22-02-2013, 10:49 AM
Programmable Logic Controllers
Programmable Logic.ppt (Size: 1.32 MB / Downloads: 34)
Industrial Automation
Common knowledge: computers are used in factories...
Robotic arm, CNC, injection molding
Not common knowledge: today this is usually accomplished with Programmable Logic Controllers (PLCs)
PLCs are the answer to a variety of needs: durability, reliability, flexibility, scalability, reprogrammability, etc..
Why should you care? Because you will run into PLCs...
Did you know? <Insert shocking Buckley statistic here>
Median starting salary for entry-level “Electrical Controls Engineer” is $57,452. (EE is $55K, HW Eng is $48K, SW Eng is $53K) [monster.com]
As long as there is industry, it will be computer controlled and engineers will earn paychecks.
Relay Logic
Conditional logic can be represented in terms of contacts and coils.
Contact: A simple input switch.
Coil: An output load, e.g., a relay or motor.
Symbolic representation called ladder logic.
To clarify: “Ladder Logic” is a notation originally used to describe/document relay logic configurations.
Later became the basis for PLC programming languages
(This parallels HW Desc. Langs. (HDLs)... VHDL was intended by DoD to document ASICs... Learn more in CSE 341 and especially CSE 490)
Ladder Logic
Power supply rails drawn as parallel vertical lines on left and right
Connection of rails implies current will flow
An output is “on” when a connection is completed and current flows through the load’s coil
PLC
The first PLC was invented by Dick Morely in 1978.
Morely designed a computer with three components: a processor, memory, and a logic solver.
“[The logic solver] allowed us to get the speed we needed in this application-specific computer to solve the perceptually simple problem of several cabinets full of relay wiring.” -Morely
Not very interesting… how about a “stay-on” variation? (When the switch is released, the light stays on)
PLC benefit: The state of an “output” in one rung may be used as a “contact” in another.
In fact, there are “internal utility relays” – virtual outputs that act as intermediate steps toward real outputs.