Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
  • G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
  • G01F23/28—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring the variations of parameters of electromagnetic or acoustic waves applied directly to the liquid or fluent solid material
  • G01F23/284—Electromagnetic waves
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/003—Transmission of data between radar, sonar or lidar systems and remote stations
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
  • G01S7/027—Constructional details of housings, e.g. form, type, material or ruggedness

Definitions

• the present invention relates to level measurement in industrial processes, wherein the invention is used for measurement of product level in a storage tank of the type used in industrial applications using a microwave level gauge. More specifically, the present invention relates to a device and a method for efficient use of power provided to the gauge from a two-wire process control loop.
• Instrumentation for the measurement of product level (either liquids or solids) in storage vessels is evolving from contact measurement techniques, such as tape and float, to non-conduct techniques.
• One technology is based on the use of microwaves, which involves transmitting microwaves towards the product surface and receiving reflected microwaves from the surface. The reflected microwaves are analyzed to determine the distance that they have travelled. Knowledge of the distance travelled allows determination of the product level.
• a 4 mA signal represents a zero reading and a 20 mA signal represents a full-scale reading.
• a transmitter in the field has sufficiently low power requirements, it is possible to power the transmitter using current from the two-wire loop.
• microwave transmitters for level gauging in the process control industry have often required a separate power source. These microwave transmitters were large and their operation required more power than could be delivered using 4-20 mA standard. Thus typical prior art microwave transmitters for level gauging required additional wiring to provide power to the unit.
• the document US 5 672 975 discloses an arrangement for providing power to a radar level gauge and for transmitting level information provided by the radar level gauge by means of a two-wire process control loop.
• the term radar level gauge is here used for a unit including an antenna unit, a microwave transmitter, a receiver, transmitter and receiver circuits and circuits for calculating a measured level..
• a two-wire radar level gauge is distinguished by that it is being supplied by power and at the same time communicating analogue and digital information through the same wires.
• a prior art two-wire radar level gauge can be coupled as is shown in Fig. 1.
• a voltage source 1 is supplying the radar level gauge 2 with power through the two-wire loop 3.
• a barrier 4 for protection against current transients and for containing an EMC-filter may be included in an interface between the gauge and the loop.
• the gauge is conveying an actual measured value to a control unit 5 by setting a current proportional to the level value measured. This current can be set in the interval 4 - 20 A.
• An antenna unit 6 is included in the gauge.
• the internal power consumption of the radar level gauge must be lower than or equal to 4 A. This lowest limit is valid for a measured value that is represented by the lowest value to be conveyed by the loop. In reality not even as much as 4 A current from the loop is available for powering the gauge. Current available for the supply of the gauge is approximately 3,5 mA. The reason for this is that the gauge is set to send a low level alarm when the reading is below the lower end of the current range 4 to 20 mA used for transmission of data via the loop.
• a two-wire loop is characterized by that only two wires are needed for connecting an instrument to a controller.
• the length of the wires may be up to several hundreds of meters.
• the wires are also used for the communication between the instrument and the controller. Said communication may be analogue in the 4 - 20 A interval as stated. Digital bi-directional communication according to e.g. the known HART protocol is also possible to use in said interval.
• Such equipment may be powered via a barrier to limit the energy that is fed out to the wires and to the gauge.
• Intrinsic safe means that the construction in itself is designed in such a way, that electric energy is not available in a sufficient amount to generate a spark, which can set fire to an explosive gas surrounding the construction. From practical reasons, this means that there is a barrier at the entrance to those parts being classified as intrinsic safe. Either a barrier 4 or a barrier 7 exemplifies this.
• a barrier 4 or a barrier 7 exemplifies this.
• parts of the equipment that must be located inside, for example, an oil container has to be intrinsic safe. As a result there is required a barrier that limits, with high security, energies possibly available at wave guides and antenna parts.
• the input voltage to the gauge at the loop end changes depending on the barrier used, supply cable characteristics, loop current, losses in the gauge and supply voltage.
• a loop power supply is located remote from the gauge, often at the same location as the receiver of data from the gauge.
• a typical remote supply voltage is 24 V and the total resistance in the loop is often 500 ohms or more. Consequently, the input voltage at the gauge terminals can vary within a 10 V interval as the voltage drop across the loop can reach 10 V at 20 mA in the loop.
• 0 o P is the current corresponding to the value measured by the gauge.
• the input terminals of the converter of figures 3 and 4 are the input terminals to the gauge.
• the power consumed is used to energize the gauge.
• the amount of power required to supply the gauge is usually a portion of the total consumed power.
• a converter of the kind used to transfer power from the loop to the gauge as shown has a built-in feature that the input power to the converter is fairly constant over a wide input voltage range. Excessive power is dissipated in a device, a current generator, controlled by the gauge to stabilize the loop current to an amplitude that corresponds to the present reading of product level. Limitations arise when the power required by the gauge exceeds the available power from the loop.
• this available power sets a limit, a threshold, for minimum operational input voltage for the gauge.
• Said threshold is here called "lift-off voltage”.
• lift-off voltage is used to indicate the voltage level that must be available for the gauge to perform as defined at a certain current. If the available voltage for the gauge is below the required lift-off voltage at said certain current the power is not high enough to energize the gauge. The value of the lift-off voltage is generally lower when high loop current is prevailing as the loop can supply required power without limitation.
• a power regulating circuit is associated with a circuit configured to sense a deficit in its capability to supply the circuits of the gauge with power and to delay the execution of a program stored in the gauge for control of the measuring of the level by the gauge.
• the program is delayed sufficiently in response to the sensing of the deficit to reduce the power required by the gauge to overcome the deficit.
• a loop current setting means is connected in series with the transducer, whereby the full loop current is passing said setting means and thus imparting a voltage drop across said setting means in the loop.
• Patent application U.S. 2002/0005713 Al discloses a device, a gauge powered from a loop, which shows a way to approximate the power consumption of a gauge to predetermined power consumption without the predetermined power consumption being exceeded. The measuring operations are then regulated in a way such that a power surplus is minimized.
• the object of the present invention is to provide a circuit for a radar level gauge powered via a two-wire control loop for sensing the surplus power transferred in the loop and for use of said surplus power in increasing performances of the gauge.
• a two-wire powered radar level gauge comprising the features: the radar level gauge uses microwaves and is coupable to a two-wire process control loop for measuring a level of a surface of a product in a tank and the radar level gauge further comprising, a microwave antenna unit directed into the tank, a microwave source for sending a microwave signal through the antenna unit into the tank, a microwave receiver for receiving a reflected microwave signal from the surface of the product in the tank, measurement circuitry coupled to the source and receiver for initiating transmission of the microwave signal and for determining product level based upon the received signal, output circuitry coupled to the two-wire process control loop for setting in the loop a desired value of a loop current (I/ 00 p) corresponding to the product level, a power supply circuitry coupled to the two-wire process control loop for receiving power from the loop and being a source of power for the microwave source, the microwave receiver, the measurement circuitry and the output circuitry and including a converter for transferring power from the loop to said power supply circuit
• a method for measuring a level of a surface of a product in a tank by means of a radar level gauge using microwaves wherein said gauge is coupled to a two-wire process control loop and the method further comprising the steps: directing an antenna unit into the tank, sending by means of a microwave source a microwave signal through the antenna unit into the tank, receiving by means of a receiver a microwave signal reflected from a surface of the product in the tank, initiating transmission of the microwave signal and determining product level based upon the received signal in a measurement circuit coupled to the microwave source and the microwave receiver, setting in an output circuitry situated in the radar level gauge and coupled to the two-wire process control loop a loop current value (I
• the invention uses the sensing circuit, which senses current generated in said current generator for allowing excessive power available to be used for powering equipment in the radar level gauge.
• the sensing circuit is used for switching the gauge between two modes, one base mode for providing basic measuring functions with power and one extended mode, wherein further functions are allowed to operate.
• the converter included in the level gauging system is transforming the voltage from the loop to an over-voltage protected and current limited low feed voltage to the electric circuits of the gauge.
• the converter can at the same time be supervised to load the loop in said extended mode with currents between 4 and 20 mA.
• Process signals (such as HART signals) from and to the gauge can be handled by the circuit (not shown) and transmitted in both directions.
• the sensing circuit it is possible to determine when lift-off voltage is reached, whereby it will be possible to utilize power available from the loop more efficiently.
• the gauge can be made to adapt its function or performance depending on available power supplied.
• the gauge can also be operational from lower input voltage. Specific areas where the detected available power can be used are the powering of circuits for:
• - Charging electrical storage means such as a capacitor or a battery, in the gauge with sensed excessive power.
• the circuit according to the invention aspects can be used for a better control of the gauge when the supply voltage is switched off or if the voltage for a short while would fall outside normal levels. Situations where circuits are locked due to uncontrolled high current level during e. g. start can be eliminated. Setting the sensing circuit to connect a low power stand by state of rest when the power supply cannot fulfil the demands for normal performances can provide this function.
• the level gauging system can be programmed to use available energy for optimum operational performance at any time and that the sensing circuit can be used for supervision of loop supply.
• the lift-off voltage can be detected at any loop current.
• the proposed circuit increases the efficiency of the gauge compared to a conventional solution as functions of the gauge can be controlled in such a way that the voltage interval in which the gauge will work is expanded.
• Fig. 1 shows a prior art two-wire radar level gauge.
• Fig. 2 schematically shows an example of a radar level gauging system for determining the level of the surface of a product in a tank.
• Fig. 3 shows a schematic view of a radar level gauging system used in the invention.
• Fig. 4 shows a schematic view of a converter for transferring power from the loop to the gauge.
• Fig. 5 schematically shows a view of a circuit controlling the loop current and for an efficient use of said current according to an aspect of the invention.
• Fig. 6a to 6c show in diagrams three examples of different operating modes of the feeding of a gauge from a loop where the vertical axis shows the input voltage U to the gauge and the horizontal axis indicates the loop current.
• a tank 11 is used for storing a product 12.
• the product may be such as oil, refined products, chemicals and liquid gas, or may be a material in powder form.
• Radar 13 is attached to the roof 14 of the tank 11.
• a microwave beam is transmitted from the radar via an antenna 15 at the interior of the tank.
• the transmitted beam is reflected from the surface 16 of the product and is received by the antenna 15.
• the microwave may be transmitted from the antenna a free radiated beam or via a wave guide (not shown), which communicates with the product.
• the radar level gauge as shown in fig. 2 is only used as an example.
• FIG 3 The location of the converter for transferring the power from the loop to the gauge in a level gauging system according to the invention is illustrated in figure 3.
• Figure 2 refer to the radar level gauge unit.
• the antenna 15 is connected via a microwave transmission line to gauge electronics 18 containing the radar transmitter, the radar receiver, the measurement circuitry and the output circuitry.
• Said gauge electronics is connected to the two-wire loop 20 over a transient protection and EMC-filter barrier 21, which in this embodiment is located at the input to the radar level gauge 2.
• a converter 22 between said barrier and the gauge electronics.
• Power to the radar level gauge is provided by a voltage supply 23 feeding the two-wire loop via a second barrier 24, if required, to render the feeding line intrinsic safe.
• receiving means 25 for receiving the value of the level of the product in the tank, wherein said value is based on a reading of the current in the loop 20.
• a prior converter 22 for the transfer of power to the gauge is described in figure 4 and illustrated by only a few blocks as depicted in said figure.
• the loop is referred to by number 20.
• 00 p is divided into two branches, one portion I DC / DC is lead through a DC/DC-converter 30 having an output 31.
• the current I D C/ DC which is not allowed to exceed 4 mA is distributed via a regulator Reg and then sent to the converter 30, which is supplying the internal electronics of the gauge illustrated by means of block 18 with an adapted voltage(s).
• the regulator Reg normally stabilizes/ limits a voltage Ul over the converter to a level, which is substantially, lower than the nominal supply voltage U 0 .
• the other portion I Sh of the loop current is let through a current generator 32.
• the current generator 32 controls the current through the loop, whereby said current is set by a comparator 33, which gets information about actual value of the current delivered from loop current measuring means 35.
• a desired current value 39 is sent from the level sensing instruments of the gauge, whereby the comparator 33 can set the current of the loop by controlling the current generator 32.
• the design of the gauge is thus a balance between functionality and performances in dependence of the power available.
• FIG 5 One embodiment of the circuit disclosed in the aspect according to the invention is shown in figure 5.
• the influence of the regulator is eliminated as it is removed at the same time as the circuit as described in figure 4 is enlarged with some components.
• a sensing circuit 38 there is included in the circuit of figure 5 a sensing circuit 38.
• the actual measured loop current represented by loop current signal 40 is compared with the desired loop current represented by desired loop current signal 39, the reference value of the current.
• the inverted error of these two signals is amplified and controls the current generator 32 in order to maintain a current in the loop that within the interval 4 - 20 mA corresponds to the desired current obtained via the current loop control which delivers the desired loop current signal 39.
• the circuit includes said sensing circuit 38, which measures the feedback Signal being loop current signal 40, the desired loop current signal 39 and further the drive signal 37 to the current generator 32. By the information obtained from these three signals 40, 39 and 37 the sensing circuit 38 determines whether current flows through the current generator 32 or not.
• Information to the sensing circuit 38 that current I sh is floating through the current generator 32 and the level of said current is sent to a power controller 41.
• Said power controller 41 is thus programmed to administer distribution of available current to said receivers of surplus power or the disconnection/ standby mode of said functions and processes when surplus power is not available.
• the level gauging system can thus be programmed to use available energy for optimum operational performance at any time.
• the circuit can also be used for supervision of loop supply.
• the lift-off voltage can be detected at any loop current.
• the sensing circuit 38 supervises said signals 40, 39 and 37 (corresponding to loop current, desired loop current and the drive signal to the current generator) detection of when the loop current control ceases to function is achieved. This occurs at a certain voltage limit (the lift-off voltage) at the input of the gauge in dependence of the current in the loop as described.
• the sensing circuit 38 sends an alarm to the power controller 41 if said limit is reached.
• the DC/DC converter must of course be designed for conversion of the higher power requested by the gauge electronic circuits.
• the circuit offers increased amount of output power at higher operational voltages, which thus is used for providing extended performances of the gauge.
• the sensing circuit 38 is included into the system to provide the use of energy available from the loop more effectively, whereby the sensing circuit 38 controls:
• FIGS. 6a, 6b and 6c Examples illustrating said supervision of the current through the loop by means of the sensing circuit are shown in figures 6a, 6b and 6c.
• the gauge is working in two power modes, which require predetermined (established during design of the gauge) and known power to the gauge supplied via the conduit 31.
• One of said power modes requiring a lower power for the gauge is a base function mode.
• A, second, extended power mode provides, beside said base function, extended measure capacity, storing of energy and other available functions earlier mentioned.
• the prerequisites in figures 6a and 6b are: supply voltage to the loop 24V and in figure 6c variable voltage; the resistance in the loop being 500 ohms.
• the power controller 41 can switch between the base mode, symbolised by means of user 42, wherein the gauge consumes 30 mW from the loop and the extended mode, symbolised by means of user 43, wherein the gauge consumes 150 mW from the loop.
• the vertical axis shows the input voltage Uioop to the gauge.
• the horizontal axis shows the current in the loop, I
• Figure 6a illustrates the currents of the gauge operating in extended mode.
• the voltage at the input to the gauge follows the load line L indicated.
• the measured value conveyed by the gauge corresponds to a current I
• O0 p 10mA.
• the voltage U ⁇ oop is then around 19 V and the current I DC / DC drawn by the gauge can be read from the diagram to amount to around 7.9 mA.
• the current generator has to generate a current around 2.1mA.
• the sensing circuit 38 detects that the current in the loop is sufficient to allow the extended mode to prevail.
• the arrow w x" indicates the point where a switch to base mode takes place.
• FIG. 6b An example of base mode operation for the gauge is illustrated in the diagram of figure 6b.
• the measured value conveyed by the gauge corresponds to a current I
• 00P 5 mA.
• 0 ⁇ p is then around 21.5 V.
• the current I D Q ⁇ C drawn by the gauge can be estimated to around 1.4 mA.
• the current generator has to generate a current around 3.6 mA.
• the sensing circuit 38 detects that the loop current is enough for base mode operation only.
• Figure 6c illustrates "lift-off", which means that the threshold for full function of the gauge is reached due to the voltage U Io0 p being to low.
• the sensing circuit 38 initiates an alarm signal on conduit 44.
• the nature of the alarm which is to be delivered via the loop when the power available from the loop is insufficient to energize the gauge, will now be further explained and defined.
• the alarm preferably involves a loop current manipulation, which a remote (in relation to the gauge) unit can distinguish as different than the loop current during normal operating situations.
• the nature of the alarm may be selected as one of the following: - a predetermined constant current below the nominal operating range;
• the nominal operating range is a loop current of 4-20 mA.
• one manipulation which is preferred in the context of this invention, is to form the alarm by setting the loop current to a value in the range 1-3 A during a time period of approximately 20 seconds. This can be detected by the remote unit as an alarm and appropriate action shall be taken as initiated in the remote unit.
• the gauge is arranged to send an alarm, which indicates that the power available from the loop is insufficient to energize the gauge, by other means than via the loop.
• Such other means could include presenting the alarm via a display in the gauge, indicating an the alarm via an optic indicator in the gauge, indicating the alarm via an acoustic indicator in the gauge, and transmitting the alarm via a remote communication channel other than the loop.
• An example of a different communication channel is radio communication means.
• the alarm generation aspect of this invention may be advantageously applicable in a similar way to other kinds of gauges of process variables in industrial processes than radar level gauges.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• General Physics & Mathematics (AREA)
• Radar, Positioning & Navigation (AREA)
• Remote Sensing (AREA)
• Electromagnetism (AREA)
• Thermal Sciences (AREA)
• Fluid Mechanics (AREA)
• Measurement Of Levels Of Liquids Or Fluent Solid Materials (AREA)

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Citations (3)

• 2004
  • 2004-01-20 DE DE112004000339T patent/DE112004000339T5/de not_active Withdrawn
  • 2004-01-20 WO PCT/SE2004/000066 patent/WO2004076986A1/en active Application Filing

Patent Citations (3)

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