EP0868658A1

EP0868658A1 - Force measurement device

Info

Publication number: EP0868658A1
Application number: EP96944006A
Authority: EP
Inventors: Andreas Pauer
Original assignee: PFISTER WAAGEN GmbH
Current assignee: PFISTER WAAGEN GmbH
Priority date: 1995-12-19
Filing date: 1996-12-19
Publication date: 1998-10-07
Also published as: WO1997022865A1; DE19547472A1

Abstract

The aim is to produce a highly overload-resistant force measuring device at low cost, with a pressure sensor located between force introduction elements, embedded essentially transversely to the direction of the force to be measured, in an elastomeric material and in force-transmitting contact therewith. To that end it is proposed that the pressure sensor (5) should comprise at least one pressure-dependent film resistor (7) mounted on a substrate plate (6) and vulcanised so as to be in direct contact with the elastomeric material (4).

Description

description

Force measuring device

The invention relates to a force measuring device with a pressure sensor arranged between force introduction parts, which is embedded essentially transversely to the direction of introduction of the force to be measured in pressure transmission contact in elastomeric material.

Such a force measuring device is known from EP-A-0 205 509 and in the form based thereon from EP-A-0 496 956. Here, a uniform hydrostatic pressure is generated via a load introduction plate or a load introduction piston in an almost completely enclosed elastomeric material, which pressure is measured by the pressure sensor. The connection of the pressure sensor in the first-mentioned publication requires a certain amount of processing on the housing part and a precise manufacture of the connection surface of the elastomeric material (in particular rubber) in order to obtain exact, reproducible measurement signals. In the second-mentioned publication, while saving the processing of the housing part, it was proposed to store the pressure sensor in a floating manner in the elastomer material. This also has the advantage that the pressure sensor itself is well protected against environmental influences. However, here too, a support ring provided with a free space is required for mounting the sensor body itself, the support surface of which must be manufactured with high accuracy for a membrane.

The pressure sensors proposed here each have a deformation body in the manner of a measuring membrane. The membrane deformation into the free space necessary to allow the deflection is picked up by strain gauges on the back of the membrane. The thin and therefore fragile sensor membrane thus forms the weakest component of the complete force measuring device, so that for the

ORIGINAL DOCUMENTS Pressure sensor overall due to the required sensitivity of its membrane, a high-precision production is required to create defined tension / pressure zones. This makes such pressure sensors relatively expensive, especially for higher pressure ranges (over a hundred bar). In addition, there are certain problems in embedding the floating pressure sensor directly in the egg-elastomeric material, since relatively high process pressures of 600 bar and more and high temperatures are used for the vulcanization of the elastomeric material (in particular rubber). Thus, the pressure range of the pressure sensors must be switched to the required process pressures, which, however, on the other hand, for small pressure ranges of e.g. B. 20 bar significantly reduces the sensitivity in the measuring range. In addition, the positioning and mounting of the pressure sensors when embedded in elastomeric material requires special measures to prevent displacement, for example directly to an adjacent force application part, or damage to the wiring at the high vulcanization pressures.

From DE-A-41 42 142 a force measuring device in the form of a force measuring disc is also known which uses the pressure sensitivity of thick film resistors in order to obtain force-proportional electrical signals. A similar force measuring device is known from DE-C-42 21 426, but there is also the problem that a considerable amount of production work has to be carried out in order to apply a force load uniformly as a pressure distribution to the thick-film resistor. For example, the last-mentioned patent specification for producing a force sensor suggests that two layers be printed in succession in order to achieve a particularly flat surface of such a thick-film resistor with a high surface quality. However, this is relatively complex, just like the proposal according to DE-A-41 42 142, namely to limit and even out the pressurization of the resistors by an additional spring body to a maximum pressure. Also this suggestion with Additional components are relatively complex to manufacture and therefore hardly suitable for series production.

Accordingly, the invention has for its object to provide a force measuring device that can be manufactured inexpensively and is also suitable for high pressures.

This object is achieved by the force measuring device with the features of claim 1.

The invention is based on the consideration that the elastomer pressure of the elastomeric material, when force is applied, provides an optimal, uniform pressurization on the sheet resistors, so that they do not need to be produced with the otherwise required particularly flat surface. This results in a particularly simple, cost-effective production, especially since the sheet resistors used as pressure sensors can consist of common, relatively inexpensive materials. So are suitable as measuring resistors for the pressure sensor

Carbon film resistors or cermet resistors or similar thick film resistors.

This design of the pressure sensor makes a separate measuring membrane as a deformation body unnecessary, since only one or more, preferably two, thick-film measuring resistors are embedded directly on a substrate plate in the elastomeric material. Two resistors outside the immediate pressure range, e.g. B. placed on the also with embedded signal evaluation board. The measuring resistors are thus connected to an active half-bridge in a manner known per se. Since pressure sensors of this type, in contrast to measuring membranes, are not subject to any significant mechanical deformations, but instead use pressure-dependent changes in the electrical properties for signal tapping, these measuring resistors in connection with the embedding and vulcanization of the substrate plate directly into the pressure-distributing egg elastomer material are extremely overload-resistant. Because of this high overload resistance, elastomer pressures of approximately 1000 bar or higher can be achieved, which corresponds to an increase in the pressures possible in the prior art by a factor of 10. This also allows a substantial reduction in the installation space of such a force measuring device. The required diameter with the root of the pressure increase factor decreases by a factor of 2 to 4 with the same nominal measuring ranges. The overall height can also be very low, since there is no support or support ring for a membrane, so that the force measuring device can be made particularly flat. It is particularly advantageous here that such pressure sensors are suitable for surviving the high process pressures and temperatures without damage when vulcanized into rubber. This possibility of vulcanizing in pressure-dependent resistors directly and thus integrating them into the usual production processes of rubber-metal bearing elements, for example for engine support bearings, makes it possible to manufacture such force measuring devices extremely cost-effectively. In addition, a very simple and cheap housing design results from the elimination of cost-intensive machining, especially since the egg-elastomer material forms part of the outer housing and can also encase signal processing boards and electrical components.

Further advantageous embodiments are the subject of the dependent claims.

A preferred exemplary embodiment is explained and described in more detail below with reference to the drawings. Here show:

Fig. 1 shows a cross section through a force measuring device;

Fig. 2 is a perspective view of one in FIG

Force measuring device used pressure sensor;

Fig. 3 shows an associated circuit diagram of the pressure sensor

Fig. 2;

Fig. 4 is an enlarged partial perspective view of the

Pressure sensor; Fig. 5 is an enlarged cross-sectional view through the

2 according to FIG. FIG. 6 shows a view according to FIG. 5 in the loaded state; FIG. 7 shows a modified embodiment according to FIG. 1 with increased measuring accuracy; Fig. 8 shows a cross section through an embodiment

Force measuring device as radial rubber bearing with associated side view; and FIG. 9 shows a modified embodiment of the radial rubber bearing according to FIG. 8.

In Fig. 1, a force measuring device 1 is shown in the form of a so-called rubber bearing or rubber-metal bearing element, as used for example as a support element for motors or machine parts. The force measuring device 1 comprises force introduction parts 2 and 3, between which a block of elastomeric material 4 is provided. On the force introduction parts 2 and 3 there are threaded pieces 2a and 3a to the outside, with which, for example, a machine part set up on the force measuring device 1 is fastened, while the thread 3a on the lower force introduction part 3 serves for fastening to a frame or the floor. A pressure sensor 5 is embedded centrally in the elastomeric material 4 and is oriented essentially transversely to the direction of introduction of the force to be measured, namely here essentially parallel to the plate-shaped force introduction parts 2 and 3. The pressure sensor 5 is in pressure transmission contact with the elastomeric material 4, so that a "measuring rubber bearing" is formed. Due to the complete vulcanization of the pressure sensor 5 in the elastomeric material, it is well protected against environmental influences, so that this force measuring device 1 can also be used in “rough” conditions, for example in a chassis of a vehicle or in a processing machine, in order to detect the force transmission parts 2 and 3 acting forces. The measurement signal detected by the pressure sensor 5 is amplified and / or processed by a measurement signal converter 11, which is in particular designed as a microprocessor or semiconductor chip embedded laterally on the circumference in the elastomeric material 4, so that the processed measurement signals are led out of the force measuring device 1 Measuring cable 13 can be delivered directly to a display or a further processing device. The measurement signal converter 11 is protected by a jacket 10 which is formed in one piece with the elastomeric material 4 during manufacture and vulcanization. Furthermore, an anti-kink sleeve 12 can also be formed in one piece from the elastomeric material 4 around the measuring cable 13, so that this connection to the force measuring device 1 is produced in a particularly simple manner. In addition, there is a high degree of pull-out resistance for the measuring cable 13 due to the direct embedding in the elastomeric material 4, in particular if the measuring cable 13 is led through a metal ring 15 to the centrally arranged pressure sensor 5. This metal ring 15 also serves to mount and hold the pressure sensor 5 during vulcanization by means of a mounting grid 14. This ensures that the pressure sensor 5 is placed exactly on the central axis of the force measuring device 1 and in a right-angled orientation. In addition, the metal ring 15 restricts the tendency for the elastomeric material 4 to bulge radially when a high force is applied.

2, the construction of the pressure sensor 5 is shown enlarged in a perspective view. The pressure sensor 5 comprises at least one substrate plate 6, preferably made of aluminum oxide or zirconium oxide ceramic. At least one sheet resistor 7 is preferably formed on this substrate plate 6, which is arranged above a similarly designed second substrate plate 6, using thick-film technology. In the preferred construction of the pressure sensor 5 from two stacked substrate plates 6, one becomes Intermediate separating layer 8 is formed by means of sealing glass, so that an integral structure of the pressure sensor 5 is formed which is connected to one another. In addition to the sheet resistors 7 (in the circuit design as an active half-bridge or as a full-bridge circuit with the measuring resistors R1, R2, R3 and R4 and the associated connecting wires 9 '), the conductor tracks 9 leading away from the sheet resistors 7 are also applied, in particular printed, using thick-film technology and branded. This results in a circuit diagram according to FIG. 3 as (Wheatstone bridge circuit). This embodiment as a full-bridge circuit is preferred, since temperature deviations due to the very close arrangement of the two substrate plates 6 and thus the four sheet resistors 7 on the flat pressure sensor 5 are minimal.

FIG. 4 shows a preferred construction of the pressure sensor 5, the separating layer 8 shown in dotted lines in FIG. 2 having a frame-like design. The frame-shaped separating layer 8 has a channel-like interruption 18 on the circumference in order to allow the air enclosed in the cavity to escape when the two substrate plates 6 melt together. This interruption 18, which serves as a ventilation opening, can optionally be closed by temperature-resistant adhesive in order to seal the central cavity. The interruption 18 can also be closed by a suitable design of the routing of the conductor tracks 9. With this design, it is also advantageous that the connection pads of the conductor tracks 9 are easily accessible, so that expensive

Vias are avoided. The separating layer 8 thus creates a defined, very small distance between the two substrate plates 6, as shown in FIG

5 is shown. This small distance of, for example, 1/10 mm between the two substrate plates 6 is to be dimensioned such that the substrate plates 6 support one another at higher pressures, which are approximately 100% above the nominal measuring range, as is shown in FIG. 6. In this bent position of the two These lie against each other in the middle of substrate plates 6, so that the bending stress of the substrate plates 6 is limited, but as the load increases, the compressive stress component increases to the layer resistances 7 applied thereon and can be measured without the substrate plates 6 being destroyed.

If no pressure is exerted on the pressure sensor 5, the substrate plates 6 are in their starting position according to FIG. 5 plane-parallel and at a defined distance from one another. With increasing pressure, a force-proportional deflection of the substrate plates 6 occurs until the deflection is so great that they touch in the middle (cf. FIG. 6) and thus support each other. A further increase in pressure due to increasing forces on the force measuring device 1 only results in a comparatively small deflection. Because of the contact, there is a kinking characteristic, but it is particularly advantageous that the force measuring device 1 can still measure in overload operation even at very high pressure ranges. The measurement signals are obtained by the pressure sensitivity of the external thick-film resistors 7, the signal deviation and thus the measurement sensitivity being very high due to the relatively high change in resistance due to the compression of the resistors. It should be noted that due to the firmly bonded connection of the pressure sensor 5 due to the vulcanization with the elastomeric material 4, negative normal stresses (tensile forces on the force introduction parts 2, 3) can also be detected.

It is possible that with a relatively thick design of the substrate material 6 and a narrow design of the frame 8 around the sheet resistors 7, the external sheet resistors react to the direct pressure effect of the elastomeric material 4. A Wheatstone bridge can thus be built up, in which, as shown in FIG. 3, the resistors R1 and R3 contribute to the bridge trimming and thus to the measurement signal formation. As shown schematically in FIGS. 5 and 6, the two substrate plates 6 can also be made relatively thin, for example with a layer thickness of approximately 0.6 mm. In the frame-shaped design of the separating layer 8, as is shown in FIG. 4, the substrate plates 6 can move towards one another under pressure, as is shown in FIG. 6. Since the lower sheet resistor 7 is arranged on the lower substrate plate 6 in the interior of the sensor 5, it is subjected to a tensile load, which results in an increase in resistance within the measuring bridge circuit. On the other hand, a reduction in resistance within the measuring bridge circuit is achieved on the upper substrate plate 6 by the direct action of pressure starting from the elastomeric material 4 and by the pressure deformation of the sheet resistance 7 lying here above. In a modification of the circuit diagram in FIG. 3, the resistances R2 and R4 thus have an increase in resistance, while the resistors R1 and R3 are subjected to an additional reduction in resistance due to the compression deformation. As a result, the signal swing of the sensors is particularly high. In this embodiment, the substrate plates 6 are mutually supported with the sheet resistors 7 above a certain pressure and thereby limit the bending stress in the substrate plates 6, as shown in FIG. 6. It goes without saying that the frame height of the separating layer 8 is greater by approximately twice the maximum deflection of the substrate plates 6 than the layer structure of the internal layer resistance 7 and the respective conductor tracks 9 in the region of the maximum substrate plate deflection. This can be achieved, for example, by printing and drying the frame-shaped glass paste for the separating layer 8 several times.

FIG. 7 shows a modified embodiment of the force measuring device 1, which has essentially the same structure as the embodiment according to FIG. 1. Accordingly, the corresponding components are provided with the same reference numerals. The main change, however, is a metal disk 20 is embedded in the elastomeric material 4 adjacent to the pressure sensor 5 above a central measuring space 19 within the metal ring 15 on the top and bottom. With this structure, better measurement accuracy in the sense of a linear and reproducible sensor signal can be achieved in a simple manner. Due to the two metal disks 20 in the vicinity of the pressure sensor 5, a largely closed space filled with elastomeric material is created to delimit the central measuring space 19, in which the elastomeric material 4 can hardly deform when pressure is applied, since a flow movement is almost completely excluded. Thus, the rubber volumes between the load introduction parts 2 and 3 and the immediate measuring range or measuring space 19 within the ring 15 are used to sense the force and thus to achieve the resilient and damping properties. For this purpose, the ring 15 can also be placed directly on one of the load introduction plates 2 or 3, so that in order to cover the central measuring space 19 adjacent to the pressure sensor 5 embedded in elastomeric material 4, the latter only needs to be covered with a single metal disk 20.

8 shows an embodiment of the force measuring device 1 in the form of a radial rubber-metal bearing, the force introduction parts 2 and 3 being formed by two sleeves which are connected by vulcanized elastomeric material 4. In this elastomeric material 4, four pressure sensors 5 are embedded here in a cross-shaped arrangement, so that the individual force components can be detected directly.

FIG. 9 shows a modified embodiment of the radial rubber-metal bearing element described above, in which the respective measuring space 19 is sealed off by molded pockets 21 in the immediate vicinity of the pressure sensor 5. These incorporated pockets 21 either on the inner or on the outer part are in turn covered with a metal disc 20 so that the central measuring space 19 around the pressure sensor 5 is completely filled with elastomeric material 4, but when the elastomeric material 4 flows out under load incorporated pocket 21 is almost completely prevented. As a result, the measuring accuracy can be increased considerably. As can be seen from the associated side view in FIG. 9, here too four pressure sensors 5 are arranged offset by 90 °, so that the force components or force vectors in their force direction can be determined exactly. The force measuring devices shown in FIGS. 8 and 9 can thus be used as a measuring bearing bush, for example for bearing journals in machine parts. In addition to the described plate rubber bearings and radial rubber bearings, the force measuring device described can also be used in the form of spherical rubber bearings or bonded disc rubber springs, as are known in railway construction.

Claims

claims

1. Force measuring device with a pressure sensor arranged between force introduction parts, which is embedded transversely to the direction of introduction of the force to be measured in pressure transmission contact in elastomeric material, characterized in that the pressure sensor (5) from at least one

Substrate plate (6) applied, pressure-dependent

Sheet resistance (7) is formed in the immediate

Contact to the elastomeric material (4) is vulcanized.

2. Force measuring device according to claim 1, characterized in that the pressure sensor (5) is formed from two substrate plates (6) arranged at a short distance above one another with an intermediate separating layer (8).

3. Force measuring device according to claim 2, characterized in that the separating layer (8) is frame-shaped.

4. Force measuring device according to claim 2 or 3, characterized in that the substrate plates (6) have the same thickness and the same external dimensions.

5. Force measuring device according to one of claims 2 to 4, characterized in that the distance between the substrate plates (6) for limiting the bending stress is dimensioned so that when the nominal measuring range is exceeded, the substrate plates (6) abut one another in the center.

6. Force measuring device according to one of claims 2 to 5, characterized in that the substrate plates (6) consist of ceramic, in particular aluminum oxide or zirconium oxide ceramic.

7. Force measuring device according to one of claims 1 to 6, characterized in that two sheet resistors (7) are provided which are connected as an active half-bridge.

8. Force measuring device according to one of claims 1 to 6, characterized in that four sheet resistors (7) are provided, which are arranged in full bridge circuit.

9. Force measuring device according to one of claims 1 to 8, characterized in that the sheet resistor (7) is applied in thick-film technology on the respective substrate plate (6) together with the associated conductor tracks (9).

10. Force measuring device according to one of claims 1 to 9, characterized in that the egg-elastomeric material (4) forms a casing (10) for a measuring signal converter (11).

11. Force measuring device according to one of claims 1 to 10, characterized in that the egg-elastomeric material (4) forms an anti-kink sleeve (12) for a measuring cable (13) leading out from the elastomeric material (4) from the pressure sensor (5).

12. Force measuring device according to one of claims 1 to 11, characterized in that the pressure sensor (5) is supported on a mounting grid (14).

13. Force measuring device according to one of claims 1 to 12, characterized in that the pressure sensor (5) is surrounded at a radial distance by a ring (15).

14. Force measuring device according to claim 13, characterized in that the ring (15) is arranged in the outer region of the elastomeric material (4).

15. Force measuring device according to claim 13, characterized in that the measuring cable (13) leading away from the pressure sensor (5) is guided through the ring (15).

16. Force measuring device according to one of claims 1 to 15, characterized in that the pressure sensor (5) by a floating in the elastomeric material embedded disc (20) is covered.

17. Force measuring device according to one of claims 1 to 16, characterized in that the pressure sensor (5) is arranged in a pocket-like recess (21) in one of the force introduction parts (2, 3).