10. PROGRAMMABLE LOGIC CONTROLLERS (PLCs)
CONTROL - Using artificial means to manipulate the world with a particular goal.
System types,
Continuous - The values to be controlled change smoothly.
e.g. the speed of a car as the gas pedal is pushed
Logical - The values to be controlled are easily described as on-off.
e.g. The car motor is on-off (like basic pneumatics).
Note: All systems are continuous but they can be treated as logical for simplicity.
Logical control types,
Conditional - A control decision is made by looking at current conditions only.
e.g. A car engine may turn on only when the key is in the ignition and the transmission is in park.
Sequential - The controller must keep track of things that change and/or know the time and/or how long since something happened.
e.g. A car with a diesel engine must wait 30 seconds after the glow plug has been active before the engine may start.
Note: We can often turn a sequential problem into a conditional by adding more sensors.
Examples:
mixed (continuous and logical) systems:
A Programmable Logic Controller (PLC) is an input/output processing computer.
Advantages of PLCs are:
- cost effective for complex systems
- flexible (easy to add new timers/counters, etc)
- computational abilities
- easy to add new components
Ladder logic was originally introduced to mimic relay logic.
