Photoacoustic gas sensor

Abstract

To provide a photoacoustic gas sensor which can precisely detect a detection target gas.SOLUTION: A photoacoustic gas sensor (1) includes: a light source (11) for emitting light; a light detector (31) for detecting the light emission amount of the light source (11); and a detection unit (41) for detecting vibration sound of gas molecules of a detection target gas which has absorbed the emitted light and outputting a detection signal. The gas sensor measures the concentration of the detection target gas on the basis of the detection signal and can correct the concentration of the measured detection target gas on the basis of the light emission amount of the light source (11).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a photoacoustic gas sensor. [Background technology]

[0002] In recent years, gas sensors (gas measuring devices) have been developed that are equipped with a light source that emits infrared rays, are configured so that the infrared rays pass through a gas containing the gas to be detected, and detect the concentration of the gas to be detected by utilizing the absorption characteristics of the infrared rays by the gas to be detected. The gas to be detected is, for example, alcohol or carbon dioxide. For example, Patent Document 1 discloses a photoacoustic gas sensor that measures the gas concentration by picking up the vibrations of gas molecules that absorb light as sound with a high-performance microphone. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2020 / 212481 Summary of the Invention [Problem to be solved by the invention]

[0004] Here, the amount of light from the light source may change due to the influence of temperature and deterioration over time. If the amount of light from the light source used in the gas sensor changes, measurement errors may occur. Therefore, there is a demand for a gas sensor that can detect the change in the amount of light from the light source and improve the measurement accuracy.

[0005] In view of the above circumstances, an object of the present disclosure is to provide a photoacoustic gas sensor capable of detecting a target gas with high accuracy. [Means for solving the problem]

[0006] (1) A photoacoustic gas sensor according to an embodiment of the present disclosure includes: A light source that emits light; a photodetector for detecting an amount of light emitted by the light source; a detection unit that detects vibration sounds of gas molecules of the detection target gas that have absorbed the emitted light and outputs a detection signal; The concentration of the detection target gas is measured based on the detection signal, and the measured concentration of the detection target gas is correctable based on the amount of light emitted by the light source.

[0007] (2) As an embodiment of the present disclosure, in (1), The measured concentration of the target gas can be corrected based on a temperature calculated from the amount of light emitted by the light source and a resistance value of a resistor included in the photodetector.

[0008] (3) As an embodiment of the present disclosure, in (1) or (2), The signal output by the photodetector uses the same physical quantity as the detection signal.

[0009] (4) As an embodiment of the present disclosure, in any one of (1) to (3), The photodetector is integral with the light source.

[0010] (5) As an embodiment of the present disclosure, in any one of (1) to (4), the light source emits the light in accordance with a drive signal; The drive signal varies in response to the signal output from the photodetector.

[0011] (6) As an embodiment of the present disclosure, in (5), The drive signal changes so that the duty ratio is greater than or equal to 10% and less than or equal to 30%.

[0012] (7) As an embodiment of the present disclosure, in (5) or (6), A driving source that outputs the driving signal and an amplifier that amplifies the signal output by the photodetector are used together, The length of the wiring electrically connecting the photodetector and the amplifier is configured to be shorter than the length of the wiring electrically connecting the light source and the drive source.

[0013] (8) As an embodiment of the present disclosure, in any one of (1) to (7), The detection unit is covered with a metal housing.

[0014] (9) As an embodiment of the present disclosure, in any one of (1) to (8), a gas cell having a hole through which a gas passes and introducing the gas from the hole into a gas detection space; the light source, the light detector, and the detection unit are disposed inside the gas detection space, The light source emits light at a drive frequency determined based on a natural frequency of a resonant mode of the gas cell.

[0015] (10) As an embodiment of the present disclosure, in (9), The gas cell is made of a conductive material, is electrically connected to a substrate that holds the light source and the detector, and is fixed at a reference potential. Effect of the Invention

[0016] According to the present disclosure, it is possible to provide a photoacoustic gas sensor capable of detecting a target gas with high accuracy. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram illustrating an example of the configuration of a photoacoustic gas sensor according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 is a diagram for explaining an example of setting the driving frequency of the light source. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] A photoacoustic gas sensor 1 (see FIG. 1) according to an embodiment of the present disclosure will be described below with reference to the drawings.

[0019] (Photoacoustic gas sensor) FIG. 1 is a diagram showing a configuration of a photoacoustic gas sensor 1 according to this embodiment. The photoacoustic gas sensor 1 measures the presence or concentration of a gas to be detected in a gas by a photoacoustic method. In this embodiment, the photoacoustic gas sensor 1 is described as a device for measuring the concentration of a gas to be detected. The photoacoustic gas sensor 1 includes a light source 11, a photodetector 31, and a detection unit 41. The photoacoustic gas sensor 1 may include a substrate 30 and a gas cell 40. The photoacoustic gas sensor 1 may also include a signal processing unit 50. In this embodiment, the photodetector 31 is integrated with the light source 11. In detail, the light source 11 and the photodetector 31 are sealed by a sealing unit 23 on one surface of the light source substrate 12. That is, the light source substrate 12, the light source 11, the photodetector 31, and the sealing unit 23 are integrated to form a light source module. FIG. 1 is a cross-sectional view showing a cross section of the photoacoustic gas sensor 1 including these components. Details of the components of the photoacoustic gas sensor 1 will be described later.

[0020] Here, the gas cell 40 has a hole 43 through which a gas (e.g., ambient air) passes, and the gas is introduced into the gas detection space 42 through the hole 43. The photoacoustic gas sensor 1 measures the concentration of a detection target gas in the introduced gas. The detection target gas is a specific gas to be detected, and may be, for example, carbon dioxide, water vapor, carbon monoxide, nitric oxide, ammonia, sulfur dioxide, alcohol, formaldehyde, methane, propane, fluorocarbons, alternative fluorocarbons, R32, R1234y, etc.

[0021] The photoacoustic gas sensor 1 measures the concentration of the detectable gas in the gas introduced into the gas detection space 42, and outputs an electrical signal indicating the measurement result. The photoacoustic method measures the detectable gas by picking up the vibrations of gas molecules that absorb light as sound (pressure change) with a high-performance microphone.

[0022] The photoacoustic gas sensor 1 is configured with a gas cell 40 on a main surface 30a of a substrate 30. Here, the main surface 30a is one of the surfaces of the substrate 30 having the largest area. As shown in FIG. 1, an orthogonal coordinate system is set so that the xy plane is parallel to the main surface 30a of the substrate 30. The z-axis direction is a direction perpendicular to the main surface 30a of the substrate 30. The z-axis direction may be referred to as a height direction or a vertical direction. The positive direction of the z-axis corresponds to the upward direction. In this embodiment, the light source 11, the photodetector 31, and the detection unit 41 are disposed inside a gas detection space 42 on the main surface 30a.

[0023] (substrate) The substrate 30 has a function of holding the gas cell 40, the light source module, and the detection unit 41. In this embodiment, the substrate 30 also holds the signal processing unit 50. In this embodiment, the substrate 30 is a PCB (printed circuit board), on which the light source 11, the detection unit 41, and the signal processing unit 50 are mounted and which also electrically connects these. The material of the substrate 30 is, for example, paper, glass cloth, ceramics, polyimide, liquid crystal polymer, etc.

[0024] (Gas Cell) The gas cell 40 has a gas detection space 42 for detecting a gas to be detected. The gas cell 40 is provided on the main surface 30a of the substrate 30. That is, the gas detection space 42 is an internal space surrounded by the outer wall of the gas cell 40 and the substrate 30. The material of the outer wall of the gas cell 40 can be, for example, metal, glass, resin, a composite material thereof, or the like. Examples of the resin include phenol resin, epoxy resin, polyimide resin, bismaleimide triazine resin, fluororesin, polyphenylene oxide resin, LCP (liquid crystal polymer), PP (polypropylene), PEEK (polyether ether ketone), PA (polyamide), PPE (polyphenylene ether), PC (polycarbonate) or PPS (polyphenylene sulfide), PMMA (polymethyl methacrylate resin), PAR (polyarylate resin), etc., and hard resins in which two or more of these are mixed, etc. Furthermore, the gas cell 40 is preferably made of a material with a small light absorption coefficient and high reflectance in order to efficiently confine the light emitted from the light source 11 within the gas cell 40. Specifically, a resin housing coated with an alloy containing aluminum, gold, or silver, a dielectric, or a laminate of these is preferable. When the inner surface of the gas cell 40 is formed by deposition or plating on the resin housing, it is possible to improve productivity and weight reduction compared to when the gas cell 40 is formed from a metal material. In addition, the gas cell 40 may be formed by cutting, but from the viewpoint of productivity, it is preferable to form the gas cell 40 by injection molding or press processing. In addition, the gas cell 40 and the substrate 30 may be mechanically joined by adhesives, screws, claws, fittings, grommets, welding, soldering, or the like.

[0025] When the gas cell 40 is made of a conductive material such as a metal and the substrate 30 and the gas cell 40 are electrically connected by solder or the like, the gas cell 40 can function as an electromagnetic wave shield by fixing the gas cell 40 to a reference potential, thereby improving the gas measurement performance. The reference potential is, for example, a ground potential.

[0026] (light source) The light source 11 emits light including a wavelength absorbed by the gas to be detected. The light source 11 is also called a light emitting unit. The light source 11 may be provided on the main surface 30a of the substrate 30 inside the gas detection space 42. In this embodiment, the light is infrared. That is, the light source 11 is composed of a light emitting element that emits infrared. The light emitting element is preferably driven by a voltage or current pulse and can be driven at high speed. For example, it is preferable that it can be driven at 15 kHz or more. The light emitting element may be, for example, an LED (Light Emitting Diode), a laser (Light Amplification by Stimulated Emission of Radiation), a MEMS heater, a lamp, or the like. In this embodiment, the light source 11 is an LED (infrared LED) that emits infrared rays. In addition, the light source 11 is preferably a quantum type infrared LED that emits infrared rays using electrons or holes in a semiconductor so that it can be driven at a high frequency.

[0027] Here, the photoacoustic gas sensor 1 may further include a drive source that outputs a drive signal to drive the light-emitting element. In this embodiment, the drive source is included in the signal processing unit 50. In other words, a part of the signal processing unit 50 functions as the drive source.

[0028] Furthermore, the light source 11 may include an auxiliary member having auxiliary optical functions such as wavelength selection, light collection, scattering, wavelength conversion, etc. Specifically, the auxiliary member is a wavelength selection filter, a lens, a phosphor, a diffraction grating, etc.

[0029] The light source 11 emits light from a light emitting surface 11a. The light emitting surface 11a is a surface of the light source 11 that contacts the light source substrate 12. The light emitting surface 11a corresponds to one of the surfaces of the light source 11 that has the largest area.

[0030] Here, the wavelength of the infrared light may be 2 μm to 12 μm. In the region of 2 μm to 12 μm, there are many absorption bands specific to various gases, and this is a wavelength band that is particularly suitable for use in the photoacoustic gas sensor 1. For example, there is an absorption band for methane at a wavelength of 3.3 μm, for carbon dioxide at a wavelength of 4.3 μm, and for alcohol (ethanol) at a wavelength of 9.5 μm.

[0031] The sealing portion 23 is made of, for example, a molded resin, but is not limited to this as long as it seals the light source 11 and the photodetector 31. When the sealing portion 23 is made of a molded resin, the light source 11 and the photodetector 31 can be integrated together with the light source substrate 12 while maintaining their positional relationship. As a material for the molded resin, for example, an epoxy resin or a phenolic resin may be used. As long as there is no problem with mounting, the sealing portion 23 may be a ceramic package or a can package. The light source substrate 12 may be, for example, a semiconductor substrate. As the semiconductor substrate, for example, a Si substrate, an InP substrate, a GaAs substrate, etc. may be used.

[0032] (Photodetector) The photodetector 31 detects the amount of light emitted by the light source 11. The photodetector 31 may be configured to include a light receiving element having sensitivity to the wavelength band of light emitted from the light source 11. The light receiving element may be a quantum type sensor such as a photodiode having a PIN structure, as a specific example. As another example, the light receiving element may be a phototransistor, a thermopile, a pyroelectric sensor, a bolometer, or the like. In this embodiment, the light receiving element is a quantum type infrared sensor. The photodetector 31 is disposed at a position where light emitted from the light source 11, which is reflected by a surface in contact with the gas detection space 42 of the light source substrate 12 and passes through the inside of the light source substrate 12, is incident. By disposing the photodetector 31 at such a position, it becomes possible to detect the amount of light emitted by the light source 11 regardless of the usage environment.

[0033] The photodetector 31 may also include a resistor. The resistance value of the resistor changes depending on the temperature. The resistor is provided, for example, in a position close to the light source 11 in the photodetector 31. The temperature of the light source 11 can be calculated from the resistance value of the resistor provided in the photodetector 31. The photodetector 31 outputs a signal corresponding to the amount of light emitted by the light source 11. The photodetector 31 may also output a signal indicating the resistance value or a signal corresponding to the temperature of the light source 11. The signal output from the photodetector 31 may use the same physical quantity as the detection signal output from the detection unit 41 described below. For example, when the detection signal is a current value, the signal output from the photodetector 31 may also be a current value. For example, when the detection signal is a voltage value, the signal output from the photodetector 31 may also be a voltage value. By using the same physical quantity, it is possible to omit the calculation process of converting different physical quantities in the process of correcting the concentration of the detected gas described below, thereby reducing the load of the calculation and improving the accuracy of the calculation.

[0034] Here, the photoacoustic gas sensor 1 may further include an amplifier that amplifies the signal output by the photodetector 31. In this embodiment, the amplifier is included in the signal processing unit 50. In other words, a part of the signal processing unit 50 functions as the amplifier.

[0035] Furthermore, the photoacoustic gas sensor 1 may have an analog-to-digital converter that converts the analog signal output by the photodetector 31 into a digital signal. In this embodiment, the analog-to-digital converter is included in the signal processing unit 50. In other words, a part of the signal processing unit 50 functions as the analog-to-digital converter.

[0036] (Detection unit) The detection unit 41 detects a change corresponding to the amount of the detection target gas in the concentration measurement of the detection target gas present in the gas (air) in the gas detection space 42. The detection unit 41 is provided on the main surface 30a of the substrate 30 inside the gas detection space 42. The light emitted from the light source 11 passes through the gas detection space 42 and is absorbed according to the amount of the detection target gas present. The detection unit 41 detects a signal based on the light emitted from the light source 11 and outputs a detection signal. The detection element may be, for example, a microphone, an air pressure sensor, or a pressure sensor. In this embodiment, the detection unit 41 is a microphone. Also, in this embodiment, the signal based on the light is a vibration sound of the gas molecules of the detection target gas that absorbs the light. That is, the detection unit 41 detects the vibration sound of the gas molecules of the detection target gas that absorbs the emitted light and outputs a detection signal. In order to suppress the influence of electromagnetic noise generated when the light source 11 is driven, the detection unit 41 is preferably configured to be covered with a metal housing.

[0037] (Gas detection space) The gas detection space 42 is separated by an outer wall and has a function of storing a gas such as air in the inner space. The gas stored in the gas detection space 42 is replaced through the hole 43.

[0038] (hole) The hole 43 is provided in a part of the outer wall (side wall and ceiling) that partitions the gas detection space 42 of the gas cell 40. Gas passes through the hole 43, and the gas in the gas detection space 42 is replaced. There may be a plurality of holes 43. Here, the hole 43 may be equipped with a dust-proof filter for dust prevention. The dust-proof filter may be, for example, a nonwoven fabric, a Teflon (registered trademark) sheet, or the like.

[0039] (Signal Processing Unit) The signal processing unit 50 may control the entire photoacoustic gas sensor 1. For example, the signal processing unit 50 may execute a calculation process to obtain the concentration of the gas to be detected. That is, the signal processing unit 50 may calculate the concentration of the gas to be detected based on the detection signal from the detection unit 41, which is a microphone. The signal processing unit 50 may be configured to include one or more processors. The processor may be, for example, a general-purpose processor or a dedicated processor specialized for a specific process, but is not limited to these and may be any processor. The signal processing unit 50 may be provided outside the gas detection space 42, or may be provided so that at least a part of it is inside the gas detection space 42. The signal processing unit 50 may be provided on the main surface 30a of the substrate 30, but may also be embedded in the substrate 30, or may be provided on the back surface opposite to the main surface 30a. The photoacoustic gas sensor 1 may not be configured to include the signal processing unit 50.

[0040] (Photoacoustic gas sensor design) Here, in the photoacoustic gas sensor 1, since sound is picked up by a microphone or the like, noise in the usage environment affects the detection accuracy. For example, if there is an air conditioner or other device in the usage environment of the photoacoustic gas sensor 1, noise having a peak at around 100 Hz may be emitted. Therefore, in order to detect the gas to be detected with high accuracy, it is necessary to avoid the influence of noise. Also, the gas detection space 42 is partitioned by an outer wall, and is provided as an internal space of a structure (cell) that is, for example, close to a rectangular parallelepiped. Therefore, by utilizing the resonance mode of the cell, it is possible to increase the sound picked up by the microphone and improve the measurement accuracy.

[0041] FIG. 2 is a diagram for explaining the setting of the driving frequency of the light source 11. The dotted line in the left diagram of FIG. 2 shows the result of a numerical calculation of the noise energy intensity in the usage environment of the photoacoustic gas sensor 1. As a model of the sound (noise) emitted from the vibration source, measurement data of the noise from the blower of the indoor unit of the Hitachi room air conditioner "RAS-AJ36D(W)" during heating operation was used. The vertical axis shows the noise energy intensity normalized by the maximum value. The horizontal axis shows the frequency, and the noise energy intensity for each frequency component is shown. By setting the driving frequency of the light source 11 to a frequency band away from the peak of the noise energy intensity, it is possible to avoid the influence of noise in the usage environment. In the example of FIG. 2, there is a sharp intensity peak near 100 Hz. Therefore, the driving frequency of the light source 11 may be set to 200 Hz or more. In addition, in order to be further away from the peak of the noise energy intensity, the driving frequency of the light source 11 may be 1 kHz or more, or may be 10 kHz or more.

[0042] Moreover, the gas cell 40 has a gas detection space 42 and a resonance mode that greatly amplifies sound waves of a specific frequency (natural frequency). It is known that the output of the detection unit 41, which is a microphone, can be increased by making the driving frequency of the light source 11 substantially equal to the natural frequency of the resonance mode of the gas cell 40. In other words, the light source 11 emits light at a driving frequency determined based on the natural frequency of the resonance mode of the gas cell 40, so that the microphone can pick up a loud sound based on the vibration of gas molecules that have absorbed the light. This makes it possible to reduce the influence of noise and detect the gas to be detected with high accuracy.

[0043] Regarding the light emission state of the light source 11, if the light emission state is ON and the non-emission state is OFF, the drive frequency of the light source 11 is determined by the ON interval. In this embodiment, a drive signal is periodically applied to the light source 11, which is an infrared LED, and the light source 11 periodically repeats ON and OFF. In the right diagram of FIG. 2, the ON interval is indicated by a period t. The drive frequency of the light source 11 is given as the reciprocal of the period t. Here, the duty ratio (the time when the light source is ON relative to the period t) is not limited, but may be set to be 10% or more and 50% or less, for example. As described above, the light source 11 operates at a high drive frequency, so that it is prone to heat generation and is susceptible to the effects of aging. Therefore, it is necessary to make corrections according to changes in the amount of light.

[0044] The photoacoustic gas sensor 1 is configured to measure the concentration of the target gas based on a detection signal from the detection unit 41, and to be able to correct the measured concentration of the target gas based on the amount of light emitted by the light source 11. Specifically, the signal processing unit 50 is configured not only to acquire the detection signal from the detection unit 41 for calculating the concentration of the target gas, but also to acquire a signal corresponding to the amount of light emitted by the light source 11 from the photodetector 31 for correction. The signal processing unit 50 may correct the measured value of the concentration of the target gas based on the amount of light emitted by a known method (for example, the method described in Japanese Patent No. 6010702). As described above, the light source 11 is susceptible to the effects of deterioration over time, but the measurement accuracy can be improved by correction based on the amount of light emitted.

[0045] In this embodiment, the photoacoustic gas sensor 1 is configured to be able to correct the measured concentration of the target gas based on the temperature calculated from the amount of light emitted by the light source 11 and the resistance value of the resistor included in the photodetector 31. Specifically, the signal processor 50 is configured to further acquire a signal corresponding to the temperature of the light source 11 from the photodetector 31 for the correction. The signal processor 50 may also perform temperature correction by a known method. As described above, the light source 11 is prone to heat generation, but the measurement accuracy can be further improved by temperature correction.

[0046] In this embodiment, the signal processing unit 50 includes a driving source that outputs a driving signal to drive the light emitting element. The light source 11 emits light in accordance with the driving signal from the driving source. The driving signal may change in accordance with the signal output from the photodetector 31. That is, the signal processing unit 50 may change the duty ratio of the driving signal, etc., in accordance with a signal in accordance with the amount of light emitted by the light source 11. In this case, it is preferable that the signal processing unit 50 changes the duty ratio within the above range so as not to cause unstable operation. That is, it is preferable that the driving signal changes so that the duty ratio is 10% or more and 30% or less.

[0047] In the photoacoustic gas sensor 1, the distance of the wiring electrically connecting the photodetector 31 and the amplifier (signal processor 50 in this embodiment) may be configured to be shorter than the distance of the wiring electrically connecting the light source 11 and the drive source (signal processor 50 in this embodiment). The signal output of the photodetector 31 is smaller than the drive signal to the light source 11 (for example, the current value is smaller or the amplitude of the voltage value is smaller). Therefore, it is preferable to arrange the photodetector 31 and the amplifier as close as possible to each other.

[0048] As described above, the photoacoustic gas sensor 1 of this embodiment has the above-mentioned configuration, whereby the photodetector 31 detects the amount of light emitted by the light source 11 and can make corrections in accordance with changes in the amount of light, thereby enabling the detection of the gas to be detected with high accuracy.

[0049] Although the embodiments of the present disclosure have been described based on the drawings and examples, it should be noted that those skilled in the art can easily make various modifications or corrections based on the present disclosure. Therefore, it should be noted that these modifications and corrections are included in the scope of the present disclosure. [Explanation of symbols]

[0050] 1. Photoacoustic gas sensor 11 Light source 11a Light-emitting surface 12 Light source board 23 Sealing part 30 Substrate 30a Main surface 31 Photodetector 40 Gas Cell 41 Detection unit 42 Gas detection space 43 holes 50 Signal processing section

Claims

1. A light source that emits light; a photodetector for detecting an amount of light emitted by the light source; a detection unit that detects vibration sounds of gas molecules of the detection target gas that have absorbed the emitted light and outputs a detection signal; a photoacoustic gas sensor configured to measure a concentration of the detection target gas based on the detection signal, and to be able to correct the measured concentration of the detection target gas based on the amount of light emitted by the light source.

2. 2. The photoacoustic gas sensor according to claim 1, wherein the measured concentration of the detection target gas can be corrected based on a temperature calculated from an amount of light emitted by the light source and a resistance value of a resistor included in the photodetector.

3. 3. The photoacoustic gas sensor according to claim 1, wherein the signal output from the photodetector uses the same physical quantity as the detection signal.

4. 3. The photoacoustic gas sensor according to claim 1, wherein the photodetector is integrated with a light source.

5. the light source emits the light in accordance with a drive signal; 3. The photoacoustic gas sensor according to claim 1, wherein the drive signal varies in response to a signal output from the photodetector.

6. 6. The photoacoustic gas sensor according to claim 5, wherein the drive signal changes so that a duty ratio is not less than 10% and not more than 50%.

7. A driving source that outputs the driving signal and an amplifier that amplifies the signal output by the photodetector are used together, 6. The photoacoustic gas sensor according to claim 5, wherein a distance of a wiring electrically connecting said photodetector and said amplifier is shorter than a distance of a wiring electrically connecting said light source and said driving source.

8. 3. The photoacoustic gas sensor according to claim 1, wherein the detection portion is covered with a metal housing.

9. a gas cell having a hole through which a gas passes and introducing the gas from the hole into a gas detection space; the light source, the light detector, and the detection unit are disposed inside the gas detection space, 3. The photoacoustic gas sensor according to claim 1, wherein the light source emits light at a drive frequency determined based on a natural frequency of a resonance mode of the gas cell.

10. 10. The photoacoustic gas sensor according to claim 9, wherein the gas cell is made of a conductive material, is electrically connected to a substrate that holds the light source and the detection portion, and is fixed to a reference potential.

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