Abstract:
An aspirated smoke detector includes a flow path and a generator of acoustic waves in the flow path. Airborne particulate matter in the flow path responds to the acoustic field by particle agglomeration; the resulting larger particles flow into a photoelectric-type smoke sensor. A sensed level of particles can be processed, or compared to one or more predetermined thresholds to establish presence of one or more predetermined conditions.

Description:
FIELD 
   The invention pertains to aspirated smoke detectors. More particularly, the invention pertains to such detectors which include a source of acoustic waves which can be used to agglomerate airborne particulate matter into larger particles that then flow into a smoke sensor. 
   BACKGROUND 
   Optical sensing techniques, usable in smoke detectors, can be classified as transmission and light scattering techniques. Transmission measurements in early fire detection require impractically long optical paths. Intensity of the scattered signal depends on many factors besides the number of particles per unit volume and intensity of incident light. Modest improvements of light scattering signal at low smoke densities can be achieved by optimizing wavelength, scattering angle, detector sensitivity, intensity of the incident light, and polarization state of the incident light. 
   Acoustic agglomeration of aerosols and colloids is a well-known technique to manage fine particulate matter in pharmaceutical, environmental and other industrial applications. Basic concept is based on forming standing wave in acoustic resonator. Acoustic (usually ultrasound) pressure forces both small and large particles to jiggle along with air molecules. However, larger particles have a larger slip factor and are not able to follow air movement (this is particularly true at ultrasonic frequencies) as well as smaller particles can. As a result, aerosol particles experience increased collision frequency as compared to collision due to thermal motion alone. 
   Each collision may result in coagulation of particles where smaller particles disappear and larger particles emerge. As larger particles get formed, they tend to move to a location of one or more nodes in the acoustic field where they start to agglomerate (a phenomena called flocculation). If the field is powerful enough they tend to levitate. In sum, if a standing wave resonant acoustic field is established in a space containing a small concentration of aerosols then in a few seconds, those particles will coagulate into larger particles at nodes of the acoustic field. 
   Optical smoke detectors are advantageous in that they will respond to smoldering-type fires and potentially can provide early warnings thereof. Such technologies are also usually readily acceptable world wide. 
   There is thus a continuing need to improve performance of optical-type smoke detectors. Preferably, sensitivity could be increased without at the same time increasing incidences of false alarms. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an apparatus which embodies the invention. 
   

   DETAILED DESCRIPTION 
   While embodiments of this invention can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention, as well as the best mode of practicing same, and is not intended to limit the invention to the specific embodiment illustrated. 
   In accordance with the invention, the size distribution function of smoke can be changed by applying a high intensity resonant acoustic field. An acoustic field forces particles to move along with the field but big particles do not follow the field as readily as small ones. Increased collision frequency of particles ultimately forms one large particle levitating in the node of the acoustic field that is much easier to detect using a conventional light scattering technique. 
   Advantageously, in embodiments of the invention, a light scattering signal can be amplified hundreds of times if all particles collapse into a single one at very low densities. This technique would work for any type of photoelectric detector. However, the preferred embodiment would be to apply the acoustic field to a flow path of an aspirated type detector. 
   In an aspect of the invention, airflow is controlled by the aspiration system rather than by environmental conditions and acoustic trapping would be under better control. In such conditions it is possible to vary the duration of levitation period and monitor growth of the resulting particle that can be correlated to the fire conditions. Another advantage of the aspirated system is that power consumption is usually not as critical as it is with spot-type photoelectric detectors. Yet another benefit is that spatial restrictions on the system are not stringent and it would be possible to confine the acoustic field to a detector enclosure. 
   Finally, additional sensing techniques can be used in combination to improve nuisance immunity of the system. This may include use of multiple color scattering signals, additional gas sensors or monitoring heating of particles upon illumination by a high-intensity light source, by photothermal beam deflection or other suitable technique all without limitation. 
   A detector  10  in accordance with the invention is illustrated in  FIG. 1 . Detector  10  includes an inflow port  12  which is coupled a flow pipe  14  carried by a housing  16 . The pipe  14  provides a bounded, internal flow path for ambient particulate carrying atmosphere. 
   In one aspect of the invention, a dust filter  18  can be included in the flow path formed by the pipe  14 . Those of skill will understand that filtering element  18  is optional. 
   Detector  10  incorporates an ultrasonic piezoelectronic transducer  20  which generates a high intensity acoustic field which can be coupled to a region  24  in the flow path formed by the pipe  14 . The region  24  can be a bounded region in which the acoustic field is generated as would be understood by those of skill in the art. 
   The transducer  20  could resonate at a 40 kilohertz rate with a selected, even, number of wave lengths. Those of skill will understand that a transducer, such as a transducer  20  can generate an ultrasonic standing wave on the order of 140 dB in the region  24 . Such a field is capable of levitating selected air borne particulate matter, for example smoke particles. 
   In accordance with the invention, when the transducer  20  generates the acoustic field in the region  24  particulate agglomeration occurs at nodes of the field therein. In one aspect of the invention, the field generated in the region  24  functions as an acoustic trap for very small particles. Alternately, it can be considered an integrating effect which creates a plurality of larger particles which then move from the field into a housing  28  for an optical-type smoke sensor or smoke detector. 
   Those of skill in the art will understand that a variety of configurations of optical smoke sensor  28  could be used in combination with the transducer  20 . In one aspect of the invention, the field generated in the region  24  could extend to an interior region of a housing  28 - 1  of the sensor  28 . In this embodiment housing  28 - 1  can be eliminated exposing the elements of sensor  28 . An expanded housing  28 - 2  could include both the field in the region  24  and the sensor  28  as shown in phantom. In this embodiment the agglomerated particles could be detected by sensor  28  while still in the acoustic field. 
   The housing  28 - 1  for  28 - 2  of the sensor  28  can incorporate a light source, for example a light emitting diode, and an off-set sensor, such as a photo diode, to detect scattering of light due to the agglomerated particles formed in region  24  by the acoustic field. 
   A control unit  30  can be coupled to the transducer and the sensor  28 . Those of skill will understand that the control unit could include a programmable processor and associated control software as well as interface circuits to properly drive the transducer  20  and to generate signals to the optical sensor  28  to energize the light source therein. 
   Signals from the optical smoke sensor  28  could be coupled to the control unit  30  for analysis and a determination as to the existence of one or more predetermined smoke related conditions. As those of skill in the art will understand, predetermined conditions could include a pre-alarm condition, or a fire alarm condition all without limitation. 
   The unit  10  can also incorporate an optional aspiration device, such as a fan  34 , which is also coupled to the control unit  30 . It will be understood that a variety of fans, blowers or other mechanical movable devices could be used all without limitation. Electronic aspirating devices also come within the spirit and scope of the present invention. 
   Control unit  30  could vary the flow rate induced by the device  34  to adjust “growth time” of the particles. Unit  30  also variably controls the transducer  20  to alter the field  24  as would be understood by those of skill in the art. 
   Additionally, a microphone  28 - 3  can be located in the vicinity of the sensing region of smoke sensor  28 . The control unit  30  could modulate illumination of the optical source in sensor  28 . Signals responsive thereto could be fed to control unit  30  from microphone  28 - 3  to provide an audible indicator thereto as to the presence of particles that absorb light. Sensor  28  could also include an ionization-type smoke sensor, a gas sensor and a thermal sensor, all coupled to control unit  30  to provide multi-criteria sensing. 
   Ambient atmosphere which has traveled through the flow path formed by pipe  14  exits detector  10  via outflow port  40 . 
   One form of an optical smoke detector, such as a detector  28 , is disclosed in the U.S. Pat. No. 5,764,142 entitled “Fire Alarm System With Smoke Particle Discrimination” assigned to the assignee hereof and incorporated by reference. 
   From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications that fall within the scope of the claims.