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Claims  |
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What is claimed is:
1. A noise cancellor for canceling noise which is generated from a driving
device and propagates to a predetermined object point, comprising:
a first sensor for detecting the noise generated from the driving device
and converting the noise to electric signals;
signal processing means for multiplying said electric signals by a
predetermined factor series to form control signals;
sound producing means for producing, in response to said control signals,
sound which interferes with the noise so as to cancel the noise at the
object point; and
a second sensor for detecting sound at the object point, converting them to
electric signals, and inputting them to the signal processing means;
said signal processing means including adaptive active control means for
changing said factor series in response to the electric signals detected
by said second sensor, and selecting means for selecting a first control
mode wherein said adaptive active control means is used or a second
control wherein said factor series is fixed, in accordance with a
predetermined condition, until the electric signals from the second sensor
have a value which falls within a predetermined range.
2. A noise cancellor according to claim 1, wherein said selecting means
includes means for determining the frequency of change from said second
control mode to said first control mode, in accordance with a difference
between the present factor series and a factor series of last time.
3. A noise cancellor according to claim 1, wherein said selecting means has
means for determining the frequency of change from said second control
mode to said first control mode, in accordance with a lapsed time after
the driving device is started.
4. A noise cancellor according to claim 1, wherein said selecting means has
means for determining the frequency of change from said second control
mode to said first control mode, in accordance with an operating condition
of the driving device or transfer functions around the driving device.
5. A noise cancellor according to claim 1, which further comprises a
control unit for sending instructions of stop and restart of the driving
device to the signal processing means, and wherein said signal processing
means includes output means for memorizing the present factor series when
said signal processing means receives the instruction of stop and for
outputting the memorized factor series as an initial value when said
signal processing means receives the instruction of restart.
6. A noise cancellor for canceling noise which is generated from a driving
device and propagates to a predetermined object point comprising:
a first sensor for detecting said noise generated from the driving device
and converting the noise to electric signals;
signal processing means for multiplying said electric signals by a
predetermined factor series to form control signals;
sounding means for producing, in response to said control signals, sound
which interferes with said noise so as to cancel the noise at said object
point; and
a second sensor for detecting sound at said object point, converting said
sound to electric signals, and inputting them to said signal processing
means;
said signal processing means including adaptive active control means for
changing said factor series in response to said electric signals detected
by said second sensor, and adjusting means for adjusting frequency of
change of said factor series in accordance with a predetermined condition,
until the electric signals from the second sensor have a value which falls
within a predetermined range.
7. A noise cancellor according to claim 6, wherein said adjusting means has
means for determining whether said factor series should be changed, based
on the following equation:
(.vertline.h.sub.m -h.sub.m-1 .vertline.)N>.epsilon.,
where h.sub.m is a factor series set last time, h.sub.m-1 is a factor
series set just before last time, N is a number for determination, and
.epsilon. is a constant.
8. A noise cancellor for canceling noise which is generated from a driving
device and propagates to a predetermined object point, comprising:
a first sensor for detecting said noise generated from the driving device
and converting said noise to electric signals;
signal processing means for multiplying said electric signals by a
predetermined factor series to form control signals;
sound producing means for producing, in response to said control signals,
sound which interferes with said noise so as to cancel said noise at said
object point;
a second sensor for detecting sound at said object point, converting the
sound to electric signals, and inputting them to said signal processing
means; and
a control unit for sending instructions of stop and restart of said driving
device;
said signal processing means including adaptive active control means for
changing the present factor series in response to said electric signals
detected by said second sensor, and output means for memorizing the
present factor series when said output means receives said instruction of
stop and for outputting, as an initial value, said factor series stored in
said output means when said output means receives said instruction of
restart.
9. A noise cancellor for canceling noise which is generated from a driving
device having a predetermined driving frequency and propagates to a
predetermined object point, comprising:
a sensor for detecting noise generated from said driving device and
converting it to time series signals;
signal processing means for storing impulse response functions responding
to only specific frequencies based on the driving frequency of said
driving device and for producing control signals by convoluting said time
series signals by said impulse response functions; and
sound producing means for producing, in response to said control signals,
sound which interferes with said noise so as to cancel said noise at said
object point.
10. A noise cancellor according to claim 9, wherein said impulse response
functions are obtained by means of inverse Fourier transform of components
of transfer functions which correspond to said driving frequency and
integral multiples thereof.
11. A noise cancellor according to claim 9, wherein said impulse response
functions are obtained by means of inverse Fourier transform of transfer
functions, which have frequencies falling within ranges over which said
driving frequency and integral multiples thereof are allowed to change.
12. A noise cancellor for canceling noise which is generated from a driving
device having a predetermined driving frequency and propagates to a
predetermined object point, comprising:
a sensor for detecting noise generated from the driving device and
converting it to time series signals;
signal processing means for holding only components of transfer functions,
which have specific frequencies based on said driving frequency of said
driving device, converting the time series signals to frequency domain
signals by means of Fourier transform, multiplying said frequency domain
signals by said components of the transfer functions, and converting the
resultant to time series control signals by means of inverse Fourier
transform; and
sound producing means for producing, in response to said time series
control signals, sound which interferes with said noise so as to cancel
said noise at the object point.
13. A noise cancellor according to claim 12, wherein said components of
said transfer functions include components corresponding to said driving
frequency, integral multiples thereof and predetermined frequency ranges
over which said driving frequency and integral multiples thereof are
allowed to change.
14. A method of canceling noise which is generated from a driving device
and propagates to an object point, comprising:
a first converting step of detecting noise and converting it to electric
signals;
a producing step of producing control signals by multiplying said electric
signals by a predetermined factor series;
a canceling step of producing, in response to the control signals, sound
which interferes with said noise so as to cancel said noise at the object
point; and
a second converting step of detecting sound at said object point to convert
said sound to electric signals;
said producing step including a selecting process of selecting adaptive
active control wherein said factor series is changed in response to said
electric signals converted by said second converting step or active
control wherein said factor series is to be fixed, in accordance with a
predetermined condition, until the electric signals from the second sensor
have a value which falls within a predetermined range.
15. A method according to claim 14, wherein said selecting step includes a
process of determining the frequency of change from said active control to
said adaptive active control, in response to a difference between the
present factor series and a factor series of last time.
16. A method according to claim 15, wherein said selecting step includes a
process of determining the frequency of change from said active control to
said adaptive active control in response to a lapsed time after the
driving device is started.
17. A method according to claim 14, wherein said selecting step includes a
process of determining the frequency of change from said fixed active
control to said adaptive active control, in accordance with an operating
condition of the driving device or acoustic transfer functions around the
driving device.
18. A method according to claim 14, wherein said producing step includes a
first process of memorizing the present factor series when a stop
instruction of said driving device is output and a second process of
outputting, as an initial value, the memorized factor series when a
restart instruction of said driving device is outputted.
19. A method of canceling noise which is generated from a driving device
and propagates to an object point, comprising:
a first converting step of detecting noise and converting it to electric
signals;
a producing step of producing control signals by multiplying said electric
signals by a predetermined factor series;
a canceling step of producing, in response to the control signals, sound
which interferes with said noise so as to cancel said noise at said object
point;
a second converting step of detecting sounds at said object point to
convert said sound to electric signals; and
a changing step of changing said factor series in response to said electric
signals converted by said second converting step, said changing step
including a process of adjusting the frequency of change of said factor
series, in accordance with a predetermined condition, until the electric
signals from the second sensor have a value which falls within a
predetermined range.
20. The method according to claim 19, wherein said frequency of change of
said factor series is determined by the following expression:
(.vertline.h.sub.m -h.sub.m-1 .vertline.)N>.epsilon.,
where h.sub.m is a factor series of last time, h.sub.m-1 is a factor series
from which said factor series of last time was changed, N is a number for
determination, and .epsilon. is a constant.
21. A method of canceling noise which is generated from a driving device
and propagates to an object point, comprising:
a converting step of detecting noise generated from said driving device and
converting it to electric signals;
a producing step of producing control signals by multiplying said
electrical signals by a predetermined factor series;
a canceling step of producing, in response to the control signals, sound
which interferes with said noise to cancel said noise at the object point;
a memorizing step of memorizing the present factor series when a stop
instruction of said driving device is outputted; and
an outputting step of outputting the stored factor series as an initial
value when a restart instruction of said driving device is outputted.
22. A method of canceling noise which is generated from a driving device
having a predetermined driving frequency and propagates to an object
point, comprising:
a step of detecting noise generated from said driving device and converting
it to time series signals;
a step of producing control signals by convoluting said time series signals
by impulse response functions which response to only specific frequencies
based on said driving frequency of said driving device; and
a step of producing, in response to the control signals, sound which
interferes with said noise to cancel said noise at said object point.
23. A method according to claim 22, wherein said impulse response functions
are obtained by means of inverse Fourier transform of components of
transfer functions which correspond to said driving frequency and integral
multiples thereof.
24. A method according to claim 22, wherein said impulse response functions
are obtained by means of inverse Fourier transform of transfer functions,
which have frequencies falling within ranges over which said driving
frequency and said integral multiples thereof are allowed to change.
25. A method of canceling noise which is generated from a driving device
having a predetermined driving frequency and propagates to a predetermined
object point, comprising:
a converting step of detecting noise generated from the driving device and
converting it to time series signals;
a processing step of holding only components of transfer functions, which
have specific frequencies based on said driving frequency of said driving
device, converting the time series signals to frequency domain signals by
means of Fourier transform, multiplying said frequency domain signals by
said components of the transfer functions, and converting the resultant to
time series control signals by means of inverse Fourier transform; and
a canceling step of producing, in response to said time series control
signals, sound which interferes with said noise so as to cancel said noise
at the object point.
26. A method according to claim 25, wherein said components of said
transfer functions include components corresponding to said driving
frequency, integral multiples thereof and predetermined frequency ranges
over which said driving frequency and integral multiples thereof are
allowed to change. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a noise cancellor, and in particular to a noise
cancellor for actively canceling noises at an object point.
2. Description of the Related Art
Driving devices such as rotating machines, except for particular devices,
generate noise when they are operating. The noise bring about various
adverse influences on the environment. Generally, however, it is extremely
difficult to obtain noiseless driving devices
Conventionally, there has been developed a noise cancellor for reducing
noise at a specific place by using an acoustic technique. With this noise
cancellor sound waves having reverse phases to and equal magnitudes to
those of the noise at the specific place are artificially produced and are
caused to interfere with the noise, thereby to actively cancel the noise
at the specific place.
For example, in order to prevent that noise generated from a driving device
as a noise source in a chamber leak out through the aperture of the
chamber, it is possible to cancel the noises at the aperture, i.e., at an
object point, by using this noise cancellor. In this case, the noise
cancellor is generally constructed such that the noise generated by the
driving device are detected by a receiver such as a microphone provided in
the chamber and are converted into electric signals which are inputted to
an arithmetic unit through an amplifier and an A/D converter. The signals
output from the arithmetic unit are inputted through a D/A convertor to a
sound generator such as a speaker provided near the aperture for producing
required sound waves.
Let it be assumed that the noise generated by the driving device be S1, the
sounds produced by the speaker be S2, the noise detected by the microphone
be R1, the noise at the object point be R2, and the transfer functions
between the driving device and the microphone, the driving device and the
object point, the speaker and the microphone, and the speaker and the
object point be T11, T12, T21 and T22, respectively, the following
equation of a two-input two-output system is obtained:
##EQU1##
Since the noise cancellor is intended to make the sound level be zero, R2
can be set to be zero. Therefore, the following equation is obtained.
S2=(R1.multidot.T12)/(T12.multidot.T21-T11.multidot.T22) (2)
As understood from Eq. 2, if the sounds S2, which is obtained by
multiplying the noise R1 detected by the microphone by a filter factor h,
may be produced from the speaker, it is possible to make R2 be 0, where
h=T12/(T12.multidot.T21-T11.multidot.T22) (3)
Therefore, when the filter factor series (impulse responses) for minimizing
the noise at the aperture of the chamber is calculated and stored in the
arithmetic unit of the noise cancellor, the optimum S2 can be obtained
from the following equation:
S2=R1.multidot.h (4)
Two noise canceling methods are considered when Eq. (4) is used.
With one method, time series signals obtained from the microphone are
converted by means of Fourier transform to obtain frequency domain signals
and the obtained signals are multiplied by transfer functions of the
frequency domain designation. Thereafter, the resultant signals are
converted again to time series signal by means of inverse Fourier
transform, and these new time series signals are input to the speaker to
produce sounds.
With this method, it is difficult to produce control sounds by the speaker
at real time, because the signals are processed in batch. Since, however,
a driving device such as a rotating machine repeatedly generates sounds
having substantially the same waves, noise can be canceled by adjusting
the timing of producing control signals in accordance with trigger signals
which synchronize with the rotation of the rotating machine.
With the other method, transfer functions are converted to so called filter
factor series (impulse responses) by means of inverse Fourier
transformation. Further, time series data to be inputted to the speaker is
obtained by convoluting the filter factor series and the time series data
which are detected through the microphone. This second method is called
FIR filter system, FIR being the abbreviation of Finite Impulse Response,
and produces control sounds at real time.
With the second method, the control sounds are given by the following
equation:
##EQU2##
where h(i) is a filter factor series, X(n-1) is a closest sample datum of
the i'th input signal, M is a tap number, i is a tap factor number, and
S2(n) is the n'th output datum.
When both methods are used, noise at the aperture of the chamber can be
actively canceled, and thus the noise generated by the driving device in
the chamber can be prevented from leaking out of the chamber through the
aperture.
With the conventional noise cancellors, however, the transfer functions
from which the filter factor series are calculated are not always
constant. In other words, the transfer functions vary according to the
temperature change in the transmission paths of the sound, the change in
the output characteristics of the speaker, the change in the
characteristics of the driving device, and the like. For example, when the
temperature in the chamber rises by heat generated from the driving
device, the speed of sound changes, and this speed change varies the
acoustic transfer functions. Further, when the speaker is continuously
energized, the temperature of the coils of the speaker becomes higher and
its resistance changes, whereby the output of the speaker and the transfer
functions vary. If the noise generating positions of the driving device
vary in the course of the operation of the device, the acoustic transfer
functions also vary. Such variation of the transfer functions reduces
effect of noise cancelation at the object point. In order to carry out
effective noise cancelation, therefore, it is necessary to alter the value
of the filter factor series according to the change of the transfer
functions.
For the purpose of overcoming the above problem, recently a noise cancellor
has been developed which is provided with an adaptive control function. In
this cancellor, another microphone is arranged at the object point, and
the filter factor series is automatically altered so that the outputs from
the microphone become zero. The filter factor series of the noise
cancellor having this control function is changed at constant time
intervals according the following equation:
h(i) new=h(i) old+Ke X(n-i) (6)
where h(i) new is the i'th FIR filter factor after the alteration, h(i) old
is the i'th factor before the alteration, K is a constant defining the
alteration ratio of h, e is an error signal which is detected by the
microphone at the object point, and X(n-i) is a closest sample datum of
the i'th input signal.
However, this noise cancellor is encountered with the problems set forth
below.
With this noise cancellor, the filter factor series is changed at constant
time intervals while K is kept constant. The reason why K is kept constant
is that the standard of changing K is not clear. However, if K is always
kept constant, the following problems occur. When the time constant of the
change of the physical factors, which determine the transfer function, is
substantially identical to the time constant of the change of the filter
factor h determined by the value of K, resonance occurs. Further, when the
time constant of the change of the filter factor h depending on K is
larger than that of the physical factors, control cannot be performed in
accordance with the change of the physical factors. If the time constant
of the change of h is rendered very small by increasing the value of K,
the robustness of the control system is reduced. When it is known in
advance that the change of the physical factors is slow, it is necessary
that the value of K be very small. Very small K, however, leads to
omission of bits or the like when signals are processed. Accordingly it is
very difficult to select the value of K.
The disadvantages occurring from the alteration of the filter factor series
at constant time intervals are as follows:
When the filter factors are altered too often by rendering the time
interval too short, the robustness of the control system is reduced. On
the contrary, when the frequency of the alteration is rendered small by
making the time interval long, the control cannot follow to the change of
the transfer function.
As described above, with the noise cancellor having an adaptive control
function, the frequency of the change of the filter factor series, that
is, the control convergence ratio of the control system is always
constant, whereby the stability and the convergence may deteriorate,
depending on the operation conditions of the noise cancellor and the
driving device.
Further, with the conventional noise cancellors, when external noise
propagates to the object point after the adjustment of the filter factor
series has been finished and the convergence of control has been attained,
the cancellors malfunction, changing the filter factors with the result
that the complete noise reduction cannot be attained. Far from that,
surplus sounds are produced at the object point.
SUMMARY OF THE INVENTION
The present invention is contrived in consideration of the above
circumstances and its object is to provide a noise cancellor and a noise
canceling method which can perform stable and efficient control of noise
in accordance with the operational condition of a driving device and the
like without malfunction due to an external noise.
In order to obtain this object, according to this invention, the control
convergence ratio of a control system is adjusted by switching control
mode, in accordance with a predetermined condition, to an adaptive active
control wherein the filter factor series is suitably changed in accordance
with inputs measured at the object point or an active control wherein the
filter factor series is kept constant. The predetermined condition
described above means, for example, the frequency of the change of the
filter factor series, which is determined by the operating condition of
the noise cancellor including the driving device, the frequency of the
change of the filter factor series, which is determined by the elapsed
time from the starting of the driving device as a noise source, or the
like.
Further, according to this invention, during the adaptive active adopting
control, the frequency of the change of the filter factor series is
altered in accordance with the operating condition.
With this invention, the alteration of the filter factor series changes the
control convergence K of the filter factor series, enabling the optimum
convergency to be automatically selected. Therefore, it is possible to
realize an adaptive active adopting control which can improve both the
stability and convergence of the noise control. After the alteration of
the filter factor series, the noise cancellor does not malfunction due to
an external noise or the like, and the filter factor series is not changed
by the external sounds or the like. This provides more stable control of
adaptive active noise cancelation.
Further, with this invention, the filter factor series includes only
specific frequency components based on the frequency of the rotation of
the driving device. Accordingly, it is possible to prevent that the
elimination control is performed in response to an external noise having
the frequencies other than said frequency components, thereby facilitating
more stable noise control.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 to 4 show a noise cancellor according to a first embodiment of the
present invention, in which:
FIG. 1 is a sectional view schematically showing the overall noise
cancellor;
FIG. 2 is a flow chart illustrating the control of the noise cancellor;
FIG. 3 is a view showing the operating conditions of a driving device; and
FIG. 4 is a flow chart illustrating the control of the noise cancellor in
response to the operating condition of the driving device;
FIG. 5 is a flow chart illustrating the process of the control of a noise
cancellor of a second embodiment, in accordance with an elapsed time after
a driving device in started;
FIGS. 6 to 9 show a noise cancellor according to a third embodiment of the
present invention, in which:
FIG. 6 is a sectional view of the overall noise cancellor;
FIG. 7A is a view showing a frequency distribution of a compressor noise;
FIG. 7B is a view showing the distribution of the components of the
acoustic transfer functions corresponding to the frequency distribution of
FIG. 7A;
FIG. 7C is a view showing a frequency distribution of control sounds;
FIG. 8A is a view showing a frequency distribution of external noise;
FIG. 8B is a view showing the distribution of the components of the
acoustic transfer functions corresponding to the frequency distribution of
FIG. 8A;
FIG. 8C is a view showing a frequency distribution of erroneous control
sounds;
FIG. 9 is a sectional view showing a device for obtaining filter factor
series;
FIG. 10 is a view showing a frequency distribution of a driving device
noise; and
FIG. 11 is a flow chart for illustrating another signal processing system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
This invention will be explained in detail by way of embodiments with
reference to the accompanying drawings.
FIG. 1 shows a noise cancellor of an embodiment of this invention, with
which noise generated from a driving device such as a compressor 10
provided in a chamber 12 is prevented from leaking out of the chamber
through the aperture 13 thereof.
The noise cancellor has a first sensor 22 such as an acceleration pickup or
a microphone, which is arranged near the compressor 10 in the chamber 12.
The sensor 22 detects the noise or vibrations generated from the
compressor 10 and converts them to electric signals which are inputted to
a digital signal processor 25 through an amplifier 23 and an A/D converter
24. The signal processor 25 uses, as FIR (Finite Impulse Responses), a
required filter factor series h(i) as described later and stored in the
signal processor 25, and produces control signals from the input signals.
The noise cancellor is provided with a speaker 30 as sound producing
means, which is located at the vicinity of the aperture 13. The speaker 30
receives the control signals from the signal processor 25 through a D/A
converter 29 and produces sounds interfering with the noises from the
compressor 10, thereby canceling the noise at the aperture 13 as an object
point. The sound pressure at the aperture 13 is detected by a second
sensor 26 such as a microphone and is converted to electric signals which
are inputtd to the signal processor 25 via an amplifier 27 and an A/D
converter 28. The signal processor 25 changes the stored filter factor
series such that the values of the output signals of the converter 28
becomes zero, namely, the sound pressure at the aperture 13 becomes zero.
The signal processor 25 is connected to a control unit 31 which controls
the start and stop of the compressor 10 and receives the instructions for
the start and stop of the compressor 10 from the unit.
The operation of the noise cancellor will be explained.
Before starting the noise cancellor, filter factors h are not set in the
signal processor 25 of the noise cancellor. In order to provide a basis
for the filter factors, the values corresponding to the filter factors
h.sub.m altered last time and the filter factors h.sub.m-1 from which
h.sub.m was altered are natually set in the signal processor 25 before
starting the cancellor.
Thereafter, the noise at the aperture 13 of the chamber 12 is continuously
canceled by using the noise cancellor, according to the processes shown in
FIG. 2.
The first sensor 22 detects noise from the compressor 10 and obtains input
signals X(n-1) (Process S1). The signal processor 25 calculates control
signals S2(n) by covoluting the input signals by the filter factor series
hm(i) as FIR filters according to Eq. (5) (Process S2). These control
signals are inputted to the speaker 29 via the D/A converter 29, and the
speaker 30 produces control sound (Process S3). At the aperture 13, the
noise from the compressor 10 and the control sound from the speaker 30
interfere with each other so as to cancel each other. Thus, if the noise
and the sound completely cancel each other, the sound pressure at the
aperture 13 is perfectly zero. In genera, however, it is very rare that
the sound pressure is completely zero in this step.
Therefore, the sound pressure at the aperture 13 is detected by the second
sensor 26 and inputted to the signal processor 25 as an error signal e
(Process S4). The signal processor 25 changes the filter factor series
h(i), based on the error signal, such that the sound pressure at the
aperture 13 becomes zero. This change is made according to Eq. (6).
Specifically, the signal processor 25 calculates the value of change
(h.sub.m -h.sub.m-1) of the filter factor series h (Process S5) and
judges, according to the following expression whether the filter factors
should be changed (Process S6):
(.vertline.h.sub.m -h.sub.m-1 .vertline.)N>.epsilon. (7)
where N is the counting number counted by a change-frequency counter 32,
and .epsilon. is a predetermined constant.
When the absolute value of the change (hm-hm-1) multiplied by N is smaller
than the constant .epsilon., it is judged that no change is necessary, and
the process is returned from Process 6 to Process 2. Accordingly, the
noise cancelation is carried out by using the same filter factor series
hm(i) as that of last time.
When it is judged that the filter factor series should be changed, the
signal processor 25 outputs Ke X (n-i) defined by Eq. 6 (Process S7) and
calculates new filter factor series h.sub.m+1, based on Eq. 6 (Process
S8). Then, the filter factor series in Process S2 are changed from h.sub.m
to h.sub.m+1, and new control signals S2(n) are calculated from the new
filter factor series.
The counter 32 is constituted such that it counts the number of clock
pulses of a constant period and the old counting number of the clock
pulses are cleared when the filter factor series are changed.
Thereafter, the above processes are repeated.
With this noise cancellor, the decision as to whether or not the filter
factor series should be altered is made according to the predetermined
condition, or according to Expression (7), and the frequency of the
alteration is determined by Expression (7) as well. When the value of the
alteration is large, the alteration is made frequently. On the contrary,
when the value thereof is small, the frequency of the alteration is small.
As a result, the optimum convergence ratio of the control system can be
set in accordance with the variation of the operational condition of the
compressor 10, the change of the acoustic transfer functions in the
chamber 12, and the like. Accordingly, this noise cancellor realizes
adaptive active noise control which satisfies both high stability and high
control convergence. If the frequency of the switching is set to be large
as the change of the operational condition and the acoustic transfer
functions is large, and if it is set to be small as the change of them is
small, the control can be performed without according to Expression (7).
Once the filter factors are determined, the change of the absolute value
.vertline.h.sub.m -h.sub.m-1 in Expression (7) is small enough. Thus, even
if an external noise propagates into the chamber 12, it can be prevented
that the filter factor series is changed by the malfunction of the noise
cancellor, thereby facilitating more stable noise control.
The noise cancellor is constructed in consideration of the fact that the
compressor 10 operates intermittently as shown in FIG. 3. When the stop
instruction to the compressor 10 is outputted while the above mentioned
noise control is being carried out, as is shown by Process S11 in FIG. 4,
the control device 31 memorizes, as fixed value, the filter factors hm and
hm-1 which are being used at this moment (Process S12). The signal
processor 25 sends a storage-finishing signal to the control device 31 at
the time when the storage of the filter factors is completed, and then the
control device 31 stops the operation of the compressor 10 (Process S13).
Thereafter, when the starting signal to the compressor 10 is applied from
the control device 31 to the signal processor 25, the processor 25 assumes
the previously stored filter factors h.sub.m and h.sub.m-1 as the initial
value and starts the noise control shown in FIG. 2 (Process S14).
According to the above described construction, while the compressor 10 is
not operated, no alteration of the filter factor series is made and no
noise is superposed on the filter factor series. When the compressor 10 is
operated again, the noise control starts by taking the previously stored
filter factor series as the initial value. Therefore, the convergence of
the noise control can be quickened as compared with the case in which the
filter factor series which is zero-cleared or mixed with noise is used as
an initial value. This means that, with this noise cancellor, a quick
change to the optimum adaptive active noise control is possible after the
compressor has started.
The concept, wherein the filter factors are fixed when the instruction to
stop a driving device as a noise source is inputted and the previously
stored filter factor series is used as the initial value when the
instruction to start the driving device is inputted, can be adopted to
other systems than the ordinary active noise cancellor and the adaptive
active noise cancellor.
With the above embodiment, the value of the change of the filter factors
(h.sub.m -h.sub.m-1) is calculated and the timing of change of the filter
factor is determined by this calculated value and the number of counting N
by the change-frequency counter 32. In some systems for noise control, the
factors affecting the transfer functions, such as the sound speed in the
chamber, the output of the speaker and the like, change in specific
characteristic. In this case, the frequency of setting the adopting
control may be changed in accordance with an elapsed time t after the
driving device and the noise cancellor are started. FIG. 5 shows a flow
chart related to a noise cancellor using this control system.
With the embodiment shown in FIG. 5, in the Process S5, the signal
processor reads out the elapsed time t and the value of a control
convergence required at the time t, from a data base which has previously
memorized the characteristics of the factors affecting the transfer
functions. Thereafter, in Process S6, the signal processor judges whether
the filter factor series should be changed at the elapsed time. The other
control processes are the same as those of FIG. 2.
The adapting characteristic of this control system is a little worse than
that of the first embodiment. However, the noise cancellor itself and the
processing routine are greatly simplified.
The first embodiment provides adaptive active control for changing the
filter factors in response to the error signals detected by the second
sensor arranged at the object point. Alternatively, when noise control is
performed with respect to a driving device such as a compressor which
generates noise with almost constant frequencies, the noise cancellor may
be constructed as shown in FIG. 6.
As seen from the wave frequency distribution chart shown in FIG. 7A, the
noise generated from a compressor 10 mostly consist of frequency
components which include the rotating frequency of the compressor 10 and
the integral multiples r1, r2, r3, r4 and so on of the rotating frequency.
On the contrary, an external noise generally contains a wide range of
frequency components as shown in FIG. 8A. The third embodiment shown in
FIG. 6 is constructed, taking this phenomenon in consideration. With this
third embodiment, the control signals are obtained by multiplying the
input signals, which is detected by a first sensor, by the filter factor
series, like the control signals with the first embodiment. However, in
this embodiment, specific frequencies based on the rotating frequency of
the compressor 10 are only used as the filter factors, thereby preventing
the affection of an external noise having frequencies other than the
specific frequencies.
The operation of the third embodiment will be explained by using a transfer
function of the frequency domain designation. The filter factor series is
determined only by using the components of the acoustic transfer functions
h1, h2, h3, h4 and so on (FIG. 7B) corresponding to the frequencies r1,
r2, r3, r4 and so on which are the most part of the frequencies of the
noise generated from the compressor 10. Thus, the control sound
corresponding to the noise from the compressor are produced by the speaker
as shown in FIG. 7C, and so called erroneous control sound is as shown in
FIG. 8C. As understood from these figures, the noise cancellor of the
third embodiment does not respond to the most part of the frequencies of
the external noise. Since the frequency components of the external noise,
to which the cancellor response, are dispersed, they have very few
influence on the noise control effects. Accordingly, the third embodiment
provides a noise cancellor which does not malfunction due to an external
noise and can carry out a stable noise | | |
