Patent Publication Number: US-6990207-B2

Title: Active noise control system

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   The present invention relates to an active noise control system for reducing undesirable noise by producing noise canceling waves which are shifted 180 degrees in phase with respect to the noise. More particularly, the invention relates to an active noise control system suitable for reducing undesirable road noise of a frequency of 100 Hz or lower, which is generated inside the cabin of a vehicle caused by shocks or vibrations during the drive of the vehicle. 
   2. Description of Related Art 
   Known active noise control system for reducing road noise of a vehicle involves deriving a signal indicative of noise by a noise detector such as a microphone, and converting and amplifying the input signal for producing noise canceling waves from an electrical acoustic converter such as a speaker. 
     FIG. 8  shows one example of a frequency characteristic of road noise produced during the drive of a vehicle on a normal road. It has been ascertained that the peak in the vicinity of 40 Hz causes most unpleasant, depressing noise.  FIG. 8  shows that high-level noises are also produced under the frequency of 30 Hz, but such does not present an audial problem as mentioned above because of the low frequency. However, these low-frequency components input to an electrical acoustic converter, which is generally a dynamic speaker, increase the amplitude of output signal, causing a distortional noise to be produced from the speaker. 
   To solve such problem, the speaker must have high performance to be able to produce large canceling waves corresponding to noise of low frequency having a large amplitude. This is, however, not practical in noise control applications in a vehicle, due to high cost and the physical size restrictions on the speaker. 
   SUMMARY OF THE INVENTION 
   The present invention has been devised to solve the problems pointed out above in the prior art, and therefore it is an object of the invention to provide an active noise control system for effectively reducing noise of a low frequency without producing an abnormal or distortional noise from a speaker. 
   To achieve the object, an active noise control system for reducing an undesirable noise according to one embodiment of the invention includes:
         a noise detector for deriving an input signal representative of the undesirable noise;   an interfering wave signal generator for processing the input signal to produce an interfering wave signal for generating a noise canceling wave;   a limiting amplifier having a specified output signal amplitude threshold, for outputting amplified interfering wave signal having an amplitude equal to or less than the specified output signal amplitude threshold; and   an electrical acoustic converter for propagating the noise canceling wave.       

   The limiting amplifier may be divided into a limiter and an amplifier. The system may also be constructed of digital circuits. 
   These and other objects and characteristics of the present invention will become further clear from the following description with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram showing the arrangement of an active noise control system according to one embodiment of the present invention; 
       FIG. 2  is a schematic diagram showing the arrangement of an active noise control system according to another embodiment of the invention; 
       FIG. 3  is a chart representing characteristics of an open-loop transfer function used in the active noise control of the invention; 
       FIG. 4  is a chart representing transfer function of a signal generator according to the invention; 
       FIG. 5  is a block diagram showing one example of the arrangement of a limiting amplifier according to the invention; 
       FIG. 6  is a schematic diagram showing the arrangement of an active noise control system according to yet another embodiment of the invention; 
       FIG. 7  is a schematic diagram showing the arrangement of an active noise control system according to a further embodiment of the invention; and 
       FIG. 8  is a chart showing a frequency characteristic of road noise detected during the drive of a vehicle. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows an active noise control system applied to a vehicle according to one embodiment of the present invention. A noise detector or a microphone  101  for detecting and converting noises into electric signals is disposed in the vicinity of the driver&#39;s seat. The signal indicative of noise is input to an interfering wave signal generator  102 , which adjusts the amplitude and phase of the noise signal for producing noise canceling waves. A limiting amplifier  103  amplifies the input signal from the signal generator  102  variably in accordance with the size of the input signal for driving an electrical acoustic converter, which is commonly a dynamic speaker  104 . The limiting amplifier  103  has a specified threshold or maximum output value associated with its output signals, and amplifies the input noise canceling wave signal so as to have an amplitude lower than the predetermined threshold level when outputted. The speaker  104  produces noise canceling acoustic waves inside the cabin  106  of the vehicle  105  in accordance with the signal output from the limiting amplifier  103 . 
   Thus a loop is formed from the noise detector  101  to the speaker  104  via the cabin  106 . The noise Vn′ at the position of the noise detector  101  can be expressed as Vn′=Vn/(1−F(s)), where F(s) is an open-loop transfer function and Vn is the noise detected in a state without the active noise control system. 
   The signal generator  102  adjusts the open-loop transfer function F(s) within the range of frequency including the low frequency of the noise to be reduced.  FIG. 3  is a graph representing the characteristics of one example of the open-loop transfer function F(s) used in the noise control of the present invention with respect to the amplitude and the phase. As shown in  FIG. 3 , the signal generator  102  processes the input noise signal to produce an interfering wave signal which has an amplitude and a phase optimal for canceling the noise signal wave at the frequency of 40 Hz. Thus the noise around 40 Hz is effectively reduced. 
   The transfer function of the signal generator  102  is shown in  FIG. 4 . As can be seen from  FIG. 4 , the signal generator  102  passes the signal component of frequencies lower than 30 Hz. Therefore, input large noise signals of low frequencies, which may be generated upon driving of the vehicle on a bumpy surface, will cause a distortional noise from the speaker  104 . Accordingly, the limiting amplifier  103  has a specified amplitude threshold for the output value and variably amplifies the input signal for producing the interfering waves in accordance with its size. Thereby, even if there is generated a large noise of a low frequency of less than 30 Hz, the noise of the predetermined frequency, which is 40 Hz in this embodiment, is actively reduced without causing the distortional sound to be produced from the speaker  104 . 
     FIG. 5  is a block diagram showing one example of a practical arrangement of the limiting amplifier  103 . A transconductance amplifier  502  has its input connected to the output of an op-amp  501 , its output being connected to the inverting input of the op-amp  501 . The trans-conductance amplifier  502  can vary the conductance in accordance with electric current at an external current terminal  503 . When a large current flows, it increases the conductance, whereas when a small current flows, it decreases the conductance. 
   To the output of the op-amp  501  is also connected a wind comparator  504 . The wind comparator  504  connects the output of the op-amp  501  to the positive side of a power source when the absolute value of the output of the op-amp  501  is within a range above a predetermined threshold. If the absolute value of the output of the op-amp  501  is below the predetermined threshold, the wind comparator  504  opens the circuit. 
   To the output of the wind comparator  504  is connected a time constant determining circuit  505  composed of a capacitor and a resistor. The time constant determining circuit  505  is connected to a constant current source  506  for generating an electric current proportional to the output voltage of the time constant determining circuit  505 . The current generated by the constant current source  506  is supplied to the external current terminal  503  of the trans-conductance amplifier  502 . 
   A resistor  507  is provided across the input terminal of the limiting amplifier  103  and the inverting input of the op-amp  501 . Across the output of the op-amp  501  and its inverting input is also provided a resistor  508 . 
   The limiting amplifier  103  operates as follows. When the output voltage of the time constant determining circuit  505  is zero, the constant current source  506  generates no electric current. The conductance of the trans-conductance amplifier  502  at this time is also zero, and therefore the limiting amplifier  103  has a constant gain which is determined by R2/R1. 
   If the output of the op-amp  501  exceeds the threshold of the wind comparator  504 , it connects the time constant determining circuit  505  to the positive side of the power source, whereby the time constant determining circuit  505  generates an output voltage. This accordingly increases the conductance of the trans-conductance amplifier  502  through the current provided from the constant current source  506 , causing the resistor to be equivalently connected across the output and the inverting input of the op-amp  501 . As a result, the gain of the limiting amplifier  103  decreases from the above-mentioned R2/R1. In the event of continuous large inputs, the gain is automatically adjusted so that the amplitude of the output signal from the op-amp  501  slightly exceeds the threshold value of the wind comparator  504 . 
   Thus, should large signals be input, the limiting amplifier  103  reduces its gain, so that it will not output a signal having a correspondingly large amplitude, whereby abnormal noise from the speaker is prevented. Also, while restricting the amplitude of the output signal, the limiting amplifier  103  automatically adjusts its gain to be maximum, whereby the noise control effect is maximally achieved while preventing abnormal noises from the speaker. It should be noted that the circuit arrangement for the limiting amplifier  103  is not limited to the example shown in  FIG. 5  and various other arrangements may be employed for achieving the same effect. 
     FIG. 2  is a diagram showing the arrangement of an active noise control system applied to a vehicle according to another embodiment of the present invention. The system according to this embodiment has substantially the same constituent elements as those of the previously described embodiment, and the description of the common elements will be omitted. As can be seen from the drawing, the positions of the signal processor  102  and the limiting amplifier  103  are inverted in this embodiment. The system operates similarly as described in the foregoing. 
     FIG. 6  is a diagram showing the arrangement of an active noise control system applied to a vehicle according to yet another embodiment of the present invention. Instead of providing the limiting amplifier  103  as in the previously described embodiments, a limiter  103   a  is provided on the upstream side of the signal generator  102  and an amplifier  103   b  is provided on the downstream side of the signal generator  102 . The system according to this embodiment has substantially the same constituent elements as those of the previously described embodiment apart from the limiting amplifier  103 , and operates as described in the foregoing. The description of the common elements will be omitted. 
   The system shown in  FIG. 6  may be modified such that the positions of the limiter  103   a  and the amplifier  103   b  are inverted, the limiter  103   a  being provided on the downstream side of the signal generator  102  while the amplifier  103   b  being provided on the upstream side of the signal generator  102 . 
     FIG. 7  is a diagram showing the arrangement of an active noise control system applied to a vehicle according to a further embodiment of the present invention. The system has an active noise control signal generator  405  constructed of digital circuits having the combined functions of the above-described limiting amplifier  103  and the signal generator  102 . Other constituent elements are identical with those of the previously described embodiment, and the description thereof will be omitted. 
   The digital filter of the active noise control signal generator  405  generates signals for producing noise canceling waves based on the A/D converted signals representative of noise detected by the microphone  101 . The limiting amplifier calculates an optimal constant gain for outputting D/A converted, amplified signals when the output value from the digital filter is below a specified threshold value. If the output value from the digital filter exceeds the threshold, the limiting amplifier varies the gain to be optimal in accordance with output value from the digital filter, converts the digital signal into an analog signal, and amplifies and outputs same to the speaker  104 . 
   According to the invention, as described above, by providing the limiting amplifier, a large noise signal at low frequencies is processed so as not to cause distortion in the speaker for producing noise canceling waves. In doing so, the limiting amplifier adjusts the gain to be maximum in accordance with the level of the noise signal, whereby an optimal noise reducing effect is achieved. 
   Although the present invention has been fully described in connection with the preferred embodiment thereof, it is to be noted that various changes and modifications apparent to those skilled in the art are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.