DE1573942C - Strain measuring element made of semiconductor material with piezoelectric resistance - Google Patents

Description

The invention relates to the measurement of mechanical stresses in a test specimen using of strain gauges made of semiconductor material with piezoelectric resistance.

In many cases, strain gauges are desired with which the stress in a test body can be measured in a main direction independently of any transverse stresses that may also be present. In known strain gauges using metal wires and foils as well as semiconductors in the form of thin rods with a square cross-section, attempts are made to eliminate the influence of transverse stresses in that only stresses in the longitudinal direction of the strain gauge, but not those in the transverse direction, are transferred to the element. These strain gauges therefore have high sensitivity to stresses in the test specimen which run in their longitudinal direction and low sensitivity to stresses in their transverse direction. Nevertheless, _ao is the case of the known strain gauges due to the Poisson effect also a sensitivity to stresses in the transverse direction of Database.

The invention aims at this disadvantage in a strain gauge, consisting of a semiconductor crystal with a piezoelectric resistor and two Avoid spaced electrodes. The peculiarity according to the invention is that the piezoelectric. Resistance coefficient of the crystal across the measuring direction is smaller than 10 percent of the piezoelectric coefficient of resistance of the crystal in the direction of measurement.

By choosing certain crystal orientations, a strain gauge can be produced, which is essentially not sensitive to transverse stresses, although these stresses affect the Measuring element are transmitted.

Semiconductor strain gauges that are not sensitive to transverse stresses in the medium the common feature that the transverse direction in the plane of the measuring element (perpendicular to the current direction) is symmetrical to the valley direction, z. B. <100> for a valley semiconductor <111> and <111> for one. Valley semiconductor <100>. Longitudinal directions for this Both types of semiconductors are suitable are (110) and <1TO>. The relative change in resistance is, ■

for the valley material <111>

AR (π _Μ )

R ₀ 2

for the valley material <100>.
- AR (π _η )

■ σ,

5 °

55

R _a

where σ is the decomposed longitudinal normal stress and π _Μ and TtJ _{1 are} piezoelectric fundamental drag coefficients.

Such a strain gauge has considerable sensitivity to stresses in a Main direction, but remains essentially due to tensions in a direction perpendicular to it unaffected, although transverse stresses are actually transferred to the measuring element.

Ideally, the piezoelectric resistor should. Standing coefficient of the measuring element in the transverse direction be essentially zero. For practical cases, however, it is sufficient if the piezoelectric The drag coefficient in the measuring direction is large compared to the drag coefficient in the transverse direction. It has been found to be satisfactory Results can be obtained when the drag coefficient in the transverse direction is less than 10 percent of the coefficient of resistance in the direction of measurement.

example

A strain gauge, which is essentially insensitive to transverse stresses, is made from a single crystal of η-germanium with the approximate dimensions 1.0 x 0.2 x 0.005 cm, which is connected to a solid steel plate. The measuring and at the same time longitudinal direction parallel to the current flow is <1TO> and the transverse direction is <001>. A mechanical tension in the measuring element of 1 · 10 ⁸ dynes per cm ² parallel to the longitudinal direction produces a change in resistance which is given by the equation:

Δ RIR ₀ = (π _44/2 ) (1 x 10 ^s ) = (69 x 10 "«) (1 x 10 ") ■; = 0.69-10-2 = 0.69%.

If a transverse voltage of the same order of magnitude is applied to the plate, the additional Change in resistance negligibly small.

Claims (3)

Claims 1. A strain gauge consisting of a semiconductor crystal with a piezoelectric resistance and two electrodes arranged at a distance from one another, characterized in that that the piezoelectric resistance coefficient of the crystal across the measuring direction is smaller than 10 percent of the piezoelectric coefficient of resistance of the crystal in the direction of measurement. 2. strain gauge according to claim 1, characterized in that the piezoelectric Coefficient of drag of the crystal in the transverse direction is substantially zero. 3. strain gauge according to claim 1 or 2, characterized in that the crystal consists of η-germanium and that the measuring direction <1TO> and the transverse direction <001> is.