EP1996918A1 - Liquid sensor - Google Patents

Liquid sensor

Info

Publication number
EP1996918A1
EP1996918A1 EP07731974A EP07731974A EP1996918A1 EP 1996918 A1 EP1996918 A1 EP 1996918A1 EP 07731974 A EP07731974 A EP 07731974A EP 07731974 A EP07731974 A EP 07731974A EP 1996918 A1 EP1996918 A1 EP 1996918A1
Authority
EP
European Patent Office
Prior art keywords
detector
electromagnetic radiation
source
wavelengths
amplifier
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP07731974A
Other languages
German (de)
French (fr)
Inventor
Peter James Dobson
Leonard Pincher
Christopher John Padbury
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Oxford University Innovation Ltd
Original Assignee
Oxford University Innovation Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Oxford University Innovation Ltd filed Critical Oxford University Innovation Ltd
Publication of EP1996918A1 publication Critical patent/EP1996918A1/en
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M3/00—Investigating fluid-tightness of structures
    • G01M3/38—Investigating fluid-tightness of structures by using light
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/314—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry with comparison of measurements at specific and non-specific wavelengths
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3563—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing solids; Preparation of samples therefor
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3577—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing liquids, e.g. polluted water
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/314—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry with comparison of measurements at specific and non-specific wavelengths
    • G01N2021/3181—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry with comparison of measurements at specific and non-specific wavelengths using LEDs
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/314—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry with comparison of measurements at specific and non-specific wavelengths
    • G01N21/3151—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry with comparison of measurements at specific and non-specific wavelengths using two sources of radiation of different wavelengths
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3554—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for determining moisture content
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
    • G01N21/94—Investigating contamination, e.g. dust

Definitions

  • the present invention relates to a device that can detect whether or not a floor or other surface has material spilt, splashed or otherwise distributed on it.
  • the device can be used to detect the presence of liquid or solids spilt on a floor.
  • the device can be mounted on the ceiling or wall of a room or may be freestanding.
  • US 6812846 discloses a spill detector which is based on machine imaging. This patent discloses the use of cameras and image processing techniques for the purpose of spill detection. A known image scene is recorded and compared with views taken by the camera on a continuing basis. Fixed background data is removed using a technique based on image subtraction and the image may be segmented to identify and remove from further analysis segments corresponding to non-spill foreground objects. This allows the image subtraction software to discount non-spill images.
  • a spill detector comprising at least one source of electromagnetic radiation which is able to emit electromagnetic radiation at two or more different pre-determined wavelengths, a detector adapted to generate a signal in response to electromagnetic radiation originating from the source, and an amplifier to amplify the signal
  • the source of electromagnetic radiation provides electromagnetic radiation at two or more different wavelengths.
  • the source provides radiation at two different wavelengths.
  • the or each source of electromagnetic radiation is a light source.
  • the light source is a light emitting infrared light of known wavelength or wavelengths.
  • a light emitting diode (LED) is particularly suitable.
  • the two different wavelengths of electromagnetic radiation are both in the infrared region.
  • the infrared radiation is provided by one or more light emitting diodes (LED) or laser diodes (LD).
  • the wavelengths of radiation provided by the source will depend upon the nature of the liquid to be detected. In another embodiment, multiple LEDs or LDs could be used for each wavelength to increase the intensity of the reflected light.
  • At least one of the wavelengths of electromagnetic radiation provided by the source must be emitted at a wavelength which is capable of absorption by the material to be detected. This is known as the absorption wavelength.
  • a material may have more than one absorption wavelength. It is therefore important that the other wavelength or wavelengths of electromagnetic radiation should be distanced from the absorption wavelength and that this other wavelength or wavelengths is not absorbed by the material. This wavelength or wavelengths are defined here as non-absorbing wavelengths.
  • a source emitting two different wavelengths Since it is preferred to utilise a source emitting two different wavelengths the following description is based on such an arrangement. However, in principle, more than one absorbing wavelength could be emitted and/or more than one non-absorbing wavelength could be emitted by the source. Thus, in another embodiment, more than two different wavelengths could be used to give an improved detection scheme. This could be used either to detect one or more different liquids or a mixture of liquids, or simply to improve the accuracy of detection of a single liquid. The use of LDs rather than LEDs can also give better sensitivity and selectivity because the emitted wavelength spread is narrower.
  • an absorbing and a non-absorbing wavelength allows a differential signal to be obtained in the detector due to reflection of the electromagnetic radiation originating from the source from the surface and the material distributed thereon. Detection of the relative intensities of the two wavelengths of reflected light originating from the source thus allows the detector to determine whether or not material is present on the surface.
  • the material is a liquid.
  • the liquid is water or contains water or is a solution of a substance in water.
  • the other non-absorbing wavelength would be at least 30 nm and more preferably 50 nm away from the absorption peak.
  • the important criteria, however, is that the second wavelength should not interact with and be absorbed by the material to be detected. It should also not cause interference with the radiation of the other wavelength.
  • the source of electromagnetic radiation may be a single source capable of emitting radiation at two different wavelengths or it may comprise two separate sources of radiation of different wavelengths.
  • the intensities of the two wavelengths emitted are preferably identical. However, it is not necessary for the intensities to be identical and in one embodiment the intensities may be different provided that the relative intensities of the two emitted wavelengths is known.
  • the detector is connected to signal processing means which measures the relative intensities of the two wavelengths detected and compares it with the two known intensities of the wavelengths emitted by the source of electromagnetic radiation.
  • the detector uses a photodiode.
  • An amplifier amplifies the signal obtained by the detector at the two wavelengths of interest. Where there is a differential signal the differential signal is thus amplified.
  • the amplifier is a lock-in amplifier or similar synchronous or heterodyne detector.
  • a differential signal can be received and amplified under two circumstances.
  • the first circumstance arises when there is a material spill and the emitted radiation at the two different wavelengths is of equal intensities but radiation at one of the frequencies is absorbed by the material. This leads to the intensities of the reflected radiation received by the detector being different.
  • the spill detector can include an algorithm which provides for a warning to be issued under such circumstances. If no signal is amplified in this case then this means that there is no material present on the surface.
  • the second situation in which a differential signal can be received by the detector arises when the two different wavelengths emitted by the source of electromagnetic radiation are emitted at different intensities.
  • the reflected radiation is therefore likely to be of different intensity at the two wavelengths regardless of whether or not there has been any absorption at one of the wavelengths.
  • an algorithm determines whether the relative intensities of the two wavelengths is the same as the original relative intensities of the two wavelengths emitted by the source or whether this value has changed. If the value has changed it is due to interaction with a material on the surface in the detection zone.
  • the spill detector may include an algorithm to generate a warning in this case.
  • the light source is one or more LEDs. It is possible that the two LEDs may have different intensities naturally and a number of methods are available for compensating for the difference in intensities. In addition to or in place of using an algorithm the following protocols could be adopted. For example, one of the two LEDs could be kept on longer than the other. Alternatively, one or more neutral density filters could be placed in the path of one of the LEDs. Equally, the current to one of the two LEDs could be reduced relative to the other. These approachs could be used for any electromagnetic source providing radiation of different intensities at the different wavelengths.
  • the source of electromagnetic radiation is a single source capable of emitting radiation at more than one wavelength.
  • the source is a modulated light source which is capable of emitting light at two wavelengths. This then permits synchronous detection techniques that eliminate the effects of other ambient light.
  • the detector is any conventional detector capable of receiving electromagnetic radiation at the two emitted wavelengths.
  • a photodetector such as a photodiode is particularly suitable for this purpose.
  • the source is adapted to emit more than two wavelengths to allow a detection of more than one type of material. In such a case the detector could be used to detect mixtures of liquids.
  • the detector of the present invention is also capable of detecting solids or gels provided that the solids or gels have an appropriate absorption peak for electromagnetic radiation. This is a significant advantage over the prior art detector which is not able to detect solids.
  • Figure 1 shows a spill detector according to the present invention
  • Figure 2 illustrates a schematic diagram of a wet floor detector.
  • source 1 of electromagnetic radiation comprises a pair of LEDs 2 which emit infrared radiation at two different frequencies X and Y.
  • the source 1 is mounted on ceiling 3 but could equally be mounted on wall 4 or be free standing so as to be placed on a cabinet 5.
  • a pool of spilt liquid 6 on floor 7 receives incident light of wavelengths X and Y which have intensities Xl and Yl, respectively.
  • the incident light is reflected as infrared radiation of wavelengths X and Y having intensities X2 and Y2 respectively.
  • the reflected light of wavelengths X and Y is received by detector 8 which includes a photodiode within a housing 9.
  • the housing 9 may also contain amplifier 10 and processing circuitry 11.
  • the detector 8 may be mounted on the ceiling 3, wall 4 or may be free standing. When a spill is detected the algorithm provides for a warning signal to be issued. In this case, light 12 is illuminated to warn of the spillage.
  • the source 1 and detector 8, respectively, may include their own power supplies or may be connected to external sources of power.
  • oscillator circuit 21 is connected to LED 1 and LED 2. Oscillator circuit 21 is also connected to lock-in amplifier 22 which in turn is connected to photodiode detector 23 and also to output circuit 24. One of LED 1 and LED 2 emits light at a wavelength which is strongly absorbed by the liquid and the other LED at a wavelength which is not.
  • the LED's light is instead reflected off the surface in the detection zone.
  • the oscillator circuit 21 oscillates at a fixed frequency square-wave with variable duty cycle. One LED is connected between the oscillator output and ground, the other LED is connected between the oscillator output and the positive power rail. The LEDs are thus lit in antiphase.
  • the photodiode detector 23 observes the area of a floor that the LEDs shine on. This is the detection area.
  • a photodiode is a light sensitive electronic device which is capable of receiving and generating a signal in response to light. In the embodiment shown in Figure 2, the photodiode generates a signal in response to light from both LED 1 and LED 2.
  • the lock-in amplifier 22 rejects any signal which is not in phase with the oscillator 21 (eg lighting flicker).
  • the output of the lock-in amplifier 22 is connected to the output circuit 24 which measures the voltage of the lock-in output and acts upon it. For example it provides a sound, flashes a warning light or sends an email or wireless message.
  • the oscillator used in the embodiment shown in Figure 2 creates a square wave oscillating between the positive power rail at a certain positive voltage and ground (zero volts). In this case, the positive rail is at 5 volts.
  • the duty cycle of the oscillator can be varied to balance the intensity of the two LEDs. This is a type of pulse width modulation.
  • the oscillator used in the embodiment shown schematically in Figure 2 is based on the 555 timer IC which is available from a number of electronic retailers. However any conventional oscillator could be used in the detector of the present invention.
  • a lock-in amplifier is used to measure a weak signal against a noisy background. Any conventional lock-in amplifier could be used in the detector of the present invention.
  • the clock input of the lock-in amplifier 22 is high (for example 5 volts in the present case)
  • the input signal is connected to the lock-in output.
  • the clock input of the lock- in is low (zero volts)
  • the input signal is inverted and then connected to the lock-in output.
  • the output of the lock-in is then time averaged. In the present case, this is achieved using a low pass filter.
  • the only signals that are not time averaged to zero by this process are signals in phase (at the same frequency) as the clock input.
  • the output of the lock-in amplifier is therefore the average amplitude of the signal in phase with the clock input.
  • Signals from the two LEDs 1 and 2 are balanced by varying the duty cycle of the oscillator so that the time averaged output of the lock-in amplifier is zero volts. This happens in the present embodiment shown in Figure 2 because the signal from one LED is inverted by the lock-in amplifier and then added to the signal from the other LED. A change in the relative intensity of the signals results in an imbalance and a non-zero voltage from the lock-in amplifier 22. This non-zero output can be used by the output circuitry to trigger a buzzer or a light.
  • an electronic switch is placed in between the oscillator output and the LEDs. In one state of the switch one LED is connected, in the other state the other LED is connected.
  • This switch can be controlled by another lower frequency oscillator or by a micro controller.
  • the lock-in amplifier would give a measurement of the signal for each LED and measurements could then be compared. The calibration process would create the same lock-in output for both LEDs.
  • the wet floor detector consists of three main units: a timer, a photodiode detector and a lock-in amplifier. These are described in more detail below.
  • the output of the 555 timer is fed into the base of two transistors (BCl 84L and BC212L) to give the required current. Without these transistors the wavefo ⁇ n is distorted and reduced in amplitude.
  • the BC184L transistor has its collector connected to 5 V and its emitter connected to the 970nm (absorbed by water) LED and a current setting resistor.
  • the BC212L transistor has its collector connected to ground (OV) and its emitter connected to the other LED (which is not absorbed by water) and a current setting resistor.
  • the current setting resistors could be varied to change the LED currents though this would move them off there design conditions specified by the manufacturer.
  • the BC184L transistor is 'turned on' when its base (and the 555 timer output since they are connected) goes to 5 V.
  • the BC212L transistor on the other hand is 'turned on' when its base goes to OV. The result of this is that the LEDs are driven in antiphase.
  • the photodiode is connected between the inputs of a 741 operational amplifier (OpAmp).
  • OpAmp operational amplifier
  • a lOMegohm resistor is connected between the inverting input of the OpAmp and its output. This serves to convert the current created by light falling on the photodiode into a voltage at the OpAmp output.
  • the output of the OpAmp is connected to a highpass filter to remove any DC offsets (passing any frequency above -IHz).
  • a Twin T notch filter with a notch frequency of 100Hz (120Hz for the US) is the next part of the circuit. This filter greatly reduces the effect of fluorescent lights flashing at twice the mains frequency.
  • the notch filter has a finite Q factor which means it doesn't just eliminate the desired frequency. This has both desirable and undesirable effects.
  • the lock-in amplifier has two inputs; a signal input connected to the photodiode circuit and a clock input connected to the 555Timer's output.
  • the signal input is connected to a 741 OpAmp unity gain buffer to remove any effects of loading the signal.
  • a resistor can be connected between the input and ground if a lower input impedance is desired.
  • the output of the buffer is connected to an inverting 741 OpAmp circuit.
  • the outputs of both the inverting amplifier and the buffer amplifier are connected to different analog switches (CMOS 4066B).
  • the clock input is connected two a pair of comparators (could use a comparator and a NOT gate instead).
  • a reference voltage (2.5V supplied by a potential divider) is connected to the other input of the comparators (LM339 Quad comparator IC).
  • One comparator has the clock input connected to its (+) input and 2.5V connected to its (-) input. This results in the comparator output going High when the clock input of the lock-in circuit goes High.
  • the output of this comparator is connected to the switch control input of the analog switch which controls the non-inverted signal (straight from the signal input buffer).
  • the other comparator is set up to go output High when the clock input of the lock- in circuit goes Low.
  • the output of this comparator is connected to the control input of the analog switch which controls the inverted signal.
  • the outputs of the two analog switches are connected together and form an output of the lock-in amplifier circuit.
  • An assembler code has been written for a microcontroller (AtTinyl5) which will take the place of the 555Timer.
  • the microcontroller also has an ADC (analog to digital) input which is to be connected to the lock-in amplifiers main output.
  • ADC analog to digital
  • the microcontroller balances the signals from the LEDs by changing the duty cycle.
  • a buzzer connected to an output pin of the microcontroller sounds.
  • This microcontroller should save cost as well by eliminating the need for the multi-turn potentiometer used by the 555Timer circuit to set the duty cycle. Abs(Vout lock-in) will be fed to the ADC of the microcontroller.
  • the calibration process will result in the x51 amplifier gain being reduced.
  • All OpAmps are currently LM741, though some may be replaced with higher quality OpAmps or amalgamated into a Quad OpAmp IC.
  • the OpAmp converting the photodiode current into a voltage could be replaced with and integrating transimpedance amplifier (eg IVC 102).
  • IVC 102 transimpedance amplifier

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  • Spectroscopy & Molecular Physics (AREA)
  • General Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
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Abstract

The present invention relates to a device that can detect whether or not a floor or other surface has material spilt, splashed or otherwise distributed on it. Thus, the device can be used to detect the presence of liquid or solids spilt on a floor. The device can be mounted on the ceiling or wall of a room or may be freestanding. The source of electromagnetic radiation provides electromagnetic radiation at two or more different wavelengths. At least one of the wavelengths of electromagnetic radiation provided by the source must be emitted at a wavelength which is capable of absorption by the material to be detected. The use of an absorbing and a non-absorbing wavelength allows a differential signal to be obtained in the detector due to reflection of the electromagnetic radiation originating from the source from the surface and the material distributed thereon. Detection of the relative intensities of the two wavelengths of reflected light originating from the source thus allows the detector to determine whether or not material is present on the surface.

Description

LIQUID SENSOR
The present invention relates to a device that can detect whether or not a floor or other surface has material spilt, splashed or otherwise distributed on it. Thus, the device can be used to detect the presence of liquid or solids spilt on a floor. The device can be mounted on the ceiling or wall of a room or may be freestanding.
US 6812846 discloses a spill detector which is based on machine imaging. This patent discloses the use of cameras and image processing techniques for the purpose of spill detection. A known image scene is recorded and compared with views taken by the camera on a continuing basis. Fixed background data is removed using a technique based on image subtraction and the image may be segmented to identify and remove from further analysis segments corresponding to non-spill foreground objects. This allows the image subtraction software to discount non-spill images.
The system of US 6812846 has the disadvantage that it requires complicated software in order to ensure correct processing of the image. There is also a risk of false alarms due to foreground objects and/or inaccurate processing. Hence, there is a need for a spill detector which is able to work in an active manner by directly analysing the spill itself.
According to the present invention, there is provided a spill detector comprising at least one source of electromagnetic radiation which is able to emit electromagnetic radiation at two or more different pre-determined wavelengths, a detector adapted to generate a signal in response to electromagnetic radiation originating from the source, and an amplifier to amplify the signal
The source of electromagnetic radiation provides electromagnetic radiation at two or more different wavelengths. Ideally, the source provides radiation at two different wavelengths. In a preferred embodiment, the or each source of electromagnetic radiation is a light source. This is a convenient source of radiation. Suitably, the light source is a light emitting infrared light of known wavelength or wavelengths. A light emitting diode (LED) is particularly suitable. In a preferred embodiment the two different wavelengths of electromagnetic radiation are both in the infrared region. Thus, in a preferred embodiment, the infrared radiation is provided by one or more light emitting diodes (LED) or laser diodes (LD). The wavelengths of radiation provided by the source will depend upon the nature of the liquid to be detected. In another embodiment, multiple LEDs or LDs could be used for each wavelength to increase the intensity of the reflected light.
At least one of the wavelengths of electromagnetic radiation provided by the source must be emitted at a wavelength which is capable of absorption by the material to be detected. This is known as the absorption wavelength. A material may have more than one absorption wavelength. It is therefore important that the other wavelength or wavelengths of electromagnetic radiation should be distanced from the absorption wavelength and that this other wavelength or wavelengths is not absorbed by the material. This wavelength or wavelengths are defined here as non-absorbing wavelengths.
Since it is preferred to utilise a source emitting two different wavelengths the following description is based on such an arrangement. However, in principle, more than one absorbing wavelength could be emitted and/or more than one non-absorbing wavelength could be emitted by the source. Thus, in another embodiment, more than two different wavelengths could be used to give an improved detection scheme. This could be used either to detect one or more different liquids or a mixture of liquids, or simply to improve the accuracy of detection of a single liquid. The use of LDs rather than LEDs can also give better sensitivity and selectivity because the emitted wavelength spread is narrower.
The use of an absorbing and a non-absorbing wavelength allows a differential signal to be obtained in the detector due to reflection of the electromagnetic radiation originating from the source from the surface and the material distributed thereon. Detection of the relative intensities of the two wavelengths of reflected light originating from the source thus allows the detector to determine whether or not material is present on the surface.
In a preferred embodiment, the material is a liquid. Examples include water; laboratory solvents such as alcohols, ethers, hydrocarbons and halogenated solvents; and fuels. Preferably, the liquid is water or contains water or is a solution of a substance in water. In the case of water, there is an absorption peak at 970 nm and thus one of the wavelengths is preferably centred at 970 nm with the other being distanced away from the absorption wavelength.
Ideally the other non-absorbing wavelength would be at least 30 nm and more preferably 50 nm away from the absorption peak. The important criteria, however, is that the second wavelength should not interact with and be absorbed by the material to be detected. It should also not cause interference with the radiation of the other wavelength.
The source of electromagnetic radiation may be a single source capable of emitting radiation at two different wavelengths or it may comprise two separate sources of radiation of different wavelengths. The intensities of the two wavelengths emitted are preferably identical. However, it is not necessary for the intensities to be identical and in one embodiment the intensities may be different provided that the relative intensities of the two emitted wavelengths is known.
The detector is connected to signal processing means which measures the relative intensities of the two wavelengths detected and compares it with the two known intensities of the wavelengths emitted by the source of electromagnetic radiation. In a preferred embodiment, the detector uses a photodiode.
An amplifier amplifies the signal obtained by the detector at the two wavelengths of interest. Where there is a differential signal the differential signal is thus amplified. In an embodiment, the amplifier is a lock-in amplifier or similar synchronous or heterodyne detector.
A differential signal can be received and amplified under two circumstances. The first circumstance arises when there is a material spill and the emitted radiation at the two different wavelengths is of equal intensities but radiation at one of the frequencies is absorbed by the material. This leads to the intensities of the reflected radiation received by the detector being different. In such a case the spill detector can include an algorithm which provides for a warning to be issued under such circumstances. If no signal is amplified in this case then this means that there is no material present on the surface.
The second situation in which a differential signal can be received by the detector arises when the two different wavelengths emitted by the source of electromagnetic radiation are emitted at different intensities. The reflected radiation is therefore likely to be of different intensity at the two wavelengths regardless of whether or not there has been any absorption at one of the wavelengths. In this case, an algorithm determines whether the relative intensities of the two wavelengths is the same as the original relative intensities of the two wavelengths emitted by the source or whether this value has changed. If the value has changed it is due to interaction with a material on the surface in the detection zone. The spill detector may include an algorithm to generate a warning in this case.
In an embodiment, the light source is one or more LEDs. It is possible that the two LEDs may have different intensities naturally and a number of methods are available for compensating for the difference in intensities. In addition to or in place of using an algorithm the following protocols could be adopted. For example, one of the two LEDs could be kept on longer than the other. Alternatively, one or more neutral density filters could be placed in the path of one of the LEDs. Equally, the current to one of the two LEDs could be reduced relative to the other. These approachs could be used for any electromagnetic source providing radiation of different intensities at the different wavelengths.
In an embodiment, the source of electromagnetic radiation is a single source capable of emitting radiation at more than one wavelength. In a preferred embodiment, the source is a modulated light source which is capable of emitting light at two wavelengths. This then permits synchronous detection techniques that eliminate the effects of other ambient light.
The detector is any conventional detector capable of receiving electromagnetic radiation at the two emitted wavelengths. A photodetector such as a photodiode is particularly suitable for this purpose. In a further embodiment of the invention, the source is adapted to emit more than two wavelengths to allow a detection of more than one type of material. In such a case the detector could be used to detect mixtures of liquids.
The detector of the present invention is also capable of detecting solids or gels provided that the solids or gels have an appropriate absorption peak for electromagnetic radiation. This is a significant advantage over the prior art detector which is not able to detect solids.
The invention will now be illustrated by the following figures in which: Figure 1 shows a spill detector according to the present invention; and Figure 2 illustrates a schematic diagram of a wet floor detector.
In Figure 1, source 1 of electromagnetic radiation comprises a pair of LEDs 2 which emit infrared radiation at two different frequencies X and Y. The source 1 is mounted on ceiling 3 but could equally be mounted on wall 4 or be free standing so as to be placed on a cabinet 5. A pool of spilt liquid 6 on floor 7 receives incident light of wavelengths X and Y which have intensities Xl and Yl, respectively. The incident light is reflected as infrared radiation of wavelengths X and Y having intensities X2 and Y2 respectively. The reflected light of wavelengths X and Y is received by detector 8 which includes a photodiode within a housing 9. The housing 9 may also contain amplifier 10 and processing circuitry 11. The detector 8 may be mounted on the ceiling 3, wall 4 or may be free standing. When a spill is detected the algorithm provides for a warning signal to be issued. In this case, light 12 is illuminated to warn of the spillage. The source 1 and detector 8, respectively, may include their own power supplies or may be connected to external sources of power.
In Figure 2, oscillator circuit 21 is connected to LED 1 and LED 2. Oscillator circuit 21 is also connected to lock-in amplifier 22 which in turn is connected to photodiode detector 23 and also to output circuit 24. One of LED 1 and LED 2 emits light at a wavelength which is strongly absorbed by the liquid and the other LED at a wavelength which is not.
Importantly, there is no direct line of sight between the LEDs and the photodiode detector. The LED's light is instead reflected off the surface in the detection zone. The oscillator circuit 21 oscillates at a fixed frequency square-wave with variable duty cycle. One LED is connected between the oscillator output and ground, the other LED is connected between the oscillator output and the positive power rail. The LEDs are thus lit in antiphase.
The photodiode detector 23 observes the area of a floor that the LEDs shine on. This is the detection area. A photodiode is a light sensitive electronic device which is capable of receiving and generating a signal in response to light. In the embodiment shown in Figure 2, the photodiode generates a signal in response to light from both LED 1 and LED 2.
The lock-in amplifier 22 rejects any signal which is not in phase with the oscillator 21 (eg lighting flicker).
The output of the lock-in amplifier 22 is connected to the output circuit 24 which measures the voltage of the lock-in output and acts upon it. For example it provides a sound, flashes a warning light or sends an email or wireless message.
The oscillator used in the embodiment shown in Figure 2 creates a square wave oscillating between the positive power rail at a certain positive voltage and ground (zero volts). In this case, the positive rail is at 5 volts. The duty cycle of the oscillator can be varied to balance the intensity of the two LEDs. This is a type of pulse width modulation. The oscillator used in the embodiment shown schematically in Figure 2 is based on the 555 timer IC which is available from a number of electronic retailers. However any conventional oscillator could be used in the detector of the present invention.
A lock-in amplifier is used to measure a weak signal against a noisy background. Any conventional lock-in amplifier could be used in the detector of the present invention. When the clock input of the lock-in amplifier 22 is high (for example 5 volts in the present case), the input signal is connected to the lock-in output. When the clock input of the lock- in is low (zero volts) the input signal is inverted and then connected to the lock-in output. The output of the lock-in is then time averaged. In the present case, this is achieved using a low pass filter. The only signals that are not time averaged to zero by this process are signals in phase (at the same frequency) as the clock input. The output of the lock-in amplifier is therefore the average amplitude of the signal in phase with the clock input.
Signals from the two LEDs 1 and 2 are balanced by varying the duty cycle of the oscillator so that the time averaged output of the lock-in amplifier is zero volts. This happens in the present embodiment shown in Figure 2 because the signal from one LED is inverted by the lock-in amplifier and then added to the signal from the other LED. A change in the relative intensity of the signals results in an imbalance and a non-zero voltage from the lock-in amplifier 22. This non-zero output can be used by the output circuitry to trigger a buzzer or a light.
In a variant of the detector shown, an electronic switch is placed in between the oscillator output and the LEDs. In one state of the switch one LED is connected, in the other state the other LED is connected. This switch can be controlled by another lower frequency oscillator or by a micro controller. The lock-in amplifier would give a measurement of the signal for each LED and measurements could then be compared. The calibration process would create the same lock-in output for both LEDs.
In one example of the invention that has successfully been employed to detect a spillage on a floor, the wet floor detector consists of three main units: a timer, a photodiode detector and a lock-in amplifier. These are described in more detail below.
1. 555Timer oscillator circuit and LED drivers
This is a simple square wave oscillator with variable duty cycle (varied by changing a potentiometer). The output of the 555 timer is fed into the base of two transistors (BCl 84L and BC212L) to give the required current. Without these transistors the wavefoπn is distorted and reduced in amplitude. The BC184L transistor has its collector connected to 5 V and its emitter connected to the 970nm (absorbed by water) LED and a current setting resistor. The BC212L transistor has its collector connected to ground (OV) and its emitter connected to the other LED (which is not absorbed by water) and a current setting resistor. The current setting resistors could be varied to change the LED currents though this would move them off there design conditions specified by the manufacturer. The BC184L transistor is 'turned on' when its base (and the 555 timer output since they are connected) goes to 5 V. The BC212L transistor on the other hand is 'turned on' when its base goes to OV. The result of this is that the LEDs are driven in antiphase.
2. Photodiode detector and input filter amplifier
The photodiode is connected between the inputs of a 741 operational amplifier (OpAmp). A lOMegohm resistor is connected between the inverting input of the OpAmp and its output. This serves to convert the current created by light falling on the photodiode into a voltage at the OpAmp output. The output of the OpAmp is connected to a highpass filter to remove any DC offsets (passing any frequency above -IHz). A Twin T notch filter with a notch frequency of 100Hz (120Hz for the US) is the next part of the circuit. This filter greatly reduces the effect of fluorescent lights flashing at twice the mains frequency. The notch filter has a finite Q factor which means it doesn't just eliminate the desired frequency. This has both desirable and undesirable effects. On the plus side it negates the need for fine-tuning the filter frequency and the use of high tolerance components, on the negative side it slightly distorts the received square wave (not too badly). However, we have found that an effective filter can be chosen easily to suit the circuitry. After the notch filter there is a simple non inverting amplifier circuit to increase the amplitude of the signal. This amplifier is currently set to amplify by a factor of 51. The output of this circuit is then connected to the signal input of the lock-in amplifier.
3. Lock-in amplifier
The lock-in amplifier has two inputs; a signal input connected to the photodiode circuit and a clock input connected to the 555Timer's output. The signal input is connected to a 741 OpAmp unity gain buffer to remove any effects of loading the signal. A resistor can be connected between the input and ground if a lower input impedance is desired. The output of the buffer is connected to an inverting 741 OpAmp circuit. The outputs of both the inverting amplifier and the buffer amplifier are connected to different analog switches (CMOS 4066B). The clock input is connected two a pair of comparators (could use a comparator and a NOT gate instead). A reference voltage (2.5V supplied by a potential divider) is connected to the other input of the comparators (LM339 Quad comparator IC). One comparator has the clock input connected to its (+) input and 2.5V connected to its (-) input. This results in the comparator output going High when the clock input of the lock-in circuit goes High. The output of this comparator is connected to the switch control input of the analog switch which controls the non-inverted signal (straight from the signal input buffer). The other comparator is set up to go output High when the clock input of the lock- in circuit goes Low. The output of this comparator is connected to the control input of the analog switch which controls the inverted signal. The outputs of the two analog switches are connected together and form an output of the lock-in amplifier circuit. The net result of this is that then the lock-in clock input is High, the input signal is 'connected' to the output and when the lock-in clock input is Low, the input is inverted and then 'connected' to the output. There is a second output of the lock-in amplifier circuit which forms the main output. This output is a lowpass filtered (~lHz) version of the other output. This serves to average the other output. The only signals to survive this lock-in process are at, or extremely close two the clock frequency (as a note frequencies with are an odd multiple of the clock frequency also survive but that's ok since it is a squarewave input).
An assembler code has been written for a microcontroller (AtTinyl5) which will take the place of the 555Timer. The microcontroller also has an ADC (analog to digital) input which is to be connected to the lock-in amplifiers main output. When powered up, the microcontroller balances the signals from the LEDs by changing the duty cycle. When the ADC input surpasses a predefined voltage (set by a potentiometer connected to another ADC input) a buzzer connected to an output pin of the microcontroller sounds. This microcontroller should save cost as well by eliminating the need for the multi-turn potentiometer used by the 555Timer circuit to set the duty cycle. Abs(Vout lock-in) will be fed to the ADC of the microcontroller. The calibration process will result in the x51 amplifier gain being reduced. All OpAmps are currently LM741, though some may be replaced with higher quality OpAmps or amalgamated into a Quad OpAmp IC. The OpAmp converting the photodiode current into a voltage could be replaced with and integrating transimpedance amplifier (eg IVC 102). The above example of a working detector employs specific electronic components chosen for their compatibility with one another and on the basis of availability. However, the specific components chosen may easily be substituted for equivalents components performing the same function without altering the working of the detector.

Claims

1. A spill detector comprising at least one source of electromagnetic radiation which is able to emit electromagnetic radiation at two or more different pre-determined wavelengths, a detector adapted to generate a signal in response to electromagnetic radiation originating from the source, and an amplifier to amplify the signal.
2. A spill detector as claimed in claim 1, wherein the or each source of electromagnetic radiation is a light source.
3. A spill detector as claimed in claim 2, wherein the light source is a light emitting infrared light of known wavelength or wavelengths.
4. A spill detector as claimed in claims 2 or 3, wherein the two different wavelengths of electromagnetic radiation are both in the infrared region.
5. A spill detector as claimed in any of claims 2, 3 or 4, wherein the infrared radiation is provided by one or more light emitting diodes (LED) or laser diodes (LD).
6. A spill detector as claimed in any preceding claim, wherein the source of electromagnetic radiation is a single course capable of emitting radiation at more than one wavelength.
7. A spill detector as claimed in any preceding claim, wherein the amplifier is a lock- in amplifier or similar synchronous or heterodyne detector.
8. A spill detector as claimed in claim 7, wherein the spill detector includes an algorithm.
9. A spill detector as claimed in any preceding claim, wherein the electromagnetic radiation is detected by any conventional detector capable of receiving electromagnetic radiation.
10. A spill detector as claimed in claim 9, including a photodiode to detect electromagnetic radiation.
EP07731974A 2006-03-20 2007-02-27 Liquid sensor Withdrawn EP1996918A1 (en)

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GBGB0605486.0A GB0605486D0 (en) 2006-03-20 2006-03-20 Liquid sensor
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US8004683B2 (en) * 2007-10-11 2011-08-23 Ecolab Usa Inc. Optical product detection sensor

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US3783284A (en) * 1971-10-28 1974-01-01 Texas Instruments Inc Method and apparatus for detection of petroleum products
GB8515519D0 (en) * 1985-06-19 1985-07-24 Graviner Ltd Gas detection
US4897551A (en) * 1988-04-11 1990-01-30 Spectral Sciences, Inc. Leak detector
WO1992007249A1 (en) * 1990-10-10 1992-04-30 Boston Advanced Technologies, Inc. Methods and sensor systems for sensing hydrocarbon-containing fluids based on fluorescence detection
US20040144163A1 (en) * 2002-11-06 2004-07-29 Kram Mark Lenard Storage tank leak detection system for petroleum products

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