1. Field of the Invention
The present invention relates generally to a load cell and more particularly to a load cell or device for measuring the axial load on a machine or machine component through a rotation bearing. The invention has particular applicability for use in a bearing assembly and for use in linear actuators and more specifically, screw-type linear actuators. Accordingly, the invention also relates to a linear actuator and a bearing assembly incorporating such load cell.
2. Description of the Prior Art
Various machines and machine components currently exist in which the ability to measure monitor axial forces acting on such machines or components is beneficial and desired. One specific example, among others, includes a variety of linear actuators such as those used in resistance welding to linearly move a welding head into a welding position to produce a desired resistance. The real time monitoring of forces exerted by the actuator or the maintenance of such forces within a predefined range would be highly beneficial and would enhance the efficient use of such actuator. Accordingly, there is a need in the art for a load cell or other device which can be used to measure or monitor the axial load on a machine or component such as a linear actuator.
The present invention relates to a load cell or force measuring device for measuring the axial force or load on a machine or a machine component. Although the device of the present invention has a wide range of potential applications, it has particular applicability for use in measuring or monitoring the force exerted by a linear actuator on a workpiece. More specifically, the present invention is directed to a load cell or force measuring device for measuring the axial force exerted by a linear actuator in a resistance welding application. Still more specifically, the load cell of the present invention measures or monitors the force exerted in a screw-type linear actuator through its rotational support bearing. Accordingly, the present invention is directed to such load cell and to a linear actuator and a bearing assembly incorporating the load cell.
In the preferred embodiment which is described with respect to a screw-type linear actuator, the load cell or force measuring device is positioned between the rotational support bearing and the bearing housing and includes a stabilizing sleeve portion and a force measuring cell portion. The force measuring cell portion includes a force receiving surface engageable with a portion of the bearing and an opposite force transmission surface engageable with a portion of the bearing housing. A flexing or force measuring area in the form of a flexing web is positioned between the force receiving and force transmission surfaces. A strain gauge is mounted in the area of the flexing web to measure the strain in the flexing web and thus, through signal amplification and calibration techniques, the level of force exerted by the bearing on the force measurement cell.
In the preferred embodiment, both the strain gauge connection board and the amplification electronics are integrated within the actuator itself. This eliminates long cables and strain gauge wire and results in improved signal-to-noise ratio and a more robust system.
Accordingly, an object of the present invention is to provide a load cell or force measuring device for measuring axial forces exerted upon a machine or machine component.
Another object of the present invention is to provide a load cell or force measuring device for a linear actuator.
A still further object of the present invention is to provide a load cell or force measuring device for measuring axial forces exerted through a rotational support bearing.
A still further object is to provide a linear actuator and/or a bearing assembly with such a load cell incorporated therein.
A still further object of the present invention is to provide a linear actuator with an axial force load cell in which the load cell measuring and amplification electronics are integrated into the actuator itself.
These and other objects of the present invention will become apparent with reference to the drawings, the description of the preferred embodiment and the appended claims.
The present invention is directed to a load cell or force measuring device for measuring or monitoring the axial load or force exerted by a machine or machine component on a work piece. Although the present invention has potential applicability for use with a variety of machines or machine components, it has particular applicability to a linear actuator such as a screw-type linear actuator of the type shown in U.S. Patent Application Publication No. US-2005-0253469-A1 and U.S. Pat. No. 6,756,707, incorporated herein by reference. The invention is also applicable to a through hole type actuator as shown in the preferred embodiment in which the screw shaft or rotating extension thereof extends through the load cell or to a closed end actuator in which the screw shaft does not extend through the load cell.
Accordingly, the preferred embodiment of the present invention will be described with respect to a screw-type linear actuator of the type shown in U.S. Patent Application Publication No. US-2005-0253469-A1. The subject matter of this published application is incorporated herein by reference and made a part hereof. An isometric view of such an actuator is shown in
In describing the load cell of the present invention, the terms “distal” and “proximal” will be used to define the various surfaces or other portions of the load cell and other components. As used herein, the term “distal” shall define the surface or portion which is closest to the force exerting end of the actuator or machine, while the term “proximal” shall define the surface or portion which is furthest from the force exerting end of the actuator or machine.
In describing the load cell 10, initial reference is made to
The bearing 11 is retained within the bearing housing 12 by a disc-shaped, externally threaded retainer ring 22. As shown best in
With continuing reference to
The bearing housing 12 is comprised of a block member having a distal end 25, a cylindrical inner surface 26 and a portion 28 at its proximal end. The portion 28 extends radially inwardly from the cylindrical surface 26 and includes an inner annular surface 29 designed for engagement with a proximal end of the load cell 10. The housing 12 includes a central opening or through hole 31 at its proximal end. This opening 31 is sufficiently large to permit the screw shaft 20 or an extension thereof to extend through the housing 12. In the preferred embodiment, the distal end 25 includes a recess 30 to receive an O-ring for connection with the main body 13 (
The load cell 10 is described best with general reference to
The proximal end of the sleeve portion 40 is integrally formed with the force measuring cell portion 41 as shown best in
The force measuring cell 41 is a generally cylindrical structure having an outer cylindrical wall or surface 46 continuous with the outer cylindrical sleeve surface 44 and an inner cylindrical wall or surface 48. The surface 48 is spaced radially inwardly from the inner cylindrical sleeve surface 42. As shown best in
The plurality of proximal surface portions include three force transfer surfaces 50 and three force measuring surfaces 51 between the surfaces 50. As shown best in
Although the preferred embodiment shows three force transfer surface portions 50 and three force measuring surface portions 51 between them, more or less of such surface portions could be provided. However, the proximal end of the load cell 10 should preferably include at least one force transfer surface 50 and at least one force measurement surface 51 adjacent to the force transfer surface 50. The preferred embodiment shows the size of the surface portions 50 to be equal to one another, the size of the surface portions 51 to be equal to one another and their respective positions and arrangement to be symmetrical. While this is a preferred construction, benefits of the invention can still be achieved with structures in which the surface portions 50 and the surface portions 51 are not equally sized and in which such surface portions 50 and 51 are not arranged symmetrically, either individually or in combination.
The force measuring cell 41 also includes a plurality of elongated flexing slots 55 corresponding to and associated with the plurality of force transmitting surface portions 50. These flexing slots define one or more flexing webs or strain measurement areas 61. As shown, each of these slots 55 extends radially through the wall of the cell 41 and between the distal surface 49 and its corresponding surface portion 50. Each of the slots 55 further extends circumferentially around the cell 41 for a distance greater than the circumferential length of its corresponding surface portion 50. With this structure and relationship, an end 56 of each slot extends past an end 52 of its corresponding surface portion 50. The wall portion of the cell 41 between the surface portion end 52 and its associated slot end 56 forms a flexing web or force or stress measurement area 61 (
In the preferred embodiment, each of the slots 55 extends radially through the wall of the cell portion 41 and is substantially of equal width in an axial direction throughout a substantial portion of its length. Further, the ends 56 of each slot are rounded and enlarged toward the surface portion 51 as shown by reference character 58. This rounded and enlarged end has the effect of directing the location of the strain created in the flexing web 61 in a desired direction, thereby facilitating measurement of the force acting on the bearing, and thus on the cell 41.
With continuing reference to
The strain gauge 60 is a strain gauge of the type known in the art to measure strains on a member which is being flexed. In the preferred embodiment, the strain gauge 60 is a conventional strain gauge manufactured by Vishay Micromeasurement and includes a pair of spaced gauge elements 64 and 65 and a plurality of solder paths 66 for providing and receiving electrical signals in a manner known in the art. During operation, and by measuring differences in electrical resistance, one of the elements 64 and 65 will measure tensile forces and the other will measure compressive forces in the flexing web 61. The results of these measurements are then compared in a conventional manner through a Wheatstone bridge or other means and the force “F” is calculated through calibration techniques known in the art. A disc shaped electronic jumper or cable board 37 (
Preferably, the number, size and position of the surface portions 50, the number, size and position of the surface portions 51 and the number, size and position of the slots 55 should be such as to provide a substantially symmetrical structure. Such a structure minimizes, if not eliminates, stress concentrations which might exist in a non-symmetrical structure. In a symmetrical structure where stress variations are minimized, only one, or at least one, strain gauge 60 is needed. In the preferred embodiment, however, two strain gauges are used and are positioned approximately diametrically opposite from one another as shown in
In the preferred embodiment, as best shown in
Specifically, the generally annular connector board 37 is positioned near the proximal end of the load cell 10 and is secured to a portion of the inner cylindrical wall 48 of the cell portion 41 by a silicon based adhesive. In this position, extremely fine gauge jumper wires are used to electronically mount and connect the strain gauge pads 66 (
Significant advantages in accordance with the present invention are also achieved by integrating the signal conditioning electronics of the circuit board 43 within the actuator itself. A principal reason is that load cells, such as the load cell in accordance with the preferred embodiment, produce a very low level signal. If this low level signal needs to be transmitted out of the actuator through long cables on the order of up to fifteen feet or more to signal conditioning electronics on a robot, noise or other interference will likely be introduced, thereby quickly reducing the signal-to-noise ratio. By integrating the signal conditioning electronics onboard the actuator, noise or other interference becomes less significant with respect to the usable signal. The output from the board 43 on the actuator can be an analog or digital signal, depending on the controller available to utilize the signal. Further, depending upon the signal conditioning within the actuator on the board 43, the user has a signal that in many cases does not need further conditioning.
While the preferred embodiment shows the load cell and strain gauges positioned between the bearing and the bearing housing, the advantages of integrating the strain gauge electronics within the actuator itself can be achieved regardless of the position of the load cell. For example, the load cell could be incorporated within the bearing itself rather than between the bearing and housing. The load cell must, however, preferably be capable of measuring axial forces on the bearing.
The present invention is directed to a load cell for preferred use to measure or monitor the axial force exerted by a linear actuator. The invention is also directed to an actuator or bearing assembly incorporating such a load cell and an actuator in which the strain gauge electronics are integrated within the actuator itself. In such an actuator, axial force applied to a work piece is transmitted through the rotor or other actuator component to a rotation bearing. In the preferred embodiment, this force is transmitted from the rotor or other component to the inner race of the bearing and then transmitted through the bearing to the outer race and from the outer race of the bearing to a force measuring cell positioned between the bearing and the bearing housing. In the preferred embodiment, this force measuring cell includes a force receiving surface in engagement with the bearing, a force transmission surface in engagement with the bearing housing and a flexing web portion or other strain measuring area between the force receiving and force transmitting surfaces. With this structure, when a force is applied by the actuator to the distal end of the inner bearing race, the web flexes in proportion to the level of the force exerted by the actuator. The level of the force is determined by use of a conventional strain gauge applied to a surface of the flexing web or other strain measuring area. By measuring the tensile and compressive stresses in selected areas of the flexing web and by comparing the measurement results and utilizing calibration techniques known in the art, the level of the force “F” can be measured and/or monitored.
While the preferred embodiment shows the flexing web 61 created by the slot in combination with the surface portion 50, it is contemplated that such web 61 or other strain measurement area could be formed by other structural configurations. Further, it is contemplated that the strain gauge 60 or other strain measuring means may be provided at other locations in the area of the web 61 or other strain measuring areas.
Although the description of the preferred embodiment has been quite specific, it is contemplated that various modifications could be made without deviating from the spirit of the present invention. Accordingly, it is intended that the scope of the present invention be dictated by the appended claims rather than by the description of the preferred embodiment.