Derivation: 9.8N/kg=9.8 (kg · m/s ²)/kg=9.8m/s²[2]
Reason for naming
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stayPhysicsWhere Newton (symbol N) is the metric unit of force.It is named after Sir Isaac Newton who founded classical mechanics.Newton is aInternational System of UnitsDerived unit, which is determined by kg · m/s ²SI Basic Unitsexport[2]。
Concept of force
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Force is avector, determined by the three factors of force size, direction and action point[3]。
Force is the interaction between objects, which can change the mechanical movement state or shape of objects.In other words, any action that can change the momentum of the object to obtain acceleration or deform the object is called force.The effect that changes the moving state of an object is called the dynamic effect of force, and the effect that causes the object to deform is called the static effect of force[3]。
Force measuring principle
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General
There are many methods to measure force, for example, force can be measured by balancing with known gravity, and force can also be measured by various physical effects (such as elastic, piezomagnetic, piezoelectric and other effects) proportional to force.Although there are many methods, they can be summarized as two methods to measure force: dynamic effect of force and static effect of force[4]。
Measure the force value by using the dynamic effect of force
The dynamic effect of force is to make the object produce acceleration.according toNewton's second lawIf the mass of the object and the acceleration obtained are measured, the force on the object will also be known[1]。
In the gravity field, the gravity of the earth causes the object to produce gravitational acceleration. This force is called“gravity”。Since the gravitational acceleration of the place where the object is located is certain, the gravity of the object is proportional to the mass of the object.Therefore, the force can be measured by the gravity of an object with known mass in the gravity field, or the gravity can be magnified to measure the force later, which is the basic principle of measuring force by using the dynamic effect of force[1]。
Force reference machine, static weight type, lever type, hydraulic force standard machine, and someMaterial testing machineIs to measure the force according to the dynamic effect[1]。
Measure the force value by using the static force effect
As mentioned before, the static effect of force is to deform the object.The force can also be calculated by measuring the deformation of the object under the action of an external force, or the changes of other physical quantities caused by the deformation.Various elastic ring standard force measuring instruments, such as dial gauge type, mercury box type, and various kinds of physical effects (such as strain, piezomagnetism, piezoelectricity, etc.)force sensor The force is measured by static effect[1]。
Force measuring method
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The commonly used force measurement methods are as follows[5]：
(1) The measured force is directly measured by balancing the measured force with known gravity or electromagnetic force.
(2) The measured force is indirectly measured by measuring the acceleration of an object with known mass under the action of the measured force.
(3) The measured force is measured by measuring the fluid pressure generated by the measured force.
(4) When the measured force tightens a vibrating string, thenatural frequencyIt will change with the size of the measured force. The measured force can be measured by measuring the change of the frequency.
(5) The measured force is measured by measuring the deformation or strain of an elastic element under the action of the measured force.
Most of the above force measurement methods are used for the measurement of static force or slowly changing force.However, the last method is applicable to the measurement of static force or dynamic force with frequency below thousands of hertz, so it is a widely used force measurement method[5]。
Force measuring device
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overview
Load cellThere are many forms of (or force measuring device). According to their conversion principles, there are resistance type, inductance type, capacitance type, piezomagnetic type, piezoelectric type, etc[6]。
Resistance strain gauge load cell
For force measurement, resistance strain gauge and group bridge can be directly pasted on the measured object, or resistance strain gauge and group bridge can be pasted on the elastic element to form various force measuring sensors.The commonly used elastic elements are column type, beam type, ring type, spoke type and other forms.The resistance strain force sensor has the advantages of simple structure, convenient manufacturing and high precision, and has been widely used in static and dynamic measurement[6]。
Other load cells
Other load cells
Sensor type
characteristic
Capacitive load cell
On the rectangular special elastic element, several through round holes are machined. Each round hole is fixed with two "T" shaped electrodes with parallel end faces. Each electrode is pasted with copper foil, which is composed of severalParallel plate capacitorComposed in parallelmeasuring circuit 。Under the action of force F, the elastic element deforms to change the electrode plate spacing, thus changing the electrical capacity[6]。
Capacitive force sensor is characterized by simple structure, high sensitivity and fast dynamic response, but it is not suitable for static force measurement because of the unavoidable charge leakage[6]。
Piezoelectric load cell
Piezoelectric force sensor has the characteristics of small size and fast dynamic response, but it also has the problem of charge leakage, and is generally not suitable for static force measurement.Avoid bearing transverse force and applying preload during use[6]。
Piezomagnetic force transducer
Some ironmagnetic material The change of permeability caused by external force is called piezomagnetic effect.It is an automatic weighing and force measuring device, which is used in metallurgy, mining, transportation and other departments.When in use, the load is conducted by the lifting ring, force transfer plate and pressure ring, so that the piezomagnetic iron core is deformed, the distribution of magnetic lines of force changes, and some magnetic lines of force cross the measurement winding and are generated in the measurement windingInduced electromotive force, and the greater the force, the greater the induced electromotive force[6]。
The piezomagnetic force sensor is characterized by: after the stress surface of silicon steel material is enlarged, thousands of tons of force can be measured, and the output electromotive force is large, even only filtering rectification is required without amplification, which is commonly used for measuring the rolling force of large rolling mills.In actual engineering measurement, lateral force interference should be prevented from damaging the laminated structure of silicon steel[6]。
Differential transformer type load cell
The elastic element in the differential transformer type force sensor is a thin wall cylinder (i.e. shell). Under the action of the external force F, the deformation causes the differential transformer's iron core medium to move slightly, and the secondary side of the transformer generates corresponding electrical signals[6]。
The characteristic of the differential transformer type force sensor is its wide working temperature range.In order to reduce the influence of transverse force or eccentric force, the height diameter ratio of the sensor should be as small as possible[6]。
