First, the definition of load cell
Taking into account the effects of the gravitational acceleration (g) and the air buoyancy (f) at the place of use, a sensor that measures the force of the mass by converting one of the measured (mass) into another measured (output).
Second, the basic principle of load cell
The elastic body (elastic element, sensitive beam) is elastically deformed under the action of an external force, so that the resistance strain gauge (converting element) attached to the surface thereof is deformed at the same time. After the resistance strain gauge is deformed, its resistance value will change (increase or decrease), and then this resistance change will be converted into an electrical signal by a corresponding measurement circuit, thereby completing the process of transforming the external force into an electrical signal.
It can be seen that the resistance strain gauges, elastomers and detection circuits are indispensable parts of the resistance strain gauge load cell. The following brief discussion of these three aspects.
1, resistance strain gauge
Resistance strain gauges mechanically distribute a resistance wire on a substrate made of an organic material. An important parameter of the strain gage is the sensitivity K. When both ends are subjected to a F-force, they will stretch, that is, they will deform. When elongated, the cross-sectional area is reduced, and the radius of the cross-section circle is reduced. There is a proportional relationship between the rate of change of resistance of the resistance strain gauge (relative resistance change) and the elongation of the resistance wire (relative change in length).
It should be noted that the sensitivity coefficient K is a constant determined by the properties of the metal wire material, and is independent of the shape and size of the strain gauge. The K value of different materials is generally between 1.7-3.6. Secondly, the value of K is a dimensionless quantity and there is no dimension.
In material mechanics, ΔL/L is referred to as strain, which is denoted as ε. It is often used to indicate that the elasticity is too large and inconvenient, and its one millionth unit is often referred to as με.
2. Elastomer
The elastomer is a specially shaped structure. There are two functions. Firstly, it receives the external force of the load cell, generates the reaction force to the external force, and achieves the relative static balance. Secondly, it provides a high-quality strain field (zone), so that the resistance strain gauges pasted in this area are compared. Ideally completes the task of converting electrical signals.
3, detection circuit
The function of the detection circuit is to change the resistance of the resistance strain gauge into a voltage output. Because the Wheatstone bridge has many advantages, such as can suppress the influence of temperature changes, can suppress lateral force interference, can easily solve the compensation problem of the load cell, etc., so the Wheatstone bridge is obtained in the load cell. A wide range of applications.
The full-bridge equal-arm bridges are used for load cells, mainly because the sensitivity of the full-bridge equal-arm bridge is the highest, and the parameters of each arm are the same, and the effects of various interferences are easily offset by each other.
Third, how to choose a load cell
The weighing sensor is regarded as the heart of the electronic scale. Its performance largely determines the accuracy and stability of the electronic scale. When designing electronic scales, the problem of how to select sensors is often encountered.
The load cell is actually a device that converts the mass signal into a measurable electrical signal output. The actual working environment of the sensor, the correct choice of sensor is crucial, it relates to the sensor's ability to work properly and its safety and service life, and even the reliability and safety of the entire scale.
The selection of the sensor range can be determined based on the comprehensive evaluation of factors such as the maximum weighing value of the scale, the number of selected sensors, the weight of the scale body, the maximum partial load that may be generated, and the dynamic load. In general, the closer the sensor's range is to the load assigned to each sensor, the higher the accuracy of its weighing. However, in actual use, due to the load imposed on the sensor, in addition to the object being called, there are loads such as the weight of the scale body, tare weight, eccentric load, and vibration impact. Therefore, when selecting the sensor range, many factors must be considered to ensure The safety and life of the sensor.
Based on experience, the sensor should generally be operated within its 30% to 70% range. However, for some weighing instruments that have a large impact during use, such as dynamic track scales, dynamic truck scales, steel scales, etc., when selecting a sensor, it is generally necessary to expand the range so that the sensor is in the range of 20% to 30%. Working within it increases the weighing reserve of the sensor to ensure the safety and life of the sensor. The choice of sensor type depends mainly on the type of weighing and installation space, to ensure proper installation, weighing safe and reliable; on the other hand, consider the sensor manufacturer's recommendations. Manufacturers generally specify the applicable range of the sensor according to the force conditions, performance indicators, installation forms, structural types, elastomer materials and other characteristics of the sensor. For example, aluminum cantilever sensors are suitable for use in pricing scales, platform scales, case scales, etc. Cantilever beam sensors are suitable for hopper scales, electronic belt scales, sorting scales, etc. Steel bridge sensors are suitable for track scales, truck scales, crane scales, etc. Column sensors are suitable for truck scales, dynamic track scales, large tonnage hopper scales Wait.
In addition, the selected sensor must meet the input sensitivity requirements of the instrument and must match the selected instrument to meet the accuracy requirements of the entire electronic scale. An electronic scale is mainly composed of a mechanical mechanism, a sensor, and an instrument. When the accuracy of the sensor is selected, the accuracy of the sensor should be slightly higher than the theoretical value, because the theory is often limited by objective conditions, such as machinery. The factors such as the strength of the structure, the performance of the instrument, and the harsh working environment of the scale all directly affect the accuracy of the scale. Therefore, it is necessary to raise the requirements from all aspects, taking into account the economic benefits and ensuring that the objectives are achieved.
Fourth, the classification of load cells
A. Points by force method
Pressure type
Pull type
Pull type
Beam type
B. According to the elastomer material
Stainless steel: It is mainly used in places where there are corrosive environments such as food, chemical, and pharmaceuticals;
Alloy steel: corrosive environment;
Aluminum alloy: generally used in small-scale, low-cost markets.
C. According to the output data category
Analog sensor: The output is an analog signal;
Digital sensor: The output is digitally encoded.
D. According to the applicable temperature points
Normal temperature: used in a normal place, no special requirements on temperature;
High temperature: used in metallurgy, nuclear power and other high temperature (usually above 100 °C) environment;
E. Press accuracy
See GB/T7551-1997 "Load Cell"
f. divided by appearance structure
Column type
Type S
Spoke
Beam type
Single point etc.
Fifth, the basic application of load cell
The load cell is mainly used in various electronic weighing instruments, industrial control fields, online control, safety overload alarm, material testing machine and other fields. Such as electronic truck scales, electronic platform scales, electronic forklifts, dynamic axle weight scales, electronic crane scales, electronic pricing scales, electronic steel scales, electronic rail scales, hopper scales, batching scales, canned scales and so on.