Solenoids in industrial automation turn an electrical signal from the PLC into an immediate mechanical action: diverting a part, locking a safety guard or holding a metal component. These electromagnetic devices are the link between the machine’s digital control and physical movement, and they are integrated through a simple digital output, with no pneumatic or hydraulic circuit.
On an automated line, each of these actions is repeated thousands of times per shift. That is why a solenoid is not chosen on its rated force alone. How it behaves under the real duty cycle, the machine temperature and the installation environment also matters.
Three functions of solenoids on an automated line
Sorting and diverting parts
In sorting systems, a linear solenoid drives diverters, stops and gates on conveyor belts. The PLC receives the signal from a sensor or a vision system and energises the solenoid to reject a defective part or send the product to its destination.
What matters here is response speed, repeatability and endurance over a very high number of operations. You can read more about this application in our article on solenoids for sorting systems.
Locking guards and safety doors
On CNC machines, robot cells and production lines, an electromagnetic locking bolt keeps the guard closed while a hazard is present, for example while a spindle is still rotating or a robot is moving. The control system only releases the lock once the machine has reached a safe state.
This function is known as guard locking and is covered by ISO 14119, the standard for interlocking devices associated with guards. We look at it in more detail below because it is the application that demands the most care at the design stage.
Holding and positioning metal parts
Holding electromagnets secure ferrous parts during handling, assembly or light machining operations. When energised they attract the part, and when the power is cut they release it, with no grippers or clamps. They are integrated into robot arms, feeders, stackers and positioning fixtures.
We explain how they are used on automated lines in our article on holding electromagnets in industrial automation.
Solenoid or pneumatic actuator: when to use each
Solenoids do not replace pneumatics in every case. Each technology has its own ground.
| Criterion | Linear solenoid | Pneumatic cylinder |
| Power supply | Electrical only | Compressed air + solenoid valve |
| Control | Directly from a PLC output | Through the solenoid valve |
| Stroke | Short (typically a few millimetres) | Medium or long |
| Force | Moderate, varies along the stroke | High and more constant |
| Maintenance | Few moving parts, no leaks or air treatment | Checking seals, leaks and air quality |
| Typical use | Diverters, interlocks, stops, releases | Long pushes, pressing, load transfer |
The solenoid is the better option for short, fast movements controlled directly by the PLC, especially on machines without a compressed air supply or where the associated air consumption is to be eliminated. For long strokes or high forces, pneumatics remains the more suitable choice.
Guard locking with electromagnetic locking bolts
Spring-applied or power-applied locking
A guard locking bolt can work in two ways:
- Spring-applied locking, power to unlock. The bolt is locked when de-energised and only opens when power is applied. If the power fails, the guard stays locked.
- Power-applied locking, spring release. The bolt only locks while it is energised. If the power fails, the guard is released.
For the protection of people, ISO 14119 generally gives preference to spring-applied locking, because a power failure does not leave a zone accessible while it may still be hazardous, such as a shaft that keeps rotating under inertia.
In our range, this logic corresponds to fail-secure locking bolts, such as the CU20/CP or the ERC30/CP, which remain locked without power.
Position monitoring
In a safety function, the control system needs to know whether the guard is actually closed and locked. That is why locking bolts with signalling contacts are used, such as our C30 model, which report their position to the controller.
Bear in mind that the locking bolt is one component of the safety function, not the complete function. The circuit architecture and the required performance level (PL) according to ISO 13849-1 are defined and validated by the machine manufacturer.
Design parameters to check before choosing a solenoid
Duty cycle and temperature
The duty cycle (ED %) indicates what percentage of the time the coil can remain energised without exceeding its maximum temperature. A solenoid rated for 100 % ED can be energised continuously. One designed for a lower ED needs off-time to dissipate heat.
A 100 % ED solenoid is not automatically the better choice. It is usually bulkier or delivers less force for the same size. The right approach is to start from the actual application profile: pulse duration, operating frequency and the temperature inside the cabinet or the machine.
Force at the critical point of the stroke
The force of a linear solenoid increases as the plunger approaches the end of its stroke. The figure to check is the force available at the point of greatest resistance in the mechanism, which is usually the start of the movement.
As a reference, the ECM range covers strokes from 3 to 12 mm and forces from 0.02 to 32 N. For mechanisms with higher loads or intensive cycles there are the high-performance linear solenoids of the ERC, ECH and CU ranges.
Environment and IP rating
The IP rating (IEC 60529) indicates how well the enclosure resists the ingress of solids and liquids. IP40, as on the ECH and CU ranges, protects against solid particles larger than 1 mm and is suitable for use inside machines and cabinets.
In areas exposed to cutting fluid, washdown or splashing, a higher rating should be considered, or the solenoid should be protected by the machine’s own enclosure. We explain this in our article on IP ratings in solenoids.
Power supply and control
Most automation applications run on direct current, normally at the machine’s control voltage. A freewheeling diode or suppression circuit should be provided at the PLC output, as the coil generates a voltage spike when it is switched off.
Applications on automated lines and in other sectors
Which solenoid to choose for each function
| Function | Device type | NAFSA references |
| Diverters and stops in sorting | High-performance linear solenoid | ERC range, CU range |
| Compact releases and interlocks | Single-acting linear solenoid | ER range, ECM range |
| Guard locking | Fail-secure locking bolt with signalling | CU20/CP, C30 |
| Holding ferrous parts | Holding electromagnet | VEM range, VM/ND range |
You can browse the full range on our page of solenoids for industrial automation.
Other automated sectors
The same principles apply beyond the classic production line. In power distribution, linear solenoids act as trip devices in medium-voltage switchgear. In medical and laboratory equipment, compact solenoids drive positioning and locking mechanisms in automated analysers.
When a custom solenoid makes sense
A dedicated development makes sense when the mounting space, the available voltage, the combination of force and stroke or the production volume does not fit a catalogue model.
At NAFSA we have been designing and manufacturing solenoids since 1996 under a quality management system certified to ISO 9001:2015. In our custom manufacturing projects we start from the machine builder’s requirements, develop prototypes and carry out functional testing before series production.
Benefits of solenoids on an automated line
For the short, repetitive movements of a machine, the solenoid offers specific advantages over other actuation technologies:
- Direct integration with the PLC. It is driven from a PLC digital output, with no solenoid valve and no pneumatic or hydraulic circuit.
- Less maintenance. It has few moving parts, and there are no leaks, seals or air treatment to check.
- Fast, repeatable response. It is well suited to diverters, stops and releases that are repeated thousands of times per shift.
- No compressed air supply. It allows machines without one to be automated and eliminates the associated air consumption.
- Defined behaviour on power loss. A spring-applied locking bolt keeps the guard closed even if the power fails.
- Status monitoring. Locking bolts with signalling contacts report their actual position to the controller.
- Holding without mechanical tooling. Holding electromagnets secure and release ferrous parts with no grippers or clamps.
In industrial automation, the solenoid is the component that physically carries out what the PLC decides. Most failures that appear once the machine is in production can be avoided by getting four things right: the function (divert, lock or hold), the real duty cycle, the force at the critical point and the IP rating.
Frequently asked questions
What are solenoids used for in industrial automation?
Mainly for three functions: diverting or stopping parts in sorting systems, locking machine guards and safety doors, and holding metal parts during handling or assembly.
Is a solenoid or a pneumatic cylinder better?
It depends on stroke and force. A solenoid is better suited to short, fast movements controlled directly by the PLC and needs no compressed air. A pneumatic cylinder is preferable for long strokes or high forces.
What does 100 % ED mean on a solenoid?
It means the coil can be energised continuously without exceeding its maximum temperature. A solenoid with a lower duty cycle needs off-periods to dissipate heat.
Which locking bolt is recommended for locking a machine guard?
For the protection of people, a bolt that locks by spring and unlocks when energised is usually preferred, so the guard stays closed in the event of a power failure. The reference standard is ISO 14119.
What IP rating does a solenoid need for a machine?
For use inside machines and cabinets, IP40 is usually sufficient. In areas with cutting fluids, washdown or splashing, a higher rating should be considered, or the solenoid should be protected by the machine enclosure.
Can solenoids be custom-made for a specific machine?
Yes. When the format, force, stroke or voltage does not fit a standard model, we develop the solenoid from the manufacturer’s specifications, with a prototype and testing before series production.