METHOD AND APPARATUS FOR CALIBRATING AND/OR CHECKING OF LOAD CELLS

Abstract: A method and an apparatus for calibrating and/or testing of load cells of heavy duty weighing systems use a force transmission device being self-adjustable without friction and having a force transmission ratio in the range of 1:50 to 1:5000 and is adapted to be placed onto a force introduction member. The load cell interposed between said force introduction member and a base may be calibrated by connecting said force transmission device through said force introduction member to said base and by applying varying calibration standards at the force introduction side of the force transmission device.

Description: Field of the Invention: The invention relates to a method and an apparatus for calibrating and/or checking of load cells incorporated in vehicle weighing systems, bunkers or similar heavy load devices. Background Art: Vehicle weighing systems, bunker or bin weighing systems or similar heavy load weighing devices generally use load cells in a measuring range of, for example .1 to 10 MN. After built-in of the load cells into the systems they are to be calibrated and periodically checked for proper function. Up to now, it was necessary to load these weighing systems with very large calibrating or test masses of considerably varying sizes. For example, a truck was put onto the platform of a vehicle weighing system to be calibrated, the truck having an exactly defined mass. Then, the truck was removed from the platform and loaded with a pre-determined calibration mass and put onto the platform again. This procedure was performed again, for example, 20 times with the truck being loaded with different calibration masses or standards each time. Furthermore, it has been proposed to use for calibrating or testing heaving load weighing systems a calibrating or testing device provided with an exactly calibrated load cell. In view of the inherent creepage in the engaging connection between the weighing system and the calibrating or testing device the force applied to the weighing system does not maintain definitely constant. Thus, the accuracy of calibrating or testing is rather low. Summary of the Invention It is an object of the invention to provide a method for calibrating and/or testing of load cells incorporated in heavy load weighing systems the method having a very high accuracy. It is a further object of the invention to provide an apparatus for calibrating and/or testing of load cells in corporated in heavy load weighing systems, the apparatus having a very simple design and being easy transportable. It is a further object of the invention to provide an apparatus for calibrating and/or testing load cells incorporated in heavy load weighing systems the apparatus having a very high accuracy. It is a still further object of the present invention to provide a method for calibrating and/or testing of load cells incorporated in heavy load weighing systems, the method being easy to apply, fast and accurate. These and other objects of the invention are achieved by an apparatus for calibrating and/or testing of load cells interposed between a base and a force introduction member of heavy duty weighing systems comprising: a force transmission device having a transmission device having a transmission ratio in the range of about 50 to 5000, being self-adjustable without friction and adapted to be placed against said force introduction member; and a connecting means for connecting said force transmission device to said base at a position close to said load cell and arranged in a direction of a force applied by said force transmission device to said load cell. According to another aspect of the invention there is provided a method for calibrating and/or testing of load cells interposed between a base and a force introduction member of a heavy duty weighing system comprising the steps of a) placing a force transmission device, which is self-adjustable without friction against said force introduction member; b) connecting said force transmission device through an opening of said force introduction member to said base; c) loading said force transmission device with a calibrating standard in the range of 50 to 5000 times smaller than the standard loading of said weighing system; d) determining the reading of said load cell; e) repeating steps c) and d) with different calibrating standards for determining a calibration or testing curve in respect of said load cell. Brief Description of the Drawing: The Figur is an elevational view partly in section of the calibrating and/or testing apparatus of the invention as applied to a vehicle weighing system. Best Mode of Carrying out the Invention: The Figur shows part of a vehicle weighing system comprising a base 12 and a platform 22 having a load cell 18 provided therebetween. The load cell 18 rests on the base 12 and the load, f.i. a truck or a similar vehicle put on the platform 22 is introduced into the load cell 18 from the platform 28 via a girder 34 provided on the bottom side of the platform 22. It will be appreciated that the weighing system must be calibrated after incorporating the load cell 18 into the system. Then, very large masses are required to calibrate the load cell 18. Thus, according to the invention there is used a force transmission device generally indicated by the reference numeral 40 which is inherently self-adjustable without friction and which allows, due to a high ratio of force transmission, and the friction less transmission of force application to use very small calibrating standards. The principal of design and operation of the force transmission apparatus 40 have been disclosed in detail in USA Patent 4,213,509, in particular Fig. 3 and associated description, however, in quite a different environment, namely for testing load cells as such, put into the apparatus. In view of the detailed disclosure in USA Patent 4,213,509 a description of the force transmission apparatus 40 used in connection with the invention is restricted to the basic design. Thus, the apparatus 40 comprises a frame 42 adapted to be firmly placed on the plateform 24, the frame 42 supports a housing 44 containing the force transmission elements, in particular wedges 48 and 52 which are guided in hydrostatic or pneumostatic bearings as explained in detail in the above mentioned USA Patent. Specifically wedge 42 is frictionlessly guided for movement in an essentially horizontal direction in a support member 36 firmlay supporting on frame 42. On the other hand, wedge 52 is arranged for movement in vertical direction between one of the end surfaces of wedge 48 and a support element 38 resting on an inner side surface of the housing 44. It should be noted that the top surface of support element 36 has a spherical shape and has engaged thereto, again by interposing a hydrostatic bearing, a fixing member 24 having a bottom surface formed complementary to the top surface of support element 36. It should be noted, that there is provided an essentially vertical through hole 26 in the support element 36 and, aligned thereto, a passage 28 in the frame 42 for loosely passing a connecting member 30, preferably a rod made of metal. The upper end of the connecting member 30 passes through the fixing member 24 as well, and is provided with a threading for having screwed thereon a nut 16. As may seen from the Figur the connecting member 30 passes a passage 4 as well, provided in the platform 22 and has its lower end provided with a threading to be screwed into a holding member 6 fixed at the base 12. It should be noted, that the threading might be provided in a bore in the base 12, thus eliminating the holding member 6. Furthermore, the upper end of the connecting member 30 may fixed to fixing member 24 in another way than by the nut 16. In order to calibrate or test the load cell 18 the wedge 52 is loaded by an exactly defined calibration mass or standard 50 in vertical direction. This calibration mass 50 may be connected to the lower end of wedge 52 by a cored 32 passed through a passage 14 in the frame 42, or such calibration mass 50 may be put onto the upper horizontal end surface of the wedge 52 or a table appropriately attached to this upper surface. Though in the Figur two wedges 42 and 52 are shown for transmitting a low force caused by the calibration mass 50 into very high force applied to the load cell 18, it should be appreciated, that the transmission apparatus 40 may use one wedge 48 only having the cord or wire 32 directly connected to the left end surface (in the Figur) the cord being deflected to the horizontal direction by means of a roll (not shown) or a similar means. Calibrating or testing the load cell 18 is accomplished according to the method of the invention as follows: First, the apparatus 40 is placed onto the platform 22 in essential alignment of throughole 26, the passages 28 and 4 and the holding member 6. Now, the connecting member 30 is introduced through an opening 56 in the top wall of the housing 44 and is passed through the throughhole 26, the passages 28 and 4 in order to engage the threading of the holding members 6. As a next step the connecting member 30 is threaded into the holding member 6 with the nut 16 fixed to the upper end of the connecting member 30. The result is a firm exactly defined transmission chain from the calibrating mass 50, via wedges 52, 48, the upper part of the support element 36 to the fixing member 24 the connecting member 30 the holding member 6 to the base 12 and the lower end of the load cell 18. On the other hand, the reaction force is transmitted through the lower part of support element 36, the frame 42 and the platform 22 to the upper end of the load cell 18. Thus, a very high force corresponding to the calibration mass 50 attached to court 32, but transmitted in a very high ratio, for example in the range of 1:100 to 1:5000 due to the transmission ratios of the two wedges 42 and 42 is applied between the upper and lower ends of the load cell 18. As explained in detail in the above mentioned USA Patent, due to the guidance of the wedges 48 and 52 in hydrostatic bearings there is no friction at all and the apparatus is self-adjusting; thus, it is the exactly defined force caused by the calibration mass 50 which is applied to the load cell 18. Now, the load cell 18 will generate an electrical signal corresponding to the force applied. By varying the calibration mass 50 various readings are derived from the load cell 18 which may be used for plotting the usual calibration curve. Alternatively, the various values of the calibration masses and their corresponding readings of the load cell 18 may be entered into an electronic evaluation device for compensating any deviations of the load cell 18 in any futur weighing procedures. It should be noted, that due to the spherical hydrostatic bearing between the top surface of the support element 36 and the fixing member 24 any deviation from the exact vertical direction of the connecting member 30 be compensated. In the forgoing description a method and an apparatus for calibrating and/or testing of load cells has been disclosed in corporated in heavy load weighing systems. Due to using a transmission device having a high transmission ratio and being self-adjustable without any friction, it is possible to calibrate and/or test such load cells using very small calibration masses. Also, the transmission apparatus is a compact device which is easily transportable as the calibration masses are.

Claims: 1. An apparatus for calibrating and/or testing load cells (18) interposed between a base (12) and a force introduction member (22) of heavy duty weighing systems comprising: a force transmission device (40) being self-adjustable without friction and adapted to be placed against said force introduction member; and a connecting member (30) for connecting said force transmission device (40) to said base (12) at a position close to said load cell and arranged in a direction of a force applied by said force transmission device (40) to said load cell (18). 2. The apparatus of claim 1 wherein said force transmission device has a transmission ratio in the range of about 50 to 5000. 3. The apparatus of claim 1 or 2 wherein said force transmission device comprises a wedge-type transmission means guided without friction in hydrostatic bearings. 4. The apparatus of cliam 3 wherein said wedge-type transmission means comprises first and second wedges (48,52), said first wedge (48) having an axis of movement extending horizontally and said second wedge (52) having an axis of movement extending vertically, a calibration mass (50) may attached to said second wedge (52) applying a force thereto in vertical direction. 5. The apparatus of claim 3 wherein said wedge-type means comprises a wedge having an axis of movement extending horizontally, a calibration mass being applicable thereto by a flexible means for pulling said wedge in said horizontal direction. 6. The apparatus of any of the preceding claims wherein said connecting member (30) has an longitudinal extension such that it extends from said force transmission device (40) to said base (12) and is provided at a lower end thereof with means (6) for fixing said connecting member (30) to said base (12). 7. The apparatus of claim 6 wherein said lower end (2) of said connecting member (30) is provided with a threading adapted to be screwed into a threading means provided in said base(12), preferably in the form of a sleeve fixedly inserted into said base and having an inner threading for engagement with said connecting member. 8. The apparatus of any of the preceding claims, wherein said wedge-type force transmission means (40) includes a support element (36) for horizontally guiding said wedge means in hydrostatic bearings said support member having a top surface being in engagement with a bottom surface of said fixing member via a hydrostatic bearing, the engaging surfaces being preferrably sperically formed. 9. A method for calibrating and/or testing of load cells interposed between a base and a force introduction member of a heavy duty weighing system comprising the steps of a) placing a force transmission device, which is self-­ adjustable without friction against said force introduction member; b) connecting said force transmission device through an opening of said force introduction member to said base; c) loading said force transmission device with a calibrating standard in the range of 50 to 5000 times smaller than the standard loading of said weighing system; d) determining the reading of said load cell; e) repeating steps c) and d) with different calibrating standards for determining a calibration or testing curve in respect of said load cell. 10. The method of claim 9 wherein said steps a) to e) are preceded by providing said base with a fixing means for a connecting member and providing said force introduction member with an opening essentially aligned in vertical direction with said fixing means. 11. A heavy duty weighing system comprising: a plurality of load cells interposed between a base means and a force introduction member; a fixing means provided at said base means close to each load cell; a passage each in said force introduction member vertically aligned with each fixing means for passing a connecting member to be fixed at said base by said fixing means and adapted to be connected to a calibrating and/or testing apparatus for said load cells.

Inventors: HAEFNER, HANS W., Häfner, Hans W.
Applicants: PFISTER GMBH
Application year: 1987
Main classification: G01G23
IPC codes: G01G23