In recent years, belt conveyors have been widely used in coal, metallurgy, electric power, building materials, petroleum, chemical and other industries. The theoretical and experimental research on belt conveyors has made great progress, but there are still low transmission efficiency systems. The structure is complex and the reliability is poor, lacking defects in automation control and communication protocols, and an effective soft start control system is not formed. Therefore, it is necessary to research and develop a new mechatronic soft start system with excellent performance, high transmission efficiency, simple mechanical structure and control system, long working life, low price and convenient installation and maintenance.
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1. Mechanical and electronic soft start control system composition and principle
It consists of a host computer, a frequency converter, and a programmable controller (PLC). In order to protect the working instrument, the inverter, programmable controller, sensor, etc. are installed in the control cabinet. The actuator is an asynchronous motor and a differential planetary reduction mechanism, and the control object is a belt conveyor.
The control system is master-slave two-level control in structure, in which the computer is the main and the PLC is the slave. After completing the software design of the control system, the computer downloads the control program to the PLC through communication with the PLC. As the industrial control host, the computer is responsible for the online modification of the PLC program, signal acquisition, data recording and processing, and control output tasks, while a large number of sequential actions are handled by the PLC.
When the control system is working, the PLC is powered on. In the process of starting the speed-regulating motor, the PLC controls the speed of the speed-regulating motor by controlling the output frequency of the inverter, so that it can reach the predetermined speed according to the set rising speed law. After it reaches the predetermined speed, the PLC turns on the main motor power through the contactor (Mc), and the main motor starts. Because of the speed synthesis of the differential gear train, there is no output at the load end. When the load starts, under the control of the PLC, the speed regulating motor decelerates according to the set rule, and the load is slowly output until the working speed is reached. When the load is soft stop, the PLC controls the output frequency of the inverter to accelerate the speed control motor according to the set curve. The main motor continues to work at the rated speed. Through the speed synthesis, the load slowly decelerates to zero. When the system is completely stopped, the PLC will disconnect the contactor from the main motor, the main motor will stop, then cut off the power of the speed regulating motor, and the system will stop completely.
2. Mechanical and electronic soft start control system hardware design
2.1 Selection of programmable controller
1. Programmable controller control system I / O point estimation
Determining the number of I/O points is a key issue in programmable controller design. After estimating the number of I/O points of the controlled object, you can select the programmable controller with the equivalent number of points, select the programmable controller of the corresponding scale and leave 100 %-15% I/0 margin.
2. Memory estimation
The memory capacity required by the user is affected by several factors: memory utilization; number of output points of the switch output; number of analog input and output points; user programming level. The ratio of the number of contacts in a block to the number of words required to store the machine language represented by the block is called memory utilization. The total number of PLC input/output points is an important basis for calculating the required memory capacity. Systems with analog control use digital transfer and arithmetic function instructions. The function instructions have lower memory utilization and occupy more memory. The pros and cons of the user program have a great impact on the length of the program and the running time. For the same system, programs written by different users may make the program length and execution time very different.
In summary, the empirical calculation formula for memory size is:
Total memory word count 2 (switch input point number + switch output point number) *10+ analog quantity point number *150
Consider the margin by 25% of the number of memory words.
3. Response time analysis
The programmable controller's sequential scan mode of operation makes it impossible to reliably accept input signals that last less than the scan period. The system response time refers to the time interval between the moment when the input signal is generated and the time when the state of the output signal is changed: System response time = input filter time + output filter time + scan period through the functions and parameters of the programmable controller Considering the trade-offs, the Siemens SIMATIC 57-200 programmable controller is selected for design, and the Siemens CPU226 and model (6ES7216-2BD21 0XB0) are selected to meet the design requirements of the control system. With the S7-200 CPU 226, the module consists of a central processing unit (CPU), power supply and digital I/O points integrated in a compact, stand-alone device. The CPU is responsible for executing the program and storing the data, and scanning the status and data of the field through the I/O components to control the industrial control tasks or processes. Inputs and outputs are the control points of the system: the input section collects signals from field devices such as sensors or switches, and the output section controls the motor and other equipment in the industrial process. The power supply supplies power to the CPU and any modules it is connected to. The communication port allows the S7-200 CPU to be connected to other devices such as a programmer or computer. When connected to a computer, it can form a multi-level control system. The status light shows the operating mode of the CPU, the current state of the local I/O, and the detected system errors.
2.2 Programmable controller and computer communication
Data communication is the transmission and reception of data within the same level or between different levels. The communication between the programmable controller and the computer is an important aspect of broadening the application of the programmable controller, and is also an effective way to accomplish multiple control tasks and achieve multi-level control. When the programmable controller is connected to a computer, the computer is only used for online programming, parameter modification, and data display. The communication between the programmable controller and the computer is performed through the RS-485 port and the RS-232 port. The information exchange mode is a string mode, and the RS-485 and RS-232 channels are used to easily configure a communication with the computer. system. The data and status of all devices are sent to the computer by the programmable controller, and the data is collected by the computer for analysis and operational status monitoring. The computer controls the initial value and set value of the programmable controller device to realize the upper control of the programmable controller by the computer. The S7-200 series PLCs are equipped with standard D-type sockets dedicated to RS-485 communication.
3. Software design of the control system
In the software design process of the control system, the program is written by computer plus V3.1 STEP Micro/WIN software. The design of the program is mainly based on the main functions of the control system flow chart and the control system. On the basis of understanding the function and control mode of the control system, according to the different working conditions of the belt conveyor, the control program of the system in each working stage is designed.
The programming mainly includes the following processes:
(1) System self-test, monitoring the status of the control system, if an error is found to the user, otherwise enter the next step.
(2) The time division start of the dual motor is realized by the control of the electromagnetic contactor. The timer is used in the program design. When the preset time is reached, the corresponding motor starts to start under the action of the PLC, and the motor reaches the set speed within the set time. The speed control of the speed control motor needs to be controlled by the programmable controller to control the output frequency of the inverter. The user can modify the start time of the main and variable speed motors in the program. The starting of the main motor and the speed regulating motor is controlled by the corresponding electromagnetic contactor.
(3) The soft start and soft stop process are realized by timer. The belt conveyor starts and stops according to the user set time and speed curve in different periods.
(4) A sequential power failure is required when parking is required, and all power sources can be directly released during an emergency stop. The sequence power is off using a timer, which is performed at intervals set by the user.
4. Summary
The innovation of this paper: This system uses the programmable controller to control the output frequency of the inverter to realize the control of the speed of the speed regulating motor. The speed of the speed-regulating motor and the main motor are synthesized by the speed of the differential planetary gear train, which enables the transmission system to be steplessly regulated in a wide range, achieving controllable starting of the load and high transmission efficiency. The system has been used in production, and its work is very stable and the effect is better.
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