Tuning Means In A Microwave Heating Apparatus

Abstract

Tuning means in a microwave heating apparatus comprises a slotted waveguide section connecting a microwave energy source to a microwave heating means, a tuning probe extending into the slotted waveguide section through the slot, a motor driven carriage slidable along the slot and mounting the tuning probe, two voltage standing wave pattern sensing probes attached to the carriage and extending into the slotted waveguide through the slot, and a control circuit connected to the sensing probes and the electric motor for tuning the apparatus by moving the tuning probe along the slot. Two further probes, which are similar to the sensing probes, may be provided together with further circuitry for indicating on a voltmeter when adjustment is necessary of the depth that the tuning probe extends into the slotted waveguide section.

Description

This invention relates to microwave heating apparatus, and is particularly concerned with tuning means for microwave heating apparatus.

Tuned or resonant microwave heating systems often offer a convenient means to efficiently couple microwave energy to heat, for example, continuous thin webs and filamentary materials. Known resonant microwave heating systems usually require some manually adjustable tuning mechanism to compenstate for detuning of the system as a result of variations in the properties of the material being heated or changes in its transport speed at the heating position.

Although many known resonant microwave heating systems maintain proper tuning for extended periods of time during operation, nevertheless manual adjustment of the tuner by the operator is often required during starting up procedures and as stated above is occasionally necessary during operation. When, however, the microwave heating system is detuned, full microwave power transfer to the material to be heated is not achieved and so incomplete heating results. Consequently, for most consistent product quality and operational convenience it would be desirable to provide a microwave heating apparatus wherein there is provided an automatically adjusted tuning means for tuning the apparatus.

It is an object of the invention to provide in a microwave heating apparatus, tuning means which will automatically tune the apparatus.

According to the invention there is provided a microwave heating apparatus, comprising a microwave energy source, and a microwave heating means, a microwave tuning means connecting the microwave energy source to the microwave heating means, the tuning means comprising a slotted waveguide section, a carriage outside the slotted waveguide section and slidable along the slot therein, a tuning probe extending into the waveguide section through the slot, means adjustably mounting the tuning probe on the carriage for adjusting the distance the tuning probe extends into the waveguide section to the correct depth for tuning, at least two sensing probes extending into the waveguide section to sense the voltage standing wave pattern, means mounting the sensing probes at a fixed distance apart from each other and from the tuning probe, and means connected to the carriage and responsive to changes in the voltage standing wave pattern sensed by the probes, for tuning the microwave heating section by moving the tuning probe along the slot.

In the accompanying drawings which illustrate, by way of example, embodiments of the invention,

FIG. 1 is a perspective view of a microwave heating apparatus,

FIG. 2 is a partly sectioned side view of the tuning means shown in FIG. 1,

FIG. 3 is a plan view of FIG. 2,

FIG. 4 is a diagram of the circuit which controls the tuning means shown in FIGS. 1 to 3,

FIG. 5 is a partly sectioned side view of another tuning means for use in the apparatus shown in FIG. 1, but which also indicates when the depth of a tuning probe in a waveguide section needs adjustment,

FIG. 6 is a plan view of FIG. 5,

FIG. 7 is a diagram of a circuit to control the tuning means shown in FIGS. 5 and , and

FIG. 8 is a diagram of a circuit to control a tuning means having three probes,

Referring to FIG. 1 there is generally shown a microwave heating apparatus, comprising a microwave energy source in the form of a magnetron 1, a microwave heating means in the form of a slotted waveguide 2, and a microwave tuning means 3 (the control circuit of which is not shown) connecting the magnetron 1 to the waveguide 2.

The output from the magnetron 1 is fed via a transition 5, isolator 6, reflected power monitor 8, tuning means 3, waveguide bend 10, and microwave transformer section 12 to the waveguide 2. The provision of an isolator 6 is optional. An exhaust cowling 14 is provided for evacuating water vapour from the waveguide 2, and the whole assembly is mounted upon a table 16.

Referring now to FIGS. 2 and 3, the tuning means 3 generally comprises a slotted waveguide section 18, a carriage 20 outside the slotted waveguide section 18 and slidable along the slot 22 therein, a tuning probe 24 extending into the waveguide section 18 through the slot 22, means 26 adjustably mounting the tuning probe 24 on the carriage 20 for adjusting the distance the tuning probe 24 extends into the waveguide section 18 to the correct depth for tuning, two sensing probes 28 and 30 extending into the waveguide section 18 to sense the voltage standing wave pattern, means 32 mounting the sensing probes 28 and 30 at a fixed distance apart from each other and from the tuning probe 24, and means, including an electric motor 34 and lead screw 36, connected to the carriage and responsive to changes in the voltage standing wave pattern sensed by the sensing probes, for tuning the apparatus of FIG. 1 for maximum power transfer from the magnetron 1 to the slotted waveguide 2 by moving the tuning probe 24 along the slot 22.

The means connected to the carriage and responsive to changes in the voltage standing wave pattern also includes a control circuit to be described later with reference to FIG. 4.

The waveguide section 18 has flanges 38 and 40 for connection to the isolator 6 (FIG. 1) and waveguide bend 10 (FIG. 1) respectively. Two brackets 42 and 44 are provided on the waveguide section 18 for mounting the electric motor 34 and rotatably mounting the lead screw 36. The brackets 42 and 44 also support slides 46 and 48, and have limit switches 50 and 52 respectively, mounted on them. The limit switches 50 and 52 when contacted by the means 32 and the carriage 20 respectively, reverse the direction of movement of the carriage 20 by reversing the direction of motion of the motor 34.

The carriage 20 is slidably mounted on the slides 46 and 48 and has an extension 54 provided with a clearance hole 58 through which the lead screw 36 extends. A push button 62 is slidably mounted in a bore 60 in the extension 54, and has a threaded portion 64 of an elongated bore 66 urged into threaded engagement with the lead screw 36 by means of compression spring 68. By pressing the push button 62 the threaded portion 64 may be disengaged from the lead screw 36 to allow the carriage 20 to slide along the waveguide section 18 manually.

The tuning probe 24 comprises a spherical end 66 and a stem 68. The means 26 comprises an adjustment screw 70, a bracket 72 mounted on the carriage 20, and a lock nut 74. The screw 70 is attached to the stem 68 and is screwed into a threaded bore 76 in the bracket 72.

The means 32 mounting the sensing probes 28 and 30 at a fixed distance apart from each other and from the tuner probe 24 comprises a mounting block 78, slidably mounted on the slides 46 and 48, and screwed to the carriage 20 by an adjusting screw 80 and nut 82. The sensing probes 28 and 30 are electrically connected to a coaxial microwave diode detector mounts 84 and 86 respectively, the output of which is fed by coaxial cables 88 and 90 respectively, to the control circuit of FIG. 4.

In operation of the apparatus is arranged as shown in FIGS. 1 to 3 and the magnetron 1 is energized, a strip of paper (not shown) with a glue line to be dried on it is passed along the slot in the slotted waveguide 2. Microwave energy passes through the isolator 6, the tuning means 3 and hence to the slotted waveguide applicator 2 containing the strip of paper with the glue line on it. Microwave energy propagated down the length of the slotted waveguide applicator 2 and which is not absorbed by the paper or glue line is reflected from the far end thereof, resulting in a standing wave pattern.

When the apparatus is started up the system is tuned manually by:

1. Adjusting the depth that the tuning probe 24 extends into waveguide section 18. The correct depth depending on the total energy loss in the applicator 2, and

2. Adjusting the distance of the tuning probe 24 along the waveguide section 18, the correct position of the probe depending on the phase of the reflected energy.

A slight variation in the properties of the material being dried will result in changes in the voltage standing wave pattern in the tuning means 3. In this embodiment of the invention the change in total energy loss due to small variations in the properties of the material being dried is considered sufficiently negligible to require only manual adjustment of the depth of the tuning probe 24 from time to time.

However, slight variations in the properties of the material being dried may significantly alter the phase of the energy reflected back to the tuner and this may require an appreciable adjustment of the position of the tuning probe 24 along the waveguide section 18. For this reason the tuning means 3 is connected to the control circuit shown in FIG. 4.

Referring now to FIG. 4, the sensing probes 28 and 30 (FIGS. 2 and 3) are connected to the inputs of amplifiers 100 and 102 respectively for amplifying voltages detected by the sensing probes 28 and 30. The sensing probes 28 and 30 are spaced apart preferably one-quarter of the wavelength of the waveguide 2, and are so disposed along the length of the slotted waveguide section 18 as to supply voltages of equal magnitude when the waveguide 2 is tuned by the tuning probe 24.

When, through changes in the properties of the paper, it is necessary to adjust the position of the tuning probe along the slotted waveguide section 18, voltages of different magnitudes will be passed from the sensing probes 28 and 30 to the amplifiers 100 and 102. The amplified signals from the amplifiers 100 and 102 are fed to a difference amplifier 104. The output signal from difference amplifier 104 is amplified by amplifier 106, an inverting amplifier with adjustable gain, whose output signal is passed to transistors Q.sub.1 and Q.sub.2 which prevent excessive loading of the amplifier 106. The signals from transistors Q.sub.1 and Q.sub.2 are passed to transistors Q.sub.3 and Q.sub.4 which function as emitter followers to provide an output control signal of low impedance for the electric motor 34. (FIGS. 2, 3 and 4)

The control signal to the electric motor 34 causes the electric motor 34 to rotate the lead screw 36 moving the tuning probe 24 and sensing probes 28 and 30 until the magnitudes of the voltages from the sensing probes 28 and 30 are equal, at which position the tuning probe 24 is at the desired position.

Should either of the limit switches 50 and 52 (FIGS. 2, 3 and 4) be closed by the carriage 20 or means 32 reaching the ends of travel along the slotted waveguide section 18, then the motor 34 is reversed for a predetermined period of time to move the carriage 30 towards the centre of the slotted waveguide section 18.

Assuming limit switch 50 is closed, then capacitor 50a is quickly charged to supply a voltage to amplifier 108. The voltage output from amplifier 108 is applied to the input of amplifier 106 to saturate this amplifier and drive the motor 34 in the appropriate direction. When the voltage across capacitor 50a drops to a predetermined value at a rate determined by the setting of variable resistance R.sub.1, the amplifier 108 ceases to be saturated and so its output drops rapidly and ceases to drive the motor 34.

Referring to FIGS. 5 and 6, similar parts to those shown in FIGS. 2 and 3 are designated by the same reference numerals, and the previous description is relied upon to describe them.

In FIGS. 5 and 6 two further probes 110 and 112 are mounted on the means 32 and are connected to coaxial microwave diode detector mounts 114 and 116 respectively. The detector mounts are provided with coaxial cables 118 and 120 respectively.

Referring to FIG. 7, similar parts to those shown in FIG. 4 are designated by the same reference numerals, and the previous description is relied upon to describe them.

As shown in FIG. 7 the apparatus shown in FIGS. 5 and 6 is connected to a similar circuit to the apparatus shown in FIGS. 2 and 3, and function in the same manner, except that probes 110 and 112 are connected to amplifier 122 and 124 respectively.

The signals from amplifiers 122 and 124 are passed to difference amplifier 126 from which a signal is passed to amplifier 128, together with a signal from amplifier 104 through diodes CR.sub.1 to CR.sub.8, in such a manner that the output of amplifier 128 is proportional to the sum of the absolute values of the signals from amplifiers 126 and 104. A signal from amplifier 128 is then passed to a voltmeter 130 from which an indication of any necessary adjustment for the depth of the tuning probe 24 in the slotted waveguide section 18 can be obtained.

Thus the circuit shown in FIG. 7 also includes means having a voltage output, and connected to the probes 28, 30, 110 and 112, for detecting any manual adjustment necessary of the distance that the tuning probe 24 extends into the slotted waveguide section 18.

The two signals that are combined in amplifier 128 can also be obtained using only three probes 28, 112 and 30 (FIGS. 5 and 6). In that case amplifier 122 (FIG. 7) is eliminated and amplifier 126 is connected as shown in FIG. 8, so that the output of amplifier 126 is proportional to two times the output of amplifier 124 minus the output of amplifier 100 minus the output of amplifier 102. The remainder of the circuit as well as its function remains unchanged from the circuit shown in FIG. 7, and similar parts thereto are disignated by the same reference numerals.

Claims

We claim:

1. In a microwave heating apparatus, comprising a microwave energy source, and a microwave heating means, a microwave tuning means connecting the microwave energy source to the microwave heating means, the tuning means comprising a slotted waveguide section, a carriage outside the slotted waveguide section and slidable along the slot therein, a tuning probe extending into the waveguide section through the slot, means adjustably mounting the tuning probe on the carriage for adjusting the distance the tuning probe extends into the waveguide section to the correct depth for tuning, at least two sensing probes extending into the waveguide section to sense the voltage standing wave pattern, means mounting the sensing probes at a fixed distance apart from each other and from the tuning probe, and means connected to the carriage and responsive to changes in the voltage standing wave pattern sensed by the probes, for tuning the microwave heating section by moving the tuning probe along the slot.

2. Tuning means according to claim 1, wherein two sensing probes are provided on the means for mounting the probes for sensing the voltage standing wave pattern, two further probes are mounted on the means for mounting the probes, a means having a voltage output, connected to the sensing probes and further probes, is for detecting any manual adjustment necessary of the distance that the tuning probe extends into the slotted waveguide section, and a voltmeter is connected to the voltage output for indicating any necessary manual adjustment of the tuning probe.

3. Tuning means according to claim 1, wherein adjustable means are provided, for maintaining the sensing means at a fixed distance from the carriage, for adjusting the fixed distance.

4. Tuning means according to claim 1, which includes two slides extending along the slotted waveguide section, and the carriage is slidably mounted on the slides.

5. Tuning means according to claim 1, which includes a lead screw rotatably mounted on and extending along the slotted waveguide section, the lead screw engaging a threaded bore in the carriage for moving the carriage along the slotted waveguide section, and an electric motor drive connected to the lead screw.

6. Tuning means according to claim 5, which includes limit switches, at each end of the movement of the carriage along the slotted waveguide section for reversing the rotation of the electric motor.