Abstract:
A method for the control of actuation force in apparatus for determining entrained gas-phase content of process streams wherein the apparatus comprise compression of the process stream, measurement of the compressive behavior, and calculation of the volume of the entrained gas phase. In the apparatus, a piston device is rapidly translated into the process stream. The inertia of the liquid adjacent to the end of the piston causes the volume of fluid close to the end of the piston to be compressed by the movement of the piston. A pressure sensor integrally incorporated in the end of the piston measures the pressure pulse caused by the rapid movement of the piston. The pressure pulse is inversely proportional to entrained gas phase content. An accelerometer incorporated within the piston measures the acceleration of the piston. Changes in acceleration due to factors such as actuator hysteresis and seal friction are compensated for by automatically changing power applied to the actuator device as necessary to maintain constant acceleration.

Description:
The present application claims priority from Provisional Application Serial No. 60/136,123, filed May 26, 1999. 
    
    
     This invention relates generally to the automatic control of actuation force of a rapidly linearly displaced rod or piston. While not so limited, it is particularly useful in apparatus for the determination of the entrained gas-phase content of liquids. Apparatus of this type can determine the volume of entrained gas by rapidly actuating and impacting the liquid with a piston so that the pressure increase against the piston is indicative of gas phase content 
     BACKGROUND OF THE INVENTION 
     Many test instruments and other devices make use of a rapidly accelerating force which acts against some material in order to measure a particular property. It is essential that acceleration of the device imposing the force should be consistent and reproducible. Maintaining consistent acceleration over time has proved to be a problem for various reasons. Among these can be mentioned wear of component parts or friction caused by small bits of trash being captured in seals. 
     Entrained gas-phase content in liquids can be measured by various direct and indirect methods. Some indirect methods include density, viscosity, and attenuation of sound waves. In general, indirect methods suffer limitations due to contributions by other factors. For example, density also depends upon general composition, and attenuation of sound waves depends upon the presence of suspended solids. Direct measurement can rely upon fluid compressibility. Simply stated, liquids are incompressible while gases are compressible. Methods relying upon fluid compressibility generally require the collection and isolation of a sample of liquid containing entrained gases. An exception to this rule is found in an earlier patent of the present inventor, U.S. Pat. No. 5,932,792. This patent describes apparatus for determining the gas content by impacting the liquid with a piston and measuring the pressure against the piston as indicative of gas content. This apparatus requires high-speed linear actuation in order to move the piston into the fluid with sufficient velocity so that the fluid adjacent to the piston, by virtue of its inertia, does not have time to move away from the end of the piston. The fluid adjacent to the end of the piston therefore serves as a dynamic containment vessel. 
     Actuators designed to produce linear motion experience changes in their linear translation behavior due to mechanical hysteresis and aging effects. For example, the magnetostrictive actuators described in my earlier patent utilize the change in length of a crystalline structure subjected to an external magnetic field. The migration of the microcrystalites within the crystal structure is actually not a smooth and reproducible phenomenon. Small changes in the movement pathway of the microcrystallites throughout the crystal structure result in slightly different elongation and relaxation characteristics from cycle to cycle, and over a multitude of cycles. Therefore, the actuator does not always produce the same linear translation when subjected to a given magnetic field. Other factors can influence the magnetic field itself, such as aging of the coil used to generate the magnetic field. Similar kinds of effects occur with other actuators such as piezoelectric and solenoid actuators. 
     In addition to changes in the linear translation characteristics of actuators, the apparatus described in my earlier patents is affected over time by other factors, such as seal friction, that can hinder piston movement to greater or lesser degrees over time. 
     Therefore, because a constant applied motivational force to an actuator incorporated within my earlier apparatus may not provide an identical linear displacement characteristic of the impacting piston from cycle to cycle, the pressure measured at the end of the impacting piston, at an otherwise constant entrained gas content, may drift over time. 
     The present invention describes a method for overcoming this problem. However, it is more broadly useful for automatic control of the intensity and acceleration of the pulsed rapid linear movement of any similar device. 
     SUMMARY OF THE INVENTION 
     I have now discovered that in an apparatus of the type described in my earlier patent, the magnitude of the pressure pulses correlate closely with the acceleration of the impacting piston at a constant entrained gas content. Therefore, independent control of the acceleration of the impacting piston will ensure that reproducible pressure pulses are obtained at a given constant entrained gas content. The acceleration can be measured directly with an accelerometer. Alternatively, the acceleration can be calculated from the change in linear displacement over time since the second derivative of displacement with respect to time is identical to acceleration. 
     The acceleration produced during an actuator cycle could be expected to proceed as follows. Initially, when the actuator is at idle, there is zero acceleration. When the actuator begins to move the impacting piston into the fluid there is an increase in acceleration. When the linear movement of the impacting piston begins to decrease to zero, the acceleration decreases and approaches zero. As the impacting piston is retracted, the acceleration is negative. Finally, when the impacting piston has again come to rest, the acceleration becomes zero. As has been observed in actual practice, the characteristics of the acceleration are much more complicated than given by the preceding description. The acceleration may actually go through several positive peaks before the acceleration becomes negative. The complex behavior in the acceleration of the impacting piston results in a multiplicity of pressure responses at the end of the impacting piston. 
     The acceleration which occurs early in an impact cycle has the most important influence on the resulting pressure pulses even though subsequent acceleration may be greater than the early acceleration. Whereas the inertia of the fluid adjacent to the end of the piston causes the fluid to serve as a dynamic containment vessel, this effect will diminish rapidly as the fluid does begin to move under the action of the piston. Therefore, the first pressure pulse is most indicative of entrained gas content, while subsequent pressure pulses reflect a combination of compression of the fluid and simple inertial acceleration of the bulk fluid. Emphasis is therefore placed on the measurement of the early acceleration for the purpose of controlling the ability of the impacting piston to measure the entrained gas content. 
     The preferred method to measure the acceleration is with a dedicated accelerometer. Alternatively, a linear displacement transducer can be used to measure the position of the impacting piston over time, and acceleration can be calculated from the shape of the displacement-time curve. 
     A principal object of the present invention is to provide a method and apparatus for the automatic control of the amount of activation energy applied to a linear actuator so that the resultant acceleration will exhibit minimal drift over time. 
     Another object is to provide a method and apparatus for automatic control of activation energy applied to a rapidly translating piston used to measure entrained gas within a liquid. 
     A further object is to combine multiple acceleration responses in order to produce a single representative value of acceleration for control of energy applied to the actuator. 
     It is an additional object of the invention to provide a method and apparatus to control the performance of an actuator by measuring the acceleration of an impacting piston, and to use that information to automatically control the actuator as necessary to maintain constant acceleration. 
     These and many other objects will become readily apparent upon reading the following detailed description, taken in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an open cross section of the apparatus, excluding any transducers for position and acceleration measurement of the impacting piston. 
     FIG. 2 is an open cross section of the apparatus shown in FIG. 1, showing selected detail. 
     FIG. 3 is a drawing of one possible accelerometer installed within the apparatus of FIG.  1 . 
     FIG. 4 is a drawing of one possible linear displacement sensor installed within the apparatus of FIG.  1 . 
     FIG. 4A represents a section along line A—A of FIG.  4 . 
     FIG. 5 is a graphical representation of an oscilloscope trace representing typical acceleration responses during an actuation cycle. 
     FIG. 6 is a graphical representation of an oscilloscope trace showing typical pressure pulse responses during an actuation cycle. 
     FIG. 7 is a schematic diagram of a system providing closed-loop control of the actuator power to maintain constant acceleration of the impacting piston. 
     FIG. 8 is a schematic diagram of a system for providing power to an actuator during an actuation cycle. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Various products of specific manufacturers found to be satisfactory in my apparatus will be noted in the following description. However, it is not my intent to endorse these particular products over similar ones from other suppliers that would be equally suitable. 
     Referring to FIGS. 1 and 2, an embodiment of the apparatus is constructed as follows. Actuator housing  1 , connecting rod housing  2 , and mounting flange  3  provide the framework for the apparatus. Actuator housing  1  is secured to mounting flange  3 ; e.g., by welding or threading. Connecting rod housing  2  slips into mounting flange  3 , and is locked in place with set screw  4 . O-ring  5  provides a seal. Actuator housing top flange  6  is welded to actuator housing  1 . In a preferred version of the apparatus, actuator  7  is any suitable actuator means capable of producing a linear actuation of about 0.013 mm (0.005 in) over a time of about 400 microseconds. In the preferred embodiment, actuator  7  is a magnetostrictive actuator. One actuator found satisfactory is Model No. AA140J025 made by Etrema Products, Inc., Ames, Iowa. Actuator  7  is attached to top cover  8  by means of bolt  9 . Top cover  8  is attached to actuator housing top flange  6  with bolts  10 . Connecting rod housing  2  and mounting flange  3  provide a means for inserting the apparatus into a process stream. Connecting rod housing  2  may have a diameter of about 38 mm (1½ in) and a length of about 250 mm (10 in). Connecting rod adapter  20  is threaded to connecting rod  11 . The assembly of connecting rod adapter  20  along with connecting rod  11  acts as a piston and contacts actuator rod  7   a . Connecting rod adapter  20  has a proximal end held against actuator rod  7   a  by a biasing means  12  which may be any suitable elastic device such as a spring or an O-ring. The biasing means  12  is held between the shoulder end  20   a  of connecting rod adapter  20  and the end of the connecting rod housing  2 . 
     Pressure transducer housing  13  is threaded to connecting rod  11 . Pressure transducer  14  is inserted into the distal end of connecting rod  11  and held in place by pressure transducer housing  13 . Pressure transducer  14  is of flush diaphragm construction, and utilizes any common methods for the sensing of pressure, including strain, piezoelectricity, and capacitance. A suitable transducer is Model No. AB100PSIS, manufactured by Data Instruments, Acton, Me. 
     The combination of the connecting rod adapter  20 , the connecting rod  11 , the pressure transducer housing  13 , and the pressure transducer  14  is called the “impacting piston” in the following discussion and description. 
     O-ring  15  provides a seal between pressure transducer housing  13  and pressure transducer  14 . O-ring  16  provides a seal between the pressure transducer housing  13  and the connector rod housing  2 . O-ring  16 , in addition to serving as a seal, acts with O-ring  17  to serve as bearings between the stationary connector rod housing  2  and the moveable assembly comprised of connecting rod adapter  20 , connecting rod  11 , and pressure transducer housing  13 . The grooves for O-rings  16  and  17 , measured in the axial direction, are wider than the O-ring diameters, thereby allowing the O-rings  16  and  17  to roll and deform elastically as the assembly comprised of connecting rod adapter  20 , connecting rod  11 , and pressure transducer housing  13  is translated in the axial direction by means of actuator  7 . Pigtails  18  and  19  provide for connection to the electronics processors. 
     FIG. 3 illustrates a modified apparatus that uses an accelerometer to measure the acceleration characteristics of the impacting piston. Accelerometer  41  is attached to base plate  40  by means of a threaded stud  42 . Base plate  40  is attached to connecting rod adapter  20  by means of a suitable adhesive, such as an epoxy. Base plate  42  is electrically insulating and serves primarily to electrically isolate accelerometer  41  from the apparatus. When actuator  7  is activated, the increase in actuator length causes connecting rod adapter  20  to move axially away from the actuator. The resultant acceleration is measured by accelerometer  41 . The resultant electrical response is carried by wires  43  to the electronics processor. A suitable accelerometer is Model 8704B, manufactured by Kistler Instrumentation Corp., Amherst, N.Y. 
     FIGS. 4 and 4A illustrate a modified apparatus which utilizes a strain gauge as a linear displacement transducer. Resistive strain elements  51  are bonded to flexible substrate  50 . Flexible substrate  50  is rigidly clamped at each end between blocks  52  and  53 . Attachment screws  54  rigidly attach the clamped flexible substrate  50  to actuator  7  on one end, and to connecting rod adapter  20  at the other end. When actuator  7  is activated, the increase in actuator length causes connecting rod adapter  20  to move axially away from the actuator  7 , which in turn causes the flexible substrate  50  and the bonded strain elements  51  to be deformed. The resistive strain elements  51  are typically oriented in a Wheatstone bridge arrangement. The deformation of the strain elements  51  causes a change in voltage across the Wheatstone bridge. The voltage change, which is proportional to linear displacement, is carried by wires  55  to the electronics processor. A preferred combination of resistive strain elements  51  and flexible substrate  50  is Model FR1020D, manufactured by Futek, Irvine, Calif. It is readily apparent that alternative means can be devised to attach other linear displacement transducers, such as linearly variable displacement transformers (LVDT), or optical gauging devices. 
     FIG. 5 illustrates an oscilloscope trace of a typical acceleration response during an actuation cycle. Actuator control voltage  60  is a low level voltage signal that controls the application of power to actuator  7  (FIG.  1 ). When actuator control voltage  60  drives high, as indicated at position  61 , power is applied to actuator  7 . Acceleration curve  62  depicts the acceleration response to the resultant actuator  7  movement. 
     The acceleration curve  62  increases through several distinct peaks. For the particular activation energy being provided to the actuator  7 , the amplitude of the first three peaks (shown at  63 ,  64 , and  65 ) are approximately 37, 32, and 60 g, at times of 70, 160, and 270 microseconds, respectively. In a preferred embodiment, a weighted sum of the first three peaks has been found to provide sufficient information for satisfactory control of the activation energy to be applied to the actuator  7 . Although the area under the acceleration vs. time curve also provides information suitable for control of the activation energy, I have discovered that the effect of acceleration on the measured pressure pulses diminishes during the actuation time. Thus, selective combination of the acceleration peaks provides the most useful information. 
     The particular acceleration response depicted in FIG. 5 is not a singular response. That is, with different applied activation energies, the number of peaks, the peak amplitudes, and the times observed for the acceleration peaks may change significantly. 
     The acceleration has been found to be entirely independent of the entrained gas content in the liquid. This result is to be expected since the resistance to motion of the impacting piston is much more dependent upon the inertia of the impacting piston, the friction of the seals  16  and  17 , and the resistance of the biasing means  12  (FIG.  1 ), than it is to the inertia of the liquid adjacent to the end of the impacting piston. 
     FIG. 6 illustrates an oscilloscope trace of a typical pressure pulse response during an actuation cycle. The actuator operating conditions were identical to those used to generate the acceleration results shown in FIG.  5 . Actuator control voltage  60  is a low-level voltage signal that controls the application of power to actuator  7  (FIG.  1 ). When actuator control voltage  60  drives high, as indicated at position  61 , power is applied to actuator  7 . Pressure pulse curve  70  depicts the pressure response to the resultant actuator  7  movement. 
     The entrained gas content was zero for the particular pressure pulse results depicted in FIG.  6 . The first pressure pulse  71  has been found to be highly sensitive to low entrained gas contents, below 5-10% by volume. The second and third pressure pulses  72  and  73  are relatively insensitive to entrained gas contents below 1-2% by volume. In a preferred embodiment, the first pressure pulse  71  is selected for low entrained gas contents, while the second and third pulses  72  and  73  are selected for measurements at higher entrained gas contents. 
     Referring to FIG. 7, a system for providing an acceleration controlled impacting piston and a measurement of entrained gas content suitable for process control is explained as follows. Apparatus  80 , previously described in FIGS. 1-4, is inserted through assembly  81  into process pipe  82 , containing the process fluid. Assembly  81  consists of pipe nipple  83  and flange  84 . Pipe nipple  83  may be welded to process pipe  82 . Flange  84  may be welded or threaded to pipe nipple  83 . Flange  84  on assembly  81 , and flange  3  on assembly  80 , are bolted together to complete the insertion of assembly  80  into process pipe  82 . Pipe nipple  83  is of suitable length so that the distal end of connecting rod  11  protrudes into process pipe  82 . 
     Microprocessor  85  passes a signal  97  (trace  60 - 61  in FIGS. 5 and 6) on a regular interval to actuator electronics  86 . Actuator electronics  86  then passes a voltage pulse to activate actuator  7  (FIG. 1) in apparatus  80  through pigtail  19 . The resulting pressure-pulse electrical signal is passed through pigtail  18  to pressure pulse amplifier electronics  96 . 
     Pressure pulse amplifier electronics  96  amplifies and conditions the pressure pulse electrical signal and combines the desired pressure pulse peak characteristics (peaks  71 - 73  in FIG. 6) as appropriate for process requirements. The conditioned pressure pulse signal is read by microprocessor  85 . Microprocessor  85  converts the pressure pulse signal to the equivalent entrained gas content by means of a calibration table, and the result is transmitted in digital and/or analog forms suitable for use in process control. 
     The acceleration electrical signal is passed through wires  43  to acceleration amplifier electronics  87 . Acceleration amplifier electronics  87  amplifies and conditions the acceleration electrical signal. As directed by timer  89 , multiplexer  88  passes the conditioned accelerometer electrical signal to peak-hold amplifiers  90 ,  91 , and  92 . Three peak-hold amplifiers are depicted in FIG.  7 . The actual number of amplifiers can be greater or smaller, depending upon processing requirements. The peak hold amplifiers  90 ,  91 ,  92 , along with the accelerometer signal routing effected with the timer  89  and multiplexer  88 , allows the individual accelerometer peaks, such as peaks  63 ,  64 , and  65  in FIG. 5 to be captured and held. The captured and held peaks are passed to acceleration shaper  93 . Acceleration shaper  93  performs a weighted addition of the captured and held peaks. For example, with the apparatus of the preferred em-as described in FIGS. 1,  2 , and  3 , an optimal combination is to fully weight peaks  63  and  65 , while adding only half of the magnitude of peak  64 . In practice, peak  64  contributes relatively less useful information for closed-loop control of the acceleration. The shaped acceleration result is passed to microprocessor  85 . The desired acceleration setpoint  94  is provided to the microprocessor  85  as a separate signal. Acceleration setpoint  94  may be conveniently produced by an external adjustable potentiometer. Microprocessor  85  compares the difference between the desired acceleration as provided by the acceleration setpoint  94  with the actual acceleration and, using standard control algorithms, calculates and passes an adjusted control signal  95  to actuator electronics  86 . Actuator electronics  86  produces an appropriately modified voltage pulse to actuator  7  upon the next actuation cycle. 
     FIG. 8 illustrates further detail of a suitable actuator electronics means  86  (FIG. 7) for providing controlled power to the actuator  7 . AC voltage  111  is provided to AC to DC converter  112  to produce DC voltage  113 . Signal  95  provided from microprocessor  85  (FIG. 7) controls the magnitude of the DC power from the AC to DC converter  112 . In the preferred embodiment, DC voltage  113  is controlled between, for example, 50 to 250 volts, as the control signal  95  is varied between, for example, 1 to 5 volts. A suitable AC to DC converter  112  is model 1/4 A12, manufactured by Ultravolt Corp., Ronkonkoma N.Y. DC voltage  113  is stored by capacitor  114 . Upon command from signal  97 , switch  110  passes the stored DC voltage from capacitor  114  through wires  19  to actuator  7  (FIG.  1 ). In the preferred embodiment, switch  110  is a commonly available mosfet (solid state switch). The electrical current available from AC to DC converter  112  is low, for example, 10 ma, whereas the current draw by actuator  7  is high, for example, 4-8 amperes. The correspondingly rapid discharge of power from capacitor  114  results in a brief application to actuator  7  of a voltage pulse which is an order of magnitude or more above the normal voltage limit of actuator  7 . Therefore, actuator  7  produces an acceleration an order of magnitude or greater than could otherwise be obtained. 
     It will be readily apparent to those skilled in the art that many minor variations in the invention can be made that have not been described herein. It is the intent of the inventor that these variations should be included within the scope of the invention if encompassed within the following claims.