Traditionally, control in hazardous areas has been achieved using pneumatics but, more recently, 4-20mA electronic systems have been used. However, with a growing demand to cut installation and maintenance costs, Ray Barnes predicts that piezo pneumatics will be more widely used.

Safety requirements for installing remote I/O systems in hazardous areas are relatively complex and have often led to the equipment being installed in expensive Eexd or Eexp enclosures. Piezo-pneumatic valves, however, have no such restrictions, enabling complete pneumatic control systems to be located inside hazardous areas, close to actuating equipment - with resulting increases in performance, efficiency and responsiveness (Fig.1).
Using piezo valves minimises the demands for electrical protection barriers, particularly when operated with Eex-compatible fieldbus systems. When operating items of electrical equipment in an Ex environment, they are normally protected by electrical barriers that operate like a fuse, preventing excess power surges to the installed equipment. The barrier and electrical equipment must be carefully matched such that power demands, inner effective inductance and inner effective capacitance of the equipment are less than that of the protection barrier.
The cost of barriers is closely related to the power rating of the associated equipment. With conventional control systems an individual barrier is required for each electrical component, for example, each electrically operated pneumatic valve would require its own barrier. But when an intrinsically safe bus system is used, a single barrier provides protection to the entire bus system. Furthermore, piezo valves, which may be operated directly from the field bus data line, require no further protection (Fig.2).
A potentially explosive environment is created when three critical ingredients are all present: a flammable substance (gas, vapour, mist or dust); oxygen (normally from the surrounding air) and an ignition source - in industrial environments this may include electrical connections, electro-magnetic and induction effects, friction and mechanical impact.
An explosion, nevertheless, will only occur if the substance-air mixture is within a certain concentration range. Anybody accustomed to home car maintenance and old enough to remember the days before computerised engine management systems, will know from experience that if the mixture is too rich or too lean combustion will not occur. The necessary conditions, the explosion limits, depend on the ambient pressure and the oxygen concentration.
To avoid explosions and the related dangers, effective precautions must be taken. These may involve: steps to prevent the formation of an explosive environment;
steps to prevent explosive environments from igniting and, steps to restrict the effects of an explosion to a safe level.
Steps 1 and 2 are effectively 'primary' or preventative explosion protection and step3 is secondary protection.
Pneumatic control valves are most commonly operated by solenoids, to facilitate the easy interfacing with electronic controllers.
While convenient and relatively low cost, solenoids are not particularly efficient: they heat up and, even with low power solenoids, typically have an electrical consumption of several watts.
In explosion-proof environments, remote control and battery-operated systems, energy consumption is particularly important and requires careful consideration when specifying products.

In comparison, piezo-pneumatic operators are the lowest energy electro-pneumatic interface available and are a major development for specifiers of pneumatic control equipment.
Pioneered by Hoerbiger-Origa, this technology has minute power demands, is extremely fast in operation and, with practically no moving parts, is extremely reliable.
At the heart of all Hoerbiger-Origa piezo operators is a tiny piezo valve, a simple 3port, 2position valve in which the air flow is switched by a piezo element. Operating rather like a bimetallic strip, the piezo element deflects in direct proportion to an applied voltage, so that the valve may either be used in a simple on/off switching function or in a proportional mode to produce an analogue pressure output.
By integrating the piezo valve into an ISO standard pilot operator (Fig.3), Hoerbiger-Origa offers users the benefits of low energy piezo technology as an alternative to solenoid operation for most standard pneumatic valves, providing significant application advantages.

Ray Barnes is with Hoerbiger-Origa, Tewkesbury, UK.