WO1990001227A1

WO1990001227A1 - Controlled output unipolar ion generator

Info

Publication number: WO1990001227A1
Application number: PCT/GB1989/000811
Authority: WO
Inventors: Christopher David Jones; John Evan Elias; David Martin Taylor
Original assignee: The Secretary Of State For Defence In Her Britannic Majesty's Government Of The United Kingdom Of Great Britain And Northern Ireland
Priority date: 1988-07-18
Filing date: 1989-07-14
Publication date: 1990-02-08
Also published as: GB9101398D0; GB8817067D0; GB2239740A; GB2239740B

Abstract

A unipolar ion generator for use in atmospheric dispersion studies which may be pre-set to give a desired output current, such output current being steady and controllable, any fluctuations in the current being corrected by use of a control circuit. The output current is unaffected by adverse weather conditions and surface electrode conditions.

Description

CONTROLLED OUPUT UNIPOLAR ION GENERATOR

This invention relates to a controlled output unipolar ion generator. Unipolar ion generators are increasingly used in the field of atmospheric dispersion studies and in particular, in the study of short range diffusion phenomena where the ionised air is employed as a tracer material to simulate the behaviour of, say, a smoke or gas cloud and the dispersion of the electrostatically charged cloud is monitored by ion collectors. The injected ions persist in the cloud for several minutes before being discharged so that the dispersion of the smoke or gas can be simulated. It is desirable in this technique to have an ion source which is capable of injecting predetermined quantities of charge into the atmosphere so that the magnitude of charge in the cloud is known.

Previously ion generators have consisted of a pointed electrode surrounded by an earthed casing which includes a fan. The pointed electrode is connected to a very high voltage supply which produces an asymmetric electric field within the generator. Air ions of one charge only accumulate near the pointed electrode resulting in a corona, some of which are then 'blown' into the atmosphere by the electric fan contained within the casing. Ion generators have been largely employed outdoors (but have cer¬ tain wind tunnel applications) and it has been found that with con¬ ventional ion generators the nature of the corona discharge depends very much on atmospheric and surface electrode conditions as well as the ambient humidity and these affect the rate at which the ions are expelled into the atmosphere. It is therefore impossible to accurately measure and therefore control the current produced by the ion generator, and in consequence the magnitude of the charge in the - cloud can only be measured relatively and not quantitatively or absolutely. There is therefore a need for a unipolar ion generator which can work reliably and efficiently and not be affected by adverse atmos¬ pheric conditions, and which is capable of injecting pre-determined quantities of ions into the atmosphere.

According to the present invention a controlled output unipolar ion generator includes an ion source which comprises a high voltage corona electrode, a counter electrode, a fan mounted coaxially behind the high voltage corona electrode and an earthed outer casing, and

SUBSTITUTE SHEET a high voltage supply characterised in that a first electrometer is con¬ nected to measure a corona current and a secondelectrometer is connectedto measure a return current to the counter electrode and a con¬ trol circuit monitors the difference current between the corona current and the return current and uses this as a control signal to control the high voltage supply so as to maintain a pre-set ion current into the atmosphere.

The first electrometer is essentially an electrometer amplifier which is flcαting at the same potential as the corona point. Hence the difference current cannot be obtained directly. Therefore the first electrometer includes an optical fibre link which provides the necessary high voltageisolation between the floating amplifier of the first electrometer and the second electrometer so that the difference current can be obtained directly. The first electrometer is preferably a small, battery-driven electrometer amplifier located inside the ion generator. The generator is therefore fitted with a connection for a battery so that the amplifier can be recharged. A head unit is preferably provided with two controls to adjust the zero setting and the 'slope' or span of the first electrometer so that any drift that occurs as the batteries run down can be corrected.

The control unit is fitted with various switches and functions including an on/off switch, a fan speed-setting knob, a multi-turn potentiometer, a direct reading liquid crystal display, an LCD function switch and a connection for a 12v battery supply. The multi-turn potentiometer allows a precisely pre-set ion current to be selected by setting this to a pre-calibrated position, and the direct reading liquid crystal display allows all the relevant variables of the ion generator to be monitored, without disturbing the ion current. The control unit isalso fitted with a pulse function so that the generator can be used to eject short bursts of ions into the atmosphere. This is a particularly useful function for simulating 'puffs' of gasses etc.

The high voltage electrode of the ion source preferably has point electrodes attached thereto which may take the form of metal needles or preferably, small bunches of carbon fibres. A metal wire or a sharp-edged disc may also be used as a point electrode. The counter electrode is preferably cylindrical and is an extension of the earthed

SUBSTITUTE SHEET outer casing being insulated from it by preferably a PTFE washer. The shape of the overall ion source is such as to minimise air disturbance, preferably being of an aerofoil shape and the training edge of the earthed outer casing is designed so as to minimise wake effects.

A combination of the above features provides an ion generator which produces a controllable, reliable and efficient source of ions into the atmosphere and is unaffected by adverse atmospheric conditions. The present invention will now be described, by way of example only, with reference to the following diagrammatic drawings, in which:- Fig 1 is a cross-section through the ion source; Fig 2 is a block diagram of the ion source in conjunction with the circuit for controlling the ion output; and Fig 3 is a circuit diagram of the control unit. As shown in Fig 1 an ion source 10 comprises an aluminium rod 11 which forms a high voltage corona electrode to which various point electrodes 12 are attached. These may be in the form of metal needles or, preferably, small bunches of carbon fibres. The aluminium rod 11 is held in place by a perspex boss 13 and metal spokes (not shown) within and coaxial with an earthed outer cylinder 14. A counter electrode 15 is an extension of the earthed outer cylinder 14 and is insulated from it by preferably a PTFE washer 16. A fan 17 is mounted coaxially behind the high voltage electrode 11. The earthed surround 14 surrounds most of the counter electrode 15 in order to minimise electrical noise and other extraneous effects.

As shown in Fig 2 a variable and controllable high voltage supply 20 is connected through preferably a limiting resistor 21, and a first electrometer 22 which includes an optical fibre link 23, to the high voltage corona electrode 11. A second electrometer 24 is connected to the counter electrode 15. A control unit 26 is connected to each of the first electrometer 22, the second electrometer 24 and the high voltage supply 20. An electrostatic voltmeter 27 is connected to the output of the high voltage supply 20 and a variable voltage supply 28 is connected to the fan 17 of the ion source 10, and controlled by the control unit 26.

SUBSTITUTE SHEET The control unit 26 is fitted with various switches and functions (not shown). These include an on/off switch 30, a fan speed-setting control 32, a multi-turn potentiometer 40, a direct reading liquid crystal digital display (not shown) an LCD function switch (not shown) a pulsing function 34 and a connection for a 12V battery supply 36. In operation, the multi-turn potentiometer is set on a pre- calibr ted dial. to_.the desired ion current. The required voltage is then applied to the high voltage corona electrode 11 which causes a corona discharge to be formed at the point electrodes 12. A constant but controllable voltage is applied to the fan 17 which causes the ions formed at the point electrodes 12 to be 'blown' into the atmosphere. The first- electrometer "22 monitors the corona current I, and the second electrometer 24 monitors the return current I₂. The difference current I, will be the ion current emitted into the atmosphere, assuming that the ions generated at the corona are either ejected into the atmosphere or return to earth via the counter electrode 15 ie I_ = I, - I₂.

Any variation in the difference current I, is. detected by the control circuit 26 which uses this to supply a control signal of the appropriate sense to the high voltage supply 20 so as to maintain a constant ion current into the atmosphere at the pre-set value.

The pulsing function 34 may be used to simulate 'puffs' of gasses etc.

It is preferable to have in the circuit in Fig 2 a limiting resistor 21 which protects the amplifier in the first electrometer 22 as well as the user of the device. It is also preferable to have an electrostatic voltmeter 27 connected to monitor the voltage of the high voltage supply to check that the ion generator is working correctly. The circuit diagram of the control unit 26 is shown at Fig 3 and works in the following way: The variable resistor 40 is pre-set to give the required output ion current. Currents I, and I_ from the first electrometer 22 and second electrometer 24 respectively are fed into resistors 42 and 44 respectively. After passing through further resistors 45 the remaining currents combine and pass through a first comparator circuit 46 and a second comparator circuit 48, the output 50 of which is fed into the EHT supply unit 20, which is

SUBSTITUTE SHEE connected to the corona electrode 12. The circuit further includes test points 30-34 to test that the circuit is working properly. The function of the circuit is to monitor the difference between I and I_j, that is I.. Fluctuations in I, are mirroed by fluctuations in the output 50 and thus in the EHT supply 20. The resultant control signal is applied to the corona electrode 12, thus causing the output current I₃ to be stabilised at the magnitude set by the variable resistor 40.

It will be realised by those skilled in the art that the present invention is not limited to atmospheric dispersion studies but can be used in applications for which any conventional ion generator would be used.

SUBSTITUTE SHEET

Claims

CLAIMSWhat is claimed is:-

1. A controlled output unipolar ion generator including an ion source comprising a high voltage corona electrode, a counter electrode, a fan mounted coaxially behind the high voltage corona electrode and anearthed outer casing, and a high voltage supply characterised in that a first electrometer is connected to measure a corona current, a second electro¬ meter is connected to measure a return current to the counter electrode, and a control circuit monitors the difference current between the corona current and the return current and uses this as a contr signal to control the high voltage supply so as to maintain a preset ion current into the atmosphere,

2. A controlledoutput unipolar ion generator as claimed in Claim 1 characterised in that the first electrometer includes an optical fibre link to enable the difference current to be obtained directly.

3. A controlled output unipolar ion generator as claimed in Claim 1 or Claim 2 characterised in that the first electrometer is a battery-driven electrometer amplifier.

4. A controlledoutput unipolar ion generator as claimed in any previous claim characterised in that it further includes a head unit which is provided with a first control to adjust a zero setting of the first electrometer and a second control to adjust a span of the first electromet

5. A controlledoutput unipolar ion generator as claimed in Claim 1 or Claim 2 characterised in that the control unit includes a multi-turn potentiometer and the preset ion current is selected by setting the multi- turn potentiometer to a precalibrated position.

6. A controlled output unipolar ion generator as claimed in any previous Claim characterised in that the control unit supply includes a liquid crystal display to display various relevant variables of the ion generator.

7. A controlled output unipolar ion generator as claimed in any previous Claim characterised in that the high voltage corona electrode ha point electrodes attached thereto.

8. A controlled output unipolar ion generator as claimed in Claim 7 characterised in that the point electrodes are metal needles.

9. A controlled output unipolar ion generator as claimed in Claim 7 characterised in that the point electrodes are bunches of carbon fibres.

10. A controlled output unipolar ion generator as claimed in Claim 7 characterised in that the point electrode is a metal wire.

SUBSTITUTE SHEET

11. A controlled output unipolar ion generator as claimed in Claim 7 characterised in that the point electrode is a sharp-edged disc.

12. A controlled output ion generator as claimed in any previous Claim characterised in that the counter electrode is an extension of the earthed outer cylinder and is insulated therefrom by a washer made from insulating material.

13. A controlled output ion generator as claimed in any previous Claim characterised in that an electrostatic voltmeter is connected in circuit between the high voltage supply and the first electro¬ meter so that the voltage supplied by the high voltage supply can be monitored.

14. A controlled output ion generator as claimed in any previous Claim characterised in that a limiting resistor is super imposed between the high voltage supply and the first electrometer so as to protect the circuitry of the first electrometer.

15. A controlled output ion generator as claimed in any previous Claim characterised in that a variable voltage supply is connected to the fan of the ion source.

16. A controlled output ion generator as claimed in any previous Claim characterised in that the earthed outer casing has an aerofoil shape.

17. A controlled output ion generator as claimed in any previous Claim characterised in that the trailing edge of the earthed outer casing is designed so as to minimise wake effects.

SUBSTITUTE SHEET