LU84072A1 - MESSWANDLER FUER EINEN KRAFTMESSER - Google Patents

Description

«S

Mett1er Instrumente AG, Greifensee (Switzerland) n

Transducer for a dynamometer

The invention relates to a transducer for a dynamometer, comprising a measuring body that is subjected to tension.

Conventional metal alloys have hitherto been used for the measuring bodies of such measuring transducers (for example in accordance with German Offenlegungsschrift 23 49 281). This involved a number of problems that arose from the material properties. Influences of temperature, in particular temperature-dependent changes in the modulus of elasticity or in length, and also creep and hysteresis effects 10 could falsify the measurements.

The present invention arose from the task of providing a measuring transducer in which creep and hysteresis effects are largely eliminated with high mechanical resilience and the independence of the 15 measurements from temperature influences is improved.

To achieve this object, the invention proposes that, in the case of a measuring transducer of the type mentioned at the outset, the measuring body comprises a band-shaped substrate made of amorphous metal, on which sensors for the load-dependent expansion are arranged. As amorphous metal (also called metallic glass), alloys produced in a known manner (e.g. US Pat. No. 5,298,382), which have, for example, the general formula FexByCrz, come into consideration. Such amorphous metals have a very low coefficient of linear expansion with temperature (it can even be zero in the practical temperature range 10), they are very tear-resistant and have little tendency to creep and hysteresis effects. What special composition for the respective application? ste is z. B. can be determined by experiments.

A preferred embodiment of the invention is characterized in that the band-shaped substrate has a constriction, in the area of which strain gauges are arranged. This design enables a very compact design (low overall height), the measuring body itself, in particular, requiring only little material.

In another advantageous embodiment, the band-shaped substrate has two constrictions lying one behind the other in the pulling direction, in the area of which strain gauges are arranged, the measurement force being introduced between the two constrictions. In this case, the advantages of a series connection of the sensors known per se, for example better zero point stability and linearity of the measuring transducer, also result.

In both of the above-mentioned embodiments, the strain gauges are preferably arranged symmetrically in pairs.

V

- 3 -

The strain gauges are expediently vapor-deposited or sputtered onto one side of the substrate. This measure means a particularly economical manufacture of the transducer and guarantees 5 good adhesion of the strain gauges to the substrate.

Embodiments of the invention are explained below with reference to the drawings. The drawings are not to scale

1 shows a first exemplary embodiment, FIG. 2 shows the corresponding bridge circuit,

3 shows a second exemplary embodiment, and FIG. 4 shows the bridge circuit for FIG. 3.

The first example shows a measuring body 10, which is clamped with its fixed end 12 into a bracket 14 of the dynamometer, which is not described in more detail. Its free end 16 is connected to a force transducer 18.

A constriction 20 with rounded transitions can be seen symmetrically to the two ends 12, 16. In the area of the constriction 20, two resistors R-j_ and R3 are arranged on one side of the measuring body 20 10. Symmetrically there too, but rotated by 90 °, two further resistors R2 and R4 are provided outside the region of the constriction 20 (reference resistors). Leads 22 connect the ends of the four resistors to contacts 24 to which the 25 lead wires are soldered (not shown).

The measuring body has, for example, the following dimensions: free length between the clamps: 20 mm, width of the ends 12, 16: 10 mm, width of the constriction 20: 5 mm, thickness: 0.05 mm. With a tensile strength of approx. 100 kg, these dimensions allow the measurement of an s - 4 - depending on the selected safety factor on the elastic limit.

Tensile force F of several kg.

The production of the measuring body 10 can be carried out as follows: · - punching out the blanks 5 - provisional fastening on a carrier - surface grinding of the blanks - application of the resistors, the lines and the contact points using one of the known photolithographic methods, 10 whereby the processing of several pieces each done simultaneously.

When the measuring body 10 is loaded with the measuring force F, the resistors R 1 and R 3 are loaded in tension. Correspondingly, the bridge of the evaluation circuit (cf. FIG. 2) fed by the voltage Ug is detuned and supplies an evaluation signal U ^ in a known manner.

The second example (FIGS. 3 and 4) shows a measuring body 30, which in turn is clamped in place at the top (end 32, bracket 34). Its lower end 36 is also connected to the bracket 34 via a biasing spring 38, the biasing spring 38 being softer than the measuring body 30, that is to say having a smaller spring constant than this.

The measuring body 30 has two constrictions 40, 42, in each of which a resistance Rx or Ry is applied, for example vapor-deposited, including contacts 44 for supply lines 46. A bore 50 can be seen in the central part 48 of the measuring body 30. It is used to attach a coupling member to introduce the measuring force 30 (not shown), which in turn is directed downwards and thus, taking into account the preload - 5 - - by the spring 38, an increase in the tensile force in the upper region 40 ( Rx) affects. This arrangement results in improved stability since, for example, temperature fluctuations have an equal effect on both resistances (Rx and Ry).

Figure 4 shows the corresponding bridge circuit for evaluation (similar to Figure 2 for the first example).

«

Analogous to the first example, the resistors could also be arranged in pairs in both constrictions 40, 42 in order to increase the signal swing (sensitivity of the evaluation circuit). The bridge circuit would then have to be modified accordingly.

If a much larger spring constant is selected for the preload spring 38 than for the measuring body 30, in addition to a better linearity between measuring force and measuring signal, a doubling of the measuring signal results:

To the same extent that resistance 40 (Rx) changes in one direction under the measuring force, resistance 42 (Ry) changes in the opposite direction.

20 The examples mentioned can be modified in various ways within the scope of the concept of the invention. In the case of the first exemplary embodiment, the measurement can in principle also be carried out with only one or two resistors. However, this is then associated with the disadvantage that a smaller measurement signal is present and the compensation of interference (e.g. temperature fluctuations) does not happen automatically.

Claims (5)

1. Measuring transducer for a dynamometer, comprising a measuring body that is subjected to tension, characterized in that the measuring body (10; 30) comprises a band-shaped substrate made of amorphous metal, on which sensors (R ^ .. 5. R ^; Rx, Ry) for the load-dependent. Stretch are arranged. 2. Measuring transducer according to claim 1, characterized in that the band-shaped substrate has a constriction (20), in the area of which strain gauges (R] _, 10 R3) are arranged. 3. Transducer according to claim 1, characterized in that the band-shaped substrate has two constrictions (40, 42) one behind the other in the pulling direction, in the area of which strain gauges (Rx, Ry) 15 are arranged, the introduction of the measuring force between the two constrictions he follows. 4. Transducer according to claim 2 or 3, characterized in that the strain gauges are arranged symmetrically in pairs. 5. Transducer according to one of claims 2 to 4, characterized * in that the strain gauges (R ^ ... R4; Rx »Ry) are vapor-deposited or sputtered onto the substrate on one side.