Abstract:
Embodiments of the present invention are directed to a MEM viscosity sensor that is configured to be operated submerged in a liquid. The MEMS viscosity sensor comprises a MEMS variable capacitor comprising a plurality of capacitor plates capable of being submerged in a liquid. An actuator places a driving force on the variable capacitor which causes relative movement between the plates, where the movement creates a shear force between each moving plate and the liquid, which damps the movement of the plate and increases the capacitor&#39;s response time to the applied force in accordance with the liquid&#39;s viscosity. To determine the actual viscosity of the liquid, a sensor is coupled to the variable capacitor for sensing the response time of the plates as an indicator of the liquid&#39;s viscosity.

Description:
FIELD OF THE INVENTION 
   The present invention is directed to a microelectromechanical system (MEMS) and in particular, to a device having a plurality of plates for measuring fluid viscosity. 
   BACKGROUND OF THE INVENTION 
   Many mechanical systems, such as vehicles, require liquids for their operation. To extend the life of the equipment, such liquids must be maintained, including the replacement of spent and degraded liquids. 
   Most systems are maintained on a scheduled basis, which is not dependent upon the actual condition of the liquid, but rather, is based upon the general understanding of the liquid&#39;s life. For example, most drivers change the oil in their vehicles at predefined mileage intervals, such as every 3000 miles. Although this is beneficial, it fails to account for other factors that may affect the condition of the liquid. For example, a failure in the equipment could cause the liquid to degrade at a faster rate than that anticipated by the normal replacement schedule. Similarly, the condition of the equipment might be such that the liquid&#39;s useful life is extended. Thus, costs can be increased due to poorly operating equipment or due to unnecessary liquid replacement. If the equipment requiring maintenance is a fleet, the expense could be quite large. 
   Currently, electrochemical sensor systems exist that allow the monitoring and measurement of the condition, or health, of such liquids. Indeed, many monitoring systems exist which measure a variety of liquid parameters, including, dielectric constant, conductivity, pH and the amount of water in the liquid. Although such measurements are useful, taken in isolation they do not necessarily identify the health of the liquid unless the user is also aware of the measurement&#39;s history. For example, if the value of a particular parameter rises or falls as the liquid degrades, the liquid&#39;s health would be unclear from a single measurement. Rather, the parameter&#39;s history would be required to accurately assess the liquid condition. 
   One solution to the problems presented by electrochemical measurements is to measure liquid viscosity that, by itself, can be a good indicator of liquid health. The measurement of viscosity does not suffer from the historical problems associated with electrochemically measured parameters of the liquid. Indeed, if viscosity increases monotonically with operating time and, if a liquid&#39;s viscosity degradation profile is known in advance, then its health can be determined by a single measurement regardless of when the measurement is taken. 
   As viscosity is not directly measured by electrochemical sensors, but rather, is measured by the application of mechanical forces, including compressive forces and shear forces, a separate measuring sensor must be used. One approach uses a vibrating quartz or piezoelectric element that measures the shift in a device&#39;s resonant frequencies in response to applied vibrations, which in turn is a measurement of the damping value Q and thus of viscosity. The measurement of Q, however, is not a linear measurement, and thus will not be useful for a wide viscosity range. Further, this manner of measuring viscosity introduces complexities because it applies both compressive and shear forces. The contributions of both components to the net response can complicate data interpretation and limit operating range. 
   A more desirable and accurate viscosity measurement is obtained with a shear force measurement. One shear force measurement technique involves the dropping of large balls through cylinders filled with the liquid to be measured. As the ball moves through the liquid, the shear force resulting between the moving balls and the liquid can be measured. Although this technique is accurate, it is riot useful for smaller systems or equipment, and thus, is limited in its applications. 
   As seen from above, although viscosity measurements are desirable, current liquid monitoring systems require separate electrochemical and viscosity sensors to monitor both liquid health, and also the other liquid characteristics available from an electrochemical sensor. Indeed, most users will not simply replace their electrochemical sensors and rely solely on a viscosity measurement. 
   A need in the industry exists for a measurement system that provides the ability to measure the health of a liquid in a system, wherein the measuring system resides within the system. A further need in the industry exists for a viscosity sensor that utilizes a shear force measurement technique that can be used in small or confined environments, and can be combined with other sensors. 
   SUMMARY OF THE DISCLOSURE 
   Embodiments of the present invention are directed to a MEM viscosity sensor that is configured to be operated submerged in a liquid. The MEMS viscosity sensor comprises a MEMS variable capacitor comprising a plurality of capacitor plates capable of being submerged in a liquid. An actuator places a driving force on the variable capacitor which causes relative movement between the plates, where the movement creates a shear force between each moving plate and the liquid, which opposes the movement of the plate and increases the capacitor&#39;s response time to the applied force in accordance with the liquid&#39;s viscosity. To determine the actual viscosity of the liquid, a sensor is coupled to the variable capacitor for sensing the response time of the plates as an indicator of the liquid&#39;s viscosity. A feature of preferred embodiments is that the liquid sensor can be small in size. The sensor can thus be placed within a system for directly measuring the health of a system liquid on a continuous basis. It can also alert the user to equipment failure as it relates to the rate of degradation of the equipment liquid. 
   The MEMS structure allows for the integration of the sensor with other electronic circuits used for monitoring liquid health, while the bulk manufacturing of MEMS devices reduces costs. 
   Another feature of preferred embodiments is that a mechanical, thermal, electromagnetic, and chemical evaluation of liquids can be integrated into a single sensor, allowing the number and cost of separate sensors to be reduced, and multiple parameters measuring the health of a liquid to be monitored by a single component. 
   The preferred system measures viscosity via the application of a shear force, which reduces measurement errors and widens measurement ranges. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The detailed description of embodiments of the invention will be made with reference to the accompanying drawings, wherein like numerals designate corresponding parts in the figures. 
       FIG. 1  is an elevation view of a viscosity sensor in accordance with a preferred embodiment of the invention. 
       FIG. 2  is an elevation view of a comb capacitor having interdigitated plates in accordance with the embodiment of  FIG. 1 . 
       FIG. 3  is a graph of viscosity sensor response time. 
       FIG. 4  is a graph depicting viscosity response time versus viscosity for various measured liquids. 
       FIG. 5  is a graph depicting the response time of the various measured liquids of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention are directed to a MEMS viscosity sensor  10  disposed on a supporting substrate  12 . The viscosity sensor  10  is configured to directly measure the ability of the liquid to reduce friction. Fabrication techniques to create such a MEMS are known in the art and are described in U.S. Pat. No. 6,159,385, and U.S. patent application, entitled Microelectromechanical System (MEMS) Devices and Fabricating Methods, Ser. No. 10/454,031, filed on Jun. 2, 2003, which are fully incorporated herein by reference. In preferred embodiments, during use the MEMS viscosity sensor is submerged in a liquid. Use of a submersible MEMS device is described in pending application Ser. No. 10/227,141 entitled Liquid Medium Submerged MEMS Device, which is fully incorporated herein by reference. 
   With reference to  FIG. 1 , in one preferred embodiment, the viscosity sensor  10  comprises a transverse, centrally located, compliant suspension  14  carrying a longitudinally-extending arm  16 . The arm  16  includes transverse ends  18  and  20  coupled to compliant, electrically conductive suspension beams  22  and  24 , via electrically insulating bridges  26  and  28 , respectively, fabricated of, for example, silicon dioxide. In preferred embodiments, the arm  16  and suspensions  14 ,  22  and  24  are mechanically coupled together to move longitudinally as a single unit with respect to the substrate  12 , and form a motion actuator. The bridges  26  and  28 , however, electrically isolate the arm  16  from the electrically conductive suspensions  22  and  24 . The suspension  14  is coupled at its opposed outer ends to anchors  30  and  32  affixed to the substrate  12 . Similarly, the outer ends of suspensions  22  and  24  are coupled to anchor pairs  34 ,  36  and  38 ,  40  respectively, affixed to the substrate  12 . 
   The sensor  10  further comprises comb sense capacitors  42 ,  44 ,  46  and  48  (also known as interdigitated capacitors) for providing to an external output circuit signals representing the displacement of the arm  16  from its rest position. A pair of comb capacitors  42  and  44  straddle the arm  16  adjacent to the right end suspension  24 . Similarly, a pair of comb capacitors  46  and  48  straddle the arm  16  adjacent to the left end suspension  22 . Since the comb capacitors  42 ,  44 ,  46  and  48  are identical, only the right hand comb capacitor  42  will be described. 
   With reference to  FIG. 2 , the comb capacitor  42  comprises a fixed member  50  having a plurality of cantilevered support members  52 . Comb fingers  54 , also referred to as comb plates, extend longitudinally from the support member  52  to provide a large surface area for interacting with liquids. The capacitor  42  further comprises a plurality of connectors  56  cantilevered from the moveable arm  16 . Comb fingers  58 , also referred to as comb plates, extend longitudinally from connectors  56 , and are configured to interleave with the comb plates  54 . Similar to fixed comb plates  54 , moveable comb plates  58  also provide a large surface area for interacting with liquids. The comb plates  54 ,  58  are made from electrically conductive materials, such as silicon on which metals or alloys may be coated or plated onto said plates. In preferred embodiments, the plates are thin enough to reduce or eliminate compressive forces that may be created by their movement through a medium, such as, a liquid. The fixed member  50 , cantilevered support members  52 , cantilevered connectors  56 , and combination of the interleaved stationary and moveable comb plates  54  and  58 , appropriately connected to a drive actuator  60 , forms a variable capacitor whose capacitance varies with the amount of overlap between plates  54  and  58 . 
   With further reference to  FIG. 2 , the sensor  10  is coupled to a drive actuator  60 , a sensor  62  and a read-out  63 , which are electrically coupled to the capacitor. In preferred embodiments, the drive actuator  60  can be an electrostatic actuator, or a Lorentz force actuator. Regardless of the type of drive actuator utilized, the drive actuator  60  causes transverse suspensions  22 ,  24  to move bridges  26 ,  28  longitudinally in the plane of  FIG. 1  such that plates  54  move parallel to plates  58 . 
   In preferred embodiments, either an electrostatic actuator or a Lorentz force actuator can be utilized to move the arm  16 , and thus cause the comb plates  54  to move relative to the fixed comb plates  58  with which the plates  54  are interleaved. Alternate actuation methods such as thermal, electromagnetic, and piezoelectric may also be used as known to those skilled in the art. A description of electrostatic actuators suitable for use in embodiments of the invention are described in U.S. Pat. No. 5,025,346, entitled Electrostatic Comb Drive Actuator, which is incorporated herein by reference. A description of Lorentz force actuators suitable for use in embodiments of the invention are described in U.S. patent application Ser. No. 10/213,951, entitled A Lorentz Force Microelectromechanical System (MEMS) and A Method for Operating Such A MEMS, which is incorporated herein by reference. In one example with a Lorentz force actuator, a magnetic field source, such as a permanent magnet or electromagnet typically disposed above or below the substrate  12 , provides a magnetic field. Connections to one or more external circuits are made via anchors  34 ,  36 ,  38  and  40  carrying the suspensions  22  and  24 , to which the anchors are electrically connected. If current flows from a connected external circuit through the suspension  22 , a Lorentz force causes the arm  16  and the moveable portions of the interconnected compliant suspensions  14 ,  22  and  24  to move laterally as indicated by the arrow  64  ( FIG. 1 ). As the current flowing through one of the suspensions  22  varies, the distance that arm  16  moves varies, to vary the overlap between the comb plates  54 , and  58 , thus varying the capacitance between them. 
   If the capacitors are immersed in a liquid, the liquid will dampen the movement of the comb plates  54 ,  58  upon the application of the driving force from the drive actuator  60 . The liquid has an initial viscosity value and a degraded viscosity value which may change as a function of operating time. The response time of the device, as determined through the capacitive sensing, provides a measure of the fluid viscosity. If the degraded viscosity value changes relative to the initial viscosity value there will be a corresponding change in time response. The measured response time will be related to the new viscosity value, which in turn, is related to the health of the liquid. 
   By connecting the capacitors to a sensor  62  that may comprise, for example, a resonant circuit or a high frequency a.c. bridge, minute displacement of the comb plates  58  may be sensed capacitatively and converted into a voltage. The sensor  62  preferably includes a timing means for sensing the time for the comb plates  58  to move from, for example, 30% of the maximum displacement to, for example, 70% of the maximum displacement. The voltage and time is used to determine the response time of the comb plates  58 , and thus, calculate a viscosity value. As shown in  FIG. 4 , such response time and viscosity of the liquid within which the sensor is immersed are related linearly as plotted along log/log axes. The viscosity may be displayed directly on a read-out  63 . Any suitable device for displaying the viscosity can be used, including, but not limited to, a digital display. 
   The viscosity sensor  10  is configured in a manner which allows at least the comb capacitor  42  to be immersed in a liquid medium. For example, as shown in  FIG. 1 , a set of walls  40  extending from the substrate surrounds the sensor, allowing the liquid&#39;s health to be directly measured. The enclosure created by the walls  40  does not include a top and thus, the sensor is exposed. In this regard, liquid can fill inside the walls and cover the comb capacitors  42 ,  44 . If the viscosity sensor  10  is submersed in liquid, the walls assist in reducing agitation of the liquid nearest the comb plates. 
   The configuration of the comb plates  54 ,  58  allow the viscosity to be measured via a shear force. Indeed, when immersed in a liquid, as the comb plates  54 ,  58  interleave due to the force applied by the drive actuator  60 , a shear force between the liquid and the surface area of the movable comb plates  58  are created as the comb plates  58  move through the liquid. The more viscous the liquid, the more resistance is encountered by the comb plates  58  as they move through the liquid. Conversely, the less viscous the liquid, the less resistance is encountered by the comb plates  58  as they move through the liquid. Due to the configuration of the comb plates  54 ,  58  only a shear force is created by the movement of the movable comb plates  58  through the liquid, and thus, the viscosity measurement is more accurate as there are no compressive forces included in the measurement. 
   As the viscosity of a liquid affects the mechanical performance of a MEMS device, various measurements can be utilized to measure the viscosity of the liquid based upon the response of the MEMS device to an applied force. In preferred embodiments, a measurement of the damping force, that is, the amount of time which is required for the connectors  56  to cease moving in response to the applied force can be used to determine the viscosity of the liquid. To measure the damping force, the step response time or the resonant frequency shift is measured. 
     FIG. 3  depicts an ideal step function representing the applied force. The graph depicts viscosity sensor response time, showing the capacitance, which is a function of position, versus time. In response to a step function input the ideal system instantaneously displaces a maximum value, which is normalized to 1. To determine the response times of various liquids, the response of a device submerged in a liquid to a step function is measured between two preset values, for example, between 30% and 70%. The results of the measurement of the viscosity for various liquids for the preset values are plotted in  FIG. 4 .  FIG. 4  depicts the viscosity of the various measured liquids as determined by the response time against a viscosity standard (straight line). As shown in  FIG. 4 , there is a high linearity of response time and viscosity measurement. The health of the liquid can be determined based upon the measured value of viscosity, compared to the healthy viscosity profile for the given liquid being measured. In the described preferred embodiments of the invention, changes in viscosity of 1% can be detected by the sensor  10 .  FIG. 5  depicts the standard response times of a MEMS device as a graph of the MEMS capacitance change in various fluids versus time. As the over-damped step response time varies positively with viscosity, the viscosity of the fluid can be plotted against the response time. Each response time (traces) depicted in  FIG. 5  represents a measurement of viscosity versus response time as plotted on  FIG. 4 . 
   Embodiments of the device can be utilized in a variety of situations in which measurements determining the health of a liquid are desired. For example, a sensor can be installed in the oil tank of a vehicle, machine, or in a separate testing apparatus to which liquid samples are brought. In operation, the drive actuator  60  causes a drive voltage to be applied to the suspensions  22 ,  24 . Assuming a Lorentz force actuator as an example, if current flows from an external circuit through the suspensions, in the presence of a magnetic field, a Lorentz force causes the arm  16  and suspensions  14 ,  22  and  24  to move as a single unit with respect to the substrate  12 . As the suspensions  14 ,  22  and  24  move, the arm  16  is linearly translated in the direction of arrow  64  ( FIG. 1 ), which in turn moves the connectors  56  and moveable comb plates  58 . This causes the comb plates  58  to interleave with the fixed comb plates  54 , thereby causing a change in the displacement and, hence capacitance, between the comb plates  58 . The change in the displacement of the comb plates  58 , as well as the response time, is measured by the measurement element  62 . The change in the displacement as a function of the response time allows the user to determine the viscosity of a liquid by correlating the response time of the measured displacement to known viscosity standards for the particular liquid and thus, to determine whether the measured liquid is healthy. A viscosity measurement which is based upon the displacement measurement of the comb plates is displayed in the read-out  63 . 
   Although the foregoing described the invention with preferred embodiments, this is not intended to limit the invention. Indeed, embodiments of this invention can be combined with other sensors and systems, such as other lubrication health sensors. In other embodiments, the viscosity sensor can be directed integrated with a stationary interdigitated sensor where the interdigitated sensor is created on the same chip by the same fabrication process for electrochemical sensing. In some embodiments, a separate element, such as a separate thin plate, can be included in addition to a sensing capacitor where the thin plate is configured to move with respect to the substrate and produce a shear force which is measured by a sensor. In still other embodiments, the viscosity sensor can be combined with a temperature sensor which distinguishes degradation from temperature dependent viscosity changes. As seen from the foregoing, the embodiments of the viscosity sensor are intended to be used as a stand alone sensor or in combination with other types of sensors. In this regard, the foregoing is intended to cover all modifications and alternative constructions falling within the spirit and scope of the invention as expressed in the appended claims, wherein no portion of the disclosure is intended, expressly or implicitly, to be dedicated to the public domain if not set forth in the claims.