The safety control system can provide safety protection for the equipment during the start-up, shutdown, process disturbance and normal maintenance operations of the production equipment. Once the equipment is in a dangerous situation, the safety control system can immediately respond and output the correct signal to put the equipment in a safe state or shut down. In some industries, the safety control system is generally referred to as ESD (Emergency Shutdown System) or SIS (Safety Instrument System). Strictly speaking, ESD refers to the logic operator in SIS, that is, the control system hardware and corresponding software, while SIS also includes peripheral instrument sensors and final actuators.
The development process of safety control system mainly depends on the development process of logic control unit. The development of logic control unit has also gone through a process from simple to complex, from low level to high level, just like the domainization of human beings. From simple relays to solid-state circuit logic systems, to safety control systems with microprocessors as the core.
The advent of relays in 1863 led to the formation of the world's first safety control system with relays as the core component. This safety control system lasted for more than 100 years until the American Digital Equipment Corporation developed the world's first PDP-14 PLC in 1969, ushering in a new era of using programming methods for industrial control. In 1975, Honeywell first launched the first generation of distributed control systems (DCS), namely the TDC-2000 system, based on the problems of PLC for discrete control.
Since then, the development direction of safety control systems has been divided into two lines: the development process of PLC control systems; the development process of DCS control systems.
According to the capacity, I/O points and scanning speed of PLC, the development process of PLC is mainly divided into three stages.
Stage 1: PLC capacity is small, I/O points are less than 120 points, and the scanning speed is 20 to 50 ms/kB. The PLC safety control system at this stage only has some simple logic operations, timing, and counting functions.
Stage 2: The PLC capacity has been expanded, the I/O points have reached 512 to 1,024 points, and the scanning speed is 5 to 6 ms/kB. In addition to the functions of the PLC control system in the first stage, the PLC control system at this time has also added arithmetic operation instructions, comparison instructions, analog quantity control, and ladder diagram programming language.
Stage 3: With the continuous expansion of the scale of integrated circuits, the PLC safety control system composed of 16-bit and 32-bit microprocessors has been further developed. At this time, the PLC capacity is very large, and the I/O points of large PLC control systems have reached 4,000 to 8,000 points, with a scanning speed of 0.47ms/kB. On the basis of the second stage, arithmetic floating-point operation instructions, PLC adjustment function instructions, graphic configuration function instructions, network, communication instructions, and sequential function language are added.
