The first automatic control system was developed to govern an industrialized process was performed based on existing items so far. These elements are called relays, which are electromagnetic devices, which are the precursors of technology based automation philosophy called "dedicated or standard automation."
Before use as control elements, relays were used only as a high power handling mechanisms especially in the field of telecommunications, but even a long time and still today, the relays are used both in machines and equipment in general as elements of control and regulation.
Relays are electromagnetic components that perform switching in its mechanical parts, and also low energy control. The relays are mainly used for processing control signals involved in the operation logic of a process.
The way to run a relay is to connect a voltage between the ends of the coil, which generates an electrical current flowing through the coil, creating a magnetic field the fluid which in turn causes the displacement of a metal plate to the core that has adhered to the coil. The metal plate on the other hand, is equipped with mechanical contacts that can be opened or closed by moving the plate, the state can acquire the contacts, either open or closed, represents the logic state that has the relay at that time, remaining this state while the voltage on the coil is applied. By interrupting the voltage of the coil, the metal plate back to its normal position by the action of a return spring, as shown in Figure 1.5.
Figure 1.5 Schematic of a relay
According to the complexity of the specific function monitoring is required depends on the number of relays to be used, to thus maintain the security conditions required by the operation of the control logic. Another important factor in determining the number of relays to use is the number of contacts that have relays, as implicitly represent the logic functions that must be taken.
The various control functions embodied by the operation of the relays, interlock with each other to be integrated so the entire logical system of automatic control.
All logic functions have to meet the relays are connected together by wires, which intercommunicate with all relays involved. The various relays are housed within a set of modular racks, and on those where it does all the wiring to interconnect the relays together. These connections are composed of a single strand wire topped with pads on both ends.
Figure 1.Racks with 6 relays
To somehow facilitate the relays can be unplugged and replaced when required, these are installed on foundations in this way to facilitate their exchange.
That automatic control system based on relays, must be ordered and housed in closed rooms where also all the documentation must exist for the connection of contacts, and location of the coils (the latter is a situation that is not always ideal met).
Although the current era is dominated by electronics, the relays are still very important in the market for various reasons, such as:
Easy adaptation to different working voltages.
Thermal insensitivity towards the environment, because the relays work reliably at temperatures ranging from -40 º C to 80 ° C.
Relatively high resistance between the contacts of work when they are disconnected.
Possibility to activate several independent circuits to each other.
Presence of a galvanic isolation between the control circuit and main circuit.
