Electronic truck scales are mainly weighing equipment that convert non-electricity gravity into electricity through the process of force and electricity conversion. In order to ensure the smooth realization of this function, it is inseparable from the important device of the weighing sensor, which is located on the weighing table. At the fulcrum, it needs to have various good mechanical properties, such as stiffness, strength and fatigue resistance, and it also needs to bear the resultant force of the load on the table. Since the load cell is usually installed at the bottom of the weighing platform, it is difficult to attract people's attention. Therefore, in many cases, its function cannot be effectively exerted, and maintenance is not in place, resulting in shortening its service life. We must pay great attention to this,
It is necessary to not only play the role of the load cell through reasonable application, but also to do a good job in troubleshooting to reduce losses.
1 Overview of electronic truck scale load cells
Electronic truck scales are usually composed of multiple components, such as carriers, weighing display instruments, load cells, connectors, limit devices and junction boxes. They are also often equipped with other external equipment, such as large-screen monitors, computers, etc. level regulated power supply, etc. Judging from the working principle of the electronic truck scale, the main thing is to place the weighed objects, trucks, etc. on the loader table. Under the influence of gravity, the gravity is transmitted to the load cell under the action of the loader, so that the elastic body of the load cell Deformation will occur and the strain gauge bridge will be difficult to maintain balance due to its attachment to the elastomer. Then the output is proportional to the weight value
The signal will be amplified after passing through the linear amplifier and converted into a digital signal by A/I. After the instrument microprocessor processes the weight signal, the weight data can be displayed. After installing the printer, the weighing data can be printed and recorded, but this requires configuring the computer to input the measurement data into the computer management system, and then comprehensively manage it. Figure 1 shows the working condition of the electronic truck scale load cell.
The load cell is an important conversion component in electronic truck scales. It can convert mechanical quantities into electrical quantities and convert the gravity acting on the measured object into voltage and current signals according to a certain ratio. It is the heart component of electronic scales. With the rapid development of science and technology, electronic scales based on load cells are also used in various industries and fields, which can ensure the accuracy of material weighing and are also very fast, especially the application of microprocessors. The increasing level of automation in industrial production makes load cells an inaccessible device in process control [1]. From the weight measurement of large tanks and hoppers, and the measurement and control of crane scales, truck scales, etc. that were unable to be weighed in the past, to the current mixing and distribution of multiple raw material batching systems, automatic detection in the production process, and powder and granular feeding amount control, etc. , load cells also play an important role, even electronic truck scales. According to the conversion method, weighing sensors mainly include photoelectric, hydraulic, electromagnetic, capacitive and resistance strain gauge types. Among them, resistance strain gauge load cells are the most used and are most used in electronic truck scales. The load cell is one of the important components in electronic truck scales. It mainly collects and converts original weighing information. At present, most electronic truck scales use resistance strain gauge to transmit data.
The sensor is the main component. Only by continuously improving its quality can the normal operation of the weighing instrument be ensured and the accuracy can be improved.
2 Specific applications of load cells in electronic truck scales
2.1 Determination of working environment
In the process of selecting sensors, it is necessary to fully consider the actual working conditions to ensure that the load cells used are consistent with the actual weighing requirements. It is necessary to carefully and carefully check the surrounding electromagnetic field environment. The sensors used must not only have strong anti-electromagnetic capabilities, but also have good shielding properties. Especially in places with harsh working environments, the sensors must also have good sealing properties. , and different types of sensors have great differences in sealing effects [2]. If there is a lot of sand and dust in the environment, it is easy to
If the sensor is short-circuited, you should choose a sensor with good sealing performance based on the installation and use environment. If there is a problem of salinity and alkali, it will easily cause corrosion to the sensor. At this time, the sensor should not only have good airtightness, but also have strong corrosion resistance.
2.2 Number and range of sensors
When determining the number of sensors, you should fully consider the purpose of the electronic truck scale, the number of support points required for the scale body, etc., and make a reasonable selection.
For the determination of the sensor range, it is mainly based on the maximum weighing value of the scale, the number of sensors, the weight of the scale body and the maximum eccentric load and dynamic load factors. Only by taking all factors into consideration can we ensure the safety of the sensor and extend its life. Determine the number of weighing sensors according to the sensor parameters. You should ensure that the sensor accuracy matches the instrument input signal, conversion circuit, etc., and take into account the sensor input sensitivity. It cannot be lower than the input sensitivity on the electronic scale instrument, even if the linearity of the weighing sensor is the best. A good weighing range is usually 2/3 of the full range. The load cell is preferably 30%-70% of the range, and the sensitivity S is 2MV/V. For the case where S is 1MV/V, taking a single load cell as an example, as long as the load cell response (output) changes and the corresponding If the ratio of changes in excitation (applied load) does not exceed S=1MV/V, the relevant regulations can be met.
2.3 Determination of sensor accuracy level
The accuracy level of the load cell is mainly determined by technical indicators such as sensor nonlinearity, creep, repeatability, hysteresis, and sensitivity. In the process of selection and application, the accuracy level of the electronic truck scale needs to be taken into consideration. Generally speaking, the total accuracy of the sensor used is the root mean square of the sum of the three indicators of nonlinearity, non-repeatability and hysteresis. Its value is higher than the accuracy of the scale. The accuracy level of the load cell is determined by a single sensitive element and a single transmitter. The characteristics of the sensor in terms of output and input are not good. If you choose differential technology that combines differential, symmetrical structure and differential circuit, it can effectively eliminate the zero value and reduce nonlinearity.
It not only improves the sensitivity, but also can complete temperature compensation to avoid interference caused by common mode errors [3]. In this way, the technical performance of the sensor can be improved and the accuracy of the load cell can be improved. The accuracy level of the load cell is expressed as a percentage of the maximum basic error within the measuring range and the full-scale ratio. In order to comply with the instrument input requirements, the sensor output signal
After the signal is amplified, A/D converted, etc., the weighing results can be displayed on the weighing display table. The sensor output signal cannot be lower than the input signal size required by the instrument, that is, the sensor output sensitivity is brought into the matching formula between the sensor and the instrument, and the calculation result cannot be lower than the input sensitivity required by the instrument. For example, if the rated force of the sensor is 100kg, the excitation voltage is 12V, and the sensitivity is 2±0.0002MV/V, then the force is 100kg. When the excitation voltage is 12V, the output is
24±0.0024MV. In order to comply with the accuracy requirements of the entire electronic truck scale, the accuracy of the sensor accuracy usually needs to be higher than the theoretical calculation value. Since theory is usually limited by objective conditions, if the strength of the scale body is insufficient, the instrument lacks good performance, and the working conditions of the scale are poor, then the accuracy of the scale will be difficult to meet the requirements. For each load cell, its maximum graduation number nLC must exceed the calibration graduation number n of the weighing instrument: nLC ≥ n. According to this regulation, the graduation number of the weighing instrument has been determined
There are restrictions, that is, if the electronic truck scale uses a C3 level (3000v) load cell, the maximum number of calibration graduations is 3000e. In this regard, it is necessary to improve the requirements in all aspects, not only to have good economic benefits, but also to ensure that expected requirements are met.
3. Troubleshooting of electronic truck scale load cells
3.1 Improper weighing leads to sensor damage
First, the alleged vehicles and objects are overloaded. Second, there will be an impact during weighing. At this time, the object's own weight will fall under the influence of gravity. The kinetic energy will cause an impact force after the object contacts the weighing platform. This will be greater than the rated load of the sensor, causing damage to the sensor. . In this regard, overload weighing should not occur during use. Protective devices such as shock absorption and anti-collision should be installed, and the rated load of the load cell should be increased. If problems such as vehicle loss or inaccurate measurement occur, the load cell is generally used. Failure occurs, which requires timely maintenance and replacement to ensure normal operation can be restored as soon as possible.
3.2 The sensor is damaged due to poor sealing
The use conditions of electronic truck scales are usually very poor. If you choose a sensor with poor sealing performance, the resistance of the resistance strain gauge will easily change under the influence of factors such as industrial dust and various corrosive media, and the weighing results will also be affected. become inaccurate. In response to this situation, a digital multimeter should be used to measure the impedance of the sensor input and output. If there is a large deviation between the measured value and the product-related technical parameters and the value marked on the certificate, it indicates that the sensor is damaged [4]. In order to solve this problem, a sensor with better sealing should be selected. For example, the sensor should be sealed with silicone or welded.