Principle of Piezomagnetic Force Measuring Device-EEWORLD

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Force measurement principle

By measuring the force, torque and pressure of mechanical parts and mechanical structures, we can analyze their stress conditions and working conditions, verify design calculations, and determine the working process and the mechanism of certain physical phenomena. It plays an important guiding role in the safe operation of equipment, automatic control and the development of design theory.

The methods of measuring force can be summarized into two types: utilizing the static effect and dynamic effect of force.

10.1.1 Measurement of stress and strain

The commonly used force measurement method is to use strain gauges and strain meters to measure the surface strain of the component, and determine the stress state of the component based on the relationship between strain, stress and force.

When the strain gauge uses an AC bridge, the output characteristics are similar to those of a DC bridge (output characteristics of a DC bridge).

The arrangement and bridge connection of strain gauges (referred to as strain gauge bridge connection) should be determined according to the force distribution of the measured object and the object being measured. Appropriate strain gauge bridge connection should also be used to eliminate the effects of temperature changes and combined loads.

When measuring tensile (compressive) strain, an appropriate bridge arrangement should be used to achieve temperature compensation (temperature compensation when measuring axial tension (compression)), eliminate the influence of bending moment (use double working pieces to eliminate the influence of temperature) and improve measurement sensitivity (use four working pieces to improve measurement sensitivity).

Commonly used stress measurement patch and bridge assembly methods:

When the specimen is subjected to bending moment, tensile strain or compressive strain will be generated on its upper and lower surfaces respectively. The bending moment can be obtained by strain measurement. When bridging the fabric, attention should be paid to utilizing the characteristics of the bridge, retaining the influence of bending strain in the output, and eliminating the strain components generated by axial tension and pressure.

2. Resistance strain gauge force measuring device

Other force sensors

1. Capacitive force sensor

It is characterized by simple structure, high sensitivity and fast dynamic response, but it is not suitable for static force measurement because charge leakage is difficult to avoid (structural principle of capacitive force sensor).

2. Piezoelectric force sensor

The previous chapter introduced the principle of piezoelectric sensors and piezoelectric vibration acceleration sensors. The structure of force sensors is similar. They are small in size and fast in dynamic response, but they also have charge leakage and are not suitable for static force measurement. During use, they should be prevented from being subjected to lateral forces and preload.

3. Piezomagnetic force measuring device

Its characteristic is that after the force-bearing surface of silicon steel material is enlarged, thousands of tons of force can be measured, and the output potential is large, and even only filtering and rectification are needed , without amplification processing. It is often used to measure the rolling force of large rolling mills. During use, it should be prevented from damaging the laminated structure of silicon steel due to lateral force interference (the working principle of the piezomagnetic force measuring device).

4. Differential transformer force sensor

Its characteristics are wide operating temperature range. In order to reduce the influence of lateral force or eccentric force, the height-to-diameter ratio of the sensor should be small. (Working principle of differential transformer force sensor)

Capacitive force sensor

Several through holes are machined on a special rectangular elastic element. Two T-shaped electrodes with parallel end faces are fixed in each hole. Copper foil is attached to each electrode, forming a measurement circuit composed of multiple parallel plate capacitors in parallel. Under the action of force F , the elastic element deforms to change the moment between the plates, thereby changing the capacitance, as shown in the left figure (capacitive force sensor).

Principle of Piezomagnetic Force Measuring Device

When some ferromagnetic materials are subjected to external forces, the phenomenon of magnetic permeability change is called piezomagnetic effect. Its inverse effect is called magnetostrictive effect. When silicon steel is compressed, its magnetic permeability decreases along the stress direction and increases along the vertical direction of the stress; when stretched, the change of magnetic permeability is just the opposite. If there are 4 symmetrical through holes on the silicon steel laminate, two coils perpendicular to each other are wound in the holes, as shown in the left figure (Figure piezomagnetic element working principle), one coil is the excitation winding and the other is the measurement winding. When there is no external force, the magnetic lines of force do not interlink with the measurement winding, and the measurement winding does not generate induced potential. When subjected to external force, the distribution of magnetic lines of force changes, part of the magnetic lines of force interlink with the measurement winding, and an induced potential is generated in the winding, and the greater the force, the greater the induced potential.

When measuring force, you can directly place the bridge on the object to be measured, or you can place the bridge on the elastic element so that the force is transmitted to the strain gauge through the elastic element. Commonly used elastic elements include column type, beam type, ring type, spoke type and other forms.

Column elastic element The force is measured through the tensile (compressive) deformation of the column elastic element surface. The pasting of the strain gauge and the connection of the bridge should eliminate the influence of eccentricity and bending moment as much as possible. Generally, the strain gauge is symmetrically pasted in the middle of the cylindrical surface with uniform stress. Column force sensor can measure loads of 0.1 to 3000 tons and is often used for rolling force measurement of large steel rolling equipment.

Beam-type elastic elements include beams of equal cross-section, beams of equal strength, and double-end fixed beams. They measure force through the bending deformation of the beam and have a simple structure and high sensitivity.

Ring-type elastic elements are divided into circular ring type and octagonal ring type. It also measures force through the bending deformation of the element and has a compact structure. Practical applications include cutting dynamometers.

Spoke-type elastic element The stress state of spoke-type elastic elements can be divided into tension and compression, bending and shear. The first two types of force measuring elastic elements are often used, and their accuracy and stability have reached a certain level. However, changes in installation conditions or movement of the force point will cause errors that are difficult to estimate. Shear-type elastic elements have the characteristics of being insensitive to loading methods, anti-eccentric loading, lateral stability, and low profile.

Reference address：Principle of Piezomagnetic Force Measuring Device

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