Strain gauges are devices that convert the measured elastic deformation of a solid body into an electrical signal. This happens due to a change in the resistance of the sensor conductor when its geometric dimensions change from tension or compression.
Strain gauge: principle of operation
The main element of the device is a strain gauge mounted on an elastic structure. Strain gauges are calibrated by stepwise loading with a given increasing force and measuring the magnitude of the electrical resistance. Then, by changing it, it will be possible to determine the values of the applied unknown load and the strain proportional to it.
Depending on the type of sensors, you can measure:
- strength;
- pressure;
- move;
- torque;
- acceleration.
Even with the most complex loading scheme of the structure, the action onthe strain gauge is reduced to stretching or compressing its lattice along a long section called the base.
Which strain gauges are used
The most common types of strain gauges with a change in active resistance under mechanical action - strain gauges.
Wire strain gauges
The simplest example is a straight piece of 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, s is the sectional area.
The glued wire is elastically deformed together with the part. At the same time, its geometric dimensions change. When compressed, the cross section of the conductor increases, and when 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 gauge led to the need to increase the length of the wire in a small measurement area. To do this, it is made in the form of a spiral (lattice) of wire, glued on both sides with insulation plates from a film of varnish or paper. To connect to the electrical circuit, the device is equipped with two copper lead wires. They are welded or soldered to the ends of the coiled wire and are strong enough to connect to an electrical circuit. The strain gauge is attached to the elastic element or the part under test with glue.
Wire load cells have the following advantages:
- simple design;
- linear dependence on strain;
- small sizes;
- low price.
Disadvantages include low sensitivity, influence of ambient temperature, the need for protection from moisture, use only in the area of elastic deformations.
The wire will deform when the adhesion force of the adhesive to it greatly exceeds 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 can be increased up to 0.05 mm. Then, during normal operation of the strain gauge, the adhesive layer will not be destroyed. In addition, the sensor works well in compression, since the wire strands are integral with the adhesive film and the part.
Foil Load Cells
The parameters and principle of operation of the foil load cell are the same as those of the wire ones. The only material is nichrome, constantan or titanium-aluminum foil. The photolithography manufacturing technology allows obtaining a complex lattice configuration and automating the process.
Compared to wire strain gauges, foil strain gauges are more sensitive, carry more current, transmit strain better, have stronger leads, and have a more complex pattern.
Semiconductor load cells
The sensitivity of the sensors is approximately 100 times higher than the wire ones, which allows them to be used frequently without amplifiers. The disadvantages are brittleness, high dependence on ambient temperature and significantparameter spread.
Strain gauge specifications
- Base - the length of the grid conductor (0.2-150 mm).
- Nominal resistance R - active resistance value (10-1000 Ohm).
- Working supply current Ip - current at which the strain gauge does not noticeably heat up. When overheated, the properties of the materials of the sensing element, the base and the adhesive layer change, distorting the readings.
- Strain factor: s=(∆R/R)/(∆L/L), where R and L are the electrical resistance and the length of the unloaded sensor, respectively; ∆R and ∆L - change in resistance and deformation from external force. For different materials, it can be positive (R increases with tension) and negative (R increases with compression). The value of s for different metals varies from -12.6 to +6.
Schemes for switching on strain gauges
To measure small electrical signals, the best option is a bridge connection, in the center of which is a voltmeter. The simplest example would be a strain gauge, the circuit of which is assembled according to the principle of an electric bridge, in one of the arms of which it is connected. Its unloaded resistance will be the same as the rest of the resistors. In this case, the device will show zero voltage.
The principle of operation of a strain gauge is to increase or decrease the value of its resistance, depending on whether the forces are compressive or tensile.
The temperature of the strain gauge has a significant effect on the accuracy of the readings. If a similar strain resistance is included in the other arm of the bridge, which will not be loaded, it will perform the function of compensating for thermal effects.
The measuring circuit must also take into account the values of the electrical resistance of the wires connected to the resistor. Their influence is reduced by adding another wire connected to one of the pins of the strain gauge and the voltmeter.
If both sensors are glued onto the elastic element in such a way that their loads differ in sign, the signal will be amplified by 2 times. If there are four sensors in the circuit with loads indicated by arrows in the diagram above, the sensitivity will increase significantly. With this connection of wire or foil strain gauges, a conventional microammeter will give readings without an electrical signal amplifier. It is important to accurately select the resistance values using a multimeter so that they are equal to each other in each arm of the electric bridge.
Application of strain gauges in engineering
- Part of the design of the scales: when weighing, the sensor body is elastically deformed, and with it the strain gauges glued to it, connected in a circuit. The electrical signal is transmitted to the meter.
- Monitoring of the stress-strain state of building structures and engineering structures in the process of their construction and operation.
- Strain gauges for measuring deformation forces during machiningmetal pressure at rolling mills and stamping presses.
- High temperature sensors for steel and other industries.
- Measuring sensors with stainless steel elastic element for operation in chemically aggressive environments.
Standard strain gauges are made in the form of washers, columns, simple or double-sided beams, S-shaped. For all structures, 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 action of parasitic forces. Their prices largely depend on this.
For strain gauges, the price ranges 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 components of different types of scales.
Conclusion
The principle of operation of all strain gauges is based on the transformation of the deformation of the elastic element into an electrical signal. For different purposes, there are different sensor designs. When choosing strain gauges, it is important to determine whether the circuits have compensation for distorting temperature readings and parasitic mechanical influences.
