In the process of producing concrete, the self compacting concrete mixer needs to accurately measure the raw materials such as aggregate, powder, liquid admixture and water. For the mixer truck without fixed outdoor workplace and requiring on-site mixing and pouring, the high-quality raw material metering process can be guaranteed by maintaining the stability and stability of equipment and frame in the metering process. The design is conducive to improve the measurement accuracy and is applied to all aspects of the mixer structure.
The belt scale device of superposition metering and conveying of stone and sand is adopted for the transportation and metering of aggregate. In the process of aggregate unloading into the belt scale, due to the drop of space, after the receiving signal of the unloading door is closed, excess aggregate enters the belt scale. Through software and electrical control, it can automatically compensate the drop of aggregate and accurately achieve the measurement target value. The belt scale uses skirt belt for aggregate conveying. The skirt structure on the side of the belt can prevent sand and gravel from leaking to the side and ensure accurate measurement. The sand metering belt adopts frequency conversion control, which can adjust the conveying speed according to the metering needs and further improve the metering accuracy.
The use of powder is less than that of aggregate. In order to achieve accurate measurement, more stringent measures need to be taken. When the mixer truck is carrying out the mixing of powder, the powder is transported through the screw conveyor with frequency conversion function and then enters the matched powder scale for measurement. The screw conveying speed is adjusted through frequency conversion control, and the material is fed quickly during initial metering to save time; When the metering is close to the target value, feed slowly to control the accuracy. In this way, the coarse and fine metering effect of powder is realized under the condition of considering efficiency. Similar to the belt scale, there is also a height drop between the screw conveyor and the powder scale. In order to improve the metering accuracy, the powder metering system (as shown in Figure 4) adopts the "one increase and one decrease" control algorithm. The process is as follows: when the powder scale is in the conveying and storage stage, the incremental control algorithm is adopted to automatically replenish the drop; When the powder scale is in the unloading stage, the system detects the memory reserves of the powder scale. If the weight exceeds the target value, the system executes the reduction control algorithm to intercept the excess weight for the next batch of batching. Due to the characteristics of fine particles and easy caking of powder, the soft connection at the discharge port of powder scale is prone to ash and material accumulation, which will have an adverse impact on the measurement accuracy. In this regard, the special-shaped soft connection specially designed for powder unloading can prevent the scale pulling phenomenon caused by powder accumulation and eliminate adverse effects. Through frequency conversion transportation, "one increase and one decrease" and anti pull connection, the accurate measurement of powder in the mixer is guaranteed.
When the vibration of frame and box is inevitable, in order to ensure the accurate measurement of water and liquid admixture, the traditional single hanging point and pull sensor measurement is not the best choice. The principle of liquid scale using pull sensor as metering element is to convert the gravity strain generated by metering weight into electrical signal to obtain the weight value, which is widely used in concrete mixing plant.
However, due to its sensitive sensing characteristics, the vibration of the equipment frame and small disturbance of external force may lead to the deviation of the measured value when measuring the small target value. In the self compacting concrete mixer with compact structure, accurate measurement requirements, complex working conditions and high mixing quality requirements, there should be a better design.
Through many tests, it is verified that the flowmeter is used for raw material measurement of water and liquid admixtures, which overcomes the shortcomings of pull sensor. The working principle of the flowmeter used in the liquid metering system is to measure the flow according to the principle of electromagnetic induction. When conductors such as water and additives move to cut magnetic lines of force in a magnetic field, an induced potential is generated on the electrodes at both ends of the pipeline.
By measuring the induced potential, combined with other known parameters, it is converted to the measured weight of water or admixture. In the flowmeter, the induced voltage signal has a linear relationship with the average flow rate, and the measurement accuracy is high; Built in measuring electrode, strong anti-interference ability; The metering process is not affected by vibration factors, and the metering process is stable.