Pulsed radar level gauging with relative phase detection
First Claim
1. A method for determining a process variable of a product in a tank based on a time delay of electromagnetic waves propagating between a predefined reference and a surface of said product, said method comprising:
- generating a transmission signal in the form of a first pulse train,generating a reference signal in the form of a second pulse train,wherein each pulse in said first and second pulse trains has an essentially identical waveform,pulses in said first and second pulse trains being delayed in relation to each other by a predefined and varying time delay,guiding said transmission signal into the tank as electromagnetic waves, and allowing said waves to propagate towards said surface,receiving a reflected signal reflected from the tank,forming a measurement signal comprising a sequence of values, each value representing a time correlation between a pulse of the reference signal and the reflected signal, said measurement signal comprising at least one amplitude peak indicating a surface echo,sampling and digitizing said measurement signal to form a digital signal,identifying a time window of said digital signal including said surface echo peak,determining a relative time period between a reference time corresponding to said predefined reference and a beginning of said time window,time-to-frequency transforming said digital signal in said time window to obtain a phase spectrum,determining a relative phase shift of said spectrum and using said relative phase shift to calculate a corresponding time shift, anddetermining said time delay by adding said relative time period and said time shift.
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Abstract
A method for determining a process variable of a product in a tank based on a time delay of electromagnetic waves. The method further comprises forming a measurement signal comprising a sequence of values, each value representing a time correlation between a pulse of a reference signal and a reflected signal, sampling and digitizing this measurement signal to form a digital signal, identifying a time window of the digital signal including the surface echo peak, determining a relative time period between a reference time corresponding to the predefined reference and a beginning of the time window, time-to-frequency transforming the digital signal in the time window to obtain a phase spectrum, determining a relative phase shift of the spectrum and using the relative phase shift to calculate a corresponding time shift, and determining the time delay by adding the relative time period and the time shift.
The invention is based on the realization that major improvement of measurement performance, compared to amplitude detection only, can be achieved by discrimination of the phase difference between the reflected signal and a reference. The detection is further independent of the pulse waveform and modulation, significantly reducing the requirements on pulse modulation.
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Citations
18 Claims
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1. A method for determining a process variable of a product in a tank based on a time delay of electromagnetic waves propagating between a predefined reference and a surface of said product, said method comprising:
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generating a transmission signal in the form of a first pulse train, generating a reference signal in the form of a second pulse train, wherein each pulse in said first and second pulse trains has an essentially identical waveform, pulses in said first and second pulse trains being delayed in relation to each other by a predefined and varying time delay, guiding said transmission signal into the tank as electromagnetic waves, and allowing said waves to propagate towards said surface, receiving a reflected signal reflected from the tank, forming a measurement signal comprising a sequence of values, each value representing a time correlation between a pulse of the reference signal and the reflected signal, said measurement signal comprising at least one amplitude peak indicating a surface echo, sampling and digitizing said measurement signal to form a digital signal, identifying a time window of said digital signal including said surface echo peak, determining a relative time period between a reference time corresponding to said predefined reference and a beginning of said time window, time-to-frequency transforming said digital signal in said time window to obtain a phase spectrum, determining a relative phase shift of said spectrum and using said relative phase shift to calculate a corresponding time shift, and determining said time delay by adding said relative time period and said time shift. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. A radar level gauge for determining a process variable of a content in a tank based on a time delay of electromagnetic waves propagating between a predefined reference and a surface of said product, said radar level gauge comprising:
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a first transmitter for transmitting an transmission signal in the form of a first pulse train, a second transmitter for transmitting a reference signal in the form of a second pulse train, each pulse in said first and second pulse trains having an essentially identical waveform, and pulses in said first and second pulse trains being delayed in relation to each other by a predefined and varying time delay, a propagation device for guiding said transmission signal as electromagnetic waves into the tank, and allowing said waves to propagate towards said surface, a receiver for receiving a reflected signal reflected in said tank, a mixer connected to said reflected signal and to said reference signal, and adapted to generate values of a measurement signal, each value representing a time correlation between a pulse of the reference signal and the reflected signal, said measurement signal comprising at least one amplitude peak indicating a surface echo, an analogue to digital converter for sampling and digitizing said measurement signal, to form a digital signal, processing circuitry arranged to identify a time window of said digital signal including said surface echo peak, determine a relative time period between a reference time corresponding to said predefined reference and a beginning of said time window, time-to-frequency transform said digital signal in said time window to obtain a phase spectrum, determine a relative phase shift of said spectrum and using said relative phase shift to calculate a corresponding time shift, and determine said time delay by adding said relative time period and said time shift. - View Dependent Claims (12, 13, 14, 15, 16, 17, 18)
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Specification