Magnetoelastic force transducer

Abstract: Three-dimensional magnetoelastic force transducer comprising a circular-cylindrical body (1) with force transmission between the two circular end surfaces of the body. The body is arranged with at least three measuring zones (M1) equidistantly placed from the axis of the body and symmetrically distributed around the body. The measuring zones are provided with holes for the excitation and measuring windings of the force transducer.

Description: The invention relates to a magnetoelastic force transducer according to the precharacterising part of claim 1. Such a magnetoelastic force transducer is known from the EP-B-0 089 916. Magnetoelastic force transducers designed in different ways more or less adapted to different measuring purposes are previously known from a plurality of patent specifications. SE-B-151 267 describes such a transducer which comprises a force-subjected magnetic core built up of magnetoelastic sheets and provided with four through-holes for two windings located perpendicular to each other. One of these is an excitation winding which is supplied from an a.c. source for generating a magnetic flux in the core, the other being a measuring winding for sensing the magnitude of the magnetic flux passing through the measuring winding. The principle of measurement is based on the property of magnetic material which, when subjected to a mechanical stress, exhibits a change in permeability in the direction of the stress. The magnitude of the flux which passes through the measuring winding in this way will then depend on the magnitude of the external force applied to the core. A magnetoelastic transducer with a solid core is described in US-A-4 802 368. The transducer consists of two identical cylindrical bodies which are held together with the aid of a stud bolt. In an axially and concentrically inner space, there is placed a bobbin with two measuring windings connected in opposition and an excitation winding supplied with alternating current. The interior of the transducer is designed such that, opposite to the respective measuring winding in each of the bodies, a thin cylindrical tubular wall is formed. When the transducer is affected, via the stud bolt, by an externally applied force, a compressive stress arises in one of these tubular walls and a tensile stress in the other. This affects the magnetic conditions such that a signal is obtained from the measuring windings which is proportional to the applied force. SE-B-300 062 describes a magnetoelastic transducer which is preferably intended to be placed in a circular recess in the magnetostrictive material perpendicular to the direction of stress. The transducer has a cylindrical shape and is provided with a magnetization device connected to an a.c. source as well as a sensing device connected to a measuring device. Both the magnetization device and the sensing device consist of an iron core with a winding applied thereon and the iron cores are each arranged in respective axial planes in the transducer, the two axial planes making an angle of either 90 or 60 degrees with each other. EP-A-0 271 832 describes a magnetoelastic force transducer equipped with means to abolish the residual output voltage at zero applied force (neutral point compensation). The core of the transducer consists of two identical cylindrical halves each provided with a axial centre hole to accommodate a stud bolt and a tubular space to accommodate the bobbin consisting of a centrally arranged excitation winding with one measuring winding each at the end faces of the excitation winding. The axial length of the bobbin is smaller than the axial length of the sum of the tubular spaces of the two core halves. This allows with the help of adjustment screws to adjust the axial position of the bobbin in order to compensate for the residual voltage. The FR-A-2 455 734 discloses a magnetoelastic force transducer for the measurement of large forces. The force transducer consists of a core body of circular ring shape with its end faces supposed to accept the force to be measured. Around the circumference of the ring are arranged a plurality of measuring zones, each with measuring winding and excitation winding. In order to compensate for uneven distribution of the force to be measured around the ring there are provided slits between the various measure zones which slits extend in the axial direction and penetrate the ring in the radial direction. In certain applications, special requirements are made on the design of the force transducer and on its properties in other respects. These requirements may, for example, be that the force transducer should be insensitive to non-desired forces in the form of lateral loads and to torques. To comply with such requirements, attempts have been made to avoid the problems by enclosing the transducer in load cell housings. These load cell housings often have built-in membrane packages which are weak in the direction of the measuring force but stiff in other directions. This means that only the measuring force is introduced into the transducer. This solution, however, leads to an increased overall height. The EP-B-0 089 916 describes a magnetoelastic force transducer with a body of solid magnetoelastic material shaped substantially as a parallelepiped at one side of which a force is introduced on a centrally located rim extending from one edge to the other edge of said side while the reaction force is introduced on the opposite side on two similar rims each located at one of two opposing edges. In the two portions between said three force introducing rims excitation windings and measuring windings are provided on legs extending parallel to the two force absorbing sides and formed by through-holes through the solid body on either side of said legs. Such a magnetoelastic force transducer is still sensitive to lateral forces and can not be built in a compact manner capable of absorbing large forces. The invention aims at developing a magnetoelastic force transducer of the afore-mentioned kind which is able to measure very high forces, has an inherent insensitivity to disturbing lateral forces by exhibiting a three-dimensional character and enjoys small dimensions which allow it to be mounted in restricted spaces or existing constructions. To achieve this aim the invention suggests a magnetoelastic force transducer according to the introductory part of claim 1, which is characterized by the features of the characterizing part of claim 1. Further developments of the invention are characterized by the features of the dependent claims. As mentioned above a magnetoelastic transducer according to the invention constitutes a solution to a problem-ridden field where there is a need for a force transducer for very high forces, where lateral loads may occur and where the available space is restricted and, to a certain extent, determined by a special application. Examples of such demands are hydroelectric power generating sets, that is, turbines and generators. These normally have vertically standing shafts with a diameter which may be between 0.6 and 2.0 m. For taking up the vertical force generated by the rotating part, the shaft is provided with a flange resting against a supporting bearing. The shaft is often provided with a guide bearing consisting of a number of bearing segments placed around the periphery of the shaft. To give the bearing segments a certain mobility, these segments are provided, in the centre of the outwardly-facing surface, with a recess for relatively small circular-cylindrical solid bodies - called "studs" within this technical field - which make contact with the surrounding ring or bearing housing of the guide bearing. The studs often have a diameter of 40-60 mm and an axial length of 30-50 mm. One of the circular end surfaces, which faces the bearing segments, is formed such that the contact with the segments consist of an annular, plane surface with an outer diameter corresponding to the diameter of the end surface. The contact of the other circular end surface of the stud with the bearing housing largely occurs within a small region around the centre of the end surface. This is achieved by making this end surface slightly curved in a direction from the periphery and towards the centre. From several points of view it would be desirable to be able to measure, with force measuring devices, the radial forces arising on the bearing segments of the guide bearing. The problem which arises in that connection is that the force measuring devices must not entail any change of the existing guide bearing constructions. An example of another application for measurement of compressive force which requires a low overall height is the weighing of containers. It is often desired to mount a load cell into the legs of an already existing container. Since the construction from the beginning did not take into consideration the accommodation of load cells, the space for these are usually minimal. Other applications of this embodiment of transducers relate to the measurement of tensile force. This transducer can also be used in connection with wire drawing. It is then desired to control the force in the wire to optimize the process while at the same time avoiding, for example, wire breakage. One embodiment of the invention comprises using the existing stud as a measuring body and forming it with measuring zones adapted for magnetoelastic measurement of the forces which the stud is to transmit. A stud thus formed will be referred to in the following as a "puck", by which is meant a magnetoelastic force transducer comprising a body of solid magnetoelastic material with measuring zones and with excitation windings which are to be supplied with alternating current and with measuring windings for sensing the forces to which the magnetoelastic force transducer is subjected. The design of the puck with introduction of force from the bearing segments to the annular surface on one of the circular end surfaces of the puck, and with transmission of force to the bearing housing via the central parts of the other, outwardly-facing, slightly curved circular end surface of the puck means that the principal stress trajectories arising when a load is applied will, at least in a region half-way between the two end surfaces, substantially extend along straight, inclined paths the inclination of which is, in principle, parallel to a conceived line between the annular plane surface of one of the end surfaces and the central part of the slightly curved other end surface. The excitation and measuring windings are to be placed in relation to the principal stress trajectories in such a way that the relationship between measuring load and disturbing loads becomes optimal, and therefore the inclination of the principal stress trajectories is determining for the location of the holes for the excitation and measuring windings. The holes are then placed preferably half-way between the two end surfaces. Calculations of how the principal stress trajectories extend show that they have a relatively long straight, inclined central part which is deflected towards the two end surfaces such that the trajectories can be said to have a truncated S-shape between the two end surfaces. As the pressure areas of the two end surfaces are displaced in relation to each other, the distance from the measuring zone to the pressure area increases. This means a reduced dependence on the planeness of the base. The classic magnetoelastic force transducer according to the above-mentioned SE-B-151 267 is, in principle, a two-dimensional force transducer with force planes parallel to the magnetoelastic sheets included in the sheet package. Such a construction exhibits an undesirable influence from lateral forces and, for thinner transducers, also an instability across the sheet package. The traditional way of increasing the resistance to lateral load is to build the transducer into a load cell housing which, with the aid of a membrane, takes up lateral forces but not the forces to be measured. By providing the puck with at least three measuring zones equidistant from the axis of the puck and symmetrically distributed around the puck, a three-dimensional force transducer is obtained. This means that the design of the transducer has an inherent stiffness and insensitivity to lateral forces. This means that a transducer is obtained which does not need a separate load cell housing with built-in membranes for taking up lateral forces. In a preferred embodiment the load cell is provided with four measuring zones with the angle 90° between the central force planes. The measuring zones of the puck are formed by removing, in some way, such a quantity of the stud material from the cylindrical surface of the solid stud and inwards that only the measuring zones remain. Then the holes for the excitation and measuring windings are bored in the measuring zones, these windings being rotated relative to the direction of the principal stress trajectories, as mentioned above. By way of example, the invention will now be described in greater detail with reference to the accompanying drawings showing in Figure 1 a side elevation of a preferred embodiment of the puck according to the invention, Figure 2 a section of the puck of Figure 1 along line A-A, that is, parallel to and half-way between the two end surfaces of the puck. Figure 1 shows the puck 1 in a preferred embodiment with four measuring zones seen from the side. The annular end surface is shown at 2. The opposite end surface 3 is slightly upwardly (convexly) curved as shown in the Figure 1, the force transmission to the bearing housing taking place within the central part of the curved end surface. To describe in the simplest way how the measuring zones are formed, reference is made to Figure 2 which shows a section through the puck at A-A in Figure 1. By means of cylindrically cutting tools B and C, centred in the same plane A-A, the volume which in plane A-A corresponds to the dashed surfaces B1 and C1 is cut away from the cylindrical surface. That part of the material of the puck which remains between the cut-away parts forms a first measuring zone M1. The cylindrically cutting tools D and E are then used to cut away the parts D1 and D2 and E1 and E2, respectively. Corresponding cuts are made on the opposite side of the puck in the same plane A-A, thus forming the other three measuring zones. One end surface M1E of the measuring zone M1 is also shown in Figure 1 together with the corresponding cuts. By means of this embodiment, a protected space for the winding coils is also created. Instead of one end surface being slightly outwardly-curved with force transmission via the central part of the end surface, it may be suitable in certain applications for the end surface to be plane with a centrally located sphere. For tensile force measurements the puck may be provided with a central through-hole into which a tension pin may be inserted, which has a flange making contact with the other circular end surface. The tensile force connection then takes place against the other end of the pin. In connection with wire drawing, a nozzle may be mounted in the hole, through which the wire is drawn. The dashed lines shown in Figure 1 largely correspond to the principal stress trajectories in a diametrical plane in the middle of two measuring zones. This also determines the location of the holes and the inclination thereof relative to the principal stress trajectories. In other embodiments (not shown) the puck may generally be provided with at least three measuring zones, as mentioned above, placed equidistantly from the axis of the puck and symmetrically distributes around the puck.

Claims: Magnetoelastic force transducer consisting of a body (1) of solid magnetoelastic material provided with excitation windings which are to be supplied with alternating current and with measuring windings for sensing the forces to which the magnetoelastic force transducer is subjected, characterized in that the body has a circular-cylindrical shape destined for force transmission between the two circular end surfaces of the body, wherein one circular end surface is formed with an annular plane surface (2) with an outer diameter corresponding to the diameter of the body and wherein the other circular end surface (3) is formed with a centrally located elevation, that, for the purpose of implementing a three-dimensional magnetoelastic force transducer, the body is provided with at least three measuring zones equidistantly placed from the axis of the body and symmetrically distributed around this axis, and that the measuring zones are provided with holes for excitation and measuring windings. Magnetoelastic force transducer according to claim 1, characterized in that each measuring zone has a shape which results when using cutting cylindrical tools (B,C), directed against each other and centred in a plane half-way between the two end surfaces, for removing, from the cylindrical surface and towards the interior of the body, an equal quantity of the material (B1, C1) of the body, with the measuring zone consisting of the material part (M1) remaining between the removed parts. Magnetoelastic force transducer according to claim 1 or 2, characterized in that the location of the holes for the excitation and measuring windings in the measuring zones is determined by the inclination of the principal stress trajectories in case a force is applied between the two end surfaces. Magnetoelastic force transducer according to any of the preceding claims, characterized in that the body is provided with four measuring zones with the angle 90° between the central force plane of the measuring zones. Magnetoelastic force transducer according to any of the preceding claims, characterized in that said centrally located elevation of the other circular end surface is obtained by curving said other circular end surface slightly outwardly from the periphery towards the centre. Magnetoelastic force transducer according to any of claims 1 to 4, characterized in that said centrally located elevation of the other circular end surface is obtained by forming said other circular end surface as a plane surface with a centrally located sphere. Magnetoelastic force transducer according to any of the preceding claims, characterized in that the body is formed with a central through-hole for a tension pin provided with a flange able to make contact with the other circular end surface. Magnetoelastic force transducer according to any of claims 1 to 6, characterized in that the body is formed with a central through-hole for mounting a nozzle therein.

Inventors: GUSTAFSSON PAER, NORDVALL JAN O, OERNHOLMER LARS, GUSTAFSSON,PAER, NORDVALL,JAN O., OERNHOLMER,LARS, Gustafsson,Pär, Örnholmer,Lars
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1993
Main classification: G01L1
IPC codes: G01L1, G01L5