WO1994029846A1

WO1994029846A1 - Vehicle operator station with three dimensional active noise cancellation

Info

Publication number: WO1994029846A1
Application number: PCT/US1994/004205
Authority: WO
Inventors: Sen M. Kuo; Mary K. Christensen; Robert A. Herold
Original assignee: Caterpillar Inc.
Priority date: 1993-06-11
Filing date: 1994-04-18
Publication date: 1994-12-22
Also published as: JPH08500457A; EP0655157A1; AU7014594A; AU671552B2

Abstract

A zone of reduced sound is formed about the head and shoulders of an operator in a vehicle by projecting a cancelling sound signal into a passively attenuated operator workplace (4), positioning monitoring microphones (8, 9) in the operator workplace (4), acoustically sensing drive related sources of sound at the source location, normalizing the sound signals and processing the normalized input and monitored signals in an adaptive multi channel filtered X least means squares algorithm.

Description

VEHICLE OPERATOR STATION WITH THREE DIMENSIONAL ACTIVE NOISE CANCELLATION

Technical Field

The invention relates to the control of the sound level in a vehicle operator station and in particular to the formation of a localized three dimensional zone surrounding an operator within a vehicle enclosure in which the sound level is reduced.

Background and Relation to the Prior Art

Active noise cancellation involves superimposing on a noise acoustic wave an opposite acoustic wave that destructively interferes with and cancels the noise wave. The cancelling acoustic wave is of equal amplitude but of opposite phase to the noise acoustic wave. The generation of the proper interference signal to produce cancellation, in the proper position at the right time requires taking into consideration a number of variables resulting in elaborate signal processing.

The active noise cancellation principle is most useful at frequencies below 500 cycles per second (Hz) . Above that frequency range, noise attenuating materials applied to surfaces are more effective. The implementation of the principle of active noise cancellation generally involves sensing of the characteristics of the noise acoustic wave, generating the cancelling acoustic wave and, through monitoring the combined waves, developing a feedback signal that keeps the cancelling wave in adjustment.

The monitoring signal is frequently called an "error" signal. Most implementations of the active noise cancellation principle also accommodate changes in the frequency and the intensity characteristics of the noise. This usually involves incorporating adaptability into a feedback from a monitoring microphone that provides information used in adjusting the cancelling wave. The computations involved in determining the adjustments to the cancelling wave are performed under a procedure known in the art as an algorithm. A number of algorithms with adaptability have evolved in the art. A survey article by J.C. Stevens entitled "An Experimental Evaluation of Adaptive Filtering Algorithms for Active Noise Control", Georgia Institute of Technology, GRTI/AERO, Atlanta, Ga., 1992 Pages 1-10, provides an illustrative description of the current capabilities in the art.

The implementation of the algorithms has been achieved in the art using digital signal processing (DSP) and fabricated in the form of single semiconductor chip active noise controller devices. Active noise cancellation systems exhibit instability under conditions where the cancelling signal gets into the noise prior to the sensing of the characteristics of that noise. Care in constructing a system is employed to prevent the situation or a modification of the algorithm can be used to accommodate it.

The active noise cancellation principle has been applied extensively in the art under conditions where the noise source is localized and the operations of sensing, cancelling and monitoring can be positioned serially as is the case in reducing noise in ducts and pipes. An illustrative example is U.S. Patent 4,987,598. When, however, there is an attempt to apply the active noise cancellation principle to three dimensional space many interdependent considerations are encountered. In three dimensional space the source of the noise is usually not localized, the complexity of the sound fields, where reflections from enclosures of various shapes may be involved, is usually significantly higher and an arrangement of sensing, cancelling and monitoring operations in serial order may not be readily achievable.

In implementing the active noise cancellation principle in vehicle operator stations further considerations require attention. The operator station is frequently enclosed in an enclosure with a relatively small volume so that there is minimum sound attenuation in the air between the walls and the occupant. The noise enters the enclosure both by the surrounding air and by vibration. There are many different types of noise associated with a vehicle such as engine, fan, hydraulic, gear drive as well as noise produced from motion of the vehicle, the sources of all of which are close to the operator station. The material out of which the vehicle is made is usually a good transmitter of vibration.

There has been some effort in the art toward reducing the noise in a three dimensional enclosure on a vehicle. In U.S. Patent 4,506,380 a system is shown for reducing sound throughout a multi-occupant enclosure wherein a possible noise condition, such as a particular engine speed, that has been measured with a tachometer is countered with a single speaker cancelling signal in the vehicle enclosure. In the system of the patent the cancelling signal is selected through a table look up operation of previously stored tachometer vs. noise data.

In an article by Perry et al, entitled "The Use of DSP for Adaptive Noise Cancellation for Road Vehicles" Paper No. 3 Session 3 Pages 331 to 338; tachometer or ignition based indirect sensing of noise is processed in a controller with a cancelling signal for an entire multi-occupant enclosure being provided through a plurality of peripherally mounted speakers and with the monitoring being through a distributed plurality of microphone pairs positioned at each seat.

In U.S. Patent 4,977,600 the principle of active noise cancellation is applied to an individual vehicle seat. In that patent cancelling speakers are positioned in wrap around portions of the seat back near the head of the occupant and flexibly mounted monitoring microphones are positioned in the vicinity of the ears of the occupant.

Heretofore in the art the considerations of applying active noise cancellation in a vehicle with an enclosed operator station has not been addressed.

Summary of the Invention

The invention provides a localized zone where the sound level is reduced in the enclosure for the operator of a vehicle. The zone is positioned about the head and shoulders of the operator.

The reduced sound zone in the vehicle enclosure is achieved by projecting an active noise cancelling signal into the region about the head and shoulders of the vehicle operator at the operator's station inside a passive sound attenuation covered enclosure. The input to the cancellation signal is a collection of acoustically sensed signals taken at the location of individual sound sources. The individual sound sources are principally those associated with the drive train of the vehicle and are sensed by microphone preferably within the housing of the particular source of noise. The cancelling signal is monitored by microphones placed in the reduced sound zone in the vicinity of the ears of the operator. The cancelling signal is maintained in effectiveness by processing the monitor signal and the normalized input signals in a processor using an adaptive filter type algorithm that performs the special functions of accommodation of multiple cancellation signal speakers and prevention of component saturation.

Brief Description of the Drawings Figure l is a schematic view of the positioning of the reduced sound zone in the operator enclosure of a vehicle.

Figure 2 is a sectional view of the passive attenuation material applied to the operator enclosure of the vehicle.

Figure 3 is a graph showing the relative effectiveness of active and passive sound attenuation vs. frequency.

Figure 4 is a sketch illustrating the acoustic paths and electrical component wiring of the invention.

Figure 5 is a diagram of a functional block representation of the algorithm employed in processing the active noise cancellation signal.

Description of the Invention

The invention involves structural and processing modifications that permit the principle of active noise cancellation to be used in producing a reduced sound zone about the head and shoulders of an operator at an operator station within a passively sound attenuated vehicle enclosure.

While the shape of the reduced sound zone and the amount of sound level reduction in it may be affected by complex sound fields involving speaker positioning frequency of the noise being cancelled and reflected from nearby objects; the reduced sound zone should have at least a theoretical lOdB sound level reduction within an essentially spheroidal shape with a diameter about 1/5 of the wavelength of the particular noise frequency. Expressed another way; the at least lOdB reduction or about 1/100 of the mean square pressure of the primary noise source, should extend about 0.1 of the wavelength from the center point of cancellation. The reduced sound level zone is about the size of the head and shoulders of an equipment operator.

The cancellation aspect of the invention is particularly valuable in controlling low frequency, up to 500 cycles per second (Hz) sound.

The structural modifications involve the positioning of the speakers, the positioning of the monitoring microphones and the positioning of the input microphones principally at the location of and within the housing for drive train related sound sources and the using of acoustic sensing for those sound sources.

The processing modifications involve normalization of multiple input signal levels, accommodation of the presence of several cancelling speakers and several monitoring microphones and the prevention of component saturation in the algorithm used in developing the cancellation signal and maintaining it in adjustment. Referring to Figure 1 there is shown a schematic view of the positioning of the reduced sound zone in the operator enclosure of the vehicle, wherein in a vehicle operator enclosure 1 having a door 2, panels of different sound reflectivity such as a windshield 3 and, in the broken out portion, an operator station 4 shown as a seat. As a result of the proximity of many sound sources such as engine, fan, transmission and hydraulic and the vibration transmitting nature of the materials used in vehicles the sound level in the enclosure 1 is very high. Passive attenuating materials on the surface of the enclosure 1 help but are less effective at lower sound frequencies. It is desirable to reduce the overall sound level for the operator but the operator must retain the ability to recognize and respond to sounds, such as emergency indications, that are essential to the operator role.

A reduced sound level zone 5, shown schematically as a zone within a dotted line in Figure 1, is produced by projecting a cancelling signal shown bounded by dashed lines from a cancelling signal speaker 6 with a second cancelling speaker 7 not visible in this figure at the far portion of the enclosure 1 projecting the cancelling signal therefrom into the zone 5. The cancelling signal is monitored by the microphones 8 and 9 near the ears of the operator shown dotted in the figure. While the illustration of Fig. 1 is of an operator in a sitting position it will be apparent from the principles set forth that the providing of a reduced sound level zone for an operator would not be limited to a particular position.

In operation, the sound level in the zone 5 is lowered by a cancellation signal from the speakers 6 and 7, not visible. The cancellation signal is developed from: at least one of a plurality of input signals, acoustically sensed at the source of predominant component sounds usually originating in connection with the drive train of the vehicle; and, from the signals from the monitoring microphones 8 and 9. The mathematical operations in developing the cancelling signal and in keeping the cancelling signal in adjustment are performed in accordance with an algorithm to be later described. The zone 5 is about the size of the head and shoulders of the operator.

Referring next to Figure 2 a sectional view is provided of the passive attenuation material that is applied to the surfaces of the enclosure 1. In Fig. 2 the surfaces of the enclosure 1, usually of metal, have a layer of a sound attenuating material 10 covering them. In some portions such as the floor, the attenuating material must also have wear properties and be of an appropriate material such as a rubber mat 11. The passive attenuation is some help in reducing the sound level but its effectiveness is minimal at the low frequencies below 500 cycles per second (Hz) .

Referring to Figure 3 a graph is shown illustrating the relative effectiveness of active and passive sound attenuation vs. frequency. In the frequencies above 500 Hz the passive attenuation is quite effective but that effectiveness falls off markedly below the 500 Hz rate. The active noise cancellation principle is most effective at the lowest frequencies up to about the 500 Hz rate. In the graph about ~ 80 cycles per second is the lower response of most off the shelf equipment. Efficient attenuation below the ~ 80 Hz rate is expected. In vehicles the use of active and passive attenuation of sound either singly or together is beneficial in reducing the sound level in an operator enclosure.

An aspect of the invention is the sensing of the sources of sound to be lowered in the zone 5 outside of the enclosure 1. The sources that can be lowered most effectively and which heavily contribute to the overall sound level in the enclosure are sources of sound associated with the drive equipment of the vehicle. These sources raise the general level of sound and have a low probability of containing information essential to the operator. In accordance with the invention the input is sensed at the source of the sound preferably within a housing, where the sound source has one, and by sensing the sound acoustically, that is the sound travels by air to a microphone. In some prior art applications, indirect sound level indicators such as tachometers and ignition pulse counters have been used to sense a measure of sound level. It has been found however that acoustic sensing avoids harmonics and is a more faithful replica of what the ear would hear.

In a vehicle environment particular care is needed with respect to letting any of the cancellation signal get into the input signals on which the cancellation signal is to be based. The feedback that results makes the cancellation system unstable. While some cancellation signal feedback can be handled by modifications of processing in the algorithm the better arrangement in this invention is to construct the system to avoid cancellation feedback by remote sensing of large noise source signals and passive attenuation materials.

In the system of this invention, the acoustic sensing at the generally remote location of the source of the sound within the source of sound housing where possible, takes advantage of the high relative signal strength of the sound input and overrides any portion of the cancelling signal if present. The passive attenuation of the enclosure 1 is helpful also in containing the cancelling signal thereby further reducing the probability of it getting into the input microphones.

Referring next to Figure 4 where a sketch is shown that illustrates the acoustic paths and electrical component wiring of the invention. In Fig. 4 the remote acoustic sensing of the sources of sound is illustrated with an illustrative three input microphones 12, 13 and 14. Microphone 12 is shown located inside a housing 15 surrounding a source of vehicle drive related noise such as an engine, not shown. There is an air separation in the housing 15 between the source of the sound and the microphone 12 that senses it. Similarly microphone 13 is located inside a housing 16 for another source of vehicle drive related noise such as a fan. There may be sources of noise that are not housed such as transmissions and hydraulic pumps. The noise from sources such as those would be sensed by positioning a microphone such as 14 in the proximity. The illustrative three input microphones 12,

13 and 14 have their signals normalized through operational or reference amplifiers 17, 18 and 19 respectively and the signals algebraically summed into a single input signal in a summing amplifier 20 in an input normalization section 21. The operational amplifiers 17, 18 and 19 are sometimes called reference amplifiers in the art. In Figure 4 reference numerals as used for like elements in Figure 1 are employed. The enclosure 1 is shown with a dashed line and acoustically it impedes any cancellation signal travel toward any input. The acoustic paths from the speakers 6 and 7 to the monitoring microphones 8 and 9 are shown as dash-dot lines. The acoustic paths illustrate that where there is more than one cancelling speaker and more than one monitoring microphone the algorithm in a controller 22 must accommodate the fact that each monitoring microphone will receive input from each speaker.

The signals from the monitoring microphones 8 and 9 are delivered by conductors 23 and 24 respectively. The outputs of the input normalizing summing amplifier 20 and the monitoring signals are inputs to the controller 22.

In the controller 22 there is an adaptive least mean squares type of algorithm embodied in integrated circuit form.

The output stages 25 and 26 deliver the cancelling signal from the controller 22 to the speakers 6 and 7 through conductors 27 and 28 respectively.

In a preferred embodiment the controller 22 is a commercially available integrated circuit. A satisfactory model for controller 22 is the TMS 320C30 Floating Point DSP manufactured by Texas Instruments Inc., Dallas, Texas. A satisfactory model of a microphone suitable for the monitoring microphones 8 and 9 is a model SM98-A made by the SHURE, CO. and a satisfactory model of a microphone suitable for the input microphones 12, 13 and 14 is a model PZM distributed by the Radio Shack Co. A satisfactory model of a cancelling speaker suitable for the speakers 6 and 7 is the Rockford Fosgate PRO-128 12" Sub Woofer. A satisfactory type of passive acoustic attenuating material for the surfaces of the enclosure 1 is 1 inch thick open cell foam plastic material. In processing, a satisfactory algorithm for use in the computations in the controller 22 is the Multi Channel Filtered X Least Mean Squares type with modifications for multiple speakers and to permit small steps in the iterations. An example is shown in Fig. 5.

Referring next to Figure 5 there is shown a diagram of a functional block representation of the algorithm employed in processing the elements of the active noise cancellation signal of the invention. The algorithm is the Multi Channel Filtered X Least Mean Squares type known in the art with the modifications of the invention.

In the diagram of Fig. 5 the elements are representations of the functions of variables that influence the cancelling signal and the monitoring or error signal. The algorithm operates by calculating an error correction for the cancelling signal, applying the correction and repeating in a series of cycles until a minimum variation is achieved. The selected input noise signals delivered through microphones 12, 13 and 14 have been normalized in stage 21.

In Fig. 4 the normalized input travels in the main paths directly from the speakers to the monitoring microphones. The adaptive filters provide adjustment to the cancellation signal. A portion of each signal in the acoustic path goes direct to the closest microphone and across to the other microphone. The main, the direct from cancellation signal 1 and the cross from cancellation signal 2 are acoustically superimposed to form error signal 1 which in turn is introduced into the controller labelled Filtered X-LMS at the input labelled error signal 1. A similar situation takes place to produce error signal 2. The changes that take place in the adaptive filters as the iteration steps take place is indicated by the dashed arrows. The size of the steps taken will influence how soon the algorithm will converge on the optimum minimum error in the cancellation signal. The size of the steps is influenced by the input and error signal power. The algorithm accommodates in the filter weighting for the multiple paths from the speaker to the monitoring microphones and for component saturation, which for example could be where a change called for would drive a speaker beyond its linear range. The algorithm accommodation is by reducing the weight of the filter.

What has been described is the formation of a zone of reduced sound level in the enclosure for the operator of a vehicle by interdependent positioning of the elements of active noise cancellation, the remote acoustic sensing of sources of drive related noise inside the individual housings, the application of passive noise attenuation to the enclosure and the modification of a standard processing algorithm.

Claims

1. A vehicle operator station (4) with a zone (5) of reduced sound level at the operator's location comprising: a passive sound attenuated operator station enclosure (1) , at least one sound cancellation speaker (6,7) , said at least one sound cancellation speaker (6,7) positioned to project sound toward an operator location in said enclosure (1) , at least one monitoring microphone (8,9), said at least one monitoring microphone (8,9) being positioned in said operator location, at least one input microphone (12,13,14) , said at least one input microphone (12,13,14) positioned within an enclosure (15) at least partially surrounding a separate source of vehicle drive related noise for acoustic sensing of said separate source of noise, signal translation means operable to normalize the signal from said at least one input microphone (12,13,14) into a single signal, and sound cancelling signal means, said sound cancelling signal means being responsive to said normalized input signal and to a signal from said at least one monitoring microphone (8,9) and operable to provide an iteratively corrected cancellation signal to each said speaker (6,7) .

2. The operator station (4) of claim 1 wherein said at least one sound cancellation speaker (6,7) is two sound cancellation speakers (6,7).

3. The operator station (4) of claim 2 wherein said at least one monitoring microphone (8,9) is two monitoring microphones (8,9).

4. The operator station (4) of claim 3 wherein said at least one input microphone (12,13,14) is three input microphones (12,13,14).

5. The operator station (4) of claim 4 wherein said signal translation means includes an operational amplifier (17,18,19) for each input microphone (12,13,14) with each operational amplifier (17,18,19) output summed in a summing amplifier (20).

6. The operator station (4) of claim 5 wherein said sound cancelling signal means includes a multi channel filtered X least mean squares algorithm modified for multiple cancellation speakers and multiple monitoring microphones.

7. In a vehicle workstation (1) the improvement for providing a reduced sound level in a localized operator space comprising: an operator enclosure (1) with a layer of passive sound attenuating material (10) thereon, active sound cancellation means consisting of at least one cancelling speaker (6,7) positioned to project sound into an operator work location (4) , at least one monitoring microphone (8,9) positioned in said operator work location (4) and at least one input microphone (12,13,14) each said at least one input microphone (12,13,14) positioned to acoustically sense a separate source of vehicle drive related sound at the location of said source and within a housing for said source, signal translating means for normalizing signals from said at least one input microphone

(12,13,14) and responsively producing normalized input signals, and signal processing means responsive to said normalized input signals and said at least one monitoring microphone (8,9) to deliver to said at least one cancelling speaker (6,7) an iteratively corrected cancellation signal.

8. A noise control system for a vehicle operator station (4) comprising: an operator enclosure (l) with a layer of passive sound attenuating material (10) essentially covering the surfaces thereof, and, means for controlling the level of sound at an operator workplace location (4) in said enclosure (1) said sound control means including, at least one sound cancellation speaker (6,7) positioned to project a cancellation sound signal into said operator workplace location (4) , at least one monitoring microphone

(8,9) positioned in said operator workplace location (4), input means including at least two separate input microphones (12,13,14) positioned at at least two separate sources of vehicle drive noise and located apart from said enclosure for acoustic sound sensing, and signal normalizing means for converting input signals from said separate input microphones (12,13,14) into a normalized input signal and signal processing means responsive to signals from said at least one monitoring microphone (8,9) and said normalized input signal in delivering to said at least one sound cancellation speaker (6,7) an iteratively corrected sound cancellation signal.

9. The system of claim 8 wherein said at least one sound cancellation speaker (6,7) is two sound cancellation speakers (6,7).

10. The system of claim 9 wherein said at least one monitoring microphone (8,9) is two monitoring microphones (8,9).

11. The system of claim 10 wherein at least one monitoring microphone (8,9) is three input microphones (12,13,14) .

12. The system of claim 11 wherein said input means said signal normalizing means includes an operational amplifier for each said input microphone signal with each operational amplifier (17,18,19) output summed in a summing amplifier (20) .

13. The system of claim 11 wherein in said signal processing means said iteratively corrected sound cancellation signal is developed in iterative signal processing in a multi channel filtered X least mean squares algorithm modified for multiple cancellation speakers (6,7) and multiple monitoring microphones (8,9).

14. The process of providing a reduced sound level in a vehicle operator work station (4) comprising the steps of: providing an operator workplace enclosure (1) with a layer of passive attenuation material (10) on and essentially covering the surfaces thereof, positioning at least one sound cancelling signal speaker (6,7) to project sound into an operator workplace (4) in said enclosure (1) , positioning at least one monitoring microphone (8,9) in said operator workplace (4), sensing at least one source of vehicle drive noise at the source thereof acoustically, normalizing input signals from acoustically sensed separate sources of sound and processing and delivering to said cancelling sound speaker (6,7) a noise cancellation signal in response to normalized input signals and monitoring microphone signals with iterative cancelling signal correction.

15. The process of claim 14 wherein in said processing step said normalized input signals and said microphone signals are processed using a multi channel filtered X least mean squares algorithm.

16. The process of claim 15 wherein said algorithm includes a modification to accommodate multiple speakers.