7. 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.