What is industrial automation? Levels from sensor to MES, networks, production types and the system life cycle

Industrial automation is the use of control systems to run machines and production processes with little human intervention: sensors measure, controllers decide, actuators act, and operators supervise from screens. In a plant it is built in levels, from field devices and controllers up to SCADA, MES and ERP, joined by industrial networks. This page explains each level, the networks, the four kinds of production, who does what on a project and the life of a control system, and points to the article that goes deeper on each topic.

What industrial automation is

Wikipedia describes automation as a range of technologies that reduce human intervention in processes by deciding in advance the criteria, relationships and actions, and building those decisions into machines. It is achieved with mechanical, hydraulic, pneumatic, electrical and electronic devices and computers, usually in combination, and ranges from a household thermostat to an industrial control system with tens of thousands of measurements and control outputs. The word itself was not widely used before 1947, when Ford set up an automation department.

The basic building block is the control loop. A controller compares a measured value with the desired value (the setpoint) and changes an input to the process so that it stays at the setpoint despite disturbances. NIST SP 800-82r3 (section 2) uses the broader term operational technology (OT) for programmable systems that interact with the physical world, industrial control systems among them, and names three ways such a system can run: open loop on fixed settings, closed loop where the output feeds back to the input, or manual mode under full human control.

An industrial control system can be a few panel-mounted controllers or a distributed control system with many thousands of field connections. In every case it takes data from sensors measuring process variables, compares it with setpoints and drives final control elements such as control valves.

The levels, from sensor to ERP

Plant architecture drawings commonly use the levels of the Purdue model, developed in the 1990s by Theodore J. Williams and the Industry-Purdue University Consortium. NIST (appendix D.1.5.1) notes that ISA-95 grew from this architecture, first published by ISA in 1992. Wikipedia lists the levels as follows:

Documents do not all draw the line in the same place. The ISA-95 summary in OPC Foundation's OPC 10030 puts PLCs and DCSs at level 2 with sub-second responses and lists SCADA and historians among the level 3 systems, while Wikipedia's article on process control puts I/O modules and controllers at level 1 and field devices at level 0. What matters on a project is the job a system does. The ISA-95 article covers the level 3 to level 4 boundary, and the IEC 62443 article shows how the same levels are grouped into security zones.

What each layer does:

• Field devices. Sensors measure flow, pressure, temperature and level; final control elements such as valves, motors and heaters act on the process. Signals reach the I/O as analog current loops (4–20 mA), on/off contacts, or digital buses that also carry diagnostics.
• Controllers. A PLC is an industrial computer hardened for plant conditions. PLCs were first developed for the car industry to replace hard-wired relay logic; the first, the Modicon 084, came out of a 1968 request from General Motors. NIST lists what PLCs do: I/O control, logic, timing, counting, PID control, communications, arithmetic and data processing. Their languages are standardised in IEC 61131-3, Programmable controllers – Part 3: Programming languages (current edition 2025). A DCS spreads control over several controllers under one supervisory system (what is a DCS). The safety instrumented system (SIS) brings the process to a safe state when set limits are exceeded and is often kept independent of the control system (what is an ICSS).
• HMI, SCADA and historian. SCADA software works at the supervisory level: the PLC runs the control loop, and the operator changes setpoints and sees alarms on the SCADA screens. A historian is a time-series database for process data. See what is SCADA, what is an HMI, what is a process historian, and for screens and alarms ISA-101 and ISA-18.2.
• MES. A manufacturing execution system tracks and documents how raw materials become finished goods and keeps the as-built record; its functions include order dispatch, downtime and OEE, quality and track and trace. See what is MES and what is OEE.
• ERP manages the business processes, with finance at its core; at level 4 it sets the plant production schedule, material use, shipping and inventory.

Industrial networks in one section

Field devices used to be wired one by one to the controller with 4–20 mA or on/off signals. A fieldbus, standardised in IEC 61158 and IEC 61784, replaces those point-to-point connections with one digital network and cuts the length and number of cables, Wikipedia notes; since 2000 several fieldbuses based on real-time Ethernet have been established and may replace the older buses in the long term. Common examples in plants:

• EtherNet/IP carries the Common Industrial Protocol (CIP) over standard, unmodified Ethernet with TCP/IP and UDP, and is managed by ODVA. ODVA lists star, linear and Device Level Ring (DLR) topologies. Ring and parallel redundancy are compared in DLR or PRP.
• Modbus is a messaging structure developed by Modicon in 1979, which the Modbus Organization calls a common denominator between manufacturers; Modbus TCP runs it over TCP/IP on port 502.
• PROFINET is the industrial Ethernet standard of PROFIBUS & PROFINET International; it defines the data exchange between IO-Controllers (typically a PLC, DCS or industrial PC) and IO-Devices such as I/O blocks, drives and sensors.
• OPC UA, released in 2008, is a platform-independent, service-oriented architecture that runs from embedded controllers to the cloud, with certificate-based authentication, signing and encryption, and an information model. Moving off older OPC DA links is covered in OPC DA and DCOM errors.

Which ports to open between levels is listed in firewall ports for FactoryTalk and Logix.

Discrete, process, batch and hybrid automation

The kind of production decides what the control system must do well.

• Discrete: event-driven, sequential control of machines and production lines. The PLC grew out of the need to replace relay and timer racks for this kind of control, first on car production lines; Wikipedia also names printing presses, and NIST names automobile assembly lines as a typical PLC application.
• Continuous process: variables that change smoothly over time, held at setpoint by feedback loops such as PID. Wikipedia names fuels, chemicals and plastics, made in very large quantities. The DCS was shaped around large continuous processes.
• Batch: set quantities of raw materials combined in a set way for a set time, as in adhesives, food, beverages and medicines. ANSI/ISA-88, approved in 1995 and adopted by the IEC as IEC 61512-1 in 1997, gives the models and terms for batch control; see the ISA-88 article.
• Hybrid: Wikipedia uses the word for applications with both batch and continuous elements. Vendors use the same three-way split: Rockwell Automation's 2025 annual report groups its markets into discrete, hybrid and process, with food and beverage, life sciences and tire as examples of hybrid markets, and ODVA lists discrete, hybrid and process applications for EtherNet/IP.

Who does what on a project

These are the roles you meet on a control system project. One company can hold more than one, so write the scope task by task, whatever the type of company.

The responsibilities of each party for security are set out in the IEC 62443 article.

The life of a control system

1. Specification. The owner's requirements; on a new plant the EPC contract may follow a FEED. For an existing plant, start from what is installed: the site survey checklist lists what to prepare.
2. Design. Control philosophy, I/O lists, network and security architecture, panels and software. Related: control panel engineering, DLR or PRP, what is a DCS.
3. Factory acceptance test (FAT). Testing at the supplier's site before installation: completeness, verification against the contract, proof of function by simulation or function test, and a final inspection. IEC 62381:2024, Automation systems in the process industry – Factory acceptance test (FAT), site acceptance test (SAT), and site integration test (SIT), gives requirements and checklists for these tests. Related: ControlLogix redundancy pre-FAT checklist.
4. Site acceptance test (SAT) and commissioning. After the FAT, the users may run a site acceptance test at the plant. Commissioning checks that every component, from single instruments up to whole systems, is installed and works to the owner's requirements before handover, and applies to expansions and revamps as well as new plants. Related: SCADA server redundancy, which includes a switchover test list.
5. Operation and maintenance. Program backups and version history, preventive checks, fault finding and security. Related: AssetCentre automatic backup, PLC and HMI backup and version control, the maintenance checklist, Logix major fault codes, PLC security hardening, vibration monitoring.
6. Obsolescence and migration. NIST puts the lifetime of OT components often at 10 to 15 years and sometimes longer, against three to five years for typical IT components. Rockwell publishes a lifecycle status for each product: Active, Active Mature (still supported, but a newer product exists), End of Life (a discontinuation date is announced, time for migration and last-time buys) and Discontinued (no longer made, though repair or exchange may still be offered). Related: obsolescence and lifecycle review, end-of-life lookup, legacy Allen-Bradley upgrade paths, PLC-5 to ControlLogix, legacy HMI/SCADA migration.

A migration starts a new specification, so the cycle begins again.

Where to start