Patent Application
20060001567
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Also, transistor components are generally more suitable for dissipating higher amount of energy.
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.
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.
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.
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.
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.
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.
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.
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.
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.
These and other aspects of the present invention will be further described with reference to the appended drawings, illustrating presently preferred embodiements.
a is a block diagram of a voltage protection circuit according to a first embodiment of the invention.
b is a block diagram of a voltage protection circuit according to a second embodiment of the invention.
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.
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.
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
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.
The voltage protection circuit 6 in
During operation, the diode 12 will thus act as a feedback control of the transistor 11, ensuring that the voltage uDS 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 iSH 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.
Of course, the transistor can instead be a P-type FET, in which case the zener diode should be connected as indicated in
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.
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.
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.
The basic principle of the circuit in
A voltage protection circuit 106 according to a further embodiment of the invention is shown in
By selecting the various components the circuit in
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.
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.
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.
An alternative to the circuit in
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.
The voltage regulator component 107 in
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.
According to the embodiment in
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.
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.
The voltage drop—input to output—uREG1 and uREG2 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.
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.
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 UDS 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.
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.
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.
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.
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.
At a certain level of voltage unbalance between supply and the circuit in
A more detailed block diagram of a specific embodiment of the regulator 15 is given in
The circuit in
The second voltage control unit 16 in
During operation, the voltage control units in
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.
