The electrical control system of the mixing plant can perform system self inspection and automatic operation of the process flow of the complete equipment. Various controls can have both serial connection and parallel overlapping timing control, and can be controlled by logical programs under certain interlocking constraints. The measuring and batching system can ensure that various raw materials are dynamically measured within the set mix ratio and specified accuracy range. The computer management system has management functions such as setting and storing various information and parameters, processing and modifying control data, monitoring production operation status and alerting common faults, recording and printing production operation data, etc. The industrial television monitoring system can monitor the operation of remote sites and important areas.
The electrical control system of the mixing plant also includes the operation control system of the aggregate consolidation device and the powder transfer device. The shovel type mixing plant has an independent shovel operation electrical device. The electrical control system of other forms of mixing plants implements centralized and unified operation and control of the operation of the complete equipment.
The operation and control of the workflow of each system device in the mixing plant have three modes: manual, semi-automatic, and fully automatic. Automatic control is the implementation of automatic and mandatory program control over the entire process flow through a set program, without the need for manual repetitive operations, to complete the entire process of concrete production. Semi automatic control is the automatic control of related processes that are connected in series, while related processes that are crossed in parallel use manual operation to control the timing sequence. It is suitable for the implementation of a certain process execution device that exceeds the design timing required by the process, resulting in fully automatic control being unable to meet the constraints of interlocking conditions, and the operation control when the control program does not need to be changed. Manual button operation, suitable for single point debugging and maintenance, as well as situations where on-site conditions do not allow continuous automatic operation.
The core technology of the control system is program control. Early mixing plants used conventional relay and contactor control methods. With the rapid development of electronic technology, it has gone through several stages such as single board computer, single-chip microcomputer, programmable controller, and industrial control computer. The microcomputer control system with PLC as the lower computer and industrial control computer as the upper computer has become a typical mode of mixing plant control system. Programmable controllers are designed for industrial control applications, with strong anti-interference ability and large driving power. They can directly control electrical devices, and their module structure is easy to combine and expand, which can adapt to the system configuration of various products. The source program can be programmed with a graphical programmer for ladder diagram programming on a personal computer, or with a handheld programmer for mnemonic programming.
Both methods allow the written program to be written into the main controller of the PLC through a communication interface, and can be modified, debugged, set breakpoints, and executed in single steps. As a lower level unit, PLC is widely used due to its wide applicability, high reliability, and high programming efficiency and debugging speed. However, if the PLC control system does not have an upper computer, there are disadvantages such as inconvenient program development, limited input of control commands and information output, and the inability to achieve more management functions due to limited memory. Using an industrial computer as the upper computer, it can store a large number of mix proportions for users to choose from, and it is very convenient to call or modify mix proportions; Can intuitively simulate the entire process dynamics of a concrete mixing plant and achieve real-time monitoring; Capable of automatically recording, storing, and printing various types of data, achieving advanced scientific management functions; Combined with other electrical devices, it can achieve automated production processes and optimized concrete quality control.
The key link of program control is the control of the opening and closing of the executing components that feed the weighing device in the measuring process. Early mechanical levers used the mercury switch, micro switch, or non-contact switch on the scale to control the opening and closing of the storage bin valve when the scale was lifted or lowered. The preset weight of the material is set using the weights and weights on the scale bar. There are also disc scales that use pendulum or spring type gauges. When the movable pointer rotates to coincide with the fixed pointer, the permanent magnet induction switch sends an electrical signal to control the opening and closing of the silo valve. The preset weight of the material is set by the position of the fixed pointer on the dial, and multiple fixed pointers can be set to achieve cumulative measurement. The working principle of the so-called mechanical electronic scale is to use sensor components to send opening and closing control signals to the output door of the mechanical lever system. In some early imported mixers, a synchronous and accurate potentiometer is installed on the movable pointer shaft of the disc scale. When a fixed voltage is applied to the two fixed ends of the potentiometer, the voltage value output by the potentiometer arm is proportional to the weight value of the material being weighed as the shaft angle displacement changes. The voltage value output by the potentiometer arm is input into a specially designed electronic circuit and compared with the preset voltage value representing the proportioning weight. When they are equal, a control signal is output to close the silo valve. There is also a mechanical electronic scale that uses an analog punch card reader to set the proportioning weight, a servo motor to drive the set value pointer, and a digital meter head to display the predetermined value.