GB2238622A

GB2238622A - Sensing device

Info

Publication number: GB2238622A
Application number: GB8925830A
Authority: GB
Inventors: John Ernest Thornton Lawrence
Original assignee: Electricity Council
Current assignee: Electricity Council
Priority date: 1989-11-15
Filing date: 1989-11-15
Publication date: 1991-06-05
Also published as: GB8925830D0

Abstract

A device for sensing a voltage field established between a pair of spaced field electrodes (not shown) comprises a sensing electrode (I, T) which can be located between the field electrodes. The sensing electrode then forms a capactive potential divider and is connected in a sensing circuit (Fig 4) to provide an output indicating the voltage on the sensing electrode which can then be displayed. As shown, an output optical signal from a diode (15) travels to a digital (3) and audible (4) display device 2 by way of an optical fibre (5). <IMAGE>

Description

SENSING DEVICE This invention relates to a sensing device, and particularly to a device for sensing a voltage field established between a pair of spaced field electrodes.

Radio frequency dielectric heating is used in many processes, either as the main heating source or to assist conventional heating, thus reducing process time and improving throughput and/or quality.

In some food processing, such as baking, even heating across the area being heated is important, and to meet this requirement, the radio frequency energy electrode system is usually adjusted both electrically and mechanically to provide the optimum conditions.

This may mean fitting loading inductors to the electrode system at various positions and/or mechanically positioning the field electrodes to provide an even voltage field.

When setting up a processing system, initial adjustments are concerned with bringing the resonant frequency on to the working frequency. Subsequent adjustments for optimising the evenness of heating are usually made empiricaly by making small changes and noting the effect on the product.

According to this invention there is provided a device for sensing a voltage field established between a pair of spaced field electrodes, comprising a sensing electrode adapted to be located between the field electrodes thereby to form a capacitive potential divider between the field electrodes, the sensing electrode being connected in a sensing circuit providing an output signal indicative of the voltage on the sensing electrode, and a display means responsive to said output signal to give an indication of the voltage on the sensing electrode.

The device of the invention can be used for rapidly determining the relative voltage field within the space between the field electrodes, and can significantly reduce the time taken to set up the field electrode system.

The invention will now be described by way of example with reference to the drawings, in which: Figure 1 is a diagrammatic view of a device according to the invention; Figure 2 is a plan view of a sensing electrode probe used in the device of Figure 1; Figure 3 is a cross-sectional view through the sensing electrode probe of Figure 2; Figure 4 is a circuit diagram of one sensing circuit for use in the device of Figure 1; and Figure 5 is a circuit diagram of another sensing circuit for use in the device of Figure 1.

Referring to Figure 1, the device comprises a probe 1 carrying a sensing electrode and associated circuitry to be described later, which can be coupled to a display means 2 which provides a digital read-out 3 and an audible output from a speaker 4, by means of an optical fibre cable 5. Two probes 1 are shown, which are used separately, one being a high power probe (see Figure 4) for use in normal operation of the heating field electrode system providing the field being sensed, and the other a low power probe (see Figure 5) for use when the heating field electrode system is being energised from a low power signal generator for setting-up or testing purposes.

Referring to Figures 2 and 3, each probe 1 comprises a metal housing 6 one wall of which comprises a sensing electrode 7 in the form of circular disc mounted on an insulating member 8 with a gap 9 between the periphery of the disc 7 and the remainder of the wall of the housing 6 such that the disc 7 is electrically isolated from the remainder of the housing 6. As shown in Figure 3, the disc 7 is secured to the member 8 by a screw 10 while the member 8 is secured to the opposite side of the housing 6 by another screw 11, it being ensured that the screws 10 and 11 to not contact each other.

The housing 6 contains sensing circuitry (not shown in Figure 3) which provides a light output signal which can be obtained at an outlet 12 on the housing 6.

For use the probe 1 is placed between a pair of spaced field electrodes of a dielectric heating apparatus to be set-up or tested, such that the sensing electrode 7 forms with the field electrodes a capacitative potential divider. With the probe shown in Figures 2 and 3 the housing 6 serves as an earth electrode at the same potential as one of the field electrodes of the heating apparatus. The sensing electrode is connected to the circuity in the housing 6, and the light output signal available at outlet 12 is indicative of the voltage on the sensing electrode 7. The output signal is supplied over the optical fibre cable 5 to the display means 2 which then provides a digital display 3 of the voltage and also an audible clicking output at a frequency dependent upon the voltage on the sensing electrode 7.By moving the probe around in the field being sensed the voltage at various points in the field can be ascertained and the heating apparatus adjusted to give the required heating profile between the field electrodes.

Referring now to Figure 4, in the circuitry in the housing 6 for the high power probe I, the sensing electrode is connected to earth (the housing 6) by way of an inductor 13, the junction between the sensing electrode 7 and the inductor 13 being connected by way of a rectifying diode 14 to a light emitting diode 15 and thence to earth, a decoupling capacitor 16 being connected in parallel with the light emitting diode 15. The light from the light emitting diode 15 provides the output signal indicative of the voltage on the sensing electrode 7, which is supplied to the cable 5 and on to the display means 2 by way of the outlet 12.

Referring now to Figure 5, this shows the circuitry in the housing 6 for the low power probe 1, with parts corresponding to parts of the circuit of Figure 4 having the same references. This circuit includes a battery 17 connected by way of a switch 18 to energise a voltage-to-frequency converter network 19 which receives the voltage signal from the diode 14 and provides a pulse output signal for the light emitting diode 15 which in response provides a pulsed output signal having a repetition frequency proportional to the voltage on the sensing electrode 7. The pulsed light output signal is again supplied over the cable 5 for display by the display means 2. The circuit includes a variable resistor 20 by which the range of the output signal can be set.

Preferably the housing 6 of the probe 1 is as slim as possible since this together with the use of a non-metallic optical fibre cable 5 will ensure a minimum disturbance to the field being sensed by the probe 1.

If required the output signal can be supplied to a display means in the form of a computer controlling a plotting device in order to obtain a plot of the field sensed.

The device of the invention can be fully portable, being powered from either internal, preferably rechargeable, batteries or from a mains supply.

Claims

1. A device for sensing a voltage field established between a pair of spaced field electrodes, comprising a sensing electrode adapted to be located between the field electrodes thereby to form a capacitive potential divider between the field electrodes, the sensing electrode being connected in a sensing circuit providing an output signal indicative of the voltage on the sensing electrode, and a display means responsive to said output signal to give an indication of the voltage on the sensing electrode.

2. A device as claimed in Claim 1, in which said output signal is a light signal which is supplied to the display means by way of an optical fibre cable.

3. A device as claimed in Claim 2, in which the output signal is a pulse signal having a repetition frequency proportional to the voltage on the sensing electrode.

4. A device as claimed in any preceding claim, in which the display means provides a digital display.

5. A device as claimed in any preceding claim, in which the display means provides an audible output.

6. A device as claimed in any one of Claims 1 to 3, in which the display means comprises a computer controlling a plotting device serving to provide a plot of the field sensed.

7. A device as claimed in any preceding claim, in which the sensing electrode is a planar member carried by and electrically isolated from a conductive housing, the sensing electrode forming all or part of one wall of the housing.

8. A device as claimed in Claim 8, in which the housing is at the same potential as one of the field electrodes.

9. A device as claimed in Claim 1 or Claim 2, in which the sensing circuit is substantially as described with reference to Figure 4 of the drawings.

10. A device as claimed in Claim 1, Claim 2 or Claim 3, in which the sensing circuit is substantially as described with reference to Figure 5 of the drawings.

11. A sensing device substantially as hereinbefore described with reference. to Figures 1 to 4, or Figures 1 to 3 and 5 of the drawings.