Temperature measuring apparatus and related improvements

An apparatus for measuring the temperature of an object. The apparatus includes a device for establishing a standing wave pattern or radiation emanating from the object and a coupling device for coupling the radiation to a detector, such as a measuring radiometer.

FIELD OF INVENTION

This invention relates to temperature measuring apparatus. The invention in particular, though not exclusively, relates to such apparatus using electromagnetic antenna cavities or cavities operating in a combination of induction field (near field) and travelling wave (far field) modes or standing-wave modes. The invention, in particular further, though not exclusively, relates to using one or more standing-wave patterns in an antenna-cavity structure to obtain a desirable form of radiometric spatial weighting function for coupling a radiometer to a region of source material whereby a temperature of the source material inside the cavity may be measured.

BACKGROUND OF INVENTION

Many industrial processes require non-invasive, non-destructive measurement of temperature within a mass of material, for example, processed food product inspection and also medical applications.

Travelling wave and induction field surface-contact type antennas have previously been used for medical applications. However, these types of antennas are not well suited to many envisaged applications. In particular, the following problems are evident:

(1) The weighting functions for this class of antenna are strongly biased to a region in direct contact with the antenna, i.e., the induction or near-field zone. The radiometric temperature measurement is, therefore, strongly biased to an object's surface region adjacent to the antenna.

(2) If there is a region of low dielectric loss material between the antenna and the glossy material of an object, such as a packaging air gap, wave modes propagating perpendicular to the antenna measurement axis may be set up, dramatically reducing the radiometric coupling to the product.

(3) If an object is electromagnetically thin in the direction of the antenna measurement axis, radiation from sources beyond the object will be coupled into the antenna together with the object material radiation giving a false temperature measurement (e.g., fluorescent lighting microwave radiation).

(4) If there is a region of low dielectric loss material between the antenna and the lossy material of an object (as in (b) above), radiation from sources around the antenna and the object will enter the antenna, again leading to a false temperature measurement.

Thermal/Infra-red imaging also only gives a surface reading and has problems in penetrating water and grease. It is also not possible to see through packaging for food.

It is an object of at least one embodiment of at least one aspect of the present invention to obviate/mitigate one or more of the disadvantages of the prior art.

It is also an object of at least one embodiment of at least one aspect of the present invention to provide a method of using natural microwave thermal radiation to measure the temperature of an object.

SUMMARY OF INVENTION

According to a first aspect of the present invention there is provided an apparatus for measuring a temperature of an object comprising:

means for establishing at least one (and preferably one) standing wave pattern of radiation emanating from the object, and

means for coupling the radiation to a measuring radiometer.

It is preferred if one or more ports, probes or loops electromagnetically couple the means for establishing the at least one standing wave pattern and the measuring radiometer. The position, size, shape, orientation and construction of the ports, probes, or loops may determine coupling to the electric and/or magnetic fields of the at least one standing wave pattern.

Preferably, the means for establishing the at least one standing wave pattern is an enclosed wall structure. Advantageously, the enclosed wall structure is substantially rectangular or cylindrical, or at least part conical or pyramidal.

Preferably, the radiation is in the microwave range.

Preferably, the microwave radiation has a frequency in the range of 0.1 GHz to 30 GHz.

Preferably, the means for establishing the at least one standing wave pattern(s) are made from electrically conducting material.

Preferably, the means for establishing the at least one standing wave pattern have low-loss, high-wave impedance areas within them that provide electromagnetic isolation.

According to a second aspect of the present invention there is provided a method of measuring the temperature of an object comprising the steps of:

establishing a standing wave pattern of radiation emanating from the object, and

coupling the radiation to a measuring radiometer.

According to a third aspect of the present invention there is provided a transport container including an apparatus according to the first aspect.

According to a fourth aspect of the present invention there is provided a transportation means including an apparatus according to the first aspect.

According to a fifth aspect of the present invention there is provided an apparatus for measuring a temperature of an object comprising:

a member having an open cavity and a plate which are moveable relative to one another so as to enclose the object within the cavity,

wherein, in use, a standing wave pattern of radiation emanating from the object is set up within the cavity, and

means for coupling the radiation to a measuring radiometer.

Preferably, the member comprises a sleeve portion.

According to a sixth aspect of the present invention there is provided a method of measuring a temperature of an object comprising the steps of:

providing a plate for receiving the object;

providing a member having an open cavity wherein, in use, the member and the plate are moveable relative to one another so as to enclose the object within the cavity,

establishing a standing wave pattern of radiation emanating from the object within the cavity, and

coupling the radiation to a measuring radiometer.

According to a seventh aspect of the present invention there is provided a transport container including an apparatus according to the fifth aspect.

According to an eighth aspect of the present invention there is provided a transportation means including an apparatus according to the fifth aspect.

According to a ninth aspect of the present invention there is provided a production line including an apparatus for measuring a temperature of an object according to the first or fifth aspects.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring initially toFIG. 1, there is shown an apparatus, generally designated5, according to an embodiment of the present invention. The apparatus5is for measuring a temperature of an object or product such as a processed food product14, and comprises means for establishing a standing wave pattern of radiation emanating from the object, and means for coupling the radiation to a detector such as a measuring radiometer11.

The means for establishing a standing wave pattern comprises an enclosed wall structure6, including a cavity10forming a measurement region. The means for coupling comprises a coupling port12provided on the wall structure6.

The coupling port12electromagnetically connects the cavity10to one or more radiation signal measuring radiometer receivers11(forming an “antenna-cavity” structure). The cavity10is made from a suitable conductive material such as copper, copper plated steel and/or silver plated brass. Such materials are chosen because they have good microwave surface conductivity. Enclosing of the measurement region provides isolation from external sources of electromagnetic radiation allowing proper measurement of an effective radiation temperature of an object or product14within the cavity10. As can be seen fromFIG. 1, the object14is substantially centrally placed within the cavity10.

A suitable cavity10size is typically 0.25 m×0.25 m×0.15 m. However, a wide range of sizes, e.g., from a few millimeters to a few meters can be used depending on the frequency and mode of the measurement. It is, for example, in embodiments of the present invention possible to monitor a temperature of an object within the apparatus5which can comprise a transport container20on a transportation means25such as a lorry, or a train, or ship, or the like.

In use, to measure the temperature of the object14, a microwave standing wave pattern of naturally occurring microwaves from the product14is set up inside the cavity10.

The microwaves are typically in the wavelength region 0.1 GHz to 30 GHz.

An apparent radiation temperature “seen” by a radiometer11connected to the cavity10will be substantially that of the material of the object14within the cavity10weighted according to a weighting function applying to the whole source (cavity and product). By weighting function is meant that although the temperatures across the whole cavity10contribute to the obtained measured temperature, the temperature reading is biased due to the standing wave arrangement in the cavity10towards a certain region of the cavity10. Therefore, by altering the properties of the standing wave pattern, different regions of the object14within different parts of the cavity10can have their temperature measured. In general, if the radiation losses of the object14are much larger than the losses in the enclosing/coupling antenna-cavity structure10, the measured temperature will be close to that of the object14and the antenna-cavity temperature has only a small effect.

It is characteristic of low-loss conducting wall enclosing structures, such as that shown in cavity10, that the electromagnetic radiation within them tends to form standing wave patterns. The form of the weighting function within an antenna-cavity—which determines the spatial form of the radiation coupling of the source material to the radiometric signal measured by the radiometer11—is determined by the standing wave pattern of the antenna-cavity and enclosed material combination. Antenna-cavity structures can thus be designed to support standing wave patterns which provide weighting functions that give stronger or weaker radiation coupling to source material at different positions in the structure. This can then allow the temperature measurement process to be biased towards regions of a product of particular importance. An example of this would be the case of a disc-shaped object or product (e.g., a food product such as a quiche), where the cooking process heats the periphery more than the center, but it is the central temperature that is particularly important for product quality and food safety.

Referring toFIG. 2, there are shown field and power density profiles across the cavity10for a given frequency of radiation. It can be clearly seen that the signal is strongest in the center of the cavity10, and therefore, the temperature reading will be predominantly biased by the temperature in that region. To measure the temperature of the object14, the object14should therefore be placed in the center of the measurement region. The arrangement inFIG. 2, therefore, provides a centrally weighted microwave temperature measurement.

It should be noted that inFIG. 2a uniform Z-field is used and there is therefore no variation in the vertical direction. If the Z-field was varied, a 3-D topographic surface would be obtained for the electric field and power density.

FIG. 3shows electric field and power density profiles for a further given frequency of higher frequency than the given frequency ofFIG. 2. InFIG. 3, the obtained temperature reading therefore tends towards a “quasi-uniform” response across the whole of the cavity10tending to give an “average” temperature for the object14within the cavity10.

Referring toFIG. 4, there is illustrated an experimental plot of cavity response versus position across a floor of a cavity of an apparatus according to a further embodiment of the present invention. As can be seen fromFIG. 4, the cavity of this embodiment exploits standing wave patterns to produce a strongly peaked central response.

Referring now toFIGS. 5(a) and5(b), there is shown an apparatus, generally designated105, according to a vet further embodiment of the present invention, for measuring a temperature of an object114and comprising: a member106having an open cavity110and a plate surface118which are moveable relative to one another so as to enclose the object114within the cavity110, wherein, in use, a standing wave pattern of radiation (such as naturally occurring microwave radiation) emanating from the object114is set up within the cavity110, and means for coupling112the radiation to a measuring radiometer111.

The member106comprises a sleeve portion which fits over and encloses object114residing on a plate surface118of an electrically conducting material. The plate surface118may comprise at least part of a conveyor belt or the like of a production line130.

In use, the plate surface118is moved in direction “A” until the object114is below the member106. The member106is then lowered in direction “B” onto the plate surface118. After a measurement has occurred, the member106is lifted and plate surface118moved on to a next object114, and so on.

FIG. 5(a) shows member106initially held above a product114whose temperature is to be measured. Member106is then lowered, as shown inFIG. 5(b), to be positioned around product114. Product114sits on conducting plate surface118. A standing wave pattern is setup within the cavity110such that a temperature of the product114can be measured by radiometer111. This nature of measurement is useful on a high speed production line as a measurement operation may only take around 0.5 seconds.

The form of radiation standing-wave pattern in an antenna-cavity structure can be strongly influenced by the positioning, orientation and form of radiometer coupling port112(a probe or loop may also be used). If two or more appropriate coupling ports112,112′ are provided, each can couple to a different antenna-cavity weighting function, and each of these ports112,112′ will then allow the measurement of a radiometric temperature having a different spatial weighting across the product114. The radiometric signals at the coupling ports112,112′, can be measured sequentially or the two coupling ports112,112′, can be measured at the same time using a synchronized 180° out of phase source reference switching configuration (not shown) in the radiometer111.

Radiometric temperatures measured with different spatial weightings are used to estimate temperature variations across a source object114.

It will be appreciated that the embodiments of the present invention hereinbefore described are given by way of example only, and are not meant to limit the scope thereof in any way.

For example, although the member106illustrated inFIGS. 5(a) and5(b) is part conical or pyramidal, it will be appreciated that the member may be of any suitable shape, and may indeed be merely rectangular.

It will further be appreciated that the form of the radiation standing wave pattern in an antenna-cavity structure is dependent on the frequency of the radiation typically in the region 0.1 GHz to 30 GHz. Radiometric temperature measurements made at different frequencies, even through the same coupling port112, will in general have different weighting functions. For example, referring to the resonant frequency expression for a rectangular cavity the low frequency resonance corresponding to standing wave modes designated by l, m, n =1, 1, 0 can have a weighting function for an appropriately polarized electric field which is strongly peaked in the center of the cavity; at higher frequency resonances around l, m, n =3, 3, 1 etc, the weighting function will become relatively uniform cross much of the cavity. The antenna-cavity dimensions must be chosen so as to provide the appropriate resonances at suitable radiometer measurement frequencies. Depending on the material of the source object, the frequency dependence of the attentuation factor may also be used to obtain change in weighting function within the material. For example, if the source object material has a high water content, the antenna-cavity could be dimensioned to support relatively high-order resonances producing near uniform weighting functions external to the source object for a range of frequencies over which the radiation attenuation in the source object varied appreciably. These different frequency radiometric temperatures could be efficiently and accurately measured with a multi-frequency-reference radiometer.