Relay control system is composed of devices and wires. It has the advantages of simple structure and low cost. At the same time, due to the simple principle, it is easy for engineers and technicians to grasp. However, the relay control system has a particularly large workload for the design and installation of the entire system for a complex system and sometimes becomes impossible to complete. The physical contact of the mechanical contact is easily damaged; the wiring is also susceptible to vibration and the like, and the reliability is deteriorated.
Because the control function is realized through the connection of the device, when the control effect needs to be changed, the hardware wiring needs to be changed, which is unfavorable to the maintenance and upgrade of the control system.
However, the operation of the relay is a reliable mechanical isolation for the control system, so it is often used in conjunction with other control devices (such as programmable controllers).