WO2003062801A1

WO2003062801A1 - Gas sensor comprising an analog evaluation circuit

Info

Publication number: WO2003062801A1
Application number: PCT/DE2003/000167
Authority: WO
Inventors: Georg Bachmaier; Oliver Hennig; Erhard Magori; Hans Meixner; Rainer Strzoda; Christian Tump
Original assignee: Siemens Aktiengesellschaft
Priority date: 2002-01-25
Filing date: 2003-01-22
Publication date: 2003-07-31
Also published as: DE10202918C1

Abstract

The invention relates to a gas sensor (1) comprising a laser diode (7) which is arranged at the beginning of an absorption measuring path (8), and a photodiode (9) which is arranged at the end of the absorption measuring path (8), the signal thereof being processed by an analog circuit (10, 13, 14, 16, 17, 22) into a surface measuring signal (23) corresponding to the surface (19) of absorption lines (12) in the absorption spectrum. Said surface measuring signal (23) is supplied to a comparator circuit (24) which compares the level of the surface measuring signal (23) with a nominal value and optionally triggers an alarm.

Description

description

Gas sensor with analog evaluation circuit

The invention relates to a gas sensor with a radiation transmitter arranged at the beginning of an absorption measurement section and an associated control circuit which tunes the emission wavelength of the radiation transmitter, and to a radiation receiver arranged at the end of the absorption measurement section, which is followed by an evaluation circuit.

Such gas sensors are generally known. Such gas sensors are described in the publication Erhard Magori, "Development of a Sensor System for Detecting Gases with Near Infrared Laser Diodes", University of the Bundeswehr, Munich, 2000. In the known gas sensors, the radiation transmitter is a semiconductor laser, the wavelength of which is periodically tuned using the control circuit. The gas present along the absorption measurement section absorbs the infrared radiation emitted by the radiation transmitter on the basis of the rotation and vibration band, so that the receiver arranged at the end of the absorption measurement section records an absorption spectrum. If the optical line is not completely saturated, the depth of the ^• absorption line can be used to determine the concentration of the associated gas molecules along the absorption measurement section. Conventional gas sensors use an analog-to-digital converter downstream of the receiver to evaluate the absorption spectrum, which converts the measurement signals generated by the radiation receiver into digital values and feeds them to a microprocessor.

In order to ensure reliable functioning of the gas sensor, conventional gas sensors are generally equipped with a device which stabilizes the temperature of the semiconductor laser serving as a radiation transmitter. The fluctuations in the operating temperature of the semiconductor laser also affect the wavelength of the emitted light that the absorption lines in the absorption spectrum seem to drift and can no longer be reliably assigned to the individual gas molecules. In addition, the known gas sensors have a high power consumption, which makes battery operation or operation in the bus system impossible without their own energy supply.

From the diploma thesis by Christian Tump, "Low-Power Gas Sensor based on the principle of absorption", University of Dortmund, 2001, gas sensors are known which are based on vertically emitting laser diodes (VCSEL) and do without the otherwise necessary temperature stabilization of the laser diode. With these gas sensors, the current consumption and the amount of circuitry have already been significantly reduced. However, a relatively powerful microcontroller together with a high-quality analog-digital converter is still necessary.

The known gas sensors are therefore not suitable for use in large numbers as portable devices.

However, there is a need for compact, portable, battery powered gas sensors. It happened in cellars with chemicals. for example, to serious accidents, because atmospheric oxygen has been displaced by chemical vapors and this was not noticed in good time by people entering the room. For this application, a compact, battery-powered oxygen sensor is required, which can always be carried in your shirt pocket, for example. Such an oxygen sensor may be installed in the respective rooms and would trigger an alarm, the oxygen concentration falls below a certain threshold .if ^'.

Another conceivable application is a methane sensor to detect leaks in good time in rooms with natural gas-powered devices. The methane sensor must be constructed in such a way that Exceeding a certain methane concentration alarm is triggered.

Based on this prior art, the invention has for its object to provide a compact, battery-powered gas sensor.

This object is achieved according to the invention in that the evaluation circuit comprises a signal processing unit which is supplied with the measurement signal supplied by the radiation receiver and generates an intermediate signal following the course of the absorption spectrum from the measurement signal, and in that the evaluation circuit has a surface integrator which is supplied with the intermediate signal and which consists of a surface measurement signal determined by the depth and width of absorption lines is generated for the intermediate signal, and that the evaluation circuit is followed by a comparison circuit which compares the surface measurement signal with a reference signal and outputs a detection signal at an output when predetermined limit values are exceeded.

In the gas sensor according to the invention, a surface measurement signal is generated with the aid of the area integrator, which is determined by the depth and width of absorption lines in the absorption spectrum. If the absorption lines can be assigned to a certain gas and are not in optical saturation, the area measurement signal is approximately proportional to the gas concentration. The circuit of the gas sensor according to the invention can be implemented with simple means without the use of digital switching elements. In addition, the gas sensor according to the invention does not require a device for stabilizing the operating temperature, since a drift in the absorption spectrum does not necessarily change the area measurement signal. The gas sensor according to the invention therefore requires little energy and can be operated from batteries. Since only a few simple components are required to manufacture the gas sensor are required, the gas sensor can be manufactured in large quantities as a compact, portable device.

In a preferred embodiment, analog components are used for the signal processing unit and the area integrator. By dispensing with digital components, such as microcontrollers and analog-digital converters, the manufacturing costs and the power consumption of the gas sensor according to the invention can be significantly reduced.

In a further preferred embodiment, a device is provided which detects the operating temperature of the radiation sensor and controls the comparison circuit in such a way that, depending on the operating temperature of the radiation receiver, an area measurement signal which is scaled differently with respect to a limit value is fed to a comparator.

In this embodiment, the temperature of the radiation transmitter is not actively regulated, which generally costs a lot of energy, but is actively compensated for by a simple circuit.

Further details are the subject of the dependent claims.

The invention is explained in detail below with reference to the attached drawing. Show it:

Figure 1 is a block diagram of a gas sensor;

Figure 2 is a block diagram of another gas sensor; and

FIG. 3 shows a block diagram of a comparison circuit that can be used for the gas sensors from FIGS. 1 and 2. FIG. 1 shows a block diagram of a gas sensor 1. The gas sensor 1 shown in FIG. 1 has a control logic 3 which is acted on by a clock signal 2 and which controls a ramp generator 4. The ramp generator 4 generates a sawtooth voltage 5, which is fed to a laser control 6. The laser controller 6 supplies a laser diode 7 with current, the time course of the current intensity corresponding to the course of the sawtooth voltage 5.

The light emitted along an absorption measurement path 8 is detected by a photodiode 9, which acts on an amplifier 10. The amplifier 10 generates a measurement signal 11, which is also sawtooth-shaped and shows a ramp-shaped course with absorption lines 12 within a period. In order to free the measurement signal 11 from high-frequency noise components, the measurement signal 11 is fed to a low-pass filter 13. The low-pass filter 13 smoothes the measurement signal 11 so that a differentiator 14 can generate a meaningful derivation signal 15 from the measurement signal 11. In order to free the derivative signal 15 from DC voltage components, the differentiator 14 is followed by a high pass 16, the output of which is connected to an integrator 17, which generates an intermediate signal 18.

The profile of the intermediate signal 18 corresponds to the course of an absorption spectrum, which was recorded with the aid of a light source, the radiation power of which is largely constant over the wavelength. If there is no optical saturation along the absorption measurement section 8, an area 19 of the absorption line 12 is approximately proportional to the column density of the associated gas molecules along the absorption measurement section 8. The absorption line area 19 is the area that is from an imaginary continuity line 20 and an absorption profile 21 of the absorption line 12 is included. A surface measurement signal 23 formed by a surface integrator 22 therefore indicates the column density of those gas molecules over whose absorption line was integrated. If the gas concentration along the absorption measurement section 8 is to be monitored, it is therefore sufficient to compare the surface measurement signal 23 with a target value and to trigger an alarm if the target value is exceeded or undershot. This task is performed by a comparison circuit 24, which is controlled by the control logic 3. The control logic 3 ensures in particular that the comparison circuit 24 compares the current value of the surface measurement signal 23 with a desired value at the end of a scan. The function of the comparison circuit 24 is also determined by a temperature measuring circuit 25, which detects the operating temperature of the laser diode 7. The temperature measuring circuit 25 provides for a scaling of the value of the surface measurement signal 23 corresponding to the operating temperature of the laser diode 7 or a corresponding scaling of the limit value, because a change in the operating temperature of the laser diode 7 also leads to a change in the wavelength of the light emitted by the laser diode 7. It can therefore happen that absorption lines 12 are no longer in the scanned wavelength range, so that the surface measurement signal 23 changes abruptly. The temperature measurement circuit 25, together with the comparison circuit 24, compensates for the changes in the surface measurement signal 23 due to the temperature drifts of the laser diode 7.

If, depending on the application, the value of the area measurement signal 23 is above or below a desired value, the comparison circuit 24 outputs an alarm signal to an amplifier 26, which amplifies it and provides it at an output 27.

FIG. 2 shows a further gas sensor 28 which has a modified signal processing unit. In the gas sensor 28, the amplifier 10 is followed by a low-pass filter 29, which extracts a ramp-shaped basic signal 30 from the measurement signal 11. The basic signal 30 corresponds to the measured signal 11 except for the averaged absorption lines 12. The measured signal 11 then becomes the basic signal 30 deducted with the aid of a subtractor 31 and in this way the intermediate signal 18 is generated. The intermediate signal 18 can then in turn be fed to the area integrator 22, which generates the area measurement signal 23 therefrom.

The remaining components of the gas sensor 1 are the same as the corresponding components of the gas sensor 1 shown in FIG. 1.

An exemplary embodiment of the comparison circuit 24 is shown in FIG.

In the comparison circuit 24, the temperature measurement _signal U _τ supplied by the temperature measurement circuit 25 is present at an input 32. At a further input 33, the comparison circuit 24 is supplied with a reference voltage U _Ref . By means of a chain 34 of resistors 35 connected to the input 33, the reference voltage 35 is divided down to a series of voltage values, each of which is applied to the inputs of comparators 36. The outputs ki to k _{7 of} the comparators 36 lead to sample-and-hold elements 37 which, when there is a corresponding signal at an input 38 of the comparison circuit 24, output a logic 1 or 0 at their output, depending on the voltage level present at their input. The control signal U _{E present} at the input 38 comes from the control logic 3 and ensures that the data are each adopted at the end of a scan. The logic values at the outputs of the sample and hold elements 37 are fed to inputs xl to x7 of a programmable decoder 39 which assigns logic values to the inputs xl to x7 and logic values at the outputs yl to y4. Switching elements 40, for example transistors, are actuated via the logic values at the outputs y1 to y4, by means of which voltage dividers formed by resistors 41 can be activated. With the aid of the voltage dividers formed by the resistors 41, the level U _{A of} the surface measurement signal 23 can be scaled such that a comparator 42 always issues an alarm signal when the Concentration of the monitored gas molecules along the absorption measurement section 8 exceeds a predetermined limit.

It should be noted that the sample comparator 42 can also be followed by a sample and hold element 43, which holds the output value of the comparator 42 for one scan period.

Furthermore, it should be noted that in a modified embodiment of the comparison circuit 24, it is not the level U _{Ä of} the surface measurement signal 23 but the reference signal U _{G that is} scaled. Of course, it is also possible to scale both the level U _{A of} the area measurement signal 23 and the level U _{G of} the reference signal.

The complex scaling of the level U _{A of} the area measurement signal 23 or level U _{G of} the reference signal is necessary insofar as the operating temperature of the laser diode 7 fluctuates and there is furthermore no linear relationship between the operating temperature of the laser diode and the level U _{A of} the area measurement signal 23. Because depending on the operating temperature of the laser diode, different absorption lines 12 can be scanned in the absorption spectrum. The area measurement signal 23 is correspondingly different. It is therefore entirely possible that, with the concentration remaining the same, the surface measurement signal 23 falls, or vice versa, as the operating temperature rises. By means of the programmable decoder 39, however, the comparison circuit 24 offers the possibility of adapting the gas sensors 1 and 28 to the respective application.

If the programmability can be dispensed with or a different scaling of the surface measurement signal 23 is required for each value of the temperature measurement signal U _τ , the decoder 39 can be dispensed with. In this case, the outputs of the sample and hold elements 37 are directly connected to the inputs of transistors 40 connected. Consequently, a number of transistors 40 corresponding to the number of sample and hold elements 37 is also necessary, but decoder 39 is not required for this.

In a preferred embodiment of the gas sensor 1 and the gas sensor 28, analog components for the laser control 6, the amplifier 10, the low pass 13, the differentiator 14, the high pass 16, the integrator 17 and the area integrator 22, as well as for the low pass 29 and uses the subtractor 31. Through these measures, both the manufacturing costs and the energy consumption of the gas sensors 1 and 28 can be reduced.

In a further modified embodiment of the gas sensors 1 and 28, the current ramps fed to the laser diode 7 have an increasing gradient towards the end of the period. This measure compensates for the effect that those absorption lines which are at wavelengths with high intensity of the laser radiation have a larger area than those absorption lines which are at wavelengths with low intensity of the laser radiation. Due to the increasing slope of the laser ramps, that part of the spectrum in which the laser radiation has high intensities is traversed faster, so that the absorption lines appear narrower in this part of the absorption spectrum. The increasing slope of the current ramps can therefore compensate for the effect of the area of the absorption line of the laser radiation which increases with increasing intensity.

Claims

claims

1. gas sensor with a radiation transmitter (7) arranged at the beginning of an absorption measurement section (8) and an associated control circuit (3) which tunes the emission wavelength of the radiation transmitter (7), and a radiation receiver (9) arranged at the end of the absorption measurement section (8), which is followed by an evaluation circuit, characterized in that the evaluation circuit comprises a signal processing unit (10, 13, 14, 16, 17, 29, 31) which is acted upon by the measurement signal (11) supplied by the radiation receiver (9) and from the measurement signal ( 11) generates an intermediate signal (18) following the course of the absorption spectrum, and that the evaluation circuit has a surface integrator (22) to which the intermediate signal (18) is applied and which determines from the intermediate signal (18) a depth and width of absorption lines (12) Surface measurement signal (23) is generated and that the evaluation circuit is followed by a comparison circuit (24) tet, which compares the surface measurement signal (23) with a reference signal and outputs a detection signal at an output (27) when predetermined limit values are exceeded.

2. Gas sensor according to claim 1, wherein the signal processing unit (10, 13, 14, 16, 17, 29, 31), the surface integrator (22) and the comparison circuit (24) are analog circuits.

3. Gas sensor according to claim 1 or 2, wherein the signal processing unit (10, 13, 14, 16, 17) comprises a low-pass filter (13) acted upon by the output signal of the radiation receiver (9), each of which has a differentiator (14), a high-pass filter (15) and an integrator (17) are connected downstream.

4. Gas sensor according to claim 1 or 2, wherein the signal processing unit (10, 29, 31) has a low-pass filter (29) acted upon by the output signal of the radiation receiver (9) and a subtractor (31) which is connected to the output signal of the low-pass filter (29 ) and with the output signal of the radiation receiver (9).

5. Gas sensor according to one of claims 1 to 4, in which the radiation transmitter (7) is acted upon by a control circuit (3) with a current ramp.

6. The gas sensor according to claim 5, wherein the slope of the current ramp varies.

7. Gas sensor according to claim 5 or 6, in which a temperature measuring device (25) generates a temperature measuring signal which characterizes the temperature of the laser diode (7).

8. Gas sensor according to claim 7, in which the temperature measuring device (25) controls the scaling of the surface measurement signal (23) carried out in the comparison circuit (24).

9. Gas sensor according to claim 8, in which the comparison circuit comprises a voltage divider chain (34) acted upon by the temperature measurement signal, the partial voltages of which are fed to comparators (36) which are connected in parallel and are connected to a decoder (39) which has different voltage dividers (41) for the Flat measurement signal (23) activated.