WO1998038502A1

WO1998038502A1 - Electronic circuits

Info

Publication number: WO1998038502A1
Application number: PCT/GB1998/000626
Authority: WO
Inventors: Peter Alfred Payne; Krishna Chandra Persaud; Richard Stone
Original assignee: Aromascan Plc
Priority date: 1997-02-27
Filing date: 1998-02-27
Publication date: 1998-09-03
Also published as: GB9704064D0

Abstract

There is disclosed an electronic circuit comprising a phase shift oscillator of which a component is a transducer.

Description

ELECTRONIC CIRCUITS

This invention relates to electronic circuits and has particular but not exclusive reference to gas sensor devices.

Gas sensor devices are known using semiconducting polymer elements the electrical properties of which, particularly resistance, change on adsorption or desorption of a gas. Different polymer materials respond to different gases and gas sensor devices can comprise circuits including a plurality of such elements the individual and/or collective response of which to a gas or a mixture of gases can be established for example by the use of a neural net which can be trained to recognise particular gases or combinations and at least to some extent interpolate or extrapolate from a given "training" set.

Such devices (but not only such devices) can be particularly sensitive to humidity, which is, to say, of course, sensitive to water in gaseous form ("gas" as used herein including "vapour"). Water vapour is common and usually not of interest except inasmuch as it can impair the detection or measurement of other gases when its effect is desired to be eliminated or compensated for. Some gas sensors of this technology are provided with humidity compensation by means of a Wheatstone bridge configuration, which may employ either ac or dc sensor interrogation techniques. (See PCT/GB96/01554).

The present invention provides new sensor circuits which may be improved over existing circuits for certain end uses in connection with the detection of gases or which may facilitate different output signal processing techniques. In particular, the signal produced is near digital, allowing for ready integration with digital processing techniques. In addition, the invention provides means for de-sensitising such sensors or indeed any associated electronic circuits to the effects of varying humidity or indeed exposure to different gases so that commonly-encountered gases in certain environments, such, for example as carbon dioxide in a brewery or distillery, can be discounted or compensated for. Carbon dioxide is not detected by semiconducting polymer sensors, but dedicated carbon dioxide sensors are readily available if required.

The invention comprises an electronic circuit comprising a phase shift oscillator of which a component is a transducer.

The transducer may be a sensor.

The transducer may be a gas-sensitive substance such as a semiconducting polymer which changes its electrical property in accordance with adsorption or desorption of a gas.

The circuit may be arranged as a gas sensor. The adsorption or desorption of gas may change the phase of the oscillator for a given frequency. The circuit may be so arranged that the frequency can be varied and the frequency for zero phase shift under gas adsorption or desorption conditions, or a quantity functionally related thereto, measured, the value of such frequency or quantity functionally related thereto being used as an indication of the presence of a particular gas.

The circuit may comprise a plurality of semiconducting polymer components.

The circuit (whether or not it is a gas-sensor circuit) may be arranged as a gas compensator with a first semiconducting component having a first response to a particular gas and a different second such sensor having a second response to said gas. Said first and second responses may be respectively large and small or zero. The circuit may comprise a multi-stage, e.g. a three or four stage R-C phase- shift network, a Wien-bridge oscillator, a Wien-bridge with long-tail pair, a twin-T filter or an astable multi-vibrator circuit, this last being capable of handling square wave signals facilitating digital processing without the need for sophisticated prior signal processing.

Embodiments of circuits according to the invention will now be described with reference to the accompanying drawings, in which :

Figure 1 is a three stage R-C phase-shift network;

Figure 2 is a four stage R-C phase-shift network;

Figure 3 is a Wien-bridge oscillator;

Figure 4 is a Wien-bridge oscillator with long-tail pair;

Figure 5 is a twin-T filter;

Figure 6 is an astable multi-vibrator circuit;

Figure 7 shows a digital logic oscillator circuit;

Figure 8 shows oscillator stability with a fixed resistor;

Figure 9 shows oscillator stability with a semiconducting polymer sensor; and

Figure 10 shows responses to gas pulses of a 1% butane/water mixture. The drawings illustrate electronic circuits comprising phase shift oscillators of which at least one component 11 is a semiconducting polymer such for example as a polypyrrole or polyindole, which changes its electrical property in accordance with adsorption or desoφtion or a gas. However, other sensors or, indeed, other forms of transducers might be employed. An example is a strain gauge.

The circuits may be arranged as gas sensors. The three and four stage R-C phase shift circuits of Figures 1 and 2, for example can have any or all of the resistors R substituted by a semiconducting polymer resistance or indeed any one or more substituted by a plurality of such resistances in series and/or parallel, as seen at R3, R4 in Figure 2, the fine line circuit being substituted for the thick lined resistors R3, R4.

Figure 3 illustrates a compensating, e.g. a water vapour compensating arrangement comprising a Wien-bridge oscillator. If Rl is chosen to be a conducting polymer sensor with a large response to humidity and R2 is chosen to be a sensor material with zero or low response to humidity, the effects of humidity will be reduced by the use of simple calculations (see PCT/GB96/01554).

The Wien-bridge oscillator with long-tail pair illustrated in Figure 4 is a well known phase-shift oscillator circuit. Any of the resistors R could be replaced by one or more series and/or parallel connected conducting polymer resistors exactly as in Figure 2.

The twin-T filter of Figure 5 is a more shaφly tuned oscillator than the Wien-bridge device. Again, any one of the resistors R can be substituted with a conducting polymer sensor element or complex.

Other oscillator circuits can be used such as the astable multi-vibrator circuit of Figure 6 which is a square wave phase shift oscillator, the square waveform facilitating digital processing without complicated signal processing as would be required from a sinusoidal waveform.

Operating frequencies would be typically in the hundreds of kilohertz region for most or all of these devices, which could be constructed as arrays, each designed to oscillate at a specified frequency when not influenced by a gas, the pattern of frequency changes being input to analysis circuitry and/or software such for example as neural net. It will be appreciated that the frequency of the circuit might be measured, or a quantity functionally related thereto might be measured. Such quantities include the observed frequency shift, or the time period associated with the measured frequency, or the observed time period shift caused by the presence of a gas. Another useful quantity is the fractional change in the time period dT/T, where T is the time period when not influenced by a gas and dT is the observed time period shift. This may be equated, at least at lower operating frequencies, to the fractional change in sensor resistance dR/R. It is possible to probe the dielectric properties of the sensor material if higher operating frequencies, probably above 1MHz, are used. However, it is more difficult to make measurements at such high frequencies.

Other gas sensitive materials can be used instead of or in addition to the semiconducting polymer materials referred to - metal oxides, for example.

Example

A digital logic oscillator circuit as shown in Figure 7 was employed.

The main benefit of this circuit is that it requires only one component - a capacitor 70 - other than the logic gates 72, 74 and the sensor 76. The type of logic used was 5 V HC, which resulted in a duty cycle of around 50% and a frequency of approximately 1/1.8 RC where R is the resistance of the sensor (ohm). The value of the capacitor (57nF) was chosen so that the circuit operated in the low kilohertz range (between 1 and 10 kHz) with an appropriate sensor. The output of the oscillator was buffered by a further inverter and the time period, which is proportional to resistance, was measured using a Hewlett Packard HP53132A frequency counter connected to an IBM PC using a IEEE 488 interface. To minimise temperature effects on the period of oscillation the oscillator circuit and the sensor were placed in a temperature controlled environment held at 30 °C ± 1 °C.

Three experiments were conducted to demonstrate the capabilities of the system. The first experiment examined the basic stability of the oscillator by using a fixed 2 kΩ resistor in place of the sensor. Figure 8 shows the results of this stability test, and as can be seen the system is stable to a few thousandths of one per cent. Note that the percentage change in time period, (dT/T)%, is equivalent to the (dR/R)% percentage change in the sensor resistance.

In the second experiment, the fixed resistor was replaced by a semiconducting polymer sensor. The sensor was connected to a gas delivery system that could be switched from dry air to dry air which had flowed over a 1% butanol in water mixture. In a similar manner to the fixed resistor equipment, the stability of the sensor in the dry air stream was evaluated. The results can be seen in Figure 9. The output is a little less stable than as shown in Figure 8, but the deviations observed are well within the acceptable circuits for a simple gas delivery system. The low frequency components of the noise in the system are mainly due to temperature fluctuations in the sensor, since the sensor substrate was not heated directly but from the convection of warm air within the incubator. This caused low frequency deviations in the time period as ambient temperature changed. The cooling effect of the gas stream also made the sensor more sensitive to flow rate. The main feature to note is that the high frequency components of the drift in Figures 8 and 9 are similar in amplitude, although the gas sensor noise is a little higher. This is possibly due to the longer lead lengths needed to connect the sensor to the oscillator.

The gas flowing across the sensor was switched from dry air to air which had been passed over the butanol solution and the response of the sensor measured. The data recorded are shown in Figure 10. The graph depicts the standard shape of response of a reversible gas sensor to a volatile. The baseline is recovered between the two pulses of solvent and the response is repeatable. In this particular case, the signal due to the adsorbed volatiles is much greater than the noise in the system : a signal to noise ratio of 2500 is observed.

The measurements described above require a small number of analogue components. Furthermore, it is possible to integrate logic onto a programmable device, such as a field programmable gate array.

Claims

1. An electronic circuit comprising a phase shift oscillator of which a component is a tranducer.

2. An electronic circuit according to claim 1 in which the transducer is a sensor.

3. An electronic circuit according to claim 1 or claim 2 in which the transducer is a gas-sensitive substance such as a semiconducting polymer which changes its electrical property in accordance with adso╧åtion or deso╧åtion of a gas.

4. A circuit according to claim 3, arranged as a gas sensor.

5. A circuit according to claim 4, in which the adso╧åtion or deso╧åtion of gas changes the phase of the oscillator for a given frequency.

6. A circuit according to claim 5, in which the circuit is so arranged that the frequency can be varied and the frequency for zero phase shift under gas adso╧åtion or deso╧åtion conditions, or a quantity functionally related thereto, measured, the value of such frequency or quantity functionally related thereto being used as an indication of the presence of a particular gas.

7. A circuit according to any one of claims 1 to 6, comprising a plurality of semiconducting polymer components.

8. A circuit according to any one of claims 1 to 7, arranged as a gas compensator with a first semiconducting component having a first response to a particular gas and a different second such sensor having a second response to said gas.

9. A circuit according to claim 8, said first and second responses being respectively large and small or zero.

10. A circuit according to any one of claims 1 to 9, comprising a multi-stage R- C phase-shift network.

11. A circuit according to any one of claims 1 to 9, comprising a Wien-bridge oscillator.

12. A circuit according to any one of claims 1 to 9, comprising a Wien-bridge oscillator with long-tail pair.

13. A circuit according to any one of claims 1 to 9, comprising a twin-T filter.

14. A circuit according to any one of claims 1 to 9, comprising an astable multi- vibrator circuit.