to a circuit for driving a mechanical device at its resonance frequency. More particularly, the present invention relates to a circuit for automatically driving a device at its resonance frequency. 
     BACKGROUND OF THE INVENTION 
     Various types of audible indicators that employ a piezoelectric or electro-mechanical transducer to generate a relatively piercing and noticeable audible tone when energized with power have been used for many applications. Such indicators are commonly used in numerous small and large appliances and alarm systems, and for other applications in which the generation of an audible signal is required. For example, for safety reasons, many heavy duty machineries such as forklifts and bulldozers include a backup alarm system that will generate a loud, and sometimes offensive, warning signal during their operation in the reverse driving mode so as to warn passersby of their movement. 
     During their operation, these alarm systems are preferably operated at or near the resonance frequency of the vibrating element even though such alarm systems may be operated at other frequencies. By operating at or near the resonance frequency, the most efficient use of available electrical energy to produce the greatest audible output is achieved. As a result, manufacturers often will test their alarm systems and, if necessary, adjust or tweak such alarm systems to produce the maximum audible output. Although this manufacturing step is implemented as a quality control step to ensure that each alarm system leaving the factory will operate at its maximum efficiency, the resonance frequency of each alarm system may later vary due to such factors as aging, and varying temperature and humidity. In light of such previously-stated problem, various alarm systems have been proposed so as to operate at or near a resonance frequency at any time during their usage. These proposed alarm systems are generally complicated and costly. 
     Accordingly, it is desirable to eliminate the above-mentioned labor-intensive manufacturing step of testing each alarm system to ensure that each alarm system leaving the factory will operate at its maximum efficiency, especially when the resonance frequency later may vary due to uncontrollable factors. By reducing such step in their manufacturing process, makers of alarm systems can effectively reduce the costs associated with the production of these alarm systems. In addition, it is also desirable to provide a simplified circuit capable of automatically driving the vibrating element of these alarm systems at or substantially near a resonance frequency so that minimal electrical energy is used to produce the greatest audible output. 
     The above-mentioned labor-intensive testing step is further associated with the production of [1] wireless RF “key fobs” for car security alarm systems and [2] remote control garage door openers. In order to transmit signals, the wireless RF key fobs and remote control garage door openers include a signal transmitting device such as an antenna. Although very little power is required to drive the antenna, it is still desirable to extend the life of the battery providing such power. Thus, prior to their shipment from the manufacturers to the wholesalers or retailers these wireless RF key fobs and remote control garage door openers are also tweeked or adjusted for maximum power efficiency. Similar to the alarm systems, maximum power efficiency of the wireless RF key fobs and remote control garage door openers is achieved when the antenna is driven at a resonance frequency. 
     Accordingly, it is also desirable to eliminate the above-mentioned labor-intensive manufacturing step of testing each wireless RF key fob or remote control garage door opener by providing a circuit capable of automatically driving the antenna at or substantially near a resonance frequency so that minimal electrical energy is used to transmit signals. 
     SUMMARY OF THE INVENTION 
     Generally, the present invention is directed to a circuit for automatically driving a mechanical device at its resonance frequency. To do so, the circuit detects non-resonance driving conditions of the mechanical device being coupled to and driven by such circuit. Based on such detection, the circuit generates a signal to drive the device at its resonance frequency. 
     More specifically, according to one aspect of the present invention, an acoustic transducer system is provided. The acoustic transducer system comprises [1] a power supply, [2] an acoustic transducer having a first electrical terminal coupled to the power supply and a second electrical terminal coupled to a reference ground, and [3] a phase-locked loop circuit detecting a phase difference between first and second signals at the first and second electrical terminals, respectively, and generating an output signal based on the detected phase difference to drive the acoustic transducer via a feedback connection forming a closed loop from the phase-locked loop circuit back to the second electrical terminal. The output signal generated by phase-locked loop circuit drives the acoustic transducer at a resonance frequency when the detected phase difference is negligible. 
     According to another aspect of the invention, a circuit automatically drives an antenna coupled to the circuit at a resonance frequency when a power supply is provided. This circuit comprises [1] a major feedback circuit providing an output signal, [2] a power amplifier driving the antenna in response to the output signal of the major feedback circuit, wherein the major feedback circuit detects a frequency difference between its output signal and a reference signal being provided to the major feedback circuit, and [3] a minor feedback circuit, coupled to the antenna and the major feedback circuit, detecting a phase difference between voltage and current signals provided by the power amplifier to drive the antenna, wherein the major feedback circuit generates the output signal based on the detected frequency and phase differences, and further wherein the power amplifier drives the antenna at the resonance frequency when the detected phase difference is negligible. 
     These and other features and advantages of the present invention will be apparent from the drawings as fully explained in the Detailed Description of the Preferred Embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and features of the present invention and many of the attendant advantages of the present invention will be readily appreciated and become better understood by reference to the detailed description when considered in connection with the accompany drawings in which like reference numerals designate like parts throughout the figures thereof and wherein: 
     FIG. 1 illustrates a first embodiment of the present invention that is capable of automatically driving an acoustic device such as the illustrated speaker at a resonance frequency. 
     FIG. 2 illustrates the first embodiment of present invention in detail especially with respect to its first and second limiters. FIG. 2A illustrates an alternative embodiment for one of the first and second limiters. 
     FIG. 3 illustrates a second embodiment of the present invention that is capable of automatically driving a signal transmitting device such as the illustrated antenna at a resonance frequency. 
     FIG. 4 illustrates a third embodiment of the present invention that is also capable of automatically driving a signal transmitting device such as the illustrated antenna at a resonance frequency. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates the first embodiment of the present invention. This first embodiment includes a circuit  100  that is capable of automatically driving an acoustic transducer, such as a speaker  10  or a piezoelectric device, coupled to the circuit  100 , at a resonance frequency when a power supply  20  is provided. Preferably the power supply  10  is a battery. For example, if the speaker  10  is a part of an alarm system installed on a bulldozer, the power supply  20  would be the battery of such bulldozer. 
     The circuit  100  of FIG. 1 includes first and second zero-crossing limiters  30 ,  40 , voltage comparators  50 ,  60 , a phase-locked loop circuit  70 , a switching device  80  and resistive elements  90 ,  91 . With respect to their electrical connections, the speaker  10  is coupled to the circuit  100  via its connection to and between nodes A, B to which the first and second limiters  30 ,  40  are also respectively coupled. The comparator  50  is coupled to and between the limiter  30  and the phase-locked loop circuit  70 . Similarly, the comparator  60  is coupled to and between the limiter  40  and the phase-locked loop circuit  70 . The phase-locked loop circuit  70  is coupled to the switching device  80  which is in turn coupled to node B so as to effectively provide a feedback connection forming a closed loop. The resistive element  90  is coupled to and between the switching device  80  and a reference ground to minimize the dissipation of electrical energy. And lastly, the resistive element  91  is coupled to and between the power supply  20  and node A to provide isolation from the power supply  20  and thus allow the limiter  30  to sense the signal at node A. 
     When the power supply  20  provides electrical energy to drive the speaker  10 , the limiters  30  and  40  detect signals at nodes A, B, respectively, and convert the detected signals at nodes A, B into first and second signals having a common zero-crossing reference so that the circuit  100  can accurately detect a phase difference between the signals at nodes A, B. Thereafter, the comparators  50 ,  60  respectively convert the first and second signals into digital signals. These digital signals are then provided to the phase-locked loop circuit  70  which detects their phase difference. Based on the detected phase difference, the phase-locked loop circuit provides an output signal to the drive the speaker  70  via the switching device  80 . 
     It should be noted that the signals at nodes A, B being detected to determine their phase difference are voltage signals at such nodes. Alternatively, voltage and current signals at nodes A, B, respectively, may also be detected to determine their phase difference. This phase difference is also equivalent to the phase difference between the detected voltage signals due to Ohm&#39;s Law. More specifically, V B =V A −IZ, where V B  is the voltage at node B, V A  is the voltage at node A, I is the current flowing from node A to node B, and Z is the complex impedance of the speaker  10 . 
     Furthermore, in a preferred embodiment, the circuit  100  also includes a lock detect circuit  71  providing an error signal that indicates whether the speaker  10  is being driven at its resonance frequency by the circuit  100 . This error signal may be used to drive a light emitting diode (LED) so as to cause the LED to turn [1] “off” when the speaker  10  is being driven at its resonance frequency and [2] “on” when the speaker  10  is not being driven at its resonance frequency, or vice versa. Thus, if the LED remains “on” for a while, this may indicate that [1] there is a fault associated with the speaker  10  so as to cause an open circuited condition between nodes A, B or [2] the switching device  80  is not working properly. If such indication is desirable, the lock detect circuit  71  is coupled to the phase locked loop circuit  70  so as to detect a phase difference between [1] one of the input signals of the phase locked loop circuit  70  and [2] the output signal of the phase locked loop circuit  70  being provided to drive the speaker  10 . The input signals of the phase locked loop circuit  70  may be the signals at nodes A, B or the output signals of the comparators  50 ,  60 . When such detected phase difference is negligible, the LED would be “off” and when the detected phase difference is not negligible, the LED would be “on.” 
     The operation of the circuit  100  is now further explained in detail with respect to FIG.  2 . More specifically, FIG. 2 illustrates preferred embodiments of the limiters  30 ,  40  and the phase-locked loop circuit  70  in detail. With respect to the limiters  30 ,  40 , the signals at nodes A, B are coupled to them via their respective capacitive elements  32 ,  42 . Thus, the circuit  100  only “sees” alternating current components of the signals at nodes A, B. Coupled to the capacitive elements  32 , 42  are resistive elements  34 ,  44 , respectively. The resistive elements  34 ,  44  are also coupled to inverting terminals of amplifiers  36 ,  46 , respectively. Preferably, the amplifiers  36 ,  46  are operational amplifiers. Each of the amplifiers  36 ,  46  also has a non-inverting terminal that is coupled to a reference voltage and an output terminal that is coupled to the respective comparator. In addition, each of the limiters  30 ,  40  further includes an additional resistive element and two diodes that are coupled to the respective amplifier in accordance with the electrical connection shown in FIG.  2 . 
     When the signals at nodes A, B are detected by the limiters  30 ,  40 , the limiters  30 ,  40  respectively convert the detected signals to first and second signals having the reference voltage as a common zero-crossing reference. In addition, minimum and maximum values of the first and second signals are substantially identical. More specifically, the minimum value of the first and second signals is the reference voltage minus the voltage drop across one of the diodes which is typically around 0.7 volt. The maximum value of the first and second signals is the reference voltage plus the voltage drop across one of the diodes. 
     Alternatively, instead of the two diodes, two metal oxide semiconductor field-effect transistors (MOSFETs) may be used to achieve the same effect by coupling the gate and drain of each MOSFET together. If so, the minimum value of the first and second signals is the reference voltage minus the voltage drop across such MOSFET which is typically between 0.8-1.0 volt depending on whether the MOSFET is a N-channel MOSFET (NMOS) or P-channel MOSFET (PMOS). Likewise, the maximum value of the first and second signals is the reference voltage plus 0.8-1.0 volt. FIG. 2A illustrates a limiter  37  that uses two MOSFETs, a PMOS  38  and a NMOS  39  instead of using two diodes. 
     Next, the first and second signals are provided to the comparators  50 ,  60 , respectively. In response, the comparators  50 ,  60  convert the first and second signals to digital signals and thereafter provide such digital signals to a phase detector  72  of the phase-locked loop circuit  70 . The phase detector  72  detects a phase difference between the digital signals. Coupled to the phase detector  72  is a low pass filter  72  of the phase-locked loop circuit  70 . The low pass filter  74  converts the detected phase difference to a voltage level. In response to such voltage level, a voltage controlled oscillator  76  of the phase-locked loop circuit  70 , which is coupled to the low pass filter  74 , generates the output signal to drive the speaker  10  via a bipolar junction transistor  82  being shown in place of the switching device  80 . Note that there is a resistive element  84  which is coupled to and between the base of the transistor  82  and the voltage controlled oscillator  76 . 
     Alternatively, a metal oxide semiconductor field-effect transistor can also be used as the switching device  80 . If so, the resistive element  84  would not be needed because the MOSFET is voltage controlled device unlike the transistor  82  which is current controlled device. Furthermore, if the circuit  100  is driving a piezoelectric device instead of the speaker  10 , the output signal from the voltage controlled oscillator  76  can be used to directly drive the piezoelectric device without relying any switching device because a current that is required to drive a piezoelectric device is much smaller than a current that is required to drive the speaker  10 . 
     When the signal at node A is leading the signal at node B, obviously there is a phase difference between such signals so as to indicate that the previous driving frequency of the speaker  10  is less the resonance frequency of the speaker  10 . In other words, the existence of the phase difference indicates that the speaker  10  is not being driven at its resonance frequency. In response, the voltage controlled oscillator  76  generates an output signal having a frequency that is higher than frequencies of both the signals at nodes A, B so as to drive the speaker  10  a little faster and thus closer to its resonance frequency. In contrast, when the signal at node A is lagging the signal at node B, this indicates that the previous driving frequency of the speaker  10  is greater than the resonance frequency of the speaker  10 . In response, the voltage controlled oscillator  76  generates an output signal having a frequency that is lower than frequencies of both the signals at nodes A, B so as to drive the speaker  10  a little slower and thus closer to its resonance frequency. More specifically, the circuit  100  drives the speaker  10  at its resonance frequency with a response time controlled by transfers functions of the low pass filter  74  and the voltage controlled oscillator  76  when the detected phase difference is negiligible. 
     Alternatively, the phase difference can also be detected by monitoring the signals at nodes A, C instead of at nodes A, B. Here, the limiter  30  remains coupled to node A but the limiter  40  is coupled node C of FIG. 2, which is between the transistor  82  (or the switching device  80  of FIG. 1) and the resistive element  90 . In addition, it should be noted that there are various types of phase-locked loop circuit that can be used for phase difference detection so as to eliminate [1] one of the comparators  50 ,  60 , [2] both of the comparators  50 ,  60  or [3] the limiters  30 ,  40  and the comparators  50 ,  60  from the circuit  100  of the present invention. Furthermore, the present invention may also be implemented as a complementary metal-oxide semiconductor (CMOS) integrated circuit or as a peripheral component of a microprocessor. If so, the speaker  10 , the limiters  30 ,  40 , the comparators  50 ,  60  and the phase-locked loop circuit  70  would be parts of such CMOS integrated circuit or such microprocessor. If the circuit  100  also includes the lock detect circuit  71 , such lock detect circuit  71  would also be a part of the CMOS integrated circuit or the microprocessor. 
     FIG. 3 illustrates a second embodiment of the present invention. This second embodiment includes a circuit  300  that is capable of automatically driving a transmitter such as an antenna  310  at a resonance frequency when a power supply is provided. The circuit  300  comprises [1] a major feedback circuit  320  that preferably includes a frequency detector  322 , a lowpass filter  324 , a voltage controlled oscillator  326 , and a frequency divider  328 , [2] a minor feedback circuit  340  that preferably includes limiters  342 ,  344 , a phase detector  346 , and comparators  348 ,  350 , [3] a power amplifier  360 , and [4] a resistive element  370 . 
     With respect to their electrical connections, the antenna  310  is coupled to the circuit  300  via its connection to and between nodes D, E to which the limiters  342 ,  344  of the minor feedback circuit  340  are also respectively coupled. In addition, the comparator  348  is coupled between the limiter  342  and the phase detector  346 . Similarly, the comparator  350  is coupled between the limiter  344  and the phase detector  346  which in turn is coupled to the lowpass filter  324  of the major feedback circuit  320 . The lowpass filter  324  is coupled to and between the frequency detector  322  and the voltage controlled oscillator  326 . Likewise, the frequency divider  328  is also coupled to the frequency detector  322  and the voltage controlled oscillator  326  which is in turn coupled to the power amplifier  360 . The output terminal of the power amplifier  360  is coupled to node D. And lastly, the resistive element  370  is coupled between node E and a reference ground. 
     Furthermore, in a preferred embodiment, the circuit  300  also includes a lock detect circuit  371  providing an error signal that indicates whether the antenna  310  is being driven at its resonance frequency by the circuit  300 . This error signal may be used to drive a light emitting diode (LED) so as to cause the LED to turn [1] “off” when the antenna  310  is being driven at its resonance frequency and [2] “on” when the antenna  310  is not being driven at its resonance frequency, or vice versa. Thus, if the LED remains “on” for a while, this may indicate that [1] there is a fault associated with the antenna  310  so as to cause an open circuited condition between nodes D, E or [2] the power amplifier  360  is not working properly. If such indication is desirable, the lock detect circuit  371  is preferably coupled to [1] one of the comparators  348 ,  350  and [2] the voltage controlled oscillator  326  of the major feedback circuit  320  so as to detect a phase difference between [a] an output signal of one of the comparators  348 , 350  and [b] an output signal of the voltage controlled oscillator  326  being provided to drive the antenna  310 . Alternatively, the lock detect circuit  371  may also be coupled to the comparators  348 ,  350  so as to detect a phase difference between their output signals. When such detected phase difference is negligible, the LED would be “off” and when the detected phase difference is not negligible, the LED would be “on.” 
     When a power supply provides electrical energy to drive the antenna  310 , the power amplifier  360  drives the antenna  310  in response to an output signal generated by voltage controlled oscillator  326  of the major feedback circuit  320 . The frequency of this output signal will be adjusted, if necessary, based on [1] a frequency difference detected by the frequency detector  322  of the major feedback circuit  320  and [2] a phase difference detected by the phase detector  346  of the minor feedback circuit  340 . With respect to the detected frequency difference, the frequency detector  322  detects a frequency difference between [a] a reference signal and [b] a signal from the frequency divider  328  whose frequency is an integral proper fraction of the frequency of the output signal of the voltage controlled oscillator  326 . The reference frequency being provided to the frequency detector  322  is preferably between 1 MHZ and 20 MHZ and can be less or more than this specified range depending on the type of frequency divider being used. With respect to the detected phase difference, the minor feedback circuit  340  detects a phase difference between signals at nodes D, E. More specifically, the limiters  342 ,  344 , which are functionally similar to the limiters  30 ,  40  of FIG. 2, respectively detect the signals at nodes D, E and convert them to first and second signals that have [1] a common zero-crossing reference and [2] minimum and maximum values which are substantially identical. These first and second signals are then provided to the comparators  348 ,  350 , respectively. In response, the comparators  348 ,  350  convert the first and second signals to digital signals and thereafter provide such digital signals to the phase detector  346  which in turn detects a phase difference between the digital signals. Based on these detected frequency and phase differences, the lowpass filter  324  generates a voltage level. In response to such voltage level, the voltage controlled oscillator  326  generates an output signal for the power amplifier  360  to drive the antenna  310 . 
     When the signal at node D is leading the signal at node E, obviously there is a phase difference between such signals so as to indicate that the previous driving frequency of the antenna  310  is less the resonance frequency of the antenna  310 . In other words, the existence of the phase difference indicates that the antenna  310  is not being driven at its resonance frequency. In response, the voltage controlled oscillator  326  generates an output signal having a frequency that is higher than frequencies of both the signals at nodes D, E so as to drive the antenna  310  a little faster and thus closer to its resonance frequency. In contrast, when the signal at node D is lagging the signal at node E, this indicates that the previous driving frequency of the antenna  310  is greater than the resonance frequency of the antenna  310 . In response, the voltage controlled oscillator  326  generates an output signal having a frequency that is lower than frequencies of both the signals at nodes D, E so as to drive the antenna  310  a little slower and thus closer to its resonance frequency. More specifically, the circuit  300  drives the antenna  310  at its resonance frequency with a response time controlled by transfers functions of the low pass filter  324  and the voltage controlled oscillator  326  when the detected phase difference is negiligible. 
     It should be noted that the signals at nodes D, E being detected to determine their phase difference are voltage signals at such nodes. Alternatively, voltage and current signals at nodes D, E, respectively, may also be detected to determine their phase difference. This phase difference is also equivalent to the phase difference between the detected voltage signals due to Ohm&#39;s Law. More specifically, V D =V E −IZ, where V D  is the voltage at node D, V E  is the voltage at node E, I is the current flowing from node D to node E, and Z is the complex impedance of the speaker  310 . 
     Moreover, it should also be noted that the lowpass filter  324  relies mainly on the detected frequency difference to generate the voltage level. The minor feedback circuit  340  has limited frequency adjustment capability because it is being implemented to account for the effect of parasitic capacitance associated with the printed circuit board upon which the antenna  310  is attached to. The presence of such parasitic capacitance effectively changes the resonance driving frequency of the antenna  310  and thus the phase difference detected by the minor feedback circuit  340  allows the lowpass filter  324  to account for the minor effect of such parasitic capacitance. 
     FIG. 4 illustrates a third embodiment of the present invention. This third embodiment includes a circuit  400  that is also capable of automatically driving a transmitter such as the antenna  310  at a resonance frequency when a power supply is provided. The circuit  400  is substantially similar the circuit  300  of FIG.  3 . The only difference is that the circuit  400  includes a power amplifier  400  having two output terminals F, G between and to which the antenna  310  is coupled. Therefore, the minor feedback circuit  340  is coupled to nodes F, G so as to detect a phase difference between signals at such nodes. Otherwise, the operation of the circuit  400  is similar to the operation of the circuit  300 . In addition, it should be noted that both the circuits  300 ,  400  can still operate without including [1] one of the comparators  348 ,  350 , [2] both of the comparators  348 ,  350  or [3] the limiters  342 ,  344  and the comparators  348 ,  350 , depending on the type of phase detector being used. Furthermore, the present invention in accordance with FIGS. 3 and 4 may also be implemented as a CMOS integrated circuit or as a peripheral component of a microprocessor. If so, both the major and minor feedback circuits  320  and  340 , respectively, would be parts of such CMOS integrated circuit or such microprocessor. If the circuit  300  also includes the lock detect circuit  371 , such lock detect circuit  371  would also be a part of the CMOS integrated circuit or the microprocessor. 
     In summary, the present invention automatically drives a mechanical device such as a speaker or an antenna at a resonance frequency when a power supply is provided. By doing so, there are several advantages associated with the present invention. First, manufacturers of alarm systems, wireless RF key fobs, remote control garage door openers, and similar devices can now eliminate the laborious and expensive “tweeking” step from the production process. Second, the life of a battery providing electrical energy to run an alarm system or a wireless RF key fob can now be maximized. Third, wireless RF key fobs or alarm systems can now generate the greatest amount of or the loudest audible signals by using optimal electrical energy, respectively. And fourth, an indication is provided if the device is not being driven at its resonance frequency. 
     With the present invention has been described in conjunction with several alternative embodiments, these embodiments are offered by way of illustration rather than by way of limitation. Those skilled in the art will be enabled by this disclosure to make various modifications and alterations to the embodiments described without departing from the spirit and scope of the present invention. Accordingly, these modifications and alterations are deemed to lie within the spirit and scope of the present invention as specified by the appended claims.