Patent Document

FIELD OF THE INVENTION 
     The present invention relates to a radar level gauge system using microwaves for measuring a level of a surface of a product in a container. More specifically, the invention relates to energy storage in such a gauge. 
     BACKGROUND OF THE INVENTION 
     Radar level gauges are suitably used for making non-contact measurements of the level of products such as process fluids, granular compounds and other materials. An example of such a radar level gauge can include a microwave unit for transmitting microwaves towards the surface and receiving microwaves reflected by the surface, processing circuitry arranged to communicate with said microwave unit and to determine said level based on a relation between transmitted and received microwaves, an interface for connecting said processing circuitry externally of said radar level gauge, and a power management circuitry providing said microwave unit and said processing circuitry with operating power. 
     In order to ensure a satisfactory signal level of the received echo, the emitted microwaves must have a sufficient power level. The processing of received signals also requires significant power, and in some cases the clock frequency of the processor is increased during the processing in order to enable high speed calculations. In combination, this results in an increased demand of power during certain parts of the measuring cycle. The power requirements are especially high for Frequency Modulated Continuous Wave (FMCW) systems. However, the provision of power is relatively difficult to achieve in practice, since energy is normally a scarce resource in the above-discussed type of gauges. 
     In particular, limited available power is a problem in systems using a two wire feeding system. Radar level gauges for measuring of a level in a tank, and other types of process sensors, are typically connected with a two-wire interface, where only two lines serve to both supply the sensor with limited power and to communicate a measured and processed measuring signal. The interface can be a 4-20 mA industrial loop with superimposed digital communication, or another two-wire fieldbus, such as Fieldbus Foundation (FF) or Profibus. Other possible interfaces include a four-wire interface, where two lines provide power, and two wires communicate measurement signals. In case of a 4-20 mA loop, the available power is thus dependent upon the signal value of the gauge, so that during periods with low signal value (e.g. around 4 mA) only a very limited power is available. Even during periods of high signal value (e.g. around 20 mA) the available power may not be sufficient to power the processing circuitry and the microwave emitter during a measurement cycle. 
     For this reason, power management in some form may be required, to distribute the available power between different components and over time. Such power management may include storage of energy in some kind of energy storage device, so that this energy can be used to boost the available power during periods of increased power requirements. The energy storage can take place in specifically designated stand-by periods, following each measurement cycle, or take place throughout the measurement cycle, during periods of low activity. 
     In the case where measurements are made in a tank containing explosive gas or liquids, or in any other situation where the sensor is located in an explosion endangered area, there is also an issue of explosion protection. Normally, either the installation is made explosion proof by some kind of encapsulation, or its outside electrical connection is made intrinsically safe (IS). The latter case requires that input power, voltage and current do not exceed levels stated by safety regulations (IS requirements). This is ensured by a so called electrical barrier, arranged in the interface to the intrinsically safe area. 
     As it is undesirable to capsulate the microwave electronics, the measurement device should operate at a relatively low voltage in order to comply with IS regulations. At such low voltages, energy storage is rendered inefficient as it requires large and slow capacitors. Typically, the minimum energy store capacitance falls within the range covered by aluminium electrolyte capacitors only. 
     An aluminium electrolyte capacitor looses roughly 20% of its initial capacity due to low temperature (−40 degrees C.) and 20% due to ageing (5000 hours). This may be compensated for at the expense of size, price and a significantly increased start-up time. In addition, if an aluminium electrolyte capacitor is used in a position where a major part of its energy is discharged during the system&#39;s active cycle, this will also significantly decrease the initial capacitance value. 
     Another major disadvantage with using large capacitors is that the life time is reduced dramatically when exposed to high temperatures. 
     Document U.S. Pat. No. 6,972,584 discloses a power decoupling circuit intended for an ultrasonic level gauge. In order to enable operation of the device, the voltage provided by the current loop is elevated by a step-up converter. Energy storage is also performed at this higher voltage level. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to address the above problems, and to provide improved energy storage in a radar level gauge without violating safety regulations (e.g. IS requirements). 
     This object is achieved with a radar level gauge, a power supply circuit and a method according to the appended claims. 
     According to a first aspect of the present invention, there is provided a radar level gauge comprising a microwave unit for transmitting microwaves into the tank, and receiving a reflection from the tank, processing circuitry connected to the microwave unit and arranged to determine the level based on a relation between transmitted microwaves and the reflection, a power interface for connecting the radar level gauge to an external power supply, and power management circuitry arranged to provide power at an operating voltage to the microwave unit and the processing circuitry. The power management circuitry includes a first voltage converter, having a low-voltage end for receiving a drive voltage from the power interface and a high-voltage end for supplying an intermediate voltage higher than the operating voltage, a temporary energy store arranged to be charged by the intermediate voltage, a second voltage converter, having a high-voltage end for receiving an input voltage from the energy store, and a low-voltage end for providing the operating voltage lower than the input voltage. 
     According to the present invention, energy is stored at a voltage higher than the voltage level of the current loop, and energy is consumed at a lower voltage level, preferably low enough to comply with IS regulations. 
     By storing energy at a higher voltage, a different type of energy store (e.g. low capacity capacitor) may be used. As a consequence, the cost and start-up time of the energy store is significantly reduced. 
     Further, as the intermediate voltage is converted down to the operating voltage, the intermediate voltage can be allowed to vary significantly, enabling a more efficient use of the energy store compared to any corresponding energy store provided at operating voltage, which typically can only be allowed to vary a few tenths of volts. 
     Another advantage is that the first voltage conversion of the drive voltage enables driving of the level gauge at a lower available drive voltage (lower required lift-off voltage). This leads to a more robust measuring device, and a quicker and less costly installation. 
     It is acknowledged that some prior art (e.g. U.S. Pat. No. 6,972,584) also teaches storage of energy at an elevated voltage level compared to the voltage of the current loop. However, such solutions have been limited to devices operating at an elevated voltage, and thus always requiring a step-up converter. The present invention is related to applications where the operating voltage is lower than the voltage of the current loop, for example due to IS regulations. In such applications, the introduction of a step-up converter, followed by a step-down converter, has been considered too inefficient to be contemplated by the person skilled in the art. The present invention is based on the surprising realization that the above mentioned advantages with energy storage at a higher voltage more than compensate the drawback of the multiple voltage conversions. 
     While it is generally difficult to encapsulate the microwave unit, as the potting material may change the properties of the circuitry, the temporary energy store may advantageously be encapsulated to eliminate risk of explosion. This allows more freedom when designing the radar level gauge to fulfill safety requirements in explosion risk applications. 
     The interface can be adapted to receive power in an intrinsically safe manner, typically by means of an electrical barrier. The interface can be a two-wire interface, arranged both to transmit measurement data to a remote location and to receive power for operation of the system. For example, the interface can be a 4-20 mA industrial loop with superimposed digital communication (HART), a Fieldbus Foundation bus, or a Profibus. Such loops are widely used to power radar level gauges. Alternatively, the interface can be four-wire interface. 
     According to one embodiment, the microwave unit is adapted to emit pulsed signals, and the processing circuitry is adapted to determine a filling level of the container based on the time between the emission of a pulsed signal and the reception of the reflected signal. This type of measuring is referred to as pulsed measuring. 
     According to a second embodiment, the microwave unit is adapted to emit waves over a range of frequencies, and the processing circuitry is adapted to determine a filling level of the container based on a mix of the emitted signal and the reflected signal. This type of measuring is referred to as FMCW (Frequency Modulated Continuous Wave). The microwave unit may also be adapted to emit pulsed waves with a number of different frequencies, referred to as MFPW (Multiple Frequency Pulsed Wave). 
     The advantages listed above are generally obtainable in any process variable sensor in an application where there are restrictions in the power supply. According to a second aspect of the present invention, therefore, there is provided a power management circuitry as disclosed above, but for use generally in any processing variable sensor. 
     According to a third aspect of the present invention, there is provided a method for providing operating power to a sensor for detecting a process variable, comprising storing energy in an temporary energy store at an intermediate voltage higher than an operating voltage required by said sensor, and converting an output voltage from said temporary energy store down to said operating voltage. 
     This method allows efficient energy storage, while enabling intrinsically safe operation of the sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention. 
         FIG. 1  is a functional block diagram of a radar level gauge in which the present invention can be implemented. 
         FIG. 2  is a schematic circuit diagram of a first embodiment of a power management circuitry according to a first embodiment of the invention. 
         FIG. 3  is a functional block diagram of a radar level gauge provided with a power management circuitry according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a schematic block diagram of a radar level gauge  10 , in which the present invention advantageously can be implemented. The radar level gauge is arranged to determine the position of the surface of a material  11  in a tank  12  (i.e. the filling level of the material  11 ). The radar level gauge  10  includes a microwave unit  13 , adapted to emit waves into the tank, and to receive reflected microwaves, processing circuitry  16  for communicating with said microwave unit and for determining a measurement result based on a relation between transmitted and received microwaves, and a power management unit  17  for providing required power to the processing circuitry and the microwave unit  13 . 
     The microwave unit  13  can comprise a microwave controller  14 , a microwave emitter/receiver  15 , and a signal transfer medium  18  connecting the emitter/receiver  13  to the controller  14 . The controller  14  is connected to the processing circuitry  16  by a data bus  20 , and is adapted to generate a microwave signal in accordance with control data from the processing circuitry  16 . The controller  14  can comprise a transmitter, a receiver, a circulator and any control circuitry required to manage these components. Further, the controller  14  can comprise an A/D-converter for digitizing a tank signal, i.e. a signal received from the tank. The emitter/receiver  15  can, as shown in  FIG. 1 , include a free radiating antenna  19  in the top of the tank, or alternatively the emitter/receiver  15  can include a probe extending into the tank. The signal transfer medium  18  can be a wire or cable, but can also include more sophisticated wave guides. In case of a explosive or otherwise dangerous content in the tank  12 , the signal transfer medium  18  may include an air tight seal passing through the tank wall. It is also possible that the controller  14  is connected directly to the emitter/receiver  15  with a suitable terminal, or that the emitter/receiver  15  is arranged on the same circuit board as the controller  14 , in which case the signal transfer medium simply may be a track on the circuit board. 
     The system  10  is connected to an interface  21 , for providing the system  10  with drive power, and possibly also for communicating a measurement result externally to the gauge system. In the illustrated example, the interface  21  is a two-wire interface, comprising two lines  22 ,  23 , and an electrical barrier  24 . The barrier  24  ensures that the area  25 , in which the gauge system  10  is installed, is intrinsically safe, i.e. that power, current and voltage transferred through the interface  21  are kept below given limits, reducing the risk of hazard. An example of such a two-wire interface, at the same time providing drive power and communicating a measurement signal, is a 4-20 mA industrial loop. 
     The power management unit  17  is connected to one of the lines  22  and is adapted to convert the voltage in the two-wire interface (typically in the order of 5-20 V), into an operating voltage suitable for the circuitry  16  and the microwave driver  14 , typically in the order of 3 V. In the simplest case, the power management unit  17  is a DC/DC step down converter and a smoothing capacitor. The power management unit is connected to the circuitry  16  via a line  26  and to the microwave driver  14  via a line  27 . 
     Both lines  22 ,  23  are further connected to a current control unit  28 , which is controlled by the processing circuitry  16  via a digital bus  29 . The bus  29  also carries communication according to the HART protocol, to be superposed in the current in the loop  22 ,  23 . The control unit  28  can be supplied with drive voltage from the power management unit  17 . 
     In use, the processing circuitry  16  controls the microwave controller  14  to generate a measurement signal to be emitted into the tank  12  by the emitter/receiver  15 . This signal can be e.g. a pulsed signal (pulsed level gauging or Multiple Frequency Pulsed Wave, MFPW), or a continuous signal with a frequency varying over a certain range (Frequency Modulated Continuous Wave, FMCW). The microwave emitter  15  acts as an adapter, enabling the signal generated in the controller  14  to propagate into the tank  12  as microwaves, which can be reflected by the surface of the material  11 . A tank signal, i.e. the emitted signal and its echo, or a mix of emitted and reflected signals, is received by the emitter/receiver  15 , and communicated to the microwave controller  14 , where it is received and A/D converted. The digitized signal is then provided to the processing circuitry  16  via bus  20 , and the processing circuitry  16  determines a measurement result based on a relation between the emitted and received waves. The measurement result is then communicated to the current control unit  28  via bus  29 , and the current flowing through the current control unit  28  is regulated so that the total current in the current loop corresponds to the measurement result. 
       FIG. 2  shows a power management circuitry  30  according to a first embodiment of the invention. This circuitry can advantageously be used as or be incorporated in the power management unit  17  in  FIG. 1 . 
     According to this embodiment, the circuitry  30  includes a DC/DC step-up converter  31 , here referred to as a boost converter, and a DC/DC step-down converter  32  connected in series. Both converters are preferably of the type that performs voltage conversion while essentially preserving the input power. (Of course, this is an ideal situation, in reality there will be a slight power loss due to conversion efficiency.) In between the two converters is provided a temporary energy store  33 . As temporary energy store it is possible to use a reservoir capacitor  33  or any other type of element or combination of elements adapted to store electrical energy when a voltage is applied over it. Of course, the temporary energy store may include other components in stead of or in addition to the capacitor  33 . For example, the temporary energy store may include a resistance in series with the capacitor  33 , in order to safeguard the capacitor against peak voltages. The resistance should preferably be so small that the voltage drop across this resistance is negligible at the typical currents. 
     The circuitry  30  preerably also comprises a diode network  38 , connected on one of the lines tio prevent energy from the energy store  33  from leaking back into the current loop  22 ,  23 . The diode network  38  may comprise one or several diodes, and simply ensures that no current is allowed to flow in the opposite direction than intended. 
     The circuitry  30  preferably also includes a current limiting unit  39 . The purpose of the current limiting unit  39  is to ensure that the power consumed by the power management unit  17  does not create a current in the loop exceeding the current value corresponding to the measurement value determined by the gauge. If, for example, the measurement result corresponds to a current in the loop of 5 mA, the current management unit  17  must not consume power so that the current in the loop exceeds 5 mA. This is ensured by the current limiting unit  39 . In a very simple case, the limiting unit  39  is just a fixed current limiter, limiting the current to the minimum value of the current loop, e.g. 4 mA. Alternatively, the current limiting unit can be controlled in accordance with the currently available current in the loop. For this purpose, a control signal  40  can be provided from the control unit  28 , or directly from the processing circuitry  16 . 
     In some situations, the energy storage in the power management circuitry  30  is too large to fulfill the IS regulations. The circuitry  30  may then be encapsulated in order to make the device explosion proof. 
     One alternative is to encapsulate the entire RLG  10 . However, it is generally difficult to meet explosion proof requirements, as the microwave unit  13  typically has a microwave cavity. Therefore, it may be desirable to encapsulate only the power management circuitry  30 , while the rest of the RLG  10  is intrinsically safe, i.e. fulfils suitable IS standard. In this case, a barrier  34  (similar in function to the barrier  24 ) may be arranged on the output side of the circuitry  30 , to ensure a limitation of extracted power and current. 
     The encapsulation may be made using a potting material. The encapsulation should preferably be free from cavities. By selecting a suitable potting material, more power can be dissipated in encapsulated small components and thus more power may actually be made available for consumption. The issue of surface temperature of specific components will in practice be transferred to an issue of whether the potting material is specified to withstand the maximum internal temperature. This means that the selected potting material needs to have good thermal conductivity or withstand high enough maximum temperatures (or both). 
     In use, the converter  31  converts the supply voltage V drive  on line  22  (typically in the order of 5-20 V, depending on factors such as line resistance) up to a higher intermediate voltage V int  (typically in the order of 25-30 V). Note that under some circumstances (with low available line voltage), the up transformation can be significant, and may be 4 or 5 times. Under other conditions, with higher available line voltage, the up-transformation may be less significant, and may be only around 25%. The capacitor  33  is therefore charged at the higher voltage V int , ensuring a short charging time. As an example, energy in the order of mWs can be stored in the capacitor  33 . At an intermediate voltage of 25 V, this corresponds to a capacitance in the order of tens of μF. Due to the relatively low requirement of capacitance, superior capacitor types like tantalum may be used, improving the robustness of the system. Such capacitors have limited temperature variation and better life span, especially at high temperatures. 
     The intermediate voltage V int  is subsequently stepped down to a lower level V op  by the step-down converter  32 . The voltage V op  can be essentially equal to the operating voltage of the processing circuitry  17  and/or microwave unit  13 , typically in the order of 3 V. 
     When the processing circuitry demands more power than is available from the interface  21 , the reservoir capacitor  33  will be discharged, thereby providing additional power needed e.g. for powering the microwave unit  13  during transmission. This will be especially important when the available current in the current loop is low (i.e. during periods of a low measurement value). 
     Optionally, the step-up converter  31  is provided with a control port  41 , and the step-down converter  32  is provided with a control port  42 , both arranged to receive a control signal  43 . This control signal  43  permits by-passing the energy storage in circuitry  30 . 
       FIG. 3  illustrates the system in  FIG. 1 , where the power management unit  17  is adapted to include a by-pass of power management circuitry  30  as mentioned above. The processing circuitry  16  here receives a monitor readout  44  from the power management unit  17  corresponding to the voltage V int  in  FIG. 2 , and returns the control signal  43  to the power management circuitry  17 . This control of the power management circuitry  17  provides the possibility to bypass the power storage in capacitor  33  during periods when no such storage is required, e.g. when a large current is available on the loop  22 ,  23 , or when the processing circuitry  16  requires an immediate voltage, e.g. during startup. 
     The monitor readout also provides a possibility to optimize the duration of the measurement cycle, in order to ensure that sufficient charging of the temporary energy store can be effected between measurements. In principle, monitor readout  46  can be used to initialize the next measurement cycle as soon as the temporary energy store is sufficiently charged. Such control would make the duration of the cycle dynamic, so that it will depend on the available power, i.e. the current in the loop. 
     The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the power management circuitry according to the invention is not necessarily provided in only one place in the radar level gauge, but may be distributed in the system. For example, the circuitry described with relation to  FIGS. 2 and 3  may be implemented directly in the microwave controller  14 .

Technology Category: 3