What are the characteristics of programmable controllers?

Some PLC modules can be hot-standby. When one host is working, the other host is also running, but it does not participate in control and is only used as a backup. Once the working host fails, the hot standby can automatically take over its work.

There are also further redundancies, using a three-in-one design, where the CPU, I/O module, and power module are all redundant or part of them are redundant. The three sets work simultaneously, and the final output depends on the result of the majority decision among the three. This can make the probability of system failure almost zero, making it foolproof. Of course, such a system is very expensive and is only used in particularly important occasions, such as the fork control system at a railway station.

(2) Software:

The working mode of the programmable controller is scanning plus interruption, which can ensure that it can work in an orderly manner and avoid the "risky competition" that often occurs in relay control systems. Its control results are always certain; it can also handle emergency controls that need to be handled urgently, ensuring that the programmable controller responds to emergency situations in a timely manner and enables the programmable controller to work reliably.

In order to monitor whether the program of the programmable controller is running normally, the programmable controller system is equipped with a "watchdog" monitoring program. When the user program starts running, first clear the "watchdog" timer and start timing. When the user program runs a cycle, check the timing value of the timer. If it times out (usually not more than 100ms), an alarm will be issued. If it times out seriously, the programmable controller can also stop working. The user can take corresponding emergency measures according to the alarm signal. If the timing value of the timer does not time out, the starting process is repeated, and the programmable controller will work normally. Obviously, with this "watchdog" monitoring program, the normal operation of the user program of the programmable controller can be guaranteed, and the "dead loop" that affects its working reliability can be avoided.

The PLC also has many instructions for preventing and detecting faults, which can generate prompt signals for whether each important module is working properly. By compiling corresponding user programs, the working status of the PLC and the system controlled by the PLC can be monitored to ensure its reliable operation.

Each time the programmable controller is powered on, it will run the self-check program and initialize the system. This is configured by the system program and the user does not need to intervene. When a fault occurs, there will be a corresponding error signal prompt.

It is the strong reliability measures in software and hardware that ensure the reliable operation of programmable controllers. Its mean time between failures can reach tens of thousands of hours; if a failure occurs, the mean time to repair is also very short, just a few hours or even a few minutes.

Someone once did a questionnaire survey on why programmable controllers are used. In the answers, most users cited the reliable operation of programmable controllers as the main reason for choosing them, that is, they regarded the reliable operation of programmable controllers as their preferred indicator.

4. Economical

The use of high-tech will surely bring huge social and economic benefits, which is the embodiment of science and technology being the primary productive force and the source of the vitality of high-tech. The same is true for programmable controllers.

Although the initial investment in using a programmable controller is larger, it is still economical in the long run. This is because:

Although the investment in using a programmable controller is large, it is small in size, occupies little space, requires less investment in auxiliary facilities, saves electricity when in use, has low operating costs, works reliably, has less downtime losses, is easy to maintain, has low maintenance costs, can be reused, and can bring added value, etc., from which greater returns can be obtained. Therefore, in most cases, its benefits are considerable.