Control and automation engineering is a specialisation. Here is hardwired relay logic (AND/OR, self-holding, interlocking), PLC structure and cycle, programming languages according to IEC 61131-3 (ladder, FBD, ST) and sensors and actuators — the link between diagram and real machine.
Introduction
Control engineering is the science of making a machine do the right thing at the right time. Historically the logic was built with relays and contactors (hardwired); today a PLC (programmable logic controller) handles the same thing in software. An electrician in the automation specialist field must be able to do both — and understand that the PLC's ladder programming is a direct digital successor to relay logic. The controlling standards are DS/EN 61131-3 (PLC programming), DS/EN 60204-1 (electrical equipment of machines) and DS/EN 60947-5-1 (control circuit devices).
Relay logic: AND, OR and the two basic circuits
All logic can be built from three elements: AND, OR and NOT. In a control circuit they correspond to the placement of contacts:
- 01AND (AND): contacts in SERIES — all must be closed before current reaches the coil. E.g. 'run only if hatch is closed AND emergency stop is pulled out'.
- 02OR (OR): contacts in PARALLEL — one of them is enough. E.g. 'start from pressing A OR pressing B'.
- 03NOT (NOT): a breaking contact (NC) — closed at rest, opens when activated. For example, a stop button or a thermal relay contact.
Self-hold — the most important base circuit
A push button provides contact only while held down. For a motor to continue running after you release the start button, self-holding is used: an auxiliary contact from the contactor itself is placed PARALLEL with the start button. Once the coil has pulled in, its own contact keeps it energised.
Interlocking — when two things must not happen at the same time
Interlocking prevents a dangerous or destructive combination. The classic example is a motor that can run both ways: the two contactors (forward and back) must NEVER pull simultaneously as that would short-circuit two phases. So each contactor's break contact is placed in the other's coil circuit — when 'forward' pulls, 'back' is blocked, and vice versa. Interlocking is also used between mains and emergency power, between doors and machines, and everywhere two states exclude each other.
PLC structure
A PLC is an industrial computer built for control. It consists of a CPU (calculates the logic), inputs and outputs (the interface to the machine) and a power supply. Inputs (DI/AI) read sensors and buttons; outputs (DO/AO) control contactors, valves, lights and motors.
- 01Digital inputs (DI): on/off signals — button, limit switch, proximity sensor.
- 02Digital outputs (DO): on/off control — relay, contactor coil, signal lamp.
- 03Analogue inputs/outputs (AI/AO): measure or control continuous values — temperature, pressure, speed (e.g. 4–20 mA or 0–10 V).
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