How to Choose a PLC for a Small Machine or Line

Knowing how to choose a PLC saves you from two expensive mistakes: buying one too small to accept the next expansion, or one so large it inflates the machine's cost for nothing. The decision comes down to a checklist: number of inputs and outputs, communication, environment, software and, above all, who gives you support nearby.

Criteria for choosing a PLC

1. Inputs and outputs (I/O)

Count every sensor, button, switch and actuator on the machine. Separate digital from analog and note the output types: relay, transistor or analog. Add a 20% to 30% reserve for tweaks and future expansion. Check that the model accepts add-on modules without replacing the controller.

2. Special functions

High-speed encoder counting, servo motion control, thermocouple inputs or temperature PID are not always in the base model. Confirm whether they are built in or need an extra module, which changes the final price.

3. Communication and protocols

List what the PLC must talk to: an HMI, drives, servos, barcode readers, supervisory software. Check for an Ethernet port, RS-485 serial and which protocols are supported, such as Modbus and OPC UA. That decides whether data collection will be simple or need gateways.

4. Programming software

Ask whether the software is free, paid or tied to a hardware key, and whether updates cost extra. Also see which languages it supports (ladder, structured text, function blocks) and whether ready-made libraries exist for your devices.

5. Environment and certifications

Temperature, dust, vibration and humidity shape the choice. If the machine has a safety function, evaluate whether the PLC or an additional safety module meets the applicable standards, with a qualified professional involved.

Local support and spare parts: the forgotten criterion

An excellent PLC that takes weeks to arrive after a failure is a line stoppage waiting to happen. Ask the supplier about replacement lead time, technical assistance in your region, training and the availability of integrators who know the platform.

Consider lifecycle too: for how many more years will the manufacturer sell and support this family? Keep at least one CPU and one module of each critical type on the shelf.

Brands you will meet in the Brazilian market

Options differ in price, ecosystem and regional presence. Always confirm availability and support in your own city before deciding:

• Siemens, Rockwell (Allen-Bradley) and Schneider Electric: broad ecosystems, common in larger plants, generally at a higher cost;
• WEG: a domestic manufacturer with good integration with its own motors and drives;
• Mitsubishi, Omron and Delta: compact lines, popular in special-purpose and mid-size machines;
• entry-level lines from various brands, fine for simple automation, but check how deep local support really is.

Questions for the supplier and red flags

1. What is the replacement lead time for CPUs and modules?
2. Does the software carry license, update or key costs?
3. Is there an integrator or service partner within a day's travel?
4. For how long will the product line be maintained?
5. Are there references of applications similar to mine?

A red flag is a supplier who cannot answer about parts lead time, or who pushes the priciest model without asking what the machine does. If you need help specifying the controls, our industrial automation work starts with an I/O survey.

Frequently asked questions

How much spare I/O should I plan for?

A 20% to 30% margin on inputs and outputs is usually reasonable, to cover design changes and small expansions without replacing the controller.

Is it worth standardizing on one PLC brand?

Yes, when possible. Standardizing cuts spare-part stock, training and maintenance time, even if one model costs slightly more.

Do I need an Ethernet PLC even on a small machine?

It is recommended, because it eases programming, HMI connection and later data collection. The price difference is usually small compared to the benefit.