Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B6/00—Heating by electric, magnetic or electromagnetic fields
  • H05B6/46—Dielectric heating
  • H05B6/48—Circuits
• • A—HUMAN NECESSITIES
  • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
  • A23B—PRESERVATION OF FOODS, FOODSTUFFS OR NON-ALCOHOLIC BEVERAGES; CHEMICAL RIPENING OF FRUIT OR VEGETABLES
  • A23B2/00—Preservation of foods or foodstuffs, in general
  • A23B2/05—Preservation of foods or foodstuffs, in general by heating using irradiation or electric treatment
  • A23B2/08—Preservation of foods or foodstuffs, in general by heating using irradiation or electric treatment using microwaves or dielectric heating

Definitions

• the present invention generally finds application in the field of radio-frequency treatment systems, and particularly relates to a device for generating an alternating radio-frequency electromagnetic field.
• the invention also relates to a method of controlling a device for generating an alternating radio-frequency electromagnetic field, and a product treatment plant comprising such generator device.
• Plants are known for treating and conditioning various types of products by application and use of alternate electromagnetic fields having various frequencies.
• plants which operate using electromagnetic fields operating in the microwave or radio-frequency range, and used for drying relatively wet products, such as hides or painted products.
• Electromagnetic fields are further used in the food processing industry, e.g. for pasteurization of products containing milk or eggs products and in general for conditioning and/or sanitization treatment of any food product.
• the applicant hereof manufactures and sells a treatment plant which uses an alternate radio-frequency electromagnetic field, such plant basically comprising a voltage generator that supplies voltage to an applicator having a pair of electrodes for producing the electromagnetic field through which the products to be treated are designed to pass.
• the applicator further comprises a capacitor, which is electrically connected to the electrodes and has a pair of opposed plates. The distance between the plates may be mechanically adjusted to vary the capacitance of the capacitor, thereby varying the supply voltage to the electrodes and, accordingly, the electromagnetic field emission power. [0008] With power adjustment, the heating temperature generated by the electromagnetic field on the products to be treated may be varied.
• the object of the present invention is to obviate the above drawbacks, by providing a device for applying a radio-frequency electromagnetic field that is highly efficient and relatively cost-effective.
• a particular object is to provide a device for applying a RF electromagnetic field that, when used in a plant for RF treatment of products in general, allows quick and dynamic regulation of product treatment temperature.
• a further object is to provide a device for applying a RF electromagnetic field that ensures high safety by preventing generation of unexpected electric discharges.
• Another object is to provide a plant for treating products by applying a RF electromagnetic field that allows effective treatment of products without affecting their inherent properties.
• Another important object of the present invention is to provide a method of controlling a plant for RF treatment of products that allows quick and immediate regulation of one or more electrical control parameters of the plant.
• the control means comprise an input connected to the electric power network, a first electronic control circuit which is connected to said input for substantially instantaneous variation of such electrical parameters and instantaneous control of the field emission power, a second electronic control circuit for regulating the operation of such oscillator, wherein the first electronic circuit has an output connected to such power supply means.
• field emission power and hence product treatment temperature, may be actually regulated without delay, as no mechanical regulation is required to set the plant to the new operating configuration.
• the invention relates to a method of controlling a device for generating an alternating electromagnetic field as defined in claim 1 1 .
• the invention relates to a plant for treating products with an alternating electromagnetic field as defined in claim 1 2.
• FIG. 1 is a schematic block view of a device for generating an electromagnetic field according to the invention
• FIG. 2 shows the current-voltage characteristic of a common diode as used in the plant of Fig. 1 ;
• FIGS. 3 to 7 show simplified wiring diagrams in various embodiments of a detail of Fig. 1 ;
• FIG. 8 is a schematic block diagram of the method of controlling a device for generating an electromagnetic field according to the invention. Detailed description of a preferred embodiment
• Fig. 1 shows a device for generating an alternate radio- frequency electromagnetic field in a predetermined working area.
• the device generally designated by numeral 1 , may be designed to generate an alternating electromagnetic field at a substantially constant frequency or, alternatively, at a frequency that falls within a predetermined band.
• Preferred frequencies for the electromagnetic field range from 5 MHz to 500 MHz. Particularly, suitable frequencies may be selected within frequency ranges admitted by international standards for civil or industrial uses, whose central values are 6.78 - 1 3.56 - 27.1 2 - 40.68 - 433.92 MHz.
• the operating frequency will range from 20 MHz to 50 MHz, as best results were experimentally found to be obtained with these lower values, e.g. about 27.1 2 MHz or 40.68 MHz.
• the device 1 may be used in any plant for RF treatment of products.
• a general plant incorporating the device of the invention may find application in the food processing industry, e.g. for sterilization, pasteurization and/or treatment of food products in general, particularly milk- and/or egg-based products, flours, grains and the like, as described and illustrated in EP2375910 and EP1 91 251 2, both by the Applicant hereof.
• the Applicant has provided an industrial hide drying plant which uses a device for generating an alternating RF electromagnetic field having flat electrodes for generating a substantially uniform electromagnetic field through which the hides to be treated are designed to pass.
• a further application of a similar plant may be treatment and drying of polymeric materials, such as profiles or the like that have undergone a surface treatment with water-based paints.
• the plant of the invention essentially differs from such prior art plants for the particular configuration of the alternate RF electromagnetic field generator device.
• the device 1 basically comprises an applicator 2 for emitting the electromagnetic field in the working area 3, an oscillator 4 for providing an alternate voltage V RF and electric current IRF having a predetermined value and a predetermined frequency and power supply means 5 for supplying a substantially DC voltage Vcc to the oscillator 4.
• the working area 3 is the area through which the products to be treated are designed to pass, and is designed according to the type of products and the particular means for feeding them through the plant.
• the feeding means may comprise one or more conduits or pipes, in case of pasty, semi-pasty, liquid or powdered products, or a belt conveyor, a roller conveyor or a similar device in case of loose products or industrial hides, polymeric profiles and large products in general.
• the device 1 further comprises control means 6 associated with the power supply means 5 for controlling the electrical parameters of the alternate voltage VRF or current IRF provided to the applicator 2 by the oscillator 4.
• control means 6 may be designed to control the electrical parameters concerning the amplitude and/or frequency of the voltage VRF and/or the current IRF provided to the applicator 2.
• the control means 6 comprise an input 7 connected to the electric power network N, a first electronic control circuit 8 which is connected to the input 7 for substantially instantaneous variation of such electrical parameters and hence for instantaneous control of the field emission power.
• the first electronic control circuit 8 further has an output 9 connected to the power supply means 5 and the control means 6 comprise a second electronic control circuit 10 for regulating the operation of the oscillator 4.
• the first control circuit 8 may control the electrical parameters of the electric signals provided to the power supply means 5.
• Such voltage variation will cause a corresponding variation of the power of the electromagnetic field emitted in the working area 3 with a particularly short delay or with no delay at all.
• the first electronic control circuit 8 may comprise an inverter 1 1 , for converting the standard voltage of the network N, having a constant frequency and a constant rms value, into an AC voltage VINV having a controlled rms value and/or a controlled frequency.
• the supply means 5 may include an amplifier circuit 12 with an input 13 connected to the output
• the amplifier circuit 12 may amplify and change the voltage
• VINV at the output of the inverter 1 1 to provide a substantially DC voltage VCC to the oscillator 4.
• the amplifier circuit 12 may comprise a voltage boost transformer 16 having an input 13 connected to the output 14 of the inverter 1 1 and a rectifier 18 downstream from the transformer 16 having an output
• the voltage V !N v at the output 14 of the inverter 1 1 may be an AC voltage, with an value ranging from 0 V to 400 V
• the output voltage VTR of the transformer 16 will also be an AC voltage with a rms value not exceeding 9000 V
• the DC voltage V C c at the output of the rectifier 18 is limited to about 12000 V.
• the oscillator 4 may comprise at least one triode 19 having two power terminals, i.e. the anode A and the cathode K respectively.
• the triode 19 may also comprise a grid terminal G which is adapted to receive a control signal designed to vary the electrical parameters of the signal between the anode A and the cathode K.
• FIG. 2 shows a voltage-current characteristic of a bias triode whose instantaneous operating point lies along a substantially straight segment T.
• the displacement of the operating point P may be controlled by varying the voltage V G in the grid terminal G of the triode 19, which is represented on the y-axis of the diagram of FIG. 2.
• the bias of the triode acting as an oscillator is such that the anode A and the cathode K are supplied with the DC voltage V C c present at the output 15 of the rectifier.
• the oscillation voltage V RF at the output of the anode A and cathode K terminals of the triode 19 may be varied by controlling the waveform of the voltage VQ in the grid terminal G.
• the use of the triode 19 as an oscillator at a predetermined frequency requires a substantially periodic voltage V G with a positive wave portion to be applied to the grid terminal G.
• the duration of the positive grid voltage VQ may be equal to a predetermined angular fraction of the oscillation period of the voltage V RF provided to the applicator 2.
• This angular fraction may be substantially close to one third of a round angle, and may particularly range from about 80 ° to about 150 °.
• the triode 19 comprises a heating filament F with a power supply circuit that is known per se and will not be further described herein.
• the second electronic control circuit 10 may be connected to the grid terminal G of the triode 19 for controlling the grid voltage V G according to the instantaneous value of the AC voltage VINV at the output of the inverter 1 1 .
• the second electronic control circuit 10 may comprise a voltage divider circuit 21 connected to the input of the applicator 2 for drawing a predetermined portion VPAR of the voltage VRF.
• the second electronic control circuit 10 may comprise a DC voltage adder circuit 22, downstream from the divider circuit 21 .
• the adder circuit 22 may be connected to the grid terminal G of the triode 19 to adapt the average value of the grid voltage VG such that the triode 19 is continuously kept oscillating.
• control means 6 may comprise a programmable central control unit PLC 23, which is adapted to control at least the output voltage VINV of the inverter 1 1 , and the second electronic control circuit 10 for varying the power emitted by the electromagnetic field generated by the applicator 2 and for ensuring constant oscillation of the triode 19.
• PLC 23 programmable central control unit
• a central control unit 23 may be programmed to control the second electronic control circuit 10 by varying the DC voltage value of the adder circuit 22 according to the instantaneous value of the voltage VRF provided to the applicator 2.
• the central control unit 23 may comprise interface means, not shown, which are used to enter and/or display the data concerning the value of the electrical parameters provided and/or detected in the first 8 and second 10 electronic control circuits.
• the central control unit 23 may comprise an alphanumeric keyboard, not shown, for entering data concerning the values of the desired electrical parameter and a display, also not shown and known per se, for displaying the data concerning the instantaneous values of such parameters.
• the divider circuit 21 may comprise the pair of inherent capacitors CAG, CGK of the triode 1 8 which are present between the anode A and the grid G and between the grid G and the cathode K respectively.
• the divider circuit 21 may also comprise an inductor LAC inserted between the anode A and the grid G of the triode 1 9 and an inductor LB for blocking the DC component, having a first terminal 24 electrically connected to the grid terminal G.
• the divider circuit 21 may differ from the diagram of FIG. 3 in the presence of an inductor L G K connected between the grid G and the cathode K of the triode 1 9.
• This configuration has the advantage of using an inductor whose maximum operating voltage is lower than that used in the diagram of FIG. 3.
• the value of the inductors L A K or L G K may be varied during calibration to adjust the value of the voltage VPAR according to the voltage VRF provided to the applicator 2.
• the voltage VPAR may be a predetermined fixed portion of the voltage V RF presented to the applicator 2.
• the voltage VPAR may range from 1 0% to 20% the value of the voltage VRF and is preferably about 1 5% the value of the voltage V RF .
• the adder circuit 22 may be connected downstream from the divider circuit 21 and may be electrically connected to the second terminal 25 of the inductor LB and the cathode K of the triode 1 9.
• FIGS. 3 to 7 schematically show three different types of adder circuits 22.
• the adder circuit 22 may be electrically equivalent to a variable resistor R.
• the adder circuit 22 may be electrically equivalent to a DC voltage generator, with a variable tension E.
• the central control unit 23 may be programmed to control the ohmic value R of the equivalent variable resistor or the value of the tension E of the equivalent generator according to the instantaneous value of the voltage V RF provided to the applicator 2.
• FIG. 6 the diagrams of FIG. 4 and FIG. 5 may be combined to form an adder circuit 22 substantially equivalent to a variable resistor R connected in series to a voltage generator E.
• the central control unit 22 may be designed to simultaneously control both the ohmic value of the equivalent resistor and the electromotive force of the generator R.
• the divider circuit 21 and the adder circuit 22 may be designed to provide a voltage V G to the grid terminal G which is substantially in phase opposition to the voltage VRF provided to the applicator 2.
• the voltage VQ at the input of the grid terminal G of the triode 1 9 is obtained by adding the variable-amplitude DC voltage to an AC signal V PA R having a substantially constant amplitude.
• the voltage V G at the grid terminal G may be obtained by the sum of a variable DC voltage provided by the adder circuit 22 to a variable AC voltage provided by the divider circuit 21 .
• the variation of the amplitude of the AC voltage provided by the divider circuit 21 may be obtained by appropriately varying the capacitance value of the capacitors CAG and CQK of the triode 1 9 between the anode A and the grid G and between the grid G and the cathode K respectively.
• the central control unit 23 may be designed to simultaneously control the capacitance values of the capacitors CAG and CQK and the ohmic value of the equivalent variable resistor R.
• control means 6 may comprise an oscillation frequency control circuit, not shown, downstream from the triode 19 and adapted to vary the voltage frequency VRF provided to the applicator 2.
• the applicator 2 may comprise at least one pair of electrodes 26, 27 for emitting and directing the electromagnetic field in the working area 3.
• the shape, type and number of electrodes may be selected according to the particular type of plant for which the device 1 is designed and particularly the electrodes may be configured as described in the above mentioned patent documents by the Applicant hereof.
• they may be formed with a pair of facing plates, placed at a predetermined distance from each other for radiating a substantially uniform electromagnetic field into the gap therebetween, or they may be annular electrodes adapted to have said product feeding conduit extending therethrough such that products may pass through the field.
• the applicator 2 may comprise a capacitor 28 with plates 29, 30 electrically connected in series to the electrical terminals of the electrodes 26, 27.
• the capacitor 28 may be designed to limit the value of the electric current provided at the output of the power supply means 5 if the electrodes 26,27 are short-circuited.
• the plates 29, 30 may be substantially flat and face each other at a fixed distance.
• the capacitor may have a fixed-value capacitance which is adapted to define an impedance with an ohmic value sufficient to limit the current generated by the triode 1 9 if the electrodes 26, 27 are short- circuited.
• the invention relates to a method of controlling the above described device 1 for generating the alternate RF electromagnetic field, as shown in FIG. 8, which essentially comprises a step a) of providing an applicator 2 for emitting the electromagnetic field in a working area 3, a step b) of providing an oscillator 4 which provides an alternate voltage VRF and an alternate current IRF having a predetermined value and a predetermined frequency to the applicator 2.
• control step d) comprises varying the electrical parameters to instantaneously vary the electromagnetic field emission power and the controlled power supply to the oscillator 4.
• control means 6 comprise an inverter 1 1 and the oscillator 4 comprises a triode 19.
• the inverter 1 1 is designed to convert the network voltage into an AC voltage having a variable rms value and/or a variable frequency.
• the triode 19 is powered with a grid voltage V G , which is controlled to ensure constant oscillation thereof as the AC voltage V !N v transformed by the inverter 1 1 is varied.
• control step d) may be obtained by varying the alternate voltage and/or frequency at the output 14 of the inverter 1 1 without requiring any mechanical adjustment that would inevitably involve plant response delays.
• the control step d) may be carried out while maintaining the waveform of the voltage V G applied to the grid G of the triode 19 substantially constant.
• the control of the voltage VQ at the grid G of the triode 1 9 will allow the triode to keep on oscillating, such that it may provide a voltage V RF adapted to supply the applicator 2, between the anode A and the cathode K.
• control step d) will control the positive portion of the grid voltage VQ, such that the latter has a duration equal to a predetermined angular fraction of the oscillation period.
• the device of the invention fulfills the intended objects and particularly meets the requirement of allowing immediate and dynamic regulation of electromagnetic field emission power, for delay-free regulation of product treatment temperature.

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Chemical & Material Sciences (AREA)
• Food Science & Technology (AREA)
• Polymers & Plastics (AREA)
• Zoology (AREA)
• Wood Science & Technology (AREA)
• Apparatus For Disinfection Or Sterilisation (AREA)
• Control Of Eletrric Generators (AREA)
• General Physics & Mathematics (AREA)
• Inverter Devices (AREA)
• Radiation-Therapy Devices (AREA)
• General Induction Heating (AREA)
• Breeding Of Plants And Reproduction By Means Of Culturing (AREA)
• Physical Or Chemical Processes And Apparatus (AREA)
• Feedback Control In General (AREA)

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