The development of industrial production and technology is inseparable from the automatic control of PLC. PLC can be broadly understood as: centralized relay extension control cabinet. In actual production applications, PLC greatly saves the cost of industrial control and strengthens the concentration of equipment. Management and automatic control, if you want to learn PLC well, first of all, the foundation of PLC needs to be solid.
1. From the composition of PLC, besides the CPU, memory and communication interface, what interfaces are directly related to the industrial site? And explain its main functions.
(1) Input interface: accept the signal of the controlled device, and drive the internal circuit to turn on or off through the photoelectric coupling device and the input circuit.
(2) Output interface: The execution result of the program is output through the photoelectric coupling device and output components (relay, thyristor, transistor) of the output interface to control the on or off of the external load.
2. What parts does the basic unit of the PLC consist of? What role does each play?
(1) CPU: The core component of the PLC, which directs the PLC to perform various tasks. Such as accepting user programs and data, diagnosing, executing execution programs, etc.;
(2) Memory: storage system and user programs and data;
(3) I/O interface: The connection between PLC and the controlled object in the industrial production site, It is used to receive the signal of the controlled device and output the execution result of the program;
(4) Communication interface: exchange information with other devices such as monitors and printers through the communication interface;
(5) Power supply.
3. What types of PLC switching output interfaces are there? What are the characteristics of each?
Thyristor output type: Under normal circumstances, it can only carry AC load, with fast response speed and high operating frequency;
Transistor output type: Under normal circumstances, it can only carry DC load, with fast response speed and high operating frequency;
Relay output type: Under normal circumstances, it can carry AC and DC loads, but its response time is long and the operating frequency is low.
4. According to the structure type, what types of PLCs are there? What are the characteristics of each?
(1) Integral type: The CPU, power supply, and I/O components are all concentrated in one chassis, with compact structure and low price. Generally, small PLCs adopt this structure;
(2) Modular type: Divide the various parts of the PLC into several separate modules, and you can choose different modules to form a system according to the needs. It has the characteristics of flexible configuration, convenient expansion and maintenance. Generally, medium and large PLCs adopt this structure. A modular PLC consists of a frame or baseplate and various modules, which are mounted on the sockets of the frame or baseplate.
(3) Stacked type: Combining the features of integral type and modular type, the CPU, power supply and I/O interface of stackable PLC are also independent modules, but they are connected by cables, so that the system not only configures Flexible and compact.
5. What is the scan cycle of PLC? What is it mainly affected by?
The scanning process of PLC includes five stages: internal processing, communication service, input processing, program execution, and output processing. The time required for one scan of these five stages is called scanning cycle. The scan cycle is related to the running speed of the CPU, the hardware configuration of the PLC and the length of the user program.
6. What method does the PLC use to execute the user program? What stages does the user program execution process include?
PLC executes the user program by means of cyclic scanning, and the execution process of the user program includes the input sampling stage, the program execution stage and the output refresh stage.
7. What are the advantages of PLC control system compared with relay control system?
(1) In terms of control method: PLC adopts program method to realize control, which is easy to change or increase control requirements, and the contacts of PLC are infinite;
(2) Working mode: PLC adopts serial working mode to improve the anti-interference ability of the system;
(3) In terms of control speed: the contact of PLC is actually a trigger, and the execution time of the instruction is in the microsecond level;
(4) Timing and counting: PLC uses semiconductor integrated circuits as timers, and clock pulses are provided by crystal oscillators, with high delay accuracy and wide range. PLC has counting function that relay system does not have;
(5) In terms of reliability and maintainability: PLC adopts microelectronic technology, which has high reliability. Its self-checking function can detect its own fault in time, and the monitoring function is convenient for debugging and maintenance.
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