Abstract:
The present invention discloses a driving circuit for electronic horn and a driving method for the electronic horn. The driving circuit includes an oscillating circuit, which generates a signal having oscillating frequency. Based on the signal, said driving circuit generates a driving signal to drive the electronic horn to produce sound. It is characterized in that said oscillating circuit includes a variable capacitor, and the oscillating frequency is changed by adjusting capacitance of the variable capacitor so as for the frequency of the driving signal to be consistent with working frequency of the electronic horn. The present invention overcomes the problem of mismatch between the frequency of circuit&#39;s driving signal and the horn&#39;s sounding diaphragm and horn&#39;s tone inflexion due to resistance change caused by vehicle vibration.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to electronic horn for vehicle, and more particularly, to a method of and circuit for adjusting frequency of driving signal of the electronic horn. 
       BACKGROUND OF THE INVENTION 
       [0002]    With development of vehicle techniques, a lot of mechanical vehicle parts have been replaced with electronic parts, and the vehicle horn has been through several modifications, a major change of which is that an electronic oscillator has been used to replace mechanical contacts so as to extend lifespan of the horn and decrease interference introduced by the electromagnetic radiation. Up to hundreds of patents have been invented in this regard. But, most of them have not been put into practice since they fail to achieve a driving circuit, frequency of which is consistent with resonant frequency of the horn&#39;s acoustic system including horn diaphragm and resonance cavity. 
         [0003]    Because the sounding diaphragm of the horn has inherent resonant frequency just as a tuning fork, a key point to the electronic horn is that signal frequency produced by the horn&#39;s electronic circuit should be consistent with that of the sounding diaphragm and the acoustic system, so as for the horn to produce optimum sound and achieve higher horn efficiency and better stability. 
         [0004]    Currently, ways of adjusting frequency for electronic horn can be classified into three categories:
       1. adjusting resistance to change oscillating frequency;   2. changing oscillating frequency by means of frequency synthesis;   3. using microcomputer chips to directly generate a signal having a particular frequency.       
 
         [0008]    Among these, the second and the third categories of ways can produce a signal frequency, which is in good match with that of horn diaphragm, but they result in higher cost. The first category of ways can produce a signal having the same frequency as the diaphragm, but the frequency might change under operation due to the resistance change since the adjustable resistor comprises a contact which, under vehicle vibration, may inevitably lead to a resistance change. 
         [0009]    Taking the first category of way for example, there is an alternative solution, in which an adjustable resistor may be used to find the match point and then replaced with a fixed resistor. By this way, no resistance change of the fixed resistor would be found when the vehicle is vibrating. However, the adjustable resistor could hardly be replaced by fixed resistors with the same resistance. For example, the adjustable resistor is tested to have a resistance value of 4.85 kΩ when the driving circuit is in a good match with the horn diaphragm. But, fixed resistors have generally discrete values such as 4.7 KΩ, or 5.1 kΩ, no fixed resistor being found to have exactly the value of 4.85 kΩ. The adjustable resistor with the tested resistance has to be replaced with a fixed resistor having approximate but different value, which might lead to a dramatic change in frequency and thus a drop of horn performance. Even if a fixed resistor having exactly the same resistance as the adjustable resistor has been found, the horn assembled with the fixed resistor may suffer a great change in performance from the test due to distributed capacitors along leads in the adjustable resistor. 
         [0010]    Therefore, the problem in respect of stability and reliability has not been solved for long time. The U.S. Pat. No. 5,266,921, titled as ‘Method and apparatus for adjusting vehicle horns’ and granted in Nov. 30, 1993 uses laser to trim resistor to avoid the problem of resistance change. But, this method has its own defect, i.e., the frequency can not be adjusted to the match point. Because a point can only be learned to be the match point only after it is passed. If the match point is passed, it means over-tuned, otherwise it is mis-tuned. 
         [0011]    Therefore, the adjusting methods mentioned above have bad industrial applicability and can not be well practiced in mass production. 
       SUMMARY OF THE INVENTION 
       [0012]    Therefore, it is an object of the present invention to provide a method of and circuit for adjusting frequency of a driving signal for driving an electronic horn. 
         [0013]    According to a first aspect, the present invention provides a driving circuit for electronic horn including an oscillating circuit, said oscillating circuit generates a signal having an oscillating frequency and, based on the signal, said driving circuit generating a driving signal to drive the electronic horn to produce sound, characterized in that said oscillating circuit includes a variable capacitor, the oscillating frequency may be changed by adjusting capacitance of the variable capacitor so as for the frequency of the driving signal to be consistent with working frequency of the electronic horn. 
         [0014]    According to a second aspect, the present invention provides a method of driving an electronic horn, said method comprising: generating, by an oscillating circuit having a variable capacitor, a signal having an oscillating frequency; generating, based on the signal, a driving signal to drive the electronic horn to produce sound, characterized in that said method comprises changing the oscillating frequency by adjusting capacitance of the variable capacitor so as for the frequency of the driving signal to be consistent with working frequency of the electronic horn. 
         [0015]    In the invention, the oscillating circuit is preferably implemented by a RC oscillator within a monolithic processor. 
         [0016]    Preferably, said oscillating circuit includes a second capacitor in parallel with the variable capacitor. More preferably, capacitance ratio of the variable capacitor and the second capacitor is approximately 1:10-15. 
         [0017]    Preferably, the oscillating frequency is approximately between 20400-33400 Hz. 
         [0018]    Preferably, said oscillator is a multi-vibrator composed of inverters. Alternatively, said oscillator is formed by gate circuits, schmitt circuit, operation amplifier or discrete elements. 
         [0019]    The present invention uses a variable capacitor to adjust frequency of the driving signal, and achieves the advantages: (1) continuously adjustable frequency; (2) no frequency drifting in a long term; (3) lower cost; (4) suitable for mass production. 
         [0020]    The above and other objects, features, and advantages of the present invention will become apparent from the following detailed description thereof, which is described with reference to the accompanying drawings in which the like reference numerals represent the same or similar elements. 
     
    
     
       BRIEF INTRODUCTION OF THE DRAWING 
         [0021]      FIG. 1  illustrates a block diagram of a driving circuit for electronic horn according to the present invention; 
           [0022]      FIG. 2  illustrates the driving circuit according to a first embodiment of present invention; 
           [0023]      FIG. 3  illustrates the driving circuit according to a second embodiment of present invention; and 
           [0024]      FIG. 4  illustrates wave diagram generated by the wave-width adjusting part as shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0025]      FIG. 1  illustrates a block diagram of a driving circuit for electronic horn according to the present invention. As shown in  FIG. 1 , the driving circuit includes frequency oscillator  12 , frequency divider  14 , pulse width adjusting unit  16  and power driving unit  18 . 
         [0026]    Frequency oscillator  12  includes a variable capacitor  13 . Frequency oscillator  12  generates a signal having an oscillating frequency. The oscillating frequency may be changed by varying capacitance of the variable capacitor  13 . 
         [0027]    In a preferable embodiment, frequency oscillator  12  oscillates in high frequency. Generally speaking, the frequency for an individual horn needs to be adjusted within ±5% of a prescribed fundamental frequency. Oscillating at higher frequency may help reduce the capacitance of the variable capacitor  13 , thus reduce the circuit volume and enhance stability of the circuit. In addition, to improve stability of the circuit and diminish environmental effects, the oscillating frequency is preferred not too high. Generally, the oscillating frequency is about 20-35 kHz, depending on individual horn sounding frequency requirements. If the oscillating frequency is higher, additional stage of the frequency divider has to be added, which may adversely affect the adjusting precision; if the oscillating frequency is lower, given same adjusting range, capacitance of the variable capacitor has to be increased, which means a larger volume of the variable capacitor, adversely affecting the horn assembly. 
         [0028]    In another preferable embodiment, a parallel connection between a fixed capacitor and the variable capacitor is adopted in order to ensure stability of the circuit and effective adjustment range and decrease adverse influence to frequency stability caused by capacitance change of the variable capacitor due to temperature change. It is preferable for the capacitance ratio of the variable capacitor and the fixed capacitor to be about 1:10-15. 
         [0029]    Frequency divider  14  receives the output signal of frequency oscillator  12  and conducts frequency division to the signal so as for the divided frequency consistent with horn&#39;s working frequency and then outputs the frequency-divided signal to pulse width adjusting unit  16 . 
         [0030]    The pulse width adjusting unit  16  conducts pulse width adjustment to the frequency-corrected signal. In order for the sound produced by the horn to meet prescribed frequency spectrum, pulse width adjustment unit needs to adjust the signal&#39;s pulse width, thereby to change their harmonic components. 
         [0031]    In addition, in condition that the horn sounding spectrum requirement is met, electro-acoustic conversion efficiency of the horn will be enhanced. Lower horn driving power will effectively have the result of energy-saving. Driven by a signal having duty cycle ratio of 5:3, more energy could be saved and better sound quality achieved. 
         [0032]    Power driving unit  18  conducts power amplification to the pulse-width-adjusted signal so as to drive the horn&#39;s electro-magnetic windings. The power driving unit may be realized by devices such as transistors. 
         [0033]      FIG. 2  illustrates a driving circuit according to a first embodiment of present invention. This driving circuit utilizes a variable capacitor to adjust the circuit&#39;s oscillating frequency, and conducts frequency division and pulse width adjustment by means of monolithic processor, and uses transistor(s) to conduct power amplification so as to drive the horn. 
         [0034]    The driving circuit includes a power supply part, a oscillating, frequency-dividing and pulse width adjusting part, and a power amplifying part. The power supply part is a series-connected regulation circuit, which consists of a diode D 1  protecting from reverse connection, a current limiting resistor R 1  and a regulating diode Z 1 . The diode D 1  has its anode connected with the positive power terminal, and its cathode side connected with resistor R 1 . A capacitor C 1  and a resistor R 2  are in parallel connection with the diode Z 1 , and then connected between the negative power terminal and resistor R 1  to provide stable voltage. The voltage of the power supply is for example between 9-48 volt, and the stable voltage thus generated is for example about 5 volt. Those skilled in the art will realize that the power supply part may be realized by other kinds of regulator. 
         [0035]    The oscillating, frequency dividing and pulse width adjusting part is formed by a monolithic processor  22 . Those skilled in the art may realize that the monolithic processor  22  could be any monolithic processor having RC oscillating function. The monolithic processor  22  has a VDD terminal connected to the cathode of the diode Z 1  and supplied with stable voltage by diode Z 1 , and a VSS terminal connected to the negative power terminal. A parallel circuit consisting of a variable capacitor C 2  and a fixed capacitor C 3  is connected between input terminal OSC 1  of monolithic processor  22  and the negative power terminal. A resistor R 3  is connected between the VDD terminal of monolithic processor  22  and input terminal OSC 1 . A clear terminal MCLR is connected to the VDD terminal. 
         [0036]    In operation, an oscillator within the monolithic processor is set to RC oscillating mode. By selecting values of resistor R 3 , capacitors C 2  and C 3 , the oscillating frequency of the RC oscillator may be set to be around 32 kHz. The capacitance of the variable capacitor C 2  may be varied to continuously adjust the resultant frequency. The capacitance ratio of the variable capacitor and the fixed capacitor is preferred to be about 1:10-15. According to individual horn sounding frequency requirement, the oscillating frequency is set to be about 20400-33400 Hz. Individual monolithic processor technical manual may be referred to select values of R and C so as to fix the oscillating frequency. 
         [0037]    Frequency division and pulse width adjustment may be achieved by means of registers within the monolithic processor. The frequency division may be conducted prior to pulse width adjustment. Alternatively, the frequency division and pulse width adjustment may be completed simultaneously. By configuring parameters of the registers, different ratio of frequency division and different ratio of duty cycle width may be selected. For an example, a 64-fold division has been done to the oscillating signal, in which the register is used to extend the high and low voltage level 64 times while the high and low voltage levels are adjusted according to a ratio of 5:3, and then output via the terminal OUT 1  of the monolithic processor. 
         [0038]    The power amplifying part includes a field-effect-transistor T 1 . The field-effect-transistor T 1  has a gate connected to signal output terminal of the monolithic processor to receive the frequency-divided and pulse-width-adjusted signal. The gate is also connected via a resistor R 4  to the negative power terminal. Field-effect-transistor T 1  has a source terminal connected to the negative power terminal. A parallel connection circuit, which is formed by a diode D 2  and a capacitor C 4 , is connected between the drain and source terminals of field-effect-transistor T 1 . The anode of diode D 2  is connected to the source terminal of field-effect-transistor T 1 . Field-effect-transistor T 1 , diode D 2  and capacitor C 4  form an absorption and protection circuit. Horn  24  is connected between the drain of field-effect-transistor T 1  and the cathode of diode D 1 . 
         [0039]    Those skilled in the art will realize that the power amplifying circuit may be implemented by electronic elements such as transistor, field-effect-transistor, insulated gate bipolar transistor (IGBT). 
         [0040]    The signal output by monolithic processor  22  is amplified via field-effect-transistor T 1  to drive the horn  24  to produce sound. 
         [0041]      FIG. 3  illustrates the driving circuit according to a second embodiment of present invention. The driving circuit provides a solution, in which a variable capacitor is used to adjust the circuit&#39;s oscillating frequency, a digital circuit to implement frequency division, a pulse combination scheme to implement pulse width adjustment, and transistor(s) to implement power amplifying. 
         [0042]    The driving circuit includes a power supply part, an oscillating part, a frequency division part, a pulse width adjusting part and a power amplifying part. 
         [0043]    The power supply part includes a regulator, in which a diode D 1  protecting from reverse connection and a current limiting resistor R 1  and a regulating diode Z 1  are connected in series. The diode D 1  has its anode connected with the positive power terminal, and its cathode connected with resistor R 1 . A capacitor C 1  is in parallel connection with the diode Z 1 , and then connected between the negative power terminal and resistor R 1 , thereby to provide stable voltage VDD. 
         [0044]    The oscillation part includes a CMOS inverting amplifier N 1  and N 2 . Between the output terminal and input terminal of the amplifier N 1  are connected a circuit including resistor R 5  and R 6 . The amplifier N 2  has an output terminal connected via a parallel circuit formed by a variable capacitor C 2  and a capacitor C 3  to the joint point of R 5  and R 6 . Thereby, amplifiers N 1  and N 2 , resistors R 5  and R 6 , and capacitors C 2  and C 3  form a multi-vibrator. The multi-vibrator outputs, at the output terminal of the amplifier N 2 , a signal having an oscillating frequency. The oscillating frequency may be calculated by f=1/2.2R 3 *(C 2 +C 3 ). The capacitance of capacitor C 2  may be varied so as to continuously adjust the oscillating frequency. The capacitance ratio between variable capacitor C 2  and fixed capacitor C 3  is preferred to be about 1:10-15. The frequency division part includes a set of flip-flops Q 1 , Q 2 , Q 3 , Q 4 , Q 5  and Q 6  connected in cascade. The oscillating frequency signal generated by the oscillation part is input, at output terminal of inverting amplifier N 2 , into the input terminal of flip-flop Q 1 . The output terminal of flip-flop Q 1  is connected with input terminal of flip-flop Q 2 . The output terminal of flip-flop Q 2  is connected with input terminal of flip-flop Q 3 , . . . , and the output terminal of flip-flop Q 5  is connected with input terminal of flip-flop Q 6 . The output terminal of flip-flop Q 6  outputs a 64-fold divided signal. It shall be noted that the number of flip-flops may not be limited to 6, and may be varied depending on desired frequency division ratio. In addition, the set of flip-flops may be connected in other ways. 
         [0045]    The pulse width adjustment part conducts pulse width adjustment in the way of pulse combination by using AND gate and OR gate. The part includes AND gate A 1  and OR gate O 1 . Output signals of flip-flop Q 4  and Q 5  are respectively input into two input terminals of AND gate A 1 . The output terminal of gate A 1  is connected to an input terminal of OR gate O 1 , and the output terminal of flip-flop Q 6  is connected to the other input terminal of OR gate O 1 . OR gate O 1  generates a pulse width adjusted signal with high to low level ratio of 5:3, which is input into the power amplifying part so as to drive horn  24  to produce sound.  FIG. 4  illustrates a wave diagram generated by the wave width adjustment part as shown in  FIG. 3 . 
         [0046]    Those skilled in the art realize that other ways of pulse combination may be adopted to generate signals with varying pulse width ratio. 
         [0047]    The power amplifying part includes a field-effect-transistor T 1 , gate of which is connected to output terminal of OR gate O 1  to receive pulse-width-adjusted signal. Other portions of the power amplification part remain the same as those of the power amplification part as shown in  FIG. 2 , and thus their description will be omitted. The signal amplified by field-effect-transistor T 1  drives the horn  24  to produce sound. 
         [0048]    It will be obvious to those skilled in the art that various changes and modifications may be made therein. 
         [0049]    For example, the oscillating circuit may be formed by a RC oscillating circuit within a monolithic processor, gate circuit (including NAND gate, NOR gate), Schmitt circuit, operation amplifier, or discrete elements. 
         [0050]    In addition, the frequency division circuit may be implemented by registers in a monolithic processor, or by cascaded flip-flops or programmable logic device. 
         [0051]    Furthermore, pulse width adjustment circuit may be implemented by registers in a monolithic processor, or by digital circuit (for example, monostable circuit, Schmitt circuit, programmable logic device). To adjust pulse width, other ways may also be adopted, such as pulse combination by means of gate circuit including NAND gate and NOR gate or diode, or operation amplifier. 
         [0052]    It is aimed, therefore, to cover in the appended claims all such changes and modifications as fall within the true spirit and scope of the invention, which is defined by the metes and bounds of the appended claims.