The Automation Hierarchy: Where Each System Sits
A layered picture of a plant's automation, from field devices to business systems, and what each layer is allowed to do.
11 min
Activities: 0 of 2
Why a layered picture helps
On most sites you will meet three kinds of control system. A programmable logic controller (PLC) runs a machine or a package. A distributed control system (DCS) runs a large continuous process. A supervisory control and data acquisition (SCADA) system watches and controls assets spread over a wide area, such as wellheads or pump stations along a pipeline.
These systems do not work alone. They pass data up to people and business systems, and they pass commands down to valves and motors. When you understand the layers, you can answer three practical questions quickly: where does this signal come from, who is allowed to change it, and what happens if the link between two layers fails?
The layers, from the field upwards
- 1
Level 0
Field devices
Transmitters, switches, valves, motors and analysers. They measure the process and change it.
- 2
Level 1
Basic control
Controllers and input and output (I/O) modules. PLCs, DCS controllers and remote terminal units (RTUs) execute the control logic here, many times per second.
- 3
Level 2
Supervisory control
Operator stations, the human-machine interface (HMI), the alarm system and the SCADA master station. People see the plant and give commands here.
- 4
Level 3
Site operations
The historian, production reporting, maintenance systems and engineering tools that serve the whole site.
- 5
Level 4
Business systems
Planning, finance and email on the corporate network. These systems need data from the plant but must never control it.
The rule behind the layers
This layered picture is often called the Purdue model, after the university where it was first described. Its central rule is simple: each layer talks mainly to the layer next to it, and the boundary between the plant layers and the business layers is strictly controlled. Time-critical work stays low. A controller at Level 1 must keep the process safe even if every computer above it stops. A historian at Level 3 may stop for a day and the plant keeps running.
Modern systems blur the picture. A smart transmitter at Level 0 carries diagnostics that a maintenance system at Level 3 wants to read. Cloud analytics want data from every level. The model is still useful, because it tells you where to put the controls when data must cross a boundary.
Match each item to its layer
Pair each piece of equipment with the layer where it normally sits.
- Pressure transmitter
- Remote terminal unit at a wellhead
- Operator alarm display
- Process historian
- Corporate finance system
A designer wants to put a pump's high-pressure stop in the SCADA master station because the logic is easier to edit there. What is the main objection?