How to Choose the Right PLC for a Packaging Machine

Leave an Engineering Allowance

Many engineers reserve roughly 15% to 25% additional I/O and usable panel capacity on a machine that is likely to change. This is a practical design allowance, not a universal manufacturer rule. The appropriate margin depends on whether the machine design is frozen, whether customer options will be added, and how difficult future expansion would be.

Check expansion at three levels:

1. Does the CPU support enough local and remote I/O modules?
2. Can the power supply and system bus support the proposed modules?
3. Is there physical DIN-rail, terminal, cable-duct, and enclosure capacity?

A controller that supports another eight modules on paper does not help when the cabinet has no space or the selected power arrangement cannot supply them.

3. Calculate the Required Response, Not Just PLC Scan Time

Packaging speed should be translated into a response-time budget. “High speed” is not a specification.

The total control response may include:

Sensor response and input filtering
+ input-module update time
+ network update time, if remote
+ PLC task or scan time
+ motion or logic processing
+ output-module update time
+ actuator or drive response
= total machine response

PLC scan time is only one part of this chain.

Suppose a film moves at 1 m/s and registration correction must begin within 2 mm of mark detection. The available control window is only 2 ms before considering sensor delay and actuator behavior. A normal cyclic input read in a heavily loaded task may be inappropriate; a high-speed input, timestamp, hardware latch, or motion-network function may be required.

For each critical event, record:

• Maximum line or film speed
• Product pitch
• Required position tolerance
• Sensor response time and input filter setting
• Local or networked I/O update time
• Task period and worst-case execution time
• Drive command and mechanical response

Ask the PLC supplier for worst-case or configured performance, not only the smallest published instruction time. Communication load, motion tasks, data logging, HMI polling, and diagnostics all affect the completed application.

Before releasing the design, prototype the fastest sequence with the intended CPU, I/O, drive, network cycle, and realistic program load.

4. Separate Simple Positioning from Coordinated Motion

The number of servo motors alone does not define the motion requirement. Four independent index axes can be easier than two axes that must remain electronically geared at high speed.

Create an axis table with these fields:

• Axis function
• Servo, stepper, VFD, or pneumatic actuator
• Independent or synchronized motion
• Position, speed, or torque control
• Registration input requirement
• Electronic gearing or camming
• Homing method
• Encoder type and feedback path
• Safety functions required at the drive
• Maximum update period and accuracy requirement

When Pulse-Train Positioning May Be Enough

Pulse and direction control can suit a small number of independent axes where the PLC and drive support the required pulse rate and signal type. It may be reasonable for an indexing conveyor, simple cut-length axis, or occasional changeover adjustment.

Check the exact transistor-output CPU and axis limits. Relay outputs cannot generate high-speed positioning pulses, and a family brochure may describe functions that are available only on specific CPUs or expansion modules.

When a Real-Time Motion Network Is the Better Choice

Use a motion-capable PLC or machine controller when the machine needs coordinated axes, electronic cam profiles, registration correction, interpolation, high-speed diagnostics, or tightly synchronized I/O. Platforms may use EtherCAT, PROFINET IRT, Sercos, SSCNET, or another deterministic vendor-supported network.

Verify these items for the exact CPU and license:

• Maximum physical, controlled, and synchronized axes
• Supported servo-drive families
• Minimum configurable motion cycle
• Cam, gearing, interpolation, and registration functions
• High-speed input integration
• Safety communication and safe-motion compatibility
• Axis commissioning and diagnostic tools

Do not assume that ordinary Ethernet communication to a servo drive provides deterministic coordinated motion. The controller, drive, protocol, and engineering software must support the required real-time motion architecture as a system.

5. Let the Risk Assessment Define the Safety System

Safety is not an optional PLC feature to add after the electrical drawings are complete.

Start with a machine risk assessment. Identify hazards around sealing jaws, knives, augers, conveyors, pneumatic mechanisms, robots, and stored energy. Define each required safety function, such as emergency stop, guard interlocking, prevention of unexpected startup, safe stop, or safely limited speed.

The required Performance Level or Safety Integrity Level must be determined using the applicable standards and market requirements. ISO 13849-1:2023 covers the design and integration of safety-related parts of control systems. IEC 62061:2021, with its applicable amendments, addresses functional safety of machinery control systems. In North America, ANSI/PMMI B155.1-2023 is specifically relevant to packaging and processing machinery.

Only after that work should you decide between safety relays, a separate safety controller, or an integrated safety PLC with safety I/O and safe network communication.

An integrated safety PLC is often attractive when the machine has several guards, zones, drives, or operating modes. It can improve diagnostics and reduce hardwiring. A small machine with a limited number of straightforward safety functions may be adequately served by properly selected safety relays. Neither choice is automatically correct.

Do not implement a required safety function in a standard PLC unless the complete design is permitted and validated under the applicable safety architecture. A controller carrying a safety designation also does not make the machine compliant by itself. Sensors, actuators, wiring, diagnostics, software, verification, and validation all belong to the safety function.