WO2001077625A1

WO2001077625A1 - System and method for remote monitoring of tank levels

Info

Publication number: WO2001077625A1
Application number: PCT/US2001/010931
Authority: WO
Inventors: Peter John Lagergren; Chester Cameron Allen
Original assignee: Firebird Data Communications, Inc.
Priority date: 2000-04-05
Filing date: 2001-04-04
Publication date: 2001-10-18
Also published as: AU2001249847A1

Abstract

Embodiments of the invention are described including a remote level monitoring device. The remote level monitoring device includes a ping source for placement on a tank containing a liquid. The ping source provides a ping signal that is transmitted into the tank. In a preferred embodiment, the ping signal is an ultrasonic signal. A transducer is provided for receiving reflections of the ping signal off of the top surface of the liquid. The difference between the time the ping is sent and the time it is received indicates the top level of the liquid in the tank. A signal indicating the level is transmitted to a central monitoring facility. Preferably, the signal is transmitted via a communications satellite. Refilling trucks are then sent to the remote site to refill the tank when the level signal indicates that refilling is necessary.

Description

SYSTEM AND METHOD FOR REMOTE MONITORING OF TANK LEVELS

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to the field of non-invasive monitoring of liquid levels in a storage tank. More specifically, the present invention relates to ultrasonic detection of the level in a storage tank and transmission of the level to a centralized monitoring facility for remote monitoring and refill, when necessary. Description of the Related Art

Providing remote power sources can entail logistical challenges. For example, oil well pumps may be place far from cities and communications facilities. These pumps must be place where the oil is. To provide the energy source for the pumps, the industry generally uses gasoline or diesel based engines to drive the pumps. Storage tanks at the site fuel these engines. To keep the oil flowing, these tanks must be replenished. However, each trip by a tanker truck is expensive. It is desirable to minimize the number trips taken by tanker trucks to the well site.

The prior art includes many techniques for measuring the amount of liquid in a storage tank. An example is the use of ultrasonic sensors. Examples of these devices can be found in Kronk, U.S. Patent No. 4,149,139 and are provided by companies such as Lundahl Instruments (http://www.lundahl.com). These devices provide accurate level measurements, but require constant monitoring and are coupled or placed into the inside of the tank. J-n remote locations, constant monitoring is not feasible. Personnel can be sent to the site, but this is contrary to the goal of avoiding trips to the site. In addition, invasive techniques create opportunities for leaks in remote tanks. With remote tanks, even a small leak can cause complete drainage because there is no one to notice the leak. It is desirable to provide a non-invasive technique for monitoring the level in remote tanks.

BRIEF SUMMARY OF THE INVENTION It is an object of the present invention to provide a non-invasive method of monitoring the level of a liquid in tank at a remote site. It is another object of the present invention to provide systems and means for monitoring levels in remote tank to maintain the level of a consumable liquid while miriimizing the number of visits to the remote tank to replenish the liquid.

It is yet another object of the invention to provide an economical system for tank level monitoring for use in remote locations that do not have ready access to communications facilities. These and other objects are provided by a remote level monitoring device. The remote level monitoring device includes a ping source for placement on a tank containing a liquid. The ping source provides a ping signal that is transmitted into the tank, h a preferred embodiment, the ping signal is an ultrasonic signal. A transducer is provided for sending the ping and receiving reflections of the ping signal from the air/liquid interface (i.e. the top surface of the liquid). The difference between the time the ping is sent and the time it is received indicates the top level of the liquid in the tank. A signal indicating the level is transmitted to a central monitoring facility. Preferably, the signal is transmitted via a communications satellite. Refilling trucks are then sent to the remote site to refill the tank only when the level signal indicates that refilling is necessary. BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, reference should be made to the following Detailed Description taken in connection with the accompanying drawings in which:

Figure 1 is a block diagram depicting an embodiment of the present invention; Figure 2 is a block diagram showing the operation of the level sensor in the embodiment of

Figure 1;

Figure 3 is a timing diagram illustrating the operation of the level sensor; and Figures 4 A and 4B are schematic diagrams of the transducer driving circuit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 depicts a preferred embodiment of the present invention. Tank 10 contains liquid

12 having a top surface 14. Ultrasonic level monitor 20 is placed on the bottom of the tank. Preferably, level monitor 20 is placed so that the ultrasonic pings (explained below) emitted by level monitor 20 propagate vertically. This makes their direction of propagation perpendicular to top surface 14 of the liquid. The level of top surface 14 is provided electronically by level monitor 20 to satellite transceiver 22. A battery (not shown) preferably powers both level monitor 20 and transceiver 22 that is kept charged by solar collection device 24. Satellite transceiver 22 transmits the level of top surface 14 via satellite 26 to receiving station 28. The interval between transmissions is selected to conserve battery power while minimizing the chance that liquid 12 will be completely consumed. An example interval for an installation such as an oil pump would be one week. Of course, greater or lesser intervals may be employed and are considered within the scope of the present invention.

The level signal received at receiving station 28 is converted by TCP/IP converter 30 from a raw signal to a message in a format suitable for transmission on the Internet 32. The Internet is selected because it is a nearly ubiquitous public network. Other networks may be used for transmission of the level data as suits the circumstances of a particular application of the invention. The level signal is provided to a central monitoring station 34. The level signal is stored and analyzed to determme when to dispatch a fuel truck to replenish liquid 12 such that tank 10 is nearly empty and the maximum amount of fuel can be delivered on each trip by the fuel truck. This I preferably determined using historical level signals to determine a usage rate and comparing the rate to the current level. If the measurement interval is one week, the usage rate combined with the current level may indicate that the fuel will be consumed in three days. By using historical usage data, the measurement interval can be maximized (thus conserving battery power) while the tank can be filled when nearly empty (thus minimizing fuel truck trips). Figure 2 shows the operation of level sensor 20. Level sensor 20 included piezoelectric transducer 42, driving circuit 44 and control logic 46. Piezoelectric transducer 42 preferably emits a very short ultrasonic signal 48 that is coupled to the lower surface of tank 10. The ultrasonic ping 48 travels to top surface 14 and is partially reflected due to the abrupt change in acoustical impedance between liquid 12 and the air in the rest of tank 10. This reflected signal 50 has the resonant frequency of piezoelectric transducer 42 because it originated from piezoelectric transducer 42. Thus, reflected signal 50 induces a signal that can be monitored by driving circuit 44. The level of the top surface 14 is determined by the time required for the ultrasonic ping 48 to travel from piezoelectric transducer 42 and return. This difference is processed by control logic 46 and provided to transceiver 22 for transmission. Preferably, several pings are sent to provide an average reading to increase accuracy. Logic circuits 46 calculate the timing for the ping and provide wake- up signals to system when a reading is to be taken.

Figure 3 is a timing diagram depicting the signals provided by ping source 42 and received by sensor 44. As shown, ping signal 48 leads the receipt of reflected signal 50 by a certain interval. In Figure 3, those intervals are ti, and t₂ for the two pings emitted. Using the speed of sound in the liquid 12, an approximate level of top surface 14 can be determined. Preferably, sensor 20 is calibrated on site for accuracy. This calibration involves setting the tank level to selected levels, measuring the level using sensor 20 and entering an adjustment factor for the difference. The adjustment factors are stored on programmable read-only memory in control logic 46 and used for calculating the sensed level.

Figures 4A and 4B are schematic diagrams of the transducer driving circuit 44 and control logic 46. The overall operation of the circuit can be described as operating in four states. The four states are defined by the status of transistors 74, 76 and 86. The states are as follows: State 0 - Wait for echo The transistors are switched as follows. Transistor 74 - Off (+ 12 Volt switch) Transistor 76 - On (Transducer input switch) Transistor 86 - Off (Input ground switch)

This state connects the transducer 72 to the comparator 102A Ain+. The inductor 84 is switched in parallel with this input. Comparator 102A Ain- provides the reference voltage for the comparator. If an echo is received by the transducer which produces a voltage pulse on Ain+ greater than on ATN- the Aout will go positive and trigger the comparator 102B Bin+. The output on Aout is a very short (~50 nanoseconds) positive 5 volt pulse. The comparator 102B is configured as a one shot comparator and will produce a+5 V output pulse having a duration that is adjustable from ~0 to 20 μS long using resistor 98 with capacitor 96. The resistors 87 and 89 set the input voltage on Ain+ of 102 A to +15 mN and the resistors 112 and 114 set the initial input voltage on Ain- to +35mN. The transistor 106, resistor 108, capacitor 110 and resistor 116 produce an additional time dependent input voltage on Ain- of about +1.5 V with a decay time constant of about 40 μS. The 102B Bout positive edge starts the next state. State 1 - Pump The transistors are switched as follows. Transistor 74 - On Transistor 76 - On Transistor 86 - On The absolute maximum current through inductor 78 is limited to about 4 Amps by resistor

80, inductor 78 and the resistance of transistors 76 and 86. However, the working current will be set by adjustment of the pump time. Comparator 90A is configured as an adjustable delay. It is used to control the time that transistor 76 remains on after Transistor 74 is turned on. During Pump time, current builds up in the inductor LI . The maximum usable current may be limited by magnetic saturation of inductor 78. The pump time is set by adjusting resistor 91. State 2 - Ping Transistor 74 is switched off.

The interruption of current in inductor 78 causes an inductive kick of as much as 300 volts. Diode 82 allows the voltage to swing negative 150 to 200 volts. Resistor 98 is used to adjust the transistor 76 turn on time to occur at the end of one full cycle. The transistor 76 off time may be adjusted from near zero to about 20 μS. The adjustments of resistors 98 and 91 interact in that the pump time (set by resistor 91) adds its time in front of the ping time (set by resistor 98). State 3 - Damp Transistors 76 and 86 are on Transistor 74 is off.

The comparator 126B is configured as a one shot timer which starts with the comparator 102B as a trigger. The output of comparator 126B Bout drives transistor 86 "on" which clamps the input of the comparator 102 A at Ain+ to ground for about 70 μS. This action also damps the transducer to reduce ringing. The turn off of transistor 86 is fairly quiet. State 0 - Return to Wait for Echo

The output of 126B also resets the comparator 126A. The 70 μS signal from 126A is used to charge the capacitor 148. The capacitor 148 will remain charged for about 4.5 milliseconds. If no echo is received during that time there will be no recharge of capacitor 148 and the output Aout of comparator 126A will produce a kick start pulse to the input Ain+ of 102A. The circuit will cycle through all states back to state 0 looking for an echo. Analog Control

At higher pulse rates, it is necessary to reduce the average power being pumped into the transducer. Driving circuit 44 automatically reduces the amount of energy in each pulse for close range echo by the reduced amount of charge time of resistor 93 into capacitor 88. This reduces the discharge time through resistor 91, which gives a reduced pump time for the charging of inductor 78 and a lower the inductive kick voltage to the transducer. In addition, the amount of charge received by capacitor 110 during the shorter pump time is reduced so that the average sensitivity of the detector (comparator 102A) is increased.

The operation of driving circuit 44 is now described in more detail. The piezoelectric transducer 42 is positioned on the surface of tank 10. Piezoelectric transducer 42 is connected to terminals 72. At the beginning of a measurement cycle (i.e., State 1 - Pump), transistors 74, 76 and 86 are conductive ("on"). This causes a large current from the +12 volt supply to flow through coil 78 via resistor 80, diode 82 and transistor 86. After a period of time, for example 2-3 μS, as determined by the value of capacitor 88, the Ain- input of comparator 90A is pulled below the voltage set on ATN+ by resistors 92 and 94. This causes the output of Comparator 90 A to go low, which shuts off transistor 76. This causes the magnetic field of coil 78 to collapse, thus placing an approximate 300V+ spike onto transducer 42. This causes a ping to be generated by transducer 42 at its characteristic frequency. A physical backlash in piezoelectric transducer 42 then causes a 300V- spike to create the signal shown in Figure 3. The low output from comparator 90A causes a high output from comparator 90B. When the output of comparator 90B was low, a voltage was set on capacitor 96 by the voltage divider comprising resistors 98 and 100. When the output of comparator 90B rises the voltage on capacitor 96 us pulled up through resistors 98 and 100 at a rate determined by the value of those resistors and the capacitance of capacitor 96. Approximately 2-3 μS after transistor 76 is turned off, the output of comparator 102B goes low, which allows the base of transistor 104 to be pulled low tlirough resistor 106. Also, the charge on capacitor 88 is discharged through diode 108. This comparator 90A to provide a high output which turns on transistor 76. This provides bias to piezoelectric device 42 and allows a signal received by the piezoelectric device to pass through to comparator 102A. The delay allows for the dissipation of the energy applied to the piezoelectric device and for exclusion of near reflections such as from the walls of tank 10. When the output of comparator 102B was high, capacitor 110 was charged through transistor 106. Also, the voltage on the Ain- input of comparator 102A is set by a voltage divider comprising resistors 112 and 114. The RC circuit including resistor 116 has a compensating effect on the signal applied to input Ain- to compensate for attenuation of a reflection signal that is received later. This reflected ping signal will have traveled farther and will provide a weaker signal. If a reflected signal is received before the Bin- input of comparator 102B rises too high, the output of comparator 102 A is high, which provides a high input signal to Bin+ of comparator 102B tlirough diode 124 through resistor 118. This causes a positive pulse at the output of comparator 102B that ends when Bin- rises above the voltage divided between resistors 120 and 122. This output of comparator 102B provides a voltage at the Bin+ input of comparator 126B that is determined by division of the output voltage by resistors 128 and 132, and diode 130. This causes the output of comparator 126B to go high. However, this signal is fed back to the Bin- input of comparator 126 through resistor 134. The rise on Bin- is slowed by the presence of capacitor 136. After a predetermined period, the Bin- input will rise above the Bin+ input, thus providing a pulse indicative of a received signal on at the output of comparator 126B. This signal is provided to processor 202 of Figure 4B.

Prior to the receipt of the signal from comparator 102B, the Bin- input to comparator 126B is pulled low through resistor 138 and diode 140 right after transistor 86 was shut off to begin listening for the return signal. The amount of time that Bout is low determines the voltage level on capacitor 136 when the signal pulse arrives. This determines the amount of time until the Bin- input of comparator 126B rises above the Bin+ input, thus lowering the output of comparator 126B. Thus, the width of the pulse provided to processor 202 from comparator 126B is proportional to the time between the delivery of the ping and receipt of a legitimate echo.

When the output pulse from comparator 126B occurs, Ain+ of comparator 126A is pulled up through resistor 142 and diode 144. This keeps the output of comparator 126A high. If no pulse is received, after a certain length of time determined by the value of resistor 146 and capacitor 148, the voltage on Ain+ of comparator 126A will fall below that on Ain- as set by the voltage divider comprising resistors 150 and 152. This pulls Aout low and thus pulls Ain- of comparator 102A low. This simulates the receipt of a signal and triggers the automatic cycling of the circuit to produce another ping. Thus the device will not wait forever for a reflectd signal that may have been lost. For example, wind or an impact may cause the top surface 14 to be choppy. This type of surface will scatter the ping signal.

When a signal is received, the output of comparator 126B starts another ping cycle by turning on transistor 86 and thus charging coil 78 as described above. The circuits of Figures 4A and 4B are powered using a +12 volt power source. The bulk of the circuitry in driving circuit 44 is powered using voltage regulator 204, which is controlled by processor 202 through transistors 206 and 208. Power through regulator 204 is only provided when a storage measurement is being taken, thus conserving power. Power to processor 202 and clock circuit is provide by regulator 160, which is a low current regulator. Processor 202 conducts a measurement ideally once a week. This period can be varied according to the needs of the situation and remain within the scope of the invention. Real-time clock 208 provides a signal INTO\ on this interval to trigger processor 202 via capacitor 216. Real-time clock maintains the time with high accuracy by using crystal 210 as a timing source. Processor 202 has a separate internal clock circuit controlled by crystal 212 and capacitors 214. The operational program is stored on programmable memory in processor 202.

At the beginning of a measurement cycle, processor provides a power-up signal on pin 18 that provides bias to the base of transistor 208 via resistors 218 and 220. After an adequate time for power-up, the circuit of Figure A is set to a known state by placing a high signal on pin 13. This charges capacitors 110 and 148 to their initial setting. After several ping cycles, the logic of processor 202 determines a liquid depth using well-known statistical analytical techniques. For example, an average of the depths after dropping anomalous readings may be used. When data is ready, a wake up signal is provided to the transmitter by raising pin 16 of processor 202 to turn on transistor 162 via resistors 164 and 166. The data are then transmitted to RS232 driver 222, which provides the necessary voltages and protocols to communicate with the transmitter using the RS232 standard. The depth data is then transmitted to central office 34 via satellite 26. Thus a low power system capable of wireless transmission of depth data is shown. After the measurement cycle, the signal on pin 18 of processor 202 is brought low, thus powering down the bulk of the circuitry.

Although specific embodiments of the present invention are described herein, they are not to be construed as limiting the scope of the invention. For example, although satellite transmission of the depth data is show, other transmission techniques may be advantageously used within the scope of the invention. Many embodiments of the invention will become apparent to those skilled in the art in light of the teachings of this specification. The scope of the invention is only limited by the claims appended hereto.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is set forth in the following claims.

Claims

1. A remote level monitoring device comprising: a ping source for placement on a tank containing a liquid, the ping source providing at least one ping at a first time; a transducer for receiving reflections of the ping off of the top surface of the liquid at a second time; a comparator for comparing the first and second times and providing a level signal indicating the level of the liquid in the tank; and a transmitter for transmitting the level signal to a central monitoring facility.

2. A device as in Claim 1 wherein the transmitter transmits the level signal using a radio frequency signal.

3. A device as in Claim 1 wherein the transmitter transmits the level signal via a communications satellite.

4. A device as in Claim 1 where in the ping source is positioned such that the field of travel of the ping signal is perpendicular to the top surface of the liquid.

5. A device as in Claim 1 wherein the ping source and the transducer are housed in a common housing.

6. A device as in Claim 1 wherein the ping is an ultrasonic ping.

7. A device as in Claim 1 wherein the transmitter is powered using a battery and the interval between transmissions of the level signal is at least one week.

8. A device as in Claim 1 where the transducer and ping source are positioned on the outer surface of the tank.

9. A remote level monitoring system comprising: a ping source for placement on a tank containing a liquid, the ping source providing at least one ping at a first time; a transducer for receiving reflections of the ping off of the top surface of the liquid at a second time; a comparator for comparing the first and second times and providing a level signal indicating the level of the liquid in the tank; a transmitter for transmitting the level signal to a central monitoring facility; and a receiving station for receiving the level signal and determining the status of the liquid in the tank.

10. A system as in Claim 9 wherein the level signal is transmitted via a satellite communications channel.

11. A system as in Claim 9 wherein the receiving station issues an alert to refill the tank when it is determined that the tank is nearly empty.

12. A method for monitoring the level of a liquid in a remote tank, comprising the steps of: periodically issuing at least one ping signal into the liquid at a first time; a receiving reflections of the ping off of the top surface of the liquid at a second time; a comparing the first and second times and providing a level signal indicating the level of the liquid in the tank; and a transmitting the level signal to a central monitoring facility.

13. A method as in Claim 12 wherein the level signal is transmitted using a radio frequency signal.

14. A method as in Claim 12 wherein the level signal is transmitted via a communications satellite.

15. A method as in Claim 12 where in the field of travel of the ping signal is perpendicular to the top surface of the liquid.

16. • A method as in Claim 12 wherein the ping signal is an ultrasonic ping signal.

17. A method as in Claim 12 wherein the interval between transmissions of the level signal is at least one week.

18. A method as in Claim 12 where a ping source for providing the ping signal and a transducer for receiving the reflections are positioned on the outer surface of the tank.

19. A method for monitoring the level of a liquid in a remote tank, comprising the steps of: periodically issuing at least one ping into the liquid at a first time; a receiving reflections of the ping off of the top surface of the liquid at a second time; a comparing the first and second times and providing a level signal indicating the level of the liquid in the tank; a transmitting the level signal to a central monitoring facility; a receiving the level signal at a receiving station; and determining the status of the liquid in the tank at the receiving station.

20. A method as in Claim 19 wherein the level signal is transmitted via a satellite communications channel.

21. A method as in Claim 19 further including the step of issuing an alert to refill the tank when it is determined that the tank is nearly empty.