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Improving patients’ quality of life with new piezo valves
Festo’s Paul Kendall explores the latest developments in piezo valve technology and how their application in medical devices can improve quality of life.
Pneumatic valves are found in a myriad of medical applications, from the precise control of fluid flow for dialysis, to pneumatically-operated surgical tools and the regulation of gas flow and pressures in medical mattresses for nursing beds. One important piece of equipment that needs accurate flow control is portable oxygen systems. There is a trend for these mobile devices to be ever smaller, driven by a demand for lighter and more energy-saving systems that are easier to carry: improving the patient’s quality of life. Piezo technology is offering a new way to design valves, enabling the miniaturisation of technical components that control and regulate the gas flows.
Piezo technology relies on specific ceramics which exhibit piezoelectric characteristics, meaning that when this material is mechanically deformed a voltage accumulates in it. This effect also occurs in reverse: applying a voltage to the ceramic will cause deformation. If a voltage is applied to a piezo bender, it will lift off a valve seat. The movement is proportional: the higher the voltage, the more the actuator bends and the more gas can flow through the valve.
In order to regain its original shape, the material must be actively discharged. Energy is only needed to change the shape of the piezo bender, or in other words, to change the charge on the bender itself. No energy is needed for piezo ceramics to maintain mechanical deformation. Even after a loss of power, the valve will remain in its current position.
Since piezo ceramics have a small capacitance (20-40nF), the amount of energy required to change the shape of the bender is very small: the power consumption is exceptionally low at only one milliwatt. This is a major advantage compared with solenoid valves, which require several watts of continuously supplied energy to maintain an open state. This higher energy level also causes the solenoid valve to heat up in the process. Its low energy consumption makes piezo technology ideal for battery-operated devices, and for applications where valves cannot be allowed to heat the gas.
However, piezo valves cannot be used as a drop-in replacement for sol-enoid valves. Piezo valves operate with higher voltages – up to 310V, compared to the 5, 12 or 24V commonly used by solenoid valves. A simple set of piezo driver electronics is therefore required to generate the necessary 250-310V from the 5, 12 or 24V power supply, as well as to charge and discharge the piezo bender. Though 310V may seem high, the corresponding current is very small – a maximum of 5mA). The piezo electronics can be designed to be electrically safe, so that the voltage instantly drops to well below 50V if the contacts are touched.
A further advantage to piezo valves is that they are non-magnetic and therefore don’t interfere with any other medical equipment – this can be critical in preserving the integrity of medical systems. These valves can also be used to control 100% oxygen gas, since their contact materials are tested and approved for oxygen compatibility.
Festo is embracing piezo technology and has recently introduced the new VEMD proportional flow control valve which provides extremely high control dynamics and is a cost-effective solution compared with traditional technologies. Furthermore, it is extremely quiet, energy-efficient and requires only minimal space.
The valve supplies inert gases and oxygen with flow rates from 0 to 20 ln/min, and is easy to control: the gas flow at the output of the valve can be adjusted simply and controlled in a linear mode by entering a setpoint between 0 and 10V. An integrated control circuit with thermal sensor makes the VEMD precise and dynamic. It responds up to 35 times faster than comparable products to a setpoint change, and is instantly ready for operation.
Piezo valves are providing efficient regulation and control of medical gases in a number of areas: for example, flow control with compact valves for ventilation, anaesthesia devices and oxygen therapy, pneumatic integration solutions for surgical devices, and closed-loop control of fluids in dental service units. Now, the combination of small dimensions, low power consumption and low costs makes the VEMD valve the best choice for many applications in medical devices such as portable oxygen devices.
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