DE20016329U1 - Bending moment sensor - Google Patents

Description

Mannesmann V D O AG Kruppstrasse 105

60388 Frankfurt
VF42RS/RA-tp

4773

Description

Bending moment sensor

The invention relates to a bending moment sensor with a bridge circuit having strain-sensitive resistors, wherein the strain-sensitive resistors are arranged directly on a component on which bending forces act directly without an intermediate carrier, wherein when the component is bent, an electrical signal corresponding to the strain of the thick-film resistors can be taken from the bridge circuit.

Bending moment sensors known per se have bridge circuits which are formed using strain-sensitive thick-film resistors which are arranged directly on a component to be subjected to bending. The resistors of the bridge circuits are arranged outside the direction of extension of a phase of the component which is neutral for bending loads and are at a predetermined distance and a predetermined angle to this phase.

However, when locally varying bending moments are applied, different resistance changes and additional torsional influences occur in the resistors, which distort the measurement result.

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The invention is therefore based on the object of specifying a bending moment sensor in which the generation of an error-free bridge signal is guaranteed under varying bending load.

According to the invention, the object is achieved in that the strain-sensitive resistors are arranged on the component in an approximately circular manner, the resistors of each bridge branch being arranged diagonally to one another in a position in which the change in resistance under bending load assumes the same values and the output signal that can be taken from the bridge circuit is only dependent on the bending load.

The invention has the advantage that the signal changes caused by different strain values in the direction of current flow through the resistors do not have to be compensated in a circuit, but are corrected solely by the placement of the resistors.

In a further development, the strain-sensitive resistors of the bridge branches are arranged outside a recess running on the surface of the component in the edge region of this recess, encompassing it.

This has the advantage that the signal behavior of the bending moment sensor can be increased in a simple manner. Due to the recess, the mechanical stresses acting on the carrier element overlap, with the strain in the main directions (longitudinal, transverse) having an unequal amount, whereby the measured signal tapped at the bridge circuit can be increased in a simple manner.

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Load-induced changes in the resistance values can be corrected in a particularly simple manner if the component has radial indentations in its edge region and the recess has radial regions, wherein a radial indentation and a radial region of the recess are each assigned to a resistor which is arranged on a connecting line of the first radius of the indentation and a second radius of the recess.

If the recess is circular, each resistor is arranged radially around the recess at approximately equal angular distances. This means that the resistors are at the same distance from the center of the hole.

The recess simply increases the signal behavior of the sensor without complex changes to the shaft geometry. Such a sensor is suitable for mass production because it can be manufactured cost-effectively and quickly.

Advantageously, the resistors arranged on the metal component are designed as thick-film resistors, whereby the sensitivity of the resistor pastes used to produce the resistors differs with respect to longitudinal and transverse expansion. This also improves the signal behavior of the sensor.

In a further development of the invention, the thick-film resistors for measuring the bending are arranged in one plane. However, it is also possible to arrange the thick-film resistors in one or more planes.

Advantageously, two bridge circuits can also be used, with the resistors of the first bridge circuit in a position T ₁ < r ₀ and the resistors of the second bridge circuit in a position

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r ₂ > r ₀ are arranged, whereby the positions T ₁ and r ₂ have the same difference in magnitude to the position r ₀ .

The invention allows for numerous embodiments. One of these will be explained in more detail with reference to the figures shown in the drawing.

It shows:

Figure 1: Top view of a component according to the invention to be subjected to bending

Figure 2: Arrangement of the strain-sensitive resistor on the component according to Figure 1

Figure 3: mechanical load on the component
Figure 4: Voltage curve in the bridge circuit

Identical features are marked with the same reference symbols.

Figure 1 shows a bending moment sensor. Identically constructed thick-film resistors R1, R2, R3, R4 are arranged on a shaft 1 made of steel or a steel alloy and cuboid-shaped, which is subjected to bending. The resistors R1, R2, R3, R4 are linked to form a bridge circuit as shown in Figure 4.

The resistance bridge is arranged over its entire extent on a dielectric 2, which rests directly on the component 1 without an intermediate carrier. A section through a strain-sensitive resistor R1 is shown in Figure 2.

As can be seen from Figure 2, there are electrical conductor tracks 5 on the dielectric 2, which are formed by a conductor track layer. An electrical resistance layer 9 extends between these conductor tracks 5, which forms the resistor R1, R2, R3 or R4 designed as a strain gauge. The end is formed by a passivation layer 6, which leaves only the part of at least one conductor track 5 serving as the contact surface 7 uncovered and serves to electrically contact the resistor R1.

The strain gauge described is manufactured using thick-film technology directly on the carrier 1.

In order to create an intimate connection between the dielectric 2 and the component 1, the dielectric 2 is applied to the shaft 1 using a printing technique using a non-conductive paste. The paste contains a glass frit that melts at a lower temperature than the material of the shaft 1. After the paste has been applied, a conductive layer is also applied using screen printing technology, which forms the conductor track 5 and the contact surface 7, on which in turn the structured resistance layer 4 forming the resistors R1, R2, R3, R4 is arranged. The shaft 1 prepared in this way is heat-treated in a high-temperature process at a temperature of approximately 750 to 900 ^° C. The glass layer sinters with the surface of the steel of the shaft 1. During this sintering, oxide bridges are formed between the dielectric 2 and the shaft 1, which ensure an inseparable connection between the shaft 1 and the dielectric 2, thereby achieving a very intimate connection between the two.

As can be seen from Figure 1, the shaft 1 has a rectangular surface 8, with a circular opening 3 formed in the center, which completely penetrates the shaft 1.

The edge of the component 1 has two semicircular edge recesses 9, 10 and 11, 12 on both sides in its longitudinal extension, with the recesses 10 and 11 and 9 and 12 being arranged opposite one another. The radii of the edge recesses 9, 10, 11, 12 correspond approximately to the radius of the opening 3.

The strain gauges R1, R2, R3, R4 are each arranged on a line 13 starting from the center of the opening 3, wherein the line 13 represents the imaginary connection between the radius of an edge recess 9, 10, 11, 12 and the radius of the opening 3.
Due to the opening 3 and the edge recesses 9, 10, 11, 12, two main strains of different magnitude occur on the surface of the shaft 1 when subjected to bending along an imaginary center line Z, which, from the point of view of the respective thick-film resistance R1 to R4, correspond to a longitudinal strain and a transverse strain.

As can be seen from Figure 3, shaft 1 is considered to be a bending beam that is firmly clamped on one side. Based on this, the X-axis shown in Figure 3 forms a flexible axis, while the axis pointing in the Y direction is a rigid axis. The shaft axis Z also corresponds to the neutral phase of shaft 1 with regard to the bending load.

According to Figure 1, the resistors R1 and R2 or R3 and R4 forming a bridge branch are arranged on both sides of the neutral phase. Since all resistors R1 to R4 are positioned at the same radial distance around the recess 3, they all have the same distance from the neutral phase in terms of magnitude, but differ in their angular position to the neutral phase.

The resistors R1, R2, R3, R4 are electrically connected to form a bridge circuit as shown in Figure 4.

The resistors behave differently under locally varying bending loads, whereby there is a position at the center of the opening 3 at which the change in resistance in all resistors R1, R2, R3, R4 is the same. This applies to any bending load about the rigid axis.

For a general bending load (moment and moment gradient) around the rigid axis (Y-axis), it follows that at the distance r ₀ of the resistors R1 and R3 to the center of the opening 3, the resistance changes AR ₁ and AR ₃ reach equal values.

The following applies to the resistance changes Δ R2 and Δ R4:

ΔR ₂ = - AR ₃ and AR ₄ = -AR ₁ (1)

When connected as shown in Figure 4, a bridge signal is obtained

U ₀ = U-(M ₁ +ARs-AR ₂ -M ₄ ) (2)

For the position r _n we get

₁ = _AR3 (3)
Taking into account Figure 1, this results in
_AR2 = _AR4 (4)
where formula 2 results in a bridge signal of

results.

A ₅ is a constant factor that depends on the exact sensor dimensions, the sensor material and the resistance properties.

M _Y (z _c ) is the value of the bending moment about the rigid axis (Y-axis in Figure).

This circuit is also insensitive to torsion, since for symmetry reasons under torsion

_AR1 = _AR2 and _AR3 = _AR4

which results in the bridge signal U _Q = 0 in formula (2).
Due to this arrangement and signal evaluation, in the position r = r ₀ this sensor is torsionally and transversely compensated for bending moments around the rigid axis. In addition, the sensor is insensitive to bending around the flexible axis.

A similar picture emerges for any locally varying bending load around the flexible axis (X-axis). The following applies:

_AR1 = _AR4 and _AR2 = _AR3

for each radial position r. This also gives a Uq = 0 for the bridge signal

Several such bridge circuits can be arranged in any order on such a sensor. Bending measurements can also be carried out if the bridge resistances are in one or more planes

They lead to the same torsionally compensated result.

A reliable measurement can also be achieved if two bridge circuits are present, with the resistances of one bridge in positions r>r ₀ and the resistances of the other bridge in positions

r<r ₀ , whereby the torsional and shear force-related errors each have different signs and the magnitudes of the errors are equal.

Claims (9)

1. Bending moment sensor, consisting of a bridge circuit having strain-sensitive resistors, the strain-sensitive resistors being arranged directly on a metallic component on which bending forces act directly without an intermediate carrier, an electrical signal corresponding to the strain of the thick-film resistors being able to be taken from the bridge circuit when the component is bent, characterized in that the strain-sensitive resistors (R1, R2, R3, R4) are arranged on the component ( 1 ) in a circle-like manner, the resistors of a bridge branch (R1, R2; R3, R4)) being arranged diagonally to one another in a position (r ₀ ) in which the change in resistance of each resistor (R1, R2, R3, R4) assumes the same values under bending, and the output signal (U _Q ) which can be taken from the bridge circuit is only dependent on the bending load. 2. Bending moment sensor according to claim 1, characterized in that the strain-sensitive resistors (R1, R2, R3, R4) of the bridge branches are arranged outside a recess ( 3 ) extending on the surface ( 8 ) of the component ( 1 ), enclosing the latter. 3. Bending moment sensor according to claim 2, characterized in that the component ( 1 ) has radial indentations ( 9 , 10 , 11 , 12 ) in its edge region and the recess ( 3 ) has radial regions, wherein a radial indentation ( 9 , 10 , 11 , 12 ) and a radial region of the recess ( 3 ) are each assigned to a resistor (R1, R2, R3, R4) which is arranged on a connecting line ( 13 ) of the first radius of the indentation ( 9 , 10 , 11 , 12 ) and a second radius of the recess ( 3 ). 4. Bending moment sensor according to claim 3, characterized in that the recess ( 3 ) is circular, each resistor (R1, R2, R3, R4) being arranged radially at approximately equal angular distances around the recess ( 3 ) and in its longitudinal extension perpendicular to the connecting line ( 13 ). 5. Bending moment sensor according to one of the preceding claims, characterized in that the resistors (R1, R2, R3, R4) are arranged on the flat surface ( 8 ) of the component ( 1 ) made of metal and having a rectangular cross-section. 6. Bending moment sensor according to claim 5, characterized in that the resistors (R1, R2, R3, R4) are designed as thick-film resistors, the sensitivity of the resistance paste used to produce the resistors being different with respect to longitudinal and transverse expansion. 7. Bending moment sensor according to claim 5 or 6, characterized in that for measuring the bending the thick-film resistors (R1, R2, R3, R4) are arranged in one plane. 8. Bending moment sensor according to claim 5 or 6, characterized in that the thick-film resistors (R1, R2, R3, R4) are arranged in two or more planes to measure the bending. 9. Bending moment sensor according to claim 1, characterized in that two bridge circuits are present, wherein the resistors of the first bridge circuit are arranged in a position r ₁ < r ₀ and the resistors of the second bridge circuit are arranged in a position r ₂ > r ₀ , wherein the positions r ₁ and r ₂ have the same difference in magnitude to the position r ₀ .