|II Working Principle|
|III How to replace the force sensor|
|IV How to improve the accuracy of the force sensor|
|V Application field|
The force sensor is primarily composed of three parts:
1. Force-sensitive components (common materials are aluminum alloy, alloy steel, and stainless steel).
2.The conversion component (the most common is the resistance strain gauge).
3.The circuit part (generally enameled wire, PCB board, etc.).
Force can produce various physical effects, so force sensors can be designed according to different needs adopting various principles and techniques.
The ways that force sensor used to obtain the force signal:
(1) The measured force causes the elastic body (such as spring, beam, bellows, diaphragm, etc.) to produce a corresponding displacement, and the force signal is obtained through displacement measurement.
(2) The strain gauge is firmly pasted on the surface of the elastic component to together constitute the sensor. The elastic component deforms when it is stressed, which changes the resistance value of the strain gauge (when strain occurs, the geometry and resistivity of the strain gauge change, resulting in a change in the resistance value ), and the force signal is obtained through resistance measurement. The strain gauge can be made of metal foil or semiconductor.
(3) The piezoelectric crystal converts the force directly into a potential difference placed on both sides of the crystal through the piezoelectric effect so as to obtain the force signal.
(4) The force causes the natural frequency of the mechanical resonance system to change, and the relevant information of the force is obtained through frequency measurement.
(5) Through the balance between the electromagnetic force and the force to be measured, the force information is obtained from the relevant electromagnetic parameters during balance.
Figure 1: Force sensor
1.Strain gauge type
There are 2 or 4 strain gauges attached to the wall of the cylinder, half of which are attached to the solid part as temperature compensation gauges, and the other half as measuring strain gauges. When there is no pressure, the four strain gauges form a balanced full-bridge circuit; when pressure acts on the inner cavity, the cylinder deforms into a "waist drum", which makes the bridge out of balance and outputs a voltage that has a certain relationship with the pressure. This kind of sensor can also use a piston to convert the measured pressure into a force and transmit it to the strain tube or transmit the measured pressure through a diaphragm in the shape of a vertical chain. The strain tube force sensor has a simple structure, convenient manufacture, and strong applicability, and has a wide range of applications in dynamic pressure measurement of rockets, artillery shells, and artillery.
Its elastic sensitive element is circular metal flat diaphragm with a fixed edge. When the diaphragm is deformed under pressure, both the radial strain and the tangential strain at the center reach a positive maximum value, while the radial strain at the edge reaches a negative maximum value, and the tangential strain becomes zero. Therefore, the two strain gauges are often attached to the positive and negative maximum strain respectively and connected to the half-bridge circuit of adjacent bridge arms to obtain greater sensitivity and temperature compensation. The use of circular foil strain gauges can maximize the use of the strain effect of the diaphragm. The non-linearity of this sensor is significant. The latest product of the diaphragm pressure sensor is to integrate the functions of the elastic sensitive component and the strain gauge into the monocrystalline silicon diaphragm, that is, the integrated circuit process is used to diffuse the resistance strip on the monocrystalline silicon diaphragm.
3.Strain beam type
When measuring small pressures, a fixed beam or an equal-strength beam structure can be used. One method is to use a diaphragm to convert the pressure into force and then transfer it to the strain beam through a force-transmitting rod. In Figure 3, the maximum strain of the fixed beam at both ends is at the two ends and the midpoint of the beam, and the strain gauges are attached to these places. There are other forms of this structure, such as cantilever beams and diaphragms or bellows.
In the combined strain pressure sensor, elastic sensitive elements can be divided into sensing elements and elastic strain elements. The sensing element converts the pressure into force and transmits it to the most sensitive part of the elastic strain element, and the strain gauge is attached to the maximum strain of the elastic strain element. Sensing elements include diaphragms, bellows, Bourdon tubes, etc., and elastic strain elements include cantilever beams, fixed beams, Π-shaped beams, ring beams, thin-walled tubes, etc. They can be combined into various types according to different needs.
The general working principle of force sensors is that they respond to the applied force and convert its value into a measurable quantity. Based on various sensing elements, there are various types of force sensors on the market. Most force sensors are designed using force sensing resistors. These sensors consist of sensing membranes and electrodes. The force-sensing resistor contains a conductive polymer film that changes its resistance in a predictable manner when a force is applied to its surface. The film is composed of conductive and non-conductive particles arranged in a matrix of sub-micron level. When force is applied to the surface of the membrane, fine particles will contact the sensor electrodes, thereby changing the resistance of the membrane. The resulting change in resistance value indicates the magnitude of the applied force.
In order to improve the performance of force-sensitive resistors, a variety of different methods are being used. For example, in order to minimize polymer drift, various electrode configurations are being tested through the use of new materials (such as carbon nanotubes) to replace polymers, test with sensors, etc.
In the actual use of the force sensor, sometimes it will encounter overload, impact, etc., which will cause the plastic deformation of the sensor and affect the measurement accuracy. In severe cases, the sensor will be damaged and cannot be used normally. At this time, the sensor needs to be replaced. What should we do at this time? What issues should be paid attention to when replacing? The precondition that the force sensor can be replaced is that the axis of applying force coincides with the axis of the force of the sensor. This is the first point.
As the rated load increases, the microvolt/division signal output by the force sensor decreases, instead of increasing the output signal as the rated load increases. This is often overlooked. Therefore, when replacing the sensor, the sensor with the same load as the original one should be used as much as possible. If you want to replace a larger load, you must pay attention to whether the scale of the weighing instrument of the electronic scale is adjustable: if it is an old-style display instrument that is not adjustable, it will become unusable due to the replacement of the sensor with a greater load, which results that the output microvolt/division signal becomes smaller, the full-scale output and display cannot be performed, and the dial adjustment cannot achieve the purpose.
If it is a force sensor with an adjustable range, you can set the range and debug after replacing it with a sensor with a larger load according to the manual. At the same time, it should be noted that if the rated load of the sensor is too large and the output microvolt/division signal is too small, this will easily reduce the sensitivity of the scale.
For the electromechanical combined scale with the S-shaped sensor installed in the second force transmission link, it should be noted that the link length after reinstalling the sensor is the same as the original link length. On the other hand, it is necessary to ensure that the first force transmission lever is horizontal and that the link rod and the first force transmission lever form a 90-degree angle perpendicular to it. If there is a deviation, it will directly affect the accuracy and sensitivity of the scale. If the link length is too long, the phenomenon of "large scale" will appear; if the length of the link is too short, the phenomenon of "small scale" will appear. At this time, it should also be noted that the link must be in a free-hanging state, and cannot rub against other objects, so as not to affect the sensitivity of the scale.
The commissioning of the electromechanical integrated scale after replacing the sensor should be carried out according to the manual of the weighing display instrument on the basis of accurate commissioning of the mechanical scale.
Whether it is an electronic scale or an electromechanical combined scale, after the sensor is replaced, it must be verified before it can be used.
The above solutions are effective for S-shaped sensors in addition to the general force sensors. The situation that the force sensor needs to be replaced is often encountered during use. As long as the above operation is followed, even if the sensor is replaced, the measurement will not be affected.
The temperature will cause the strain signal (resistance) of the 4 strain gauges to change in the same direction and degree. Because two positive strains and two negative strains are included in the equation, the temperature will not produce an output signal. The remaining small residual errors can be corrected by connecting to the special nickel metal on the Wheatstone bridge. In addition, strain gauges need temperature-to-sensitivity compensation (TCS). When the temperature changes, the E modulus of the material will decrease, resulting in strain. In addition, the sensitivity of the strain gauge depends on temperature. Compensation of resistance at high temperature will produce a greater voltage drop. This will reduce the output signal of the Wheatstone bridge. Under load conditions, the linearity error will also change. This can be done by optimizing the elastomer material and structure and selecting precise measurement points.
Figure 2: Compensation of force sensor
The choice of force sensor mainly depends on the type of weighing and installation space, to ensure proper installation, safe and reliable weighing; on the other hand, the manufacturer’s recommendations must be considered. For sensor manufacturers, it generally stipulates the force situation, performance index, installation form, structure form, the material of elastomer, etc. of the sensor.
For example, aluminum alloy cantilever beam sensors are suitable for electronic price calculation scales, platform scales, case scales, etc.; steel cantilever beam sensors are suitable for electronic belt scales, sorting scales, etc.; steel bridge sensors are suitable for railroad scales, truck scales, etc.; column type is suitable for truck scales, dynamic rail scales, large-tonnage hopper scales, etc.
Force sensors are mainly used in various electronic weighing instruments, industrial control fields, online control, safety overload alarms, material testing machines, and other fields.