Sizing a PLC
A PLC is specified by its input and output count and type, its processing and memory capability, and the communication protocols it must speak. Count the I/O the application needs, then add spare capacity — expansion is normal, and a controller with no free I/O forces a hardware change for a small process modification.
- Digital inputs and outputs for switches, sensors, valves and contactors
- Analogue inputs and outputs for transmitters, control valves and drive references
- Specialist modules for thermocouples, RTDs, high-speed counting and motion
- Communication protocols must match the drives, instruments and any supervisory system already on site
Signal standards
The 4–20 mA current loop remains the industrial standard for analogue signals, and for a good reason: current is unaffected by voltage drop over long cable runs, and the live zero at 4 mA distinguishes a genuine zero reading from a broken wire. Voltage signals are simpler but are susceptible to drop and interference over distance.
| Signal | Characteristic | Typical use |
|---|---|---|
| 4–20 mA | Immune to voltage drop; live zero detects wire break | Process transmitters over long runs |
| 0–10 V | Simple, but degrades over distance | Short in-panel connections, some drive references |
| Digital 24 V DC | The industrial standard for discrete signalling | Switches, proximity sensors, valve feedback |
| Fieldbus and industrial Ethernet | Multiple devices on one cable, with diagnostics | Distributed I/O, drives, instruments |
Sensors and detection
Sensor choice follows the target material, the sensing distance, the environment and the required switching speed.
- Inductive proximity sensors detect metal only, at short range, and are extremely robust
- Capacitive sensors detect non-metallic materials and suit level detection through a vessel wall
- Photoelectric sensors cover longer ranges; through-beam is the most reliable, diffuse the simplest to install
- Ultrasonic sensors suit level measurement and targets that reflect light poorly
- Confirm output type — PNP or NPN, normally open or normally closed — against the PLC input card
Variable frequency drives
A VFD is sized on motor current rather than on motor power alone, and on the load type. A variable torque load such as a fan or centrifugal pump is far less demanding than a constant torque load such as a conveyor or positive displacement pump, which needs full torque at low speed.
- Match the drive rating to motor full-load current, and confirm the load type
- Confirm whether the motor is suitable for inverter operation
- Include braking resistors where a high-inertia load must decelerate quickly
- Specify EMC filtering, and use screened motor cable with the screen terminated correctly at both ends
- Account for enclosure heat — drives dissipate significant heat inside a panel
Panel devices and terminals
Pushbuttons, selector switches, indicator lamps and terminal blocks are individually inexpensive and collectively determine how workable the panel is. Standardise on one terminal system and one device range rather than mixing, so spares are simple and wiring is consistent.
- Confirm panel cut-out diameter for operator devices, and the IP rating on the front face
- Emergency stop devices must be the correct latching, twist-release type and wired to the safety system, not to the PLC alone
- Terminal blocks should suit the conductor size, with disconnect terminals where isolation for testing is needed
- Fused terminals protect individual instrument circuits without tripping a whole group
What to include in a control components enquiry
- I/O count by type, with the spare capacity required
- Communication protocols in use on site
- Signal types and voltage levels for each interface
- Sensor targets, ranges, environment and required output type
- Motor data and load type for any drives
- Panel device requirements and IP rating
- Preferred manufacturer where the site standardises on one, quantity and delivery city