The development of low cost computer has brought the Programmable Logic Controller (PLC). PLC´s have gained popularity on the factory floor and industrial control process, 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.
- Programming is easier and reduce downtime.
- Reliable components make these likely to operate for years before failure.
There are different types of control such as continuous and discrete controls.
In the continuous control systems the values to be controlled change smoothly and they can be linear or non linear. Linear means that can be described with a simple differential equation (e.g. PID), while in the non-linear systems the mathematics become much more complex (e.g. MRAC or FUZZY LOGIC).
In the discrete control systems, they could be conditional or/and sequential. Conditional or logical means that the value to be controlled are described, with BOOLEAN and EXPERT SYSTEMS, as on-off. The sequential control is a logical control that will keep track of time and previous events, event and temporal based using TIMERS and COUNTERS.
Logical and sequential control is preferred for system design. These systems are more stable, and often lower cost. Most continuous systems can be controlled logically.
When a process is controlled by a PLC it uses inputs from sensors to make decisions and update outputs to drive actuators. That means that the process will change over time because the actuators will drive the system to new states or modes of operation.
The control loop is a continuous cycle of the PLC reading inputs, solving the ladder logic, and then changing the outputs. Like any other computer this does not happen instantly. When power is turned on initially the PLC does a quick sanity check to ensure that the hardware is working properly. If there is a problem the PLC will halt and indicate there is an error. If the PLC passes the sanity check it will then scan all the inputs. After the inputs are stored in memory, the ladder logic will be solved using the stored values (not the current values). When the ladder logic is complete the outputs will be scanned and changed. After this the system goes back to do the sanity check and the loop continues indefinitely.
Many PLC configurations are available such as rack, mini, micro, but the most essential components of all them are:
Power Supply: this can be built into the PLC or can be an external unit. Common voltages required by the PLC are 24 Vdc and 220 Vac.
CPU: Central Processing Unit, this is the brain where ladder logic is stored and processed.
I/O Units: These are the input/output cards provided so that the PLC can monitor the process and initiate actions.
Input cards can be digital or analog, and common input sensors can be proximity switches (using inductance, capacitance, light or mechanical mechanisms), temperature sensors PTC´s, potentiometers for measuring angular positions...
Output cards have typically 8 to 16 outputs and these outputs can be relays, transistors or triacs. Relays are the most flexible but they are slower and they will wear out after millions of cycles, relay outputs are often called dry contacts. Transistors are limited to DC outputs, and Triacs are limited to AC outputs. Popular actuators are solenoid valves, lights, motor starters, servomotors...
Indicator lights: These indicate the status of the PLC including power on, program running and a fault.
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