Communications: directive radio wave systems and devices (e.g. – Determining distance – Material level within container
Reexamination Certificate
2003-02-28
2004-01-20
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Determining distance
Material level within container
C342S118000
Reexamination Certificate
active
06680690
ABSTRACT:
TECHNICAL FIELD
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.
BACKGROUND AND RELATED ART
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.
Often used in the process control industry are the 4-20 mA process control loops. In these loops a 4 mA signal represents a zero reading and a 20 mA signal represents a full-scale reading. Further, if a transmitter in the field has sufficiently low power requirements, it is possible to power the transmitter using current from the two-wire loop. However, 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 U.S. Pat. No. 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 mA. An antenna unit
6
is included in the gauge.
To make it possible to span the loop current over the whole 4-20 mA interval, the internal power consumption of the radar level gauge must be lower than or equal to 4 mA. 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 mA 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 mA interval as stated. Digital bi-directional communication according to e.g. the known HART protocol is also possible to use in said interval.
Equipment which is to be located in explosive environments is subjected to authority demands. It is common that equipment is then designed and certified as “explosive safe” or “intrinsic safe”.
Safety against explosion is guaranteed, in general, by use of a casing, which complies with certain requirements. 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.
For a radar level gauge, 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.
Document SE 0203456-9, not yet published at the filing date of this application, discloses the use of a DC/DC-converter for supplying the gauge with power from the loop. The content of said document is hereby incorporated into this description by reference.
FIGS. 3 and 4
illustrate a converter according to the disclosure of said document. The converter of said disclosure further has the purpose to isolate the gauge from the loop. The total power consumed in the gauge is U
loop
*I
loop
. See
FIG. 4
(
FIGS. 3 and 4
are described more in detail in the embodiments below). The total current I
loop
is divided into the current I
DC/DC
, which is used for powering the measurement circuits of the gauge, and a shunt current I
sh
. Said total current I
loop
is the current corresponding to the value measured by the gauge. As seen from the loop, the input terminals of the converter of
FIGS. 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. In reality, this available power sets a limit, a threshold, for minimum operational input voltage for the gauge. Said threshold is here called “lift-off voltage”. The term 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 way to solve the problem with insufficient power to the gauge is described in document U.S. Pat. No. 6,014,100. In said document the measurement of the level of the product is made in an active cycle between energy storing cycles. The transmission/receive unit is completely switched off during a temporary power store cycle and i
Abrahamsson Pär
Nilsson Valter
Alsomiri Isam
SAAB Marine Electronics AB
Swidler Berlin Shereff & Friedman, LLP
Tarcza Thomas H.
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