What is closed-loop control in LPBF, and how does it differ from monitoring?

Monitoring is measuring and recording: the machine takes process data and saves it for later review. Closed-loop control is measuring, comparing to a reference, and acting, correcting a parameter while manufacturing continues. The difference isn't one of degree: in the first case, the defect is already present in the part when you notice it; in the second, it may never occur.

Almost everything the industry sells as "process control" is the first.

The three levels, in order

Level	What it does	What it gives you	What it doesn't give you
Logging	Saves process variables layer by layer	Traceability and evidence for the client	No reaction
Monitoring	Also alerts when a variable goes out of range	You find out during build	The part is already being manufactured incorrectly
Closed Loop	Compares to a setpoint and corrects the parameter	The process returns to the point without stopping	Only acts on what it can correct

All three are useful. Recording is mandatory if your client requires evidence. Monitoring allows you to abort and avoid wasting another forty hours. But only the third changes the outcome of the part inside the machine.

What can be measured during manufacturing

An LPBF build leaves traces in several ways, and not all of them serve the same purpose:

Atmosphere. Residual oxygen and pressure in the chamber. It is measured and corrected effectively: purge more, increase the flow rate.
Gas Flow. The flow rate that sweeps away fumes and splashes, and the filter's differential pressure, which indicates how clogged it is. Correctable by increasing the power of the recirculation pump.
Actual Laser Power. The output, not the requested power. Corrected by adjusting the setpoint.
The Fusion Pool. Its size and emission, captured by a sensor coaxial to the beam, at thousands of samples per second. It is the richest signal and the most difficult to interpret.
The Freshly Spread Layer. One image per layer detects powder deficiency, coating drag, and part displacement.

Why closed-loop fusion baths are difficult

Here is the true boundary of the sector, and it's best not to embellish it.

It's incredibly fast. The beam travels meters per second. To correct within the same pass, the chain of measuring, deciding, and acting has to be completed in microseconds. That requires dedicated electronics, not software running on a PC.

The signal doesn't mean just one thing. A brighter bath could indicate excessive power, an area with less material under the beam that dissipates less effectively, or an overhang. Correcting without identifying the cause worsens the part.

You have to know what's right. A closed loop needs a set of instructions. And those instructions depend on the material, the layer thickness, and the specific geometry. Without a process foundation built on testing, there's nothing to compare it to.

That's why the useful question to ask a manufacturer isn't "Do you have closed-loop control?", to which almost everyone answers yes. It's this: "On what variables, with what reaction time, and what exactly does it correct?"

What to ask, specifically

What variables are recorded and how frequently?
Which ones trigger an alert, and which ones does the machine take action on its own?
If it takes action, what does it modify: power, speed, gas flow, or all of them?
How long does it take from when a variable deviates until it is corrected?
Can the historical data be exported, or is it stored within the machine?
Can the setpoint be defined for custom materials, or only for those in the catalog?

The sixth one provides the most information about the rest. If you can't set the reference for your material, the closed loop they're selling you only works with theirs.

What is resolved today

There are closed loops that have been working for years and that no one boasts about because they don't sound as impressive: regulating the inert atmosphere, controlling the gas flow to prevent filter clogging, and stabilizing laser power against thermal drift. They aren't flashy. They're what prevent most actual failures.

The rest — layer-by-layer geometric correction starting from the fusion bath — is what the sector is heading towards, and where it is wise to be wary of anyone who takes it for granted.

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If you want the full context of how the machine works underneath, it's in what is a metal 3D printer. What needs to be checked periodically for the instructions to be meaningful is in how to calibrate the laser and its optics. And our specific position on this, with what we do and what we don't yet, is in closed-loop process control.

And if you want to see what each piece of equipment comes with as standard and what is optional, it's in the specifications of ALBA 300 and ALBA 500.