Electronic circuit for tuning vibratory transducers

1. Field of the Invention

The present invention relates to the tuning of vibratory transducers, and, more particularly, to the tuning of speakers, including the speakers used in back-up alarms typically found on commercial vehicles and mobile equipment.

2. Description of the Prior Art

Vibratory transducers are devices that convert electric energy to kinetic energy. The resultant motion may by employed for a variety of purposes, including sound generation. This energy conversion is most efficient when the transducer is operating at its resonance frequency. Each vibratory transducer has its own particular resonance frequency to which it must be tuned for peak efficiency and maximum performance.

Vibratory transducers may be found, for example, in the speakers of back-up alarms which are coupled to commercial vehicles and other mobile equipment to warn passersby that the vehicle is operating in reverse. When activated, the back-up alarm generates a warning tone at a particular frequency. As described above, this frequency is preferably the resonance frequency of the transducer, thereby allowing the alarm's speaker to produce the loudest possible volume for a given energy input.

Vibratory transducers may be tuned during manufacture by, for example, placing a potentiometer within the speaker circuit and adjusting the potentiometer until the resonance frequency is reached. After initial tuning, the potentiometer is sometimes sealed to prevent accidental or undesirable adjustments away from the resonance frequency. Unfortunately, a transducer's resonance frequency will change with age and conditions of operation, including temperature and humidity. Thus, it is desirable to regularly re-tune a transducer. Even were the tuning mechanism not sealed, however, manual tuning is a time-consuming and inefficient process. More desirably, the transducer would re-tune itself. Various methods and apparatuses have been set forth which accomplish self-tuning. One common method involves comparing the phase of an input signal to the phase of the corresponding output signal, with zero phase difference indicating resonance. Other methods involve equating resonance with peak velocity or maximum displacement or vibration. These methods are generally suited to particular transducer applications or operating conditions, can require substantial additional hardware, and may yield results of questionable accuracy. Furthermore, many such methods require an intermediate input device, such as a microphone, for driving the speaker, which adds complexity, mass, and expense.

The method and apparatus of the present invention allow vibratory transducers, particularly speakers, including the speakers associated with back-up alarms, to frequently and regularly re-tune themselves to their resonance frequency. Broadly, the method and apparatus of the present invention operate by comparing the rising and falling edges of a test voltage waveform produced by the stimulated transducer.

More specifically, if the rising edge of a pulse of the voltage waveform is lower than the falling edge of the pulse, then the transducer is operating below its resonance frequency. If the rising edge of the voltage pulse is higher than the falling edge, then the transducer is operating above its resonance frequency. The closer the voltages are to being equivalent, the closer the transducer is to its resonance frequency. If the transducer is operating either above or below resonance, a simple electronic circuit in accordance with the present invention adjusts the operating frequency accordingly to more closely approach the resonance frequency.

By physically incorporating the simple electronic circuitry of the present invention into existing driver circuitry, the transducer is directly stimulated and tested without the need for microphones or other intermediate input devices commonly required by other methods and apparatuses. Thus, a vibratory transducer may be checked for resonance, and adjusted if needed, automatically, efficiently, without human intervention, and at frequent and regular intervals.

These and other important aspects of the present invention are more fully described in the section entitled DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT, below.

A preferred embodiment of the present invention is described in detail below with reference to the attached drawing figures, wherein:

FIG. 1 is a flowchart illustrating the steps in a preferred embodiment of the method of the present invention.

FIG. 2 is a waveform progression illustrating the relationship between resonance frequency and the rising and falling edge voltages of test waveforms.

FIG. 3 is a block diagram superimposed over a detailed circuit schematic illustrating a tuning circuit constructed in accordance with a preferred embodiment of the present invention.

1. The Method

Referring to FIG. 1, a flowchart is shown which illustrates a method of finding and maintaining the resonance frequency of a vibratory transducer. Such a method has application in maintaining peak efficiency and maximum performance in any vibratory transducer having the required waveform, including the speakers of common back-up alarms used on commercial vehicles and other mobile equipment.

The method of the present invention involves a process of successive approximation. Multiple test iterations are needed to accurately estimate the resonance frequency. Each successive test iteration results in a closer approximation until the resonance frequency has been tightly bracketed to a pre-determined degree of accuracy. FIG. 1 illustrates the steps involved in performing a single test iteration, a series of which constitute a single resonance check. A resonance check may be initiated as often and as regularly as is desirable and practical. The particular order of certain steps is not critical and may be altered without departing from the scope of the present invention.

Before beginning, a default resonance frequency should be set. This frequency will be used in the unlikely event that the method described below never finds the transducer's actual resonance frequency.

The first step 101 is to set both an initial test frequency and upper and lower limits to the range of test frequencies. Preferably, the initial test frequency will be near the middle of the test frequency range and based upon the typical resonance frequency for the type of transducer being tested. That is, the initial test frequency should be near the expected resonance frequency.

As described below, each resonance check preferably includes up to two attempts to find resonance. If the first attempt fails, using an initial test frequency that is near the expected resonance frequency, then a second attempt is made using either the upper or lower (preferably lower) test range limit as the initial test frequency.

The first step 101 also includes establishing the magnitude of the change in test frequency between successive test iterations. A smaller magnitude change will yield a more accurate result but will require that a greater number of frequencies be tested than if a larger magnitude change is used. Preferably, the system will employ frequency changes of varying magnitudes. Jumps in frequency should be relatively large until resonance has been passed-over once; this yields a very coarse approximation of the resonance frequency. Subsequent frequency changes should be smaller as the system refines its initial approximation. This method strikes a balance between speed, associated with large changes in frequency, and accuracy, associated with smaller changes.

Once the resonance frequency has been bracketed to the established degree of accuracy, the system will proceed to oscillate between increasing and decreasing the test frequency from one side of resonance to the other. A limit should be set on the number of such oscillations to be performed. It may be preferable, depending on the application and hardware used, to allow for more than one oscillation.

The second step 102 is to stimulate the speaker or other vibratory transducer with the test signal in order to produce a test waveform for analysis. Preferably, this is accomplished by directly stimulating the speaker itself without the use of any intermediate input device, such as a microphone.

The third step 103 is to examine a period of the test waveform by comparing its rising edge voltage to its falling edge voltage, and, based upon the comparison result, adjusting the test frequency to be used in the next test iteration. If the transducer is operating below its resonance frequency 104 , then the rising edge will be at a lower voltage than the falling edge. In this case, the test frequency must be increased for the next test iteration. If the transducer is operating above its resonance frequency 105 , then the rising edge will be at a higher voltage than the falling edge. In this case, the test frequency must be decreased for the next test iteration.

The effect of the third step 103 on the output waveform of a transducer under test is loosely illustrated in FIG. 2, wherein the resonance frequency 111 is located near the center of the illustration, and frequencies below 112 and above 113 resonance are located to the left and right, respectively, of the resonance frequency 111 . Within limits, the greater the difference between the tested frequency and the resonance frequency 111 (as shown in FIG. 2, the frequencies progressively farther left 114 or right 115 of center), the greater the difference will be between the rising 116 and falling 117 edge voltages of a pulse of the waveform. Thus, as the tested frequency is increased or decreased in successive test iterations to more closely approximate the resonance frequency 111 , the difference between the rising 116 and falling 117 edge voltages decreases until, at just above 118 or below 119 resonance, the rising 116 and falling 117 edge voltages are approximately equal.

Referring again to FIG. 1, the third step 103 also addresses the case where either the rising and falling voltages are equal or an error has occurred. Pinpointing the resonance frequency is unlikely and, in fact, the method of the present invention is not meant to find the exact resonance frequency. However, if by luck the resonance frequency is pinpointed, then the current series of test iterations ends with the fifth step 105 .

Whether resonance can ever be pinpointed depends on the method or device used to compare the rising and falling edge voltages. Certain comparators, for example, are limited to one of two possible results—higher or lower—even though the edge voltages are exactly equal. Therefore, in order to pinpoint resonance, the system must be capable of recognizing equal rising and falling edge voltages.

Assuming that an error has not occurred and the exact resonance frequency has not been pinpointed, the fourth step 104 is to determine whether either the minimum or the maximum test frequency will be exceeded or whether the test frequency will approach the resonance frequency so closely that successive test iterations merely oscillate between increasing and decreasing the test frequency. This latter occurrence results in an estimation of the resonance frequency as a bracketed region between two known frequencies. The accuracy of this estimate (i.e., the size of the bracketed region), as discussed above, will depend upon the magnitude of the change in test frequency between tests. The number of test iterations following the first bracketing oscillation will be limited to a predetermined number after which the current test iteration will end with the fifth step 105 .

If the first attempt to find resonance, using an initial test frequency based on the expected location of resonance, is unsuccessful, a second attempt is initiated. The second attempt will use as its initial test frequency one of the test range limits, preferably the lower limit. If the lower limit is used, then successive test iterations will employ progressively increasing test frequencies until either resonance is found or the upper limit is reached. It is also possible that the initial test frequency be the upper limit, in which case successive test iterations will employ progressively decreasing test frequencies until either resonance is found or the lower limit is reached.

If, after the second attempt, resonance has not been found, then the last known resonance frequency is used. Until a resonance frequency is found, a pre-loaded default frequency is used.

In the fifth and final step 105 the resonance check ends and the process is put on hold until another resonance check is called for. This will occur at some point whether the resonance check was successful in finding resonance or not. If the fifth step 105 occurs because the minimum or maximum test frequencies have been exceeded, then the range of frequencies tested by the next resonance check should be expanded accordingly.

This process may be repeated as often and with as much regularity as is practical and desirable in order to maintain the speaker or other vibratory transducer operating at its resonance frequency. It may be desirable between resonance checks to apply negative feedback to the transducer under test in order to control its output.

2. The Electronic Self-Tuning Circuit 10

Referring to FIG. 3, a self-tuning circuit 10 is shown which may be used to implement the above described method and which is therefore useful for finding and maintaining the resonance frequency of any vibratory transducer that exhibits the required waveform. The self-tuning circuit 10 comprises eight major function blocks: a connection protection block 20 ; a voltage regulation block 30 ; a brown-out protection block 40 ; a transducer circuitry block 50 ; a negative feedback block 60 ; an amplification block 70 ; a comparator block 80 ; and a microcontroller block 90 . Such a self-tuning circuit 10 .

The connection protection block 20 connects the circuit 10 to a power source and provides circuit protection in the event the transducer is reverse-connected to the power source. A preferred embodiment of the connection protection block 20 is shown comprising a diode 21 which functions to prevent a reverse flow of current through the circuit 10 .

The voltage regulation block 30 provides regulation of circuit voltage levels generally, and, more particularly, protection against both high voltage spikes and low voltage brownouts. The voltage regulation block 30 is well-known in the art. A preferred embodiment of the voltage regulation block 30 is shown comprising: first and second capacitors 31 , 32 ; a resistor 33 ; a zener diode 34 ; and a transient voltage suppressor 35 . The first and second capacitors 31 , 32 shunt high voltage spikes and act as voltage reservoirs in the event of a voltage brownout. The resistor 33 serves a current limiting function. The zener diode 34 regulates circuit voltage at 6.8 Volts. The transient voltage suppressor 35 behaves as a short circuit in the event of a voltage spike, thereby safely dissipating the voltage spike to ground.

The brownout protection block 40 provides voltage brownout, or low voltage, protection for the microcontroller block 90 , described below. The brownout protection block 40 is well-known in the art. A preferred embodiment of the brownout protection block 40 is shown comprising: first, second, and third resistors 41 , 42 , 43 ; and a bipolar junction transistor 44 . The first and second resistors 41 , 42 combine to provide a common voltage divider network.

In the event that the voltage power supply becomes too low, the base-emitter junction of the bipolar junction transistor 44 will cease being forward biased thereby causing current to stop flowing through the third resistor 43 . This will, in turn, cause the microcontroller block 90 , described below, to go to ground and reset itself.

The transducer circuitry block 50 provides a connection for interfacing a vibratory transducer with the circuit 10 of the present invention. A preferred embodiment of the transducer circuitry block 50 is shown comprising: first and second connection points 51 , 52 ; a power transistor 53 ; first, second, third, and fourth resistors 54 , 55 , 56 , 57 . A transducer connected to the first connection point 51 will sound louder than the same transducer connected to the second connection point 52 . The appropriate connection point for a given transducer depends on the transducer's decibel output and power rating. When coupled to the first connection point 51 , the characteristic waveform voltage (peak-to-peak) measured at a certain point 58 in the circuit 10 increases as the power source voltage increases. If the power source voltage becomes too great, the ability of the amplifier block 70 to process the waveform will be exceeded. When connected to the second connection point 52 , however, this problem does not arise.

The power transistor 53 is driven by the test signal produced by the microcontroller block 90 . The power transistor 53 effectively behaves as an on-off switch that controls current flowing from the power source through the transducer.

The first resistor 54 is a jumper connection operable to connect the terminals not occupied by the transducer. The second and third resistors 55 , 56 are necessary for the proper operation of the respectively attached transistors 53 , 62 . The fourth resistor 57 is a current limiting resistor, and the loudness of a connected transducer will vary inversely with the resistive value of the fourth resistor 57 .

The negative feedback block 60 provides negative feedback which prevents a connected transducer from increasing in loudness as the power source voltage increases. In order to prevent distortion of the characteristic waveform during the tuning process, no negative feedback is applied until the resonance frequency is found, which may take approximately 0.006 seconds. The preferred negative feedback loop 60 comprises a resistor 61 and a transistor 62 . The transistor 62 is a bipolar junction transistor.

The operational amplifier block 70 functions to buffer and amplify the characteristic waveform before it is introduced into the comparator block 80 . The operational amplifier block 70 comprises a first resistor 71 ; a second resistor 72 ; and an operational amplifier 73 . The first and second resistors 71 , 72 combine with the operational amplifier 73 to determine the gain characteristic of the operational amplifier block 70 .

The comparator block 80 compares the rising and falling edges of the amplified waveform which was originally taken from a certain point 58 in the circuit 10 . The comparator block 80 comprises first, second, and third resistors 81 , 82 , 83 ; first and second diodes 84 , 85 ; a capacitor 86 ; and a comparator 87 . The second and third resistors 82 , 83 form a common voltage divider which is used to adjust the voltage at the comparator 87 . The capacitor 86 is used to capture a “snapshot” of the test voltage. The comparator 87 functions to assign either a 0 or a 1 value to the relationship between the rising and rising edges of the test waveform. It is this assigned value which determines the response of the microcontroller block 90 .

The microcontroller block 90 comprises a microcontroller 91 which generates the test waveform and determines, based upon the results output by the comparator block 80 , the proper test frequency. The microcontroller 91 includes computer code which can be found at the end of the section entitled DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT and before the section entitled CLAIMS. The computer code comprises six major subroutines, including: START; BEEP; PITCH, MAKEWAVE, ADJUST, MARKTIME, TOOHIGH, and TOOLOW. Furthermore, the computer code defines and makes use of seven flags, or indicators, which mark the circuit's progress toward determining the resonance frequency.

The following variables are referred to in the written description and in the computer program code which appears at the end of the written description:

The START subroutine calibrates the internal oscillator to 4 MHz, sets CALVAL to an initial test frequency, sets MINVAL to a lower test frequency limit and MAXVAL to an upper test frequency limit, sets RESCOPY to a default frequency in the event resonance is not found, and clears all Flags.

The BEEP subroutine performs two major operations. First, it sets FREESTART to the number of iterations the transducer will be allowed to tune freely in order to accommodate sluggish reaction times of the comparator 87 . By allowing the transducer to tune freely, without referring to the progress flags, resonance can be located more accurately. Second, BEEP determines how many times PITCH will be called.

Each call to the PITCH subroutine generates one test signal to stimulate the transducer being tested. If Flag 6 =1 then resonance has been found and the program code will jump to the MARKTIME subroutine wherein negative feedback is maintained. If Flag 6 =0, then PITCH calls MAKEWAVE wherein the rising and falling edges of the test waveform are captured and compared. Based on the results of the comparison carried out by the MAKEWAVE subroutine, the ADJUST subroutine adjusts the test frequency for the next test iteration.

If Flag 6 =0 then the tuning process is still in progress. In the MAKEWAVE subroutine, the capacitor 86 will “record” the voltage of the waveform's rising edge. This voltage is captured, microseconds later, in a “snapshot”. At the same time, the comparator 87 is comparing the rising and falling edge voltages. The result of this comparison is stored in COMPAR. If COMPAR=0, then the rising edge voltage is lower than that of the falling edge. If COMPAR=1, then the rising voltage is higher than that of the falling edge.

If the resonance frequency has been bracketed, Flag 7 allows a number of test iterations to pass before resonance is marked with Flag 3 (below resonance) and Flag 4 (above resonance). The FREETUNE register is used to keep track of the test iterations completed before resonance is marked. Flag 7 is set when FREETUNE is finished.

In the ADJUST subroutine, the value in the COMPAR register is read to determine whether the frequency should be raised (COMPAR=0), in which case CALVAL is decremented, or lowered (COMPAR=1), in which case CALVAL is incremented. This may be done without setting Flags 3 and 4 , depending on whether Flag 7 has been set or not. There is a limit to how far CALVAL can be incremented or decremented, translating into a frequency range within which resonance should be found. Every time CALVAL is changed, it is checked against MINVAL or MAXVAL to determine whether it has exceeded the established test frequency range. If so, ADJUST calls either the TOOHI or TOOLOW subroutines, whichever is appropriate. The circuit 10 will then be given one more opportunity to find the resonance frequency. Flag 2 keeps track of this. Appropriate Flags are reset and the largest allowed value is loaded into CALVAL because the circuit 10 finds resonance easiest when it starts below resonance and sweeps upward to it.

If the circuit 10 fails to find resonance on this last attempt, the last found resonance frequency is loaded into the CALVAL register from RESCOPY. Every time resonance is found, a copy of that frequency value is loaded into RESCOPY for use in the event that resonance cannot be found on the next complete tuning attempt. A default value is loaded into RESCOPY initially in case resonance is never found.

The TOOHI and TOOLOW subroutines have optional lines of code that make it easy to determine whether the circuit is finding resonance or not. These optional lines of code will cause the transducer to vibrate, or the alarm tone to sound, at a very high or a very low frequency, depending on whether resonance has been missed on the high or low sides, respectively.

At the beginning of the MARKTIME subroutine are lines of code which reject frequency oscillation between adjacent alarm tones. The first time that MARKTIME is entered, the CALVAL value that controls the frequency of the alarm tone is checked to determine how it compares to the frequency of the previous alarm tone.

From the preceding description, it can be seen that both the method and electronic self-tuning circuit of the present invention provide for fast, accurate, and automated determination of resonance frequency and, thereby, maintenance of peak performance in a vibratory transducer.

Although the invention has been described with reference to the preferred embodiment illustrated in the attached drawings, it is noted that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims. Depending on the particular application and context, for example, it may be deemed preferable to compare voltages other than the leading and trailing edge voltages. It may also be preferable to deduce the distance and direction to resonance based upon the waveform's slope and the slope's rate of change. Furthermore, although described for purposes of illustration as being applied to the electroacoustic speaker of a common back-up alarm, the present invention has application to any vibratory transducers.

The following is computer program code for a preferred embodiment of the electronic self-tuning circuit of the present invention, as described above. This program code was written specifically for use with a PIC12C508 microcontroller, but is readily adaptable to other types of controllers and control systems.