Increased reliance on automation will be pivotal in helping manufacturing facilities to raise productivity levels and reduce wastage. Upholding the ongoing operation of machinery and robotic systems and avoiding downtime due to the replacement or repair of constituent components, must be the highest priorities – or else the profitability of the facility will be severely impacted upon. Likewise, rapidly responsive safety mechanisms need to be put in place to protect valuable equipment from damage and factory staff from the prospect of injury.
With this in mind, it is clear that the relays incorporated into such machinery/systems should be chosen wisely. As well as attaining strong performance benchmarks, they must exhibit exceptional robustness and prolonged working lifespans. The following article will look at the key aspects that relays —both electromechanical and semiconductor based— need to possess if they are to be successfully utilized in a modern industrial context.
Understanding relays’ operation and their usage
Electromechanical relays have been employed in electronic/electrical hardware for many decades, providing both system switching and safety related tasks. How they function is simple enough to grasp. The applying of a small current will result in the generation of a magnetic field within the relay’s coil. This will in turn activate a switching mechanism via which a circuit running at a higher current can be controlled while maintaining electrical isolation between the two circuits.
Solid-state relays are likewise able to provide isolated switching, but here another approach is taken for delivering the isolation (usually based on an optoelectronic-oriented arrangement) instead. As we will see later, there are times when use of electromechanical relays will prove to be most apt, and others where solid-state relays will be the best option.
Relays in either of these two forms will be deployed extensively throughout industrial sites. They can be found in control panels, industrial drives, and programmable logic controllers (PLC). Increasingly, they are being used for managing the movement and ensuring the safe operation of robotic systems, in particular for ‘light curtain’ application – where they are used to stop a robot arm from moving if a human comes too close to it.
Selecting the right relay
Relay reliability is a very high priority when it is going to be integrated into some form of industrial equipment. If a malfunction event occurs, then it is likely to result in very serious consequences. Should the output of a facility’s production lines end up being halted, then the company’s revenue will suffer during the downtime that ensues. Exposure to extreme temperatures and heavy vibrations will, of course, potentially need to be taken into account as these will regularly be present in factories and processing plants.
The risk of failure must be mitigated as much as possible, with only minimal ongoing maintenance being required. This is especially important in situations where it will be difficult to access the equipment in which the relay is contained after installation.
Operational costs can be a concern too, with power consumption becoming increasingly critical. Therefore, it is advisable to pick devices that will help to minimize factory utility bills. There may also be space constraints to contend with, and in such cases a compact sized relay solution will need to be specified.
Thought needs to be given to the electrical properties of the relay. Compliance with the IEC-61010 insulation standard is likely to be expected, so that lives of factory operatives are not put in danger. Next a decision will need to be made on what type of casing will be appropriate. The conducting of IEC-60335-1 glow wire testing will confirm that plastic material from which the relay casing is made is not flammable and is therefore acceptable to use in industrial systems. As industrial drives and motors are inductive loads, they can be prone to high inrush currents. Rugged relay solutions are thus essential here, otherwise the contacts may melt together.
Another pivotal trait the relays must have if they are to be deployed in challenging industrial settings is the ability to deal with liquid ingress. Devices that have wash-tight sealed housings may need to be specified, in accordance with the IEC-61810 standard. They may also need to be flux-proof so that they have adequate protection during the soldering process. If a relay specification mentions RTIII then this signifies that it is wash-tight, while RTII confirms that it is flux-proof and there will be no problem when it is being soldered to the PCB. Wash-tight will only be used if absolutely necessary, with flux-proof being preferred, as outgassing from the housing can lead to a damaging of the contacts. The soldering of PCB-mount relays onto a board will require a second separate soldering process. Through-hole relays are thus preferred as a conventional reflow process can be followed, which will be cheaper.
The number of poles (which determines how many contacts there are and how many circuits can be controlled) is a further consideration. The more poles that can be accommodated into a relay device, the greater the space saving derived will be. There are additional upshots from this – in terms of a lower overall bill-of-material cost, a smaller power budget, etc.
Given that industrial systems can potentially stay in service for many years, there is the issue of legacy equipment still needing to be kept running. The sourcing of drop-in replacement relays for outdated models (that have the same form factor with equivalent operational parameters) can hence be an issue that needs to be attended to.
Electromechanical or solid-state
When deciding on whether to use an electromechanical or a solid-state relay, the first question that ought to be asked is how many switch cycles the selected relay will need to complete over the course of an average day. The higher cycle durability of solid-state relays may be the deciding attribute here. If cycling is very high (above 100,000 cycles during the relay’s working life) then the reliability of an electromechanical one might not be enough. Where solid-state relays have another significant edge is in relation to ongoing vibrational forces. They perform much better in such scenarios, as they do not have any moving parts. Low noise is another advantage to be aware of. In comparison to their electromechanical counterparts, solid-state relays are markedly more expensive. Nevertheless, this may be justified if their use avoids replacements needing to be made, with equipment thereby running for longer.
There are plenty of circumstances where electromechanical relays show themselves to be the best route to take though. Solid-state relays will get hotter when there is intense switching activity, so accompanying heatsinks will be mandated, whereas electromechanical relays have a lower contact resistance and so this will not be necessary here. The increasing expense associated with heatsink inclusion, plus the extra space that will be taken up, can preclude solid-state relays from certain applications. Another issue which should be mentioned is that solid-state relays have greater vulnerability to electrostatic discharge (ESD) strikes —and this is something that is likely to be commonplace in industrial locations.