 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates to active noise control (ANC) systems that are used
for attenuating undesirable noise, and more particularly, to ANC systems
utilized for canceling noise produced by internal combustion engines,
where the noise contains multiple closely spaced sinusoidal frequency
components having amplitudes and frequencies that vary in relationship
with the rotational speed of the engine.
Conventional active noise control (ANC) systems attenuate undesirable noise
by producing and superimposing noise canceling waves, which are
substantially equal in amplitude and frequency content, but shifted 180
degrees in phase with respect to the undesirable noise. As used in the
present specification and the appended claims, the term noise is hereby
defined to include both acoustic waves and mechanical vibrations
propagating from a noise source.
Recently, ANC has been accomplished by employing modern digital signal
processing and adaptive filtering techniques. Typically, an input sensor
is utilized to derive a signal representative of the undesirable noise
generated by a source. This signal is then applied to the input of an
adaptive filter and is transformed by the filter characteristics into an
output signal used for driving a cancellation transducer or actuator such
as an acoustic speaker or electromechanical vibrator. The speaker or
vibrator produces canceling waves or vibrations that are superimposed with
the undesirable noise generated by the source. The observed or residual
noise level resulting from the superposition of the undesirable noise and
the canceling waves is then measured with an error sensor, which develops
a corresponding error feedback signal. This feedback signal provides the
basis for modifying the characteristics of the adaptive filter to minimize
the overall level of the observed or residual noise.
Such systems have been successfully applied to attenuate, for example,
repetitive noise generated by fans or electric motors and random noise
propagating down heating and air conditioning ducts. The nature of
acoustic and vibrational noise generated by an internal combustion engine
differs quite significantly from the nature of the repetitive or random
noise encountered in the past.
Engine generated noise generally contains a large number sinusoidal noise
components having amplitudes and frequencies that are functionally related
to the rotational speed of the engine. These frequency components have
been found to be the even and odd harmonics of the fundamental frequency
of engine rotation (in revolutions per second), as well as half-order
multiples or sub-harmonics interposed between the even and odd noise
harmonics. Consequently, at low engine speeds, the difference in frequency
between adjacent noise components (i.e. those noise components immediately
preceding or following each other in the frequency domain) can become
quite small, for example, as little as 5 Hz at engine idle. In addition,
the amplitude, frequency, and phase of the engine generated noise
components can vary quite rapidly in response to changes in engine
rotational speed brought about by acceleration or deceleration of engine.
Also, engines having differing numbers of cylinders generate noise
characterized by different dominant frequency components due to the
difference in their firing frequencies. Finally, engine generated noise
can have different amplitude and frequency characteristics depending upon
the particular type of noise, for example, acoustic noise propagating from
the engine intake or exhaust system, or mechanical vibrations produced by
operation of the engine, which are transmitted to a vehicle frame.
Consequently, there exists a need for a flexible active noise control
system that can be tailored to effectively attenuate undesirable noise
containing multiple sinusoidal frequency components, particularly in
applications where the difference in frequency separating these noise
components is small in comparison with the values of their individual
frequencies, and where the amplitude, frequency, and phase of the
sinusoidal noise components can change quite abruptly, such as in noise
generated by an internal combustion engine during periods of rapid engine
acceleration or deceleration.
SUMMARY OF THE INVENTION
The present invention provides an active noise control (ANC) system for
attenuating engine generated noise, where the noise contains at least one
sinusoidal noise component having an amplitude and frequency that vary in
relation to changes in engine rotational speed. The ANC system includes a
means for generating at least one generator output signal that contains
one or more sinusoidal signals each having a frequency corresponding to
the frequency of a respective noise components. A separate control signal
having an amplitude that depends upon engine rotational speed is produced
to correspond with each generator output signal. Each generator output
signal is then multiplied by its respective control signal to produce a
corresponding filter input signal. Each filter input signal passes to a
respective adaptive filter, where it is filtered to produce a
corresponding filter output signal according to the adjustable filtering
characteristics of the adaptive filter. Noise canceling waves are
generated by a noise canceling actuator in response to each filter output
signal, and the canceling waves are superimposed with the undesirable
noise generated by the engine. The level of residual noise resulting from
this superposition is sensed, and an error signal is developed to
represent the residual noise level. The filtering characteristics of each
adaptive filter are adaptively adjusted based upon the error signal to
minimize the residual noise level.
When the engine noise contains multiple sinusoidal noise components, it is
preferable that the sinusoidal signal components contained in the
generator output signals be partitioned according to the amplitude
behavior of their corresponding noise components with changes in engine
rotational speed. The amplitude of each control signal can then be
determined in accordance with a respective predetermined function based
upon the sensed engine rotational speed, where each predetermined function
is made correlative of the amplitude behavior of those noise components
corresponding to the signal components contained in the respective
generator output signal that is multiplied by the control signal. This
effectuates a predetermined scaling of the amplitude of each generator
output signal based upon the engine rotational speed, which has been found
to significantly improve the ability of the ANC system to attenuate engine
noise components having amplitudes that behave in a similar fashion with
changes in engine rotational speed. This is because the amplitudes of the
sinusoidal signal components entering the adaptive filters are scaled to
more closely match the amplitude behavior of their corresponding engine
noise components. As a consequence, the adaptive filters require less time
to converge in adapting the filter output signal amplitudes to achieve an
acceptable level engine noise attenuation.
According to another aspect of the invention, the rotational position of
the engine in an operating cycle is sensed, and engine rotational speed is
derived by determining the time rate of change of the engine rotational
position. Each generator output signal can then be conveniently derived
from a respective predetermined schedule of values based upon the sensed
angular rotational position of the engine in the operating cycle.
The invention claimed in the present application is disclosed in
conjunction with two additional inventions that may be implemented for
improving the performance of active noise control systems used for
attenuating engine generated noise. One of these additional inventions is
associated with implementing the ANC system so that sinusoidal signal
components corresponding to noise components that are adjacent with
respect to frequency are contained in different ones of the generator
output signals. The other additional invention is related to the scaling
of filter adaptation factors based upon engine rotational acceleration
and/or engine rotational speed. These inventions are respectively claimed
in related U.S. Patent Applications having Attorney Docket Numbers G-9338
and G-10099, which were filed contemporaneously with the present
application, and are also assigned to the same assignee.
These and other aspects and advantages of the invention may be best
understood by reference to the following detailed description of the
preferred embodiments when considered in conjunction with the accompanying
drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an active noise control system having a
multi-channel electronic noise controller for attenuating different forms
of undesirable noise generated by an engine;
FIG. 2 is a block diagram representative of the electronic components
employed in implementing the noise controller shown in FIG. 1;
FIG. 3 is a block diagram model containing a parallel configuration of
signal generator and adaptive filter pairs representing signal processing
steps that are programmed into and carried out by the digital signal
processor of FIG. 2 for active noise control;
FIG. 4 is a block diagram representing the modeling components contained
within each of the adaptive filters AF.sub.j shown in FIG. 3, where j=1,
2, . . . , J;
FIG. 5 is a block diagram illustrating an off-line training process for the
compensation E filter included within each of the adaptive filters
AF.sub.j illustrated in FIG. 4;
FIG. 6 is a block diagram for a model programmed into the digital signal
processor, which includes a filter controller for use in conjunction with
the parallel configuration of signal generator and adaptive filter pairs
employed for attenuating engine generated exhaust noise;
FIG. 7A-C illustrate typical values for control signals produced by the
filter controller in the model shown in FIG. 6 as a function of the
rotational speed and/or acceleration of the engine when canceling exhaust
noise;
FIG. 8 is a flow diagram representative of the steps executed by a routine
programmed into the digital signal processor to perform the signal
generating, adaptive filtering, and control functions of the model
configuration shown in the FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, there is shown schematically an internal combustion
engine, generally designated as 10, with its associated air intake system
12 and exhaust system 14. A rotatable throttle valve 16 is included within
the air intake system 12 for regulating air flow to the engine 10. Also
shown are two sensors generally associated with the electronic control of
engine performance. The first is a standard throttle position sensor 18,
such as a potentiometer, which is connected to throttle valve 16 for
developing an electrical signal TP related to the degree or percent of
throttle valve opening. The second is a conventional engine rotation
sensor, in this case shown as a toothed wheel 42 mounted on the engine
crankshaft, and an electromagnetic sensor 44 that produces a SPEED signal
having pulses corresponding to the movement of teeth on wheel 42 past
sensor 44. As shown, toothed wheel 42 has six symmetrically spaced teeth,
which produce six equally spaced pulses in the engine SPEED signal for
every complete revolution of the engine 10. This particular toothed wheel
is merely exemplary, and wheels having different numbers of teeth can be
just as easily used, or alternatively, any other known type of sensor or
transducer capable of producing outputs pulses in response to the rotation
of the engine can be employed.
During the operation of engine 10, acoustic pressure waves are generated
and propagate away from the engine through the ducts and tubes forming the
air intake and exhaust systems. Eventually, these pressure waves propagate
from openings in the intake and exhaust systems as observable engine
induction noise 20 and exhaust noise 22. In addition, the engine generates
undesirable noise in the form of mechanical vibrations 24, which are
transferred to a mounting frame 40 used to support engine 10.
In general, engine generated noise contains a large number of sinusoidal
components having amplitudes and frequencies that vary in relation to the
rotational speed of the engine. The frequencies of these components have
been found to be even, odd, and half-order multiples of the fundamental
frequency of rotation of the engine (in revolutions per second).
Consequently, at low engine speeds, the difference in frequency between
adjacent noise components can become quite small (in the order of 5 Hz)
making them difficult to distinguish. In addition, the amplitude and
frequency of the engine generated noise components can vary quite rapidly
in response to abrupt changes in engine acceleration or deceleration
brought about by variations in engine loading or operator demand for
engine output power. Also, engines having differing numbers of cylinders
generate noise characterized by different dominant frequency components,
due to the difference firing frequencies. Finally, the type of engine
noise can have different frequency components depending upon the source of
the engine noise, i.e. acoustic waves propagating from the engine intake
or exhaust systems, or mechanical waves or vibrations transmitted from the
operating engine to the vehicle frame.
Consequently, to be practical an active noise control system for canceling
different forms of engine generated noise must be capable of selectively
attenuating a large number of noise frequency components, some of which
can have relatively small differences in frequency. It is also necessary
that such a system have the ability to accurately track and adapt to rapid
variations in the amplitude, frequency, and phase of engine generated
noise components that have been found to occur at different rotational
speeds and/or during abrupt acceleration or deceleration of the engine.
As will now be described, the present invention is directed toward
providing an active noise control system having the above mentioned
capabilities. The general components of such an active noise control
system are shown in FIG. 1. For illustrating a few of the many different
applications that are possible, electronic noise controller 26 is shown as
a multi-channel device having three separate channels, with each channel
operating to attenuate one of the different forms of engine noise
discussed above, i.e. intake induction noise, exhaust noise, and
vibrational noise.
One channel of the noise controller 26 is utilized to attenuate the engine
generated induction noise propagating inside the air intake system 12. As
will be described, the electronic noise controller 26 generates a
canceling OUTPUT.sub.1 waveform based upon the input engine SPEED signal.
This OUTPUT.sub.1 signal drives a canceling actuator 28, which in this
case is an audio speaker, which produces canceling acoustic waves that are
superpositioned with the engine generated induction noise. Sensor or
transducer 30, in this case an acoustic microphone, is positioned in the
air intake system 12 to measure the level of the residual or attenuated
induction noise remaining in the air intake system 12 after the
superposition of the canceling acoustic waves. Sensor 30 develops an
ERROR.sub.1 signal representing the level of the residual induction noise,
which is directed back to the induction noise channel of the electronic
noise controller 26. This ERROR.sub.1 signal provides the basis for
minimizing the observed or residual induction noise 20 propagating out of
engine intake system 12.
A second channel of the noise controller 26 is employed to cancel exhaust
noise. The operations described above for the induction noise application
are duplicated, except that a noise canceling signal OUTPUT.sub.2 is
produced to drive the exhaust noise canceling actuator 32 (in this case an
acoustic speaker) positioned to generate and propagate acoustic waves in
the exhaust system, and an error sensor 34 (in this case an acoustic
microphone) for developing an ERROR.sub.2 signal representing the level of
residual exhaust noise propagating from engine 10.
Similarly, for canceling engine generated vibrational noise 24, a third
channel of the noise controller 26 produces noise canceling signal
OUTPUT.sub.3 to drive an electromechanical vibrator 36, shown here as
being disposed between engine 10 and mounting frame 40. Electromechanical
vibrator 36 may be any type of actuator known to those skilled in the art
of active noise control, which is capable of producing the required
out-of-phase canceling vibrations for superposition with the engine
generated vibrations transmitted to mounting frame 40. For example, a
commercially available Model LAV 2-3/5-6 actuator manufactured by Aura,
Inc could be used as shown in FIG. 1, or alternatively, a Model 203B
Shaker supplied by Ling Electronics, Inc. could be mounted on frame 40 for
producing the required out-of-phase canceling vibrations. For this
channel, an error feedback signal ERROR.sub.3 representing the residual
vibrations transferred to the mounting frame 40 is developed by an error
sensor 38, which in this case is a standard accelerometer attached to the
mounting frame 40.
Referring now to FIG. 2, the electronic circuitry within the noise
controller 26 will now be described in terms of a block diagram containing
standard well known electronic components present in the second channel 46
in the noise controller. The first and third channels, 48 and 50
respectively, contain the same components adapted to provide the
appropriate input and output levels for their particular cancellation
actuators and error sensors, and accordingly, only the components within
the second channel will be described to avoid unnecessary duplication in
the specification.
It will be recognized that the implementation shown in FIG. 2 is merely
exemplary and is not intended to limit the present invention, since other
variations in the hardware are possible, as evident in the numerous
patents, texts, and publications directed toward the subject of active
noise control, see for example, "Hardware and Software Considerations for
Active Noise Control", M. C. Allie, C. D. Bremigan, L. J. Eriksson, and R.
A. Greiner, 1988, IEEE, CH 2561-9/88/0000-2598, pp. 2598-2601.
One of the principal electronic component in the preferred implementation
of noise controller 26 is a digital signal processor (DSP) designated by
numeral 52. Digital signal processors are commercially available, such as
the Motorola 56000, and typically include a central processing unit (CPU)
for carrying out instructions and arithmetic operations, random access
memory (RAM) for storing data, a programmable read only memory (PROM) for
storing program instructions, and clock or timing circuitry, used for
example, to establishing the data sampling rate at which the DSP operates
For the multiple channel operation illustrated in FIGS. 1 and 2, the DSP
52 is programmed to function as one or more adaptive filters for each
channel and it operates sequentially to perform the necessary steps or
operations for each channel within the established data sampling rate (2.5
KHz in the present embodiment).
As described previously, an indication of the angular rotational position
of the engine is preferably provided to the electronic noise controller 26
by the SPEED signal developed by the electromagnetic speed sensor 44. The
SPEED signal contains pulses generated by the movement of toothed wheel 42
past electromagnetic sensor 44. After entering the noise controller 26,
the SPEED signal is passed to standard conditioning circuitry 146, where
the pulses are shaped or squared up into a format compatible with the
digital circuitry that follows. These formatted digital pulses represent a
measure of the angular rotation of the crankshaft and are passed to a
standard frequency multiplier/divider circuit 148, which generates a fixed
or predetermined number of pulses during one complete rotational cycle of
the engine. The pulses from the frequency multiplier/divider 148 are then
counted by a conventional modulo counter 150, to provide a digital output
signal designated as COUNT. This digital COUNT signal is then used as a
reference input signal to the DSP 52 representing the time-varying degree
of engine rotation through a complete engine cycle. As such, it will be
recognized that the value of the COUNT signal will be functionally related
to the frequencies of sinusoidal noise components generated by the engine.
In general, the number of teeth on wheel 42, the frequency
multiplier/divider, and the modulo counter are selected to provide an
integer count ranging in value from 0, to a maximum value of MAX, each
time the engine completes a cycle. A complete cycle in a four-stroke
engine being two full revolutions of the engine crankshaft. The value of
COUNT then represents the time-varying angular rotational position of the
engine in an operating cycle or the fractional portion of an engine cycle
completed at any given time (the cycle position). Based upon the value of
the COUNT reference input signal, the DSP 52 is able to generate signals
containing different sinusoidal components having frequencies that
correspond to those of the sinusoidal noise components generated by the
engine.
In addition to the SPEED signal, the other analog signals directed to the
noise controller 26 are sampled at the rate established by DSP 52 and
digitized for further use within the DSP 52. Sets of sample values for the
digitized input signals are retained in the RAM memory of DSP 52 for use
in computing sample values for digital output signals in accordance with
the programmed adaptive filters in each channel. The computed digital
output signal samples from DSP 52 are then converted into analog form and
appropriately amplified to drive the channel cancellation actuators.
With regard to analog inputs signals directed to the electronic noise
controller 26, the analog throttle position signal TP from sensor 18 is
first passed through amplifier 152, and then converted into a digital
input signal TP(n) for the DSP 52 by the action of sample and hold circuit
154 and analog-to-digital converter 156. TP(n) then represents the nth or
most recent digitized sample value for the analog throttle position signal
TP, TP(n-1) represents the digitized sample value for TP obtained during
the previous sampling period, and likewise for earlier sample values of
the throttle position signal. Although not required to implement the
present invention, the digitized throttle position signal is shown as an
input to the DSP 52 for completeness, since it provides an indication of
engine loading, and may be used to improve ANC performance as described in
co-pending U.S application Ser. No. 07/565,395 filed Aug. 10, 1990 and
assigned to the same assignee as the present application.
The analog ERROR.sub.2 developed by microphone sensor 34 is first amplified
by a variable gain amplifier designated as 158 and then passed through a
bandpass filter 160 having, for example, a passband from approximately 20
to 700 Hz in this particular implementation. Bandpass filter 160 acts as
an anti-aliasing filter and removes any direct current from the amplified
ERROR.sub.2 signal. The filtered ERROR.sub.2 signal is then applied to
sample and hold circuit 162, which acts in conjunction with
analog-to-digital converter 164 to provide a digitized sample ER(n) of the
analog ERROR.sub.2 signal to the DSP 52, where as stated previously, n
represents the nth or most re sampled value.
Based upon the value of the digitized ER(n) sample, the DSP 52 supplies a
digital GAIN signal to digital-to-analog converter 166, which in turn
controls the gain of amplifier 158 to maintain the amplitude of the
amplified analog ERROR.sub.2 signal within upper and lower limits
determined by the input capability of sample and hold circuit 162 and
analog-to-digital converter 164. This form of automatic gain control is
well known in the art and is commonly used in DSP and microprocessor
interfacing circuitry when digitizing an analog signal having an amplitude
that can vary over a large dynamic range, such as the ERROR2 signal in the
present embodiment.
Sequential digital sample values for an output noise canceling signal . . .
, Y.sub.T (n-2), Y.sub.T (n-1), and Y.sub.T (n) are computed by the DSP 52
in accordance with the above described input signals and the
characteristics of the adaptive filters programmed into the DSP 52 for the
second channel. These digital output samples are directed to
digital-to-analog converter 168, where a corresponding analog waveform is
produced. The analog waveform is then passed through lowpass filter 170,
which has an upper cutoff frequency of approximately 700 Hz in this
particular implementation. The lowpass filter acts as a smoothing filter
to remove any high frequency components introduced by the
digital-to-analog conversion process. Next, the filtered analog waveform
is amplified by power amplifier 172 to produce the final output noise
canceling waveform designated as OUTPUT.sub.2. The OUTPUT.sub.2 signal
drives the cancellation actuator (speaker) 32 to produce the noise
canceling waves that are superimposed with and attenuate the undesirable
engine exhaust noise.
Depending upon the amplitude of the noise being attenuated, it may be
desirable to prevent the amplitude of the noise canceling waveform from
becoming saturated or clipped at an upper limit related to the physical
size of the cancellation actuator. An approach that may be used to prevent
such clipping or saturation of the output noise canceling waveform is
described in U.S. patent application Ser. No. 07/842,880 filed Feb. 26,
1992, which is a continuation-in-part of U.S. patent application Ser. No.
07/565,395 filed Aug. 10, 1990, and U.S. patent application Ser. No.
07/620,801 filed Dec. 3, 1990, now abandoned.
Referring now to FIG. 3, there is shown a block diagram model for a
generalized parallel configuration of signal generator and adaptive filter
pairs that represents signal processing steps programmed into and carried
out by the DSP 52 for the second channel of the noise controller 26. It
will be recognized that the other channels of noise controller 26 can be
programmed to have similar configurations and signal processing steps.
In general, the parallel configuration of FIG. 3 is shown to include a
total of J signal generators SG.sub.1, SG.sub.2, . . . , and SG.sub.J,
designated by the respective numerals 200, 202, and 204, and J
correspondingly paired adaptive filters AF.sub.1, AF.sub.2, . . . , and
AF.sub.J, designated respectively as 206, 208, and 210. Based upon the
value of the common input reference signal COUNT, each signal generator
SG.sub.j synthesizes a sampled output signal X.sub.j (n), which is then
used as | | |
