How to choose a multihead weigher reasonably

Author: Smartweigh–Multihead Weighter

The multihead weigher is a power-to-electricity conversion device that can convert the force into electronic signals, and is the core component of the multihead weigher. There are many kinds of sensors that can complete the force-electrical change, generally including resistance strain force type, magnetic field force type and capacitive sensor. The importance of the magnetic field force type is the electronic analytical balance, the capacitor sensor is part of the multihead weigher, and the resistance strain force type weight machine is commonly used in most of the weight machine products.

The resistance strain multihead weigher is simple in structure, high in precision, and has a wide range of usability, and can be applied in a relatively poor natural environment. Therefore, the resistance strain multihead weigher is obtained in the multihead weigher. The resistance strain multihead weigher is mainly composed of polyurethane elastomer, resistance strain gauge and compensation power circuit.

Polyurethane elastomer is the stressed part of the multihead weigher, made of high quality carbon steel and high quality aluminium alloy profiles. The resistance strain gauge is made of metal material foil etched into grid data type, and the four resistance strain gauges are glued to the polyurethane elastomer by the bridge structure method. In the case of powerlessness, the 4 resistors of the bridge circuit have the same value, the bridge circuit is in a balanced state, and the output is zero.

When the polyurethane elastomer is deformed by force, the resistance strain gauge is also deformed. During the whole process of the polyurethane elastomer being subjected to force and bending, two resistance strain gauges are stretched, the iron wire is stretched, and the resistance value increases, and the other two are subjected to force, and the resistance value decreases. In this way, the originally balanced bridge circuit is out of balance, and there is a working voltage difference on both sides of the bridge circuit. The working voltage difference is related to the magnitude of the force on the polyurethane elastomer. Check the working voltage difference to obtain the magnitude of the sensor force, the working voltage After the data signal is checked and calculated by the instrument panel, in order to better utilize the settings of various multihead weigher structures, the multihead weigher is composed of various structural forms, and the name of the sensor is usually also called according to its appearance design.

For example, stacking chain sensor (important electronic car balance), cantilever beam type (ground balance, warehouse scale, electronic car balance), column type (electronic car balance, warehouse scale), car type (scale), s-type ( warehouse scales) etc. A multihead weigher medium can often list sensors in multiple structural forms. If the sensor is selected properly, it helps to improve the multihead weigher characteristics.

There are many specifications and models of resistance strain multihead weighers, ranging from several hundred grams to several hundred tons. When choosing the measurement range of the multihead weigher, it must be clarified according to the size of the commonly used multihead weigher. The rule of thumb is as follows: total sensor load (maximum allowable load of individual sensors x number of sensors) = 1/2~2/3 of the maximum weight of the multihead weigher.

The accuracy level of the multihead weigher is divided into four levels: a, b, c, and d. Different grades have different margins of error. Class A sensors are specified max.

The number after the grade represents the metrological verification value, the larger the data, the better the quality of the sensor. For example, C2 means C grade, 2000 metrological verification values ​​C5 means C grade, 5000 metrological verification values. Obviously C5 is higher than C2.

Common grades of sensors are C3 and C5, and these two grades of sensors can be used to make multihead weighers with an accuracy grade of III. The error of multihead weigher is mainly caused by discrete system error, lag error, repeatability error, stress relaxation, extra error of zero point temperature and extra error of rated output temperature. The digital sensors that have appeared in recent years put the A/D conversion power supply circuit and the CPU power supply circuit into the sensor. The output of the sensor is not the analog working voltage data signal, but the net weight analog signal solved by the solution, which has the following advantages: 1. Instrument panel The data signals of each digital sensor can be collected separately, calculated according to the linear equation, and each sensor can be independently calibrated, and the possibility of adjusting the error of the four corners at one time is very high.

The biggest headache in multihead weighers using digital and analog sensors is the four-corner error adjustment, which usually requires multiple calibrations to specify, each time moving a heavy standard weight, which is time-consuming and labor-intensive. 2. Because the instrument panel can detect the data signals of all sensors, the problems of all sensors can be seen from the instrument panel, which is convenient for maintenance. 3. The digital sensor transmits the analog signal through the 485 interface, and the transmission is long distance without being affected.

Get rid of the difficult and susceptible problems of pulse signal transmission. 4. Various errors of the sensor can be adjusted according to the microcontroller inside the digital sensor, so that the output sensor data information is more correct. The multihead weigher is called the central nervous system of the multihead weigher, and its characteristics largely determine the accuracy and reliability of the multihead weigher.

When designing a multihead weigher, the question of how to use the sensors is often encountered. A multihead weigher is actually a device that converts a quality data signal into an electronic signal output that can be accurately measured. The first thing to consider when using a sensor is the specific office environment in which the sensor is located.

This is particularly important for the proper use of sensors, and it is related to whether the sensor can work properly and other safety and service life, and even the reliability and safety factor of all weight machines. The harm caused by the natural environment to the sensor has the following aspects: (1) The high temperature natural environment causes the sensor to melt the coating material, spot welding, and structural changes in the thermal stress of the polyurethane elastomer. Sensors working in high temperature natural environment often choose heat-resistant sensors, and must also add thermal insulation, water cooling, air cooling and other equipment.

(2) Hazards of smoke and humidity to short-circuit faults of sensors. In the natural environment here, a highly airtight sensor should be used. Different sensors have different sealing methods, and the sealing performance is very different.

General sealing includes filling of sealant and mechanical equipment for coating rubber sheet, electric welding (arc welding machine, etc. electron beam welding) for sealing sealing and nitrogen filling sealing for vacuum packaging. From the actual effect of sealing, the electric welding sealing is the best, and the filling and sealing dosage is poor. For the sensor working in a clean and dry natural environment in the room, you can choose the sensor with adhesive sealing. For the sensor working in the natural environment with high humidity and smoke, you must choose the pulse shock absorber heat sealing or Pulse shock absorber welding sealing, vacuum packaging nitrogen filled sensor.

(3) In the natural environment with high corrosion, such as humidity, cold, acid and alkali, which cause damage to the polyurethane elastomer, short-circuit failure and other hazards to the sensor, the outer layer should be selected for electrostatic spraying or stainless steel plate cover, which has good corrosion resistance and good sealing performance. sensor. (4) Harm of the magnetic field to the sensor output chaotic data signal. In this case, the shielding property of the solution sensor is strictly checked to see if it has excellent electromagnetic immunity.

(5) Flammability, flammability, and explosion not only cause advanced hazards to sensors, but also bring great threats to other mechanical equipment and life safety. Therefore, sensors working in flammable, flammable, and explosive natural environments clearly specify the characteristics of explosion-proof type: explosion-proof sensors must be used in flammable, flammable, and explosive natural environments. The sealing cover of this kind of sensor should not only consider the tightness, but also fully consider the compressive strength of the explosion-proof type and the moisture-proof, waterproof and explosion-proof type of the cable outlet.

Secondly, the selection of the total number of sensors and the measurement range: the selection of the total number of sensors depends on the main purpose of the multihead weigher, the level of the supporting points of the scale body (the number of supporting points must be based on the gravity center point of the overlapping scale body geometry and the benchmark of the specific gravity center point) . Generally speaking, some fulcrums of the scale use some sensors, but unique scales such as electronic hook scales only select one sensor, and some electromechanical engineering fusion scales should clearly use the number of sensors according to the specific situation. The selection of the measuring range of the sensor can be evaluated according to factors such as the size of the scale, the number of sensors, the weight of the scale itself, and the possible large wheel weight and load.

Generally speaking, the closer the measurement range of the sensor is to the load of each sensor, the higher the weighing accuracy. However, in specific applications, in addition to being called objects, there are also the weight of the scale itself, tare weight, wheel weight and vibration shock. Therefore, when using a sensor measurement range, many factors should be considered to ensure the safety and longevity of the sensor.

The calculation method of the measuring range of the sensor has been clarified after many experiments after taking into account the various elements that endanger the scale body. The formula is calculated as follows: C=K-0K-1K-2K-3(Wmax+W)/N. C- The rated range of the individual sensor W- The weight of the scale itself Wmax- It is called the highest value of the net weight of the object N- The total number of fulcrums selected by the scale K-0- The commercial insurance index, generally 1.2~1.3 K-1- of the intermediary Shock index K-2-scale gravity center point offset index K-3-air pressure index.

For example, for a 30t electronic floor scale, the maximum weighing is 30t, the weight of the scale itself is 1.9t, 4 sensors are selected, and according to the specific situation at the time, the commercial insurance index K-0=1.25, the impact index K-1=1.18, and the center of gravity are selected. Point deviation index K-2-=1.03, air pressure index K-3=1.02 Solution: According to the calculation method of the sensor measurement range: c=K-0K-1K-2K-3(Wmax+W)/N. c=1.25×1.18×1.03×1.02×(30+1.9)/4=12.36t. Therefore, the measuring range of the sensor is 15t (the loading capacity of the sensor is generally only 10T, 15T, 20t, 25t, 30t, 40t, 50t, etc., unless it is a unique customization).

According to work experience, the work of the weight machine is generally within its measurement range of 30%~70%, but the weight machine with greater impact in the whole process of application, such as dynamic track balance, dynamic electronic car balance, stainless steel plate scale, etc. , When using a sensor, generally expand its measurement range, so that the sensor works within 20% to 30% of its measurement range. Again, the application fields of a variety of sensors must be considered. The key to the selection of the sensor form is the type of weight and the setting of the indoor space, to ensure the proper setting, the weight is reliable, on the other hand, the manufacturer's recommendations must be considered. Manufacturers generally require the sensor's application field according to the sensor's endurance, performance parameters, installation method, structural form, polyurethane elastomer material and other characteristics Beam sensors are suitable for steel accumulation and release chain sensors such as material scales, electronic belt scales, and screening scales.

Ultimately, the level of accuracy of the sensor must be chosen. The accuracy level of the sensor includes the sensor's nonlinearity, stress relaxation, stress relaxation repair, lag, repeatability, sensitivity and other performance indicators. When using a sensor, not only the accuracy regulations of the electronic designation, but also its cost must be considered.

The selection of sensor levels must consider the following two criteria 1. Consider the provisions of the instrument panel input. The weighing indicator displays the information weighing result after the output data signal of the multihead weigher becomes larger and the A/D conversion is resolved. Therefore, the output data signal of the multihead weigher must be larger than the input condition size specified by the instrument panel. The output sensitivity of the multihead weigher is brought into the matching formula between the sensor and the instrument panel, and the calculation result must be greater than the input sensitivity specified by the instrument panel.

The matching formula of the multihead weigher and the instrument panel: the output sensitivity of the weight meter * the power supply voltage of encouragement * the size of the scale, the degree of myopia of the weight meter * the number of sensors * the measuring range of the sensor. For example, a quantitative packaging machine with a weight of 25kg and a scale with a large myopia of 1000 measuring ranges select 3 L-BE-25 sensors with a measuring range of 25kg and a sensitivity of 2.0±0.008mV/V, select AD4325 instrument panel for scales with stone arch bridge electrical working pressure of 12V. Asks if the selected sensor is to be combined with the dashboard.

Solution: The input sensitivity of the AD4325 instrument panel is 0.6μV/d, so according to the matching formula between the multihead weigher and the instrument panel, the specific input data signal of the instrument panel is 2×12×25/1000×3×25=8μV/d>0.6μV/d. Therefore, the selected multihead weigher can take into account the regulation of the input sensitivity of the instrument panel, which can be combined with the selection of the instrument panel. 2. Consider the regulations on the accuracy of electronic titles.

An electronic representation is mainly composed of three parts: scale, sensor and instrument panel. When choosing the accuracy of the multihead weigher, the accuracy of the multihead weigher is slightly higher than the calculated value of the basic theory. The basic theory is usually limited by objective reasons, such as scales. The compressive strength of the scale is slightly poor, the characteristics of the instrument panel are very good, the office environment of the scale is extreme and other factors.

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