The basic requirements and special requirements of the weighing system of engineering machinery mixing equipment are put forward. Several problems need to be considered when selecting the sensor for the weighing system of engineering machinery mixing equipment. The influence of the protective structure of the sensor on the operational reliability of the engineering machinery mixing equipment is analyzed. It is pointed out that the degree of protection represented by the IP code cannot cover the full protection requirements of the engineering machinery mixing equipment for the sensor. Different types of mixing equipment are recommended for sensors with different protection capabilities.
The selection of sensors in the weighing system usually takes into account the weight limit of the weighing system, the accuracy, the 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 is only the different weighing systems that have different requirements and different working conditions, and the issues to be considered are different.
Construction machinery mixing equipment usually includes concrete mixing plant (station), asphalt mixing plant, stabilized soil mixing plant and offshore concrete mixing ship, among which concrete mixing building (station) is the most typical. The following is an example of concrete mixing building (station).

First, the basic requirements of the concrete mixing plant (station) symmetric heavy system
1.1. Accurate weighing error has a great influence on the strength of concrete, especially the water-cement ratio measurement accuracy, because the strength and water-cement ratio are linear. Relevant 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 work cycle of the mixing plant.

1.3, the type of multi-weighing pre-selection of more types, the transformation should be convenient to adapt to a variety of ratios and different capacity requirements.

1.4. Simple structure The weighing device should have a simple structure, firm and reliable performance, stable performance and easy operation.

Obviously, the use of the sensor electronic weighing system is more satisfactory than the mechanical scale. Therefore, load cells have become more and more widely used in concrete mixing plant stations. However, objectively speaking, compared with mechanical lever scales, there is still much work to be done in the sensor electronic weighing system in terms of “firm and reliable performance”.

Second, the operating conditions of the load cell in the concrete mixing plant
A big difference from the electronic scales commonly used for business measurement is that the load cells in the concrete mixing plant are in quite harsh operating conditions, and the stress environment is very complicated, compared with the operating environment of general electronic products. , there is greater randomness.

2.1, ambient temperature and humidity
Concrete mixing stations are usually installed in the open air, and the sensors may be exposed to the sun and rain, and the temperature changes drastically. Many engineering projects are in mountainous or remote areas where natural conditions are rather harsh. Therefore, a larger temperature range and higher humidity conditions must be considered. Concrete requires water during the production process. In the process of water transportation and weighing, a lot of water vapor is also generated, and a relatively humid environment is formed in a certain small range. In the temperature-controlled mixing building, there are different requirements for high temperature conditions and low temperature conditions. In the summer, when running in low temperature conditions, it is necessary to pass cold air below zero and stir with ice. At this time, condensation will appear in the building, which shows the high humidity in the building.

2.2, dust
Concrete requires a large amount of cement, coal ash and an appropriate amount of admixture in the production process. These powders produce dust during transport and weighing. Even in aggregates, dust is generated during transportation. Some of these dusts will adhere to the sensor surface. Under the combined action of dust and moisture, the sensor will be subject to more severe corrosion.
Therefore, the damage of the powder scale sensor is usually more frequent than the sensors of other scales.

2.3, shock and vibration
During the feeding process, the sand and gravel will have an impact. The sensor should be able to withstand an acceleration of 5g. During the agitation process, continuous vibration is generated, and the vibration generates fatigue damage.

2.4, artificial environment
The human environment is one of the factors that must be considered when designing product reliability. Concrete mixing buildings are generally installed at the construction site. Temporary workers are heavily used on the construction site, and a considerable number of temporary workers have low levels of literacy and lack the necessary skills. In the maintenance and cleaning of equipment, it is very likely that the sensor is subjected to sputtering of high-pressure water, and overload caused by misoperation. Obviously, the sensor must be designed in such a long-term and reliable environment under such environmental conditions.
The above basic requirements and operating conditions can be used as the basis for the selection of load cells for concrete mixing plants (stations).

Third, the concrete mixing plant (station) needs to consider several issues when using the weighing sensor
3.1. Determination of the load capacity of the load cell
The load cell load capacity is usually calculated as follows
Sensor rated load = hopper weight + rated weight
(0.6~0.7)×sensor×only
In fact, people often need to consider the impact load and the selected safety factor when choosing the sensor capacity. The choice of safety factor is closely related to the sensitivity of the sensor. The sensitivity of strain gauge load cells commonly used at home and abroad is mostly 2mV/V, but there are also 1mV/V, such as column sensors; also 3mV/V, such as partial cantilever beam sensors and plate ring sensors; torsion ring sensors It is usually 2.85mV/V. The sensors currently used in the mixing floor are basically 2mV/V.

3.2, the choice of the accuracy of the load cell
The accuracy of the sensor is chosen to meet the accuracy requirements of the weighing system, and it is not necessary to unilaterally pursue an excessive sensor accuracy level. When multiple sensors are used in combination, the combined error is calculated as follows
Δr=δ/√n
Where δ is the accuracy of a single sensor and n is the number of sensors used in combination.
At present, the S-type sensor, cantilever beam sensor and plate-ring sensor commonly used in the upstairs are mostly excellent in linear, hysteresis, repeatability, sensitivity temperature influence and creep. Most manufacturers are better than 0.05%, most manufacturers. Better than 0.03%, some manufacturers are better than 0.02%. The combined error of its single sensor is close to or better than 0.1%. The combined error of multiple sensors 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 specified the static accuracy of their weighing systems to be 0.1% and 0.3%, respectively. In this way, for a scale using a single sensor, a single indicator with a 0.05% sensor can meet the requirements of a 0.3% precision scale. For scales with more than three sensors, a 0.05% sensor with a single indicator can meet the 0.1% accuracy requirement.
It should be pointed out that the above precision is the static precision of the weighing system, and the concrete national standard requires the dynamic precision. This is because the raw material is continuously feeding the weighing mechanism and the weighing error is obviously increased under the impact of gravity.
Whether the above-mentioned 0.1% and 0.3% static accuracy can guarantee the dynamic accuracy of 1% and 2% respectively is also related to the design of the feeding system.
At present, many manufacturers stipulate that the accuracy of their weighing system is ±1% for cement, water and admixture, and ±2% for sand and stone in the whole weighing range of 0~rated weighing. 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 relative error in the range of 0-20%. Therefore, it is also the 20~100% scale specified in the industry standard of concrete mixing building (station). The measuring section satisfies ±1% of cement, water and admixture, and the measurement deviation of ±2% of sand and stone is more practical. A more reasonable method of accuracy labeling is expressed in terms of the weighing instrument industry. As an accumulating hopper scale, the level markings of automatic weighing accuracy should be 1.0 and 2.0. The calibration items for this type of scale are compatible with the application, including material testing (determining cumulative errors) and static testing.
The 1.0 scale is usually designed to be 1000 divisions, and the 2.0 scale is designed to be 500 divisions. It can be seen that the error of the 1.0-scale 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 0.1% and 0.3% static accuracy specified by the previous manufacturers. However, the accuracy requirement in the low range is indeed improved. In this case, it is necessary to confirm whether the selected sensor can also meet the requirements in the low range.
The easiest way to select the sensor according to the scale of the scale is to use a 1000-division sensor for the 1000-division scale and a 500-division sensor for the 500-division scale. The accuracy of the national standard for load cells is expressed in terms of the number of divisions, but for many reasons, most manufacturers still use a single indicator to indicate the accuracy of the sensor. The user then calculates the comprehensive error based on the individual accuracy index. It is a little troublesome to use.
The accuracy of the concrete mixing plant weighing system in developed countries is generally expressed by the number of divisions, which is specified as 1000 divisions. 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 placed higher and higher requirements on the performance of concrete. Concrete technology has also entered the high-tech era. The proportion of high-performance concrete applications continues to increase. In addition to the correct selection of raw materials and the determination of reasonable process parameters for the production of high-performance concrete, the control of the construction process is also very important. The accuracy of the batching system in the concrete mixing plant is an important part of this.

3.3. Selection of sensor structure form
Commonly used pull sensors are S-shaped, plate-ring type and center cross rib ring type. The center cross rib ring sensor has high precision and excellent anti-offset performance, but the price is high, and it is usually only used in high-precision measurement. S-type sensors and plate-ring sensors are commonly used in mixing upstairs. Among them, the S-type sensor is most used because of its high precision, strong anti-offset capability, and the ability to with overload protection and wide measuring range.
Commonly used pressure sensors are cantilever beam, spoke type, column type, bridge type, twist ring type and the like. Considering the factors such as accuracy, range, installation method and price, most mixing plant manufacturers have chosen cantilever beam.

3.4, load type considerations
Concrete production mainly uses sand, stone, cement and water, admixtures, admixtures. The impact of several stone scales in several materials is the largest. In the large family of electronic scales, this type of impact is not the largest, and the average sensor can withstand.
Overload condition.
In the case of the author's investigation, it is sometimes the case that the sensor is damaged due to overload. For example, the control system fails, causing a large amount of material to pour down, causing overload. There is also an overload caused by human factors during use, especially when the small-range sensor is overloaded and damaged by the operator stepping on the scale. Therefore, the overload capability of the sensor and the sensor's presence or absence of overload protection have a certain impact on the reliable operation of the symmetric heavy system.
Two of the sensor's performance indicators are related to this, one is to allow overload, the other is extreme overload.
Allowing overload means that the sensor's performance index does not change after the load is removed. Extreme overload means that the sensor does not produce harmful permanent mechanical deformation under this load.
The allowable overload of a typical sensor is 150%, and the ultimate overload is between 200% and 300%. Some sensors with overload protection may exceed a range. For example, CFCKN-1 sensor of Putian Sensor Factory allows overloading of up to 500% due to its special design. Such sensors work reliably even under frequent overload conditions.

3.5, the degree of protection of the sensor
The degree of protection of the sensor is usually expressed in IP. General sensor manufacturers claim that their products have reached IP67 level, and some products of some manufacturers have reached IP68 level.
We know that the IP code represents the degree of enclosure protection for electrical products that do not exceed 72.5kV. The national standard "GB4208-93 enclosure protection grade" stipulates that IP67 means that the product can be dust-proof and prevent short-term immersion; IP68 means that the product can be dust-proof and prevent continuous diving. It should be pointed out that such protection does not include external influences or environmental conditions such as mechanical damage, rust, moisture. The protection involved is usually stipulated by the relevant product standard. Unlike the general electrical products and secondary instrument products, the load cell is also a force-carrying component that is constantly subjected to force and deformation during operation. In addition, it may be subject to mechanical damage such as vibration, shock or impact, more severe corrosion caused by 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 ore sensor is selected on the ore scale of a mine. A large-scale steel mill's ladle scale uses an internationally renowned brand of sensors, all of which are welded and sealed, which is very beautiful and reaches IP68. However, the actual service life at the site is very short. Try the products of Putian Sensor Factory as a last resort. The result was their accident and the service life exceeded the imported sensor. The sensor on a large steel mill's ladle scale has been in operation for seven years and is still in operation today.
In both cases, the main difference is the difference in protection. First, there is a shell or no shell; the second is the weld design. The third is the sealing material.
In general, the design of the outer casing is superior to the design without the outer casing; in the case of the weld design, the force of the weld is to be avoided as much as possible. In case of unavoidable, it is necessary to check the weld strength, especially the fatigue strength. Fine cracks caused by fatigue are an important cause of moisture ingress and sensor failure. The reason why the above-mentioned international famous brand welding seal sensor has a short service life is that it relies too much on the welding seal. The internal strain gauge is only thin and one layer of rubber seal. Once the weld crack occurs, the sensor will quickly fail under the action of moisture. In terms of sealing materials, it is important to note 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 environmental requirements, and its severity is not lower than IP67.
Affected by price competition, many sensor manufacturers have removed the protective casing, especially the pull sensor. At present, the S-type sensor and the plate-and-loop sensor produced in China almost cancel the outer casing.
In the case of the author's investigation, it is sometimes the case that the sensor is damaged due to overload. For example, the control system fails, causing a large amount of material to pour down, causing overload. There is also an overload caused by human factors during use, especially when the small-range sensor is overloaded and damaged by the operator stepping on the scale. Therefore, the overload capability of the sensor and the sensor's presence or absence of overload protection have a certain impact on the reliable operation of the symmetric heavy system.
Two of the sensor's performance indicators are related to this, one is to allow overload, the other is extreme overload.
Allowing overload means that the sensor's performance index does not change after the load is removed. Extreme overload means that the sensor does not produce harmful permanent mechanical deformation under this load.
The allowable overload of a typical sensor is 150%, and the ultimate overload is between 200% and 300%. Some sensors with overload protection may exceed a range. For example, the JLBS sensor of Handan Jinnuo Sensor Factory has been designed to allow overloads of up to 500%. Such sensors work reliably even under frequent overload conditions.

4. Subdivision of the selection of symmetric heavy sensors for different types of mixing buildings
The concrete mixing plant used in large and medium-sized water conservancy projects, urban commercial concrete mixing buildings, small water conservancy projects, and concrete mixing buildings used in county road construction are very different in terms of size, duration of use, and environmental conditions at the installation site. The performance requirements of the concrete produced are also very different. Therefore, there are actually different requirements for the reliability and protection level of the sensor, which should be treated differently. Sensors with high reliability and high degree of protection should be selected for agitating buildings with long continuous operation time, harsh environmental conditions and high performance concrete. The rest can be appropriately reduced. For concrete mixers operating at sea, the need to prevent salt spray is also considered.