"Accuracy is the politeness of kings!" Nowadays the relevance of this medieval French aphorism is only increasing. Instruments based on strain gauges are increasingly being used for accurate measurement calculations in production and in everyday life.
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What is strain gauge and what are strain gauges for?
Strain gauge (from Latin tensus - stressed) is a method and technique for measuring the stress-strain state of the measured object or structure. The fact is that it is impossible to directly measure the mechanical stress, so the task is to measure the deformation of the object and calculate the stress using special techniques that take into account the physical properties of the material.
The strain gauges are based on the strain effect, which is the property of solid materials to change their resistance under different deformations. Strain gauges are devices that measure the elastic deformation of a solid body and convert its value into an electrical signal. This process occurs when the resistance of the sensor conductor changes as it is stretched and compressed. They are a core element in devices for measuring strain of solids (e.g. machine parts, constructions, buildings).
Design and principle of operation
Strain gauge consists of a strain gauge, equipped with special contacts, which are fixed on the front part of measuring panel. In the process of measurement, sensitive contacts of the panel touch the object. Their deformation occurs, which is measured and converted into an electrical signal transmitted to the processing and display elements of the measured value of the strain gauge sensor.
Depending on the scope of functional use, the sensors differ in both types and types of measured values. An important factor is the required measurement accuracy. For example, the load cell of a truck scale at a bakery exit is absolutely no match for an electronic pharmacy scale, where every hundredth of a gram is important.
Let's take a closer look at the types and kinds of modern strain gauges.
Torque sensors
Torque sensors are designed to measure torque on rotating parts of systems such as an engine crankshaft or steering column. Torque sensors can determine both static and dynamic torque in a contact or contactless (telemetry) manner.
Beam, cantilever and edge type load cells
These types of sensors are usually based on a parallelogram design with an integrated bending element for high sensitivity and linearity of measurement. Strain gauges in them are fixed on the sensitive parts of the elastic element of the sensor and connected according to the scheme of a full bridge.
Structurally, the girder strain gauge has special holes for uneven load distribution and detection of compressive and tensile strains. For maximum effect, strain gauges are strictly oriented on the surface of the beam at its thinnest point by special marks. Highly accurate and reliable sensors of this type are used to create multi-sensor measuring systems in platform or hopper scales. They also found their application in weight batchers, bulk and liquid product packers, cable tension meters and other force load meters.
Tensile and Compressive Load Cells
Tensile and compression load cells are generally S-shaped, made of aluminum and stainless steel. Designed for hopper scales and weighers with a measuring range of 0.2 to 20 tons. S-shaped tensile and compression load cells can be used in machines for the production of cables, fabrics and fibers to control the tensile force of these materials.
Wire and foil strain gauges
Wired Strain gauges are made in the form of a spiral of small diameter wire and are attached to an elastic element or the part under investigation by means of adhesive. Their characteristics are:
- ease of fabrication;
- linear dependence on the strain;
- small dimensions and price.
Of the disadvantages note the low sensitivity, the influence of temperature and humidity of the environment on the error of measurement, the possibility of application only in the field of elastic deformation.
Foil strain gauges are currently the most common type of strain gauges due to their high metrological qualities and manufacturability. This became available due to the photolithographic technology of their production. An advanced technology makes it possible to produce single strain gauges with a 0.3 mm base, specialized strain gauge sockets and chains of strain gauges with a wide operating temperature range ranging from -240 to +1100 ºС, depending on the properties of materials of the measuring lattice.
Advantages and disadvantages of load cells
Strain gauges are widely used because of their properties:
- the possibility of monolithic connection of the strain gauge with the part under study;
- small thickness of a measuring element that provides high accuracy of measurements with an error of 1-3 %;
- convenience of mounting, both on flat and curvilinear surfaces;
- capability to measure dynamic deformations changing with frequency up to 50000 Hz;
- possibility of measurements in difficult environmental conditions in the temperature range from -240 to +1100˚C;
- Possibility to measure parameters simultaneously in many points of the parts;
- possibility of measuring deformations of objects located at big distances from strain measurement systems;
- possibility of measuring deformations in moving (rotating) parts.
Of the disadvantages it should be noted:
- the influence of meteorological conditions (temperature and humidity) on the sensitivity of sensors;
- slight changes in the resistance of the measuring elements (about 1%) require the use of signal amplifiers.
- When strain gauges work in a high-temperature or corrosive environment, special measures for their protection are required.
Basic connection diagrams
Let's consider an example of strain gauges connection to domestic or industrial scales. The standard load cell for scales has four differently colored wires: two inputs are power (+Ex, -Ex), the other two are measurement outputs (+Sig, -Sig). There are also variants with five wires, where an additional wire serves as a shield for all the others. The essence of the beam-type weight measuring sensor is quite simple. Power is applied to the inputs, and voltage is taken from the outputs. The magnitude of the voltage depends on the load applied to the measurement sensor.
If the length of wires from the weight load cell to the ADC unit is significant, the resistance of the wires themselves will affect the reading of the scales. In this case it is advisable to add a feedback circuit, which compensates for the voltage drop by correcting the error from the resistance of the wires, introduced into the measuring circuit. In this case, the wiring diagram will have three pairs of wires: power, measurement and loss compensation.
Examples of using strain gauge sensors
- A component in the construction of scales.
- measurement of deformation forces during metal forming on forging presses and rolling mills.
- Monitoring of stress - strain states of building structures and constructions during their erection and operation.
- High-temperature sensors made of heat-resistant alloyed steel for metallurgical plants.
- with stainless steel resilient element for measurements in chemically aggressive environments.
- for pressure measurement in oil and gas pipelines.
Simplicity, convenience and manufacturability of load cells are the main factors for their further active implementation, both in metrological processes and use in everyday life as measuring elements of household appliances.
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