


Summary:The automatic batching system takes the industrial control machine as the upper host, and Siemens PLC, frequency converter and weighing instrument as the lower slaves. To realize the communication management and control of each slave, the RS-232 asynchronous communication port of the industrial control machine is connected to the PLC after level conversion to form a physical channel for communication between the upper and lower units.
In real life, no matter what the foodstuffs are, the foodstuffs are not the only ones that can be used in the foodstuffs.
In real life, whether in the food processing industry or in the engineering manufacturing industry are involved in the dosage link, its economic and technical indicators for the quality and cost of the entire project has a pivotal relationship. In the actual project, the failure rate of the system controlled by microcontroller is very high, the precision is not high, the error is big, which seriously restricts the quality of the product, so as to restrict the quality of the whole company's project. So the design of high-speed, efficient, accurate, stable performance and intelligent dosage system is imperative. In this paper, the stable performance of PLC, high-precision weighing instrumentation to design automatic dosage control system, the system can be realized from the dosage measurement, to the material out of the whole process of production automation, intelligence.
I. System automatic dosage control program
The feeding system adopts master-slave structure. Industrial control machine for the upper host, Siemens PLC, frequency converter and weighing instrument for the lower slave. The host is in the dominant position to realize the communication management and control of the slaves. The RS-232 asynchronous communication port of the industrial computer is connected with PLC after level conversion to form a physical channel for the communication between the upper and lower bits; another RS-232 port of the host is connected with the communication port of the weighing instrument to form the second physical channel. The upper computer adopts polling method to communicate with the slaves one by one. The upper computer transmits the result of task planning to PLC, and in the process of PLC control, the upper computer uses the upper computer connection commands to monitor the operation status and data area content of the lower computer, and reads the internal status of PLC and the real-time data of the weighing instrument in real time, and displays them on the upper computer.
Hardware Design
The dosage control system has the following functions: ① fully automatic dosage, after setting the formula, the weighing system automatically weighs and dosages according to the formula without operator intervention; ② report function, which can generate daily, real-time, monthly and yearly reports, etc.; ③ dynamically increase and modify the reports, the system, through the setup of permissions, gives the technician or operator the right to modify and increase the formula, and at the same time, records the date of the modification and the operator's name, the date of the modification, and the date of the modification. The system gives the technician or operator the right to modify or add the formula by setting the authority, and records the date and operator number of the modification at the same time; ④ Power failure recovery function, the weighing control system can recover the measurement records before power failure in case of sudden power failure; ⑤ LAN sharing function, the host computer can share the data in LAN, which is convenient for the supervisor of the workshop to understand the progress of the project and other situations. The whole automatic batching system is composed of industrial control machine, PLC, industrial weighing instrument, frequency converter, vibration motor, mixer, weighing module, conveyor belt and other parts.
The upper computer provides human-computer interaction interface, completes the control information input, data management, data display, storage, statistics and reports, etc. The upper computer adopts IPC810 industrial computer, its main work is as follows: the host computer firstly reads a certain number of formula according to the operator's instruction, and then sends out the instruction to PLC to start the batching according to the proportion and sequence of the ingredients in the formula, enabling the PLC to start a specific formula, and the PLC to start the batching. Then, according to the proportion and sequence of the ingredients in the formula, it sends a command to PLC to start batching, which enables PLC to start a specific inverter. In the process of batching, the host computer of industrial control reads the status words of PLC in real time by polling, on one hand, to understand the operation status of PLC and PLC subordinate equipments; on the other hand, it reads the weighing data of the weighing instrument installed on the batching machine in real time, and according to the batching strategy, the host computer sends out the instruction of stopping the batching to the PLC when the weighing is close to the set value of the recipe. When all the materials on a formula are finished, the whole dosage process is suspended, waiting for the operator's instruction.
The whole feeding system is composed of industrial control machine, PLC, industrial weighing instrument, frequency converter, vibration motor, mixer, weighing module, conveyor belt and other parts. The dosing control system is composed of programmable controller, solenoid valve, regulating valve and so on. In the control of the system, analog control methods and switching control methods are involved. Among them, the control of the motor start and stop signals belongs to the switching control, while the liquid flow control belongs to the analog control, the system adopts a programmable controller integrating analog and switching, when the system collects the flow signals converted into current signals and sends them to the programmable controller, which completes the corresponding operation according to the user's requirements.
(A) host computer selection
The upper computer adopts Advantech IPC-610H, Advantech technology is strong, Advantech industrial computer with its stable appearance, complete after-sales service, limited expansion performance, high speed computing speed to attract the majority of users. Its chassis adopts an all-steel structure with anti-shock, anti-vibration, anti-electromagnetic interference and other effects, built-in special power supply has a strong anti-interference ability, the use of the base plate + CPU card structure, has a strong scalability, so that the reliability of the system has been effectively safeguarded.
(ii) PLC selection
PLC selection of Siemens S7-200, it is a strong command system; some models also provide FP-BASIC language programming CPU and a variety of intelligent modules for the development of complex systems provide software means; FP series of various PLC are configured with the communication mechanism, due to the consistency of their use of the application layer of the communication protocol, which constitutes a multilevel PLC network and the development of the PLC network application program. This brings convenience to the composition of multi-level PLC networks and the development of PLC network applications. At the same time provides advanced programming features, easy to configure, easy to install, can easily replace simple relays and complex medium-sized automation applications. The PPI communication protocol is adopted between it and the upper computer, which can transmit data conveniently and effectively.
(C) Selection of weighing instrument
The weighing instrument adopts XK3201, which has excellent zero stability and gain stability due to the adoption of recognized excellent preamplifier (made in USA) as the input amplifier of the weighing module, so that this indicator has superb zero stability and gain stability.
With a high speed A/D conversion processing of 100/sec, it also responds quickly to instantaneous weight changes, thus greatly improving the performance of the metering indicator. In all input ranges, the display accuracy of 1/10,000 can be guaranteed.
(D) Frequency converter selection
Automatic batching system due to the batching plant has a variety of raw materials, in order to batching convenience, we set up a number of conveyor belts, so the batching machine a PLC needs to be linked to a number of inverters. For this purpose, Profibus bus is used for communication between PLC and inverter, the special Profibus communication module is inserted into the inverter, and the slave address of the inverter is set, and then it is connected to PLC through Profibus connector, PLC realizes to send and receive telegrams to this inverter through programming, and sends the control words to the inverter, and reads the status words from the inverter at the same time. CPU315-2DP acts as the Profibus master, and each inverter communicating with the master can be regarded as a Profibus slave. When communicating, the master selects the slave to transmit data according to the address characters in the communication message. The slaves themselves cannot send data on their own initiative, and there is no direct transmission of information between the slaves. The inverter uses the Panasonic VFOC series inverter communication function to communicate from a PLC, computer, etc., allowing operation, frequency control, monitoring, and parameter setting. Using RS485 communication, multiple control is also possible, and the PID function allows simple control of operating temperature, pressure, and flow rate without the need for an external regulator such as a thermostat. With the auto-tuning function, the setpoint can also be adjusted automatically.
(1) frequency converter due to metering screw motor load for constant torque load, and requires a large starting torque, while taking into account the accuracy of the metering operation; according to these characteristics, this system selects MM440. MM440 is a new generation of multifunctional standard frequency converter that can be widely used. Adopting high-performance vector control technology, it provides low-speed high torque output and good dynamic characteristics, and at the same time, it has strong overload capacity, which can meet a wide range of applications. With standard RS485 interface, using the internal USS standard protocol of MM440, it can be conveniently composed of USS serial master-slave control dosage system, with hardware support up to l15.2Kb/s, which can be composed of low-cost, high-performance network communication control dosage system to meet the majority of application occasions. Meanwhile, optional PROFIBUS-DP/Device-Net communication module realizes the diversity of communication.
(2) PLC because the batching machine adopts RS-485 serial bus control based on USS protocol, s7-200 is fully realized. This batching in the selection of the new CPU226CN, S7.200CN inherited the s7-200 good quality and excellent performance, suitable for all walks of life, a variety of occasions in the detection, monitoring and control of the automation process. s7-200CN series of powerful features make it whether in independent operation or connected to a network are able to realize the complexity of the control function, so it has a very high cost-effective.
(3) weighing instrument HMI in order to facilitate the dosage process parameters and process parameters for setting or monitoring, the choice of Siemens TPI70A as a weighing instrument.
Software design
(I) PLC program design
During the batching operation, PLC communicates with the upper computer in real time to ensure the consistency between the data displayed on the interface and the actual data on the site, and the operation commands and setting parameters issued by the operator on the upper computer can be sent to PLC in real time, and the main work of PLC includes: ① receiving the commands sent by the upper computer, and controlling the starting, stopping, and speeding up and slowing down of vibration motors through the frequency inverter; ② writing the operation status of frequency inverter into the memory data area in real time for the worker to set up and monitor. Write the operation status of the frequency converter into the memory data area for the industrial control machine to read; ③ Prepare the various states of itself in the form of status words to be read by the industrial control machine in real time.
When the operator starts the system, the industrial control machine sends a signal to PLC to start adding materials, PLC controls the inverter to drive the motor to add materials quickly, at the same time, the industrial control host continuously reads the weighing data of the weighing instrument through the serial port, when the weight value is close to the set value, the industrial control host sends a control instruction to PLC to stop adding materials, at this time, PLC controls the inverter to add materials slowly, and estimates the residue of the raw materials on the transfer mechanism in advance, and sets a control instruction to stop adding materials, at this time, PLC controls the inverter to add slowly. At this time, PLC controls the frequency converter to add slowly and estimate the residue of raw materials on the conveyor in advance. When the difference between the set value and the actual feeding and the residue of raw materials on the conveyor are quite the same, PLC will issue a real stop command, which is executed by the frequency converter, thus controlling the motor to stop, and after stopping, there is no residue of raw materials on the conveyor, and the accuracy of the dosage meets the requirements.
(1) The main program block is used to call USS-INIT and USS-CTRL instruction, USS-INIT instruction defines the baud rate and communication mode when communicating; USS-CTRL instruction defines the communication address of each frequency converter respectively, as well as the setting and displaying of each frequency converter are reflected.
(2) Logic action block-dosing is a sequential control with each link detecting the execution as a condition, one step after another. It is used to control the action of various external solenoid valves and motors, as well as to control the metering time of each metering screw.
(3)Parameter setting according to some actual operation requirements, through a few multiplication commands on the metering screw motor metering time is set, in the frequency setting and display through a few multiplication and division commands, the advantage is in the parameter setting process is clear, at a glance, if in the process of operation of time and frequency setting problems, you can easily find the problem point through the monitoring.
(4) Alarm block is mainly used to display the lower limit of each storage reactor and mixing the lower limit of the alarm display.
(ii) Industrial control host software design
The main tasks of the industrial control machine are as follows:
1、Provide animation display of dosage process.
2, to PLC control commands, read the PLC operation status.
3、Reading the weighing signal on the weighing meter, according to the weighing data, sending instruction to PLC.
4、Increase and modify the formula.
5、System fault auxiliary alarm and other functions.
(C) touch screen program design
This system touch screen software program includes login interface, system main menu, recipe, operation, alarm, report interface, as shown in the figure below. Switching between screens can be realized by using the system main menu, and can also be switched by the button on the sub-screen.
In the automatic dosage process of real production, this control system has high precision of liquid and material proportioning, uniform and adjustable feeding speed, and free setting of liquid dosage speed. It realizes the switching and selecting between several liquid dosing tanks and unattended automatic dosing, which meets the requirements of on-site process and brings great convenience to the real production.