Piezo technology in pneumatic valves
Piezo valves are often a better alternative to conventional solenoid valves, especially in the areas of flow and pressure control and as directly controlled proportional valves.
Piezo valves are small, lightweight, extremely precise, very durable and incredibly fast — and, above all, they save energy. For example, piezo valves do not need any energy to maintain a switching status. They therefore generate virtually no heat. In ATEX applications, many piezo valves are classified as intrinsically safe. What’s more, piezo valves can potentially be operated without any noise. Another key advantage is that they always work proportionally — and are very wear-resistant, too.
These properties make piezo valves ideal for use in, for example, the semiconductor industry. Here, their accuracy and ability to quickly reach preselected set points ensure precise metering of even the smallest amounts of air or gas, and precise regulation of the pressure and vacuum used to press silicon wafers onto a polishing table. Other areas of use include adhesive applications with very accurate metering in small-parts assembly or gentle and safe speed control for pneumatic cylinders. Applications in medical technology, laboratory automation and even motor vehicles also benefit from piezo valves.
Piezo elements are electromechanical transducers. With the so-called direct piezoelectric effect, a piezo element converts mechanical forces (pressure, tensile stress or acceleration) into a measurable voltage. The inverse piezoelectric effect is precisely the opposite: a piezo element is deformed when a voltage is applied to it, thus generating mechanical motion or oscillations.
The piezoelectric effect was discovered in 1880 by the brothers Jacques and Pierre Curie. They found that, when subjected to a mechanical load, certain non-conductive materials produce electrical charges on their surface, which has been rendered conductive.
Piezoelectric materials, usually special ceramic objects with surfaces which have been rendered conductive, convert electrical energy into mechanical energy and vice versa. The lattice structure of the molecules in these piezoceramics is asymmetrical below the Curie temperature (TC) and is thus a dipole. Under the influence of strong electric fields, it is possible to permanently polarise piezoceramics, or, in other words, give them a preferred direction. The ceramic material then has piezoelectric properties and changes shape when a voltage is applied. 3D deformation takes place along the field lines. Since the ceramic materials have a constant volume, shrinkage occurs in the material at right angles to the field lines.
The advantage of piezo-based drives lies in the fact that they can be energised with almost zero power. In electrical terms, a piezo element is a capacitor consisting of two electrically conductive plates and the ceramic piezo material which functions as a dielectric. Current only flows while the capacitor is charging, and the flow drops to zero when charging is complete. Since electrical power is calculated as voltage x current, the power will be zero if no more current flows. In applications that need to be extremely energy efficient, it is even possible to recover the charging energy when the drive is reset. This can then be used again for the next charging operation.
Types and versions of piezo transducers and their applications
Depending on the needs of a particular application, the effect described earlier can be exploited using various types of transducers. Disc transducers, bender actuators and piezo stacks are the basic forms from which piezo elements with more or less complex structures can be derived.
The bender actuator has a rectangular shape. Its primary element is a piece of piezoceramic material which has been rendered conductive on both surfaces. This ceramic material is entirely joined on one side to a substrate which is also conductive. The conductive surfaces of the ceramic layer and the substrate function as electrodes. If a voltage is now applied to these electrodes, the ceramic material expands in the direction of the electric field. Since in most applications bender actuators are fixed at the front end, this results in a bending motion at the free end. Bender actuators are available in multiple versions with different forces and actuator motion, and are highly suitable for use in pneumatic valves. Typical characteristic data include deflection amounting to several tenths of a millimetre and forces of up to 1 N. One special variant which is often used is the trimorph, which has a second ceramic layer on the reverse side of the substrate. This increases the performance of the transducer and can be used in a wider temperature range thanks to its symmetry. Applications for bender actuators can be found in circular knitting machines, blind reading devices (Braille modules) and pneumatic valves — with the latter especially in proportional valves for pressure and flow control.
A disc transducer is also a very simple piezo element. It takes the form of a thin ceramic disc which is bonded to a metal substrate. In order to generate an electric field, the circular area on the surface of the disc has to be metalised. If a voltage is now applied to this substrate and the electrode on the ceramic, the ceramic (as is also the case with the bender actuator) expands in the direction of the electric field and the disc becomes thicker, while at the same time its diameter becomes smaller. Together, the metallised area and the passive substrate act like a bimetallic strip and cause the overall system to bend in a spherical direction. This bending effect is used in, among other applications, high-frequency loudspeakers, sensors, micropumps, fans and ultrasound generators, such as are often used in automobile distance sensors.
Stack transducers (piezo stacks) are stacked piezo discs which are connected in series mechanically and in parallel electrically. In contrast to disc transducers, operation is not triggered by the bending of a composite material but by direct expansion in the direction of the field. This configuration allows only short strokes — a maximum of 0.2% of the overall height — but with enormous actuating forces up to several kN. Applications can be found in the areas of liquid valves, such as diesel fuel-injection systems, and micropositioning.
Operation of piezo valves
Piezo elements in the shape of bender actuators are primarily used in pneumatic valves. The performance of piezo valves depends on the strength of the electric field: the greater the field strength, the better the performance of the actuator and the valve. Compared to solenoid valves, piezo valves need no holding current to maintain a switching state. The higher supply voltage required by piezo valves in comparison with solenoid valves is of significance only during the switch-on phase. Even then, the switch-on energy consumed is well below the actuation power levels normal in pneumatics.
This switch-on energy ‘E’ can be calculated as an approximation using the formula E = CV2/2, where C is the capacitance of the transducer and V is the control voltage. Values usually lie between 0.5 and 5 mW because the capacitance of the transducer is generally around 30 nF, while the control voltage can be up to 300 VDC.
Important to know: the switch-on energy of piezo valves is always specified in milliwatt seconds only. It is not possible, as it is with solenoid valves, to specify power ratings in watts.
When a piezo valve has been switched on and the connection to the power supply is then interrupted, the valve status is maintained because the charge carriers are no longer capable of flowing away due to the interruption. To reset the valve, the charge must be actively removed from the transducer. This can be achieved through buffer storage in another system (energy recovery) or by converting the energy to heat (short circuit). A changeover switch instead of an on-off switch is therefore required in order to operate the valve.
High-performance valves need to be operated using high voltage. The principle of a boost converter has proved itself ideal for generating this high voltage. A boost converter is extremely inexpensive and requires little space. With this device, the very high induction voltage generated during the cyclical switch-off of a coil (storage choke) is fed via a diode and stored in a capacitor — in the simplest case, the piezo transducer can also be used as a capacitor.
This circuit allows an output voltage of 300 V with an input voltage of as little as 1 V. The oscillator for the switch can often be realised by using the microprocessor already present in the system controller. However, there are also ready-made integrated circuits especially for this application. Modules of this kind also manage output voltage regulation and ensure maximum efficiency, which is well over 80%.
Advantages of piezo valves
In the world of electrically controlled pneumatic valves, solenoid valves are the absolute standard with a market share of almost 100%. Nevertheless, piezo valves offer many advantages over the prominent solenoid valves and open up entirely new areas of application.
Low energy consumption
Thanks to their capacitive principle, piezo valves require virtually no energy to maintain an active state. The valves do not generate heat, provided that high-frequency control is not used due to the fact that switch-on energy is frequently required. The energy balance increases along with the required switching frequency.
Piezo technology is ideal for use in the ‘very low power’ range of battery-powered devices. Compared to solenoid valves, it can increase the service life of a battery pack several times over in some cases.
Intrinsic safety is increasingly specified as the required degree of protection for environments with potentially explosive atmospheres. An electrical system is intrinsically safe if the greatest amount of energy it can store is not enough to cause ignition of the atmosphere in the event of a fault. Piezo valves are an ideal way of meeting this requirement, thus resulting in a large number of potential applications.
Piezo valves can be incredibly fast, easily reaching the sub-microsecond range. These valves are the ideal solution for applications where speed plays a decisive role. Applications include high-speed sorting systems and, in particular, closed-loop control circuits, as this type of circuit usually works better the faster the individual components react.
Proportionality is an intrinsic characteristic of piezo technology. Since ultimately all pneumatic processes in an application are analog, this is an unbeatable advantage: there is no need for pulse width modulation and the associated noise problems as a means of trying to achieve a certain proportionality when switching solenoid valves. This means that piezo valves are very resistant to wear and need only minimal energy input. Combined with their short response times, the proportionality of piezo valves make them ideal actuators for all higher level control systems.
Piezo technology can also be used without any risk of failure in areas with a high magnetic field strength, for example, magnetic resonance tomography (MRT).
The fact that housings are usually plastic, and, in particular, the absence of iron and copper, make them very portable.
This technology can be mass produced if large quantities are required, for example, piezo-ignited lighters, which are available for very little money.
Long service life
When a system is designed correctly, piezo drives can achieve an unusually high number of operating cycles. They consist of a single solid-state working component with no further wearing parts which might be subject to friction.
The one restriction
The advantages described above cannot all be exploited at the same time in a single valve. Valves are generally designed for a particular application in which individual, specific benefits come to the fore.
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