Pictures of the Future Fall 2005
Corporate Technology – Piezo Technology
Promising Power
Piezo technology offers prime examples of sensor and actuator systems. Siemens has made key contributions to almost every area of the technology’s application—the latest is the revolutionary piezo injector for diesel engines.
We’ve come a long way from the first piezo actuators developed in Siemens labs (below left) to their successful use in diesel engines (above: injector and injection process). Today, clean-room technology is needed to manufacture them (below)
At first glance, there’s nothing special about this ceramic cube, which is three centimeters high and seven millimeters square. But when viewed through a microscope, the cube reveals a very special characteristic. It consists of 340 thin layers, each 0.08 mm thick, separated by metal layers (see graphic above). Made of lead zirconate titanate, the cube responds "piezoelectrically"—it expands within a fraction of a millisecond when subjected to a voltage—and its stack stretches by 0.04 mm. As a result, this tiny block exerts a force sufficient to lift 220 kg.
"In the past, it was assumed that piezo technology could be used to process signals, but not to create any significant mechanical energy," says Max Guntersdorfer, formerly responsible for Microsystems at Siemens Corporate Technology (CT). Today, the piezo actuator is ushering in a new era: It can exert force and move objects. But it was initially developed for only one application: the piezo injector. In September 2000, Siemens VDO Automotive started mass production of piezo injectors for diesel engines and immediately became the technology leader in that sector. In comparison with the solenoid valves previously used, piezo injectors can be controlled much more precisely, so they can inject fuel in several exactly proportioned amounts during an engine cycle. This can lead to fuel savings of 20 to 30 %, as well as sharply reduced exhaust emissions.
The first piezo injector concept was developed by Siemens researchers as early as 1980. Over the years, well over 100 patents have resulted. In 1996, the Board of Siemens Automotive Systems Group decided to focus on bringing injector nozzles for diesel engines—rather than gasoline engines—to market maturity, despite Siemens having no experience at all in this sector. Even today, Hans Meixner and his team still delight in the coup they pulled off. "Everyone knew we were working on piezo injection technology for gasoline engines, but no one expected us to bring diesel injectors to market," says the former Corporate Department Head of the Sensor & Actuator Systems Competence Center at CT. The piezo actuators’ multilayer technology made it possible. Materials researchers from Siemens and colleagues from Siemens Matsushita Components (now Epcos AG) solved the high-voltage problem. Such voltages would be needed to significantly deform larger components. In contrast, 160 V is sufficient to trigger the piezo effect in an individual thin layer—so the trick is to stack lots of these layers and switch them in parallel order.
Developing a stack with sufficient load capacity and service life proved to be demanding, however. Even today, Epcos is the only company to mass-produce piezo stacks for injection valves. A piezo injector for gasoline engines is just around the corner. "Every automaker and supplier is backing this technology," reports Meixner, who was nominated for the 2005 German Future Prize (see In Brief), along with Klaus Egger, Member of the Group Board at Siemens VDO, and Friedrich Boecking of Robert Bosch GmbH.
This success story is ultimately based on the long-term experience that Siemens has gathered in piezo technology and its implementation in groundbreaking inventions. As far back as the 1950s, a team established its expertise in electroceramics and PTC thermistors. At the beginning of the 1960s, the team focused on the development of piezo materials. The main objective was to produce an extremely stable and selective filter for telecommunications technology. With piezo filters, a very large number of calls could be transmitted per channel. "Thanks to its superior concept, Siemens won the race against Bell," says Guntersdorfer. The materials research that had been conducted for this 100 kHz filter under the leadership of Helmut Thomann soon began to yield products. Siemens researchers developed piezoceramic films, which were used in telephone microphones from 1972, and piezo membranes later also replaced loudspeakers and bells. "That was piezo’s very first application as a sound generator," Guntersdorfer says. Today, these buzzers chirp away in numerous electrical appliances and generate ring tones in mobile phones.
The Piezo Tree
The field of piezo technology is extensive and has many branches. The "piezo tree" already bears numerous fruits, but continues to develop new blossoms. Piezo elements serve, for instance, as filters for electromagnetic waves (surface acoustic wave filters, filter ceramics). They convert electromagnetic signals into sound (loudspeakers, buzzers) and vice versa (telephone microphones, knock sensors), and they transform mechanical pressure into electrical signals (batteryless wireless technology, piezo igniter). Some elements serve as ultrasonic sensors, while others create ultrasound in echo sounders, in medical diagnostics, in flow-rate meters or for fluid atomization. Piezo actuators are a new, fast-growing branch that enables applications ranging from piezo injector valves right up to powerful piezo motors. Such motors are already available, for instance, from Elliptec Resonant Actuator AG—a start-up that emerged from Siemens—for the toy and consumer market. Multilayer actuators are also suitable for a completely new type of power window for automobiles that contains integrated sensor technology
Siemens was also the leader in developing piezo transducers for ultrasound diagnostics in healthcare and for the first inkjet printers, in which a jet of ink was ejected from thin piezo tubes. Corporate Research provided the momentum for many of these innovations. For example, Andreas Kappel, who had previously worked on the development of piezo inkjet printers and has registered numerous patents since the 1990s, also gave piezo injectors a very big push. He developed hydraulic translators that could increase the stroke of a piezo actuator and compensate for changes of length due to temperature. "The inkjet printer tubes are fundamentally nothing other than mini injection nozzles," says Kappel, who received the coveted Siemens accolade "Inventor of the Year" in 2002.
Another coup was pulled off by the Piezo Laboratory, in the form of the surface acoustic wave (SAW) filter made of lithium niobate, which replaced electromagnetic filters in Grundig television sets in 1977 and markedly improved picture and sound quality. Unlike the large, costly 100 kHz filters for telecommunications technology, SAWs were small and inexpensive. Although they quickly became popular, they didn’t make real money until the mobile phone boom took off. "Mobile phones would be unthinkable without SAWs," says Guntersdorfer.
In 1989, the piezo team scored a huge success in automotive engineering with the knock sensor, an electronic ear for the engine that optimizes fuel efficiency. "There are now 40 million engines equipped with Siemens knock sensors worldwide," reports physicist Randolf Mock. Mock considerably advanced the development of the sensor by means of computerized simulations.
Batteryless radio technology is a new development from the "piezo works." By simply pressing a light switch, a piezoelectric energy transducer is able to generate enough energy to transmit a radio signal to the light source. In 2001, Siemens employees founded start-up EnOcean in order to market the invention. In the meantime, other ideas have been implemented, including a tire sensor that works without batteries.
Siemens researchers have also discovered that piezo actuators are good at damping sound, for instance in magnetic resonance tomographs. Because they’re also vibration sensors, they can first register vibrations and then counteract them with the appropriate movement. Therefore, as "Learning Soft Sensors," piezo elements could, in many cases, render other sensors obsolete in the near future. In this capacity, for instance, the actuator in the injection nozzle could provide more information on cylinder pressure. "Piezo has become an important fundamental technology; and no one can predict its outcome," says Kappel.
Ute Kehse