EP1127253A1

EP1127253A1 - Capacitive measuring sensor and method for operating same

Info

Publication number: EP1127253A1
Application number: EP99960864A
Authority: EP
Inventors: Günter DOEMENS; Markus Gilch
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1998-11-06
Filing date: 1999-11-05
Publication date: 2001-08-29
Also published as: WO2000028293A1; US6633172B1

Abstract

The invention relates to a capacitive measuring sensor which instead of detecting the distance between plane-parallel electrode surfaces detects a change in an angle between electrode surfaces positioned at an acute angle to each other and indicates capacitance changes in the form of a sensor signal. The sensor is preferably elongated, in its end regions has two fixing elements connected to a body to be measured and consists of two capacitor electrodes whose mutual angle changes are effected by a displacement between points of fixation on the object to be measured.

Description

description

Capacitive sensor and operating method

The invention relates to a capacitive sensor or sensor for measuring a force, a torque or an associated extension or torsion. Such a sensor is attached to a component on which the measured variables are to be detected.

Various types of capacitive torque or force sensors are known in the prior art. Torques and forces are generally measured in the industrial sector today with measuring cells which are equipped with strain gauges or the strain gauges are applied directly to the measuring point on a component. Due to the material combinations and the small size, the bonding of strain gauges requires a high degree of accuracy during execution. This is very cost-intensive, particularly in series production or in non-stationary applications, such as in automobiles. In the case of strain gauges, the adhesive points have significant weaknesses in terms of long-term behavior, since their functionality is greatly impaired by the influence of moisture and a permanent load resistance cannot be guaranteed.

Certain capacitive torque and force sensors are equipped with planar comb-shaped electrode structures, which convert a force or a torque with corresponding expansion or torsion into a change in capacitance. The displacements that occur here are only a few micrometers. The electrode spacing is changed in accordance with a shift at the measuring location. This application of a sensor is much easier compared to strain gauges and its long-term stability is also significantly better. This type of sensors can also be used on site, ie essentially in mobile units or systems, can be easily installed.

A general disadvantage of previous sensors is that their electrode spacing is used as a measured variable, since the setting of small electrode spacings is problematic. In addition, plane-parallel capacitor electrodes must be moved towards or away from one another while maintaining their mutual alignment. In the case of capacitive sensors, whose electrode structure is comb-shaped, two comb structures, which are insulated from one another, must be mounted and guided extremely parallel to one another. Furthermore, high precision in the division is required in the manufacture of the comb structures. These aspects limit a cost reduction that runs over the quantity. Furthermore, the minimum electrode spacings and thus the sensitivity of the sensor principle are limited by tolerances due to assembly.

The invention has for its object to provide a capacitive sensor with a simple structure, the sensitivity of which is improved compared to the prior art.

This problem is solved by the combination of features of claim 1.

The invention changes from the capacitive sensor principle according to the state of the art, in which the change in the distance from plane-parallel electrodes is evaluated, and uses the angle changes of electrodes or electrode surfaces arranged at an acute or wedge shape relative to one another. Measuring signals, in particular changes in capacitance to determine paths / displacements, torsions, as well as strains or forces and torques, can be determined. The only precision technology required here is the production of very flat, effective electrode surfaces that are opposite and capacitive work together. The entire arrangement can be referred to as a wedge or as a wedge-shaped structure. Sensors based on this principle do not have to be calibrated due to the fact that the capacitance follows the relationship c _* 1 / d (c = capacitance, d = electrode spacing) very precisely. The sensor arrangement has extremely low stray capacitances and low second-order capacitances. The already very low temperature dependence of capacitive sensors is further improved by the transition from an absolutely uniform parallel change in distance to a change in angle. A capacitance or its reciprocal is measured with corresponding changes when the electrode spacing changes.

In principle, the sensor is elongated and partly plate-shaped. It is in its end areas by means of a two-point connection, i.e. a non-positive connection, connected at each end to a body to be measured, for example a shaft. The orientation of the elongated sensor relative to the body to be measured occurs in such a way that displacements due to expansion or torsion offset the fastening points of the two-point connection between the sensor and the body to be measured. To display the capacitive sensor, a first electrode is connected to a measurement object by means of a fastening element. A carrier for second electrodes is also rigidly connected to this fastening element. The rigid support is therefore not directly connected to the measuring body. The first electrode is flat in its central area between the fastening points and is not parallel to the second electrode located on the carrier.

In an advantageous embodiment, the first electrode is only metallically conductive on the surface. The second electrode is expediently applied to an insulating carrier. All electrodes are shown as flat as possible on their effective surface. The advantages of such a capacitive measuring sensor consist in a high output capacity with a small construction volume. There is also a high degree of linearity (1 / C). A temperature drift of the zero point is very low. Thermal expansion is minimal. The sensor does not require any complex adjustment during assembly (precision adjustment of electrodes). Furthermore, the specimen scatter is low with regard to the transmission behavior, so that no calibration is necessary.

In order to achieve highly precise planarity, the parts of the capacitive sensor that represent the electrodes are produced by thin-film technology.

A further advantageous embodiment of the invention provides that the first electrode is simple and that two second electrodes cooperate capacitively on the carrier.

For miniaturization and for improved data transmission, an integrated circuit for capacitance measurement can be attached to the carrier, which outputs an output signal representative of the capacitance or its reciprocal.

To protect against aggressive environmental influences, the sensor can preferably be enclosed in a housing.

If the surface of the electrodes is flat and the imaginary intersection of the electrodes set at an angle or the electrode surfaces is equal to the pivot point for changes in angle, then a simple mathematical relationship applies to changes in the distance of the electrodes with respect to a change in capacitance.

Displacements or paths, torsions, strains, forces and

Torques can be measured reliably with such a sensor. Further exemplary embodiments are described below with the aid of schematic figures.

FIG. 1 shows a capacitive torque sensor for torsion angle measurement with an equivalent circuit diagram for the electrodes,

FIG. 2 shows a further schematic representation of the geometric relationships on the sensor with the derivation of the transfer function of the sensor,

Figure 3 shows the sensor in longitudinal section with housing, Figure 4 shows the sensor on a flex plate, for direct torsion angle detection,

FIG. 5 shows the sensor attached to a shaft, FIG. 6 shows a sensor arranged for force or strain measurement relative to the direction of force,

FIG. 7 shows an embodiment of a sensor for capacitive torsion angle measurement,

Figure 8 shows based on Füg. 7 shows a plan view of a carrier 16 without a first electrode 1,

Figure 9 shows a further embodiment of a sensor according to the invention.

In order to avoid the insulated assembly of the electrodes according to the prior art to one another, the wedge-shaped structure representing the sensor is verified according to the invention from an insulating and a metallic conductive part. The insulating carrier 16 carries two second electrodes 2, 3 applied using thin-film technology. The electrodes 2, 3 are capacitively coupled to a metallic part of the body of the first electrode 1 which is opposite at an acute angle. The first electrode located at the top in FIG. 1 is the counter electrode to the electrodes 2, 3. A capacitance is measured between the electrodes 2, 3. Edge areas or fastening elements 4, 5 outside of weak points 10 can consist of non-conductive materials. Direct contact between opposing electrodes that are angled are not employed, so that the acute-angled opening of the arrangement of Figure 1 does not fall below a minimum angle. In addition, the expansion of the electrodes 2, 3 on the insulating carrier 16 in the direction of the rigid mutual connection between the carrier 16 and the first electrode 1 is sufficiently limited.

FIG. 1 contains a representation of an equivalent circuit diagram with electrodes 1, 2, 3, between which a capacitance C is measured. The direction of movement 15 shown in FIG. 1, in this case of the first electrode 1, ensures an angle change between the electrodes 1 and 2 or 1 and 3, corresponding to the direction of movement 15 '. The corresponding capacities are connected in series. Also shown are: torsion direction 15, fastening elements 4, 5, weak points 10 and capacitance C. Fastening points 6 for the display of the fastening position on the measurement object are not shown.

In FIG. 2, the transfer function of the capacitive sensor is derived using a height h (deflection) / path x (electrode length) diagram. In the diagram, the x direction means the longitudinal extension of the electrodes 1, 2, 3. It should be noted that the geometric design of the sensor is expediently carried out as described here in order to achieve a sufficient distance between the fastening points 6. The rectangular, elongated flat design of the electrodes 1, 2, 3 is expedient, but could also be shaped differently. In the diagram in FIG. 2, the length of effective electrode areas corresponds to the path between 1 ^ and 1. A height h (x) can be represented by h ₀ ± kT or ho + Δh. The dependence of the torque T on a capacity C or a capacity change dC is shown in the formulas in FIG. The formula T shown in FIG. 2 below or the transfer function of the capacitive sensor for the torque measurement results from the resolution according to the torque T. The sizes used mean individual: k = elastic constant, εg = dielectric constant, b = width of the electrode, T = torque, C = capacitance, l ₂ -l _ι = length of the electrode; the indexing of the capacity for the idle state was made with 0 and for the operating state with T.

In Figure 3, a sensor is shown in plan view in a sectional view. The following are shown in detail: a one-piece first electrode 1 located at the bottom, which is designed to be electrically conductive at least in the central region. The first electrode has two fastening points 6 on fastening elements 4, 5, which are formed on opposite ends of the first electrode 1. Accordingly, contact to the body to be measured only exists in the end regions of the first electrode 1 located at the bottom in FIG Electrodes 2, 3 are detectable. In this case, the sensor is perpendicular to the surface of a body to be measured. The surface of the body to be measured lies, for example, in FIG. 3 in the paper plane and the sensor is mounted perpendicularly thereon, so that its width extends perpendicular to the paper plane and its longitudinal extension runs from left to right. If, according to the direction of movement 15, 15 ^>, a displacement is brought about on the object surface due to a force or a torque to be measured, a resulting change in capacitance can be detected with the sensor shown in FIG. 3, since the fastening points 6 shift relative to one another . This happens, for example, in such a way that the left attachment point 6 is static and the right attachment point 6 shifts in accordance with the direction of movement 5. Accordingly, there is a change in the capacitance of the electrode system in that the first electrode 1 changes the angle relative to the electrodes 2, 3. Due to the weak points 10 there is a transmission a torque from the right fastening point 6 on the fastening element 5 to the central region of the first electrode 1 is excluded. The relatively small changes in position with occurring forces, torques or strains thus cause an angle change due to movements at the weak points 10 of the first electrode 1 located at the bottom in FIG. 3. At least the left-hand point, shown in FIG rigid support 16 is attached, rigidly connected to the measurement object. The second attachment point 6 is preferably also rigidly connected to the measurement object. The middle part of the first electrode 1 should have no contact with the measurement object in order to be freely movable. The reference numeral 7 is representative of a housing that partially covers and closes the sensor flexibly and is placed on the first electrode 1. The housing 7 is intended to protect against harmful environmental influences. It must at least be dust and moisture proof.

FIG. 4 shows a representation corresponding to FIG. 3 of a capacitive sensor on a so-called flex plate 8, as is used on a transmission in automobile construction. The fastening points 6, the idling carrier 16, the first electrode 1 and the second electrodes 2, 3 are again shown. The direction of the torque T is indicated by an arrow. Torques or strains on the output of a gear unit can thus be measured.

5 shows the use of a capacitive sensor according to the invention on a shaft 9, with a torque T being measured. The torque T in turn causes strains occurring in the torsion direction 15, which can be detected in the area between the two fastening points 6 by corresponding displacement of these fastening points 6 relative to one another. The mode of operation is constant in that the contact of the sensor to the shaft 9 only in the end regions of the first electrode 1 in the Surrounding the attachment points 6 is present. The rigid support 16 is simultaneously rigidly connected to the first electrode 1 or to the fastening element 4 in the lower region. In this way, using a capacitive sensor, torques on a shaft can be measured and quantified by direct detection of the torsion angle.

FIG. 6 shows a capacitive sensor for the detection of a path by measuring or changing the angle. The structure corresponds to the description so far, the sensor being oriented with its longitudinal extent, in this case at 45 °, to the direction of force. This is due to the connection that the largest measurable relative displacements between the fastening points 6 occur at 45 °. The angled version of the first electrode 1 with its central part shown in all the figures does not amount to an operating or an idle state. What is essential is the constant wedge-shaped or acute-angle mutual orientation of the opposing electrodes 1, 2, 3 or torques can be measured in any direction according to the direction of movement 15, provided that the fastening points 6 are not positioned too close to one another.

In the arrangement shown in FIG. 4, the torque T increases the angle at the capacitive sensor. When looking at the cut sensor in FIG. 3, a direction of movement 15 of the right fastening point 6 on the fastening element 5 downward will also result in an increase in the angle in the sensor. If the right attachment point 6 in FIG. 3 is moved upwards from the idle state in the operating state, this means a reduction in the opening angle of the sensor. For an optimal measurement, the angle at the sensor must not exceed predetermined maximum values. A height difference Δh at the sensor according to FIG. 2 is 20 μm, for example. FIG. 7 shows a sectional illustration of a capacitive sensor according to the present invention. The first electrode 1 is positioned on the fastening elements 4 and 5 corresponding to the fastening points 6 on a measurement object in such a way that optimal or desired displacements between the fastening points 6 are brought about when the measurement object is subjected to mechanical stress. The one-piece first electrode 1 contains the fastening elements 4, 5, weak points 10 being interposed. On the insulating carrier 16, which is rigidly connected to the fastening element 4, but has no contact with the measurement object, an evaluation electronics 13 (chip) is applied in addition to the second electrodes 2, 3. An output signal 14 is discharged via a corresponding line. After the opening of the electrodes 1 and 2, 3 positioned at an acute angle to one another is very small, the first electrode 1 is shaped in such a way that an output signal can be tapped off by means of a line. The evaluation electronics 13 are in a recess in the carrier 16 together with the second electrodes

2,3 shown on a layer of adhesive 11 with high-resistance silicon applied thereon.

FIG. 8 shows a top view of the carrier 16 without the first electrode 1. The second electrodes 2, 3 are shown, which cooperate capacitively with the first electrode 1. The capacitance is tapped by the second electrodes 2, 3 and processed accordingly in an electronic evaluation unit 13. It can be seen in FIG. 8 that the individual elements on the carrier 16 are located on the continuous layer of silicon 12,

Figure 9 shows a further embodiment of the invention similar to Figure 7. In Figure 9, in contrast to Figure 7, the insulating support 16 is provided with fasteners 4,5. The first electrode 1 is, just as previously described, connected to the fastening element 4 in the end region. The first Electrode 1 is permanently angled to avoid contact between electrodes 1, 2, 3 and to represent an angle in the idle state. The electrodes are likewise made of metal here and are accordingly electrically insulated from the measurement object or from the fastening points 6. In this case, the first electrode 1 has no contact with the measurement object and the central region of the insulating carrier 16 is designed to be freely movable.

As mentioned, sensors with this principle do not have to be calibrated. One reason for this is the extremely low stray capacitance compared to other capacitive principles, as well as low second-order capacitance. The very low temperature dependency of capacitive sensors is further improved by the transition from a change in distance to a change in angle.

Particular advantages lie in the fact that instead of changing the distance or the overlapping area of a plate capacitor with parallel electrodes, changes in the angle of a sensor with capacitor electrodes arranged at an acute angle to one another are carried out or evaluated in the invention. An essential advantage can be seen through the use of a carrier 16 and a first electrode 1 in connection with an integrated circuit placed directly next to one another for the measurement of the capacitance. In this way, a microsystem can be implemented.

Claims

claims

1. Capacitive measuring sensor consisting of at least two corresponding flat capacitor electrodes which are positioned at an acute angle to one another and in which a path to be measured can be converted into a corresponding change in angle with a correspondingly evaluable change in capacitance.

2. Capacitive sensor according to claim 1, wherein

a first electrode (1) is formed in one piece at opposite ends, each with a fastening element (4, 5),

the fastening elements (4, 5) are rigidly connected to points on a measurement object, the relative change in position of which is to be measured,

the second electrode (2, 3) corresponding to the first electrode (1) is shown insulated on a carrier (16) which is rigidly connected to a fastening element, - the position of the first electrode (1) by relative movements between the fastening points (6 ) is changeable, resulting in a change in angle between the corresponding electrodes.

3. Sensor according to claim 2, wherein joints or weak points (10) are present between the first electrode (1) and the fastening elements (4, 5).

4. Sensor according to one of the preceding claims, wherein the angular electrodes have flat surfaces and their imaginary intersection represents the fulcrum for a change of wikens between the electrodes, so that the following relationship applies:

C = ε ₀ bl ₂ / (ho + Δh) • In l ₂ / lα with li, 1 ₂ as the start and end point of the electrodes from the pivot point, b the width, ε ₀ the dielectric constant, h ₀ the exit opening and Δh the path change.

5. Sensor according to one of claims 2 to 4, wherein the first electrode (1) is designed in the form of a metallically conductive layer on an insulating carrier (16).

6. Sensor according to one of claims 2 to 5, wherein the electrodes are designed in the form of thin layers.

7. Sensor according to one of claims 2 to 6, wherein two second electrodes (2, 3) are present, which are capacitively connected via a first electrode (1).

8.Measuring transducer according to one of claims 2 to 7, wherein the carrier (16) additionally has an integrated circuit (IC) for data acquisition.

9. Sensor according to claim 8, wherein insulated electrodes and evaluation electronics are realized in a single IC.

10.Messaufnehmer according to any one of the preceding claims, wherein the sensor is enclosed by a housing (7) which is dust and moisture proof.

11. Sensor according to one of the preceding claims, wherein a fastening element is stationary relative to the path to be measured and a displacement acts on the other fastening element which corresponds to the path to be measured.

12. Sensor according to one of the preceding claims, wherein the fastening elements (4,5) are applied to the surface of a measurement object.

13. Sensor according to one of the preceding claims, wherein the fastening elements (4,5) in the end regions of the Carrier (16) are attached and the first electrode (1) is rigidly connected to a fastening element.

14. Sensor according to claim 13, wherein the first electrode (1) between attachment and free. End has an angle to represent an acute angle within the sensor in the idle state.

15. A method for using a sensor according to one of claims 1 to 14, wherein changes in angle between capacitor electrodes result in a change in capacitance, which is converted into a change in path, which in turn is converted into forces, torques or strains in combination with material and geometry parameters of a test object.

16. The method according to claim 15, wherein the positioning of the sensor is predetermined by means of its fastening elements relative to the path to be measured according to the measurement task.