Piezo technology is very simple in principle: apply an electric charge to a piezo crystal and it changes shape; remove the charge and the piezo crystal returns to its original shape. Making use of this technology takes a little more cunning, but the rewards are worth it; devices utilising piezo technology offer fast, quiet operation with a long life and low power consumption. Moreover, mass production techniques mean that they are affordable for incorporation within everyday consumer products.

Typically piezo mechanisms are based around piezo-ceramic crystals bonded to metal substrates to form slender cantilevered beams (unimorphs) that bend in a similar way to bimetallic strips. Piezo actuators are characterised by low-power consumption, small movements and low forces. Nonetheless, this does not infer that they are only suitable for lightweight applications, as Piezo actuators have been used successfully within lock mechanisms having a holding force of 4kN or more.

It is the knowledge of how best to apply piezo technology and piezo-based actuators that Servocell sees as its competitive advantage. This UK-based company develops miniature piezo-based actuators that OEM customers can build into innovative locks. It has also developed its own ‘ready to go’ range of locks that it can sell directly and that OEMs or distributors can badge as their own. While this was not Servocell’s original intention, the security industry is currently benefiting from strong demand for existing products and is therefore reluctant to devote resources to new product development. As an example of the type of lock products that can be developed, Servocell has already produced a ‘ready to go’ cabinet lock that is about the size of a beer mat and just 12mm high.

Furthermore, following a flotation on London’s Alternative Investments Market (AIM) in 2006, Servocell is in a position to offer fixed-price development projects. This means that the company can effectively underwrite product developments undertaken on behalf of customers, subject to there being an acceptable business case. Six such projects are currently underway, and Servocell reports that the success rate so far has been good.

Active Latch modules

Prior to the stock market flotation, Servocell had already developed the AL1 Active Latch module (Fig.1). This standard actuator product grew from a market demand for a low-power release mechanism that incorporated all of the control electronics within the same housing. The AL1 can be networked, powered and controlled via a USB port on a computer – or over a telephone line. Thanks to a small number of moving parts and the solid-state actuator, the AL1 is rated for a life of 1.5million cycles.

One product to make use of the AL1 is the Beloxx Becode cabinet handle (Fig.2). Recently launched, this is to be sold through both Adolf Würth and Burg.

However, an issue to be aware of with piezo-ceramics is that they need to be kept dry, otherwise the performance degrades. So for AL1 applications where there is a risk of contamination by moisture, care has to be taken to ensure that the AL1 is adequately sealed. This in itself is not particularly challenging, and the AL1 has been used successfully in applications such as the Salvo system from Castell Safety International that prevents lorries driving away from loading bays until unloading/loading is complete and the bay has been made safe.

Piezo-based locks offer the advantage of costing little to install, as they consume just a few milliwatts – which is on a par with light-emitting diodes (LEDs). What this means is that they can be battery-powered or solar powered and they can receive instructions via a radio link rather than conventional hard-wiring. Alternatively, they can receive their power and signals from a fieldbus. But the low cost of installing the actuators may be of little consequence in some outdoor applications due to the cost of weatherproofing the assembly in which they are installed.

From waterproof to weatherproof

With this in mind, Servocell decided to develop the AL2, a fully weatherproof actuator. At first the company designed a new actuator that was, in essence, a more compact, sealed version of the AL1. But Simon Powell, Servocell’s chief executive, explains why a waterproof actuator falls short of the requirement for a vapour-tight product: “Initially we thought that filling the actuator with silicone oil and sealing it with elastomeric seals would suffice. However, water vapour finds it easy to penetrate polymers – rather like a cat making its way along a crowded street – so you have to seal with metal, glass or ceramic. For the moving part we therefore developed a nickel bellows seal – which is now manufactured in China – and the electrical feedthrough is sealed with a glass frit. The stainless steel body is fully laser welded to complete the seal, and a desiccant pill inside absorbs any residual moisture.”

Unfortunately the inability of elastomeric seals to prevent the ingress of water vapour was not realised until the prototypes were being tested in an environmental chamber – which was one of the final checks prior to the design being approved for manufacture. Throughout the AL2 project, the aim had been to develop a product capable of surviving at 55°C and 85percent relative humidity for 2000 hours, so there was no question of the specification being refined to match the performance that could be achieved.

A crucial component in the redesigned assembly is the nickel bellows seal, which is manufactured by being electrodeposited onto a sacrificial aluminium mandrel that is subsequently etched away. The bellows seal is 30mm long to give a stroke capacity of 4mm and is designed to last for 4million cycles. Actuators under test have operated for over 1billion cycles without failure, and the complete AL2 module will be warranted for 500000 cycles (Fig.3).

In order to maintain the same overall dimensions for the AL2 actuator as the aborted design (10mm wide, 14mm high and 50mm long), despite the relatively long bellows seal, the internal latching module had to be made considerably more compact at just 12mm long. This was achieved by using a mechanism that is similar in concept to a ‘pip pin’ (quick-release pin), with four 2mm balls that are only permitted to move radially inwards to release the sprung latch when the unimorph is in the correct state. This is more costly to manufacture than the equivalent mechanism in the AL1 Active Latch module, but sourcing the main metal-injection-moulded (MIM) stainless steel component from India has helped to keep costs at an acceptable level.

As a result of the more sophisticated design, the AL2 is considerably more expensive than the AL1, the costs being approximately E13 for the AL2 and E4.5–7.5 for the AL1, depending on the specification. Much of the additional cost is due to the measures required to seal the device. Servocell is therefore leading a research project, backed by the UK’s Department of Trade and Industry, to find a coating that will make the unimorph resistant to moisture. Such a coating would eliminate the need for hermetic sealing and enable actuator modules to be manufactured at a much reduced cost.

Release-under-load

Having developed the AL2, Servocell has now started work on the AL5. Unlike the AL1 and AL2 that cannot be released if they are under load, the AL5 will be a release-under-load mechanism. This new module may therefore be better suited to, say, machinery applications, as any lock using this module will be guaranteed to open, even if, for example, mechanical damage to a guard means the lock is continuously loaded, or an operator is already trying to open a guard when the interlock is released.

The AL5 is currently at the proof-of-concept stage and Servocell is discussing the specification with a number of customers prior to commencing the detailed development phase.