102 C O R P O R A T E T E C H N O L O G Y | P i e z o Te c h n o l o g y
We’ve come a long way from the first piezo actua- tors developed in Siemens labs (above) to their suc- cessful use in diesel en- gines (right: injector and injection process). Today, clean-room technology is needed to manufacture them (facing page).
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.
t f i r s t g l a n c e , t h e r e ’ s n o t h i n g s p e c i a l a b o u t t h i s c e r a m i c A cube, which is three centime- ters high and seven millimeters square. But when viewed through a microscope, the cube reveals a very special characteristic. It con- sists of 340 thin layers, each 0.08 millimeters thick, separated by metal layers (see graphic above). Made of lead zirconate titanate, the cube responds “piezoelectri- cally” — it expands within a frac- tion of a millisecond when sub- jected to a voltage — and its stack stretches by 0.04 millimeters. As a result, this tiny block exerts a force sufficient to lift 220 kilograms.
“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 Microsys- tems at Siemens Corporate Tech- nology (CT). Today, the piezo actu- ator 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 immedi- ately became the technology leader in that sector. In compari- son with the solenoid valves previ- ously 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 percent, as well as sharply re- duced exhaust emissions.
The first piezo injector concept was developed by Siemens re- searchers 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 bring- ing injector nozzles for diesel engines — rather than gasoline engines — to market maturity, de- spite Siemens having no experi- ence at all in this sector. Even to- day, Hans Meixner and his team still delight in the coup they pulled off. “Everyone knew we were
working on piezo injection tech- nology for gasoline engines, but no one expected us to bring diesel injectors to market,” says the for- mer Corporate Department Head of the Sensor & Actuator Systems Competence Center at CT. The piezo actuators’ multilayer tech- nology made it possible. Materials researchers from Siemens and col- leagues from Siemens Matsushita Components (now Epcos AG) solved the high-voltage problem. Such voltages would be needed to significantly deform larger compo- nents. In contrast, 160 volts is suf- ficient 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 suffi- cient load capacity and service life proved to be demanding, how- ever. 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 au- tomaker and supplier is backing this technology,” reports Meixner, who was nominated for the 2005 German Future Prize (see p. 4), 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 experi- ence that Siemens has gathered in piezo technology and its imple- mentation in groundbreaking in- ventions. As far back as the 1950s,
a team established its expertise in electroceramics and PTC ther- mistors. At the beginning of the 1960s, the team focused on the development of piezo materials. The main objective was to pro- duce an extremely stable and se- lective filter for telecommunica- tions 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 de- veloped piezoceramic films, which
CO R P O R AT E
were used in telephone micro- phones 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 appli- ances and generate ring tones in mobile phones.
proved picture and sound quality. Unlike the large, costly 100 kHz fil- ters for telecommunications tech- nology, SAWs were small and inex- pensive. 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.
Siemens was also the leader in developing piezo transducers for ultrasound diagnostics in health- care and for the first inkjet print- ers, in which a jet of ink was ejected from thin piezo tubes. Cor- porate Research provided the mo- mentum for many of these inno-
vations Kappel, worked
. For example, Andreas who had previously on the development of
piezo inkjet printers and has regis-
In 1989, the piezo team scored a huge success in automotive en- gineering 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 physi- cist Randolf Mock. Mock consider- ably advanced the development of the sensor by means of computer- ized 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, in- cluding a tire sensor that works without batteries.
tered numerous patents since the 1990s, also gave piezo injectors a very big push. He developed hy- draulic translators that could in- crease the stroke of a piezo actua- tor and compensate for changes of length due to temperature. “The inkjet printer tubes are fun- damentally nothing other than mini injection nozzles,” says Kap- pel, 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 nio- bate, which replaced electromag- netic filters in Grundig television sets in 1977 and markedly im-
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 coun- teract them with the appropriate movement. Therefore, as “Learn- ing Soft Sensors,” piezo elements could, in many cases, render other sensors obsolete in the near fu- ture. In this capacity, for instance, the actuator in the injection nozzle could provide more information on cylinder pressure. “Piezo has become an important fundamen- tal technology; and no one can p r e d i c t i t s o u t c o m e , ” s a y s K a p p e l . ■ U t e K e h s e
Ultrasound imaging systems
Inhaler Fuel atomizer Spray can replacement
Flow-rate meter Echo sounder
Liquids / gases
Knock sensor Crash sensor
Level gauge Burglar alarms Conveyor belt sensor
Orientation systems for the blind
Piezo injection valves (diesel /
gasoline engine) Inkjet printer Insulin pump Piezo motors
Surface acoustic waves (SAW)
Ceramic films (sound)
Lithium niobate filter (for mobile phones, television sets etc.)
Batteryless wire- less technology
Pushbutton / piezo igniter
Selective filter for telecommunications technology
Ring tone / buzzer for mobile phones
Blood pressure microphone
■ In use ■ Close to market launch
Examples for applications of the piezo technology
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 electro- magnetic signals into sound (loudspeakers, buzzers) and vice versa (telephone microphones, knock sensors), and they transform mechanical pressure into electrical signals (batteryless wireless tech- nology, piezo igniter). Some elements serve as ultrasonic sensors, while others create ultra- sound in echo sounders, in medical diagnostics, in flow-rate meters or for fluid atomization. Piezo actuators are a new, fast- growing branch that enables appli- cations ranging from piezo injector valves right up to powerful piezo motors. Such motors are already available, for instance, from Elliptec Reso- nant Actuator AG — a start-up that emerged from Siemens — for the toy and consumer market. Multilayer actuators are also suit- able for a completely new type of power window for automobiles that contains integrated sensor technology.
Pictures of the Future | Fall 2005