Abstract:
A radar level gauge using microwaves for measuring a level of a surface of a product in a container, comprising a two-wire interface for connecting said measurement circuitry externally of said radar level gauge. A voltage protection circuitry connected between the two lines of the two-wire interface, the voltage protection circuitry including a transistor having an input, an output and a control terminal, a voltage regulator component connected between the output and the control terminal in such a way that the regulating characteristics of the voltage regulator component determines the voltage required to open the transistor, and a resistor connected between the control terminal and the input of the transistor.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a voltage protected sensor for gauging a process variable, and more specifically a radar level gauge with a voltage protection circuit.  
       BACKGROUND OF THE INVENTION  
       [0002]     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 power and to communicate a measured and processed measuring signal. The interface can be a 4-20 mA industrial loop. Other possible interfaces include a four-wire interface, where two lines provide power, and two wires communicate measurement signals.  
         [0003]     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, special protection is required. Normally, either the installation is made explosion proof by some kind of casement, or its outside electrical connection is made intrinsically safe. The latter case requires that input power, voltage and current do not exceed levels stated by safety regulations. This is ensured by a so called electrical barrier, arranged in the interface to the intrinsically safe area.  
         [0004]     Such a barrier can comprise fuses on each line for protection against excessive input power/current, and at least one voltage protection circuit, here referred to as a voltage shunt, connected between the two lines for protection against excessive voltages. The voltage shunt is typically based on one or several zener diodes. Further, resistors are typically connected on each line, to protect the area against excessive output power/current.  
         [0005]     In addition to the safety aspect, the barrier should have little or no impact on the normal function of the sensor equipment. It is therefore important to reduce any current through the voltage shunt, as such current consumption would disturb the power supply and the reliability of the sensor signal.  
         [0006]     For this purpose, a series voltage regulator is normally arranged prior to the electrical barrier in order to ensure that the voltage over the shunt during operation is smaller than the threshold voltage, thus avoiding any current through the diode. The series voltage regulator enables the sensor to handle a greater input voltage range.  
         [0007]     The operation output voltage from the preceding regulator is preferably as close to the limiting threshold voltage of the shunt as possible, in order to provide the sensor electronics with the required power. However, the voltage regulation characteristics of the diode can make a safety margin necessary, thus reducing the maximum operation voltage from the regulator.  
       SUMMARY OF THE INVENTION  
       [0008]     A first aspect of the invention relates to a radar level gauge using microwaves for measuring a level of a surface of a product in a container, comprising a microwave emitter for transmitting microwaves towards the surface and receiving microwaves reflected by the surface, measurement circuitry arranged to determine said level based on a relation between transmitted and received microwaves, a signal transfer medium coupled at a first end to said microwave emitter and coupled at second end to said measurement circuitry, a two-wire interface for connecting said measurement circuitry externally of said radar level gauge, and voltage protection circuitry connected between the two lines of the two-wire interface. The voltage protection circuitry includes a transistor having an input, an output and a control terminal, a voltage regulator component connected between the output and the control terminal in such a way that the regulating characteristics of the voltage regulator component determines the voltage required to open the transistor, and a resistor connected between the control terminal and the input of the transistor.  
         [0009]     This aspect of the invention can more generally be directed to any sensor for gauging a process variable having a two-wire interface provided with such a voltage protection circuit.  
         [0010]     In such a voltage protection circuit, almost all power is dissipated in the transistor (preferably a power transistor), enabling the use of a low power voltage regulating diode. A low power diode, for example a zener diode, can be chosen to have accurate voltage regulation at very low currents. This characteristics will then decide the characteristics of the entire circuit, providing a high power protection circuit with improved characteristics.  
         [0011]     Also, transistor components are generally more suitable for dissipating higher amount of energy.  
         [0012]     In case the transistor is a field effect transistor (e.g. a MOSFET), the control terminal is the gate, the output is the drain and the input is the source. In case the transistor is a bipolar transistor, the control terminal is the base, the output is the collector, and the input is the emitter.  
         [0013]     A second aspect of the invention relates to a radar level gauge using microwaves for measuring a level of a surface of a product in a container, comprising a microwave emitter for transmitting microwaves towards the surface and receiving microwaves reflected by the surface, measurement circuitry arranged to determine said level based on a relation between transmitted and received microwaves, a signal transfer medium coupled at a first end to said microwave emitter and coupled at second end to said measurement circuitry, a two-wire interface for connecting said measurement circuitry externally of said radar level gauge, and voltage protection circuitry connected between the two lines of the two-wire interface. The voltage protection circuitry includes a voltage regulator device having a cathode and an anode, a first diode connected with its anode to the first line and its cathode to the cathode of the regulator device, a second diode connected with its cathode to the second line and its anode to the anode of the regulator device, a third diode connected with its cathode to the cathode of the first diode and its anode to ground, a fourth diode connected with its anode to the anode of the second diode and its cathode to ground.  
         [0014]     This aspect of the invention can more generally be directed to any sensor for gauging a process variable having a two-wire interface provided with such a voltage protection circuitry.  
         [0015]     By selecting the components appropriately, such a voltage protection circuit can be made symmetrical with respect to the ground connection. Such a circuit will allow connection to a power supply independent of its ground connection, e.g. with positive ground connection as well as negative.  
         [0016]     The voltage regulator component can be embodied by a voltage protection circuit according to the first embodiment of the invention, but this is not a requirement. On the contrary, the voltage protection circuit according to the second aspect of the invention by itself represents significant advantages over prior art.  
         [0017]     A third aspect of the invention relates to radar level gauge using microwaves for measuring a level of a surface of a product in a container, comprising a microwave emitter for transmitting microwaves towards the surface and receiving microwaves reflected by the surface, measurement circuitry arranged to determine said level based on a relation between transmitted and received microwaves, a signal transfer medium coupled at a first end to said microwave emitter and coupled at second end to said measurement circuitry, a two-wire interface for connecting said measurement circuitry externally of said radar level gauge. The gauge further comprises voltage protection circuitry connected between the two lines of the two-wire interface, said voltage protection circuit being symmetric with respect to ground, a first current controlled voltage regulator connected on the first line, and a second current controlled voltage regulator connected on the second line, said voltage regulators being controlled by a regulating current flowing through the voltage barrier.  
         [0018]     This aspect of the invention can more generally be directed to any sensor for gauging a process variable having a two-wire interface provided with such a voltage protection circuitry and voltage regulators.  
         [0019]     By dividing the voltage regulation preceding the voltage shunt on the two lines, the symmetry of the circuit can be ensured. As the regulators are both controlled by a small regulating threshold current through the voltage protection circuit, they will enable adjustment of the voltage drop over each line in order to reduce current consumption to ground due to differences in ground potential between the communication interface and the power supply.  
         [0020]     The circuit configuration may include a symmetric voltage protection circuit according to the second aspect of the invention, but this is not a requirement. On the contrary, the regulator configuration according to the third aspect of the invention by itself represents significant advantages over prior art. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     These and other aspects of the present invention will be further described with reference to the appended drawings, illustrating presently preferred embodiements.  
         [0022]      FIG. 1  is a block diagram of a radar level gauge system including a two-wire interface provided with an electrical barrier.  
         [0023]      FIG. 2   a  is a block diagram of a voltage protection circuit according to a first embodiment of the invention.  
         [0024]      FIG. 2   b  is a block diagram of a voltage protection circuit according to a second embodiment of the invention.  
         [0025]      FIG. 3  is a block diagram of a voltage protection circuit according to a third embodiment of the invention.  
         [0026]      FIG. 4  is a block diagram of a voltage protection circuit according to a fourth embodiment of the invention.  
         [0027]      FIG. 5  is a block diagram of a sensor provided with a voltage protection circuit according to a fifth embodiment of the invention.  
         [0028]      FIG. 6  is a more detailed circuit diagram of a voltage regulator in  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0029]      FIG. 1  shows an example of a radar level gauge system  30 , including a microwave emitter  31  extending into a tank  32 , a measurement circuitry  33 , and a signal transfer medium  34  connecting the emitter  31  to the circuitry  33 . The microwave emitter  31  is arranged to act as an adapter, transmitting microwaves into the tank  32  to be reflected by a surface of a material  35  in the tank. The microwaves can be guided into the tank by a probe, as shown in  FIG. 1 , or the emitter can be a free radiating antenna in the top of the tank. The reflected waves are fed by the signal transfer medium  34  back to the circuitry  33 , and a measurement result based on this signal is determined. The result typically represents a level indication of the material  35 . The signal transfer medium  34  can comprise a wire or cable transmitting a pulse signal from the circuitry  33 , as well as a wave guide connecting to the emitter  31 . It is also possible that the circuitry  33  is connected directly to the emitter  31  with a suitable terminal, which then constitutes the transfer medium  34 , or that the emitter is arranged on the same circuit board as the circuitry  33 , in which the transfer medium  34  can be a conducting path on this circuit board.  
         [0030]     The circuitry  33  is further connected to a two-wire interface  36 , comprising two lines  1 ,  2 , and an electrical barrier  3 . The barrier ensures that the area  37 , in which the gauge system is installed, is intrinsically safe, i.e. that power, current and voltage are kept below given limits, reducing the risk of hazard.  
         [0031]     The interface  36  is typically arranged to provide power to the measurement circuitry  33 , and possibly also to communicate a measurement result from the circuitry  33 . An example of such a two-wire connection, at the same time providing drive power and communicating a measurement signal, is a 4-20 mA industrial loop.  
         [0032]     If advantageous, the measurement circuitry  33  can be connected to means for storing energy, such as a capacitor or battery  38 , enabling storage of surplus energy during periods of low power consumption  
         [0033]     The barrier  3  comprises fuses  4 ,  5  on each line  1 ,  2 , a voltage protection circuit  6  between the lines, and resistors  7 ,  8  connected on each line  1 ,  2  after the voltage protection circuit  6 . During operation, the fuses  4 ,  5  protect the area  36  from excessive input power/current, the circuit  6  protects the area  36  from excessive voltages, and the resistors  7 ,  8  protects the area  36  from excessive output current/power. In order to comply with safety regulations for intrinsically safe areas the voltage protection circuit has to be redundant. For example, in order to comply with intrinsically safe category “ia” (e.g. according to EN50020), three parallel voltage protection circuits  6  are required. The circuit  6  is also referred to as a voltage shunt, and the voltage between the two lines USHUNT as the shunt voltage.  
         [0034]     The voltage protection circuit  6  in  FIG. 1  can, according to an embodiment of the invention, comprise a circuit  10  as shown in  FIG. 2   a . The illustrated circuit  10  comprises an N-type field effect transistor  11  and a zener diode  12  having its cathode connected to the drain of the transistor, and its anode connected to the gate of the transistor. Further, a resistor  13  is connected between the source and gate of the transistor.  
         [0035]     During operation, the diode  12  will thus act as a feedback control of the transistor  11 , ensuring that the voltage u DS  over the circuit is kept close to, but not exceeding, the threshold voltage or regulator voltage of the zener diode  12  plus the voltage over the resistor  13 . The resistor is chosen such that this voltage, resulting from the threshold current i SH  of the diode, typically in the order of μA, is below the gate threshold of the transistor  11 . As soon as the voltage over the circuit exceeds a certain level, the voltage over the diode will exceed the threshold voltage, and the current iSH through the diode will increase rapidly. This will cause the voltage over the resistor, and thus the gate voltage, to increase, and cause the transistor to open. The transistor will thus handle the energy dissipation resulting from such an over voltage.  
         [0036]     Of course, the transistor can instead be a P-type FET, in which case the zener diode should be connected as indicated in  FIG. 2   b , showing an alternative circuit  10 ′.  
         [0037]     The transistor does not need to be a FET, but any type of transistor can be used, such as an NPN or PNP bipolar transistor, in which case the diode is connected between the base and the collector with suitable polarity. The function of the circuit would be similar, although the bipolar transistor is current controlled instead of voltage controlled.  
         [0038]     As the circuit  10 ,  10 ′ has similar characteristics as a regulator diode, the terminal  25  where current iSH enters the circuit during operation will be referred to as a cathode, and the terminal  26  where current exits the circuit during operation will be referred to as an anode. It should be noted that the polarity of the circuit  10 ,  10 ′ coincides with that of the voltage regulator component  12 .  
         [0039]     The voltage regulator component, which has been illustrated by a zener diode  12 , can be replaced with any other diode having the desired threshold characteristics, such as an avalanche diode, or a more complex component, such as an integrated circuit.  
         [0040]     The basic principle of the circuit in  FIGS. 2   a  and  2   b  is to bypass the greater part of the current flowing through the circuit via the transistor. This enables the use of a relatively low power diode, e.g. in the order of 0.1 Watt, typically offering significantly better voltage regulation than more powerful diodes. The characteristics of the diode are then transferred to the entire circuit.  
         [0041]     A voltage protection circuit  106  according to a further embodiment of the invention is shown in  FIG. 3 , connected between the lines  101 ,  102  of a two-wire sensor. A voltage regulator component  107 , such as a zener diode, acts as a voltage protection between the lines  101 ,  102 . A first diode  108  is connected with its cathode to the cathode of the voltage regulator component (here the cathode of the zener diode) and its anode to the first line. A second diode  109  is connected with its anode to the anode of the voltage regulator component (here the anode of the zener diode) and its anode to the second line. The cathode of a third diode  110  is connected to the cathode of the voltage regulator component, and the anode of a fourth diode  111  is connected to the anode of the voltage regulator component. The anode of the third diode and the cathode of the fourth diode are connected to ground  112  via a pair of zener diodes  113  connected in series cathode to cathode. A fifth  114  diode is connected with its cathode to the first line  101  and its anode to the second line  102 .  
         [0042]     By selecting the various components the circuit in  FIG. 3  can be made symmetric with respect to the ground connection  112 . Further, the circuit will provide a given limiting voltage not only for the voltage between the lines  101 ,  102 , but also between each line  101 ,  102  and ground  112 .  
         [0043]     A voltage between the lines greater than the limiting voltage of voltage regulator component  107  will cause a current through diodes  108 ,  107  and  109 . If the polarity of the line voltage is reversed, a current will immediately flow through diode  114 .  
         [0044]     A voltage between the first line  102  and ground  112  greater than the limiting voltage of voltage regulator component  107  will cause a current through diodes  108 ,  107 ,  111  and  113 , or through diodes  113 ,  110 ,  107 ,  109  and  114  depending on polarity.  
         [0045]     Finally, a voltage between the second line  102  and ground  112  greater than the limiting voltage of voltage regulator component  107  will cause a current through diodes  114 ,  108 ,  107 ,  111  and  113 , or through diodes  113 ,  110 ,  107  and  109 , again depending on polarity.  
         [0046]     An alternative to the circuit in  FIG. 3  is shown in  FIG. 4 , and is denoted  106 ′. Here, the two diodes  108  and  109  have been replaced by voltage regulator diodes (e.g. zener diodes)  116  and  117 , and the fifth diode  114  has been eliminated.  
         [0047]     Here, the voltage between the lines  101  and  102  is regulated by the limiting voltages of diodes  116 ,  107  and  117  (regardless of polarity). The diodes  116  and  117  preferably have the same limiting voltage as the voltage regulator component  107 . The voltage between line  101  and ground  112  is now regulated by the limiting voltage of the diode combination  116 ,  107 ,  111  and  113 , or  113 ,  110 ,  116  depending on polarity. The voltage between line  102  and ground  112  is now regulated by the limiting voltage of diode combination  117 ,  111  and  113  or  113 ,  110 ,  107  and  117 , depending on polarity.  
         [0048]     The voltage regulator component  107  in  FIGS. 3 and 4  can be a voltage protection circuit as according to the first aspect of the invention, e.g. a circuit as disclosed in  FIG. 2 . However, it may also, as indicated above, be a zener diode or any other type of voltage regulator diode.  
         [0049]     As was mentioned before, an electrical barrier is typically preceded by a voltage regulator, in order to ensure that the voltage over the shunt does not exceed the threshold voltage.  FIG. 5  shows how such a regulator can be implemented without loosing the symmetry of the voltage limitation.  
         [0050]     According to the embodiment in  FIG. 5 , the two-wire sensor of  FIG. 1  has been provided with a voltage shunt  106  according to  FIG. 4 , where the voltage regulator component  107  has been substituted with a circuit  10  according to  FIG. 2   a . Components corresponding to the components in previous figures have been given identical reference numerals. Further, the circuit is provided with two voltage regulators  15 ,  16 , one on each line  1 ,  2 . Note that, as mentioned above, several identical voltage shunts may be provided in parallel between the lines  1  and  2 , in order to comply with safety regulations, such as category “ia”.  
         [0051]     In a system with a grounded supply, the output voltage of the regulators  15 ,  16  dependes on how the voltage shunt is connected. The barrier should ensure that the voltage between any one of the lines  1 ,  2  and ground does not exceed a given limit. The purpose of the regulators is to keep the current comsumption as low as possible, preferably equal to the regulating threshold current of the circuit  10 .  
         [0052]     The voltage regulators  15  and  16  are complement to each other, with regulator  15  connected in series with the positive line  1  and regulator  16  connected in series with the negative line  2 .  
         [0053]     The voltage drop—input to output—u REG1  and u REG2  generated on each voltage regulator  15 ,  16  is controlled by a small regulation current in adherent control inputs  17  and  18 . Further, as the control input  17 ,  18  is connected to the regulator terminal facing the sensor (the output  19  of regulator  15  and the input  20  of regulator  16 ), the voltage difference between these two terminals is close to zero.  
         [0054]     An increased control current drawn from the control terminal  17  on voltage regulator  15  will produce a proportionally increased voltage drop over the regulator  15 . Operation is similar for regulator  16  but control current is conducted into control terminal  18 .  
         [0055]     Control terminals  17  and  18  on voltage regulators  15  and  16  are each connected to opposite side of the voltage protection circuit  10 . As soon as the voltage U DS  present between the cathode and anode of voltage protection circuit  10  is above the regulation threshold, a control current essentially equal to the threshold current will be conducted from control terminal  17  through voltage protection circuit  10  into control terminal  18 .  
         [0056]     At low supply input voltage between line  1  and line  2 , the voltage regulators will be saturated (left open). The reason is that there will be no control current flowing through the circuit  10 , as the voltage over the circuit  10  is below the regulation threshold. Increasing the input voltage until regulation threshold of voltage protection  10  is reached will create a regulation current through the circuit  10 , and thus a control current to the voltage regulators  15 ,  16 . The voltage drop over each voltage regulator  15 ,  16  will from this point on increase in direct proportion to further increased supply voltage, thus keeping output voltage regulated to a voltage equal to the threshold voltage of voltage protection circuit  10 .  
         [0057]     The very low current required for regulation (order of μA) will be conducted through the voltage regulation diode  12  in circuit  10 , as the voltage generated over the resistor  13  connected to the control terminal of the transistor  11  is too low to activate the transistor device. No additional current will be conducted through diodes  116  and  117 , since voltage between control terminals and output on the voltage regulators is zero.  
         [0058]     The proposed circuitry also monitors current being conducted through the voltage protection circuit  10  to ground  112 , in order to automatically adjust absorbed voltage and eliminate any such current. This enables maintaining a low current consumption through circuit  10  equal to the regulation current required.  
         [0059]     Typically, one supply pole of the power supply is bonded to ground, which introduces another ground connection in the system. A voltage difference between the different ground connections, together with voltage drops in the supply lines  1 ,  2 , can cause a current through the voltage protection circuit  10  to ground  112 . Depending on polarity of supply with respect to ground, such a current will be conducted either through diode  110  or  111  further to bidirectional diode  113  and to ground  112 . This current will be added through diode  110  or removed through diode  111  to the current through circuit  10 . According to this embodiment of the invention, such a change in the current through circuit  10  will reduce voltage drop over one regulator and increase voltage drop over the other. This regulation will thus rebalance the barrier with respect to the ground connection of the supply, thus removing the voltage difference and the current to ground  112 . This will ensure that current consumption is kept low, within values required from regulation and for a satisfactory performace of the sensor interface.  
         [0060]     At a certain level of voltage unbalance between supply and the circuit in  FIG. 5 , one voltage regulator may be saturated and unable to lower its voltage drop any more, in which case the output voltage to the sensor will decrease. Current consumption will still be kept low beyond this point as regulation still is operational.  
         [0061]     A more detailed block diagram of a specific embodiment of the regulator  15  is given in  FIG. 6 . Again, similar components have been given identical reference numerals as in previous figures.  
         [0062]     The circuit in  FIG. 6  comprises two bipolar transistors, an NPN transistor  21  and a PNP transistor  22 . The incoming line  1   a  is connected to the collector of the NPN transistor  21 , while its emitter is connected to the outgoing line  1   b . The emitter of the PNP transistor  22  is connected to the gate of the NPN transistor  21 , while its collector is connected to the outgoing line  1   b . A resistor  23  is connected between the base of transistor  21  and the incoming line  1   a , and a capacitor is connected between the incoming line  1   a  and the gate of transistor  22 , which gate also acts as the control terminal  17 .  
         [0063]     The second voltage control unit  16  in  FIG. 5  can be similar to the circuit shown in  FIG. 6 , but with the PNP and NPN transistors having switched places.  
         [0064]     During operation, the voltage control units in  FIG. 5  will ensure that the shunt voltage u SHUNT  over the sensor is held at a level close to the breakthrough voltage of the zener diode in the shunt circuit. Also, the control units will ensure that the shunt voltage is symmetrical around ground. As mentioned above, the voltage protection circuit  106 ′ will further ensure that both lines are voltage protected with respect to ground.  
         [0065]     In the above, the invention has primarily been described with reference to a radar level gauge system. However, it should be noted that the invention is equally applicable to other types of sensor arrangements.