The weighbridge is a weighing device that uses the principle of force electricity transformation to convert non electric gravity into electric quantity. The key device to achieve this purpose is the weighing sensor (called primary instrument element). It is located on the force fulcrum of the weighing table and must have good mechanical properties such as stiffness, strength and fatigue resistance.
1. Working environment
The working environment of the sensor must be considered when selecting the sensor, and the corresponding load cell can meet the necessary weighing requirements.
Carefully inspect the surrounding electromagnetic environment. Select sensors with good electromagnetic resistance and good shielding; Due to the high temperature environment in summer and dusty weather in spring and autumn in Xinjiang, sensors with high airtightness should be selected, and the sealing effect of different types of sensors is different. Sand and dust are easy to cause short circuit to the sensor, so the sensor with high airtightness should be selected according to the local actual installation and use environment; The sensor is easy to be corroded by salt and alkali, so the sensor with good corrosion resistance and airtightness shall be selected.
2. Number and range of sensors
The number of sensors shall be selected according to the purpose of the weighbridge and the number of support points required by the scale body. Several sensors shall be selected when there are several supporting points on the scale body. The sensor range can be selected according to the maximum weighing value of the scale, the number of selected sensors, the weight of the scale body, the maximum eccentric load that can be generated and the comprehensive evaluation of dynamic load factors. Many factors should be considered to ensure the safety and life of the sensor. The number of installed load cells shall be selected according to the sensor parameters (1) to ensure that the sensor accuracy can meet the requirements of instrument input signal and conversion circuit direction matching. (2) Select by sensor input sensitivity. It is greater than or equal to the input sensitivity required by the electronic scale instrument. Precisely speaking, it is to use a weighing range with the best linearity of the weighing sensor. Only 2/3 of the full range should be used. Generally, it can be within the range of 30% - 70%. The sensitivity of the load cell is considered as S=2mv/v. What if the sensitivity is considered as S=1mv/v? For a weighing instrument that also uses a single weighing sensor, let's compare it: when the sensitivity of the weighing sensor is S=1mv/v, the excitation voltage V=12V, the maximum weighing capacity Emax=150kg, the maximum weighing capacity Max=150kg, and the maximum net output signal is: the excitation voltage X sensitivity XEMmaxax=12vX 1mv/vX115500kkgg=12mv, when the sensitivity of the weighing sensor is S=2mv/v, the excitation voltage V=12V, the maximum weighing capacity Emax=300kg, and the maximum weighing capacity Max=150kg, The maximum net output signal is: excitation voltage X sensitivity XMEmaxax=12vX 2mv/v X310500kkgg=12mv If the minimum static load Emin of the load cell with sensitivity S=1mv/v is 6kg, when the accuracy level is C3, that is, when nLC=3000, there is a calibrated graduation value V=Emaxn-LCEmin=1503k0g0-06kg=0 048kg, while the calibration graduation value of the platform scale as described above is e=0.05kg. In this way, the calibration scale value of the load cell and the scale scale value are basically close under the premise of S=1mv/V. Thus, we can see that the provisions of nLC&ne are valid only when the ratio of the change of load cell response (output) to the change of corresponding excitation (applied load) is not greater than S=1mv/v.
3. Form and purpose of load cell
According to the characteristics of the output signal of the sensor, the weighing sensor can be equipped with analog sensor or digital sensor. As long as the installation is suitable, the weighing is safe and reliable.
4. Common technical faults and solutions of load cell
4.1 The sensor is damaged due to improper weighing. (1) The alleged vehicle (or object) is seriously overloaded. (2) In the weighing process, there is impact. At this time, the weight of the object plus the kinetic energy generated by gravity drop makes the impact force generated when the object contacts the weighing platform greatly exceed the rated load of the sensor, causing damage to the sensor.
Solution: (1) Overload weighing is strictly prohibited. (2) Install shock or impact protection for the weighing platform. (3) Increase the rated load of the weighing sensor. The electronic automobile weighing instrument may lose the vehicle and measure incorrectly when it is used, most of which are caused by the failure of the weighing sensor. After the sensor is confirmed to be faulty, the dynamic balance can be restored to normal operation by replacing the sensor.
4.2 The sensor is damaged due to improper sealing method. Electronic automobile scales are often used in harsh environments. If sensors with poor sealing performance are used, due to industrial dust, various corrosive media and other factors, the resistance value of the resistance strain gauge is easily changed, resulting in errors in the weighing results. At this time, the digital multimeter can be used to measure the input and output impedance of the sensor. When the measured value has a large deviation from the technical parameters provided by the product or the value marked on the certificate of conformity, it can be determined that the sensor is damaged. At this time, the sensor with good sealing performance should be replaced (such as the weighing sensor with silicone sealing or even welding sealing).
4.3 Weighing deviation due to moisture. When the sensor is affected with damp, the display instrument often fails to return to zero automatically, and the number changes back and forth. After manual reset, the number will still jump, and the jumping number will fluctuate irregularly within a certain range in the empty scale state. When the input and output impedances are measured with a multimeter, the measured values do not exceed the tolerance. In this case, the following methods can be used for judgment:
Dismantle all the load cells and put them into the measuring circuit one by one. When the scale is empty, the sensor that is not affected with moisture will automatically return to zero immediately and the displayed value will be stable. However, the sensor affected with damp may have digital jitter and cannot return to zero. After manual reset, the above phenomenon will occur again.
If the above methods are still unable to judge, the standard metering method can be used for resolution. The method is to calibrate the load of all sensors one by one with standard weights (or standard comparators). The measured value displayed by the sensor that is not affected with damp is the value of the standard weight gradually loaded, and the measured value displayed by the sensor that is affected with damp will have a large deviation from the value of the standard weight (or the reference object) (the displayed deviation of the weighbridge or track scale is generally<5 t). Therefore, in addition to selecting appropriate sealing methods in places prone to moisture, grease the entire sensor before installation. When all sensors are installed, it is necessary to thoroughly seal the contact between sensor and installation base, junction port, junction box seam and other places prone to moisture with grease.
4.4 Influence of other factors. Due to the restriction of the objective environment, the power line, signal line, shield wire, grounding wire, etc. of many electronic weighing instruments (especially large electronic track scale) are connected with the sensor and the display through the threading pipe. Sometimes the threading pipe must be buried underground, and the role of rain is easy to cause short circuit, open circuit and grounding of the wire. The reduction of insulation performance between lines will also cause certain errors in weighing measurement.
5. Conclusion
The load cell is the heart of the weighbridge. At present, the technical development of the load cell is changing with each passing day. Before selecting the sensor, it is very important to consider the specific working environment used by the load cell. It is related to whether the sensor can work normally, its safety and service life, and even the reliability and safety of the entire weighing instrument.