Resistive Particle Sensors Having Measuring Electrodes

Abstract

A sensor for determining the concentration of particles in gases, in particular of soot particles, has at least one substrate element, and a measuring area between at least one first and one second measuring electrode, the two measuring electrodes being configured so that by applying a voltage between the measuring electrodes, an asymmetric electric field is formed on the measuring area. The sides of the first and second measuring electrodes, facing one another, may not be parallel to one another, for example. Furthermore, at least one measuring electrode may have a structure along the side facing the other measuring electrode or along the finger electrodes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows an exemplary embodiment of a particle sensor having measuring electrodes situated on a substrate element, in top view.

FIG. 1 b shows another exemplary embodiment of a particle sensor having measuring electrodes situated on a substrate element, in top view, and

FIG. 2 a shows another exemplary embodiment of a particle sensor having measuring electrodes situated on a substrate element, in top view.

FIG. 2 b shows another exemplary embodiment of a particle sensor having measuring electrodes situated on a substrate element, in top view.

FIG. 2 c shows another exemplary embodiment of a particle sensor having measuring electrodes situated on a substrate element, in top view.

DETAILED DESCRIPTION

In a first exemplary embodiment according to FIG. 1a, sensor 1 for determining the concentration of particles in gases, in particular of soot particles, has a substrate element 5, on which a first 10 and a second 15 measuring electrode are situated as a measuring device. The space between measuring electrodes 10, 15 is used as measuring area 12, on which the particles to be detected are deposited. The two measuring electrodes 10, 15 are connectable to a measuring and control unit (not shown in the figures) via contacts 20, 25 and a voltage may be applied to them. The measured value changes as a function of the state of particle deposition on measuring area 20. The measured value of resistance (impedance) or current intensity value measured via measuring electrodes 10, 15 is a function of the measuring mode.

As explained previously, the soot concentration in a gas may ultimately be determined from the measured values. The two measuring electrodes 10, 15 are configured according to the exemplary embodiment and/or exemplary method of the present invention in such a way that by applying a voltage between measuring electrodes 10, 15 an asymmetric electric field is formed on measuring area 12. A symmetric electric field is characterized in that the field has a constant direction and intensity all over the field. Such a field is formed, for example, by the interdigital comb electrodes known from the related art. The individual finger electrodes are typically implemented by unstructured, linear track conductors, which are all parallel to one another. This results in a constant electric field between the finger electrodes.

However, as FIG. 1 shows, in sensor 1, sides 30, 35 of first 10 and second 15 measuring electrodes, facing one another, are not parallel to one another. Instead, the distance between first 10 and second 15 measuring electrode decreases or increases continuously along the electrode. This creates an area having sides 30, 35 of measuring electrodes 10, 15, situated closely next to one another, and an area having sides 30, 35 of measuring electrodes 10, 15 situated wide apart. The transition from one area to the other is smooth and continuous. A non-constant field is created by applying a voltage. Particles that deposit on measuring area 12 of sensor 1 cause a reduction in resistance between measuring electrodes 10, 15 by forming conductive paths and thus create a sensor current.

A conductive path is first produced in the area of sides 30, 35 situated close to one another. Since the distance between measuring electrodes 10, 15 is very narrow at this point, a relatively slight particle deposition is sufficient for forming a conductive path and triggering a measuring signal. The sensitivity of sensor 1 is thus increased. As further particles deposit, conductive paths are also formed between sides 30, 35 of measuring electrodes 10, 15, which are farther apart. Due to the percolation characteristics of the deposited soot, whenever an additional conductive path is completed, a stronger increase in conductivity of the entire measuring area 12 takes place, which may be determined via measuring electrodes 10, 15. A stronger signal increase is thus achieved over a longer time period than would be possible in the case of measuring electrodes arranged in parallel. After short-circuiting measuring electrodes 10, 15 along all sides 30, 35, further deposits additionally keep increasing the conductivity continuously, i.e., measurement is also possible during this phase. Since the special configuration and arrangement of measuring electrodes 10, 15 allows a larger measuring area 12 to be formed for particle deposition, higher currents may also be achieved before they reach the saturation range compared to previously known interdigital measuring electrodes. The sensor signal is thus strengthened.

A varying distance between the finger electrodes may also be achieved in a conventional interdigital comb structure by modifying its shape. As FIG. 1b shows, at least one measuring electrode 10, 15 may have finger electrodes 40 having varying widths. While in FIG. 1a, first and second measuring electrodes 10, 15 are triangular, in FIG. 2b individual finger electrodes 40 of a measuring electrode 10, 15 are triangular. The distance between two adjacent finger electrodes 40 thus changes continuously along the length of finger electrodes 40. This yields the same advantageous effects as described for the first embodiment. The pointed design also produces areas having a controlled direction of preferential growth of the deposited soot particles.

All exemplary embodiments described so far have constantly smooth, unstructured sides of measuring electrodes 10, 15 or of individual finger electrodes 40. Alternatively (FIG. 2a) or additionally (FIGS. 2b, 2c), i.e., combined to form a varying distance between measuring electrodes 10, 15 or finger electrodes 40, it is proposed that at least one measuring electrode 10, 15 has a structure 45 along side 30, 35 facing the other measuring electrode 15, 10 or along finger electrodes 40. Structure 45 is formed by regularly arranged tips, squares, dots, or other geometric shapes. Such structures 45 on the electrode sides result in increased field step-up when a voltage is applied. Structured finger electrodes 40, as in FIG. 2a, alone result in a non-constant electric field on measuring area 12. This increase in field step-up causes polarizable or already charged particles to deposit preferentially compared to electrodes without structured sides for the same voltage applied. The particle deposition rate thus increases due to the increased field gradients. Consequently, higher sensor currents are achieved for a given particle concentration. This may make the use of simplified measuring electronics in the control unit for signal analysis possible, since leakage currents or EMC (electromagnetic compatibility) currents have only a slight interfering effect.

Summarizing, measuring electrodes 10, 15 are configured in all embodiments in such a way that by applying a voltage between measuring electrodes 10, 15 an asymmetric electric field is formed on measuring area 12. The asymmetric electric field is an electric field that is non-homogeneous in space. The special design of the field distribution makes targeted particle deposition control in space possible. In particular, the formation of conductive paths in preferred areas may be controlled. Path growth over time may also be steered in a desired direction. If necessary, more than two measuring electrodes 10, 15 may be provided for this purpose, for example, at least one central electrode (not shown in the figures) may be additionally provided between first and second measuring electrodes 10, 15. The geometric shape of and the potential applied to all electrodes is to be adapted to the desired field distribution.

Claims

1-8. (canceled)

9. A sensor for determining a concentration of particles in gases, comprising: at least one substrate element;a first measuring electrode; anda second measuring electrode, wherein there is a measuring area between the first measuring electrode and the second measuring electrode, wherein the first and second measuring electrodes are arranged so that by applying a voltage between the measuring electrodes an asymmetric electric field is formed on the measuring area.

10. The sensor of claim 9, wherein sides of the first and second measuring electrodes, facing one another, are not parallel to one another.

11. The sensor of claim 9, wherein a distance between the first and second measuring electrodes one of increases and decreases continuously along the electrodes.

12. The sensor of claim 9, wherein the first and second measuring electrodes together form an interdigital comb structure, and wherein at least one of the measuring electrodes includes finger electrodes having varying widths.

13. The sensor of claim 12, wherein one of the following is satisfied: (i) at least one of the measuring electrodes has a triangle form; and (ii) the finger electrodes of at least one of the measuring electrodes has the triangle form.

14. The sensor of claim 12, wherein at least one measuring electrode has one of (i) a structure along a side facing the other measuring electrode, and (ii) a structure along the finger electrodes.

15. The sensor of claim 14, wherein the structure is formed by at least one of tips, squares, dots, and geometric shapes that are regularly arranged.

16. The sensor of claim 9, wherein at least one central electrode is provided between the first measuring electrode and the second measuring electrode.

17. The sensor of claim 9, wherein the particles include soot particles.