DE19831414C1 - Filter comprises particle load detector providing continual indication of filter condition in safety-critical applications without interruption to filter operation - Google Patents

Abstract

The filter assembly has a particle filter and/or an adsorption filter. The filter assembly incorporates a mechanical resonator (18), which may be formed by a piezo-ceramic resonator (20) or a flexural resonator. The mechanical resonator is charged with a given quantity of the particle and/or adsorbent filter medium (34).

Description

The invention relates to a filter device with a Particle filter and / or with an absorption filter.

The loading status of particle filters can be checked using a Differential pressure measuring device can be determined. For this purpose the pressure difference between the upstream and downstream sides of the filter measured. With increasing particle loading, it builds up namely in the filter, a so-called particle cake, through which the open, d. H. the filter area of the filter is reduced and thus the pressure drop through the filter is increased.

The loading status of particle filters can also be determined, for example, by the Application of electromagnetic radiation can be determined in the z. B. is visible light or gamma radiation. Here, the filter material is introduced into the radiation and the attenuation of the intensity of the transmitted radiation measured. In this method, the expenditure on equipment is relative large.  

DE 295 07 061 U1 describes a device for testing Filter media for dust, containing a holder for a Filter medium that has a front and a back, a raw gas duct that connects to the front of the filter medium is connected to a gas supply via a mixing section the raw gas channel is connected, a dust metering device is connected to the mixing section, and a suction channel that the back of the filter medium is connected and the one Filters and an extraction system are connected downstream. At this known device, the raw gas channel is arranged horizontally, are the gas supply, the dust metering device and the Extraction system can be controlled electrically, and is the device with a measuring and control unit equipped with the Gas supply, the dust metering device and the suction system is electrically connected. This device can with at least be equipped with an electronically queried pressure transducer, which is electrically connected to the measuring and control unit. Likewise, it is possible that this device with at least equipped with an electronically queried dust measuring device is, which is electrically connected to the measuring and control unit. This at least one dust measuring device can be optical sensors contain.

From the Patent Abstract of Japan 63-274422 (A) is a detector for an air filter known, wherein in a flow channel of the Air filter a fan is arranged. In the flow channel also arranged a piezo element. The flow pressure of the Air flowing through the flow channel creates a on the piezo element electrical voltage on one with the piezo element interconnected electronic circuit is present. The The air filter is activated accordingly Air flow reduced. This reduction in air flow is associated with  the piezo element and in the electronic circuit processed to the respective loading of the air filter determine.

The loading state of adsorption filters is measured usually by taking breakthrough curves. Here the concentration of the substance to be filtered is measured  Harmful gas downstream after the adsorption filter. This Concentration measurement of the harmful gas takes place, for example, with the help of Gas detection measuring devices such as flame ionization detectors, electrochemical gas sensors or the like. With these Gas detection measuring devices, the residual activity of the - Sorption layer of the adsorption filter measured. To accommodate the said breakthrough curves it is necessary to use the adsorption filter to be measured in suitable measuring equipment.

Common to these procedures is that they are destructive Measurements or tests. That means that the respective Filter after this test is no longer usable. If necessary, the filter can be regenerable, i. H. be regenerated. The relatively high cost of such a measurement are often out of proportion to the cost of one unused new filter.

Especially when using such filters of the type mentioned above in security areas or when observing safety-relevant laws and requirements require about the respective load status of the filter at all times to be informed - if possible without the filtration process interrupt - in order to exchange the respective one in time Filters or a regeneration of the same.

The invention lies in the knowledge of these circumstances, the task based on a filter device of the type mentioned create a measurement with simple means at any time the loading condition is possible.

This task is the beginning of a filter device mentioned type according to the invention solved in that the Filter device for determining their loading condition  has mechanical resonator with a certain Provide quantity of a particle and / or adsorber filter medium is.

The filter device according to the invention advantageously enables Way at any time a determination of their loading condition. Particle and / or harmful gas loading, advantageously maintaining the measurement of the loading condition during the filtration process without destroying the filter device is possible. Further advantages of the invention Filter device consist in its inexpensive Manufacturability, in their maintenance-free and through integration of the mechanical resonator in the filter device, in the Interchangeability with each filter change. Through the Combination of the mechanical resonator with a defined, relatively small quantity of the filter medium, the results Advantage that the filter device according to the invention for the respective filter medium, d. H. both for particle or Dust pollution as well as for specific harmful gases in simple Way is applicable. E.g. can the invention Filter device for the passenger compartment of a motor vehicle be provided.

In the filter device according to the invention, the mechanical Resonator preferably a piezoceramic resonator. Such Piezoceramic resonators come in a variety of different types Training as required and inexpensive available. The mechanical resonator can be designed as a bending oscillator. Such bending vibrators are, for example, as tube vibrators or as multi-layered leaf vibrator. Piezoceramic As is known, resonators consist of pre-polarized ones Piezoceramics based on a ferroelectric polycrystalline ceramic material. They are with electrodes  Mistake. By deformation of the piezoceramic can Electrodes an electrical voltage can be tapped (= Piezo effect). By applying an external electrical voltage is on the corresponding electrodes of the piezoceramic mechanical deformation of the same is possible (= inverse piezoelectric effect). Piezoceramic resonators have depending on the design, weight and material Resonance frequency as well as a certain amplitude of the Resonance frequency, which is known from the quality of the Excitation frequency and the excitation amplitude is dependent. By Apply a specific AC voltage with a defined one Excitation frequency and amplitude become one Piezoceramic resonator consequently vibrated. The the resonance frequency that arises is - as already mentioned has been - from the design used of the piezoceramic Dependent on resonators. The resonance frequencies of such Piezoceramic resonators are typically 30 kHz to 15 MHz.

According to the mechanical or piezoceramic Resonator from the particle filter and / or from the adsorption filter be provided separately, but it is preferred if the mechanical resonator integral part of the filter device is. Such training has the advantage that respective inexpensively available mechanical Resonator with the appropriate filter device to given Time is interchangeable and can be replaced by a new one.

According to the mechanical resonator with a defined small amount of adsorber material. To For this purpose, an adsorber granulate can be used on the mechanical resonator be glued in place with an adhesive. It is the same possible that a nonwoven attached to the mechanical resonator  is. This can be an electret fleece, which on mechanical resonator is glued.

According to the invention, this is used to implement a load sensor a defined small amount of the respective active Filter material fixed on the piezoceramic resonator. At Pollutant gas or odor filters thus become an adsorbent medium such as activated carbon and particle filters, for. B. a Non-woven fabric attached to the piezoceramic resonator. Are there of course depending on the design and the location also consider aerodynamic criteria. As a result of the small Overall dimensions of the respective piezoceramic resonator can this advantageously both an integral part of the Filter device and arranged in the outer area of the same be - as has already been mentioned -.

The mechanical resonator is expediently on a holder provided the connection contacts for an excitation and Has evaluation circuit. By appropriate arrangement of the Connection contacts is both a vibration excitation accordingly the inverse piezo effect as well as an amplitude measurement possible according to the normal piezo effect. By putting on a suitable external electrical voltage to the excitation The piezoceramic resonator is suitable for connecting contacts deformable. The induced corresponding to these deformations Voltage can be on the connection contacts for the evaluation circuit be tapped. The absolute amount of the induced voltage is depending on the vibration amplitude of the piezoceramic Resonators and its piezoelectric characteristics like that piezoelectric coupling factor and the charge Voltage constants, which in turn depend on the direction of polarization within the crystal structure of those used Piezoceramic depends.  

The mechanical resonator forming a load sensor, the for example, could also be formed from a quartz crystal, but preferably formed by a piezoceramic resonator is combined with the filter device in such a way that with the filter medium of the filter device also the Resonator with particles to be filtered and / or with too adsorbing harmful gas is loaded. By this loading, i.e. H. the natural frequency changes due to the mass increase or increase, d. H. the resonance property of the piezoceramic resonator. In The first approximation is the frequency change to the mass increase proportional. That leads to a defined constant Excitation frequency, d. H. Eigen-resonance frequency, to one decreasing vibration amplitude. That means that Resonance increase of the piezoceramic resonator accordingly decreases. This decrease in the vibration amplitude or According to the invention, excessive resonance can be used to determine the Loading state of the filter device are used, the Excitation and evaluation circuit on the output side of an oscillator and on the input side a rectifier circuit and one with the Rectifier circuit connected threshold circuit with a Signal output has.

The period between the two extreme load conditions of one Filter device, namely between the unloaded and the fully loaded filter device, is from the concentration of the Harmful gas in the air flowing through the filter. In order to is the amplitude of the induced voltage of the piezoceramic Resonators a measure of the mass occupancy of the filter device itself. Due to the fact that commercially available piezoceramic Resonators have a Curie temperature of at least 300 ° C, is the filter device according to the invention also for desorbable  Filters can be used if the filter is desorbed by a suitable filter Annealing of the filter medium is carried out.

Through the mechanical resonator or through the piezoceramic A resonator of the filter device according to the invention results robust and compact structure, easy adaptability of the Resonators to the respective filter material by simple selection of the mechanical resonator in a defined small amount to be attached adsorber material or nonwoven, and by the fault-resistant and inexpensive measuring system, with which the resonator with which the filter device can be exchanged. According to the invention is an integral measurement of harmful gas and / or of particles possible. Due to the fact that the sensor and the measuring system is integrated in the piezoceramic resonator, there is the advantage of minimal effort for control of the piezoceramic resonator and for evaluating the Output parameters of the piezoceramic resonator.

The control and evaluation electronics are more advantageous Simply trained, it can be used with the Filter device according to the invention in motor vehicles without problems be integrated into their on-board electronics.

Further details, features and advantages result from the following description of one shown in the drawing Design of the filter device according to the invention or essential details or properties of the same. Show it:

Fig. 1 is a perspective view of an embodiment of the filter device with a polarized filter,

Fig. 2 is an enlarged representation of the detail II in FIG. 1 showing an embodiment of the mechanical or piezoceramic resonator of the filter device shown in FIG. 1,

Fig. 3 shows the piezoceramic resonator of FIG. 2 in combination with an associated excitation and evaluation circuit,

Fig. 4 shows a configuration of the piezoelectric ceramic resonator as a piezoceramic tube,

Fig. 5 shows the piezoceramic resonator of Fig. 4, by applying a DC voltage to illustrate the mechanical bending of the piezoceramic resonator of the application of the voltage

Fig. 6 in a spatial representation of a two-layered platelet-shaped piezoelectric ceramic resonator in the unloaded state,

Fig. 7 shows the piezoceramic resonator of Figure 6 after application of a voltage -., Similar to the piezoceramic resonator of FIG. 5

Fig. 8 shows the piezoceramic resonator of FIGS. 4 and 5, when an AC voltage is applied thereto,

Fig. 9 shows the functional relationship between the amplitude A and the time of the excitation ac t U1 and U2 of the induced alternating voltage of the respective piezoelectric ceramic resonator, and

Fig. 10 shows the functional relationship between the resonant amplitude Ra and the excitation frequency f of the piezoelectric ceramic resonator, depending on the respective mass occupancy of the resonator.

Fig. 1 shows a spatial representation of an embodiment of the filter device 10 with a polarized particulate filter 12 and the adsorption filter 14 which are provided in a frame 16. A mechanical resonator 18 , which is designed as a piezoceramic resonator 20 , is arranged on the upstream side of the filter device 10 . The mechanical or piezoceramic resonator 18 , 20 is fastened to a holder 22 , from which it projects cantilevered as a bending oscillator. The bracket 22 is attached to the peripheral frame 16 of the filter device 10 or an integral part thereof. On the bracket 22 , contacts 24 , 26 and 28 are attached, which are interconnected with the piezoceramic resonator 20 .

As can be seen from FIG. 2, in which the same details are denoted by the same reference numerals as in FIG. 1, the piezoceramic resonator 20 , which has two piezoceramic plates 30 and 32 which are connected to one another, is of a defined small amount Adsorber material 34 provided. The adsorption filter 14 is provided with the same adsorber material. The adsorber material 34 is, for example, activated carbon (approx. 46 mg) based on coconut with a specific surface area (BET surface area) of 1000 m ² / g to 1600 m ² / g, which is porous on the one hand due to a powdery thermoplastic Bending oscillator forming piezoceramic resonator 20 is glued. Depending on the relative humidity, the activated carbon, for example, adsorbs approx. 3 mg of n-butane, which is used as a test gas for harmful gases containing hydrocarbons.

The adsorber material 34 can - if the filter device 10 is in particular a particle filter 12 - be replaced by a non-woven material which is attached to the piezoceramic resonator 20 .

Fig. 3 shows the mechanical or piezo-ceramic resonator 18, 20 according to FIGS. 1 and 2 together with the associated excitation and evaluation circuit 36, the output side of an oscillator 38 and the input side of a rectifier circuit 40 and means connected to the rectifier circuit 40 threshold 42 has a signal output 44 . The oscillator 38 of the excitation and evaluation circuit 36 supplies the required excitation voltage with the required excitation frequency to the connection contacts 24 and 26 of the piezoceramic resonator 20 . A corresponding alternating voltage is induced in the piezoceramic resonator 20 , which is tapped at the connecting contacts 26 and 28 and converted into a direct voltage in the rectifier circuit 40 . The threshold circuit 42 connected downstream of the rectifier circuit serves to control the DC voltage present at the output of the rectifier circuit 40 . If this DC voltage drops below a predetermined voltage value, a warning signal is emitted via the signal output 44 . The signal output 44 is connected, for example, to a light-emitting diode 46 or the like, which lights up and thus emits an optical warning sign when the voltage at the signal output 44 drops below the specified value mentioned. In this way, it is indicated when a change of the filter device 10 or a desorption thereof is required.

FIGS. 4 and 5 schematically illustrate in a greatly enlarged scale, not to scale, an embodiment of the piezo-ceramic resonator 20 in the form of a tube oscillator with the inner surface 48 which completely covers the inner electrode 50, and with two diametrically opposite external electrodes 52 and 54. E.g. Such a tube vibrator can have an outer diameter of 2.57 mm, a wall thickness of 0.3 mm and a tube length of 8.7 mm. Such a tube vibrator has a resonance frequency of 74 kHz. E.g. Tube vibrators with an outer diameter of 3.57 mm, a wall thickness of 0.37 mm and a tube length of 4.9 mm are also commercially available. Such tube vibrators of the latter type have a resonance frequency of 98 kHz. Fig. 4 shows such a tube oscillator in the inactive condition, ie, without an excitation voltage is applied thereto. In contrast, FIG. 5 shows the piezoceramic resonator 20 designed as a tube oscillator when a direct voltage U = is applied between the electrodes 50 and 52 . This leads to a corresponding bending of the tube vibrator. This bending corresponding to the inverse piezoelectric effect results in an induced voltage due to the piezo effect between the electrodes 50 and 54 , which voltage is tapped and evaluated in the evaluation circuit 36 (see FIG. 3). For this purpose, the common inner electrode 50 is connected to the connection contact 26 , the outer electrode 52 to the connection contact 24 and the outer electrode 54 to the connection contact 28 of the holder 22 .

Figs. 6 and 7 illustrate highly enlarged and not to scale an embodiment of the piezo-ceramic resonator 20 in the non-excited state (see Fig. 6) and excited by applying a DC voltage state (see Fig. 7), which - as described above in connection has been described with Fig. 2 - has two interconnected piezoceramic plates 30 and 32 . An internal electrode 50 is also provided between the two plates 30 and 32 in this configuration of the piezoceramic resonator 20 . The two outer electrodes 52 and 54 are provided on the outer main surfaces of the piezoceramic resonator 20 facing away from one another.

Fig. 8 shows a greatly enlarged schematic view of the constructed as a tube vibrator piezoceramic resonator 20 shown in Figs. 4 and 5, when an excitation AC voltage is applied with a defined excitation frequency between the inner electrode 50 and an outer electrode 52. This leads to an oscillating deflection of the piezoceramic resonator 20 , which is indicated in FIG. 8 by the curved double arrow 56 . For this purpose it is of course necessary to clamp the piezoceramic resonator 20 on one side in a holder 22 (see FIGS. 1, 2 and 3) so that the free end of the piezoceramic resonator 20 removed from the holder can oscillate freely (curved double arrow 56 in Fig. 8). This oscillating movement of the piezoceramic resonator 20 induces an output voltage which is in phase with the excitation voltage between the electrodes 50 and 52 , as can be seen in FIG. 9, in accordance with the piezoelectric effect between the inner electrode 50 and the second outer electrode 54 . In Fig. 9, the excitation voltage is designated U1 and the induced output voltage U2.

Fig. 10 illustrates the relationship of the resonance amplitude Ra and the excitation frequency f as a function of the mass density of the piezoelectric ceramic resonator 20. Curve 58 illustrates the resonance signal response of piezoceramic resonator 20 without loading the same, and dashed curve 60 illustrates the resonance signal response when the piezoceramic resonator is loaded accordingly. As curve 58 shows, at the resonance frequency of the piezoceramic resonator 20, the oscillation amplitude rises steeply due to excessive resonance. The slope or the quality of the piezoceramic resonator and its operating parameters, ie the piezo constants, are to be adapted to the filter material according to the requirements of the measurement conditions, for example the number of masses and the capacity. The resonance increase, ie the difference between the excitation amplitude and the resonance amplitude, decreases with increasing mass occupancy of the piezoceramic resonator 20 . From this, a signal for the loading state of the piezoceramic resonator 20 and thus for the loading state of the filter device 10 can be derived directly.

If the piezoceramic resonator 20 is, for example, part of a particle filter 12 , the piezoceramic resonator 20 can be provided, for example, with a PP or PC electret non-woven material which is adhered to the surface of the piezoceramic resonator with a thermoplastic such as hot melt. The electret fleece stores dust particles, as a result of which the mass increases and the resonance frequency of the piezoceramic resonator decreases, as was explained above in connection with FIG. 10. If the piezoceramic resonator 20 is used in an adsorption filter 14 , the piezoceramic resonator 20 can be combined with activated carbon based on coconut or the like, as has also already been explained. The activated carbon adsorbs harmful gas, which leads to an increase in mass. This increase in mass results in a change in the resonance properties of the piezoceramic resonator, ie the resonance frequency and in particular the amplitude of the resonance signal response decrease. In a first approximation, the change in frequency is proportional to the change in mass. If the excitation frequency is kept constant, this leads to a decrease in the oscillation amplitude. The resonance increase therefore decreases. This decrease in the excessive resonance is evaluated in the excitation and evaluation circuit 36 . This can be part of the on-board electronics if the filter device 10 is used, for example, in the passenger compartment of a motor vehicle.

Reference list

10th

Filter device

12th

Particle filter (from

10th

)

14

Adsorption filter (from

10th

)

16

Frame (from

10th

)

18th

mechanical resonator

20th

piezoceramic resonator

22

Bracket (from

18th

,

20th

)

24th

Connection contact (at

22

)

26

Connection contact (at

22

)

28

Connection contact (at

22

)

30th

piezoceramic plate (from

20th

)

32

piezoceramic plate (from

20th

)

34

Adsorber material (an

18th

)

36

Excitation and evaluation circuit (for

10th

)

38

Oscillator (from

36

)

40

Rectifier circuit (from

36

)

42

Threshold switching (from

36

)

44

Signal output (from

42

)

46

LED (on

44

)

48

Inner surface (of

20th

)

50

Inner electrode (on

48

)

52

Outer electrode (from

20th

)

54

Outer electrode (from

20th

)

56

curved double arrow

58

Curve

60

Curve

Claims (10)

1. Filter device with a particle filter ( 12 ) and / or with an adsorption filter ( 14 ), characterized in that the filter device ( 12 ) has a mechanical resonator ( 18 ) for determining its loading state , which resonates with a certain quantity of a particle and / or adsorber filter medium ( 34 ) is provided. 2. Filter device according to claim 1, characterized in that the mechanical resonator ( 18 ) is a piezoceramic resonator ( 20 ). 3. Filter device according to claim 1 or 2, characterized in that the mechanical resonator ( 18 ) is designed as a bending oscillator. 4. Filter device according to one of claims 1 to 3, characterized in that the mechanical resonator ( 18 ) is an integral part of the filter device ( 10 ). 5. Filter device according to claim 1, characterized in that the mechanical resonator ( 18 ) is provided with a defined small amount of adsorber material ( 34 ). 6. Filter device according to claim 5, characterized in that an adsorber granulate is glued to the mechanical resonator ( 18 ) by means of an adhesive. 7. Filter device according to claim 1, characterized in that a nonwoven is attached to the mechanical resonator ( 18 ). 8. Filter device according to claim 7, characterized in that an electret fleece is glued to the mechanical resonator ( 18 ). 9. Filter device according to one of claims 1 to 8, characterized in that the mechanical resonator ( 18 ) is provided on a holder ( 22 ) having the connecting contacts ( 24 , 26 , 28 ) for an excitation and evaluation circuit ( 36 ). 10. Filter device according to claim 9, characterized in that the excitation and evaluation circuit ( 36 ) on the output side an oscillator ( 38 ) and on the input side a rectifier circuit ( 40 ) and a threshold circuit ( 42 ) connected to the rectifier circuit ( 40 ) with a signal output ( 44 ) having.