RESONATOR SENSORS EMPLOYING PIEZOELECTRIC BENDERS FOR FLUID PROPERTY SENSING

Abstract: Resonator sensors namely piezoelectric benders (50, 52, 54) are used for sensing fluid viscosity and density. The benders (50, 52, 54) are inserted into a liquid to be monitored. An input voltage is applied to the bender causing it to vibrate in the liquid at a resonant frequency. Output voltage amplitude or frequency are monitored to determine changes in viscosity and density, respectively. The device may be used as a liquid level sensor and to control alteration of the viscosity of the fluid as, for example, by connection with a supply of ink or solvent in an ink jet printing device.

Description: RESONATOR SENSORS EMPLOYING PIEZOELECTRIC BENDERS FOR FLUID PROPERTY SENSING This invention relates to the field of fluid property sensing by which parameters such as fluid level, viscosity and density may be determined for a gas or liquid The prior art for such devices is extensive, mcluding devices dedicated to liquid level sensing, viscosity determination or density measurement The present invention relates to the class of sensmg devices generally known as resonating sensors The advantages of resonating sensors have been well documented and several commercial devices exist These include the vibrating wire viscometer, such as disclosed by James M Goodwin in Journal Of Physics. Volume 6, 1973, the Dynatrol Viscometer manufactured by Automation Products, Inc ; tuning fork type viscometers, for example, of the type disclosed by M. R Fisch, et al , J Acoustic Soc Am . Volume 60, No. 3, September 1976, p 623, and tuning fork type liquid level detectors, such as those manufactured by Endress & Hauser and Automation Products, Inc These pπor art devices have several disadvantages They require separate transducers in addition to a vibrating structure They are assemblies of several components requiring exotic materials and/or intricate machining thereby increasing costs Further, such devices are susceptible to corrosive materials, are larger than may be desired for some applications and have limited sensitivity ranges Perhaps most significant, is the fact that these devices offer only one function, for example, either viscometry or level detection, but not both It is desirable in fluid handling systems to monitor the viscosity of the fluid to ascertain changes in its viscosity which may indicate the need foi corrective action It is also nccessan to monitor the level of the fluid, which in the case of liquids, may mdicatc the need for replenishment Although there are many apphcations which have these requirements, one environment for which the present mvention in suited, is ink jet prmtmg In typical commercial ink jet prmtmg systems, an electrically-conductive, oftentimes corrosive, markmg fluid or ink is utilized for prmtmg information on products These may include date codes, bar codes, lot information and the like Ink jet printers are well known devices and will not be discussed in detail in this disclosure In order to keep such devices runnmg properly over extended periods of time, the properties of the markmg fluid must be monitored and modified when deviations from standard parameters are detected Thus, the viscosity and/or density of the markmg fluid must be monitored as well as the level of the fluid in the various reservoirs in which the markmg fluid are kept In such systems, it is important to provide measurements of the fluid properties and to do so in an efficient, compact and economical way Many of the prior art devices mentioned above can perform the desired functions combination, but cannot do so as efficiently as would be desired The present mvention provides fluid property sensmg apparatus and methods for monitoring properties of fluid including liquid level, viscosity and density According to the present invention, a piezoelectric bender is adapted to this purpose The term piezoelectric bender is meant to describe a class of devices which are commercially available, such as piezoelectric ceramic wafers which function as transducers between mechanical and electrical energy One source of such devices is Morgan Matioc, Inc ol Bedlord, Ohio Such devices are often used as audio tone generators and as stra gauges for measurmg tensile or compressive force, etc Piezoelectric benders, so-called because they are a composite structure which bends when a voltage is apphed, and, conversely generate a voltage when stramed One type of bender manufactured by Morgan Matroc is known in the trade as a Bimorph. The Bimorph is a composite structure of two transverse-expander piezoelectric plates bonded together This structure enhances the bend g characteristics of the device and simplifies the electromcs required to operate it According to the present mvention, piezoelectric benders are employed for the dual purpose of detectmg liquid level a fluid system and for monitoring the viscosity and/or density of the fluid The preferred embodiment accomplishes this by driving the piezoelectric bender at a voltage and frequency which cause it to resonate By monitoring the output voltage from the bender, it is possible to momtor changes m the viscosity of a liquid or gas Should the viscosity of such a liquid approach zero, this signifies that the device is no longer immersed in liquid, thereby performing level sensmg as well By measurmg the natural resonant frequency instead of output voltage, the density of a liquid can be monitored In general, the mvention relates to resonator sensors which may be implemented as fluid property sensors by measurmg the effects of viscous damping and mass loadmg on the vibrating structure of the sensor and convertmg these measurements to the fluid property values of interest via predetermined relationships These effects can be measured by monitoring an output magnitude and/or frequency. Other suitable techniques include measuring quality factor (Q), logarithmic decrement of free oscillation, decay time constant of free oscillation, electπcal impedance characteristics (motional impedance and resistance), dnve amphtude required to maintain constant output amphtude, and phase difference between input and output signals. The present invention provides a new class of fluid sensmg devices which are small, simple in construction, low in cost and which can accurately momtor properties of fluids including liquid level, viscosity and density. An apparatus embodying the mvention may be used m ink jet printing systems where the properties of liquid inks used in printing can be carefully monitored and altered when variations from desired specifications are detected. The mvention will now be described further by way of example with reference to the accompanying drawings in which: Figure 1 illustrates a piezoelectric bender; Figure 2 illustrates the use of such piezoelectric benders in an ink jet prmtmg system for use in monitoring the level and viscosity of ink: Figure 3 illustrates, in block diagram form, the manner in which the piezoelectric bender is operated to obtain useful information therefrom; Figure 4 is a data plot illustrating the manner m which the data obtained is utilized to determme viscosity and liquid level, and Figure 5 is a block diagram similar to Figure 3 for a smgle port bender. Referring to Figure 1 there is shown in cross-section the basic structure of a piezoelectric bender suitable for use accordmg to the present mvention Such devices are commercially available from a number of sources including Morgan Matroc, Inc of Bedford, Ohio Piezoelectric benders are piezoelectric ceramic devices made from lead metamobate and lead zirconate titanate compositions Such devices are available m a number of different configurations, mcludmg thin piezoelectric disks and double-plate ceramic elements. The device shown m Figure 1 is a cantilevered piezoelectric bender Unlike standard piezoelectπc crystals that only expand or contract, the composite structure of the device shown m Figure 1 causes it to bend when a voltage is apphed Conversely, when the element is stramed or flexed from its initial position, it generates a voltage. The bender illustrated is sold under the trade name Bimorph Two transverse-expander piezoelectric plates are bonded together They are identified in the drawmg as elements 10 and 12 Elements 10 and 12 are electrically isolated to simplify the electronics required to obta the result descπbed here Standard wire tacking techniques are used to attach the leads 14, 16 and 18 required for external connection to control circuitry The plates 10 and 12 are encapsulated in a pottmg material 20 In the embodiment illustrated the pottmg material forms a base or support element which can be used for mounimg the sensor to a vessel which the fluid to be monitored is contamed In the illustrated embodiment, a Bimorph device, the pottmg materials further serves to clamp one end of the bender to provide a cantilever structure This structure is not required, but us a preferred embodiment due to its simplicity The Figure 1 embodiment us preferred because it enhances the vibrating structure of the bender and because it is a suitable geometry to mterface with fluids The pottmg material also provides electrical insulation and protects the plates from corrosion The pottmg material may preferably be formed of a quartz glass composition although other materials, mcludmg epoxies, plastics or composites may also be suitable Referrmg to Figure 2, there is illustrated a fluid handling system in which the present mvention may be employed For purposes of exemplifying the mvention only, the illustration in Figure 2 relates to a fluid handling system of a cont uous ink jet prmtei Such a device mcludes a mixing vessel 30 containing a supply of markmg fluid or ink 32 A vessel 34 contams a supply of fresh ink which is used periodically to refill the mixing vessel as ink is consumed durmg the prmtmg processes A third vessel 36 contams a supply of solvent used to maintain the composition of the ink m mixing vessel 30 within desired parameters The solvent is added periodically to make up for evaporative losses as the ink in the mixing vessel 30 is recycled from a prmtmg head 40 The ink from the mixing vessel 30 is conducted by conduit 38 to a remotely located pπnthead 40 Selected drops are electrically charged and then deflected onto a surface bemg marked Uncharged drops are returned to the mixing vessel 30 by means of a collector 42 and a return conduit 44. Such an arrangement is typical for continuous-type ink jet printers and forms no part of the present invention. For more information concerning such systems, the reader is referred to U.S. Patent Nos. 5,424,766 and 4,860,027 hereby incorporated by reference. At the mixing vessel 30 it is desired to monitor the liquid level therein as well as the viscosity of the ink. This determines when it is necessary to provide additional fresh ink and/or solvent to prevent the ink from becoming too viscous due to evaporative losses. According to the present invention, this may be accomplished by using piezoelectric benders of the type illustrated in Figure 1 operated in a manner described hereafter. More specifically, the mixing vessel is preferably outfitted with three benders 50, 52 and 54. Bender 50 provides a dual function, namely, it determines both fluid level and viscosity. Benders 52 and 54, although capable of determining viscosity, are principally used to determine the level of the fluid in the mixing vessel 30. Additional benders in 56 and 58 may be provided in the fresh ink and solvent vessels 34 and 36 respectively for the same purpose, namely fluid level detection and, if desired, viscosity measurement. As illustrated in Figure 2, the encapsulated benders are mounted to the side of the vessels forming probes protruding into the vessels through openings provided therefor. The potting material 20 to which the one end of the plates is secured, serves as a surface for mounting the device to the vessel wall and to seal the vessel to prevent leakage. Thus, the piezoelectric plates 10 and 12. clamped at one end by the potting material 20 protrude into the mixing vessel to interact with any fluids contamed therem. The electrical leads 14, 16 and 18 extendmg from the benders 50 through 58, are connected to the control circuitry described hereafter for measurmg the viscosity and liquid level in the vessels With reference to Figures 1 and 3, the manner which the benders are employed to sense viscosity and fluid level will be explained As indicated previously, when the plates 10 has a voltage applied thereto, the plates 10 and 12 are caused to bend Applying an AC voltage causes vibration of the benders Accordmg to the present mvention, viscosity is measured by measurmg the effects of viscous damping on the vibration of the benders The preferred embodiment, illustrated m Figure 3, applies a voltage havmg an amplitude and frequency to optimize resonant vibration, of the bender That is, the vibrating portion of the bender is driven at its fundamental resonant frequency by applying to it a voltage signal of proper amphtude and frequency. The frequency of the voltage is dependent on the environment in which the sensor operates There are, however, many standard methods for determining the correct frequency Referrmg more specifically to Figure 3, there is illustrated an embodiment, which is presently preferred, for driving the bender at its natural resonance frequency by means of a feedback loop. In the illustrated embodiment, the bender is configured as two separate crystals, thus using three electrical connections The electrical connection on lead 16 is grounded to prevent cross-talk between the two plates 10 and 12 An AC voltage meter 60 is positioned to receive the output signal from lead 18 from the lower plate 12 of the bender. This meter provides data representmg the amphtude and frequency of the output signal This signal is also supplied to a controller 61 which is used to operate the mechanisms for addmg additional ink and solvent to the mixing vessel 30 m the case of an ink jet prmtmg apphcation of Figure 2 In other apphcations, other control circuitry may be employed to respond to the output signal 18 from the bender The output 18 is also provided to an amplifier 62 which amplifies the output signal to a level required to drive the bender without saturat g components. In order to drive the bender at is natural resonant frequency (for a given temperature and apphcation environment) the phase of the signal from the amplifier 62 must be shifted by approximately ninety degrees, as is well known m the electπcal arts This is accomplished by a phase shift circuit 64 which receives the output signal from the amplifier 62 and which, in turn, outputs a phase shifted signal as an input to the bender via lead 14, to plate 10 When the sensor is to be used with conductive fluids, such as contmuous jet inks, the fluid bemg monitored must be grounded to prevent cross-talk between the mput and output sides of the bender Alternatively a ground shield may be placed around the bender as part of the sensor structure Thus, this feedback circuitry is used to operate the bender by maintaining it at its natural resonant frequency Changes in the amphtude of the voltage obtamed on output lead 18 will then vary as a function of changes m the fluid bemg sensed as will now be described Due to configuration and electronic variables, however, the fascias may need to be optimized at slightly more or less than ninety degrees, but this can be easily determined for any given apphcation. . Referrmg to Figure 4. the manner m which the output voltage on lead 18 is utilized to determine viscosity and fluid levels will be expla cd Viscosity is measured by measurmg the effects of viscous damping on the vibration of the vibrating structure. In the present invention, the vibrating structure is the bonded plates 10 and 12. The output signal on lead 18 is proportional to the amphtude of vibration of the bender. This output signal can be correlated to viscosity as illustrated in Figure 4. As indicated, the voltage is an inverse function of viscosity. Specifically: V πns = 0.1 / (A *sqrt (n) + B) where: A and B are empirically determined constants; V„ is the root mean square voltage and n is viscosity. As viscosity increases, the output voltage decreases and vice versa. Very low viscosity values indicate that the sensor is no longer in a liquid. Thus, a liquid level detection function is also performed. The values A and B are unique to the sensor used to collect the data. The vibrating structure is driven at its fundamental resonant frequency by placing a voltage signal of proper amphtude and frequency on the bender. The frequency is dependent upon the environment in which the sensor operates and, for example, temperature can have a material affect on the frequency. By fitting an equation correlating output voltage and viscosity, a convenient method is provided for calibrating the sensor. Ideally, the manufacturing process for the benders produces sensors with minimal sample variation and it is not expected that individual calibration will be required. Because the performance of the benders is highly temperature dependent, temperature compensation algorithms must be used and arc easily developed by those skilled in th s art by fitting an equation to temperature cahbration data An alternative is to use a cahbration or reference sensor mamtamed at the same temperature Thus, for example, m this case of Figure 2, the output voltage from the bender 50 used to monitor the level and viscosity of the ink m the mixing vessel 30 may be compared with the output of the bender 56 m vessel 34 which contams only fresh ink In this manner, the temperature effect on the sensors may be nullified, provided that the two vessels are mamtamed at the same temperature As indicated previously, the sensor of the present mvention detects both the viscosity of a fluid in a vessel and hquid level m a vessel s ce the viscosity of a gas is significantly less than that of most fluids of interest It is for this reason that the benders 52 and 54 may be provided in the mixing vessel 30. Thus, when the ink level in the vessel is at or above the level of a bender, the viscosity readmg determined from the curve of Figure 4, will indicate the presence of a hquid On the other hand, when the bender is no longer immersed m a hquid, but is operating m air, its viscosity value will drop significantly, almost to zero, mdicatmg that the hquid is below the level of the bender Thus, benders 50, 52 and 54 m tank 30 provide an indication of the hquid level the vessel 30 by signalling a ma|or viscosity change as liquid drops below their positions Where the benders are intended for use with control systems as, for example, in the embodiment illustrated m Figure 2, the voltage output on lead 18 of Figure 3, is typically converted to an analog DC signal and then to a digital signal and provided to a micro-processor based controller 61 which, for example, will compare the mformation agamst a selected value or reference measurement and, when called for, add fresh ink or solvent to the mixing vessel 30 The reference measurement may be provided from bender 56 In addition, it can be programmed to determme alarm conditions such as a lack of mk in the mixmg vessel 30 as would be the case for example, if bender 50 were to signal a very low viscosity Thus, if fresh mk reservoir 34 was empty, eventually the mixmg vessel 30 would empty and require that prmtmg cease order to prevent unmtended operation of the system In summary, when the voltage output on lead 18 is less than a certain threshold (on the order of 80% of the voltage for air) this is an indication that hquid is present When the voltage amphtude is above that threshold, it means that the bender is no longer immersed m hquid and therefore the fluid level m the vessel has fallen below the position of the bender In the event that the voltage indicates that hquid is present at the bender, than the voltage magnitude may be used to determme the viscosity of the hquid This mformation can be used by a controller to modify that viscosity by the addition of solvent, fresh ink or other components as desired In addition to hquid level and viscosity, the present mvention can be used to make density measurements Density can be measured by measurmg the effects of mass loadmg on the vibration of the bender structure To do so, it is necessary to monitor the frequency of the output voltage signal rather than the magnitude This signal has a frequency equal to the fundamental resonant frequency of the vibrating structure which can be correlated to density The mam effect ot mass loadmg is to reduce the fundamental resonant frequency The density measurement capability ot the present mvention can be used to distinguish liquids from gas because the density of gas is, ot course, significantly less than that of a hquid In order to calibrate density with the frequency of the output voltage, a graph similar to Figure 4 is produced from data points taken for a given system. The graph would have density on its horizontal axis and frequency on its vertical axis. For a constant viscosity, the density of a fluid being monitored by the bender is determined by the equation: D=A f](l/Q) -B] "2 where A and B are constants Q is the resonant Q value and f is frequency As indicated previously, other piezoelectric ceramic devices can be used according to this invention, including another Morgan Matroc bender sold under the trade name Unimorph® which is a thin piezoelectric bender with two surfaces of dissimilar composition. Unlike the Bimorph® bender which is a two-port device, the Unimorph is a one-port device. A method for measuring the effects of viscous damping and mass loading that does not require two ports must be used to employ a one-port bender as a fluid properties sensor. One such method is that of measuring the changes in motional impedance or resistance as described for example, in "Acoustic Wave Micro Sensors" Analv. Chem. 1993 #65, pp. 987-996. Motional resistance, R, is the change in circuit resistance due to viscous damping. It can be measured using a network analyzer or equivalent circuit. Viscosity, v, is related to motional resistance as follows: v = K fR Λ 2 / r where: K is a constant, R is the motional resistance, r is density, and f is the resonant frequency of the bender. Referrmg to Figure 5, there is shown a block diagram of a circuit for use with a one- port bender in order to determme viscosity by measurmg motional resistance R The one-port or Unimorph device 70 has dissimilar surfaces 72 and 74 One surface is electrically connected via lead 76 to a network analyzer or similar circuit mdicated at 78 For example, a Hewlett Packard HP 8751 A Network Analyzer is suitable The other surface is grounded via lead 80 In operation, a voltage from the analyzer is apphed to the bender via lead 76 The analyzer, as is well known, measures the resistance of the circuit, which m this case us the motional resistance R The viscosity is then computed using the above formula

Claims: CLAIMS 1 A method of monitormg and mamtammg the level of hquid m a vessel comprismg the steps of : (a) locatmg at least the vibrating portion (10, 12) of a piezoelectric bender (50, 52, 54) m said vessel conta mg said hquid, (b) applymg a stimulation voltage to said bender (50, 52, 54) at an amphtude and frequency to cause the bender to resonate; (c) monitormg an output voltage from said bender, said output voltage amphtude varymg as an mverse function of the hquid viscosity whereby an mcrease m hquid viscosity produces a decrease m said output voltage amphtude and vice-versa; (d) detecting when the output voltage amphtude indicates that the bender is no longer m contact with said hquid; and (e) replenishing the hquid in said vessel when the hquid is no longer in contact with said bender. 2 A method as claimed in claim 1 , wherein the step (b) includes the substep of employmg a feedback circuit (62, 64) to amplify and phase shift said output voltage to produce said stimulation voltage apphed to said bender 3. A method of monitormg the density of a fluid comprismg the steps of (a) locatmg at least the vibratmg portion (10, 12) of a piezoelectric bender (50, 52, 54) withm said fluid: (b) applymg a stimulation voltage to said bender at an amphtude and frequency to cause the bender to resonate: and (c) monitoring an output voltage frequency from said bender, said output voltage frequency varymg as an mverse function of the fluid density whereby an mcrease in fluid density produces a decrease m said output voltage frequency and vice- versa. 4 A method as claimed m claim 3 further mcludmg the steps ot. (d) positioning the vibratmg portion (10, 12) of said bender in a vessel contammg a hquid; and (e) monitormg the hquid level in said vessel by detecting when the output voltage frequency mdicates that the bender is no longer contact with said hquid. 5. A method for monitormg the viscosity of a fluid comprismg (a) locating the vibrating portion of a piezoelectric bender (50, 52, 54) in contact with said fluid; (b) applymg a stimulation voltage to said bender (50, 52, 54) at an amphtude and frequency to cause said bender to resonate and to produce an output voltage, the amphtude of which varies as an mverse function of fluid viscosity, whereby an increase fluid viscosity produces a decrease output voltage amphtude and vice-versa: and (c) altermg the fluid viscosity responsive to the amphtude of said output voltage 6 Apparatus for monitormg the viscosity of a fluid comprising (a) a piezoelectric bender (50, 52, 54, 56, 58) having an clement ( 10. 12) which vibrates upon apphcation of a stimulation voltage thereto and which is positioned to contact said fluid; (b) means (62, 64) for applying a stimulation voltage to said bender at an amphtude and frequency to cause said bender element (10, 12) to resonate and to produce an output voltage, the amphtude of which varies as an inverse function of fluid viscosity, whereby an increase in fluid viscosity produces a decrease in output voltage amphtude and vice-versa; and (c) means (34, 36, 61) for altering the fluid viscosity responsive to the amplitude of said output voltage. 7. Apparatus for monitoring the viscosity of a hquid in a vessel (30) comprising: (a) a piezoelectric bender (50, 52, 54) having an element (10, 12) which vibrates upon apphcation of a stimulation voltage thereto and which is positioned to contact said hquid; (b) means (62, 64) for applying a stimulation voltage to said bender at an amphtude and frequency to cause said bender element to resonate and to produce an output voltage, the amplitude of which varies as an inverse function of hquid viscosity, whereby an increase in viscosity produces a decrease in output voltage amphtude and vice-versa; and (c) means (34, 36, 61) for altering the hquid viscosity responsive to the amphtude of said output voltage. 8. An apparatus as claimed in claim 1, further including means (61) for detecting the level of the hquid in said vessel (30) responsive to a change in said output voltage indicating that said bender is no longer in contact with said liquid. 9 An apparatus as claimed m claim 8 mcludmg means (34, 36, 61) for add g hquid to said vessel (30) responsive to said means for detectmg 10. Apparatus for monitoring the density of a hquid comprismg (a) a piezoelectric bender (50, 52. 54) havmg an element (10, 12) which vibrates upon apphcation of a stimulation voltage thereto and which is positioned to contact said fluid, (b) means (62, 64) for applymg a stimulation voltage to said bender at an amphtude and frequency to cause said bender element ( 10, 12) to resonate and to produce an output voltage, the frequency of which vanes as an mverse function of liquid density, whereby an mcrease in density produces a decrease m output voltage frequency and vice-versa; (c) a vessel (30) which said hquid is contamed; and (d) means (61) for monitormg the hquid level in said vessel by detectmg a change in said output voltage frequency which mdicates that said bender is no longer contact with said hquid 11 A resonator sensor for monitormg changes in the properties of a fluid such as density and viscosity comprismg: (a) a piezoelectric bender (50, 52. 54), (b) means (62. 64) for causmg said bender to vibrate in said fluid at a resonant frequency: (c) means (61) tor monitormg the changes in electrical characteristics of the vibratmg bender to determme changes m said fluid properties: and (d) means (34, 36, 61) tor altering the hquid viscosity responsive to said changes said electrical characteristics 12 A sensor as claimed m claim 1 1 wherem said piezoelectric bender is a one-port device (70) and the electπcal property monitored is related to the change in the motional resistance of said fluid 13 A sensor as claimed m claim 1 1 wherem said piezoelectric bender is a two-port device ( 10, 12) and the electrical characteristic monitored is voltage amphtude or voltage frequency at resonance

Inventors: ANDERSON PHILIP D, ANDERSON, PHILIP, D.
Applicants: GEN ELECTRIC CO PLC, THE GENERAL ELECTRIC COMPANY, PLC
Application year: 1997
Main classification: G01F23
IPC codes: B41J2, G01F23, G01N9, G01N11