Abstract:
A new high precision volume gauging system for measuring the volume of a propellant VL enclosed at a first pressure PU within a propellant tank ( 40 ) of a volume VT. The improved precision compared with prior art is achieved in that it comprises a high precision pressure sensor ( 90 ) which is comprised of a reference chamber ( 115 ) that is connected to the propellant tank ( 40 ) by a communication line ( 140 ), a valve ( 150 ) for controlling the gas flow through the line ( 140 ), and a high precision differential pressure sensor ( 95 ) that is arranged to record the pressure difference between the reference chamber ( 115 ) and the propellant tank ( 40 ) to which it is connected through a communication line ( 130 ).

Description:
FIELD OF THE INVENTION 
   The present invention relates to an advanced volume gauging device. More specifically, the invention relates to a high precision miniaturized volume gauging device. 
   PRIOR ART 
   It is a well-known problem to measure the amount of remaining propellant in a tank in zero gravity environments. The reason is that in the absence of gravity, the liquid will float around freely inside the tank. Significant benefits can be obtained by developing reliable volume gauging systems with better accuracy. 
   For a communication satellite in geosynchronous orbit, only 10% uncertainly in the estimation of remaining propellant, which is not uncommon, can lead to that more than a year of the satellite life is lost. This leads to a very high cost penalty due to the estimate error, keeping in mind the huge cost for a communication satellite. 
   In U.S. Pat. No. 4,987,775 Chobotov et al disclose a volume gauging system based on thermodynamic principles. This system  10  is shown in  FIG. 1 , and it includes a pressurisation tank  20  of volume V p . The tank  20  includes a pressurisation gas at pressure P p  and temperature T p . The system  10  further includes a propellant tank  40  of volume V T . The tank  40  includes a generally liquid propellant occupying a volume V L . The portion of the tank  40  unoccupied by the liquid phase of the propellant has an ullage volume V u , a pressure P u  and a temperature T u . The tanks  20  and  40  are interconnected by a gas line  50 . Gas flow through the line  50  is controlled by an injection valve  60 . Additionally, the pressure P p  within the tank  20  is monitored by a first absolute pressure transducer  65  in communication therewith. Similarly, a second absolute pressure transducer  70  monitors the pressure P u  within the ullage volume V u  of the tank  40 . The temperatures T p  and T u  of the tanks  20  and  40  are ascertained by temperature sensors (not shown) operatively coupled thereto. 
   The propellant measurement system  10  is adapted to determine the ullage volume V u  of the tank  40  and thereby determine the volume of remaining propellant V L  through the expression V L =V T −V u . The ullage volume V u  is determined in the following manner. First, the pressure P p  is chosen to be larger than the pressure P u  in order that the pressurisation gas within the tank  20  flows into the tank  40  upon opening of the valve  60 . The valve  60  is opened until a suitably measurable increase occurs in the pressure P u  within the chamber  40 . The valve  60  is then closed and the changes in the pressures P p  and P u  are determined from the pressure transducers  65  and  70 . The ullage volume V u  may now be determined by noting that during the above process gas is conserved within the system  10 . Accordingly, from fundamental thermodynamic equations assuming an isothermal process and that the propellant is incompressible: 
                     P   p     ⁢     V   p         T   p       +         P   u     ⁢     V   u         T   u         =           (       P   p     -     d   ⁢           ⁢     P   p         )     ⁢     V   p         T   p       +         (       P   u     +     d   ⁢           ⁢     P   u         )     ⁢     V   u         T   u                 [   1   ]             
 
where
     dP p =the change in P p  as measured by the first pressure transducer  65 .   dP u =the change in P u  as measured by the second pressure transducer  70 .   

   After simple algebra, 
                   d   ⁢           ⁢     P   p     ⁢     V   p         T   p       =       d   ⁢           ⁢     P   u     ⁢     V   u         T   u         ⁢                   [   2   ]                 Hence   ,     ⁢                                       V   u     =       d   ⁢           ⁢     P   p     ⁢     V   p     ⁢     T   u         d   ⁢           ⁢     P   u     ⁢     T   p           ⁢                   [   3   ]             
 
   From which the volume of propellant remaining in the tank  40  may be expressed as: 
                 V   L     =         V   T     -     V   u       =       V   T     -       d   ⁢           ⁢     P   p     ⁢     V   p     ⁢     T   u         d   ⁢           ⁢     P   u     ⁢     T   p               ⁢                   [   4   ]             
 
   To achieve results with high accuracy when the tank  40  is nearly empty, dP u  has to be recorded with very high requirement on resolution over a pressure range from a few bars up to 22 bars. No commercially available pressure sensor meets the requirements. Among space qualified sensors, the performance of best sensors is far from the requirements. The traditional approach is to take a good sensor and then improve the performance with new signal conditioner electronics where the rapid technological progress permits new designs with higher performance. This approach will probably not work in this case, as error sources in the sensor internal design become dominant. The errors may be of several types, long term drift, linearity, hysteresis, etc. This indicates that an alternative sensor concept must be used, which is directly tailored for the dP u  applications. 
   One possible way to accomplish such a sensor is described in JP 57035743, and shown in FIG.  2 . This particular sensor  90  is intended for measuring small fluctuations in atmospheric pressure, and is constructed as follows. A space which has been surrounded by a first vessel  100  and a second vessel  110  is divided into two parts by a flexible film body  120 , and a first chamber  105  and a reference chamber  115  are formed by the flexible film body  120  and the vessel  100 , and the flexible film body  120  and the vessel  110 , respectively. On the vessel  100  and the vessel  110  are provided communicating holes (or lines)  130 ,  140  by which the respective chambers  105 ,  115  communicate to the open air, and on the communicating hole  140  is provided an electromagnetic valve  150  for opening and closing between the reference chamber  115  and the open air. A pressure sensor  160  detects and measures pressure of a difference between the chamber  105  and  115  through the flexible film body  120 . A measuring signal processing part  170  receives a signal which has been sent from the pressure sensor  160 , converts it to a variation of pressure by means of signal processing, sends it out to a display recording part  180 , also sends out an opening and closing indication signal to an opening and closing means driving part  190  whenever a variation of pressure attains to a set value, and instantaneously opens the electromagnetic valve  150 . Generally, the concept of this sensor may be described as a differential pressure sensor  95  measuring a pressure difference between the closed reference chamber  115  and the surrounding atmosphere. 
   The sensor concept presented in JP 57035743 may be designed such that a huge increase in sensitivity (in a limited but selectable range) is achieved, compared to a conventional differential pressure sensor. A numerical example gives the following results. The pressure on the frontside and the backside will be absolutely equal if the valve  150  is open long enough. The pressure in reference chamber  115  should be within 0.1% of 22 bars if the tank pressure is 22 bars. Assume that the pressure sensor membrane  120  has 100-mbar sensitivity for a full-scale deflection and that the deflection may be measured with 0.1% accuracy. The end result is that with an absolute pressure of 22 bars a pressure change of 0.022 mbar can be detected. The resolution is 10 −6 , which is far beyond what can be achieved with any conventional pressure sensor today. 
   SUMMARY OF THE INVENTION 
   The present invention aims toward a self-contained miniaturized volume gauging device, which can be mounted on/inside, the tank wall. Such a device has three major advantages compared with existing systems. Firstly, the sample volume will have the same temperature as the tank volume, which relaxes the temperature measurement requirements. Secondly, the propellant tank walls provide additional radiation shielding for the integrated electronics. Thirdly, the proposed device will be both lighter and smaller compared with conventional systems. The device shall include the sample volume, gas injection system, super-high precision pressure sensor and electronics for control, signal conditioning and digital interface to the spacecraft. 
   An object of the present invention therefore is to provide a new miniaturized volume gauging system. 
   Another object of the present invention is to provide a new method for measuring the remaining fuel in a propellant tank using a dP pressure sensor. 
   These objects and other objects of the invention are achieved by the volume gauging device and the method as defined in the claims. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  schematically shows an existing propellant gauging system. 
       FIG. 2  shows an existing high precision pressure sensor. 
       FIG. 3  schematically shows a propellant gauging system according to the present invention. 
       FIG. 4  shows one embodiment of the miniaturized fuel gauging device of the invention. 
       FIG. 5  shows one embodiment of a michromechanical dP sensor according to one embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the invention will now be described with reference to the figures in which members having the same function as in prior art will be given the same number. 
     FIG. 3  shows a block diagram of one embodiment of the invention. The fuel gauging system  200  comprises all parts shown in  FIG. 1 , and one high precision pressure sensor  90  according to  FIG. 2 , which pressure sensor  90  is coupled to the propellant tank  40  by the communicating holes  130 ,  140 . The system further comprises a processing/control unit  210  for calculating the volume of the remaining fuel VL and controlling the gauging cycle. A line  230  connects the pressurisation tank  20  with a high pressure source (HPS) and the loading of high pressure gas into the pressurisation tank  20  is controlled by a valve  220 . 
   The system may further comprise filters to prevent liquids inside the gas system and temperature sensors for measuring the temperatures in the pressurisation tank  20  and the propellant tank  40 . But as the present invention aims toward a miniaturized fuel gauging device, which can be mounted on/inside the tank wall, the gas in the pressurisation tank  20  will approximately have the same temperature as the gas in the propellant tank  40 , whereby the temperature measurements may be omitted. 
   When a determination of remaining propellant shall be performed the following sequence is activated by the processing/control unit  210 . Valve  220  is opened and the pressurisation tank  20  is filled with gas to a high pressure (P p ), then the valve  220  is closed and the pressure transducer  65  registers the pressure Pp. At the same time absolute pressure (P u ) is registered in the propellant tank  40  by the pressure transducer  75 , and the valve  150  is closed such that the reference chamber  115  will remain at the pressure P u . Thereafter the injection valve  60  is opened and the high pressure gas from the pressurisation tank  20  is injected into the propellant tank  40 . The high precision pressure sensor  90  registers the resulting small increase of the absolute pressure dP u  in the propellant tank  40 , the injection valve  60  is closed and the processing/control unit  210  calculates the volume of the remaining propellant using equation [5] below. As the pressure in the pressurisation tank  20  now is equal to the pressure in the propellant tank  40 , dP p  in equation [4] may be replaced by (P p −(P u +dP u ) whereby: 
                 V   L     =         V   T     -     V   u       =       V   T     -         (       P   p     -     (       P   u     +     d   ⁢           ⁢     P   u         )       )     ⁢     V   p     ⁢     T   u         d   ⁢           ⁢     P   u     ⁢     T   p               ⁢                   [   5   ]             
 
   When a volume gauging system is installed in/on a propellant tank, it will also replace the usual pressure measurements for tank monitoring. Thus, the pressure measurement system shall enable two kinds of pressure data, dP u  pressure valves for volume gauging and absolute tank pressure for house-keeping 
   Requirements on a volume gauging system may be: 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               Tank volume: 
               10-T.B.D. liter 
             
             
                 
               P u  measurements: 
               pressure range 2-22 bar 
             
             
                 
                 
               resolution 0.01 bar 
             
             
                 
                 
               accuracy 0.1% 
             
             
                 
               dP u  measurements: 
               diff. pressure range ±100 mbar 
             
             
                 
                 
               resolution 0.1 mbar 
             
             
                 
                 
               accuracy 0.1% with ±0.1 bar range 
             
             
                 
                 
               response time &lt;100 mS 
             
             
                 
                 
               sampling rate 5 s/s 
             
             
                 
               P p  measurements: 
               pressure range 10-200 bar 
             
             
                 
                 
               resolution 0.1 bar 
             
             
                 
                 
               accuracy 0.1% 
             
             
                 
                 
             
           
        
       
     
   
   The requirements on fast response time and sampling rate originates from the fact that the tank pressure value are of significant importance for the accuracy of the dP u  measurement after a gas sample injection. The pressure conditions are not in steady state conditions. 
     FIG. 4  shows an exemplary embodiment of a self-contained miniaturized volume gauging device  490 , which is intended to be mounted directly on the tank wall. This embodiment comprises a main body  500  on which a pressurisation tank  20  is arranged. 
   The main body  500  comprises a communication portion  505  that is arranged to mate a hole in the wall of a propellant tank  40 . An injection valve  60  is mounted on the main body  500  inside the pressurisation tank  20 . A first line  230  extends from an outer surface of the main body  500  to the pressurisation tank  20 , through which first line  230  loading of high-pressure gas into the pressurisation tank  20  is performed. A high-pressure valve  220  (not shown in the figure) is in this embodiment arranged separately from the volume-gauging device  490  and connected to the line  230 . A second gas line  50  extends through the main body  500  terminating at one end in the propellant tank  40  and at the other end at the injection valve  60 . A micromechanical pressure sensor unit  510  is arranged in the main body  500 . The pressure sensor unit  510  comprises one P u  sensor, one P p  sensor and one dP u  sensor. The P u  sensor and the dP u  sensor communicates with the propellant tank  40  via a third gas line  520 , and the P p  sensor communicates with the pressurisation tank  20  via a fourth gas line  530 . An electrical connector for connecting the pressure sensor unit  510  and the injection valve  60  to an external control unit (not shown), is arranged on the side of the main body  500 . To prevent propellant from entering the lines  520  and  50 , they are each provided with a protection filter  540  and  550  respectively.  FIG. 5  further shows a number of sealing rings that prevent gas or propellant leakage in the system. 
   In addition to the vastly increased sensitivity, the proposed self-contained miniaturized volume gauging device  490  is considerably smaller and lighter than existing systems built up from discrete components. However, for micro-satellites and the like, even smaller devices are needed, and as the propellant tank  40  in such systems is much smaller, the pressurisation tank  20  may be extremely small, a self-contained all micromechanical volume gauging device may be applicable. 
   A practical realisation of a micromechanical dP-sensor which may be used in the above embodiments is shown in FIG.  5 . The P u  sensor and the P p  sensor of the micromechanical pressure sensor unit  510  are not shown here, as they may be considered trivial to one skilled in art. This dP-sensor is based on bonded micromachined wafers. The material is most likely silicon but other more corrosion resistant materials such as quartz or silicon carbide can also be used. The device works as follows. Wafer A  300  and wafer B  310  form the pressure sensor and the valve elements. A large cavity  320  is formed on wafer A  300  by suitable etching methods. The bottom of the cavity becomes a flexible membrane  120 . Two metal planes  330  or electrodes between wafer A  300  and B  310  act as a capacitor where the capacitance changes when the membrane bends. The electrodes can be accessed via two through-plated holes  340 . This is the pressure sensor part. 
   A reference chamber  115  is connected to the valve through a small channel  140 . The volume of the reference chamber  115  is much larger than expected as it also is connected to a buffer volume  350 . This volume has two good effects on the system. It reduces the sensitivity for valve leakage during the measurement period and also the effects of the flexible membrane  120  deflection which otherwise could cause a small increase of the locked reference pressure. A valve seat  360  is formed in wafer A  300  through wet etching of a shallow cavity with a ringshaped ridge. The gas entrance is through a wet etched through hole  370 . The hole is etched from the outside. A valve cap  380  is formed in wafer B  310 , it is a square shaped block suspended all around by a thin flexible membrane  390 . The valve cap  380  may be moved against or from the valve seat by changing the length of valve actuators  400 ,  410 ,  420 . The actuators  400 ,  410 ,  420  may be piezoelectric elements where the total length can be changed by a control voltage. The valve cap  380  opens when the central actuator  410  contracts or when the surrounding actuators  400 ,  420  elongate. The central actuator  410  is mechanically connected to the surrounding by use of a third silicon wafer  430 . 
   A fourth silicon wafer  440  with a filter structure protects the fragile sensor membrane  120  from liquids or particles.