Abstract:
A dust sensing apparatus uses a transducer element oscillating at a resonant frequency to detect changes in dust concentration. The transducer is fabricated from a polymer material which oscillates when a voltage is applied. The presence of dust affects the oscillation frequency. Dampening of the frequency by the dust presence changes the electrical resistance provided by the transducer. Electronic circuitry computes the level of dust by measuring and controlling the level of the AC signal required to maintain the oscillation frequency by monitoring the resistance.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. provisional Pat. application No. 60/042,765, entitled Apparatus and Method for Sensing Dust, filed Mar. 20, 1997. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None 
    
    
     BACKGROUND OF THE INVENTION 
     Sensors of small, airborne particles such as dust typically incorporate a mechanical, electrical, or optical sensing mechanism from which a output, or response signal, is measured. The magnitude of this response signal is proportional to the dust quantity. Physical qualities of such dust, such as size, density, and concentration are determined from the effect on this output signal. 
     Known methods used to effect such a dust sensing system include optical sensors, such as that disclosed in U.S. Pat. No. 5,136,750 in which the flux received is clouded by intervening particulate matter, and mirror based systems, such as that disclosed by U.S. Pat. No. 5,412,221 which receive light reflected from a mirror onto which particles have accumulated. Other methods include mechanical and electronic stimulation of a sensing member, typically an electrostrictive or piezoelectric member, in which particulate accumulation affects the mass of the sensor. 
     Such devices are disclosed in U.S. Pat. No. 3,653,253, which suggests sampling the mass of an accumulating piezoelectric crystal at two successive points to approximate the particulate mass collected, and U.S. Pat. No. 3,715,911, which discloses coating a sensory member with a tacky substance to accumulate airborne particles. 
     Often such devices are prone to high maintenance, as cumulative particle accumulation minimizes the sensitivity, therefore mandating frequent cleaning and/or calibration to effect accurate readings. Such devices are also expensive to manufacture, requiring specialized optical or electronic components such as piezoelectric crystals, photoresistors, and photosensors. Additionally, such devices tend to require large amounts of, or precisely metered, electrical power, limiting effectiveness with regard to household AC or common battery power. Further, point based sensors such as piezoelectric crystals have a small sensitivity area, limiting effectiveness and requiring a plurality of sensors when applied to an area. It would be beneficial to utilize a sensor capable of being manufactured from a planar material in which the electronic sensing properties are uniform along the area of the sensor. 
     SUMMARY OF THE INVENTION 
     The present invention provides a dust sensing apparatus which operates by oscillating a transducer substrate located in a sensing environment and determining the dust presence from the dampening effect such dust has on the oscillation frequency. By utilizing a conductive polymer such as poly-vinylidene-fluoride, an inexpensive yet effective sensor can be developed. 
     Such a substrate is treated to provide conductive portions in a particular pattern. Source electrodes are then attached to the non-conductive portions, and ground electrodes connected to the conductive portions. An AC voltage applied to the source electrodes will then create a piezoelectric effect causing substrate to deform. Rapid, alternating deformations caused by the AC voltage produce oscillatory, vibrational movement. This oscillation tends toward an inherent resonant frequency depending on the placement of the electrodes and the substrate material. 
     As dust presence dampens the oscillation frequency, a feedback circuit increases the voltage to drive the oscillation frequency back towards resonance. An output signal from the transducer is proportional to the amount of dust accumulated on the transducer, and also provides the feedback. The constant vibration serves to shake dust off the sensor and prevents cumulative build up, allowing the transducer to restore resonant frequency when the dust presence subsides. 
    
    
     DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a transducer according to a first embodiment; 
     FIG. 2 is a transducer according to a second embodiment; 
     FIG. 3 is a transducer according to a third embodiment; and 
     FIG. 4 is a schematic of the control and sensor logic circuit for use with the transducer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A dust sensing transducer apparatus according to the present invention applies an AC input signal, or voltage, to a substrate which has certain dielectric properties. This substrate is treated by irradiating or layering to form certain conductive portions, and such that an electrical field is induced within the substrate when such a signal is applied. Input electrodes are positioned on the non-treated surfaces of the substrate, and ground electrodes may be attached to the treated portions, such that the electric field so induced causes oscillatory movement of the substrate. Due to the electrical properties of the substrate, the resistance between the input electrodes varies with the oscillation rate, which is dampened by dust. The level of this resistance is linearly transferable to the quantity of dust on the sensor, and hence indicates the dust present in the sensing environment. 
     According to a first embodiment, a bi-morph design structure 20 is shown in FIG. 1. The bi-morph design structure 20 comprises a top layer 22 of a polymer sensor type material such as poly-vinylidene-fluoride (PVDF), a middle layer 24 of a conductive polymer, and a bottom layer 26 of a polymer sensor type material such as PVDF. The input signals 64 are applied to an electrode 21 on the top PVDF layer 22 and an electrode 23 on bottom PVDF layer 26. Electrodes 21 and 23 are of sufficient area so as to create an electric field capable of producing oscillatory movement of the substrate 20. The middle conductive layer 24 is grounded. 
     The bi-morph design structure 20 may also be created by irradiating the inner surfaces of the top PVDF layer 22 and the bottom PVDF layer 26 so that a resulting layer of conductive carbon remains on each inner surface. The inner surfaces may then be mated together to form a conductive carbon layer 24 between the top PVDF layer 22 and the bottom PVDF layer 26. The input signals 64 are then applied to the top PVDF layer 22 and the bottom PVDF layer 26 through electrodes 21 &amp; 23, while the conductive carbon layer 24 is grounded. 
     The input signals 64 that are applied to the top PVDF layer 22 and the bottom PVDF layer 26 create an electric field which induces a piezoelectric effect in the two layers causing them to repeatedly deform and thereby cause the bi-morph design structure 20 to vibrate at its resonant frequency. 
     Referring to FIG. 2, a second embodiment comprising a half-morph design structure 30 is shown. The half-morph design structure 30 comprises a single layer 32 of PVDF of which one half of its top surface 34 and the opposing one half of its bottom surface 36 has been irradiated. The irradiated surfaces are grounded. The input signals 64 are applied to the electrodes 31 &amp; 33 on the halves of the top and bottom surfaces of the PVDF slab 32 which have not been irradiated. The input signals 64 induce a piezoelectric effect which cause the PVDF slab 32 to repeatedly deform and thereby cause the half-morph design structure 30 to vibrate at its resonant frequency. 
     A third embodiment comprises a mono-morph design structure 40 as shown in FIG. 3. The mono-morph design structure 40 comprises a single layer 42 of PVDF which has had two traces 44 on its top surface irradiated. The irradiated traces are grounded. The input signals 64 are applied to the electrodes 41 &amp; 43 on the outer areas on the top surface of the PVDF slab 42 which have not been irradiated. The input signals 64 induce a piezoelectric effect which causes the PVDF slab 42 to repeatedly deform and thereby cause the mono-morph design structure 40 to vibrate at its resonant frequency. 
     Although poly-vinylidene-fluoride is disclosed in the above embodiments, alternative piezoelectric crystalline materials, such as other polyvinyl compounds, a polymer matrix comprising antimony or bismuth, or other polymer matrix doped or irradiated sufficiently to sustain an electric field, could be used for fabrication of the transducer, thereby allowing for different response characteristics depending on the dielectric and resistive properties of the material chosen. 
     Referring to FIG. 4, a block diagram of a dust sensor apparatus 10 comprising control circuit 60 and sensor logic circuit 62 connected to the transducer 12 is shown. The transducer 12 within this dust sensor apparatus 10 may be any of the embodiments described above. 
     An input voltage signal V is generated by control circuit 60 and applied to electrodes 66, 68 of transducer 12, which oscillates in response to the frequency of input voltage signal V at a resonant frequency. An output signal 70 representing the resistance of the oscillating transducer 12 provided between the electrodes 66 &amp; 68 is received by sensor logic circuit 62 which produces an indication of the level of dust presence and concentration. Dampening of the oscillation rate in response to dust causes transducer output signal 70 to vary. An increase in the presence of dust or particles increases the dampening effect, and thus causes a proportional effect on the output signal 70, as resistance of the transducer 12 increases. The output signal 70 is also monitored by control circuit 60 via a feedback 72. In response to the output signal 70 indicating increased resistance due to dampening, control circuit 60 increases the input voltage signal V to drive the oscillation rate back to the resonant frequency. As the dust presence subsides, dust accumulated on the transducer 12 is shaken off, output signal 70 indicates decreasing resistance, and the resonant frequency is again approached. Since output signal 70 indicates decreased resistance as resonance is approached, control circuit reduces input voltage signal V accordingly to maintain resonance. 
     While the above embodiments describe a response to an AC voltage signal, the input signal and feedback could comprise other waveforms such as a square wave, sawtooth wave, or other extensions of a pure sine wave, and need not be a voltage source but rather any signal that produces a calibratable movement of the substrate in response to particles. 
     It should be understood that the invention is not limited to the particular embodiments shown and described herein, and that various changes and modifications may be made without departing from the spirit and scope of the present invention as defined by the following claims.