Application of load cell in cement industry

The basic requirements and special requirements of the weighing system of construction machinery mixing equipment, several issues that need to be considered when selecting sensors for the weighing system of construction machinery mixing equipment are put forward, and the impact of the sensor protection structure on the operation reliability of construction machinery mixing equipment is analyzed. Point out that the protection level represented by the IP code cannot cover all the protection requirements for the sensors of the construction machinery mixing equipment. It is recommended to use sensors with different protection capabilities for different types of mixing equipment.
The selection of sensors in a weighing system usually takes into account the measurement limit, accuracy, installation space of the sensor, the possible influence of the surrounding environment on the sensor, the type of loading, and the life of the sensor. The weighing system of construction machinery mixing equipment is no exception. It's just that different weighing systems have different requirements and working conditions, so the focus of the issues to be considered is different.
Construction machinery mixing equipment usually includes a concrete mixing plant (station), an asphalt mixing plant, a stabilized soil mixing plant, and an offshore concrete mixing vessel, among which concrete mixing plant (station)^ is a typical example. Let's take a concrete mixing plant (station) as an example for analysis.

1. Basic requirements for weighing system of concrete mixing plant (station)
1.1. Accurate weighing error has a great influence on the strength of concrete, especially the measurement accuracy of water-cement ratio, because the relationship between strength and water-cement ratio is linear. Related national standards stipulate that the dynamic measurement accuracy of cement, water, admixtures, and admixtures is ±1%, and the dynamic measurement accuracy of sand and stone materials is ±2%.
1.2. Quickly meet the requirements of the working cycle of the mixing plant.
1.3. There are many kinds of weighing values. There are many kinds of pre-selected weighing values, and the change should be convenient to meet the requirements of various ratios and different capacities.
1.4. Simple structure The weighing device should be simple in structure, firm and reliable, stable in performance and easy to operate.
Obviously, the sensor electronic weighing system can better meet the requirements than the mechanical scale. Therefore, load cells have been more and more widely used in concrete mixing plant stations. However, objectively speaking, compared with the mechanical lever scale, the sensor electronic weighing system still has a lot of work to be done in terms of "firm and reliable, stable performance".

2. Operating conditions of the load cell in the concrete mixing plant
A big difference from the electronic scales generally used for commercial measurement is that the load cells in the concrete mixing plant are in very harsh operating conditions, and the stress environment is complex. Compared with the operating environment of general electronic products , There is greater randomness.
2.1. Ambient temperature and humidity
The concrete mixing building station is usually installed in the open air, the sensor may be exposed to the sun and rain, and the temperature may change drastically. Many engineering construction projects are in mountainous or remote areas with very harsh natural conditions. Therefore, a larger temperature range and higher humidity conditions must be considered. Concrete needs water in the production process. A lot of water vapor will also be generated in the process of water transportation and weighing, forming a relatively humid environment in a certain small area. In the temperature-controlled mixing plant, there are different requirements for high temperature and low temperature conditions. When running in low temperature conditions in summer, cold wind below zero and ice should be added to stir. At this time, condensation water will appear in the building, which shows the high humidity in the building.
2.2, dust
In the production process of concrete, a large amount of cement, coal fly ash and an appropriate amount of additives are required. These powders will generate dust during the transportation and weighing process. Even if it is aggregate, dust is generated during transportation. Some of these dust will adhere to the surface of the sensor. Under the combined action of dust and moisture, the sensor will be severely corroded.
Therefore, the damage of the sensor of the powder scale is usually more frequent than the sensor of other scales.
2.3 Shock and vibration
During the feeding process, sand and gravel will have an impact. The sensor should be able to withstand 5g acceleration. During the stirring process, continuous vibration will occur, and vibration will cause fatigue damage.
2.4, man-made environment
The man-made environment is one of the factors that must be considered in product reliability design. The concrete mixing plant is generally installed at the construction site. A large number of temporary workers are used on the construction site, and a considerable number of them have a low education level and lack the necessary skills. In the maintenance and cleaning of the equipment, it is very likely that the sensor will be sputtered by high-pressure water, and the wrong operation will cause overload. Obviously, for the sensor to operate reliably for a long time under such environmental conditions, some special designs are required.
The above basic requirements and operating conditions can be used as the basis for the selection of load cells for concrete mixing plants (stations).

3. Several issues that need to be considered when selecting weighing sensors for concrete mixing plants (stations)
3.1. Determination of load capacity of load cell
The load capacity of the load cell is usually calculated as follows
Sensor rated load = hopper weight + rated weighing weight
(0.6~0.7) × sensor × number
In fact, when selecting the capacity of the sensor, people often have to consider the size of the impact load and the selected safety factor. The choice of the safety factor is closely related to the sensitivity of the sensor. Most common strain gauge load cells at home and abroad have a sensitivity of 2mV/V, but there are also 1mV/V, such as column sensors; there are also 3mV/V, such as some cantilever beam sensors and plate ring sensors; twisted ring sensors It is usually 2.85mV/V. The sensors currently used in the mixing tower are basically 2mV/V.
3.2 Selection of accuracy of load cell
The accuracy of the sensor is selected to meet the accuracy requirements of the weighing system, and it is not necessary to one-sidedly pursue a high level of sensor accuracy. When multiple sensors are used in combination, the overall error is calculated as follows
δr=δ／√n
In the formula, δ is the accuracy of a single sensor, and n is the number of sensors used in combination.
At present, the main indicators of linearity, hysteresis, repeatability, sensitivity, temperature effect, and creep of S-type sensors, cantilever beam sensors, and plate-ring sensors commonly used upstairs are better than 0.05% by most manufacturers. Better than 0.03%, some manufacturers are better than 0.02%. The overall error of its single sensor is close to or better than 0.1%. After multiple sensors are combined, the overall error is even smaller. It can be said that the products of general load cell manufacturers can meet the requirements.
In the past, many mixing plant manufacturers set the static accuracy of their weighing systems to be 0.1% and 0.3% respectively. In this way, for a scale that uses a single sensor, a sensor with a single index of 0.05% can meet the requirements of a scale with 0.3% accuracy. For a scale using more than three sensors, a sensor with a single index of 0.05% can also meet the 0.1% accuracy requirement.
It should be pointed out that the above accuracy is the static accuracy of the weighing system, while the national standard for concrete requires dynamic accuracy. This is due to the continuous supply of raw materials to the weighing mechanism. The weighing error increases significantly under the impact of gravity.
Whether the static accuracy of 0.1% and 0.3% can guarantee the dynamic accuracy of 1% and 2%, respectively, is also related to the design of the feeding system.
Nowadays, many manufacturers stipulate that the accuracy of their weighing systems is ±1% for cement, water, admixtures, and ±2% for sand and stone within the entire weighing range of 0~rated weighing value. It should be noted that the national standard for ready-mixed concrete requires physical measurement accuracy, while the general measurement instrument has a relatively large error in the range of 0-20%. Therefore, it is still the 20-100% scale specified in the industry standard for concrete mixing buildings (stations). The measurement section meets the requirement of ±1% for cement, water, and additives, and the allowable deviation of ±2% for sand and stone is more practical. A more reasonable precision labeling method is to use the terminology of the weighing instrument industry to express it, that is, as a cumulative hopper scale, the automatic weighing accuracy grade signs 1.0 and 2.0 should be used. The verification items of this type of scale are compatible with the purpose, including both material testing (to determine the cumulative error) and static verification.
Usually the 1.0-level scale is designed to be 1000 divisions, and the 2.0-level scale is designed to be 500 divisions. It can be seen that the error of the 1.0-level scale at the high-range end is 0.15%, and the error of the 2.0-level scale is 0.3%, which is not higher than the static accuracy of 0.1% and 0.3% specified by the previous manufacturer. But the accuracy requirements in the low range range are indeed improved. In this case, it is necessary to confirm whether the selected sensor can also meet the requirements in the low range range.
The simple way to select the sensor according to the number of divisions of the scale is to use a 1000-division sensor for a 1000-division scale and a 500-division sensor for a 500-division scale. In the national standard of load cell, the accuracy is expressed by the number of divisions. However, due to various reasons, most manufacturers still use a single index to express the accuracy of the sensor. The user then calculates the comprehensive error based on the single precision index. It is a little troublesome to choose.
The accuracy of the weighing system of the concrete mixing plant in developed countries is generally expressed by the number of divisions, which is specified as 1000 divisions. It may be related to their high proportion of high-performance concrete.
With the continuous development of economy and technology, super high-rise buildings, super-long bridges, large-scale water conservancy projects and other buildings exposed to harsh environments have put forward higher and higher requirements for the performance of concrete, and concrete technology has also entered the high-tech era , The application proportion of high-performance concrete is continuously increasing. In addition to the correct selection of raw materials and determination of reasonable process parameters in the production of high-performance concrete, the control of the construction process is also important. The accuracy of the batching system in the concrete mixing plant is an important part of it.
3.3. Selection of sensor structure
Commonly used pull sensors are S-shaped, plate-ring type and central cross-rib plate-ring type. The central cross-rib plate ring sensor has high accuracy and excellent anti-eccentric load performance, but the price is relatively high, and it is usually only used in high-precision measurement occasions. S-type sensors and plate-ring sensors are commonly used upstairs. Among them, the S-type sensor is often used because of its high accuracy, strong anti-eccentric load ability, and also can be equipped with overload protection, wide measuring range and other advantages.
Commonly used pressure sensors are cantilever type, spoke type, column type, bridge type, torsion ring type and so on. Considering factors such as accuracy, measuring range, installation method, and price, most mixing plant manufacturers choose the cantilever beam type.
3.4. Consideration of load type
Concrete production mainly uses sand, stone, cement, water, admixtures, and admixtures. Among several materials, the impact of several stone scales is large. In the large family of electronic scales, this type of impact is not considered large, and general sensors can withstand it.
Overload situation.
According to the situation investigated by the author, the sensor is damaged due to overload from time to time. For example, the control system fails, causing a large amount of material to be poured down, causing overload. There are also overloads caused by human factors during use, especially small-range sensors that are overloaded and damaged by the operator stepping on the scale frame. Therefore, the overload capacity of the sensor, whether the sensor has overload protection or not, still have a certain influence on the reliable operation of the weighing system.
Two of the sensor's performance indicators are related to this, one is allowable overload, and the other is limit overload.
Allowable overload means that the performance index of the sensor remains unchanged after the load is removed. Limit overload means that the sensor will not produce harmful mechanical deformation under this load.
Generally, the allowable overload of the sensor is 150%, and the limit overload is between 200% and 300%. Some sensors with overload protection may exceed a range. For example, the CFCKN-1 sensor of Putian Sensor Factory allows overload up to 500% due to its special design. This type of sensor can work reliably even under frequent overload conditions.
3.5, the protection level of the sensor
The protection level of the sensor is usually expressed by IP. Generally, sensor manufacturers claim that their products reach the IP67 level, and some of the products of a few manufacturers reach the IP68 level.
We know that the IP code represents the enclosure protection level of electrical products whose voltage does not exceed 72.5kV. According to the national standard "GB4208-93 Enclosure Protection Grade", IP67 means that the product can prevent dust and short-term immersion; IP68 means that the product can prevent dust and continuous diving. It should be pointed out that such protection does not include external influences or environmental conditions such as mechanical damage, rust, and humidity. This type of protection is usually stipulated by the relevant product standards. The load cell is different from ordinary electrical products and secondary instrument products in that it is also a force-bearing component, which is constantly subjected to force and deforms during operation. In addition, it may be subject to mechanical damage such as vibration, impact or impact, severe corrosion under the combined action of dust and moisture, and operation under very humid environmental conditions. This is very different from the test conditions specified in GB4208-93.
For example: an imported sensor is used on the ore scale of a mine, and an internationally renowned brand sensor is used on the ladle scale of a large steel mill, all of which are welded and sealed, very beautiful, and reach IP68 level. But the actual service life on site is very short. As a last resort, putian sensor factory products were tried. The result was beyond their surprise, and the service life exceeded that of imported sensors. Among them, the sensor on the ladle scale of a large steel mill has been in operation for 7 years and is still in operation.
In these two examples, the main point is the difference in protection. One is the shell or no shell; the other is the weld design. The third is the sealing material.
Generally speaking, a design with a shell is better than a design without a shell; in the weld design, it is necessary to avoid stress on the weld as much as possible. In unavoidable situations, the weld strength, especially the fatigue strength, must be checked. Micro cracks caused by fatigue are an important reason for the ingress of moisture and sensor failure. The aforementioned reason for the short service life of a well-known international brand welded sealed sensor in the field is that it relies too much on welding and sealing. The internal strain gauge is only thinly sealed with a layer of glue. Once the weld cracks, the sensor will quickly fail under the action of moisture. In terms of sealing materials, it should be reminded that a sensor that can pass the IP67 half-hour immersion test may not pass the 12-cycle damp heat test in the sensor standard. It should be said that the damp heat test specified in the sensor standard is more in line with the sensor's use environment requirements, and its severity is not lower than IP67.
Affected by price competition, many sensor manufacturers have cancelled protective casings, especially pull-type sensors. At present, almost all the S-type sensors and plate-ring sensors produced in China have eliminated their shells.
According to the situation investigated by the author, the sensor is damaged due to overload from time to time. For example, the control system fails, causing a large amount of material to be poured down, causing overload. There are also overloads caused by human factors during use, especially small-range sensors that are overloaded and damaged by the operator stepping on the scale frame. Therefore, the overload capacity of the sensor, whether the sensor has overload protection or not, still have a certain influence on the reliable operation of the weighing system.
Two of the sensor's performance indicators are related to this, one is allowable overload, and the other is limit overload.
Allowable overload means that the performance index of the sensor remains unchanged after the load is removed. Limit overload means that the sensor will not produce harmful mechanical deformation under this load.
Generally, the allowable overload of the sensor is 150%, and the limit overload is between 200% and 300%. Some sensors with overload protection may exceed a range. For example, the JLBS sensor of Bengbu Jinnuo Sensor Instrument Factory is specially designed to allow overload up to 500%. This type of sensor can work reliably even under frequent overload conditions.

4. Subdivision of selection of weighing sensors for different types of mixing plants
Used in concrete mixing towers of large and medium-sized water conservancy projects, urban commercial concrete mixing towers, small water conservancy projects and county-level highway construction