Abstract:
A sensing device for increasing sensor accuracy in stagnant air or gas environments has a piezoelectric air mover positioned proximate a sensing element. The piezoelectric air mover blows or draws air or gas from the environment across the sensing element to eliminate the stagnant gas condition and thereby increase the speed and accuracy of measurements with the sensing element.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The field of the invention relates to sensing devices, and more particularly to a device and method for increasing the accuracy of measurements obtained with sensing devices in slow moving or still gas environments. 
         [0002]    Sensing devices for measuring temperature, humidity or other gas parameters are generally known in the field, such as a wall thermostat or automobile cabin sensor. These devices have poor contact between the sensor element inside an enclosure and the gas or air outside the enclosure. Such conventional sensor devices usually rely on convection and diffusion caused by gradients in temperature, moisture or whatever the measured value is to cause movement of the gas molecules and convey the quantity to be measured through vents in the enclosure and to the sensor element. These gradients introduce errors into the measurement and make the sensor stow to respond to the ambient changes it is supposed to be measuring. 
         [0003]    Gas has been drawn across sensors using conventional fans and pumps having rotating parts. However, these conventional gas movers are difficult to miniaturize and as a result many potential applications cannot be accessed using rotary fan technology. Further rotary fans and pumps are relatively expensive compared to the associated sensors. Power management for fans and pumps used in these applications is also a problem which is difficult to resolve, adding to the miniaturization and portability problem. 
         [0004]    Sensors used for thermostats and automotive climate control are inaccurate when the surrounding gas is stagnant or still, and there are no cost-efficient, sufficiently small fans or air movers known. Accordingly, a need exists for an air mover that is sized relative to a sensor to push or draw a sensed gas across the sensor for a wide range of applications including portable, battery powered miniaturized products. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    A sensing apparatus according to one embodiment of the invention includes a piezoelectric air mover oriented to draw or blow a gas for sensing across a sensor element used to sense a particular gas parameter. The piezoelectric air mover advantageously uses relatively little power to produce sufficient flow to the air or gas being measured by the sensor element to increase the accuracy of the measurements. 
         [0006]    In another embodiment, a duct is provided containing at least one sensor and a piezoelectric air mover. The piezoelectric air mover is adapted to generate a flow of air or gas environment through the duct by rapid vibration of the piezoelectric element within the duct. The air or gas is caused to move across the sensor so that inaccuracy due to stagnant conditions is eliminated. 
         [0007]    In a further alternative embodiment of the invention, a piezoelectric bellows is formed by two opposed plates of piezoelectric elements supported on a housing. A vent in the housing permits air or other gas to be drawn in and expelled as a result of the piezoelectric element vibration bellows movement. A sensor is mounted inside the bellows for detecting characteristics of the moving gas. 
         [0008]    In a still further embodiment of the invention, a sensing device includes a piezoelectric element with a vane attached. The vane is mounted to the piezoelectric element in a way that causes the vane to oscillate in response to the piezoelectric element vibration. A sensor is mounted to the vane in a manner which results in air or gas movement over the sensor when the piezoelectric element causes the vane to move. 
         [0009]    The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and benefits obtained by its uses, reference is made to the accompanying drawings and descriptive matter. The accompanying drawings are intended to show examples of the many forms of the invention. The drawings are not intended as showing the limits of all of the ways the invention can be made and used. Changes to and substitutions of the various components of the invention can of course be made. The invention resides as well in sub-combinations and sub-systems of the elements described and in methods of using them. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic drawing of a sensing device incorporating an air mover in accordance with one embodiment of the invention; 
           [0011]      FIG. 2   a  is a top view of a sensing device in accordance with a further embodiment of the invention; 
           [0012]      FIG. 2   b  is a sectional side elevation view of a the sensing device of  FIG. 2   a  taken along line A-A; 
           [0013]      FIG. 3   a  is a side elevation view of a still further embodiment of a sensing device in accordance with the invention; and 
           [0014]      FIG. 3   b  is a top plan view of the device of  FIG. 3   a.    
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Referring now to the drawings in which like reference numerals are used to indicate the same or related elements,  FIG. 1  illustrates a duct  14  of a sensing device  10  having inlet  12  and outlet  16 . A piezoelectric element  20  is mounted inside the duct  14 . Piezoelectric air mover  20  includes piezoelectric actuator  24  and vane  26 . Vane  26  is attached to piezoelectric actuator  24  in a manner such that vibration or oscillation of the piezoelectric actuator  24  causes oscillating movement of the vane  26  in the directions shown by arrow M. The oscillating movement of the vane  26  in turn causes the ambient gas or air to flow  30  in the tube  14  to be in the direction of the arrows. The piezoelectric actuator  24  and vane  26  may be made of conventional piezoelectric materials and MEMS materials, such as silicon, quartz and other compatible materials. Vane  26  should be selected from a material which is movable in response to the piezoelectric actuator  24  motion, while being sufficiently rigid to cause the surrounding gas  30  to flow in response to the motion of the vane  26 . 
         [0016]    Piezoelectric air mover  20  is connected to a power source (not shown) which is preferably located outside the duct  14 , so as to reduce the disturbances to flow within the duct  14 , but a power source or portion thereof may be mounted within the duct  14  as well. The power source for the air mover  20  may be a portable source, such as a battery, or a fixed source, such as a residential power circuit. The power source provides the energy needed to cause the piezoelectric elements of the piezoelectric air mover  20  to oscillate. Power management may be effected by switching the piezoelectric air mover  20  on and off to coincide with readings from sensing elements  40 ,  45 . The piezoelectric air mover  20  can be powered shortly before a sensor reading is obtained to generate gas flow  30 , and powered off after sufficient gas flow  30  is generated to provide a good sensor reading. 
         [0017]    The piezoelectric actuator  24  and vane  26  illustrated in  FIG. 1  create a unidirectional gas flow through a suitably designed duct  14 . Sensor elements  40 ,  45  are positioned in the duct  14  within the gas flow  30  generated when the air mover  20  is operating. As shown in  FIG. 1 , the sensor elements  40 ,  45  can be positioned in proximity to either the inlet  12  or outlet  16 . Further, depending on the application, the sensing device  10  can include only one or both sensor elements  40 ,  45 , or additional sensor elements  40 . As will be appreciated, the ability of moving gas or air to transmit a condition or characteristic to one or more sensors is many times greater than that of a stagnant or still gas. Accordingly, use of the sensing device  10  more closely couples the ambient gas or air being sensed to the sensing elements  40 ,  45 , thereby reducing measurement errors and sensor response time. 
         [0018]    Embodiments including an inlet-mounted sensor  40  are preferred for providing higher accuracy readings, since if there are any changes to air condition produced by the piezoelectric air mover  20  itself these will not be detected by the sensors  40  when sensors  40  are positioned at the inlet  12 . However, sensors  45  mounted between the air mover  20  and outlet  16  still provide improved performance and higher accuracy relative to sensors operating in stagnant or still air conditions. The piezoelectric air mover  20  is configured and operated so as to give an adequate, but not excessive gas flow  30  through duct  14 . Modest flow rates save power and reduces the ingress of dust which may accumulate in the duct over time. Where power is a primary concern the air mover  30  may be switched off between measurements and powered back up a suitable time in advance of the next measurement. 
         [0019]    Other types of piezoelectric air mover  20  produce a bi-directional, or reciprocating flow  30 , moving the gas rapidly back and forth. Such movers can be used with advantage of reducing build up of dust etc. in the measuring area. 
         [0020]      FIGS. 2   a  and  2   b  illustrate one such piezoelectric air mover  20  in the form of a bellows  22 . Piezoelectric plates  28  on the top and bottom of the bellows  22  are connected by a wall  29  to form a bellows chamber. An outlet  18  is provided in the wall of the bellows chamber. The piezoelectric plates  28  are energized to cause the plates  28  to oscillate. The oscillation of the plates  28  in turn operates the bellows  22  to blow and draw ambient gas in and out of outlet  18 . In this instance the sensor(s)  40 ,  45  can be placed in proximity to the outlet  18  of the bellows  22  in the path of reciprocating gas flow  30  generated by the piezoelectric air mover  20 . Sensors  40 ,  45  may be located within the bellows  22  chamber (sensor  40 ), or externally (sensor  45 ) within the path of gas flow  30 , or both sensors  40 ,  45  may be used together. 
         [0021]    In a further embodiment of the piezoelectric air mover  20 , the piezoelectric air mover can be constructed in the form of a pump. In such case, the outlet  18  of bellows  22  includes a one way valve (not shown) which enables higher pressures to be generated than a rotary fan type construction. The use of a piezo-actuated pump is desirable where gas samples must be drawn through tubing or through some other item or construction which offers high resistance to motion such as a filter element. 
         [0022]    A still further embodiment of the invention is shown in  FIGS. 3   a  and  3   b  in which a piezoelectric air mover  20  has a sensor  40  mounted directly to vane  26  of air mover  20 . The piezoelectric air mover  20  may optionally be mounted inside a duct  14  as in  FIG. 1 , but it is not necessary, since sensor  40  is exposed to moving gas by virtue of movement of the vane  26  caused by piezoelectric actuator  24 . Leads  42 ,  44  for sensor  40  may be etched, bonded or applied in another known manner to vane  26  and piezoelectric  24  for connection with a circuit for analyzing (not shown) the sensor  40  readings. 
         [0023]    By drawing a sensed air or gas  30  across sensors  40 ,  45  at some speed, the measurement time delays observed in conventional sensors positioned in stagnant gases are eliminated. The piezoelectric air movers  20  are small, for example, 2.5 cm or less in length and smaller still in other directions. In the case of a bellows or pump type piezoelectric air mover  20 , it may be about 2 cm in diameter or less. The piezoelectric air movers  20  have minimal weight and consume less than 30 mW of power. Such air movers  20  are ideal for use in a number of applications, including gas measuring equipment, wall mounted temperature and humidity sensors automotive AC and heat sensors environmental monitoring sensors, environmental validation sensors, portable gas or air monitors and many other applications in which gas characteristics are measured, particularly temperature and humidity. The piezoelectric air mover  20  may require power conditioning circuitry to make the mover supply voltage and waveform suitable and to convert battery voltage to the high drive voltage required by piezoelectric actuators  24 ,  28 . It will be appreciated how to implement such power conditioning and power management, depending on the selected application for the piezoelectric air movers  20 . 
         [0024]    The construction and arrangement of the sensing device, as described herein and shown in the appended figures, is illustrative only. Although only a few embodiments of the invention have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g. variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the appended claims. Accordingly, all such modifications are intended to be included within the scope of the present invention as defined in the appended claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. In the claims, any means-plus-function clause is intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Other substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the preferred and other exemplary embodiments without departing from the spirit of the embodiments of the invention as expressed in the appended claims. Therefore, the technical scope of the present invention encompasses not only those embodiments described above, but all that fall within the scope of the appended claims.