Control engineering has evolved over time. In the past humans were the main method for controlling a system. More recently electricity has been used for control and early electrical control was based on relays.
These relays allow power to be switched on and off without a mechanical switch. It is common to use relays to make simple logical control decisions. The development of low cost computer has brought the most recent revolution, the Programmable Logic Controller (PLC).
The advent of the PLC began in the 1970s, and has become the most common choice for manufacturing controls.
PLCs have been gaining popularity on the factory floor and will probably remain predominant for some time to come.
Most of this is because of the advantages they offer.
- Cost effective for controlling complex systems.
- Flexible and can be reapplied to control other systems quickly and easily.
- Computational abilities allow more sophisticated control.
- Trouble shooting aids make programming easier and reduce downtime.
- Reliable components make these likely to operate for years before failure.
Ladder logic is the main programming method used for PLCs. As mentioned before, ladder logic has been developed to mimic relay logic. The decision to use the relay logic diagrams was a strategic one.
By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and tradespeople was greatly reduced. Modern control systems still include relays, but these are rarely used for logic.
A relay is a simple device that uses a magnetic field to control a switch, as pictured in below Figure. When a voltage is applied to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch.
The contact that closes when the coil is energized is called normally open. The normally closed contacts touch when the input coil is not energized. Relays are normally drawn in schematic form using a circle to represent the input coil.
The output contacts are shown with two parallel lines. Normally open contacts are shown as two lines, and will be open (non-conducting) when the input is not energized. Normally closed contacts are shown with two lines with a diagonal line through them.
When the input coil is not energized the normally closed contacts will be closed (conducting).
Relays are used to let one power source close a switch for another (often high current) power source, while keeping them isolated.
An example of a relay in a simple control application is shown in below Figure.
In this system the first relay on the left is used as normally closed, and will allow current to flow until a voltage is applied to the input A.
The second relay is normally open and will not allow current to flow until a voltage is applied to the input B. If current is flowing through the first two relays then current will flow through the coil in the third relay, and close the switch for output C.
This circuit would normally be drawn in the ladder logic form. This can be read logically as C will be on if A is off and B is on.
The example in above Figure does not show the entire control system, but only the logic. When we consider a PLC there are inputs, outputs, and the logic. Above Figure shows a more complete representation of the PLC.
Here there are two inputs from push buttons. We can imagine the inputs as activating 24V DC relay coils in the PLC. This in turn drives an output relay that switches 115V AC, that will turn on a light.
Note, in actual PLCs inputs are never relays, but outputs are often relays. The ladder logic in the PLC is actually a computer program that the user can enter and change.
Notice that both of the input push buttons are normally open, but the ladder logic inside the PLC has one normally open contact, and one normally closed contact.
Do not think that the ladder logic in the PLC needs to match the inputs or outputs. Many beginners will get caught trying to make the ladder logic match the input types.