Impedance Matching Probe for Radar Level Gauge Deadzone Reduction
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Solution Overview
Problem
Radar level gauge systems face challenges in accurately determining the filling level of products in tanks due to impedance transitions, which result in signal loss and reduced measurable distance, especially when using single conductor probes passing through nozzles, leading to a 'deadzone' near the tank ceiling and limited maximum detection distance.
Innovation Solution
A guided wave radar level gauge system with an impedance matching arrangement that gradually changes impedance along the probe, reducing the radial extension of the electromagnetic field within the nozzle and providing a smooth transition to the probe's impedance below the nozzle, effectively minimizing signal loss and reflections, and allowing for more accurate filling level measurements closer to the tank ceiling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single conductor probe is used to guide electromagnetic signals into the tank, then contact measurement capability is provided, but impedance transition at the nozzle end causes signal reflection and creates a deadzone near the tank ceiling
Solution Approach 1:
An impedance matching arrangement is introduced as an intermediary component between the feed-through and the single conductor probe. This arrangement includes a coaxial section with gradually varying dimensions that acts as a transition zone, mediating the impedance mismatch between the 50Ω feed-through and the higher impedance probe, thereby reducing signal reflection and eliminating the deadzone.
Solution Approach 2:
The impedance matching arrangement utilizes gradual geometric parameter changes along its length. The coaxial section's inner and outer conductor dimensions vary continuously to transform the impedance from 50Ω at the feed-through end to the probe's impedance at the probe end. This gradual parameter change prevents abrupt impedance transitions and reduces signal reflection.
2Measurement precision
If the probe impedance is increased to 300-400Ω for single line probe, then probe performance is improved, but impedance step at feed-through connection causes signal loss and reduces maximum measurable distance
Solution Approach 1:
The impedance matching arrangement employs continuous geometric parameter changes along its length. The coaxial section's inner conductor diameter and outer conductor diameter vary gradually to transform impedance from 50Ω at the feed-through interface to the probe's 300-400Ω impedance. This gradual transformation minimizes reflection and maximizes power transfer, reducing signal loss.
Solution Approach 2:
The impedance matching arrangement is positioned locally at the critical transition zone between feed-through and probe. By concentrating the impedance transformation function in this specific location, the design optimizes signal transmission at the most critical point while leaving the rest of the probe structure unchanged.
3Ease of operation
If feed-through impedance is kept at 50Ω to match coaxial cable, then ease of connection is maintained, but impedance mismatch with high impedance probe causes strong echo signal that interferes with surface echo detection
Solution Approach 1:
The impedance matching arrangement serves as an intermediary transition section between the 50Ω feed-through and the high impedance probe. This intermediate structure with gradually varying dimensions mediates the impedance mismatch, reducing the strength of the spurious echo signal from the transition zone and preventing it from masking the weak surface echo signal.
Solution Approach 2:
By implementing gradual dimensional changes in the coaxial section of the impedance matching arrangement, the impedance transitions smoothly from 50Ω to the probe's higher impedance. This gradual parameter change reduces the abruptness of the impedance step, thereby minimizing the reflected echo signal strength.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution reduces the 'deadzone' near the tank ceiling, increases the maximum detectable distance, and enhances the accuracy of filling level measurements by minimizing signal loss and reflections, enabling reliable measurements closer to the top of the tank.
Implementation Method 1
the single conductor probe extending through the nozzle towards and into the product contained in the tank, for guiding a transmitted signal from the transceiver towards a surface of the product, and for returning a surface echo signal resulting from reflection of the transmitted signal at the surface back towards the transceiver
Implementation Method 2
an impedance matching arrangement that gradually changes impedance along the probe, reducing the radial extension of the electromagnetic field within the nozzle and providing a smooth transition to the probe's impedance below the nozzle, effectively minimizing signal loss and reflections
Implementation Method 3
the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein
Data Source
Figure 1a~1b
Figure 2a
Figure 2b
AI summary
A radar level gauge system (1), for determining a filling level of a product (6) contained in a tank (5), comprising a transceiver (10) arranged outside the tank for generating, transmitting and receiving electromagnetic signals; a single conductor probe (3) arranged inside the tank and electrically connected to the transceiver via a feed-through (15; 19) arranged at the top of a nozzle (16) of the tank. The single conductor probe (3) extends through the nozzle (16) towards and into the product contained in the tank, for guiding a transmitted signal (ST) from the transceiver towards a surface of the product, and for returning a surface echo signal (SR) resulting from reflection of the transmitted signal at the surface back towards the transceiver. An impedance matching arrangement (35; 40; 44; 48) is provided to the probe (3) and extends along a portion of the probe. An extension of the impedance matching arrangement (35; 40; 44; 48), in a direction perpendicular to the probe (3), is constant or changes with increasing distance from the feed-through (15; 19) with a first rate of change inside the nozzle (16), and changes with increasing distance from the feed-through (15; 19) with a second, negative rate of change below the nozzle (16) over a distance corresponding to at least a quarter of a wavelength of the transmitted signal (ST). The second rate of change is more negative than the first rate of change.