Advanced volume gauging device

[0001] The present invention relates to an advanced volume gauging device. More specifically, the invention relates to a high precision miniaturized volume gauging device.

[0002] 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.

[0003] 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.

[0004] 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.

[0005] 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: $\begin{array}{cc}\frac{P_p{V}_p}{T_p}+\frac{P_u{V}_u}{T_u}=\frac{\left(P_p-d{P}_p\right)V_p}{T_p}+\frac{\left(P_u+d{P}_u\right)V_u}{T_u}& \left[1\right]\end{array}$
where

• [0006] dP_p=the change in P_p as measured by the first pressure transducer 65.
• [0007] dP_u=the change in P_u as measured by the second pressure transducer 70.

[0008] After simple algebra, $\begin{array}{cc}\frac{d{P}_p{V}_p}{T_p}=\frac{d{P}_u{V}_u}{T_u}& \left[2\right]\\ \mathrm{Hence},& \\ V_u=\frac{d{P}_p{V}_p{T}_u}{d{P}_u{T}_p}& \left[3\right]\end{array}$

[0009] From which the volume of propellant remaining in the tank 40 may be expressed as: $\begin{array}{cc}{V}_L=V_T-V_u=V_T-\frac{d{P}_p{V}_p{T}_u}{d{P}_u{T}_p}& \left[4\right]\end{array}$

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] An object of the present invention therefore is to provide a new miniaturized volume gauging system.

[0015] 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.

[0016] These objects and other objects of the invention are achieved by the volume gauging device and the method as defined in the claims.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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: $\begin{array}{cc}{V}_L=V_T-V_u=V_T-\frac{\left(P_p-\left(P_u+d{P}_u\right)\right)V_p{T}_u}{d{P}_u{T}_p}& \left[5\right]\end{array}$

[0026] 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

[0027] Requirements on a volume gauging system may be:

[0028]

	Tank volume:	10-T.B.D. liter
	Pu measurements:	pressure range 2-22 bar
		resolution 0.01 bar
		accuracy 0.1%
	dPu measurements:	diff. pressure range ±100 mbar
		resolution 0.1 mbar
		accuracy 0.1% with ±0.1 bar range
		response time <100 mS
		sampling rate 5 s/s
	Pp measurements:	pressure range 10-200 bar
		resolution 0.1 bar
		accuracy 0.1%

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] A fourth silicon wafer 440 with a filter structure protects the fragile sensor membrane 120 from liquids or particles.