Strain gage sensors: description, user manual, specifications and reviews

Strain gauges are devices that convert the measured elastic deformation of a solid into an electrical signal. This is due to a change in the resistance of the sensor conductor when its geometric dimensions change from stretching or compression.

Strain gage sensor: principle of operation

The main element of the device is a strain gage fixed to an elastic structure. The load cells are calibrated, stepwise loaded with a given increasing force and measuring the electrical resistance. Then by its change it will be possible to determine the values of the applied unknown load and the deformation proportional to it.

Depending on the type of sensors, you can measure:

• Force;
• pressure;
• Moving;
• torque;
• acceleration.

Even with the most complicated structure loading scheme, the action on the strain gage reduces to stretching or compressing its lattice along a long section called the base.

What are the strain gauges used for?

The most common types of strain gauges with a change in active resistance under mechanical action are strain gauges.

Wire strain gauges

The simplest example is a straight section of a thin wire, which is attached to the part under study. Its resistance is: r = pL / s, where p is the resistivity, L is the length, and s is the cross-sectional area.

The glued wire is elastically deformed together with the workpiece. This changes its geometric dimensions. When compressed, the cross-section of the conductor increases, and when it is stretched, it decreases. Therefore, the change in resistance changes sign depending on the direction of deformation. The characteristic is linear.

The low sensitivity of the strain gage led to the need to increase the length of the wire in a small measuring section. To do this, it is made in the form of a spiral (lattice) made of wire, glued on both sides with plates of insulation from a film of lacquer or paper. For connection to an electrical circuit the device is equipped with two copper lead wires. They are welded or soldered to the ends of a wire spiral and strong enough to connect to the electrical circuit. The strain gage is attached to the elastic element or the part to be tested with an adhesive.

Wire strain gauges have the following advantages:

• Simplicity of design;
• Linear dependence on deformation;
• Small size;
• Low price.

The disadvantages include low sensitivity, the influence of environmental temperature, the need for protection from moisture, and application only in the field of elastic deformations.

The wire will deform if the bonding strength of the adhesive is much greater than the force required to stretch it. The ratio of the bonding surface to the cross-sectional area should be 160 to 200, which corresponds to its diameter of 0.02-0.025 mm. It is possible to increase it to 0.05 mm. Then, during normal operation of the strain gage, the adhesive layer does not collapse. In addition, the sensor works well for compression, since the wires of the wire are integral with the adhesive film and the part.

Strain gauges from foil

Parameters and principle of action of the foil strain gage are the same as those of wire gages. Only the material is a foil of nichrome, constantan or titanium-aluminum. The technology of photolithography makes it possible to obtain a complex lattice configuration and automate the process.

In comparison with wire, foil strain gauges are more sensitive, they pass more current, better transmit deformation, have stronger conclusions and a more complicated drawing.

Semiconductor strain gauges

The sensitivity of the sensors is about 100 times higher than the wire, which makes it often possible to use them without amplifiers. The disadvantages are fragility, a large dependence on the ambient temperature and a wide range of parameters.

Characteristics of strain gages

1. Base - the length of the conductor of the lattice (0.2-150 mm).
2. The nominal resistance R is the value of the active resistance (10-1000 Ohm).
3. Operating current I _p is the current at which the strain gage is not noticeably heated. In case of overheating, the properties of the materials of the sensitive element, the substrate and the adhesive layer, which distort the readings, change.
4. The coefficient of sensitivity is: s = (ΔR / R) / (ΔL / L), where R and L are the electrical resistance and the length of the unloaded sensor, respectively; ΔR and ΔL - change of resistance and deformation from external force. For different materials, it can be positive (R increases with stretching) and negative (R increases with compression). The value of s for different metals varies from -12.6 to +6.

Schemes of inclusion of strain gauges

To measure small electrical signals, the best option is a bridge switch, in the center of which is a voltmeter. The simplest example is a strain gage sensor, the circuit of which is assembled on the principle of an electric bridge, to one of whose shoulders it is connected. Its resistance in an unloaded state will be the same as for the rest of the resistors. In this case, the instrument will display zero voltage. The principle of the tensometric sensor is to increase or decrease the value of its resistance, depending on whether the forces are compressive or tensile.

The accuracy of the readings is significantly affected by the temperature of the strain gage. If you include a similar tensor resistance in the other arm of the bridge, which will not be loaded, it will perform the function of compensating for thermal effects.

In the measuring circuit, the electrical resistances of the wires connected to the resistor must also be taken into account. Their influence is reduced by adding one more wire connected to any lead of the strain gage and a voltmeter.

If the two elements are glued to the elastic element in such a way that their loads differ in sign, the signal will increase by a factor of 2. If there are four sensors in the circuit with the loads indicated in the diagram above by arrows, the sensitivity will increase significantly. With this connection of wire or foil strain gauges an ordinary microammeter will give an indication without an amplifier of electrical signals. It is important to accurately match the resistance values with a multimeter so that they are equal in each arm of the electric bridge.

The use of strain gauges in technology

1. Part of the design of the balance: during weighing, the sensor body is elastically deformed, and together with it the strain gauges attached to it are connected to the circuit. The electrical signal is transmitted to the measuring device.
2. Monitoring of the stress-strain state of building structures and engineering structures in the process of their erection and operation.
3. Strain gauges for measuring the strain force in the treatment of metals by pressure in rolling mills and stamping presses.
4. High-temperature sensors for metallurgical and other enterprises.
5. Measuring sensors with an elastic element of stainless steel for work in a chemically aggressive environment.

Standard strain gauges are made in the form of washers, columns, simple or bilateral beams, S-shaped. For all designs, it is important that the force is applied in one direction: from top to bottom or vice versa. Under severe operating conditions, special designs make it possible to eliminate the effect of parasitic forces. Their prices largely depend on this.

On the strain gauges the price is from hundreds of rubles to hundreds of thousands. Much depends on the manufacturer, design, materials, manufacturing technology, the values of the measured parameters, additional electronic equipment. For the most part, they are the constituent parts of scales of different types.

Conclusion

The principle of operation of all strain gauges is based on the transformation of the deformation of an elastic element into an electrical signal. For different purposes, there are different sensor designs. When strain gauges are selected, it is important to determine whether there is compensation in the circuits for the distorted readings of temperature and parasitic mechanical influences.