Views: 258 Author: Site Editor Publish Time: 2023-11-28 Origin: Site
There have been great numbers of universal equipments , special equipments and production lines in China's mechanical manufacturing industry. In the development of the modern mechanical automation technology, microelectronics technology can be applied. With the application of PLC, touch screen and numerical control device, the automation of these mechanical equipments can be improved, and also computer's advantages such as powerful functions, flexible programming and high generality and relay's simple control, powerful anti-jamming ability and low price can be ntegrated together. Therefore, this is a new way of low cost and high efficiency to develop and apply mechanical automation technology.
Plate shearing machine is a type of processing equipment which cuts off plates and simultaneously implements automatic counting according to processing requirements, and has been widely used in plate processing system. In a plate shearing machine, the processes such as plate material length testing, plate material feed, compression, feeding, blanking, and length adjustment have to be accurately implemented according to certain steps. Moreover, the plates of different length, different thickness and different material as well as the requirements on each action stroke, order and tool position are different. Therefore, the requirement on the automation, flexibility, and reliability of plate shearing machine control system is very high. Such a high requirement can't be satisfied by the traditional relay contactor control system of complex wire connection, single function, and low reliability.
Programmable controller (PLC) is a digital operational operating system and designed especially for industrial environment. In PLC, a programmable memory is used for storing and executing many operation instructions such as logic operation sequential control, timing, counting and arithmetic operations within it, and also inputting/outputting and controlling all kinds of production machineries or processes. Importantly, it has been widely applied in various industrial control occasions because of its rich functions, easy-to-use, reliability in work, and tangible benefits.
In this paper, the application of PLC in the design of the control system of plate shearing machine is mainly introduced, and also the application of PLC in drilling machine/drill hole and pressing machine is presented.
The Working Process of Plate Shearing Machine and Its Requirements on Electric Control System Plate shearing machine has been applied in many metal processing and sheet cutting operations. It is a type of automation processing equipment, which can precisely control the processing size of plates, automatically and circularly cut off and process large plates, and also enters next process through feeding car. Before plate shearing machine is designed, its shear capacity, production rate and safety are necessarily considered.
Fig. 1 The working flow of plate shearing machine
The working flow of plate shearing machine is as shown in above figure. Therefore, it can be known that plate shearing machine is composed by feeding car, operating platform, pressure clamp, scissor, limit switch, pressure relay device, etc. At the beginning, pressure clamp and scissor are in the upper limit position, and limit switches SQ1 and SQ2 is on. After the start button is pressed, the working flow is as follows.
Plates move to the right until switch SQ4 at the lower limit position stops.
After pressure clamp descends and compresses tightly plates, pressure relay is on, and pressure clamp keeps compacted Scissor descends, and SQ3 is on after plates are cut off.
Pressure clamp and scissor ascend simultaneously, and they will stop ascending after touching SQ1 and SQ2 respectively.
After all above parts stop working, the work of next cycle begins; work will stop after N blocks of materials are cut off and return to the preliminary test state.
The requirements of the working process on the electric control system are as follows: (1) the delivery of plates and the ascending and descending of pressure clamp and scissor are driven by a motor respectively, and the three motors can turn forward and backward; (2) after power is on, the states of all working mechanisms are tested, making all working mechanisms in their initial position;(3) automatic counting can be implemented after each plate is cut off, and the machine can stop at the initial state if count value is equal to set value; (4) shearing size can be set, and plates are ensured to be compacted when plates are sheared; (5) power supply interruption keeping function and necessary protection measures are available.
2.2 The PLC Control System Designed with Sequential Control Method
PLC is a special computer cored at microprocessor and used as digital controller. It is mainly composed by CPU module, I/O module, power components and programmer. The working pattern of PLC is mainly scan-round. For specific control objects, the work process can be generally divided into five stages: internal processing, communication service, input processing (sampling), program execution, and refreshing output. The programming languages commonly used by PLC include ladder diagram and instruction lists, and the program design methods mainly include experience design and sequential control design. Sequence control means that all executive mechanisms automatically and orderly operate in production according to the sequence prescribed by productive technology and the internal state/time sequence under the action of all input signals. Sequence control design is also called as stepping control design method, and its basic idea is dividing a working cycle of the system into several stages with connected sequence (these stages are named as steps), and also each step is represented with programming components such as auxiliary relay M and state S. The essence of sequence control design is to control the programming components such as auxiliary relay M of each step with input X,and then use them for control output Y.
Sequence control design method features simple program structure, high accuracy, and no-competition adventure, etc. Steps are divided according to the state of output Y; there is a very simple "and" logic relationship between M and Y; the output circuit design is also very simple. Because M is on/off in turn, the memory, interlocking and other problems in experience design method are basically solved actually.
2.2.1 I/O Address Allocation and Wiring Diagram
Input device is mainly travel switch, and has six input points totally. In output equipment, scissor,pressure clamp and feeding car are driven by a motor respectively; the motors driving scissor and pressure clamp are required to move forward and backward and five contactor contacts are needed totally; the overload and short-circuit protection of motor are implemented with thermal relay and fuse protector and also five output points are needed totally.
2.2.2 Sequence Control Function Diagram
Through analysis, it can be known that this system is a multi-step sequence control system that designs programs with sequence control design method, making each step strictly implemented according to sequence. Thus, the advantages such as simple structure, easy programming and no competition of the system can be realized. When the plates of each car are counted by the counter, the counting value is set according to the needs of user.
PLC, because of its excellent performances, has been increasingly more extensively applied in many kinds of industries (e.g. machine tool processing, textile machinery, jig dyeing machine, plastic rubber machinery, pressing machine, injection molding machine, packaging machinery, printing machinery, food machinery, medical machinery, and woodworking machinery) and especially in the mechanical manufacturing industry.
In the following figure, six uniformly-distributed holes used by special drilling machine for processing discoid parts are shown. To realize automation, the design of the control system has to be made according to the work characteristics and component motion control requirements of drilling machine. According to the above mentioned advantages of PLC, the design of the special drilling machine control system can be made with PLC.
3.1 Design of Main Circuit
Four three-phase cage asynchronous motors are applied, so as to implement the drilling and ascending of big drill and small drill, the rotation of parts, and the clamping and relaxing of parts,respectively. The drill holes of big drill and small drill, the ascending motor, and the motors of clamped and relaxed parts are required to move forward and backward and the overload and short-circuit protection are implemented with thermal relay and fuse protector.
3.2 I/O Address Allocation and Wiring Diagram
Input devices mainly include travel switch and pressure relay and possess 8 input points totally.Output device is the contactor controlling motor and possess 7 input points totally.
3.3 Sequence Control Function Diagram
The application of PLC in the mechanical manufacturing industry has been particularly prominent. In this paper, the plate shearing machine and special drilling machine using PLC are mainly introduced, and the whole working process of these mechanical equipments accords with the requirement of sequence control. The programmable controller is used for controlling plate shearing machine and special drilling machine in this paper. Therefore, the shortcomings such as non-counting function, poor reliability, complex fixed connection and poor anti-interference ability of the traditional control method are solved, and also competition and adventure problems are solved because of the application of sequence control design, making the flow of the system more flexible and accurate. In addition, motors can be driven with a frequency converter. Thus, equipments can be promoted to work at different speed by changing the parameters of the frequency converter. Meanwhile, human-machine interface can be implemented with touch screen, making the control system more convenient, efficient, and expressive and also simplified as PLC control program.
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