Device for magnetostrictive position measurement

Abstract

Device for magnetostrictive position measurement, comprising a tube (2; 2'; 2") made of a silicone material and having a longitudinal axis in which a magnetostrictive waveguide (10) extending in the longitudinal direction of the tube (2; 2', 2'') is generally centered by a guide device (6), wherein the guide device (6) is formed integrally with the tube (2; 2'; 2") and is configured to contact the waveguide (10) at most in a guide zone (12) which comprises less than 20% of the surface of the waveguide (10), characterized in that the guide zone (12) extends along a helix.

Description

[0001] The invention relates to a device for magnetostrictive position measurement (distance measurement) according to the preamble of claim 1, which can be used in particular for magnetostrictive level measurement.

[0002] Magnetostrictive position measurement has long been an established technique (see, e.g., US 3,898,555 A), in which the position of a movable magnet is detected using a magnetostrictive waveguide via the Wiedemann effect. This type of position measurement was also applied to level measurement a few years later, with the position magnet being embedded in a float (e.g., US 4,939,457 B1). The known setup consists of a protective tube or hose, shielded from the liquid stored in the tank, in which a magnetostrictive waveguide is electrically insulated.

[0003] It was recognized early on (US 4,939,457 B1, WO 98 / 28598 A1) that using a rigid protective tube for tall tanks is impractical due to high transportation and installation costs. US 4,939,457 B1 therefore proposes a measuring system with a coilable hose containing the magnetostrictive waveguide in the form of a wire, along with all necessary additional components. This hose is unwound at the installation site and protected by bolted pipe segments, preventing the hose from coming into contact with the stored liquid. WO 98 / 28598 A1 presents a further development of this configuration, along with numerous detailed improvements, in which external protection is provided by a hermetically sealed metallic corrugated hose, which can also be encased in a metallic fabric.This or a similar design is currently used by most manufacturers of such magnetostrictive level measuring devices for larger tanks.

[0004] One problem is that the mechanical torsional wave generated in the waveguide by the Wiedemann effect is damped. At longer measurement lengths, this can cause the signal intensity to drop to the noise threshold, at which point reliable operation is no longer guaranteed. The damping of the mechanical wave comprises several components: firstly, the intrinsic damping in the waveguide material, which in turn can depend on the magnetization state of the magnetostrictive material; and secondly, the losses caused by the contact between the waveguide and the electrically insulating support structure. These latter losses can be influenced by a suitable choice of material and shape for the support structure, as well as by an axial preload on the waveguide.

[0005] To minimize unwanted attenuation, a number of configurations for guiding the magnetostrictive waveguide have been devised. DE 33 43 310 A discloses a support structure in which an annular support "made of suitable plastic" is held in position by spacer tubes. According to US 4,939,457 B1 and WO 90 / 13 789 A1, a helically wound plastic tube is used as a support for a retractable probe. This plastic tube contains the return conductor required for measurement, which simultaneously presses the plastic tube against the inner wall of a retractable hose. The material of the helical plastic tube is specified as soft rubber. DE 197 53 805 A1, which describes a device according to the preamble of claim 1, specifies various bearing arrangements for the magnetostrictive waveguide.Two key considerations are cited: firstly, minimizing the contact area by using spaced supports, and secondly, minimizing energy transfer by keeping the solid-to-air ratio as low as possible. Various configurations using foam- or sponge-like materials are disclosed. In document WO 98 / 28598 A1, a guide for the magnetostrictive waveguide is proposed using a flexible insulating tube with a woven section as an inner support made of a hard material such as ceramic or glass, acting as a cushion. US 6,559,636 B1 discloses several support options. One consists of a support tube made of flexible material (silicone) with constrictions, another of rubber rings with spacers. Rectangular supports and three sharp points are also shown as supports.

[0006] In applications involving elevated tanks, long measuring lengths are required. For this purpose, it is advantageous to design a coilable probe for magnetostrictive position measurement to minimize transport and installation costs. For such long measuring lengths, it is essential to keep the damping of the mechanical shaft low. Furthermore, the cost-effective manufacturing and ease of assembly of the support structure also play a crucial role.

[0007] The object of the invention is to create a probe for magnetostrictive position measurement that allows the greatest possible measuring length, is inexpensive to manufacture and can be easily transported to its place of use.

[0008] This problem is solved by a device for magnetostrictive position measurement with the features of claim 1. Advantageous embodiments of the invention are set out in the dependent claims.

[0009] The device according to the invention for magnetostrictive position measurement comprises a tube made of a silicone material with a longitudinal axis, in which a magnetostrictive waveguide extending longitudinally along the tube is guided in a generally centered manner by a guide element. The guide element is formed integrally with the tube and is configured to contact the waveguide only in a guide zone that constitutes less than 20% of the waveguide's surface (preferably less than 10%, 5%, or 2% of the waveguide's surface). The longitudinal direction of the tube refers to an unwound, straight path of the tube. Typically, the magnetostrictive waveguide is designed as a cylindrical metal wire whose outer surface is the waveguide's surface. According to the invention, the guide zone extends along a helical path.It is possible to manufacture a silicone tube with a helical guide inside the tube, made from the same material, in a single, cost-effective operation. If the guide zone follows a helical path, designs are also conceivable where the guide zone comprises more than 20% of the waveguide's surface area.

[0010] The choice of material for the hose and the guide device (silicone) which is formed in one piece with the hose, and the small area of ​​the guide zone result in low damping.

[0011] In advantageous embodiments of the invention, the guide zone is configured such that, when precisely aligned with the longitudinal axis of the hose, the magnetostrictive waveguide would not touch the guide zone at all (or only very minimally). This is achieved, for example, if the magnetostrictive waveguide has a nominally circular cross-section and the distance of the point in the guide zone closest to the longitudinal axis of the hose to the longitudinal axis of the hose is greater than the radius of the magnetostrictive waveguide. In this case, the guide zone does not encompass a cylindrical free space in the region of the longitudinal axis of the hose in which the magnetostrictive waveguide can be located without touching the guide zone anywhere. In practice, the magnetostrictive waveguide does not have a perfectly straight path but occasionally touches the guide zone.From a metrological perspective, however, the deviation from a straight-line geometry has no effect. An advantage is that, due to the minimal contact between the magnetostrictive waveguide and the guide zone, the damping of the mechanical shaft is low.

[0012] Preferably, at least one reinforcing thread is embedded in the wall of the hose. The term "reinforcing thread" is used here in a general sense. The reinforcing thread is flexible, allowing the hose to be wound up, but it does not need to be thin and can be made of a wide variety of materials, such as metals, fiberglass, or plastic fibers, including composite materials. Both monofilament and multifilament reinforcing threads are suitable, the latter also being twisted or braided, for example. Reinforcing threads relieve strain on the hose, increase its dimensional stability, and can significantly reduce the hose's thermal expansion. The resulting improvement in properties can be crucial in suspended applications within tall tanks.

[0013] The wall of the tube can also contain at least one electrical conductor, for example, the return conductor required alongside the waveguide for magnetostrictive position measurement. The electrical conductor can also function as a reinforcing filament. It is also conceivable to embed electrical conductors and non-conductive reinforcing filaments.

[0014] Furthermore, the outer wall of the hose can be provided with at least one longitudinal recess into which a reinforcing thread or an electrical conductor is inserted. This makes it possible, for example, to connect a reinforcing thread or an electrical conductor of the type mentioned above to the hose without having to embed it in the wall during the hose's manufacture. A hose with both embedded and externally inserted reinforcing threads or electrical conductors is also conceivable.

[0015] The device according to the invention becomes more robust if the hose is surrounded by a liquid-tight protective sleeve. The protective sleeve can, for example, be designed as a corrugated hose made of non-magnetic stainless steel. A float with a magnet, which follows the fill level in a tank, can be guided on the corrugated hose. Furthermore, the corrugated hose with the silicone hose arranged within it can be coiled up for transport purposes.

[0016] As already indicated, the device according to the invention can be used for magnetostrictive level measurement in a tank designed for storing a liquid. The device is suspended from an upper part of the tank (e.g., a tank dome) and carries a float equipped with a magnet. The magnetostrictive waveguide is connected to measuring electronics, such as are generally known from the prior art. The hose containing the magnetostrictive waveguide and, optionally, a surrounding protective hose, is preferably supplied to the tank in a coiled state before installation.

[0017] The invention is described below with reference to exemplary embodiments. The drawings show in Fig. 1: a schematic longitudinal section through a first embodiment of the device according to the invention with a hose having a helical guide device for a magnetostrictive waveguide and surrounded by a corrugated hose, Fig. 2: a schematic cross-section through the hose according to Fig. 1, which is provided on its outside with three recesses for electrical conductors, Fig. 3: a schematic cross-section through a hose according to a variant in which three reinforcing threads are embedded in the hose wall and three recesses are provided for electrical conductors, and Fig. 4: a schematic cross-section through a hose according to another variant in which three reinforcing threads or electrical conductors are embedded in the hose wall.

[0018] In the Fig. Figure 1 shows an embodiment of a device 1 for magnetostrictive position measurement in a schematic longitudinal section.

[0019] The device 1 comprises a tube 2 with a wall 4 made of a silicone material, surrounding an interior 5. A guide 6 for a magnetostrictive waveguide is integrally formed with the wall 4; for clarity, the waveguide is shown in Fig. 1 is not shown. The guide device 6, which is made of the same silicone material as the wall 4, runs in a helical shape in the exemplary embodiment, which in Fig. 1 is not particularly easy to see and based on Fig. 2 is explained.

[0020] In the exemplary embodiment, hose 2 is surrounded by a hermetically sealed, flexible corrugated hose 8 made of a non-magnetic stainless steel alloy, which is also part of the device 1. The corrugated hose 8 protects hose 2 from contact with the medium in a tank when the device 1 is installed in a substantially vertical orientation in a tank and used for magnetostrictive level measurement. A float equipped with a magnet, located on the liquid surface of the medium, can be guided by the corrugated hose 8. Hose 2 and the corrugated hose 8 can, in principle, be manufactured in any length. Typically, their length corresponds to the height of the tank in which the device 1 is mounted. Before installation in the tank, the corrugated hose 8 can be coiled together with hose 2 to facilitate transport.

[0021] In Fig. Figure 2 shows a schematic cross-section through hose 2, in a plane perpendicular to the (in Fig. 1. Longitudinal axis of the hose (not shown for clarity). 2. The guide device 6 is helical, which is why it is shown in the sectional view according to... Fig. 2 only projects from the wall 4 into the interior 5 of the hose 2 at one point. In the exemplary embodiment, the cross-sectional area of ​​the guide device 6 is largely triangular, as shown in Fig. 2 shown. Due to the helical shape of the guide device 6, this triangle would form a different angle in a cross-section through the hose 2 in a different plane than in Fig. 2 are located.

[0022] The Fig. Figure 2 shows the magnetostrictive waveguide, which is arranged along the longitudinal axis of the tube 2, labeled 10, and depicted as a filled dot. In the exemplary embodiment, the magnetostrictive waveguide 10 consists of a bare metal wire without insulation on its surface. The magnetostrictive waveguide 10 is centered by the helical guide device 6. At the tip of the Fig. Within the triangle representing the guide device 6, there is a guide zone 12 where the guide device 6 can contact the magnetostrictive waveguide 10. In three dimensions and along the tube 2, the guide zone 12 is thus designed as a narrow band with a helical shape. In this embodiment, the area of ​​the guide zone 12 is less than 20% of the surface area of ​​the magnetostrictive waveguide 10, but it can, in principle, be larger.

[0023] In Fig. In Figure 2, the magnetostrictive waveguide 10 is surrounded by a circle 13 that extends to the guide zone 12 and symbolizes a free passage in the interior 5 of the tube 2. This free passage is significantly larger than the cross-sectional area of ​​the magnetostrictive waveguide 10 in order to minimize the contact areas and normal forces between the guide device 6 and the magnetostrictive waveguide 10. The size of the actual contact points between the magnetostrictive waveguide 10 (which in practice does not run exactly on the longitudinal axis of the tube 2 and occasionally touches the guide zone 12) and the guide zone 12, as well as the magnitude of the normal force and the material quality of the guide device 6, determine the damping effect. In the configuration shown, this effect is minimized.

[0024] The tube 2 with the guide device 6 can be manufactured, for example, by extrusion. It is practical to simultaneously incorporate a pull thread (e.g., made of plastic). After the tube 2 with the guide device 6 has been drawn into the corrugated tube 8, the magnetostrictive waveguide 10 can then be pulled into the interior of the tube 2 using this pull thread, provided it is held centered by the guide device 6. This offers the significant advantage that the sensitive waveguide 10 is not exposed to the risk of kinking. Finally, the magnetostrictive waveguide 10 is electrically connected at both ends.

[0025] In the Fig. In the embodiment shown in Figure 2, three recesses 16 extending longitudinally along the outer surface 14 of the hose 2 are provided. These recesses serve to guide electrical conductors 18 (shown as black dots). The recesses 16 enable reproducible assembly of the return line for the magnetostrictive waveguide 10 and, for example, leads for temperature sensors. Depending on the requirements, a person skilled in the art can also provide a different number of recesses 16, or recesses 16 can remain free, i.e., not occupied by conductors. Because the electrical conductors 18 are fixed in the recesses 16, inserting the assembly into the corrugated hose 8 is much easier and more reproducible than without the recesses 16. Furthermore, undesirable lateral deflection of the hose 2 is reduced, which would otherwise lead to an increase in damping due to the resulting increase in normal forces.

[0026] The Fig. 3 is a representation similar to Fig. 2, but for a variant of the device. Here, the hose is designated 2', while the other reference numerals remain the same. In hose 2', three reinforcing threads 20 (represented as black dots) are additionally incorporated into the silicone material of the wall 4, which can be done during manufacturing. The reinforcing threads 20 can be made of, for example, plastic, glass, or metal and serve for stiffening and strain relief.

[0027] Furthermore, the reinforcing threads 20 limit the thermal expansion of the silicone material to a fraction of its natural expansion. Limiting the differences in expansion is important because, with a long tube 2 (corresponding to a large measuring length), significant temperature-dependent expansions and contractions would occur, potentially leading to device failure. At low temperatures, the magnetostrictive waveguide 10 could even be exposed if the tube 2 contracts, potentially causing short circuits or increased attenuation. Suitable materials for the reinforcing threads include, for example, glass fibers, glass fiber braids, or plastic fibers or braids.

[0028] The Fig.Figure 4 shows another variant in a schematic cross-section. The hose is labelled 2" here, while the other reference numerals remain the same. In this embodiment, three electrical conductors 22 are directly embedded in the silicone material of the wall 4, which can be done during the manufacture of the hose 2", while no electrical conductors are provided on the outer surface 14 of the hose 2".

[0029] The electrical conductors 22 consist of stranded wire or stranded wire and also act as stabilizing threads, limiting the thermal expansion of the silicone material. However, with this variant, the assembly of the electrical connections to the ends of the electrical conductors 22 embedded in the silicone material is more difficult.

Claims

[1] Device for magnetostrictive position measurement, comprising a tube (2; 2'; 2") made of a silicone material and having a longitudinal axis in which a magnetostrictive waveguide (10) extending in the longitudinal direction of the tube (2; 2', 2'') is generally centered by a guide device (6), wherein the guide device (6) is formed integrally with the tube (2; 2'; 2") and is configured to contact the waveguide (10) only in a guide zone (12) which comprises less than 20% of the surface of the waveguide (10), characterized by , that the guide zone (12) runs along a helix. [2] Device according to claim 1, characterized by, that the magnetostrictive waveguide (10) has a nominal circular cross-section and the distance of the point of the guide zone (12) closest to the longitudinal axis of the tube (2; 2'; 2") to the longitudinal axis of the tube (2; 2'; 2") is greater than the radius of the magnetostrictive waveguide (10). [3] Device according to one of claims 1 to 2, characterized by , that at least one reinforcing thread (20) is embedded in the wall (4) of the tube (2'). [4] Device according to any one of claims 1 to 3, characterized by , that at least one electrical conductor (22) is embedded in the wall (4) of the hose (2"). [5] Device according to any one of claims 1 to 4, characterized by , that the wall (4) of the hose (2; 2') is provided on its outside (14) with at least one recess (16) extending in the longitudinal direction of the hose (2, 2') into which a reinforcing thread or an electrical conductor (18) is inserted. [6] Device according to any one of claims 1 to 5, characterized by , that the hose (2; 2') is surrounded by a liquid-tight protective hose. [7] Device according to claim 6, characterized by , that the protective hose is designed as a corrugated hose (8) made of non-magnetic stainless steel.

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