WO1990013800A1

WO1990013800A1 - Electronic pressure sensor

Info

Publication number: WO1990013800A1
Application number: PCT/EP1990/000702
Authority: WO
Inventors: Heinz Wellhausen
Original assignee: Ab Elektronik Gmbh
Priority date: 1989-05-03
Filing date: 1990-05-02
Publication date: 1990-11-15
Also published as: DE3914555A1

Abstract

The invention relates to a pressure sensor with which an electric signal proportional to pressure may be generated, where, when an elastomeric body (1) is distorted, an electric current passing through it experiences a changing electrical resistance; the elastomer of the body (1) has a constant electrical conductivity independent of pressure. In the neutral state the body's small contact area (5) with a high contact resistance comes into contact with an electrically conductive pressure area (2) and the contact area (5) may be enlarged by applying a force P on the body towards the pressure area (2) with the distortion of the body, so that a reversible reduction in the contact resistance (R) may be produced. The body is spherical or cylindrical in shape.

Description

Electronic pressure sensor

The invention relates to a pressure sensor with which an electrical, pressure-proportional signal can be generated in that an electrical current conducted through this body experiences a variable electrical resistance when a elastomeric body is deformed.

For some time now, research has been concerned with improving the capabilities and possible uses of so-called tactile sensors (tactile or pressure sensors) (see articles by DARIO, P. and DE ROSSI, D .; "Tactile Sensors and the Gripping Challenge IEEE SPECTRUM 1985, pp. 46 - 52).

The tasks to be performed in the development are defined to design sensors with high spatial resolution and sensitivity, wide bandwidth, good linearity and negligible hysteresis, which also work in environments with rough influences. On p. 48, lsp., Of the aforementioned article, some sensors are described which work with elastomeric bodies with piezoelectric properties. However, these sensors apparently could not fulfill the intended task, because of their success - Rich market launch has not become known.

To solve the above-mentioned problems, a new concept is proposed according to the invention, in which the pressure sensors of the type mentioned at the outset have the following characteristics:

a) The elastomer of the body has a constant, pressure-independent electrical conductivity, b) in the neutral state of a small contact area and high contact resistance, the body contacts an electrically conductive contact area, the contact area being increased when a force P is applied to the body and / or to the contact area with reversible deformation of the body,

c) which produces a reversible reduction in the contact resistance between the body and the pressure surface.

* • _

The concept of the invention is, on the one hand, to apply a more elastic, conductive body with a small contact surface, which is designed, for example, as a truncated cone or a spherical cap, to a hard, e.g. metal surface, so that the body deforms and the pressure surface between the body and the hard surface increases. In kinematic reversal, it is also possible to make the extended area out of an elastomer

To produce material of the type mentioned at the outset as a type of cushion and to press it into the elastic cushion surface with a small, hard and conductive body and thus to enlarge the pressure surface. The general idea of the invention is accordingly that an elastic material with constant conductance deforms when the pressure changes and the deformation changes the pressure surface and thus the electrical resistance.

Embodiments of the invention and the details of the subclaims are explained with reference to the figures. The figures show in detail:

Figure 1 shows the basic structure of a sensor in which a spherical body is located between two conductive pressure plates; Figure ia the geometric parameters according to FigLir 1;

FIG. 2 shows a second arrangement in which the body has a cylindrical shape and two conductive plates can be pressed on;

Figure 3 shows a third arrangement in which a ball or a cylindrical body is located between three plates;

FIG. 4 shows an embodiment in which a body is embedded in an elastic, non-conductive material;

FIG. 5 shows a first evaluation circuit of the sensor current;

FIG. 6 shows a second evaluation circuit for a network of sensors;

Figures 7 and 8 embodiments for the arrangement and attachment of bodies;

FIG. 9 shows a last embodiment,

Figure 10 shows an embodiment in kinematic reversal.

1 and 1a show a body 1, consisting of an elastomer with a defined electrical conductivity, which is located between two flat, conductive plates 2. The plates 2 are connected to electrical connections 3 and 4. The electrical resistance R of the body depends not only on its specific conductance k not only on the length 1 of the body but also on its cross-sectional area A.

This cross-sectional area A is made variable. The relationship applies R = l / (k ^ • A). ( 1 )

If an elastic ball - body 1 - is placed between two flat, highly conductive plates 2, this is referred to as a neutral state, with a small contact area and a high contact resistance. It is assumed that the elastomer of the body 1 does not change its specific conductance when pressure is applied. If a force P presses on the upper circuit board 2 and thus on the body 1, the body deforms and the contact surface 5 with respect to the boards 2 is enlarged. In FIG. 1 a, the difference in length caused is denoted by d1, while the radius of the contact surface 5 is denoted by r. The elasticity of the elastomeric material of the ball provides a reproducible dependence on the path change dl in relation to the transition resistance. The change in length itself is irrelevant, but only the resulting change in area.

If the deformation caused by crushing is neglected, the relationship between the path length dl, the radius r _{0 of} the sphere and the radius r of the contact surface A can be found according to FIG.

(r ₀ -dl) ² + V ² = ro ² r ² = 2 • r ₀ • dl (1 - dl / (2 • r ₀ )) r ² ~ 2 • r ₀ - dl (2)

This makes the contact surface 5 (A):

A = r ² • π = 2 • r ₀ • dl • π

The conductance G of the arrangement is then:

G = k ^• 2 ^• r ₀ ^• π • dl / 1 ^~ k • π • dl with r ₀ ~ 1 /; The conductance is therefore directly proportional to the change in length dl! If a voltage U is applied to the sensor, the current I

I = U - G = U «k« π - dl.

It should be noted that the current dependence is linearly dependent on the change in length, but is practically independent of the diameter of the ball. As a result, the sensor has basically the same sensitivity for every size, so it can be as small as desired.

The only non-linear variable is the spring force of the body 1. In order to cover the same distance, ever greater force changes are necessary. This causes the sensor sensitivity to be greatest initially, since only small forces oppose the deformation. This is a major advantage of the sensor principle.

The use of the material Simit (trademark) from Carl Freudenberg of type 50 VMQ 141 002 has proven to be advantageous. This has a specific resistance of about 10 Ohm • cm: Here there is the following dependency between current and change in length:

I = 31 • U • dl [mA / (V • mm)].

This means that regardless of the size of the sensor, a current change of at least 310 μA occurs at a voltage of 1 V and a path change of 0.01 mm

As is readily apparent, the inventive principle can also be implemented for other bodies provided with a pointed or rounded surface, for example for a cylindrical body as shown in Figure 2 is shown. Here, the body 1 rests as a cylindrical strand on two electrically separated plates 2. If a deforming force P is applied from above, for example via a non-conductive plate (not shown), the body 1 deforms and the pressure surface on both plates 2 is increased in the same way. FIG. 9 shows an embodiment in which the body 1 is designed as a pyramid or truncated cone. The tip presses on a conductive plate 2 and deforms when a force P is applied to the base of the pyramid. The dependence on deformation, force and change in resistance can be determined empirically in a simple manner.

A further embodiment that can be used in practice is shown in FIG. In this case, an approximately spectacle-shaped configuration for the body 1 is selected in cross section. Two hemispherical individual bodies are conductively connected to one another by a connection 6. The spherically rounded parts of the hemisphere rest on two contact plates, 2, 2 '. The body itself is additionally embedded in an elastic, non-conductive material 7, for example in the coating of an upper side of a robot gripping arm. In ^'contacting a surface of the contacts 2 2'auf meet these and the body 1 begins to deform. Both contacts are on the same side, so that the printing effect doubles.

It is also possible to connect several bodies to form a star connection. FIG. 3 shows an exemplary embodiment. A directional dependence of the pressure load can be achieved here, since several contact surfaces 2 distributed in space have been used. The sensor current is evaluated with a simple circuit according to FIG. 5. The current I through the sensor with the resistance designation R3 also flows through the resistor R and causes a voltage drop corresponding to its size, which appears as a change in the output voltage:

U2 = Ui - I ^• R-.

An arrangement of many sensors can be evaluated using a crossbar distributor according to FIG. 6. The evaluation can be made possible here by connecting a diode in series with each sensor. It serves for decoupling, but otherwise has no significant influence on the current. According to the current state of the art, it is not difficult to query at least 10 sensors in succession within one millisecond.

Linearity and accuracy play a secondary role for a push button sensor of the aforementioned type. It is essential that the sensor responds even with low forces and can be built very small. The individual bodies can be less than 1 mm in size:

An arrangement which is particularly suitable for the construction of measurement network circuits is shown in FIGS. 7 and 8. In this case, individual spherical and cylindrical bodies 1, 1 ′ are pierced and threaded onto a conductive strand 11 or a conductive wire so tightly that good contact is established between the interior of the body and the strand. An arrangement of pressure surfaces 2, 2 'associated with the ball arrangement is then produced by arranging separate surface elements below each body 1, which are supported by a non-conductive base. In addition, you can Rubber supports 8 are installed between the strands, which prevent undesirable deformation of these strands. If a force P now presses on the arrangement of body 1, 1 'via an arbitrarily shaped structure 9, some bodies become strong, others little, others not deformed at all. This arrangement can be tapped in the usual way via a query unit coupled to electronic data processing, so that the individually addressable bodies can be converted into a corresponding value matrix and stored. With the aid of this value matrix, evaluations can then be carried out via corresponding programs, so that the shape of the structure 9 can be recognized from the printing parameters. For example, a light ball can be distinguished from an equally heavy ring structure.

The arrangement of the sensors can, for example, be arranged on a surface-shaped sensing arm, so that, for example, a robot distinguishes the parts lying on a conveyor belt with the criteria "ring-shaped" and "spherical" and removes these parts in a controlled manner by * corresponding control of gripping arms and can sort. As already indicated, such row-column arrangements of sensor bodies can also be embedded in elastic, non-conductive supports, so that a sensor arrangement is created which simulates the surface of the human skin and in principle also mimics its "feel". In this case, the sensors are largely protected against external influences, since they only protrude from their substance with their contact surface and even consist of a material that is largely insensitive to common, destructive influences that are not tolerated by other sensors. FIG. 10 shows an embodiment in which a pillow 22 is placed on a conductive base 20 via a contact layer 21. The pillow consists of an elastomeric, conductive substance, similar to that described for the body 1, 1 ¹ . A metal tip 23 penetrates into the surface of the cushion 22 using a force P, the pressure area between the cushion 22 and the metal tip 23 increasing. This reduces the electrical resistance. This arrangement can also be arranged in configurations similar to those in FIGS. 6 to 8 above and thus as a basis for. serve a pressure sensor.

In the case of the exemplary embodiment according to FIG. 10, the "body" is accordingly the elastomeric cushion 22, while the pressure surface is the metal tip 23.

Claims

- ..0 claims

1. Pressure sensor with which an electrical, pressure-proportional signal can be generated, by deforming an elastomeric body (1, l ¹ ), an electrical current conducted through this body experiences a variable electrical resistance, characterized in that a) the Elastomer of the body (1, 1 '; 22)) has a constant, pressure-independent electrical conductivity,

b) the body (1, 1 ', 22) neutral state with a small contact area (5) and a high contact resistance contacts an electrically conductive contact surface (2, 2'; 23), the contact area (5) when a force is applied P is to be enlarged on the body and / or on the pressure surface while deforming the body or the pressure surface,

c) which produces a reversible reduction in the transition resistance (R) between the body and the pressure surface.

2. Pressure sensor according to claim 1, characterized in that the body (1) is designed as a ball, cylinder or truncated cone.

3. Pressure sensor according to claim 1, characterized in that the pressure surface is designed as a body (23) with at least one pointed or rounded small contact surface with which the pressure surface is on 1 presses on a body which is shaped as a plate-shaped element (cushion 22).

'-!

4. Pressure sensor according to claim 1 or 2, characterized in that the body (1) is embedded in a non-conductive, elastic support (10), the contact surface being open to the conductive pressure surface.

10 5. Pressure sensor according to claim 1, characterized in that a plurality of sensors or bodies (1) in at least one row, preferably in rows and columns, arranged in a network-like manner and addressed electronically can be interrogated.

15

6. Pressure sensor according to claim 5, characterized in that the sensors are decoupled by diodes and can be queried via a crossbar distributor.

20 7. Pressure sensor according to claim 5, characterized in that the body (1, 1 ') are pierced and threaded on conductive strands (11) or wires.

8. Pressure sensor according to claim 5, characterized in 25 that the pressure surfaces as separate surface elements

(2, 2 ') are arranged below each body and are supported by a base (12).

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