Industrial Automation and PLC Control¶
Industrial automation is the control of a physical production process by a programmable controller that reads sensors, decides, and drives actuators without a person in the loop. A programmable logic controller executes its program on a fixed repeating scan, so the machine's behaviour is deterministic and can be reasoned about, tested, and handed to a maintenance technician who did not write it.
How a PLC works¶
The controller runs a scan cycle, and the cycle is the whole design:
- Read inputs. Every field input - photoelectric sensors, proximity switches, limit switches, analog transducers - is sampled at once into an input image table.
- Evaluate the program. The control program runs top to bottom against that frozen snapshot. Inputs do not change mid-evaluation.
- Write outputs. The resulting output image is driven to the valve manifolds, motor starters, and indicators in one pass.
Freezing the inputs is what makes the model tractable. A rung cannot see a sensor change halfway through evaluation, so the logic behaves the same way every scan and a fault is reproducible rather than intermittent.
Ladder logic is the language most lines are written in. Rungs sit between two vertical rails; conditions on the left, an output coil on the right. It is a direct descendant of relay wiring diagrams, which is why it is drawn rather than typed and why an electrician can read it.
State is where students first meet real difficulty. A station that only reacts to inputs is straightforward. A station that must remember - which storage channel holds a part, which parts have been gauged, where the sequence was interrupted - has a model of the world that can diverge from the world. Recovering from that divergence is the manual/automatic/reset problem, and it is the part of the curriculum that transfers most directly to industry.
When to use PLC-based control¶
- The process is sequential and repetitive. Feed, place, gauge, sort - the class of problem PLCs were built for.
- Uptime matters more than unit cost. Field-serviceable terminals and standard hardware beat a cheaper custom board when the line has to run for years.
- Non-authors will maintain it. Ladder logic is readable by maintenance staff; a C firmware image is not.
- Timing must be predictable. A fixed scan gives a bounded worst-case response, which an interrupt-driven general-purpose system does not.
- Safety functions are involved. Interlocks, emergency stops, and lockout/tagout integrate with industrial controllers as designed features rather than as additions.
What PLC control cannot do¶
- Complex computation. Vision processing, model fitting, and floating-point-heavy work belong on a PC or a dedicated controller alongside the PLC, not in ladder logic.
- Fix a bad sensor picture. The controller acts on what it is told. A misaligned sensor produces correct behaviour against a false world state, and nothing in the program can detect it.
- Move without power to the actuators. On a pneumatic line, no compressed air means the sequence advances in software and the machine stands still.
- Substitute for mechanical adjustment. Belt tension, shock absorbers, gripper pressure, and traverse travel are set by hand. No amount of programming compensates for a manipulator that overshoots.
Where automation is taught at MPaCT Lab, Flagstaff, Arizona¶
The MPaCT Lab at Northern Arizona University runs two complementary automation platforms in Flagstaff, Arizona.
| Platform | Controller | What it teaches |
|---|---|---|
| Amatrol 870 Mechatronics Learning System | Allen-Bradley CompactLogix or Siemens S7-300 | Station and line sequencing, sensor adjustment, go/no-go gauging, programmable storage, LOTO |
| Amatrol Smart Robot Workcell | FANUC R-30iB Plus | Teach pendant programming, motion sequencing, cell integration, fault recovery |
The 870 line teaches control of a process by a controller. The workcell teaches control of a manipulator by a controller. Industry needs both, and the handshake between them - a robot and a PLC exchanging I/O to agree that a part is ready - is where cell integration actually lives.
Both platforms are designated educational equipment and are accessed through course enrolment or the Lab Manager, not through the instrument reservation system that covers the characterization tools.
Frequently asked questions¶
What is a programmable logic controller?¶
An industrial computer that reads sensor inputs, evaluates a control program on a fixed repeating scan, and drives outputs to actuators. What distinguishes it from a general-purpose computer is the execution model and the packaging: the program runs to completion on every scan with deterministic timing, the I/O terminates on field-serviceable screw terminals, and the hardware is specified for continuous operation in electrical noise, vibration, and plant temperature swings.
What is the difference between a PLC and a microcontroller?¶
Execution model and serviceability, more than processing power. A microcontroller runs code an engineer wrote and structured freely, and is usually soldered to a custom board. A PLC runs a fixed scan cycle - read all inputs, evaluate the whole program, write all outputs - which makes timing predictable and reasoning about state simpler, and it is wired to field terminals a maintenance technician can trace and replace. A microcontroller is cheaper per unit; a PLC is cheaper to keep running for twenty years.
What is ladder logic?¶
A graphical PLC programming language in which control relationships are drawn as rungs between two vertical power rails. Input conditions sit on the left of each rung and an output coil on the right, so a rung reads as "if these conditions are true, energise this output". It looks the way it does because it replaced physical relay wiring diagrams, which is also why electricians can read it without being taught a programming language.
Why does sensor adjustment matter more than the program?¶
Because a correct program acting on a wrong input produces confidently wrong behaviour. A photoelectric sensor aimed slightly off reports a part present when the fixture is empty; an infrared sensor misaligned at a storage channel reports occupied when it is clear; an analog gauging sensor with its threshold set wrong passes parts that should be rejected. None of these raise a fault. The station runs its sequence perfectly against a false picture of the world, which is why most automation faults are found at the sensor rather than in the code.
Why does my station stop mid-cycle?¶
In order of frequency: a sensor that never reported the condition the sequence is waiting for, low or absent compressed air so a pneumatic actuator never reaches its end position, an interlock or emergency stop that is open, or controller state that no longer matches the machine after a manual intervention. Check the input image in the controller against what is physically true before reading the program. A stopped station is usually waiting correctly for something that did not happen.
Does automation training at NAU use real industrial controllers?¶
Yes. The MPaCT Lab in Flagstaff, Arizona runs an Amatrol 870 Mechatronics Learning System on Allen-Bradley CompactLogix or Siemens S7-300 processors, and a Smart Robot Workcell built around a FANUC LR Mate 200iD/4S on an R-30iB Plus controller. These are production controllers and a production robot, not educational analogues, so the pendant, the programming environment, and the safety configuration are the ones a graduate meets on a plant floor.
Request time on this instrument¶
MPaCT Lab - Building 98E, South Engineering Lab
561 E Pine Knoll Dr, Flagstaff, AZ 86001
Phone: 928-523-2343 · Email: mpct.nano@nau.edu