A force transducer is disclosed, along with a way of fastening the force transducer, a mounting arrangement for a force transducer, as well as a weighing scale in which a force transducer is installed by means of a mounting arrangement.
Known force-measuring cells of weighing scales are equipped with a force transducer that is on one side connected to a scale housing serving as support base and on the other side to a weighing pan carrier through which the force to be measured is introduced. Force transducers can be configured in a variety of ways. Widely used are transducer designs with an elastically deformable body as a core element, or with a device that performs a force compensation, in most cases by means of a counterbalancing force which is generated by a current-regulated electromagnet and is acting through a lever mechanism.
To give an example, a force transducer described in [1], DE 199 39 633 A1 and referred to as “counterforce” or “force receiver” in [2], EP 0 670 479 A1, has an elastically deformable body connecting a housing-connected fixed part of the transducer to a force-application part or, in the case of a weighing scale, a weighing-load application part. The force transducer has transverse grooves at the transitions between the deformable body and the parts that serve to connect the force transducer to the scale housing and the weighing pan carrier. The transverse grooves serve to mechanically uncouple the deformable body in which the deformations effected by the applied forces are measured by means of sensors, such as strain gauges.
The deformable body can be configured as a parallelogram-shaped measuring element with an arrangement of guide members resembling a parallelogram (see for example [3], EP 0 511 521 A1).
The analog signal representing the measurement can be generated by means of strain gauges connected to each other in a bridge circuit. The signal is normally digitized in a converter circuit and subsequently put through further processing steps. The principal structure of a bridge circuit with strain gauges is described, e.g., in [4], U. Tietze, Ch. Schenk, Halbleiterschaltungstechnik, 11 th edition, first reprint, Springer Verlag, Berlin 1999, pages 1242–1243.
To support the further processing of the digitized measuring signal, the measuring cell described in [2] has a memory module in which characteristic parameters are stored that are specific to the individual measuring cell and are used to correct the measuring signals.
As described in [5], published patent application Ser. No. GB 1 462 808, the aforementioned correction applies in particular to errors that are caused by non-linearities, hysteresis phenomena, temperature and creep effects. The calibration- and compensation data required for the correction are determined during production at the factory through specific test and measuring procedures and are stored in the memory module.
According to [1], the measuring accuracy of force transducers in weighing cells depends to a significant degree on the hysteresis properties of the force transducer. The hysteresis of a force transducer can be caused by several factors that occur in most cases in combination with each other.
Further according to [1], mechanical design measures that have heretofore been used to lessen the hysteresis effects have been directed mainly at avoiding plastic deformations of the material of the force transducer. As a means of avoiding friction between the fixed housing-connected part of the force transducer and the mounting base to which it is connected, the two parts were securely bolted together, joined through contact surfaces finished to a high degree of planarity and cleaned of grease and other contaminations. To limit the hysteresis error of the scale, specific requirements are set for the machining of the contact surfaces and for the tightening torque of the screw bolts.
In the practical implementation of these requirements, it was found that the screw connection introduced variable amounts of stress in the material, with an additional unfavorable effect on the hysteresis.
As a solution, it was proposed in [1] to insert a shim element of a flat elastic seal material between the part of the force transducer that is installed in a fixed condition relative to the housing and the mounting base to which the fixed transducer part is connected. However, besides the cost of the seal material, the long-term behavior of the shim element is of concern in this solution. In some cases, a scale that is equipped with an elastic shim element may have to be recalibrated after a relatively short time period.
Furthermore, modern types of scales often have modular force-measuring cells that can be exchanged by a service technician at the user location without the need to send the scale to the factory. The use of an elastic shim element can hardly be recommended for these scales without a recalibration after the exchange of the force-measuring cell, because the behavior of the elastic shim element can change strongly with the exchange of the force-measuring cell.