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Process Control

PLC · HMI · Sequencing

Control is the part the customer never sees and feels constantly. A well-sequenced machine feels calm. A badly sequenced one feels like it's about to do something you didn't ask for, and operators are never wrong about that feeling.

The loop
Σ CONTROLP · I · D PROCESS setpoint actual measure → compare → correct → measure again, forever

Everything in this discipline is a variation on measure, compare, correct. The complexity isn't the loop — it's that a real machine runs dozens of them at once, they interact, and the interesting failures happen at the seams between them rather than inside any one.

HMI — the operator's whole experience
Live operator screen. Conveyor speed, mode selection, navigation. Someone stands in front of this for eight hours.
01Live operator screen. Conveyor speed, mode selection, navigation. Someone stands in front of this for eight hours.
The same screen in development. Everything easy here is a decision that has to be made now.
02The same screen in development. Everything easy here is a decision that has to be made now.
Commissioning the sequence against the real machine, which has opinions the simulation did not.
03Commissioning the sequence against the real machine, which has opinions the simulation did not.
The process being controlled. Product, motion, and the constraints that decide the whole sequence.
04The process being controlled. Product, motion, and the constraints that decide the whole sequence.
Operator stations

Before there was a touchscreen there were pushbuttons, and the good news is that the discipline never changed. Position, color and labeling are the whole interface. Green means running, red means stopped, and if you ever invert that to save a lamp you will hurt someone.

Hard operator station. Power on, motor on, motor off, selector. Legible from across the aisle and operable wearing gloves.
05Hard operator station. Power on, motor on, motor off, selector. Legible from across the aisle and operable wearing gloves.
Drive keypad alongside the pilot devices — local control for the person standing at the machine rather than at the HMI.
06Drive keypad alongside the pilot devices — local control for the person standing at the machine rather than at the HMI.
Touchscreen station with the hard-wired emergency stop kept separate, where it belongs. Safety does not go through the screen.
07Touchscreen station with the hard-wired emergency stop kept separate, where it belongs. Safety does not go through the screen.
HMI mounted at the machine. Everything the operator needs at the point they need it.
08HMI mounted at the machine. Everything the operator needs at the point they need it.
Process plant

Discrete automation counts parts. Process counts flow, pressure and temperature, and it never stops to let you work on it — which changes what a control system has to be. A palletizer can be stopped and restarted. A process line has a state you have to bring it back through.

Plant interior. Pumps, headers, insulated lines and a control panel bolted to the middle of it.
09Plant interior. Pumps, headers, insulated lines and a control panel bolted to the middle of it.
Control valve with its positioner. This is where the loop stops being arithmetic and becomes a stem moving against pressure.
10Control valve with its positioner. This is where the loop stops being arithmetic and becomes a stem moving against pressure.
Pump set. Two of them, because in process you don't get to shut down to change one.
11Pump set. Two of them, because in process you don't get to shut down to change one.
The reality of the environment — everything crowded, everything hot, and every valve someone's idea of accessible.
12The reality of the environment — everything crowded, everything hot, and every valve someone's idea of accessible.
Skids

Process equipment often ships as a skid: frame, piping, instruments, valves and its own control panel, built and wired in the shop so the site work is four bolts and a conduit run. Everything you can do on a bench is cheaper than doing it on a plant floor.

Stainless frame with the process piping in. Built at bench height, where it can still be reached from all four sides.
13Stainless frame with the process piping in. Built at bench height, where it can still be reached from all four sides.
The skid's own control station mounted to the frame — it arrives on site as one thing that already knows how to run itself.
14The skid's own control station mounted to the frame — it arrives on site as one thing that already knows how to run itself.
That panel's interior, wired out before the frame ever moves.
15That panel's interior, wired out before the frame ever moves.
Crated and ready. Control valve, positioner, panel, and the loose parts bagged and tied down — because the thing that goes missing in transit is always the small one.
16Crated and ready. Control valve, positioner, panel, and the loose parts bagged and tied down — because the thing that goes missing in transit is always the small one.
How I approach an HMI
Sequencing

Most machines that "stop randomly" are sequenced by whoever needed it working that afternoon. A state machine that's explicit about every transition — including the illegal ones — costs a day up front and saves the entire life of the machine.

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