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How much do you know about the installation of weighing sensors?

Time:2020-10-13 10:48:38 Number of hits:

How much do you know about the installation of weighing sensors?




Weighing sensor is used to weight signal and pressure signal is converted into electricity signal conversion device, weighing sensor of using metal resistance strain gauge measuring bridge road, by using metal resistance wire under the action of tension elongation and the principle of resistance increase, namely metal resistance varies with the effect of variation in the strain and made of (strain, is the size of the change). The following is a small series for you to introduce the relevant knowledge of the weighing sensor, including: weighing sensor installation precautions, shortcomings, composition, principle, classification, error analysis, selection of the four elements.




Precautions for the installation of weighing sensors


1. Weigh sensor should be handled lightly, especially for the small capacity sensor with aluminum alloy as the elastomer, any impact or drop caused by vibration is likely to cause a large output error;


2. During the design and installation of the loading device, the axes of the loading force on the weighing sensor shall coincide to minimize the influence of tilted load and eccentric load;


3. In terms of level adjustment. If the weight sensor is used, the mounting plane of the base shall be adjusted to the level by the level instrument. If multiple sensors are measured at the same time, the mounting surface of their base should be kept on a horizontal plane as far as possible. The main purpose of this is to ensure that the force borne by each sensor is basically the same.


4. Determine the rated load of the used sensor according to the range selection of the weight sensor in its description;


5. To prevent chemical corrosion. Apply vaseline to the external surface of the weighing sensor when installing. Should avoid direct sunlight and ambient temperature changes in the use of the platform;


6. Add a copper braided bypass device at both ends of the loading device of the weight sensor;


7. The cable should not be lengthened by itself. When the extension is really needed, solder the joint with moisture-proof sealant;


8. It is better to use some baffles around the weighing sensor to cover the sensor. The purpose of doing so is to prevent debris from falling into the moving part of the sensor and affecting its measurement accuracy;


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9. The cable of the sensor should be far away from the strong power power line or the place with pulse wave. In case of race, the cable of the weighing sensor should be inserted into the iron pipe separately and the connection distance should be shortened as far as possible;


10. The rated load of the weight sensor should be determined according to the range of the weight sensor in the description. Although the weight sensor has certain overload capacity, it should be avoided in the process of installation and use.


11. In high-precision applications, the weight sensor and meter should be used after 30 minutes of preheating.




Disadvantages of weighing sensors


1. High output impedance and poor load capacity


Capacity by the capacitance weighing sensor electrode geometry size limit is not easy to do a lot, such as general as dozens to hundreds of micro method, even only a few small method, therefore, the capacitance weighing sensor output impedance is high, so the load ability is poor, vulnerable to interference impact on unstable phenomenon, serious when even unable to work, good shielding measures must be taken, thus causing inconvenience to the design and use, capacitive reactance is big also requires sensors insulation resistance value is extremely high, otherwise the insulating parts will affect the performance of the instrument, as a bypass resistance to also pay special attention to the surrounding environment such as temperature, cleanliness, etc. If high frequency power supply is used, the output resistance of capacitive weighing sensor can be reduced. However, the high frequency amplification and sensor are far more complex than the low frequency, and the parasitic capacitance has a great influence, so it is not easy to ensure the stability of work.


The output characteristic is nonlinear


Capacitance weighing sensor output characteristic is non-linear, although the difference to improve, but can't completely eliminate, other types of capacitance sensor only ignored the edge effect of electric field, a linear output characteristic, otherwise the edge effect produced by the additional capacitance will be on the sensor capacitor stack directly, make the output characteristic of nonlinear.


Third, the parasitic capacitance has great influence


Early small capacitance of the capacitance weighing sensor and connection wire capacitance sensor and the electronic circuit, electronic circuit and stray capacitance sensor capacitance of the plate with the surrounding conductors so-called parasitic capacitance lack is bigger, not only reduces the sensitivity of the sensor, and the capacitance is often random changes, to make the instrument work is not very stable, affect the measurement precision, so the selection of cable, installation, connection has strict requirements. For example, a high-frequency wire with good shielding property and small self-distributed capacitance is used as the lead wire. The lead wire is thick and short, and the stray capacitance of the instrument should be small and stable. Otherwise, high measurement accuracy cannot be guaranteed.




Composition of weighing sensor


1. Sensitive element


A component that directly senses the measured (mass) and outputs other quantities that have a definite relationship with the measured, such as an elastomer of a resistance-strain weight sensor, converts the mass of the measured object into a deformation; The elastomer of the capacitive weighing sensor converts the measured mass into displacement.


2. Transform components


Also known as the sensing element, it is to transform the output of the sensitive element into the signal that is easy to measure. For example, the resistance strain gauge (or resistance strain gauge) of the resistance strain type weighing sensor converts the deformation of the elastomer into the change of the resistance amount. The capacitor of capacitive weighing sensor converts the displacement of elastomer into the change of capacitance. Sometimes some elements have the functions of both sensing element and changing element, such as the piezoelectric material of voltage weighing sensor. Under the action of external load, the deformation occurs and the electric quantity is output.


3. Measuring element


The output of the changing element is converted into electrical signals to facilitate further transmission, processing, display, recording or control, such as the bridge circuit in the resistance strain weight sensor and the charge preamplifier in the piezoelectric weight sensor.


4. Auxiliary power supply


Provide energy for the electrical signal output of the sensor. General weighing sensors work without external power supply chain, therefore, as a product must clearly indicate the requirement of power supply, but not as part of the weighing sensor, some sensors, such as magnetoelectric speed sensor, because of his output energy is larger, so don't need auxiliary power can work normally, so not all sensors have the auxiliary power.




Principle of weight sensor


Resistance strain type weighing sensor is based on the principle that elastomer (elastic element, sensitive beam) elastic deformation under the action of external force, made in its surface resistance strain gages (cell) also along with the deformation and deformation resistance strain gauge, its value will change (increase/decrease), and then through the corresponding measurement circuit convert the resistance to electrical signals (voltage or current), so as to complete the process of external force transform into electrical signals.




Classification of weighing sensors


I. Photoelectric type


Include grating type and code disc type two kinds.


1, raster sensor using grating moire fringe formed by the angular displacement is converted into photoelectric signal, grating has two pieces, one for fixed grating, another for mounting on the dial shaft moving grating, add on the bearing was tested through the force transmission lever system make the dial shaft rotation, driven in a moving grating, the moire stripe also move, using photocell, conversion circuit and display instrument, can calculate the number of moire pattern through the grating is measured the size of the Angle of rotation, so as to determine the analyte and read a quality;


2, code disc sensor encoder (symbol) is a piece of transparent glass on the dial shaft, bearing according to certain coding method of the code of black and white alternate with, and on the bearing being measured by force leverage to dial shaft rotates, the encoder has turned a certain Angle, the cell will receive light signals into electrical signals through the encoder, and then by the digital processing circuit, the final show on the display measured quality of digital, on behalf of the photoelectric sensor was mainly used in mechanical and electrical combination scales.


Two, hydraulic type


When subjected to the action of gravity P of the measured object, the pressure of hydraulic oil increases, and the degree of increase is proportional to P. By measuring the increase value of the pressure, the mass of the measured object can be determined. The hydraulic sensor has a simple and firm structure, a large measuring range, but its accuracy is generally not more than 1/100.


Iii. Capacitor type


It USES capacitor oscillation frequency of the oscillation circuit and plate spacing d f direct proportion relationship between work, plate have two piece, a fixed, another piece of mobile, analyte in bearing load, the plate spring deflection, change the distance between the two plates, the oscillation frequency of the circuit changes, the change of the measured frequency can be calculated bearing on the quality of the object under test. Capacitive sensor has low power consumption, low cost and accuracy of 1/200 ~ 1/500.


Electromagnetic force type


It USES the principle of balancing the load and electromagnetic force on the bearing platform. When the measured object is placed on the bearing platform, one end of the lever slopes upward. Photoelectric detect the gradient signal, after amplification into the coil, electromagnetic force, make the lever back to equilibrium state and to produce electromagnetic balance of current digital conversion, analyte determined quality, electromagnetic force sensor with high accuracy, can reach 1/2000 ~ 1/60000, but weigh range in only a few milligrams to 10 kilograms.


5. Form of magnetic pole change


Analyte ferromagnetic components in a mechanical deformation under the action of gravity arises, internal stress and permeability change, winding in the ferromagnetic components (pole) on both sides of the induction voltage of secondary coil changes, measure the voltage variation of the force can be calculated to the pole, and then determine the quality of the object under test, magnetic pole change form the sensor's accuracy is not high, generally at 1/100, is suitable for the work of large-tonnage weighing, weighing scope for decades to tens of thousands of kilograms.


Vi. Vibration mode


Vibrating wire sensor of the elastic element is the string wire, when the analyte bearing on the stage with a v-shaped string wire intersection is pulled down, and increased left string tension, right string tension decreases, and two of the strings of different changes in the natural frequency, the frequency difference of two of the strings, the quality of the analyte can be calculated, vibrating string type sensor of high accuracy, can reach 1/1000 ~ 1/10000, scope of weighing 100 grams to several hundred kilograms, but the structure is complex, processing is difficult, the cost is high.


Seven, the gyro ceremony


Rotor installed in the frame, with angular velocity around X axis stable rotation, the inner frame by bearing and outer frame connection, and can be around the horizontal axis Y tilt rotation, the outer frame by universal coupling and machine seat connection, and can be around the vertical axis Z rotation. Rotor axis (the X-axis) state, when the external force is not a level one end of the rotor shaft under external force (P / 2), tilt and rotate around the vertical axis Z (precession), precession angular velocity omega is proportional to the force P / 2, measured by the method of detecting frequency omega, can calculate the external force size, then calculate and force of the quality of the object under test.


Viii. Resistance strain type


Using the principle that the resistance of the strain gauge changes when it deforms, it is mainly composed of four parts: elastic element, resistance strain gauge, measuring circuit and transmission cable.


Nine, plate ring type


Plate ring type weighing transducer structure with clear stress streamline distribution, output, high sensitivity, elastic body as a whole, simple structure, steady state, the advantages of easy processing, now also in sensor production accounts for a larger proportion, and the design of the structure of the sensor formula is not very perfect, because of the elastomer strain calculation is more complex, often see it as an elastomer ring type when the design estimate, especially for 1 t and the following range of plate ring type sensor design calculation error is bigger, at the same time also tend to appear larger nonlinear error.

Error analysis of weighing sensor


1, weighing sensor using error is operator, this also means that the cause of many, for example, the temperature do not occur at the same time, fault place everything probe or probe and measuring insulation between incorrect address, some application error everything else in the air or other gas purification process of fault, using the error also hit fault is placed in the transmitter, and positive or negative pressure will affect the correct reading form;


2, the characteristic error is inherent in the equipment itself, it is the equipment, the recognized handling function characteristics and the real characteristics of the difference, this error includes the DC drift value, the slope of the incorrect or diagonal shape;


3, the dynamic characteristics of the mistakes many sensors and calibration are applicable under the condition of static, this means using the input parameter is static or similar to static, many sensors have stronger damping, so they won't to hasten to echo of the input parameter changes, such as, talent echo temperature thermistor needs several seconds step changes;


4. The thermistor does not immediately jump to a new impedance or suddenly change. Instead, it slowly changes to a new value, and then, if the delayed weighing sensor responds to the rapid change in temperature, the output waveform will be distorted because of the dynamic errors involved. The factors of dynamic error include echo time, amplitude distortion and phase distortion.


5, insert error is when the system into a sensor, due to changes in the measurement parameters, the mistake is common in the electronic measurement will appear such problem, but in other ways to measure will appear similar problems, such as a voltmeter measure the voltage in the circuit, it will certainly have an inherent impedance, is bigger than the loop impedance many, may render loop load, at this time, the reading will have a lot of mistakes, why this type of mistakes is to use a for system (e.g., pressure system) transmitter is too big; It may be that the dynamic characteristics of the system are too slow, or that there is too much thermal energy in the system due to self-heating.


6. Environmental errors are due to the environment in which the sensor is used. Elements of the weighing sensor include temperature, or swing, sensation, altitude, chemical evaporation, or other elements, which often affect the characteristics of the sensor, so in practical application, these elements are always classified and grouped together.




Weighing sensor selection four elements


1. Choice of stability


Sensors use after a period of time, the ability of its performance remains the same as stability, factors affecting the stability of sensor long-term except its own structure, mainly is the use of the environment, therefore, to make the sensor has good stability, must want to have a stronger ability to adapt the environment, before choosing sensor, deal with its using environment investigation, according to the use of specific environment to choose suitable sensors, or take appropriate measures to reduce the influence of the environment.


2. Selection of sensitivity


Usually, within the scope of the sensor is linear, hope the sensitivity is higher, the better, because only high sensitivity, and the corresponding output signal is measured value is bigger, is beneficial to the signal processing, but high sensitivity, has nothing to do with the measured outside noise will be amplified system amplification, affect the accuracy of measurement, therefore, requires sensor itself should have high signal-to-noise ratio, to minimize the introduction from outside interference signals.


3. Selection of sensor quantity and range


Sensor number selection is based on the purposes of electronic weighing apparatus, the scale body needs support points (support points shall be according to the actual make the geometric center of gravity and center of gravity superposition principle and determine), the choice of sensor range on the basis of the scale biggest value, choose the number of sensors, the balance of body weight, can produce the biggest partial load and dynamic load factor comprehensive evaluation to determine, according to the experience, general should make sensors work within the range of 30% ~ 70%, but has a larger impact weighing apparatus, when choosing sensor, general to make the sensor work within the range of 20% ~ 30%, In order to ensure the use and life of the sensor.


4. Selection of accuracy


Accuracy is an important performance index of the sensor, it is related to the whole measure system is an important link in measuring accuracy, the higher the precision of the sensor, its price is more expensive, therefore, the precision of the sensor as long as can meet the requirements of the precision of measuring system, don't choose too high, so that you can to meet the same of the many sensors measuring purpose is cheaper and simple sensor, if the purpose of measurement is the qualitative analysis, choose high repeat precision sensors can, unfavorable choose high precision absolute value; If it is for quantitative analysis, accurate measurement values must be obtained, then use the sensor weighing sensor whose accuracy level can meet the requirements.


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