Easing control system design with interface relays
Norman Carnt, Technical Manager, Finder plc

The design and commissioning of industrial control systems is a complex task, and one of the most challenging aspects is at the interface to the electrical system itself. The input and output interfaces of a control system are largely defined by the programmable logic controllers (PLCs) or industrial computers that are now widely used for programming systems. As the price of silicon has fallen, industrial controllers have become commodity items, and offer more than enough computing and logic processing for most applications. The drawback is that they generally integrate fixed and inflexible interface devices.
The weakness is therefore in the hardware, at the point where computing power meets the real world – where the environment can be hostile, and where isolating distances and current carrying capacity count. Because it is too costly to customise the packaged input and output devices of a controller to suit specific applications, the majority of industrial controllers of whatever type are equipped with inputs and outputs that are only suited for a fairly limited range of applications.
Standardisation and cost pressure dictates that the output configuration is almost always limited to a single pole Normally Open contact, open collector transistor or triac, and this is usually of limited current handling capability and often offers only a low degree of isolation. To address this challenge, Finder has developed a range of Interface Relay modules, which provide a bridge between the controller and the real world, and have been optimised to match the characteristics of the application.

Interface relays are characteristically a fully-featured, ready-built assembly comprising the relay, the socket and appropriate coil indication and coil suppression - either built into the socket or provided through the deployment of one of Finder’s wide range of plug-in modules.
By identifying a relatively limited number of relay/socket/module combinations, and by giving the combination a single part number, the design engineer has access to a range of ideally matched components that are ready assembled, and ready to accept wiring, which also brings savings in inventory and purchasing time and costs.
In reality, the need for more than one mutually isolated output contact is a common requirement within control systems. This is often a consequence of controlling devices driven from different supply voltages, or sometimes results from the need to prove the operation of the final electromechanical device by feeding a reliable proving signal back into the system logic.
Load diversity

By adopting and sensibly applying the most appropriate interface relay module or modules, the equipment designer can easily provide volt-free contacts from a single changeover to four changeovers. At the same time, a choice of contact ratings in the range 5A to 16A allows the control of diverse loads in the medium power and upward range. At the lower power end of the spectrum more specialised relays - fitted with more exotic contact materials - can be chosen to ensure the extra reliability and integrity that is necessary where low level circuits are to be interfaced. The connection and routing of temperature sensor circuits and conductive level probe sensing circuits are examples of this. A leakage suppression version is available to optimise performance at the input device, and the range as a whole offers the ability to cope with interfacing mismatched input devices into a standard input card. One of the interface series has also recently been expanded to offer solid=state relay outputs, which will switch down to 0.05mA and are particularly useful for high-speed operation or where virtually unlimited numbers of switching operations are required.
Maintainability is becoming even more important today, when “down time” of machinery and the time of skilled engineers can be very costly.  PLCs and other controllers invariably feature soldered-in output devices, while the field replaceability of the relay element of an interface module affords quick and easy maintenance, particularly for those applications where exceptionally longevity would otherwise have posed a problem for the non-replaceable output of a conventional controller.

Modularity

The modularity feature allows an interface module with particular characteristics to be selected channel by channel to match the particular load, without requiring over- or under-specification to allow the same part to be used for each channel. It also offers the possibility of individually upgrading a channel to a different specification during the lifetime of the system.
The industry standard DIN rail mounting allows for the popular and universal mounting of modules adjacent to each other and to other devices. When several modules are mounted together, multi-way jumper links make for easy and cost effective common circuit interconnection of blocks of interface modules.
The Finder range of interface modules can be driven by virtually any output configuration commonly seen in industrial controllers - AC or DC driven for voltages in the range 5 - 240V. A specialised version that can be driven by either AC or DC has proved particularly popular for battery standby applications.

Versatility

Where space is an issue, high density mounting is a possibility with module widths ranging from a modest 27mm to a super slim 6.2mm. Other useful features include an integral coil indication LED for the instant recognition of status and the simplification of diagnostics. All of the modules are supplied with marking tags fitted as standard, and also with a retain/release clip as standard. Many types are also available with screwless terminal options.
The facility for identification labels greatly aids commissioning and maintenance, while integrated relay ejector mechanisms provide for quick and safe replacement of the relay element, while an elevated or raised terminal position facilitates easy wiring
With the current level of concern over issues of safety and compliance with EU Directives, considerable effort has been expended in this area. For electrical safety, insulation coordination to Overvoltage Category III,  Pollution Degree 3 makes these products suitable for the majority of domestic, commercial, and industrial applications.
Internal suppression circuitry addresses the issue of EMC by controlling electro- magnetic emissions and providing protection to associated electronics. Type approval tests for EMC Immunity to 4kV (Burst) and 2kv/4kV (Surge) give the user the highest degree of conformity and confidence to permit their incorporation within equipment required to meet the EMC Directive.

Conclusion

For the control system designer, the availability of a professional range of interface modules can be a very useful tool for restoring features and flexibility to powerful, but hardware impoverished, microprocessor based control solutions. The Finder range of interface modules is based on the company’s world-class, systems-proven range of electrical relays and sockets, and therefore offers a greater level of confidence - as well as a wider level of power and isolation specifications - compared with electronic solutions.