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What Is The Function And Principle OF The Force Sensor

What Is The Function And Principle OF The Force Sensor

December 11 , 2023 0

What is a force sensor. A force sensor is a device used to detect and measure force, which can convert the force it feels into an electrical signal or other forms of signal, so as to achieve accurate control and measurement of force. Force sensors have a wide range of applications in many fields, such as industrial automation, robotics, medical devices, smart home and so on.

Basic Principles OF Force Sensors

A force sensor is a component that converts the magnitude of a force into an associated electrical signal. Force sensors can detect mechanical quantities such as tension, pressure, weight, torque, strain and internal stress. It has become an indispensable core component of power equipment, construction machinery, various working machinery and industrial automation systems.
The force sensor is mainly composed of three parts:
1, force sensitive components (common materials are aluminum alloy, alloy steel, stainless steel).
2. Conversion elements (most commonly resistance strain gauges).
3. Circuit part (generally enameled wire, PCB board, etc.).
Force can produce a variety of physical effects, so different principles and technologies can be used to design force sensors according to different needs.
The way the force sensor obtains the force signal:
(1) The measured force causes the elastomer (such as the spring, beam, bellows, diaphragm, etc.) to produce the corresponding displacement, and the force signal is obtained through the displacement measurement.
(2) The strain gauge is firmly pasted on the surface of the elastic element to form the sensor. The elastic element deforms when it is stressed, thus changing the resistance value of the strain gauge (when strain occurs, the geometry and resistivity of the strain gauge change, causing the resistance value to change), and the force signal is measured through the resistance. Strain gauges can be made of metal foil or semiconductors.
(3) The piezoelectric crystal converts the force directly into the potential difference placed on both sides of the crystal through the piezoelectric effect, so as to obtain the force signal.
(4) The force changes the natural frequency of the mechanical resonance system, and the relevant information of the force is obtained through the frequency measurement.
(5) Through the balance between the electromagnetic force and the measured force, the force information is obtained from the relevant electromagnetic parameters at the time of balance.

Classification OF Force Sensors

Strain Gauge Formula

Two or four strain gauges are attached to the cylinder wall, one half of which is attached to the solid part as a temperature compensation plate and the other half as a measuring strain gauge. When there is no pressure, four strain gauges form a balanced full-bridge circuit; When the pressure acts on the inner cavity, the cylinder is deformed into a "waist drum", so that the bridge is out of balance, and the output voltage has a certain relationship with the pressure. The sensor can also convert the measured pressure to a strain tube using a piston or transfer the measured pressure through a diaphragm in the shape of a vertical chain. The strain tube force sensor has the advantages of simple structure, convenient manufacture and strong applicability.

Diaphragm Type

The elastic sensitive element is a round metal planar diaphragm with a fixed edge. When the diaphragm is deformed under pressure, the radial strain and tangential strain at the center reach the positive maximum value, while the radial strain at the edge reaches the negative maximum value, and the tangential strain becomes zero. Therefore, two strain gauges are often attached to the positive and negative maximum strains respectively and connected to the half-bridge circuit of the adjacent bridge arm to obtain greater sensitivity and temperature compensation. The use of circular foil strain gauges maximizes the strain effect of the diaphragm. The nonlinearity of the sensor is significant. The resistance strips were diffused on monocrystalline silicon membrane by integrated circuit technology.

Strain Beam Type

When measuring small pressures, a fixed beam or equal strength beam structure can be used. One method is to use the diaphragm to convert the pressure, and then transfer it to the strain beam through the force transfer rod. In Figure 3, the maximum strain of a beam fixed at both ends is at the ends and midpoint of the beam, where the strain gauge is attached. There are other forms of this structure, such as cantilever beams and diaphragms or bellows.

Combined Type

In the combined strain pressure sensor, the elastic sensitive element can be divided into sensing element and elastic strain element. The sensing element transfers the pressure conversion force 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 diaphragm, bellows, bourdon tube, etc. Elastic strain elements include cantilever beam, fixed beam, Π beam, ring beam, thin-walled tube, etc., which can be combined into a variety of types according to different needs.

Working Principle OF Force Sensor

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 a sensing membrane and electrodes. A force resistor contains a conductive polymer film that changes its resistance in a predictable manner when a force is applied to its surface. The film consists of conductive and non-conductive particles arranged in a submicron matrix. When a force is applied to the surface of the film, the fine particles touch the sensor electrode, changing the resistance of the film. The resulting change in resistance value indicates the magnitude of the applied force.

How Do I Replace The Force Sensor

In practical use, the force sensor sometimes encounters overload, impact, etc., resulting in plastic deformation of the sensor, affecting the measurement accuracy. In severe cases, the sensor will be damaged and cannot be used normally. The sensor needs to be replaced. What to do at this time? What should we pay attention to when replacing? The premise that the force sensor can be replaced is that the force axis and the force axis of the sensor are reconnected. That's number one.

With the increase of rated load, the output signal of the force sensor decreases, rather than increasing with the increase of rated load. This is often overlooked. Therefore, when replacing the sensor, you should try to use the same sensor load as the original sensor. If you want to replace a larger load, be sure to pay attention to whether the scale of the weighing instrument of the electronic scale is adjustable: if it is an old display instrument that is not adjustable, it will become impossible to use the sensor with a larger load because of replacement, resulting in the output microvolt/cell signal becoming smaller, unable to carry out full scale output and display, and dial adjustment can not achieve its purpose.

If it is a force sensor with adjustable range, you can replace the sensor with a larger load according to the instructions and set the range debugging. At the same time, it should be noted that if the rated load of the sensor is too large, the output microvolt/cell signal is too small, which is easy to reduce the sensitivity of the scale.

For the electromechanical scale with the S-type sensor mounted on the second force transfer link, it should be noted that the length of the link after the sensor is reinstalled is the same as the length of the original link. On the other hand, it is necessary to ensure that the first transmission rod is horizontal, and the connecting rod is perpendicular to the first transmission rod at a 90 degree Angle. If there is a deviation, it will directly affect the accuracy and sensitivity of the scale. If the link length is too long, there will be "large-scale" phenomenon; If the length of the link is too short, there will be a "small scale" phenomenon. At this time, it is also important to note that the connecting rod must be in a free suspension state and cannot have friction with other objects.

After replacing the sensor of the electromechanical integrated scale, the debugging should be carried out according to the operating instructions of the weighing display instrument on the basis of accurate debugging of the mechanical scale.

Whether it is an electronic scale or an electromechanical scale, after replacing the sensor, it must be verified before it can be used.

The above solution is effective for S-type sensors in addition to general force sensors. In the course of use, it is often encountered that the force sensor needs to be replaced. As long as the above operations are followed, even replacing the sensor will not affect the measurement.

How To Improve The Accuracy OF The Force Sensor

Temperature will cause the strain signal (resistance) of the four strain gauges to change in the same direction and to the same degree. Because the equation contains two positive strains and two negative strains, the temperature does not produce an output signal. The remaining small residual errors can be corrected by a special nickel metal attached to the Wheatstone bridge. In addition, strain gauges require temperature-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 the temperature. The compensating resistance will produce a large voltage drop at high temperature. This will reduce the output signal of the Wheatstone bridge. The linear error also changes under load conditions. This can be achieved by optimizing elastomer materials and structures and selecting precise measurement points.

Application Areas OF Force Sensors

The choice of force sensor mainly depends on the weighing type and installation space to ensure correct installation and safe and reliable weighing; On the other hand, the manufacturer's proposal must be taken into account. For sensor manufacturers, the general provisions of the sensor stress, performance indicators, installation form, structural form, elastomer material and so on.

For example, aluminum alloy cantilever beam sensor is suitable for electronic pricing scales, platform scales, box scales, etc.; Steel cantilever beam sensor is suitable for electronic belt scale, sorting scale, etc. The steel bridge sensor is suitable for railway scales, automobile scales, etc. Column type is suitable for automobile scale, dynamic track scale, large tonnage hopper scale and so on.

Force sensors are mainly used in various electronic weighing instruments, industrial control fields, online control, safety overload alarm, material testing machine and other fields.