In the process of producing concrete, self compacting concrete mixer needs accurate measurement of raw materials such as aggregate, powder, liquid admixture and water. For the mixer truck which has no fixed outdoor work place and needs to be mixed and poured on site, it is necessary to keep the stability and stability of the equipment and frame in the measurement process to ensure the high-quality raw material measurement process. It can be used in all aspects of mixer structure.
Aggregate transport and measurement, the use of gravel and sand overlay metering conveyor belt scale device. In the process of discharging the aggregate into the belt scale, due to the existence of space gap, after the receiving signal of the discharge door is closed, there is surplus aggregate into the belt scale. Through software and electrical control, it can automatically compensate the drop of aggregate and accurately achieve the target value of measurement. Belt weigher uses skirt belt for aggregate conveying. The skirt structure on the side of belt can prevent sand and stone 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 needs of metering, so as to further improve the metering accuracy.
The usage of powder is less than that of aggregate. In order to achieve accurate measurement, more strict measures need to be taken. When the mixing truck is mixing the powder, the powder is transported by the screw conveyor with frequency conversion function and then measured in the matching powder scale. The screw conveying speed is adjusted by frequency conversion control, so that the material can be delivered quickly in the initial metering, saving time; When the measurement is close to the target value, the feeding speed is slow to control the accuracy. In this way, the coarse and fine metering effect of powder is realized under the condition of efficiency.
Similar to the belt scale, there is 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 stage of conveying and storing, the incremental control algorithm is used to make up the drop automatically; When the powder scale is in the unloading stage, the system detects the storage capacity of the powder scale. If the weight exceeds the target value, the system implements the decrement control algorithm to intercept the excess weight for the next batch of ingredients. Due to the characteristics of fine particles and easy caking of powder itself, the soft connection at the outlet of powder scale is prone to dust and material accumulation, which will have an adverse impact on the measurement accuracy. In this regard, the use of special-shaped soft connection designed for powder unloading can prevent the scale pulling phenomenon caused by powder accumulation and eliminate adverse effects. Through the measures of frequency conversion transportation, "one increase and one decrease" and anti pull connection, the accurate measurement of powder in the mixer can be ensured.
In the case of inevitable vibration of frame and box, in order to ensure the accurate measurement of water and liquid additives, the traditional single hanging point and pull sensor measurement method is not the best choice. The principle of liquid scale using pull type sensor as measuring element is that the gravity strain produced by measuring weight is induced and converted into electrical signal to get the weight value, which is widely used in concrete mixing station. However, due to its sensitive characteristics, the vibration of the equipment frame and the small disturbance of external force may lead to the deviation of the measurement value when measuring the micro 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, water and liquid admixtures are measured by flowmeter, which overcomes the shortcomings of pull sensor.
The working principle of the flowmeter used in the liquid measurement system is to measure the flow according to the principle of electromagnetic induction. When conductors such as water and additives cut the magnetic lines of force in a magnetic field, an induced potential is generated on the electrodes at both ends of the pipe. By measuring the induced potential, combined with other known parameters, it can be converted into the weight of water or admixture. In the flowmeter, there is a linear relationship between the induced voltage signal and the average flow velocity, and the measurement accuracy is high; The measuring electrode is built in and has strong anti-interference ability; The measurement process is not affected by vibration factors, and the measurement process is stable.
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