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OSCILLATING INERTIAL MICROBALANCE
AND METHOD OF PASSIVE
BACKGROUND OF INVENTION 5
This invention relates to oscillating inertial microbalance mass measurement devices and, more specifically, to mass measurement devices that reduce the adverse effects that temperature, pressure, force, and modulus of elasticity have 10 on the true indication of mass. It is known in the art that vibrating systems have a large dependency on their environment, materials of construction, and method of manufacturing.
A significant contribution to the adverse effects can come 15 from a non-ideal oscillating element that is at the heart of the classical oscillating inertial microbalance. Exemplary construction and operation of such oscillating inertial microbalance mass measurement devices are disclosed in U.S. Pat. Nos. 4,391,338, 6,444,927, and 6,784,381 which patents 20 hereby are incorporated herein by reference in their entirety.
An ideal oscillating element is one that has the ability to indicate the mass of the collected matter without the need to compensate for external outside acting forces such as temperature, pressure, modulus of elasticity, external vibration 25 or any other force that may create an adverse effect on the true indication of the mass.
For instance, a non-ideal oscillating element can exhibit a change in the modulus of elasticity with temperature. It is difficult to precisely determine how to apply corrections for 30 temperature when the adverse effects are comprised of those from the fluid properties of the sampled gas and those errors contributed by a change in the modulus of elasticity of the oscillating element has with temperature.
The problems associated with maintaining the gas stream and the oscillating element at a constant temperature are discussed in U.S. Pat. No. 6,080,939 and methods for controlling the same are discussed in U.S. Pat. No. 6,444, 927
Further, instruments used to measure a particular parameter may be affected by the variation of other parameters. For example, measurement of the mass of material deposited on an oscillating inertial microbalance may be adversely affected by a variation in temperature and/or pressure and/or 45 changes in modulus of elasticity of the oscillating element of the oscillating inertial microbalance.
A microbalance, examples of which are described in U.S. Pat. Nos. 3,926,271 and 4,391,338, typically comprises an oscillating element mounted with one end fixed and the 50 other end free. The free end typically has a filter (or other mass-receiving element) mounted thereto.
When a microbalance is configured with a hollow oscillating element, the fluid is typically drawn through the filter and through the oscillating element, thereby trapping sus- 55 pended particles within the fluid in or on the filter.
The resulting increase in the mass of the filter results in a decrease of the resonant frequency of the oscillating element. The decrease in the resonant frequency of the oscillating element is related to the increase in mass of the filter, 60 which in turn is representative of the mass of the suspended particles trapped in or on the filter.
Because the oscillating element has the ability to continually indicate the mass of the suspended particles it is an ideal means for indicating the change in mass of the suspended 65 particles trapped in or on the filter in near real time or over a measured period of time.
A microbalance, an example of which is described in U.S. Pat. No. 6,205,842, attempts to address the problem of the adverse mass indication as a result of the volatile components in the sample stream. The adverse mass indication is compensated for by having two substantially similar mass detectors where a particulate removal means is provided on one of the mass sensors to which the other is compared. A switching means is provided and equations are utilized to remove said adverse effects, enabled by the use of the switching means.
An attempt to remove the mechanical complexity of the switching means mentioned above is noted in U.S. Pat. No. 6,502,450, wherein only one sensor is used with the same particle removing means in a single flow path. The switching of a particle removal means and associated timing gives rise to the apparent loss in mass by using a formula. A further improvement of this patent, or a simplification to remove the adverse effects of volitization of the gas, is described in U.S. Pat. No. 6,651,480.
The goal of the three aforementioned patents is to address the adverse effects created by gas volitization in an inertial microbalance. They are not designed to remove the sensitivities an inertial microbalance has to the modulus of elasticity with temperature and the density change within the hollow oscillating element as a result of temperature and pressure. None of these patents provide for a continuous indication of the mass in near real time. Instead, they require that the particulate laden sample stream be interrupted, results in the disruption of the sampling cycle which can in turn potentially lead to the missing of an episode wherein a large concentration of the particulate sample is ignored.
Additionally, as the temperature of the microbalance's oscillating element changes, the resonant frequency of the oscillating element may change, even though the mass in or on the filter secured to the oscillating element may remain unchanged. As the measured mass is based on the resonant frequency, an error is introduced in the mass determination. This temperature sensitivity results primarily from a change in the modulus of elasticity of the materials from which the oscillating element is made. One way of addressing the concern of the temperature sensitivity of the microbalance, or other instrument, is to select a material of construction that has minimal sensitivity to changes in temperature.
For a microbalance, great care can be applied to the formulation of the material from which the oscillating element is made to attain the desired characteristics while attempting to optimize manufacturability and minimize the temperature sensitivity of the desired variables.
In particular, one way of reducing the temperature sensitivity of the microbalance known in the prior art is to use a shaped oscillating element made of a material having a low temperature coefficient of elastic modulus. In the end, compromises must be made at the expense of the accuracy, manufacturability and cost of the entire system.
U.S. Pat. No. 4,836,314 describes the selection of a material for an oscillating element fabricated from a glass of a specific composition. Additionally, U.S. Pat. No. 6,080, 939 describes a process of heat treatment and material combination of a metallic material. Both of these patents fail to completely address the adverse effects of temperature sensitivity having to do with the modulus of elasticity of the oscillating element.
A method of compensation of adverse effects is described in U.S. Pat. No. 5,604,335 wherein the mass is only measured when the system is in a quiescent state wherein no flow is delivered to the oscillator. The dual sensor system fails to subject both oscillators to substantially the same
conditions. Our invention specifically prescribes that both oscillators be subjected to substantially the same conditions.
U.S. Pat. No. 4,836,314 teaches a method of selection of the material of construction for reducing the thermal coefficient of elastic modulus over a selected temperature range. 5 The patent shows a "recipe" for a glass alloy to accomplish a fairly low thermal coefficient of elastic modulus. It is important to note that a "fairly low" thermal coefficient of elastic modulus is not sufficiently "low" enough to support the resolution and accuracy demands that are the object of 10 this invention.
U.S. Pat. No. 6,080,939 teaches a similar method of material selection and or treatment for a metallic oscillating element construction. It should be noted that the methods described in U.S. Pat. No. 6,080,939 are well known in the 15 art of the manufacture of bourdon tube pressure gages and precision mechanical resonating structures, precision springs, tuning forks, vibration based pressure transducers, vibrating densitometers, and other precision elastic components. 20
Nearly every high quality mechanical bourdon tube type pressure gage, Heise, Aschcroft, Rosemount, and others utilize materials and procedures very similar to those discussed in U.S. Pat. No. 4,836,314.
U.S. Pat. Nos. 3,946,615 and 4,048,846 detail methods of 25 addressing the thermal coefficient of elastic modulus with specific heat treatment methods with materials similar to Ni-Span-C. The now public manufacturing methods, from 1959, for the Bulova "Accutron" watch, that utilized a mechanical tuning fork, required materials and procedures 30 similar to those discussed in U.S. Pat. Nos. 3,946,615 and 4,048,846 as well.
A vibrating level detection system for the indication of pressure including the compensation of the thermal coeffi- 35 cient of the elastic modulus are described in U.S. Pat. No. 4,311,053. It should be noted that this device actually takes advantage of the change in the resonant frequency of the system with pressure.
U.S. Pat. No. 6,502,450 states in part that "[t]o compen- 4Q sate for instrument effects in direct mass measurements, a differential particulate mass measurement microbalance employing a pair of oscillating quartz crystal detectors has previously been proposed."
In this previously proposed approach, a particle laden gas 45 stream impacts upon the first detector and a particle free gas stream impacts the second detector. The second mass detector is used as a reference to cancel out detector instrument effects from a mass reading provided by the first detector. However, the first and second detectors are not of a geom- 50 etry capable of addressing the additional problem of the density change of the fluid within the hollow oscillating element and therefore can not compensate for these effects. Additionally, the idea of impacting a clean and particulate laden gas stream requires removing particulate from one 55 stream and leaving the other stream intact. This scheme requires a second particulate collection means. The subject invention, however, does NOT require a second particulate collection means.
U.S. Pat. No. 5,571,945 discloses a similar measurement go approach employing a pressure sensor to measure a pressure differential between a pair of particulate matter collectors. This patent also requires removing particulate from one stream and leaving the other stream intact. Additionally, this scheme requires a second collection means. 65
Neither of these two aforementioned patents adequately addresses the adverse effects of indicating a mass because
both subject the "reference" resonator to a change in state resulting from the collection of a "clean" sample.
U.S. Pat. No. 5,349,844 discloses a similar approach for use with a filter that is caused to oscillate in a direction substantially perpendicular to a plane of the filter.
SUMMARY OF INVENTION
One of the embodiments of the present invention is directed to an oscillating inertial microbalance comprising: a base; two elastic oscillating elements substantially equal in properties mounted at one end of said base; an enclosure; a means for exiting said oscillating elements; a means for measuring the resulting vibrations and a filter; wherein the mass of a particulate collected on the filter is measured by performing the steps of: forcing said oscillating elements into resonance, measuring the frequency of each of said oscillating elements, and comparing the results of the data derived from each said measurement. Another embodiment of the present invention is directed to an oscillating inertial microbalance wherein said oscillating elements are selected from the following group: nickel alloys, Ni-Span-C, NiSpan-D, Inconel, quartz, and quartz-glass alloys. Another embodiment of the present invention is directed to an oscillating inertial microbalance wherein the elastic oscillating elements have a ratio of the major and minor diameters of at least 1.5:1 to minimize precession. Another embodiment of the present invention is directed to an oscillating inertial microbalance wherein the data derived from the pressure, temperature and acceleration are measured. Another embodiment of the present invention is directed to an oscillating inertial microbalance wherein said two elastic oscillating elements are not equal but have a known relationship to each other. In any system, where one wishes to remove "common mode" noise errors, it is not uncommon to utilize mechanical elements of differing sizes or characteristics as long as there can be found a constant relationship between them. As such it should be obvious to one skilled in mechanics that one could utilize elastic oscillating elements that differ in physical size but behave in a known manner compared to the elastic oscillating element responsible for collecting the mass. With this we can even see that one of the elements could be micro machined as indicated in U.S. Pat. No. 6,444,927 and U.S. Pat. No. 6,784,381.
Further, the two elastic oscillating elements can be mounted in such a way as to minimize energy transfer from one elastic oscillating element to the other elastic oscillating element. Consider two elastic oscillating elements mounted in the same plane as compared to those oscillating in orthogonal planes. It will be clear to those skilled in the art that an orthogonally mounted system will minimize the energy transfer between the two elastic oscillating elements.
Another embodiment of the present invention is directed to a method of operating a microbalance, the microbalance including two inertial oscillating elements with substantially equal properties and with at least one having a filter mounted at one end thereof, the method comprising: inducing resonance in said oscillating elements; measuring the frequency of each of said oscillating elements; and comparing the results of the data derived from each said measurement.
BRIEF DESCRIPTION OF DRAWINGS
Exemplary embodiments of the invention will now be described with reference to the attached drawings, in which:
FIG. 1 is a section view of one embodiment of an idealized passively compensated microbalance showing an exemplary installation and related equipment;
FIG. 2 is a side view of a classical single oscillating element microbalance illustrating the principles of operation 5 thereto;
FIG. 3 is a simplified sectional side view of a microbalance in accordance with the present invention, illustrating the principles of operation thereto;
FIG. 4 is a section view of another embodiment of an 10 idealized passively compensated microbalance showing an exemplary installation and related equipment;
FIG. 5 is a schematic view showing the data processing of the signals to attain a passively corrected indication of the collected mass; 15
FIG. 6 is a sectional view, showing the ratio of the major axis to minor axis being at least 1.5:1, of elastic oscillating element of the microbalance of FIG. 1; and
FIG. 7 is an alternate sectional view, showing the ratio of the major axis to minor axis being at least 1.5:1, of the elastic 20 oscillating element of the microbalance of FIG. 1;
FIG. 8 is a schematic view of a data processor and related components with the microbalance of FIG. 1.
DETAILED DESCRIPTION 25
The present invention provides for a passive method and apparatus which overcomes problems found in the prior art.
In several embodiments of the present invention there is no need to utilize complex methods and or formulae for the 30 heat treatment of the oscillating element as illustrated in U.S. Pat. No. 6,080,939. Additionally, schemes such as those in U.S. Pat. No. 6,080,939 sport elastic oscillating elements with "low" thermoelastic coefficients that are not "low" enough to support the accuracy and resolution requirements 35 of the subject invention. It is therefore important to select a material that exhibits low losses of energy per unit oscillation. Such systems are said to have a "High Q", and provide a passive means of removing the effects of a non zero thermoelastic coefficient. 40
The present invention significantly reduces the temperature sensitivity by utilizing a differential mode wherein two or more resonators are utilized and only one is subjected to the collected mass. This is often referred to as removing "common mode" errors. The "common mode" being those 45 elements that change in a known predictable manner in both devices that can be easily subtracted from one another.
In harsh industrial environments a three axis accelerometer can be utilized to enable one to mathematically remove the adverse effects caused by external vibration. The meth- 50 ods of vibration compensation are well known in the art and are presently applied in vibration compensated sensors such as those detailed in U.S. Pat. No. 6,498,996 and many of the references cited in that patent as well as many optical related vibration compensation means, such as those described in 55 U.S. Pat. No. 5,777,741 and references cited in that patent.
According to one aspect of the invention, the temperature sensitivity of an instrument, for example the microbalance as previously described, is reduced by maintaining the instrument, or at least a temperature-sensitive element 60 thereof, at a constant temperature. This is achieved by: applying heat to the instrument; measuring a parameter that is indicative of the temperature of the instrument; and controlling the amount of heat applied to the instrument to maintain the measured parameter substantially constant. 65
While this process sounds simple to attain in practice, maintaining a constant temperature of a system that draws
air into the filter is nearly impossible due to the fact that the sampled air may not have a sufficient "residence time" in the heated oscillating element tubes, or the inlets thereto, to attain the desired temperature. For example one could not expect to heat air from 20 Deg C. to 100 Deg C. in a 1 inch long soda straw. However, if the straw is 20 inches long the residence time for the air that the heating element has heat is sufficient.
One way this control may be accomplished is by affixing a resistive heater on the oscillating element of the microbalance. The resistive heater can be wound onto the oscillating element, vacuum deposited, or applied by any other means.
Some glass formulations allow embedding platinum heater windings directly within the glass. Radiant or other types of heating, such as convective and conductive, can also be employed. A radiant heater would be positioned appropriately next to or around the oscillating element to provide heat thereto. The parameter that is used to control the amount of heat supplied to the heater can be the resistance of the heating element (which is dependent on the temperature), or the output of an appropriately positioned temperature sensor. This method is addressed in U.S. Pat. No. 6,444,927.
Embodiments of the present invention serve to eliminate the need for active temperature control of the oscillating member by significantly reducing the temperature sensitivity. By employing a differential mode, wherein two resonators are utilized, and only one is subjected to the collected mass, the need to maintain the oscillating element at a constant temperature may be reduced or removed.
For example, consider two perfectly equal tuning forks in an environmental chamber that can change temperature and pressure. Assume both tuning forks remain free of any contamination or particulate. If we operate the environmental chamber (condition A) at a high pressure and low temperature the two tuning forks will resonate at the exact same rate of 410 Hz. The difference between them is ZERO. If we now lower the pressure and increase the heat (Condition B) the difference between the two tuning forks will again be ZERO but the frequency will now be 415 Hz. While the frequency went from 410 Hz to 415 Hz the DIFFERENCE was ZERO. This is the essence of a differential system, wherein the "common mode" error, the difference between 410 Hz and 415 Hz is removed by virtue of measuring the difference between two perfectly equal tuning forks. If one of the tuning forks has a 1 gram mass added to it the difference in frequency it exhibits, compared to the other tuning fork, would be solely indicative of the mass alone. This DIFFERENTIAL method allows the user to determine the mass on tuning fork in any reasonable condition.
For example, consider the same two tuning forks but now they are exactly the same except for their resonant frequencies being 410 Hz and 310 Hz at Condition A and exhibiting 415 Hz and 315 Hz at condition B. The difference at Condition A and Condition B is the same, thereby removing the "common mode" error thus illustrating that the two tuning forks do not need to be exactly the same in every respect. In this case we can conclude, and experiments agree with the theory, that the two elastic oscillating elements need only to be completely characterized in order to remove the "common mode" errors. This can even be shown to be true utilizing an elastic oscillating element described in this patent as well as a micro structure elastic oscillating element as described in U.S. Pat. No. 6,784,381 held by this applicant.
A further error in the use of microbalances having hollow oscillating elements is caused by temperature and/or pressure changes in the fluid located within the cavity of the hollow oscillating element.
As the temperature or pressure of the fluid within the 5 cavity of the hollow oscillating element varies, so will the density of the fluid. Assuming that the interior volume of the cavity of the hollow oscillating element remains substantially constant, a variation in the density of the fluid will result in a variation of the mass of the fluid located in the 10 cavity of the hollow oscillating element. That is, the effective mass of the oscillating element will vary with temperature and/or pressure changes in the fluid located therein. This variation in the effective mass of the oscillating element will in turn affect the resonant frequency of the oscillating 15 element and indicate a change in mass when, in fact, the mass is constant.
Embodiments of the present invention significantly reduce the temperature and/or pressure sensitivity by employing a differential mode wherein two resonators are 20 utilized and only one is subjected to the collected mass. Both resonators are in fluid communication with each other thereby removing the need to maintain the pressure and/or temperature of the fluid within the oscillating element constant or to compensate for those variables. 25
The pressure-dependent error frequently manifests itself as a perceived negative mass over time. As the filter element loads up with particulate or other forms of flow impeding elements, the pressure within the cavity of the hollow oscillating element will decrease due to the increased resis- 30 tance of the filter element. As the pressure of the fluid decreases, so does the density, reducing the mass of the column of fluid within the cavity of the hollow oscillating element. This, in turn, will increase the resonant frequency of the oscillating element, indicating a false reduction in the 35 mass of the filter element and its entrapped matter. These false reductions in mass readings plague prior art devices, such as those described in U.S. Pat. No. 4,391,388.
Similarly, if the fluid gets colder, the density of the fluid column in the cavity of the hollow oscillating element will 40 increase, increasing the mass within the cavity of the hollow oscillating element. The resonant frequency of the oscillating element will be correspondingly lower, thereby indicating an erroneously high mass. The reverse is true if the fluid temperature increases. 45
By utilizing a differential mode, or "common mode", wherein two resonators are utilized and only one is subjected to the collected mass, we remove the need to maintain the pressure and/or temperature of the fluid within the oscillating element constant or to compensate for those variables. 50
Referring to FIG. 2, the principle of operation of a single oscillating element microbalance 10 can be represented by m =k/f2 where m is the mass in grams, f is the frequency in Hz, and k is the spring constant in g*Hz2.
For a particular single oscillating element microbalance, 55 the spring constant can be determined by using two values of m, one for "zero" mass (i.e. the system mass only), and one for an additional mass added to the "zero" mass. The system mass is of course not actually zero—we "tare" the system mass out for purposes of convenience much like a 60 post office scale is zeroed before the letters are placed in a box on the scale. This ensures that the mass of the letters alone are considered, and not the mass of the box.
The equation for the spring constant can be derived as follows: k=(ml-m0)/(l/fl2-l/f02) Using exemplary values 65 of ml =0.075 g, m2=0, fl=250 Hz and f0=311.314 Hz yields a value of k=13,200 g*HZ2. We can see that we can
determine the spring constant, k, for two oscillating elements as well as one. We can mathematically combine these two spring constants and combine them to a new term such as Bulk K. The equation for the Bulk K is determined in much the same manner as above.
The actual system mass at this temperature can now be determined by substituting the determined value of k and the observed value of f for the "zero" mass condition. Using these two values, we arrive at a system mass of 0.136199 g.
Referring now to FIG. 3 we see two hollow oscillating elements, 1 and 2. These two oscillating elements are excited into resonance utilizing methods well known in the art. Oscillating element 1, called the "collector", has a filter collection means 3 attached while oscillating element 2, called the "reference", has none. Both oscillating elements 1 and 2 are subjected to substantially the same operating environment including the pressure or vacuum within the hollow oscillating elements indicated at 5. As such we can determine a Bulk K value for the system comprised of two oscillating elements.
Referring now to FIG. 5, illustrated is a frequency measuring means commonly known in the art at 1 and 2. These two measured resonant frequency values come from the resonance of 1, the "collector", and the resonance of 2, the "reference". The absolute value of the square of the difference between 1 and 2 combined with the Bulk K results in an indication of the mass of the collected particulate without the errors contributed by changes in pressure, temperature, modulus of elasticity and even external vibration as all of these changes impact the resonance of both 1 and 2 equally. The "common mode" errors are removed and or minimized utilizing a differential arrangement as described in this patent.
Referring now to FIGS. 1 and 4, a microbalance 10 comprises two elastic oscillating elements 12a and 12b, an enclosure 39 and a base assembly 14.
The oscillating elements 12a and 12b may be made of known materials used in the manufacture of microbalances, but they may also be made of other materials such as nickel alloys, Ni-Span-C, Ni-Span-D, Inconel, quartz, and quartzglass alloys. For example, a nickel-cobalt alloy may be used for its properties of increased strength and decreased temperature coefficient of elasticity. By applying the temperature compensation described herein, it is possible to use materials with less-restrictive temperature dependencies of the modulus of elasticity, since embodiments of the present invention reduce the sensitivity of the instrument to external temperature variations.
Additionally, the oscillating element 12 may be manufactured using the electroforming manufacturing method. The electroforming method is similar to the method utilized for chrome plating automobile bumpers except that an appropriately shaped form is used on which to plate the material. The use of electroforming materials from which to make the oscillating elements 12a and 12b will inherently result in the two oscillating elements being substantially equal in properties providing for simplified manufacturing methods to produce substantially equal thermoelastic coefficients yielding an ideal assembly for running the embodiments in a differential mode. Utilizing separate oscillating elements requires more demanding manufacturing methods to ensure both oscillating elements exhibit substantially the same thermoelastic coefficients, or are well characterized to enable mathematically correlating them to one another to enhance the quality of the "common mode" error reduction. Additionally, an electroformed oscillating element offers a lower temperature dependence over the normal operating