Basic Principles and Applications of Constant Temperature Humidity Box Programmable Controllers Basic characteristics From the discussion of the working principle of PLC, PLC inputs and outputs are physically separated from each other, and the connection between them is by running programs stored in its memory. achieve. Its relevance to entry and exit does not depend on physical processes, not on lines; it depends on the information process and uses soft logic. Its work is based on good information.
Information is different from matter and energy and has its own laws. Information is easy to handle, easy to pass, and easy to store; information can be reused, and so on. It is because of these characteristics of information that determines the basic characteristics of PLC.
Here are four features of PLC:
2.1 rich features
PLC's features are very rich. This is mainly related to its rich instruction system for processing information and internal devices for storing information.
Its instructions are as many as tens or hundreds, and can handle a wide range of logic problems, and can also perform various types of data operations. It can also be done by ordinary computers.
Its internal devices, that is, the data storage area in memory, are numerous and have huge capacity. I/O relays can be used to store incoming and outgoing information, which can be as low as several tens, hundreds, or many thousands, tens, or even tens of thousands. This means that it can carry out so many I/O point in and out information transformations and carry out such a large-scale control.
Its internal relays are equivalent to intermediate relays. One bit in memory can be used as an intermediate relay.
Its counters and timers are also many, which is beyond the reach of relay circuits. A small box or module can have hundreds or thousands of internal timers and counters. This is also because as long as a word in memory, plus some flags, can become timers, counters, so it is so much.
In addition, these internal devices can also be set to hold power, or lose power, and remain unchanged, that is, they are cleared after power-on. To meet different use requirements. These are also difficult relays.
Its data storage area can also be used to store large amounts of data. Information of several hundred, several thousand, or tens of thousands of words can be stored, and it is not lost after a power failure.
PLC also has a wealth of external equipment to establish a friendly human-machine interface for information exchange. Can be fed into the program, fed into the data, read out the program, read out the data. And reading and writing can be done on pictures and pictures. After the data is read out, it can be dumped and printed. Data can be typed, cards can be read, and so on.
The PLC also has a communication interface that can be linked or networked with a computer and exchange information with a computer. It can also be networked on its own to form a larger, more geographically-wide control system than a single machine can.
PLC also has a powerful self-test function that allows self-diagnosis. The result can be recorded automatically. This adds transparency and convenience to its maintenance.
The rich functionality provides possibilities for the widespread use of PLCs; at the same time, it also creates conditions for the automation, remote control and intelligent control of industrial systems.
Such as the PLC is a collection of rich functions in one, is not the other electrical controller; is the traditional relay control circuit can not be compared.
2.2 Easy to use It is very convenient to use PLC to control the system. This is because: First, the establishment of the PLC control logic is a program that replaces the hardware wiring with a program. Compiling the program is more convenient than wiring, changing the program than changing the wiring, of course, much more convenient!
Second, the PLC's hardware is highly integrated and has been integrated into various miniaturized modules. Moreover, these modules are supporting and have been serialized and standardized. All kinds of control system required modules, PLC manufacturers have more than stock, can be purchased on the market. Therefore, the hardware system configuration and construction are also very convenient.
For this reason, this word "can" is used for a programmable controller. For software, its program can be edited and it is not difficult to edit. For hardware, its configuration is variable and it is also easy to change.
Specifically speaking, PLC is convenient in five aspects:
(1) Convenient configuration: It can be connected to the control system needs to determine which PLC to use, which type, what module, how many modules, after the confirmation, to the market order purchase.
(2) Easy installation: PLC hardware is simple to install and easy to assemble. The external wiring is provided with a connector, and the wiring is simple, and once it is connected, when the module is replaced, the connector can be installed on the new module, and no wiring is required. Do not connect any internal lines, just do some necessary DIP switch settings or software settings, and prepare a good user program to work.
(3) Convenient programming: Although there are no actual relays, time relays, and counters inside the PLC, it passes through the program (software) and the system memory, but these devices actually exist. The large number of them is unimaginable in a relay control system. Even with a small PLC, the number of internal relays can be thousands, and time relays and counts are hundreds. Moreover, the contacts of these relays can be used indefinitely. There are as many internal logic devices in the PLC as there are no restrictions on users. The only consideration is the entry point. And this internal entry point is used more often than not. Large-scale PLC control points can reach more than 10,000 points, how can there be such a big reality system? If it is not enough, it can be networked and controlled without any restrictions. PLC's instruction system is also very rich, and it is not difficult to implement all kinds of switching and analog control. The PLC also has a memory area for storing data, which can store all the information to be saved in the control process. In short, due to the strong functions of the PLC, its role in the control system is exerted, and the restrictions imposed on it are not the PLC itself but the imagination of the people, or other hardware facilities supporting them.
PLC peripherals are very rich, there are many types of programmers, and they are more convenient to use. There are also data monitors that can monitor the work of the PLC. There are also many softwares that use PLCs. Not only can they use ladder diagrams similar to relay circuit designs, but they can also use BASIC and C languages, as well as natural languages. These also provide convenience for PLC programming.
The PLC program is also easy to store, transplant and reuse. After the program of the PLC for a certain type of product is perfected, any such product can be used. Produce one, copy one. This is much more convenient and simpler than wiring and commissioning of relay circuit station equipment.
(4) Easy maintenance: This is because:
1 PLC work is reliable, there are not many cases of failure, which greatly reduces the workload of maintenance. This will be further described in the third feature of PLC.
2 Even if the PLC fails, maintenance is very convenient. This is because PLCs are equipped with a lot of fault prompting signals, such as the lack of battery voltage for the PLC to support memory to maintain data, and accordingly there is a low voltage signal indication. Moreover, the PLC itself can also record fault conditions. Therefore, the PLC failed and it is easy to diagnose. At the same time, troubleshooting after troubleshooting is also simple. According to the module, the spare parts market of the module can be purchased and simple replacement can be performed. As for the software, there will be no failure after debugging, and more adjustments will be made based on the use of experience to make it perfect.
(5) to facilitate the use of: PLC for a device, if the device is no longer used, its use of the PLC can also be used for other devices, as long as the adaptation of the program, you can do it. If the original equipment is different from the new equipment, some of its modules can be reused.
2.3 Reliable operation The control of the system using the PLC is very reliable. This is because PLC has taken many measures in both hardware and software to ensure that it can work reliably. In fact, if the PLC is not reliable, it cannot be used in an industrial environment and it cannot be a PLC.
(1) In terms of hardware:
The input and output circuits of the PLC are electrically isolated from the internal CPU. Its information is transmitted by optocoupler devices or electromagnetic devices. Moreover, the CPU board also has shielding measures against electromagnetic interference. It can ensure that the PLC program can operate normally without external electrical and magnetic interference.
The components used by the PLC are mostly non-contact, highly integrated, and not too numerous in quantity, which also provides a material basis for their reliable operation.
In the mechanical structure design and manufacturing process, in order to enable the PLC to work safely and reliably, many measures have also been taken to ensure that the PLC is resistant to vibration and impact. The ambient temperature can be as high as 50 degrees Celsius, and some PLCs can be as high as 80--90 degrees.
Some PLC modules can be hot-backed, one host working, and the other host running, but do not participate in control, only for backup. In the event of a failure of the work host, Hot Standby can automatically take over its work.
There are even more redundant, three-in-one design, CPU, I / O modules, power modules are redundant or part of the redundancy. Three sets work at the same time, and the final output depends on the outcome of the majority of the three decisions. This allows the system to have nearly zero chance of failure and is completely safe. Of course, the cost of such a system is very high, and it is only used for particularly important occasions, such as the railroad fork control system.
(2) Software:
The working mode of the PLC is scanning and interrupting, which can ensure that it can work in an orderly manner, avoiding the "risk competition" that often occurs in the relay control system, and the control result is always determined; and the emergency control can be handled urgently. It ensures the PLC's timely response to emergency situations and enables the PLC to work reliably.
In order to monitor whether the PLC running program is normal, the PLC system has set up the "Watchdog" monitoring program. When starting the user program, clear the "watchdog" timer and start timing. When the user program finishes running in a loop, check the timer's timer value. If the timeout (usually does not exceed 100ms), the alarm. A serious timeout can also make the PLC stop working. The user can take corresponding emergency measures according to the alarm signal. If the timer value does not time out, repeat the initial process and the PLC will work normally. Obviously, with this "watchdog" monitoring program, the normal operation of the PLC user program can be guaranteed, and the "endless loop" can be avoided to affect the reliability of its work.
PLC also has a lot of instructions to prevent and detect faults, so as to generate prompt signals for normal operation of important modules. The corresponding user program can be compiled to monitor the working status of the PLC and the system controlled by the PLC to ensure its reliable operation.
Every time the PLC is powered on, it also runs a self-test program and initializes the system. This is the system program configuration, users can not interfere. There is a corresponding error signal when there is a fault.
It is PLC that has powerful reliability measures in terms of software and hardware, and it ensures that the PLC has reliable working characteristics. Its average time between failures can reach more than tens of thousands of hours; the average time to repair the failure is also very short, a few hours or even minutes.
There have been questions about why you would use a PLC survey. In the answer, most users regard the reliability of the PLC as the main reason for choosing it, that is, the PLC can work reliably as its preferred indicator.
2.4 Economical Use of High-tech The use of high-tech will inevitably bring about huge social and economic benefits. This is a manifestation of science and technology as the primary productive force, and is also where the high-tech vitality lies. The same is true of PLC.
Although the first investment in the use of PLC is larger, the use of PLC is still economical in the overall and long-term perspective. This is because:
Although the investment in using the PLC is large, it is small in size, takes up less space, requires less auxiliary facilities, saves electricity during operation, and has low operating costs. It also has reliable work and less downtime for maintenance; it has simple maintenance and low maintenance costs; Use and bring additional value, etc., can get greater returns. So, in most cases, its benefits are substantial references http://
