Safe piezo valves control gas with precision

4 min read

Tom Shelley reports on a breakthrough in the precision control of inflammable and other gases

Tom Shelley reports on a breakthrough in the precision control of inflammable and other gases By devising a mechanism by which a gas valve can be opened using only a few mJ, it has proved possible to develop low cost, proportional, silent controls unit for domestic gas cookers and other appliances. This allows gas appliances to be built, with sophisticated control features only previously available for electric units including increased safety. The mechanism is applicable to precise control of any kind of gas, including process chemical gases and air, in which case it is then possible to increase flow rates and further reduce costs. The development is a joint one between Servocell, which makes piezoelectric actuators among other products, and Diamond H controls. According to Simon Powell, Servocell's technology director and innovation guru, whose inventions have graced the pages of Eureka before, "Lots of people have had a go at making piezoelectric gas valves for domestic cookers, but they have always come up against hysteresis." Hysteresis in any system leads to calibration drift and increasingly sloppy control. Bimorph actuators, with sufficient force to overcome the problem are too expensive. What is needed is a solution that is low cost, has low capacitance and thus little stored energy, and which closes in the event of electrical failure. Solenoids use too much electric power to be sufficiently safe, and if electric motors are used, drive electronics have to be sealed against gas, also for safety reasons. These requirements made the design task distinctly non trivial, and Simon Powell told us that two years into development, the prototype design had 43 components, leading to an agreed decision to scrap it and start again. Another year was then spent coming up with the present design, which has only four components. The main actuator is of the hairpin design described in Eureka's March 2002 cover story. The actuator in the valve has 1mm of movement. But the real breakthrough is in the design of the valve diaphragm, which is pressed against an 'O' ring by a piece of spring steel to form a dish shape. The dish shape seals particularly well against an 'O' ring. Gas pressure normally presses the diaphragm shut but it can be opened by exerting only 0.03 to 0.04N of force by peeling it back from one side. The membrane is made out of optically smooth DuPont 'Kapton' backed by a spring steel blade. Once gas starts to pass under the edge, lifting force goes to zero. The original diaphragm design tended to ping up but the present design has been extensively finite element modelled to ensure proper proportional control. Control loop feedback is from a pressure sensor on the downstream side of the valve so as to ensure a constant, even pressure in gas supplied to the burners. An integral strainer on the inlet side should prevent possible malfunction from particulate contaminants in the mains gas supply. Natural Gas can contain such particles and these can cause sealing issues in all types of gas controls. Life is unknown but long, because as Simon Powell put it, "We have never managed to wear one of the test valves out." Simon Simpson, business development manager of Diamond H said his company and their OEM customers were intending to offer a life time guarantee on the valves. In the cookers, all the valves, from one to nine will be in single blocks incorporating the electronics. Advantages to manufacturers include the doing away with the need to cut holes in metal or glass ceramic for knob shafts. Simon Simpson said it would help cooker manufacturers to produce products with ,"The sort of professional looking finish customers are demanding as a result of watching Delia Smith and other television chefs" working in their shiny, high tech kitchens. A less obvious but big additional plus is that going to electronic control allows enhanced safety. It is, for example, possible to incorporate an automatic flame failure and re-ignition sequence, which can be made to shut down after a pre-determined number of attempts, and also diagnostics that can be used to warn cooker users of possible blockage problems and their likely causes. The electronics is 5V powered and uses very little current and so is amenable to battery backup. Further advantage stems from the ability to call up different programs appropriate to different gases. The valve controller only needs to be told what gas is being supplied and what pressure it is at, and the valve will recalibrate itself. Cookers can thus work equally well on UK mains natural gas at 20mbar, US mains natural gas at 10mbar, or liquefied petroleum gas. If the cooker is switched from using one gas supply to another, it only requires a technician to switch the controller to the appropriate program. The valve is EC approved and approved by Advantia (formerly British Gas). It is currently in the process of being approved by the Canadian Standards Association. Flow rates for use on cookers are up to 10 litres/minute/port. Higher flow rates may be achieved in the present product by changing the shape of the 'O' ring. The electronics are rated at 105 deg C. Noise is less than 7dBA, which is important to cooker designers, since any noise produced, by, for example, clicking solenoids, tends to be amplified by the metal box construction. The valve body is made of aluminium, and Simon Simpson said that is was additionally suitable for use controlling gas flow in hearths and domestic boilers, allowing them to be digitally controlled in 'Smart Home' systems, when customers start to want these. Simon Powell said the valve is suitable for controlling flows of any gas in any system. If the gas is not inflammable, flow rate can be greatly enhanced by removing the steel blade, which "Allows it to open right up." Servocell Diamond H Controls Eureka says: Sophisticated electronic control of gas fired appliances can only enhance safety Pointers * Valves can be programmed to control the delivery of any gas, inflammable or non inflammable * Flow rates for cooker and domestic appliance applications are up to 10 litres/minute/burner * Noise output is less than 7dBA