Strain gauges are devices that convert the measured elastic deformation of a solid into an electrical signal. This occurs 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. The load cells are calibrated, stepwise loading with a given increasing force and measuring the value of 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:
- force;
- pressure;
- moving
- torque;
- acceleration.
Even with the most complex design loading scheme, the action on the strain gauge is reduced to stretching or compressing its grating along a long section called the base.
What strain gauges are used
The most common types of strain gauges with a change in resistance during mechanical stress are strain gauges.
Wire strain gages
The simplest example is a straight line of thin wire, which is mounted on the test piece. Its resistance is: r = pL / s, where p is the resistivity, L is the length, s is the cross-sectional area.
The glued wire is elastically deformed along with the part. At the same time, its geometric dimensions change. With compression, the cross section of the conductor increases, and with tension it decreases. Therefore, a change in resistance changes sign depending on the direction of deformation. The characteristic is linear.
The low sensitivity of the strain gauge has 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 an electrical circuit, the device is equipped with two copper lead wires. They are welded or soldered to the ends of the wire spiral and are strong enough to connect to the electrical circuit. The strain gauge is mounted on the elastic element or the test piece with glue.
Wire strain gauges have the following advantages:
- simplicity of design;
- linear dependence on deformation;
- small sizes;
- low price.
The disadvantages include low sensitivity, the influence of the temperature of the medium, the need for protection from moisture, use only in the field of elastic deformations.
The wire will deform in the case when the adhesive force of glue with it significantly exceeds the forces required for its stretching. 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 allowed to increase to 0.05 mm. Then, during normal operation of the strain gauge, the adhesive layer will not collapse. In addition, the sensor works well in compression, since the filaments of wire are integral with the adhesive film and the part.
Foil load cells
The parameters and the principle of operation of the foil strain gauge are the same as for wire gauges. The only material is nichrome, constantan or titanium-aluminum foil. Photolithography manufacturing technology allows you to get a complex lattice configuration and automate the process.
Compared to wire, foil strain gauge sensors are more sensitive, they pass more current, they transmit strain better, they have more durable conclusions and a more complicated pattern.
Semiconductor load cells
The sensitivity of the sensors is approximately 100 times higher than wire sensors, which often allows them to be used without amplifiers. The disadvantages are fragility, a large dependence on ambient temperature and a significant variation in parameters.
Strain gages characteristics
- Base - the length of the grating conductor (0.2-150 mm).
- Nominal resistance R is the value of active resistance (10-1000 Ohms).
- The operating supply current I p is the current at which the strain gauge does not noticeably heat up. When overheating, the properties of the materials of the sensitive element, base and adhesive layer, distorting the readings, change.
- Strain sensitivity coefficient: 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.
Strain gauge inclusion circuits
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, into one of the shoulders of which it is connected. Its resistance in the unloaded state will be the same as that of the other resistors. In this case, the device will show zero voltage.
The principle of operation of the strain gauge 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 gauge. If a similar strain resistance is included in the other shoulder of the bridge, which will not be loaded, it will perform the function of compensation during thermal influences.
The measurement circuit should also take into account the values of the electrical resistances of the wires connected to the resistor. Their influence is reduced by the addition of another wire connected to a terminal of the strain gauge and a voltmeter.
If both sensors are glued to an elastic element so 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 circuit above, the sensitivity will increase significantly. With this connection of wire or foil strain gauges, a conventional microammeter will give readings without an electric signal amplifier. It is important to accurately select the resistance values with a multimeter so that they are equal to each other in each arm of the electric bridge.
Application of strain gauges in technology
- Part of the design of the scale: when weighing, the sensor body is elastically deformed, and with it the strain gages glued to it, connected to the circuit. An electrical signal is transmitted to the meter.
- Monitoring the stress-strain state of building structures and engineering structures in the process of their construction and operation.
- Strain gages for measuring the strain force during metal forming in rolling mills and stamping presses.
- High temperature sensors for metallurgical and other enterprises.
- Measuring sensors with an elastic element made of stainless steel for operation in a chemically aggressive environment.
Standard strain gauges are made in the form of washers, columns, single 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 to a large extent 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, measured values, additional electronic equipment. For the most part, they are components of various types of scales.
Conclusion
The principle of operation of all load cells is based on the transformation of the deformation of the elastic element into an electrical signal. For different purposes, there are their own sensor designs. When strain gauge sensors are selected, it is important to determine if the circuitry contains compensation for distorting readings of temperature and spurious mechanical influences.