For decadesR&D teams have done their best to improve relays. Soyou would think that the humble relay had now reached perfectionright? Wrong. In pursuance of extreme reliability and the best performance in the smallest housingdevelopment continues at pace. Tetsuya Fujiwara reports.
The development and production of industrial relays started half a century ago. The trend towards miniaturisation began with Omron’s MY relaywhich was introduced in 1966 and since that time over 500million pieces have been sold around the world.
Subsequentlythe introduction of smaller-sizedhigh-power relays for use as PLC interfaces revolutionized the field of industrial automation.
Continuing development has now led to the introduction of the first 6mm plug-in relaythe G2RVwhich was designed to meet market demand for reduced control-panel space while following machine modularisation and decentralised automation trends.
Eventuallythe future will see the emergence of the micro-machined relaywhich is still a mechanical relay with coil and mechanical contactsbut it is made using MEMS (micro electro mechanical systems) technology similar to that used in the semiconductor industry.
Naturallysuch advanced designs will increase both the reliability and functionality of relays.
In order to ensure the high quality and reliability of relaysspecial production software (SPC – Statistic Process Control) has been developedwhich automatically adjusts key processes in accordance with measurement data.
But software and accurate control of production parameters are not the only keys to achieving high quality and reliability.
The high quality of equipment used and the experience of engineers employed in the areas of development and manufacturing also play important roles. The root cause of any failure has to be tracked down and the findings fed back to the development group.
In the case of Omronthe result of these activities has reduced the failure rate of relays in the market today to well below 2ppm (0.0002percent).
Setting the bar higher
Quality and reliability are taken very seriously these days and an uncompromising approach at Omron has led to the redefining of these two words.
Normally defined as the ability of the product to operate satisfactorily within given specifications and functionalityquality now means the ability to meet the specification always.
Similarlyreliability is normally defined as the ability of the product to operate over a period of time under permitted conditions. But it is now defined as the ability to last beyond the specification.
Statisticallyreliability has to be very high to fully meet the specification and it does in fact exceed the specification by far.
A Quality First philosophy can only be achieved by continuous improvement of products and production processes. This translates directly into increased reliability and customer satisfaction. Basically this can be divided in three parts:
- Failure in infancy.
- Failure in maturity.
- Failure due to wear.
To achieve high levels of product quality it is essential to reduce failures in infancy to an absolute minimum.
Most failures in infancy are caused by relays not making contactby broken wiresor by incorrect assembly of the ac coil causing vibration. These failures mainly occur due to mishandling.
The only way to reduce these failures substantially is by having 100percent traceability throughout the complete production process.
Electrical durability
Electrical durability is another key factor contributing towards improved reliability and it depends on the type of load switching.
With high-load switching (more than 1Apower switchingeg for a brakelampor heater)electrical durability mainly depends on the materials from which the contacts are made and the strength of the magnetic coil. Contact materials have their own particular physical characteristics.
Where potential welding is a distinct possibilitya high-performance coil helps to avoid welding of relay contacts by applying a combination of magnetic and spring forces.
With low-load switching (below 10mA signal switchingeg with I/O interfaces for PLCs)electrical durability is determined by how long the relay maintains low contact resistance. The main factor in contact resistance is pressuresize of contact area and the condition of surface.
The general assumption is that minimum load endurance can easily be achieved using gold-clad contactswhereas gold cladding only makes the contact resistance low and helps to avoid corrosioneg oxidisationof the contacts.
Howeverthis initial condition is worsened by switching and results in fouling of the contact surface. The key to maintaining good conductivity for the lifetime of the relay is to use contacts of a different shape.
With this in mindrelays are available with various contact shapes and customers can choose the right one for their specific application and required level of reliability for low load switching.
As an examplethe bifurcated twin crossbar contacts with gold cladding are recommended for emergency and interlock circuits.
The lifetime of a relay is specified as a guaranteed number of operationswhich means that the relay works perfectly even beyond what the specification says it should.
Expressed another wayin Omron’s internal procedures for electrical durabilitythe specification exceeds the number of operations (as calculated by the Weibull statistical methodwhich is commonly used in life data analysis) by 30percent. This means that the relay should work perfectly for at least 1.3 times the number of specified operations.
As the trend towards downsizing panels and equipment continuestoday’s customers need smaller relayswhich is why a slim industrial relay has been developed.
Despite being only 6mm in widthit fully meets the industrial standards set by its predecessors (the MY and G2RS).
Plug-in terminal
Called the G2RVit incorporates decades of experience and has been designed with a plug-in terminal to achieve maximum strength for handling and maintenance. Plug-in relays can be replaced in their socket without the risk of bending (coil) pins.
Moreoverthey have a secure connection and a large contact area between the socket and the relay terminal. This gives a far more reliable connectionespecially in equipment where the problem of vibration occurs.
The G2RV’s features help users to quickly recognise its statusperformance and condition. A LED shows when voltage is applied to the coilwhile a mechanical indicator shows the actual contact movements.
Furthermorethe relay housing is fully transparent. An additional feature that also makes life easier for users and helps to drive down labour costs are push-in terminals that require no tools for wiring.
The G2RV was designed with (high) power switching very much in mind and has more than double electrical durability of average 6mm relays.
Tetsuya Fujiwara is supervisor of Omron Relay & Devices Corporation in Japan. Omron Europe BV is based in HoofddorpThe Netherlands. www.omron-industrial.com
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