Abstract:
A method and device for assessing rheological properties of a fluid, including lubricity and viscosity. The invention utilizes a tube into which the fluid is introduced, and relies on tracking the movement of particles intentionally introduced into the fluid to assess the rheological properties of the fluid. The method and device generally entail introducing at least one solid particle into the fluid to cause the particle to flow through the portion of the tube, and assessing a rheological property of the fluid within the tube by tracking the movement of the particle through the portion relative to the flow of the fluid through the portion.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/588,669, filed Jul. 19, 2004. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0002]     This invention was made with Government support under Agreement No. W56HZV-05-C-0126, awarded by the U.S. Department of Defense. The Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     The present invention generally relates to devices and methods for measuring properties of fluids. More particularly, this invention relates to a sensing device equipped with a tube through which a fluid flows for sensing lubricity, viscosity, and other rheological properties of the fluid.  
         [0004]     Viscosity and lubricity are important fluid parameters for a variety of fluids, including fuels, lubricants, adhesives, paints, oils, tars, electrophoresis gels, syrups, etc. For example viscosity, which is the internal resistance to flow exhibited by a fluid, is a key fluid parameter for lubricants such as automotive engine oils, whose viscosities change over time to the detriment of the components they lubricate. While oil quality sensors based on measuring the dielectric constant or electrical resistance of a lubricant have been developed and are commercially available, viscosity provides a better indication of the condition of an oil (and other lubricants) and when the oil should be replaced. Lubricity, or the coefficient of friction of a fluid, is often employed to characterize lubricants, fuels, diesel fuel additives, bearings, and load bearing surfaces. As with viscosity, the lubricity of fuels and lubricants often changes over time, such as from contamination from water and particulate matter. Consequently, there has been efforts to develop viscosity and lubricity sensors for fuel and lubricating system applications, including engine oil and fuel systems. For example, lubricity has been measured using slipping disks, bearings, shafts, and balls, which typically involve a long testing process requiring a relatively large amount of sample fluid. Techniques developed to measure viscosity have used capillary force, moving paddles, blades, vibrating tuning forks, and hollow tubes or cantilevers immersed in a fluid. More recently, rheometers and viscometers have been developed with a vibrating micromachined silicon cantilever immersed in the fluid of interest, with the resultant damping of the cantilever vibration being used to indicate viscosity.  
         [0005]     Viscosity measuring techniques that rely on a vibrating structure require that the vibrating structure be inserted into the fluid being tested so that the fluid surrounds the structure. In contrast, commonly-assigned U.S. Pat. No. 6,647,778 to Sparks discloses a sensing device capable of sensing the viscosity of a fluid flowing through a microelectromechanical system (MEMS). Sparks&#39; sensing device is used in combination with a micromachined resonating tube, preferably of the type disclosed in commonly-assigned U.S. Pat. No. 6,477,901 to Tadigadapa et al. and adapted for resonant sensing of mass flow and density of a fluid flowing through the tube. One embodiment of Sparks&#39; sensing device incorporates second and third micromachined tubes having bridge portions adapted to deflect in response to a pressure change of the fluid flowing therethrough. Sparks ascertains the viscosity of the fluid flowing through the tubes by comparing the pressures of the fluid within the second and third tubes.  
         [0006]     U.S. patent application Ser. No. 10/710,106 to Sparks also discloses a method and device for assessing the viscosity of a fluid. Similar to U.S. Pat. No. 6,647,778 to Sparks, Ser. No. 10/710,106 to Sparks utilizes a vibrating tube into which the fluid is introduced, but differs by sensing the influence that the fluid has on the vibrational movement of the tube to assess the viscosity of the fluid. More particularly, Ser. No. 10/710,106 to Sparks entails introducing a fluid of interest into a passage within a freestanding portion of a tube, vibrating the freestanding portion of the tube at or near a resonant frequency thereof, sensing movement of the freestanding portion of the tube, and then assessing the viscosity of the fluid by ascertaining the damping effect the fluid has on the vibrational movement of the freestanding portion at or near the resonant frequency. The damping effect can be ascertained in reference to, for example, the quality (Q) factor or peak amplitude of the freestanding portion at the resonant frequency, or an amplitude-versus-frequency plot of the freestanding portion in the vicinity of the resonant frequency.  
         [0007]     Notwithstanding the above advancements, there is an ongoing need for techniques by which viscosity and lubricity can be measured, particularly more quickly and using smaller sample sizes than possible with existing lubricity measurement techniques.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     The present invention provides a method and device for assessing rheological properties of a fluid, including lubricity and viscosity. The invention utilizes a tube into which the fluid is introduced, and relies on tracking the movement of the fluid through the tube, either alone or with particles intentionally introduced into the fluid, to assess the Theological properties of the fluid.  
         [0009]     According to the method of the invention, the fluid and optionally at least one solid particle suspended in the fluid is flowed through a passage within a portion of a tube, and a Theological property of the fluid within the tube is assessed by tracking movement of at least one of the fluid and the particle through the portion. For example, if the particle is suspended in the fluid, the assessing step comprises tracking the movement of the particle through the portion relative to the flow of the fluid through the portion, such as by a technique chosen from the group consisting of optical, infrared, ultrasonic, electrical, magnetic, and resonance sensing techniques. If the particle is not suspended in the fluid, movement of the fluid through the freestanding portion can be tracked by vibrating a freestanding portion of the tube at or near a resonant frequency thereof and monitoring changes in at least one of the signal peak gain and resonant frequency of the freestanding portion relative to time.  
         [0010]     The sensing device of this invention comprises a tube supported by a substrate and comprising a fluid inlet, a fluid outlet, and a portion between the fluid inlet and the fluid outlet so as to define a continuous passage through the tube, optionally at least one solid particle in the fluid and flowing through the passage, and means for assessing a rheological property of the fluid within the tube by tracking movement of at least one of the fluid and the particle through the portion.  
         [0011]     According to the invention, the viscosity and lubricity of a fluid can be assessed by observing the movement of a fluid through a resonating tube, and by observing the movement (or lack thereof) of particle(s) suspended in a fluid flowing through a tube portion and the influence that the particle or particles have on the fluid, such as the density, flow rate, pressure drop, etc., of the fluid. Various sensing techniques can be employed to sense the presence and movement of the particles through the tube portion and alter the behavior of the particles in order to enhance the sensitivity of the device to the Theological properties of the fluid within the tube. Advantageously, the device can be fabricated from a variety of materials using micromachining processes, enabling miniaturization of the device.  
         [0012]     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIGS. 1 and 2  are perspective and cross-sectional views, respectively, of a sensing device with a resonating micromachined tube through which a fluid of interest flows in accordance with an embodiment of this invention.  
         [0014]      FIG. 3  is a cross-sectional view of the sensing device of  FIG. 1  modified to magnetically alter the movement of particles being carried by a fluid flowing through the micromachined tube in accordance with further embodiments of this invention.  
         [0015]      FIG. 4  is a cross-sectional view of the sensing device of  FIGS. 1 through 3  in a fluid system that enables bidirectional flow of the fluid through the micromachined tube in accordance with another embodiment of this invention.  
         [0016]      FIGS. 5 and 6  are graphs representing two modes for sensing the presence and transit time of a particle flowing through the sensing device of  FIGS. 1 through 4  in accordance with this invention.  
         [0017]      FIG. 7  schematically represents a sensing system that makes use of a sensing device of this invention.  
         [0018]      FIG. 8  is a cross-sectional view of the sensing device of  FIGS. 1 through 2  in a fluid system that enables time-based sensing of viscosity of the fluid through the micromachined tube in accordance with another embodiment of this invention.  
         [0019]      FIG. 9  is a graph representing a mode for sensing the viscosity of the fluid flowing to and then through the sensing device of  FIG. 8 .  
         [0020]      FIG. 10  is a cross-sectional view of a sensing device with a stationary micromachined tube through which a fluid of interest flows in accordance with still another embodiment of this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]      FIGS. 1 and 2  represent a viscosity sensing device  10  suitable for use in various embodiments of the present invention. The device  10 , which may be termed a rheometer, is represented as being fabricated on a substrate  12 , which can be formed of silicon or another semiconductor material, quartz, glass, ceramic, metal, or a composite material. A tube  14  is supported by the substrate  12  so as to have a freestanding portion  16  suspended above a surface  18  of the substrate  12 , depicted in  FIGS. 1 and 2  as being defined by a recess in the substrate  12 . The freestanding portion  16  of the tube  14  is generally U-shaped, though other shapes—both simpler and more complex—are within the scope of this invention. The tube  14  defines a passage  20  through which a fluid can flow. Fluid is able to enter the device  10  through a fluid inlet  26  and exits the tube  14  through a fluid outlet  28  (one of which can be seen in  FIG. 2 ) etched or otherwise formed in a surface of the substrate  12  opposite the tube  14 .  
         [0022]     According to a preferred aspect of the invention, the tube  14  is micromachined from silicon or another semiconductor material, quartz, glass, ceramic, metal or composite material. As used herein, micromachining is a technique for forming very small elements by bulk etching a substrate (e.g., a silicon wafer), or by surface thin-film etching, the latter of which generally involves depositing a thin film (e.g., polysilicon or metal) on a sacrificial layer (e.g., oxide layer) on a substrate surface and then selectively removing portions of the sacrificial layer to free the deposited thin film. The tube  14  can either be fabricated entirely from layers of the chosen materials deposited on the substrate  12 , or fabricated in part by etching the substrate  12 . The shape and size of the tube  14  can be chosen to provide an adequate flow capacity for the fluid and to have suitable vibration parameters for the intended fluids to be evaluated with the device  10 . Because micromachining technologies are employed to fabricate the tube  14 , the size of the tube  14  can be extremely small, such as lengths of about 0.5 mm and cross-sectional areas of about 250 μm 2 , with smaller and larger tubes also being within the scope of this invention. Particularly suitable configurations and processes for fabricating resonant mass flow and density sensors using micromachining techniques are disclosed in commonly-assigned U.S. Pat. No. 6,477,901 to Tadigadapa et al., incorporated herein by reference, which uses wafer bonding and silicon etching techniques to produce a suspended silicon tube on a wafer.  
         [0023]     According to Tadigadapa et al., micromachined tubes of the type shown in  FIGS. 1 and 2  can be vibrated at or near resonance to determine the mass flow rate and density of a fluid flowing through the tube using Coriolis force principles. In the embodiments shown in  FIGS. 1 through 7  and  9 , the present invention also makes use of Coriolis force principles, though not solely for determining mass flow rate and density but also for the purpose of ascertaining the viscosity, lubricity, or other rheological property of a fluid flowing through the tube  14 . As in Tadigadapa et al., the freestanding portion  16  is vibrated in a direction perpendicular to the surface  18  of the substrate  12 , preferably at or near its resonant frequency. During half of the vibration cycle in which the tube  14  moves upward, the freestanding portion  16  has upward momentum as the fluid travels around the tube bends, and the fluid flowing out of the freestanding portion  16  resists having its vertical motion decreased by pushing up on that part of the freestanding portion  16  nearest the fluid outlet  28 . The resulting force causes the freestanding portion  16  of the tube  14  to twist. As the tube  14  moves downward during the second half of its vibration cycle, the freestanding portion  16  twists in the opposite direction. This twisting characteristic is referred to as the Coriolis effect, and the degree to which the freestanding portion  16  of the tube  14  deflects during a vibration cycle as a result of the Coriolis effect can be correlated to the mass flow rate of the fluid flowing through the tube  14 , while the density of the fluid is proportional to the frequency of vibration at resonance.  
         [0024]     The resonant frequency of the tube  14  is controlled by its mechanical design (shape, size, construction and materials). Resonant frequencies will generally be in the range of about 1 kHz to about 100 kHz for tubes of the type fabricating in accordance with Tadigadapa et al. The amplitude of vibration is preferably adjusted through means used to vibrate the tube  14 . As shown in  FIGS. 1 and 2 , a drive electrode  22  is located beneath the tube  14  on the surface  18  of the substrate  12 . As depicted in  FIGS. 1 and 2 , the tube  14  is formed of an electrically-conductive material, such as doped silicon, and can therefore serve as an electrode that can be capacitively coupled to the drive electrode  22 , enabling the electrode  22  to electrostatically drive the tube  14 . However, it is foreseeable that the tube  14  could be formed of a nonconductive material, and a separate electrode formed on the tube  14  opposite the electrode  22  for vibrating the tube  14  electrostatically. An alternative driving technique is to provide a piezoelectric element on an upper surface of the tube  14  to generate alternating forces in the plane of the tube  14  that flex the freestanding portion  16  of the tube  14  in directions normal to the plane of the tube  14 . Other alternatives are to drive the freestanding portion  16  of the tube  14  magnetically, thermally, piezoresistively, thermally, optically, or by another actuation technique. Also shown in  FIGS. 1 and 2  are sensing electrodes  24  for providing feedback to the drive electrode  22  to enable the vibration frequency to be controlled with appropriate circuitry (not shown) while also sensing the deflection of the tube  14  relative to the substrate  12 . The sensing electrodes  24  can sense the tube  14  capacitively, electrostatically, magnetically, piezoelectrically, piezoresistively, thermally, optically, or in any other suitable manner capable of sensing the proximity or motion of the tube  14 .  
         [0025]     In  FIG. 2 , the sensing device  10  is shown enclosed by a cap  30  to form a sensing package. The cap  30  allows for vacuum packaging that reduces air damping of the tube vibration. A variety of package and wafer-level methods exist to vacuum package devices. These include solder or weld hermetic packages, and wafer bonding using glass frit, solder, eutectic alloy, adhesive, and anodic bonding. A preferred material for the cap  30  is silicon, allowing silicon-to-silicon bonding techniques to be used, though it is foreseeable that a variety of other materials could be used for the cap  30 , including metals and glass materials, that latter including borosilicate glass (e.g., Pyrex). Input and output signals to the device  10  are made through bond pads  32  (only one of which is shown) outside the cap  30 . Since metal runners are used to transmit the electrical signals, and the capacitive signals produced by the tube  14  are relatively small, wafer to wafer bonding methods are preferred. Therefore, in the preferred embodiment of this invention, the bond between the cap  30  and the substrate  12  is hermetic, and the enclosure formed by the substrate  12  and cap  30  is evacuated to enable the tube  14  to be driven efficiently at high quality (Q) values without damping. In such an embodiment, a getter material is preferably placed in the enclosure to assist in reducing and maintaining a low cavity pressure. As an alternative to a hermetically sealed package, the tube  14  could be enclosed such that a vacuum can be drawn when desired through the use of a pump.  
         [0026]     According to one approach of the invention, a micromachined resonating tube capable of sensing mass flow rate and density in accordance with Tadigadapa et al. is sufficiently sensitive to exhibit detectible changes in the resonant frequency of the tube when one or more particles of sufficient density and/or suitable material is present in the fluid flowing through the tube. With this capability, the viscosity or lubricity of a fluid within the micromachined resonating tube  14  can be ascertained by observing the effect that such particles have on the flow of the fluid and/or the time required for the particle(s) to travel through the tube  14  relative to the flow rate of the fluid. As represented in  FIG. 3  (which is a section along the extent of the tube  14 ), particles  34  such as microbeads are represented as having been intentionally added to the fluid so as to flow with the fluid through the tube passage  20  within the freestanding portion  16  of the tube  14 . As suitable size for the particles  34  is a diameter of about one to one hundred micrometers, though larger and smaller particles  34  are also within the scope of this invention. While multiple particles  34  are represented in  FIG. 3 , it is foreseeable that a single particle  34  relatively large in relation to the passage  20  could be used. The functional intent of the particles  34  is to interact with the fluid and/or the walls of the passage  20  in a manner that enables rheological properties such as viscosity and lubricity of the fluid to be detected. For example, interaction of the particles  34  with the fluid that impedes the flow of the fluid through the tube  14  can be proportionally correlated to the viscosity of the fluid, and interactions between particles  34  and between the particles  34  and the walls of the passage  20  that impede the movement of the particles  34  through the tube  14  can be proportionally correlated to the lubricity of the fluid. Therefore, by using a fixed pressure or flow rate through the freestanding portion  16  and tracking the flow of the fluid through the tube  14  and/or the motion of the particles  34  within the tube  14 , the lubricity and/or viscosity of the fluid can be determined.  
         [0027]     In the embodiment of  FIG. 3 , in which the resonant frequency of the freestanding portion  16  of the tube  14  is known to correspond to the density of the fluid within the freestanding portion  16 , the use of particles  34  with densities different than (e.g., greater than) the fluid enables the presence of the particles  34  within the freestanding portion  16  to be detected by sensing a change in the resonant frequency of the freestanding portion  16  caused by the particles  34  entering (and leaving) the vibrating freestanding portion  16  of the tube  14 . The time duration that the particles  34  are present in the freestanding portion  16  can also be sensed, enabling the presence and speed of the particles  34  to be monitored. As an example,  FIG. 5  represents the effect that a single particle  34  of greater density than the fluid can have on the signal peak gain of the sensing device  10 , resulting in loss of resonance during that period of time in which the particle  34  is in transit through the vibrating freestanding portion  16  of the tube  14 . A similar decrease in quality factor (Q) of the tube  14  may also occur and be similarly tracked. By measuring the length of this disturbance, the transit time of the particle  34  can be readily ascertained. Alternatively, and as represented in  FIG. 6 , if the sensing device  10  is operated with feedback so that the drive electrode  22  maintains the freestanding portion  16  at resonance, the shift in frequency necessary to maintain the tube portion  16  at resonance can be correlated to a change in density of the material within the freestanding portion  16 , again evidencing the presence of the particles  34  within the freestanding portion  16 . In each case, the width of the peak corresponds to the transit time of the particle  34 , which in turn is proportional to the speed that the particles  34  are moving through the tube  14 . Because wider peaks correspond to greater resistance to movement of the particles  34  through the tube  14  resulting from frictional forces between the particles  34  and walls of the tube passage  20 , the peak width an be correlated to the lubricity or viscosity of the fluid.  
         [0028]     The efficacy of the technique described above can be appreciated on the basis of the densities of fluids relative to materials that can be used to form the particles  34 . In an investigation leading to the invention, various fluids were flowed through silicon and glass tubes formed in accordance with Tadigadapa et al. The measured densities (g/cc) of the different fluid-tube combinations were as follows:  
                                                                     Silicon Tube   Glass Tube                                        Coolant/antifreeze   1.12622   1.12598           Water   0.99904   0.99902           Motor Oil (10W40)   0.87785   0.87780           Kerosene   0.85450   0.85435           Diesel Fuel   0.82258   0.82255           Methanol   0.79592   0.79597           Gasoline   0.75954   0.75940                      
 
         [0029]     The density of steel is about 7.83 gm/cc and of hard rubber is about 1.95 gm/cc, suggesting that a single spherical particle  34  as small as about one hundred micrometers in diameter can cause a measurable change in the density, and therefore resonant frequency, of the tube  14  for the purpose of sensing and tracking the motion of the particle  34  through the tube  14 . In addition to steel and hard ruber, a variety of other materials could be used, particularly iron, nickel, cobalt, titanium and their alloys (including iron-nickel-cobalt alloys such as KOVAR®), stainless steels, low carbon steels, glass, ceramics, and various other materials meet the functional requirements of this invention. Preferred, sizes, densities, and materials for the particles  34  will depend on the size of the tube passage  20  and the sensing technique used, though it is believed that particles  34  with diameters on the order of about 50 to about 100 micrometers are suitable for use in a tube  14  whose passage  20  has cross-sectional dimensions of about 500 by about 200 micrometers.  
         [0030]     The configuration and etching of the tube  14  through which the fluid flows can be modified to increase the interaction of the particles  34  with the tube  14  and thereby improve the sensitivity of the device  10 . For example, the walls of the passage  20  can be corrugated or otherwise made rough during micromachining of the tube  14  to increase particle-wall interactions. If the tube  14  is fabricated by plasma etching, a two-step etch and passivation process often used in micromachining techniques can be adjusted to scallop the sidewalls of the tube  14 . Further modifications to the geometry of the passage  20 , including obstructions that protrude into the passage  20 , can also be incorporated into the design of the tube  14  to increase particle-wall interactions. Alternatively or in addition, the particles  34  can be formed to have rough surfaces or nonspherical shapes to enhance drag on the walls of the tube passage  20  by increasing surface area.  
         [0031]      FIG. 3  further represents the use of a magnetic field to promote the interaction of the particles  34  with the walls of the passage  20 . In particular, a magnetic field-generating device  44  in the form of a magnet film or coil, electromagnet, or charged plate located externally to the device  10  or integrated onto the device  10  can be used to apply a constant or variable force on particles  34  formed of a ferromagnetic material (such as iron, nickel, steel, etc.) so that, depending on the strength of the force, interaction of the particles  34  with the walls of the passage  20  is promoted and movement of the particles  34  within the passage  20  is slowed or even stopped. In the latter case, one or more particles  34  of adequate size can create a sufficient obstruction to cause a pressure drop or flow rate change in the tube  14  by which viscosity can be measured. Alternatively, by generating a magnetic field strength that slows the particles  34  and pulls them into contact with the walls of the passage  20 , the friction of the particles  34  with the passage walls and the resulting effect on particle motion can be used to measure lubricity of the fluid. By applying a variable magnetic field, both a static and kinematic or dynamic coefficient of friction/lubricity can be measured. As another alternative, it is foreseeable that the device  10  could be oriented such that gravity alone is sufficient to pull dense particles  34  into contact with a wall of the passage  20 . With each of these approaches, not only does the device  10  function to track the movement of the particles  34 , but the device  10  can also function to influence the movement of the particles  34  through the tube  14 .  
         [0032]     The sensing device  10  of  FIG. 3  can be adapted for use with a variety of sources for the fluid. For example, the fluid could be introduced into the device  10  with a syringe, syringe pump, pipette, or other type of pump, as can the particles  34  if suspended in the fluid from a second source. Premixed or presorted particles can be added to a fluid sample of known volume and mixed together prior to injection into the device  10 . In  FIG. 4 , the device  10  is shown installed between pairs of pumps  36  and  38  and filters  40  and  42 , the latter of which may be magnetic filters or mechanical filters such as simple screens or sieves attached to fittings that connect the pumps  36  and  38  to the device  10 .  FIG. 4  also shows a magnet  44  placed in proximity to the tube  14  to promote physical interaction between the particles  34  and the walls of the passage  20 . With the embodiment of  FIG. 4 , the particles  34  can be introduced into a closed fluid circuit through a valve  46 , and the flow of the fluid can be periodically reversed to move the particles  34  back and forth through the device  10  and its resonating tube  14  under controlled constant or various selected field and temperature conditions. Reversing the movement of the particles  34  from one filter  40 / 42  to the other  42 / 40  can be employed to pass the particles  34  through the tube passage  20  any number of times. The field strength of the magnet  44  can be varied while maintaining the fluid at a substantially constant temperature to measure an average transit time for the particles  34  through the passage  20 . Alternatively or in addition, the temperature of the fluid can be varied with a suitable heating element (not shown) while maintaining a constant field strength with the magnet  44  to obtain additional particle transit time/speed values indicative of temperature-related changes in lubricity and/or viscosity. When all particles  34  have exited the passage  20  and have been trapped against one of the filters  40  or  42 , the density, specific gravity or chemical concentration of the fluid can be measured in accordance with Tadigadapa et al. by continuing the flow of an additional amount of the fluid through the resonating tube  14 . In this manner the lubricity, viscosity, and density of the fluid can be measured, as well as the presence of any contaminants by comparing the sensed density to a known density value for the fluid from theoretical or empirical data. As an alternative to the embodiment shown in  FIG. 4 , it is foreseeable that a circulating fluid circuit could be employed to repeatedly pass the particles  34  through the passage  20 .  
         [0033]      FIG. 7  represents a suitable test system for employing the sensing device  10  of this invention. The source of the fluid is represented as a syringe pump  50 , though from the foregoing it is understood that a variety of fluid sources could be used. The system incorporates various components described in reference to  FIG. 4 , which are identified in  FIG. 7  with the same reference numbers. As seen in  FIG. 7 , the fluid passes through a filter  40  before the introduction of the particles  34  through a valve  46  and subsequent delivery of the particle-containing fluid to the sensing device  10 . Downstream of the device  10 , the fluid is accumulated in a reservoir  52  after flowing through a second filter  42 . In this manner, it is possible to periodically reverse the flow of the fluid through the device  10  between the pump  50  and the reservoir  52 . Alternatively or eventually, the fluid can be removed from the system through a drain  54 . Pressure sensors  56  are provided by which the pressure drop through the system and accumulation of particles  34  at the filters  40  and  42  can be monitored. The system is also shown equipped with a magnet  44  for the purpose described for the embodiment of  FIGS. 3 and 4 , namely, to promote interaction of the particles  34  with the walls of the passage  20  to enhance the sensitivity of the device  10  to viscosity and lubricity effects.  
         [0034]     Because viscosity is influenced by fluid temperature, the device  10  is shown in  FIG. 7  as also being equipped with a thermal control system  60  for sensing and regulating the temperature of the fluid flowing through the tube  14 . The fluid temperature will also influence the Young&#39;s and shear moduli of the materials from which the tube  14  is fabricated, causing the resonant frequency of the tube  14  to shift. By sensing the temperature of the tube  14 , and therefore effectively the fluid within the tube  14 , appropriate signal processing with on-chip circuitry  62  can be performed on the output of the device  10  to compensate for these temperature effects. The thermal control system  60  also enables the measurement of fluid properties and particle speed with respect to field strength at different temperatures, as discussed with reference to  FIG. 4 . The thermal control system  60  can comprise one or more Peltier (thermoelectric) panels, electrical resistance heaters, or other thermal devices. The thermal control system  60  can also include heat sinks and fans to achieve cooling and heating of the fluid within the device  10  to a desired temperature.  
         [0035]     Finally, the system of  FIG. 7  is shown as including a system controller  58  to monitor and control various components of the system. For example, feedback control of the thermal control system  60  enables the temperature of the fluid to be held at a stable temperature to within a few millidegrees Celsius. The system controller  58  also makes use of feedback from the pressure sensors  56 , pump  50 , and magnet  44  to enable the operation of the pump  50  and magnet  44  to be regulated with respect to each other and test parameters programmed into the system via the system controller  58 . Finally, the system controller  58  can receive the output of the device  10 , such as the frequency, amplitude, and phase of the freestanding tube portion  16  sensed by the sensing electrodes  24 , to control the drive electrode  22  for the purpose of maintaining the tube  14  at resonance.  
         [0036]      FIG. 8  represents another embodiment of the invention in which the viscosity of the fluid flowing through the device  10  of  FIGS. 1 and 2  can be measured without the use of particles  34 . Instead of sensing the presence and transit of particles through the tube  14  by sensing the influence that particles would have on the vibration characteristics of the tube  14 ,  FIG. 9  represents a fluid system that enables time-based sensing of viscosity of the fluid flowing through the micromachined tube  14 . In particular, the fluid is delivered by a suitable source  70  (e.g., any of those cited for the pumps  36  and  50  of previous embodiments) to a passage  74  through a suitable valve  72 . The passage  74  can be formed by a tube separately formed and attached to the device  10  or integrally formed within the substrate  12  of the device  10 . The timing of the opening of the valve  72  can be controlled and monitored by a controller  78  that also communicates with the device  10 , as represented by an electrical connection to the pad  32  on the device  10 . As an optional aspect of this embodiment, particles  34  of the type described previously can be controllably introduced into the fluid within the passage  74  through a second valve  76 , such as in the same manner described in reference to  FIGS. 4 and 7 . As represented in  FIG. 9 , the viscosity of the fluid can be measured by timing the interval required for the fluid (with or without particles  34 ) to travel from the valve  72  to the freestanding portion  16 , the latter event being sensed by a change in the resonant frequency of the tube  14  (e.g., resulting from the difference in density of the air initially within the tube  14  and the fluid that subsequently enters the tube  14 ). As in  FIG. 7 , the temperature of the tube  14  can be sensed and controlled to maintain a constant fluid temperature or vary the temperature of the fluid to more fully characterize the viscosity of the fluid. Using the same tube  14  and device  10  or a separate tube within the same or separate device, the lubricity of the fluid can also be measured to provide a single system capable of measuring the viscosity, density, and lubricity of the fluid.  
         [0037]      FIG. 10  represents an embodiment of the invention in which a technique other than a resonance-based approach is used to monitor the motion of one or more particles  134  within a fluid flowing through a micromachined tube  114 . Because tracking of the particles  34  is by means other than the resonant frequency of the tube  114 , the tube  114  can be stationary during operation of the sensing device  110 . As such, the passage  120  of the tube  114  in  FIG. 10  can be defined by a trench formed in a silicon, plastic, or glass substrate  112  and a cover  130  bonded to the substrate  112  to enclose the trench. As one example of this embodiment, the particles  134  can be formed of a ferromagnetic material and the presence and movement of the particles  134  through the tube  114  are monitored with a magnetic field generated by a magnet  144 . In another approach, the cover  130  is formed of glass or another material that is sufficiently optically translucent or transparent to permit optical monitoring of the particles  34  as they flow through the passage  120 , such as with a camera scope  144 . Metal runners  146  can be used to magnetically sense the passing of particles  134  formed from a ferromagnetic material. In view of the above, it can be appreciated that, depending on the properties of the particles  134  (e.g., temperature, density, electrical, magnetic, etc.), a variety of techniques can be used to track the particles  134  through the tube  114 , including infrared, ultrasonic, electrical, and magnetic sensing techniques. Filters of the type shown in  FIGS. 4 and 7  can also be used with this embodiment of the invention to repeatedly flow the same fluid back and forth through the sensing device  110 .  
         [0038]     As with the device  10  of  FIGS. 1 through 7 , devices  110  in accordance with  FIG. 10  are able to ascertain the viscosity or lubricity of a fluid within the tube  114  by observing the effect that the particles  134  have on the flow of the fluid and/or the time required for the particles to travel through the tube  114  relative to the flow rate of the fluid. As was previously noted with reference to  FIG. 3 , while multiple particles  134  are represented in  FIG. 10 , a single particle  134  relatively large in relation to the passage  120  could be used. Also in accordance with the previously-described device  10 , the operation of the device  110  is on the basis that interactions between the particles  134  and the fluid impede the flow of the fluid through the tube  114  and can be proportionally correlated to the viscosity of the fluid, and interactions between the particles  134  and the walls of the passage  120  impede the movement of the particles  134  through the tube  114  and can be proportionally correlated to the lubricity of the fluid. Therefore, by using a fixed pressure or flow rate through the tube  114  and sensing and tracking the movement of the particles  134  through the tube  114 , the lubricity and/or viscosity of the fluid can be determined. Because the functional intent of the particles  134  is to interact with the fluid and/or the walls of the passage  120  in a manner that enables the viscosity and lubricity of the fluid to be detected, any one or more of the various approaches noted previously for promoting particle-wall interactions can be employed in the embodiment of  FIG. 10  to enhance the sensitivity of the device  110 .  
         [0039]     In view of the above, the devices  10  and  110  of this invention are able to measure the viscosity and/or lubricity of a fluid, as well as detect changes in viscosity and lubricity that may occur over time such as when a lubricant or fuel breaks down or becomes contaminated. In the case of automotive engines, monitoring of the engine oil in this manner can be used to indicate when an oil change is needed, and fuel can be monitored to assess the condition of the fuel and fuel system, including wear of fuel system components such as fuel injectors. Historical data can be saved and compared to real-time data to determine if a lubricant, fuel, or lubrication or fuel system components have degraded or need replacement. Advantageously, the sensing devices  10  and  110  of this invention can be made extremely thin, permitting their installation on an engine as an aftermarket sensor module for truck and automotive applications. By also monitoring the resonant frequency of the tube  14  of the device  10 , the density of a fuel or lubricant can also be determined and used to indicate contamination.  
         [0040]     While the invention has been described in terms of certain embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.