GB2350004A - Frequency swept radar gauge - Google Patents

Abstract

A radar gauge 10 comprising a microwave generator operable to produce a microwave over a swept range of frequencies. The microwave generator comprises a clocked direct digital synthesiser (DDS) 42 which generates a numerically controlled frequency sweep between a first frequency limit and a second frequency limit which is lower than the first limit. A digital to analogue converter 34 and freqency multiplier 38 receives an output signal of the DDS 42 and produces therefrom an output microwave which is swept over a swept range of frequencies. An antenna 20 transmits the swept frequency microwave and receives microwaves reflected back to the antenna 20 by an object. A differentiator circuit 43 receives the transmitted and received microwaves, and derives therefrom, a beat frequency which is a result of interference between the transmitted and received microwaves. This beat frequency is indicative of the distance of the object from the antenna 20. A coder/decoder 44 converts the output signal of the differentiator 43 into a digital signal suitable for indicating the distance of the object from the antenna 20.

Description

2350004 RADAR GAUGE The present invention relates to a radar gauge. It

relates particularly, but not exclusively, to a radar gauge for use in a hazardous environment.

Equipment for determining the depth of a liquid stored in a container has evolved from C1 utilising contact techniques to preferable non-contact techniques such as ultrasound and radar. Liquid contained in containers such as industrial storage tanks may be stored at high temperatures and pressures. Such extreme conditions mean that some non contact techniques are unsuitable. For example, ultrasonic gauges do not usually work at temperatures in excess of approximately 200 degrees Celsius, or at pressures higher than approximately 7 x 10' Pa. In addition, ultrasonic gauges cannot measure the depth of a product when turbulence or foaming of the product occurs.

0 Radar gauges, however, work well at high pressures and temperatures. The gauges do not need to be recalibrated, and are very reliable and accurate. Such radar-based measurement systems determine the distance of objects by measuring the time taken for radio waves to be reflected by the object and returned to the system. When measuring the level of a liquid in a container, the radar detects the surface of the liquid product, and the bottom of the tank. Thus the depth of the liquid may be calculated from these two measurements. Radar gauges typically use frequency modulated continuous waves, where the difference between the transmitted and received frequency of the radio wave is proportional to the distance travelled by the wave.

An example of such a system is disclosed in published International Patent Application No. WO-Al-9610734 (Rosemount Inc.), which discloses the use of a microwave level gauge for use in measuring the level of a product in an industrial storage tank The 1. 1 g - gauge includes a microwave feed-horn directed into the storage tank, circuitry spaced C W 0 apart from the feed-horn, and a microwave waveguide extending therebetween. A 0 115 microprocessor identifies echoes from the microwave signals which are generated and ZP sensed by the microwave transducer. The microprocessor determines the height of the tn product in the storage tank based upon a microwave echo from the product, and a 2 microwave echo from the feed-horn. The microwave source provides microwave radiation to a voltage-controlled oscillator (VCO). A disadvantage of using a VCO is Z:It) 0 that changes in ambient temperature affect the frequency of the microwaves generated.

I Z:' This leads to inaccurate distance, and hence product levels, measurements.

An aim of the present inve ntion is to provide a radar gauge with improved accuracy, C, Z:) particularly, but not exclusively, for use in hazardous environments. Another aim of the invention is to provide a radar gauge that is easier to install than other radar gauges.

ID According to a first aspect of the invention there- is provided a radar gauge comprising Z;' Z) 0 a microwave generator operable to produce a microwave over a swept range of frequencies, an antenna operable to transmit the generated microwave towards an object, and operable to receive microwaves reflected back to the antenna by the object, a differentiator means operable to receive the microwaves transmitted and received by the antenna and operable to derive therefrom, a beat frequency which is a result of interference between the transmitted and received microwaves, and thereby produce an output signal indicative of the time of flight (t) of the transmitted and received microwaves, and hence the distance of the object from the antenna, a coder/decoder operable to receive the output signal of the differentiator means and convert the output signal of the differentiator means into a digital signal, and an outpuit means for indicating the distance of the object from the antenna.

I Preferably the microwave generator comprises a clocked direct digital synthesiser (DDS) which is operable to generate a numerically controlled frequency sweep between a first frequency limit and a second frequency limit which is lower than the first limit, and a digital to analogue converter operable to receive an output signal of the DDS and produce therefrom an output analogue reference signal over a swept ranoe I-D of frequencies.

Preferably the means for generating microwave signals includes digital electronics. An advantage of this is that the gaug Y e is not susceptible to extremes of temperature, as is the case with analogue radar gauges.

The digital electronics are preferably modular in nature, and may be replaced in situ.

I 3 The radar gauge preferably uses the frequency modulated continuous wave (FMCW) microwave radar principle, operating at an average frequency of approximately 10 0 0 GHz. A high stability radio frequency is preferably generated. This signal may be linearly ramped between a lower and an upper frequency. The lower frequency may be approximately 9.5 GHz and the upper frequency may be approximately 10.5 GHz. The signal may subsequently be fed to an antenna from where it may be launched into free C.

space.

The frequency sweep required for the system to function as an FMCW radar is preferably generated very accurately using a direct digital synthesis (DDS) technique at low frequencies. These frequencies may be of the order of 10 MHz. The frequencies may be multiplied to approximately 1.25 GHz, and may be smoothed using a phaselocked loop. A microwave frequency multiplier chain may provide a second multiplication stage with an output that may be in the range 9.5 to 10.5 GFIZ, to the antenna.

The DDS technique preferably generates the frequency sweep by numerical computation, with both the sweep rate and frequency ultimately preferably being locked to a master crystal oscillator. The master crystal oscillator may be specified to have very high stability - a higher stability than that of a delay line without sophisticated temperature compensation. The DDS system may include a microprocessor which may include a DDS chip with data stored representative of the mode, starting frequency and sweep rate.

tn A clock pulse is preferably supplied to start the frequency sweep. This pulse may also be output from the RF synthesis box as a trigger output pulse which may enable a t>t!> digital signal processor board and sampling oscilloscopes to be synchronised to the 0 0 Z frequency sweep. The microprocessor then times 100 ms of sweep. Subsequently the DDS chip is reset to sweep in the opposite direction. Alternating sweep directions means there is no large frequency discontinuity between sweeps, which improves the performance available from the subsequent phase locked loop. Measurements made on alternate sweeps may also be averaged to eliminate any Doppler shift errors on a a liquids in bulk.

moving target. This is particularly important when measuring movine.

4 An envelope detector on an output line may differentiate between radiated and received microwave signals. This differentiator provides a major cost advantage in that it gives C.

an output directly at base band (i.e., a few kHz) by using components which are much I cheaper than microwave mixers. The differentiator/amplifier circuit may include 5 switched options for adjusting the gain of each stage and settina the maximum bandwidth.

The atenna may be a rod antenna, and is preferably a dielectric antenna. The antenna may be coated with PTFE in order to resist build-up of dirt and contaminants on its surface.

The microwave signal emitted from the antenna may radiate outwards. The same antenna may then collect the reflected microwaves and the gauge may compare the 4D I'D difference between the outward and return microwaves by generating a beat signal the I frequency of which is proportional to the distance travelled by the microwaves. The radar gauge may be used in order to obtain a value indicative of the depth of a product contained in a container. The product may be a liquid.

The beat frequency is preferably processed by methods such as Fourier Transform techniques and peak location algorithms are employed. A peak location algorithm may be used to locate digitally the peak frequency corresponding to the product level reflection. The depth of a liquid in the container may thus be calculated and displayed on a display that is part of the gauge system. Preferably the display is a liquid crystal display device.

The radar gauge has no moving parts within the gauge. Thus no problems associated r> 0 with mechanical wear and tear exist, and the radar level gauge is extremely robust and reliable. Both power and communications are distributed via a single 4-strand cable.

The radar gauge is advantag rp r> eously modular in nature, allowing for a wide range of options such as different flange sizes and adapters to suit a wide range of process connections. A 'watchdog' feature may also included in the gauge, which may warn of t) tD 0 any part of the system that needs attention.

In order to determine the level of a product in a container, the gauge is preferably located at, or close to, the top of a container such as a storage tank. It may, however, be located towards the side of the container or tank. The radar gauge may be adapted to 0 0 accept signals from additional sensors in order to provide temperature, pressure and C> water interface inforinatio.p. The antenna may be mounted remotely. For example, a coaxial cable may be used to connect the antenna to the radar unit.

The RF coupler preferably comprises first and second sections. The first section includes a first washer and a first member, and the second section includes a second washer and a second member. The first section of the coupler is located in the body of the housing. The second section of the coupler is located in the flange. The flange in 0 CI turn may be attached to an antenna holder to which the antenna is attached. The flange (including the second section of the coupler) is fitted onto an opening in a container C such as a stora.

ge tank, so that the antenna is directed into the container. The coupler is dimensioned and arranged so that microwaves generated pass through the first and 0 0 second sections of the coupler, from where the microwaves are transmitted by the antenna.

The first washer, second washer and second member are preferably circularly symmetric. These may be cylindrical or have different shaped cross-sections. The first member is preferably a tapered cylinder, and may have a stepped profile. All of the section components may have formed therein an aperture extending perpendicular to the axis of symmetry of a component. Apertures, or windows, are transparent to microwave radiation and may achieve this by having a rectangular cross-section. At least a portion of the microwave transparent window includes quartz glass.

The second member may have formed therein a hole or vent that extends from the cylindrical surface of the member to the microwave transparent window. The vent acts as a pressure relief. Products stored in the container, to which the radar gauge is fitted, may be under higher pressure than that outside the container.

The quartz glass window is preferably held in place by means of a sealant such as 1 solvent-less fluorosilicone elastomer paste. The quartz glass window may also be held in place as a result of the dimensions of a frame in which the window sits. For 6 example, the windows may be tapered such that the quartz volumes may not fall into the container.

The coupler may be made of stainless steel, or other suitable material.

According to a second aspect of the invention there is provided a method of measuring ID C the distance of an object from an antenna, comprising the steps of generating a ZD microwave at a swept range of frequencies transmitting the microwave from the antenna in the vicinity of the object, receiving at the antenna, microwaves reflected by C the object., differentiating between the transmitted and received microwaves by deriving therefrom a beat frequency due to interference between the transmitted and received microwaves, producing an output signal indicative of the beat frequency and hence the time of flight (t), of the transmitted and received microwaves and therefore the distance of the object from the antenna, and converting the output signal into a digital signal, indicative of the distance of the object from the antenna.

Preferably the step of generating a microwave which is swept over a range of frequencies comprises the steps of generating digitally a numerically controlled frequency sweep between a first frequency limit and a second higher frequency limit, at a precise rate of change, using a direct digital synthesiser (DDS), converting the digital output of the DDS to an analogue signal, and multiplying the frequency of the analogue signal to frequencies suitable for transmission by the antenna.

A number of embodiments of the invention will now be described, by way of example only, with reference to the accompanying Figures, in which: Figure la shows diagrammatically a radar level gauge, constructed in accordance with C2 C, the present invention, coupled to a container; Figure lb shows a typical profile graph of the contents and back round signals of the C 9 container of Figure 1; Figure Ic shows a diagram illustrating frequency modulation of a radar signal; ZD In Figure 2 shows a block diagram of the radar gauge of Figure 1; 7 Figure 3 shows schematically the radar gauge of Figure 1; Figures 4a and 4b show schematically, typical profile graphs of contents and background signals of the container of Figurel;

C) Fiaure 5 shows a cross-section of an alternative design of antenna suitable for the 0 Z:) gauge shown in figure 3; 4:1 C Fi-ure 6 shows a cross-section of the upper and lower RF assemblies of the gauge W 0 shown in Figure 3; C Figure 7 shows a simplified diagram of the upper and lower RF assemblies shown in Figure 6; Figure 8 shows a cross-section of the lower RF assembly, the antenna holder, and the I antenna of the gauge of Figure 6; Z r Figure 9 shows a cross-section of the flame-proof housing and the upper and lower RF 4D 41 assemblies of the gauge shown in Figure 6; Figure 10 shows engineering drawings of an upper (RF) washer of the gauge shown in 4: C) 0 Figure 6; Fi-ure 11 shows engineering drawings of an upper RF holder of the gauge shown in 0 0 0 C) Figure 6; Figure 12 shows engineering drawings of the lower RF (antenna) holder of the gauge shown in Figure 6; -)0 Figure 13 shows engineering drawings of the lower (antenna) washer of the gauge 0 4 C It, shown in Figure 6; and Figure 14 shows engineering drawings of the quartz plug of the gauge shown in Fiaure 6.

Referring, to the Figures, Figure la shows radar gauge 10 coupled to a container 12.

0 0 0 C' Container 12 may contain, for example, petroleum or other flammable liquid. Gauge I 8 includes the following components (best seen in figure 3): a dielectric rod antenna 0 0 20, a pressure plate or flange 24 with a microwave transparent pressure- sealed window 4n 28a, and a flame-proof electronics box 22 having a microwave transparent window 28b through which microwaves are transmitted. The flameproof electronics box 22 is located on the top of the flange 24. Flange 24 may have a diameter of 2, 3, 4, 6, or 8 C> 0 inches (approximately 5.1, 7.6, 10.2, 15.2 or 20.3 cm). It may, however, have a larger diameter. The dielectric rod antenna 20 bolts onto the underside of flange 24. The flame-proof enclosure 22 contains the RF board, base band boards, LCD display, power regulation, and intrinsically safe output links 26 for IR, Modbus (or other protocol such as HART) and 4-20 n1A interfaces. The unit is fully protected for harsh environments and flammable atmospheres.

The gauge system 10 may be made from stainless steel for, for example, marine 0 1-5 environments, or from aluminium for, for example, land-based applications. In normal operation, the gauge 10 is mounted towards the top of the container 12, with its antenna 20 pointing down towards.the surface of the product contained within the container 12.

The principle of measuring distance using the radar level gauge 10 is described as follows. A high frequency electromagnetic wave is emitted from the antenna 20. This wave reflects off an object in the storage tank 12 or the storage tank itself. The antenna C1 C1 20 collects a portion of the reflected wave. If the time of flight, t, of the wave is measured, then the distance travelled by the wave from the antenna 20 to the liquid surface 14 is equal to t12c, where c is the speed of light in a vacuum.

C If the frequency of the radar source is changed in a known manner, then knowing both the frequency of the source and that of the reflected wave allows the time of flight, t, and hence the liquid level, L, to be measured. One of the optimum ways of achieving this is to ramp the frequency in a linear manner. This technique is known as frequency modulation (FM). If the radar is emitting continuously, then this is known as a Z frequency modulation continuous wave (FMCW) radar.

In FMCW, the frequency is increased at a given rate between two frequency limits.

This is shown as the solid line in Figure lc. The frequency is then ramped down again 9 (shown by the broken line in Figure 1c), and the cycle begins again. Considering only the increasing frequency ramp, the microwave is launched by the antenna 20 towards the taroet and is reflected. Thus a second reflected frequency ramp is detected, identical to the first but delayed by a time t, the time of flight. Since both ramps have the same linearity, if one looks at q.;ie two frequencies at some particular time, say tO, then the difference in the frequencies is proportional to t, and hence to the distance travelled by the wave. FMCW radar essentially extracts the frequency difference and computes the distance. In reality, the speed of light is a large number, and consequently, the time of flight is very short, especially when compared with the rate of change of frequency of C 0 the ramp. Thus the two ramps are very close together, and the differences in frequency are a small fraction of the microwave frequency, f The technique used to produce a spectrum of peaks from the scene of interest is called a Fourier Transform.

A typical Fourier Transform 18 of a container is shown in Figure lb. In a real world environment, all objects in the path of the microwave reflect a signal back to the antenna 20. Each signal has different strengths depending on the composition of the object, the geometry of the object, and the distance to the object. Thus the container profile 18 is likely to show multiple peaks at different frequency differences, of which only one is likely to be of interest. For example, there are two peaks of interest in this case: the liquid surface 14, and the base of the container 16. The other peaks are )0 internal reflections inside the radar, or reflections from surfaces of the container close to the antenna. There may, however, be other peaks of interest, such as oiUwater emulsion layers.

In many container environments there are other obstacles which may be detected by the radar. These objects may be cross members, inspection ladders or other metallic structures inside the storage tank. Each object may contribute a peak to the container profile 18. To be useful the radar gauge 10 must identify the correct peak corresponding to the liquid surface 14.

The unit 10, with appropriate sensors, can also profile the temperature and pressure of a product in the container 12. The density, volume and weight of the product contained in container 12 may also be calculated.

In order to generate frequency modulated continuous waves, the following functions are required:

1) A swept frequency source, 2) An antenna to launch the wave towards the target, 3) A technique to compare the return sweep with that transmitted, and 4) A suitable method to extract the frequency difference produced by the target from amongst many other signals.

W In a preferred embodiment of the invention, as many of these functions as possible are carried out di itally and at low frequency. There are a number of advantages with this 9 c) approach over existing radar gauge systems:

Z:1 0 1) The components are relatively cheap as many are in mass production for mobile. communications applications, 2) The majority of the electronics assembly uses standard PCB techniques, and 3) The system has high precision.

Z3 A block diagram of the gauge system 10 is shown in Figure 2. The system is modular t> 0 0 in design. The radar gauge 10 comprises:

0 1. Hardware, including microwave signal generation and detection hardware, digital signal processing hardware, and a microwave antenna 20 mounted so that it radiates CD to, and receives from, the surface 14 of the product in the container.

2. Software, running on the gauge's central processing unit (CPU) 32.

tn 3. Power supply circuitry for all hardware.

The microwave antenna 20 is coupled to the output of the microwave signal generation b 0 hardware (30, 32, 33, 34, 36, 38, 40, 42, 44, and 46) via microwave waveguide structures which may include a solid flame-proof barrier 28. The flame-proof barrier (which acts as a window to microwaves) allows the antenna 20 to be placed in a 11 hazardous zone while the rest of the electronics is sealed within the flame-proof and explosion-proof housing 22. The gauge includes a microprocessor 32 (CPU) which performs various functions to control the gauge hardware, make measurements, and 0 calculate results using digital signal processing algorithms.

C t5 The CPU programs 33 control registers in the direct digital synthesiser (DDS) 42 at regular intervals so that the DDS 42 generates a numerically controlled frequency sweep from a lower to an upper frequency, or alternately from an upper to a lower frequency, at a precise rate of change. When the frequency sweep has endured for a predetermined length of time, the CPU 32 reprograms the DDS 42 to sweep the 0 0 frequency band at the same rate in the opposite direction. Consequently the gauge 10 g is continually generating a frequency sweep alternately either up or down between lower and upper frequency limits.

The output from the DDS 42, in digital form, is converted to analogue form by a digital to analogue converter (DAC) 34, and this analogue frequency reference is input to a phase-locked loop (PLQ 36. The PLL 36 multiplies the frequency reference of the DDS and smoothes out the discrete frequency jumps between each clock cycle of the DDS 42. The low-microwave frequency output of the PLL 36 is input to a microwave frequency multiplier 38 which multiplies the signal up to the microwave sweeping frequency band, of about 10 GHz centre frequency, as fed to the antenna 20.

A microwave coupling structure 40 samples the microwave signal transmitted to the antenna 20. A detector 41 detects the amplitude, and through interference of the transmitted signal and signals received back from the antenna, a beat frequency is a enerated which is fed into the differentiator circuit 43. The differentiator circuit 43 provides a frequency response that rises with frequency, and hence compensates for target distance from the antenna 20, since the beat frequency is proportional to target distance, whereas the received signal falls off with distance.

The output from the differentiator circuit 43 is then fed into a coder/decoder (codec) 44 and converted into sampled digital form, which can be processed by the CPU 32. The codec 44 also provides a digital-toanalogue output 45 that may be used for test and n 0 diagnostic purposes. The output may be used, for example, to display the result of a 0 12 Fast Fourier Transform on a display device such as an oscilloscope or a LCD 52 connected to the codec 44. The sampling frequency of the codec 44 is controlled by the master clock 46, as is the DDS 42, therefore any deviation in frequency of the master clock 46 is automatically compensated for. A serial input/output communications port 5 47 is provided for diagnostic, test, and remote line interfacing to the gauge 10.

ZD The software 33a, 33b, 33c, running on the CPU 32 performs all necessary interrupt service functions (33b) including periodic reprogramming of the DDS 42 frequency I sweep. The software reads sampled data from the codec 44 and performs digital si al C; gn processing algorithms to identify the signal received from the product surface 14 and Z calculate the range. The software provides a well-defined set of control and measurement subroutines that may be called by application-level software (33a) to carry out measurements.

All measurement-related microwave frequencies and sampling rates of the codec 44 are locked to a high-stability master oscillator 50. The use of a direct digital synthesis 0 technique means that frequencies are accurately determined, and frequency sweep rates are highly linear, which contributes to achievement of high range measurement C 4") accuracy, and eliminates the need for high-stability lengths of microwave reference transmission lines.

As the radar level gauge 10 uses digital components, it is possible to display the profile 18 of the container in real time on a display device 52. Calculations are perfon-ned in the digital domain whenever possible. This allows a feature unique to this system: it can subtract the background signals returned from the container 12 and obstructions within the container 12. The advantage of this is that most of the spurious peaks can be removed from the Fourier Transform (FT) trace. Figures 4a and 4b show typical graphs of the profile of the container 12. In Figure 4a, an unprocessed profile is shown. This shows peaks due to reflections from the container walls, the contents within the container, and from obstructions within the container. Since one is only interested in the peaks due to reflections related to the actual contents within the container 12, the other peaks not related to the actual contents can be regarded as "background signals"

0 Figure 4b shows a profile of a container once the background features have been

13 subtracted. It can be seen from Figure 4b, that peaks 54 and 56 (corresponding to the 1 1 liquid surface 14 and the container bottom 16 respectively) are easily identifiable.

Another advantage of performing calculations mainly in the digital domain, is that the 0 In 0 trace 18 can be displayed on the LCD 52 (or other display device) located at the front of the box 22. There are a"number of advantages, particularly at installation and set up, to this facility. The main one being that the level of the contents of the container 12 0 which the gauge 10 is detecting can be checked visually on the display device 52. With the appropriate peak which is to be detected identified, the peaks representing the background features associated with the particular container 12, can be stored in a

Flash EEPROM in the electronic circuits of the gauge 10. This enables the signals representing the background features to be elin-dnated automatically. The background is stored in the system, even when the gauge 10 is disconnected from the power supply. A curve-fitting algorithm is then used to identify the peak that is indicative of the level of contents to the desired resolution.

The best background profile to store is that of an empty container. This is not always possible at the time of installation of the gauge 10. The gauge 10 can be enabled so as

0 to store profiles of the background features as the container progressively empties, r> until a trace representing an empty container is produced and stored. This trace can then be used as a reference against which the contents of the container can be 20 calculated as described above.

In a preferred embodiment of the invention, the antenna 20 is a dielectric rod antenna with a PTFE coated surface. The dielectric rod antenna offers several advantages over other types of antenna. Firstly, the antenna may be designed to fit into a 4 inch (10 cm) aperture, making it suitable fordeployment in still pipes, and small openings in the 25 roof of a typical storage tank.

A longitudinal cross-section of a rod antenna is shown in Figure 5. The length (a) of 0 0 the rod antenna shown is 0.64 metres, and the transverse cross-section of the antenna is a rectangle measuring 0.25 metres (b) by 0. 198 metres (c).

14 Another advantage of narrow rod antenna 20 is that it produces a broad beam of microwaves with a relatively slow reduction of sensitivity as one moves off axis. This means that relatively large areas may be sampled, and that directing of the beam is simple because complicated alignment procedures, required by some radar systems, are not necessary. The gauge 10 is thus easier to install than systems that use other antenna types. The FUE rod 20 is also unaffected by most chemicals. The antenna's PUE surface resists surface build-up and offers a degree of self-cleaning.

There is, however, an option of using other sizes and types of antenna, depending on 0 C the level of performance required. For example, a horn antenna may be used. The structure of a horn antenna has the advantage of being mechanically simple, and the 1 microwave properties are well understood and amenable to calculation. A narrow angle C> horn gives the lowest phase front error across the aperture, and hence the highest gain and narrowest beam width for a given aperture size. At 10 GHz, an aperture of 20OMM diameter gives a gain of 25.6 dB. A corrugated horn may be used to reduce off-axis C spurious side-lobe levels, but fabrication of such an antenna is fairly expensive and there is a corresponding reduction in gain.

The gauge 10 is shown in greater detail in Figures 6 to 14. Referring to Figure 6, the C 0 housing 22 is fitted to a flange 24 so that the microwave transparent window 28a is 1 aligned with the microwave transparent window 28b, in order that microwaves may 0 pass to, and from, the housing 22 to, and from, the antenna 20. The window 28a is located in lower window holder 60a. The window holder 60a is in contact with lower (antenna) washer 62a. The window holder 60a and washer 62a together form the lower C RF assembly 68a. The window 28b is located in upper window holder 60b. The window holder 60b is in contact with upper washer 62b. The window holder 60b and washer 62b together form the upper RF assembly 68b. A simplified diagram of the Zn upper and lower RF assemblies 68a and 68b is shown in Figure 7.

c The lower RF assembly 68a fits into flange 24 via screws 72, as shown in Figure 6.

The upper RF assembly 68b is located at the base of the flameproof housing 22, and is 0 attached via screws 74. The upper and lower RF assemblies 68b and 68a respectively, collectively constitute a microwave coupler.

The window holders 60a, 68b and washers 62a, 62b have circular crosssections. The upper window holder 60b has a stepped profile, resulting in the diameter of the 0 window holder 60b in contact with washer 62b being of larger diameter than that of the window holder 60a in contact with the lower RF assembly 68a. Each of the windows 28a and 28b has a rectangular cross-section, and quartz plugs 76a, 76b are located in the space between windows 28a and 28b. Each quartz plue, 76a, 76b, is one- quarter wavelength thick. The quartz plugs do not fill the space between the windows 28a and 28b completely, so that spaces 66a and 66b sit above and below the quartz pluas 76a, 76b.

Detailed engineering drawings of RF window holders 60a, 60b, washers 62a, 62b, and 0 0 0 quartz plugs 76, together with dimensions of the other parts, are shown in Figures 10 to 14.

The quartz plugs 76a and 76b must be mechanically retained in the respective window 0 holders 60a and 60b, so that it is impossible for them to fall out. The quartz plugs 76a, 76b are thus cemented in place with sealant 84. The continuous cement path of the sealant is at least 10 min. The sealant 84 is required to be deployed in one shot. No partial cure which would allow a sealant interface to develop, or topping up voids is permitted according to BASEEFA standards. These regulations do not specify a minimum thickness of sealant 84, so it may only be a few micrometers thick. The sealant 84 used is a special solvent-less fluorosilicone elastomer paste. The thickness of each quartz plug 76a, 76b provides approximately 8mm of the required 1Omm path length of cement. In addition to the use of a sealant84, each quartz plug 76a, 76b is shaped so that pressure acting from the underside of the plug 76a, 76b tends to It> compress seal the joint.

In this example, all parts of the upper and lower RF assemblies 68a, 68b and the flange 24 are made from stainless steel, so that differential thermal expansions between the parts do not occur. In order to ensure good electrical contact between the RF assembly 68b located in the housing 22, and the RF assembly 68a located in the flange 24, both assemblies 68a, 68b are machined together (flat) on final assembly.

0 16 A hole 64 is formed in the RF window holder 60a extending from one side of the holder 60a to the space 66 located above the quartz plug 76a. The hole 64 provides pressure relief between the upper and lower RF assemblies 68b, 86a respectively. This is required because the contents stored in the container 12 may be contained at 5 pressures considerably higher than atmospheric pressure.

ZD - The antenna 20 that is fitted to flanae 24 via antenna holder 78 may be left inside the ZP container 12. The lower RF assembly 68a is constructed to form a seal between the container 12 and the environment. The lower RF assembly may be left in place on the container, whilst the housing 22 and upper RF assembly can be removed. In order to keep the microwave transparent window 28a clean while it is left on the container12 with the housing 22 removed, a removable temporary cap (not shown) may be fitted over the lower RF assembly 68a. The cap is removed in order to re-fit housing 22 onto the flange 24 and the lower RF assembly 68a.

C The invention has been described by way of a number of embodiments, and it will be appreciated that variation may be made to the aforementioned embodiments without departing from the scope of the invention.

Claims (8)

17 CLAIMS

1. A radar gauge comprising a microwave generator operable to produce a microwave over a swept range of frequencies, an antenna operable to transmit the generated microwave towards an object, and operable to receive microwaves reflected back to the antenna by the object, a differentiator means operable to receive the microwaves transmitted and received by the antenna and operable to derive therefrom, a beat frequency which is a result of interference between the transmitted and received microwaves, and thereby produce an output signal indicative of the time of flight (t) of the transmitted and received microwaves, and hence the distance of the object from the antenna, a coder/decoder operable to receive the output signal of the differentiator means and convert the output signal of the differentiator means into a digital signal, D and an outpuit means for indicating the distance of the object from the antenna.

C)

2. A radar gauge according to claim I wherein the microwave generator comprises a clocked direct digital synthesiser (DDS) which is operable to generate a numerically controlled frequency sweep between a first frequency limit and a second frequency limit which is lower than the first limit, and a digital to analogue converter operable to receive an output signal of the DDS and produce therefrom an output analogue reference signal over a swept range of frequencies.

3. A gauge according to Claim 2 wherein there is provided a phase-lockedloop circuit operable to receive the refere!pce signal and operable to smooth out frequency jumps in the reference signal between each clock cycle of the DDS.

A gauge according to Claim 3 wherein there is provided a microwave frequency multiplier operable to receive an output signal of the phaselocked loop circuit and produce therefrom a microwave at a higher frequency than the reference signal and operable to output the microwave signal to the antenna.

5. A method of measuring the distance of an object from an antenna, comprising the steps of generating a microwave at a swept range of frequencies transmittin- the microwave from the antenna in the vicinity of the object, receiving at the antenna, microwaves reflected by the object., differentiating I between the transmitted and received microwaves by deriving therefrom a beat frequency due to interference between the transmitted and received microwaves, producing an output signal indicative of the beat frequency and hence the time of flight (t), of the transmitted and received microwaves and therefore the distance of the object from the antenna, and converting the output gital signal, indicative of the distance of the object from the signal into a di., antenna.

6. A method accordinc, to Claim 5 wherein the step of generating a microwave M I 4 which is swept over a range of frequencies comprises the steps of generating Z z:1 digitally a numerically controlled frequency sweep between a first frequency limit and a second higher frequency limit, at a precise rate of change, using a direct digital synthesiser (DDS), converting the digital output of the DDS to an analogue signal, and multiplying the frequency of the analogue signal to frequencies suitable for transmission by the antenna.

7. A gauge substantially as herein described with reference to, and as shown in any one of the accompanying drawings.

8. A method substantially as herein described with reference to any one of the accompanying drawings.