Patent Application
20160146924
Disclosed embodiments relate to radar level gauge systems that use electromagnetic waves for measuring the level of a product in a container, and more specifically to intrinsic safety devices that limit the energy at the probe in the system particularly during fault conditions so that a combustible material in the tank is prevented from igniting during the fault conditions.
Radar level gauges (RLGs) are commonly used for measurements of the level of products such as process fluids, granular materials and other materials. An example of such a radar level gauge includes a transceiver for transmitting and receiving microwaves, a propagation device (e.g., an antenna or a guided wave probe (i.e. transmission line suspended from top to bottom in the tank) arranged to direct microwaves and to couple returned microwaves affected by the product surface to the transceiver, timing circuitry adapted to control the transceiver and to determine the level based on a time relation between microwaves transmitted and received by the transceiver, and an interface arranged to receive power and to connect the radar level gauge externally thereof.
When level measurements are made by a probe in a tank containing a combustible material that is generally a gas or a liquid, or in other situations where the RLG is located in an explosion endangered area, it is required that the RLG system be designed for the hazardous location. This requirement is generally fulfilled using an intrinsically safe (IS) design, in which a barrier device, such as a zener diode, is implemented to limit voltages, current and power at the radio frequency (RF) output that is supplied to the probe which extends into the tank.
More generally, RF outputs, including those used in guided wave radar (GWR) applications, need an RF output which is IS for hazardous location applications, including applications which where the RLG 130 is installed in an explosion proof enclosure.
This Summary is provided to comply with 37 C.F.R. §1.73, presenting a summary to briefly indicate the nature and substance of this Disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.
Disclosed embodiments describe intrinsic safety (IS) barrier circuits with series blocking capacitor(s) and clamping diode(s) coupled between the radar gauge and the probe (or waveguide) in a radar level gauge (RLG) system which renders the RF output of the RF input/output of the RLG for transmissions a protected RF output and thus the probe coupled thereto limited in energy. As a result, even during fault conditions, the probe is prevented from igniting a combustible material in the tank due to the limited energy. Disclosed IS barrier circuits remove the need for conventional galvanic isolation or bulky IS barriers. As an added benefit, disclosed IS barriers also provide electrostatic discharge (ESD) protection to the electronics of the RLG.
Disclosed IS barriers essentially also do not add significant high frequency attenuation to the RF output. In contrast, conventional IS barriers add unwanted impedance (and thus attenuation) to the RF signal. Typically, clamping (zener) diodes cannot be added to an RF output since they add unwanted capacitance and too much RF attenuation. It is recognized that the impedance of a capacitor is equal to 1/(2πfc), where c is the capacitance and f is the frequency. Therefore disclosed IS barrier circuits add a relatively large low frequency (AC mains frequency) impedance, which allows the use of an IS barrier having small size, low power, low capacitance ESD diode(s) resulting in an IS RF output with low RF attenuation.
Disclosed embodiments are described with reference to the attached figures, wherein like reference numerals, are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate aspects disclosed herein. Several disclosed aspects are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the embodiments disclosed herein.
One having ordinary skill in the relevant art, however, will readily recognize that the disclosed embodiments can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring aspects disclosed herein. Disclosed embodiments are not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with this Disclosure.
Also, the terms “coupled to” or “couples with” (and the like) as used herein without further qualification are intended to describe either an indirect or direct electrical connection. Thus, if a first device “couples” to a second device, that connection can be through a direct electrical connection where there are only parasitics in the pathway, or through an indirect electrical connection via intervening items including other devices and connections. For indirect coupling, the intervening item generally does not modify the information of a signal but may adjust its current level, voltage level, and/or power level.
The IS barrier circuit 250 is shown between the coaxial connector 218 and the transceiver 220. The IS barrier circuit 250 receives the RF output 230a from the RF input/output of the RLG 230 and outputs a protected RF output 219. A flange (not shown) may also be present on the top of the tank 205. In operation of the GWR system 200, a transmitted pulse from the transceiver 220 is launched along probe 240 which returns as the reflected pulse shown that is processed by the processor 215. The transmitted pulse may be at about 1.5 GHz. Although generally described for GWR applications, disclosed IS barrier circuits can also be applied to protect the electronics in other systems including the electronics in non-contact radar systems. In all such systems, it is recognized that during fault conditions, including faults from a source of high voltage (e.g., AC mains supply) fed to the device(s) needs to be energy limited to help keep the RF output which enters the hazardous location/area IS.
Barrier circuit 250′ provides high impedance from the AC mains frequencies provided by the source of AC voltage 110, typically 250 VAC at 50 or 60 Hz, that in
Zener diodes can be replaced by other shunting diodes, or signal diodes arranged to limit the voltage. As noted above, C1 and C2 can be replaced by a single capacitor for certain lower levels of IS in accordance with IS standards. D2, D3, D4 can be replaced by 2 diode pairs, 1 diode pair, or as explained below even a single diode depending on the level of IS needed. A chassis ground is shown on the low side of D2, D3, D4, as well as R1 and GDT 330, while an analog ground (AGND) is shown on the low side of D1 and for ASIC radar sensor circuit 260.
It is recognized herein that a single diode for voltage clamping can be used if in normal operating modes the RF signal has a relatively low voltage amplitude. A diode will clamp in the positive polarity (i.e. to reverse bias the diode) only when the voltage of the RF signal exceeds the diode's breakdown voltage. The same diode will also clamp in the negative direction (i.e. to forward bias the diode) when the voltage of the RF signal is negative provided it is higher in amplitude than its forward conducting voltage typically below 1V (e.g., about 0.6 to 0.7 V at room temperature for a silicon pn diode). For example, if the RF signal voltage is normally always greater than −1V, a single diode can thus be used to clamp RF signals for either polarity. Additional diodes such as shown in
Since the RF frequency used by GWR system 200 or 280 is relatively high, such as about 1.5 GHz, it is recognized that small value capacitors, such as on the order of several hundred pFs can be used for the blocking capacitors shown as C1 and C2 without adding any significant attenuation to the RF signal. Use of small value capacitors result in essentially no power dissipation across D2, D3, D4 during continuous radar operation, and very low power dissipation during fault conditions.
The barrier circuit 250′ can be expanded to include other components, such as by adding the resistor shown as R1 and a gas discharge tube (GDT) 330 shown for static discharge protection of a probe or antenna coupled to the RF output 219, without compromising the IS. Another capacitor C3 is also shown in
While various disclosed embodiments have been described above, it should be understood that they have been presented by way of example only, and not as a limitation. Numerous changes to the disclosed embodiments can be made in accordance with the Disclosure herein without departing from the spirit or scope of this Disclosure. Thus, the breadth and scope of this Disclosure should not be limited by any of the above-described embodiments. Rather, the scope of this Disclosure should be defined in accordance with the following claims and their equivalents.
Although disclosed embodiments have been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. While a particular feature may have been disclosed with respect to only one of several implementations, such a feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.
This application claims the benefit of Provisional Application Ser. No. 62/085,112 entitled “INTRINSIC SAFETY BARRIER CIRCUIT WITH SERIES BLOCKING CAPACITOR”, filed Nov. 26, 2014, which is herein incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62085112 | Nov 2014 | US |
