﻿ Principle of operation of the strain gage

# How does the strain gage work?

The strain gage is called the sensor transforming izmeryaeyomy deformation of solid bodies to the electric signal. Let's consider the principle of action and the device of the strain gage of the soprotivyoleniye transforming deformation (stretching or compression) to change of electrical resistance of conductors or poluyoprovodnik.

Double-support strain gages: with cylindrical support and with flat support.

Change of resistance of the conductor of the strain gage at action of deformation is explained by 2 reasons: change of the geoyometrichesky sizes (lengths, diameter of the wire) and izmeneyoniy unit resistance of material of the strain gage.

Operation of the strain gage is characterized by coefficient of the tenzochuvstvitelnost (S) which is equal private from division of otnosiyotelny change of resistance of the strain gage into its otnosiyotelny deformation:

where L and R — respectively length and resistance of the sensor in the absence of mechanical stress;

? L and? R — respectively change of length and resistance as a result of impact of external deformiruyuyoshchy effort.

The formula can be presented in the form:

The coefficient of the tenzochuvstvitelnost is the dimensionless quantity and can be as positive (for materials at which resistance at stretching increases), and negative (for materials at which resistance at stretching decreases). For different metals the value S fluctuates from — 12,6 (nickel) up to +6 (platinum-iridium). The size of rated resistance of the strain gage is in limits of 50 — 1000 Ohms.

Conduction strain gages do of the metal wire (nickel, the thaw, the constantan, etc.) with a diameter of 0,015 - 0,05 mm. Structurally they are carried out not pasted and pasted.

Figure 1. The constructive scheme of execution of not pasted strain gage.

Design of not pasted strain gage is given in fig. 1. The sensing device in it is mute являет­ся the round wire.

The pasted strain gage represents the thin proyovolochka put in the form of the grid (fig. 2) and which is pasted over on both sides with insulating plates from tissue paper, the film of varnish or glue.

For definition of stretching or compression the plate is strongly pasted on the part surface by means of special glue (for example, BF-2, BF-4, bakelitovy glue, etc.), strain gages perceive deformations of the outside voyolokn of the part and react to stretching and compression. In case of stretching resistance of the delay of the strain gage increases, and at compression - decreases.

Wire sensors have the small weight and dimensions, practically bezyneryotsionna and therefore can measure quickly changing deformations.

Figure 2. The constructive scheme of the pasted strain gage.

Wire strain gages can be placed in hard-to-reach spots, they are simple on the design and are cheap. All perechislenyony advantages of strain gages caused their wide circulation.

It is necessary to carry the small veliyochina of relative change of the soproyotivleniye and, therefore, small sensitivity to shortcomings of wire tenzoyodatchik. In this regard when using strain gages it is necessary to apply metering circuits of vysoyoky sensitivity and complexity.

Wire strain gages primenyayot at measurement of deformations in parts of mechanisms as the main elements of sensors of efforts, weight, pressure, the torsion torque, etc.

The strain gage is the constituent of the electric tenzoyometr — the device for measurement in solid bodies of the deformations arising under the influence of loadings.

For measurement of deforyomation sensors turn on in full-wave or potentiometer circuits. Strain gages allow to measure both static, and dynamic deformations.

In need of registration of difficult deformations apply the power supply of measuring bridges tension of high frequency; registration at the same time производит­ся, for example, on the oscillograph that does installation much more difficult.