Pneumatic valves made with piezo technology offer many advantages. They are small, lightweight, extremely precise, durable, fast, and save energy. Piezo valves do not need energy to maintain a switching status, and therefore generate almost no heat. What's more, piezo valves can potentially be operated without any noise. Another key advantage is that they always work proportionally.
Piezo valves can be a better alternative to conventional solenoid valves, especially in applications requiring directly controlled proportional valves. They provide gentle and safe speed control for pneumatic cylinders, and work well in medical applications, laboratory automation, manufacturing, and even motor vehicles.
What is a Piezo Element?
Piezo elements are electromechanical transducers. With the 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. Basic piezo elements can take the form of disc transducers, bender actuators, and piezo stacks.
The bender actuator has a rectangular shape. Its primary element is a piece of piezo ceramic material that has been rendered conductive on both surfaces (Figure 1). This ceramic material is joined on one side to a conductive substrate. 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. In most applications bender actuators are fixed at the front end, so 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 well suited for use in pneumatic valves.
A disc transducer is also a very simple piezo element. It takes the form of a thin ceramic disc bonded to a metal substrate. To generate an electric field, the circular area on the surface of the disc must be metalized. 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 metallized area and the passive substrate act like a bimetallic strip and cause the overall system to bend in a spherical direction. This bending effect can be used in micropumps.
Stack transducers (piezo stacks) are stacked piezo discs that 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 — maximum 0.2% of the overall height — but with enormous actuating forces ranging up to several kilonewtons. Applications include liquid valves and micro positioning.
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. Unlike solenoid valves, piezo valves do not need holding current to maintain a switching state. The piezo valves require a higher supply voltage than do solenoid valves, but that is significant only during the switch-on phase. Even then, the switch-on energy consumed is well below the actuation power levels normal in pneumatics.
A boost converter has proven to be ideal for generating the high voltage needed to operate high-performance valves. Boost converters are inexpensive and require 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 realized by using the microprocessor already present in the system controller. However, there are also integrated circuits ready-made 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. Nevertheless, piezo valves offer many advantages over the prominent solenoid valves and open entirely new areas of application.
Low energy consumption and no heat generation: Thanks to their capacitive principle, piezo valves require virtually no energy to maintain an active state. The valves do not generate heat if high-frequency control is not used (because that requires frequent switch-on energy). 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 (Figure 2).
Intrinsic safety: “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, which makes them usable in many potential applications.
Switching speed: Piezo valves can be incredibly fast, easily reaching the sub-microsecond range. These valves are often the best solution for applications where speed plays a decisive role. Such applications include high-speed sorting systems and closed-loop control circuits in general, as this type of circuit usually works better the faster the individual components react.
Proportionality: 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 well suited to function as actuators for all higher-level control systems.
Anti-magnetic: Piezo technology can also be used without any risk of failure in areas with a high magnetic field strength, such as in magnetic resonance tomography (MRT).
Minimal weight: The fact that piezo valve housings are usually plastic (without iron and copper) make them very portable.
Low costs: This technology can be mass-produced if large quantities are required. For example, piezo-ignited lighters 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 wearing parts that might be subject to friction.
It's important to note that these advantages cannot all be exploited at the same time in a single valve. Valves are generally designed for particular applications, and each would make use of its own specific benefits.
Piezo valves are an excellent choice for many applications that require precise positioning. These include manufacturing systems, surgical tools, laboratory automation, and use in non-destructive handling of fragile workpieces.
A unique solution for handling sensitive workpieces is the so-called “speed controller” for double-acting pneumatic cylinders. This makes it possible to execute speed-controlled jerk-free motions with a pneumatic cylinder. For this purpose, four piezo valves are combined with a controller unit to form a system that can be connected to a double-acting cylinder. Speed is controlled by maintaining a constant cylinder exhaust flow rate. This low-cost system works without an expensive displacement encoder. What's more, it allows smooth starting and braking.
Piezo valve technology is used in Festo's Bionic Handling Assistant. Piezo valves were chosen because of their ability to meter compressed air precisely and purposefully. In addition, the installation space required was considerably less than for other valves.
Another example is the use of piezo valves in a welding system that joins several individual components into four complete car doors within 30 seconds (Figure 3). The system requires speed and maximum precision — each door has to be welded accurately down to the last millimeter, processing a total of 148 welding points in 30 seconds. The cylinders in the welding tool are controlled by piezoelectric pressure regulators first for position, and later, during the welding process, for pressure.
This article was written by Hannes Wirtl, Head of Development Piezo Valves, and Ulrich Sixt, Product Management Piezo Valves, Festo AG & Co. KG, Germany. For more information, Click Here .