Pictures of the Future    Spring 2006

Future Motors Take Shape

Siemens researchers are developing new kinds of drives made of piezoceramics, plastics and nickel-titanium alloys. Voltages or temperature changes can be used to deform these smart materials in a controlled fashion. This characteristic enables them to power motors while also acting as sensors.

_{Siemens researcher Gottlieb with his powerful piezo motor. Such motors could one day replace conventional electric motors in cars (below)}

It’s a stroke of genius small enough to fit in the palm of your hand. Dr. Bernhard Gottlieb, a physicist at Siemens Corporate Technology (CT), displays an object that could be the corner of a modern picture frame. But the device is actually a new kind of motor and piezoelectric drive in a compact steel casing. It is strong enough to close a car window, but can also detect the slightest mechanical resistance. Known as a PAD (Piezoelectric Actuator Drive), the device has other advantages over conventional electric motors: It needs no gearbox, revolves smoothly even at low speeds, and runs almost noiselessly. What’s more, it responds very swiftly and stops with a precision of a few fractions of a second of arc, or about 1 cm over a distance of 2 km. And that’s not all: The PAD also saves energy, because it consumes no power when stopped.

Gottlieb can think of many uses for this idea. The new motor could almost noiselessly open and close windows and air vents or position welding lasers with pinpoint precision. It would be small enough to serve as a medical robot that could distinguish between arteries and bones thanks to its high sensitivity. What’s more, the touch of a finger would suffice to move it aside when the surgeon wants to take over manually. PADs are being considered as drives for SIPLACE automatic placement machines that insert electronic components into printed circuit boards. They may also compete with electric motors in automotive applications. "They’re suitable for use as drives for electromechanical parking brakes, as power-window lifts with pinch protection, for seat adjustment, or as air vent actuators," says Roland Keller, PAD project manager at Siemens VDO. "They could also adjust the air volume in an airbag precisely to the occupant’s weight." He adds that there are already some 30 potentially interesting automotive applications, and that initial negotiations with automakers are already under way.

The operating principle of the new motor is based on piezoceramic materials, which can be made to expand or contract by applying an a.c. voltage (see Pictures of the Future, Fall 2005, Piezo). These minuscule movements, of the order of a hair’s width, power the motor. Conversely, the slightest mechanical pressure causes a shift in electric charges—which can be detected as a change in voltage. Piezo materials consequently function both as a motor and as a pressure sensor. "The real challenge was how to translate the microscopic expansions of the piezo elements into macroscopic movements," says Gottlieb. To achieve this, the researchers mounted two almost match-sized piezo elements—each capable of exerting 220 kg of force—at right angles to each other on a steel ring. When out-of-phase a.c. voltages are applied to each of the two piezo elements, their combined movements cause a ring to follow a slightly oscillating orbital path around a tightly fitting motor shaft. As a result of the motion, the shaft and the ring come into contact. The revolving contact point between the ring and the shaft causes the shaft to rotate. The process is a bit like inverting the principle of the Hula-Hoop. Using a laser, the scientists also carved tiny gear teeth into the shaft and the ring. "With these miniature gear teeth we can achieve torques of up to 7 Nm," says Gottlieb. This arrangement makes the PAD much stronger than the piezo motors currently available for such mundane tasks as moving doll eyes or actuating toy train components.

Thanks to this breakthrough design, Gottlieb was honored as one of twelve Siemens Inventors of the Year 2005. Others who made significant contributions to this development include Dr. Tim Schwebel, Carsten Wallenhauer and project manager Dr. Andreas Kappel, as well as a group headed by Joachim Heinzl, Professor at the Technical University of Munich. A Siemens team under Prof. Hans Meixner originally developed the piezo elements for piezo-controlled fuel injection in diesel and gasoline engines. Suitable materials and process technologies were provided by ceramics experts headed by Dr. Carsten Schuh. And the latter was no simple task. To move a single valve using a modest 160 V, for instance, several hundred ultra-thin piezo sheets had to be assembled into a single stack. Hans Meixner, Dr. Klaus Egger of Siemens VDO and Friedrich Böcking of Robert Bosch GmbH were jointly awarded the German Future Prize 2005 by the German president for their piezo injection technology.

Good-bye Gearboxes. Michael Mönch, who works in Strategic Marketing at Siemens CT, also thinks piezo technology is a winner. "There is broad agreement that drive systems based on molecular forces will be key for future motor generations," he says. The technology’s potential derives above all from its integral sensory capabilities and its capacity to exert forces large enough, in many cases, to eliminate the need for a gearbox.

These materials also include metal alloys with shape memory. Wires or spring coils made of Shape Memory Alloys (SMAs) can be cold-formed into a wide range of shapes, which they return to when heated—no matter how much they may have been deformed in the meantime. A hair-thin titanium-nickel wire, for example, can change by one-twentieth of its length and support a weight of 100 g. These metal alloys are already in common use in household appliances, such as dishwashers and coffee makers made by Bosch and Siemens Hausgeräte GmbH (see Pictures of the Future, Spring 2003, Adaptronics). But they can also be used to widen human arteries or unfold solar arrays in outer space.

"We also intend to use these smart actuators in drives," says Dr. Heinz Zeininger of Siemens CT in Erlangen. Zeiniger combines wires and springs into powerful motion devices. One such device is made from two wires that are tied together. When current is passed through one wire, it heats up and stretches the second wire. Current is then passed through the stretched wire, which contracts, stretching the first wire as it does so. The sequence is then repeated. In the laboratory, this two-wire drive is already setting crankshafts in motion. A suitable sheathing material ensures rapid cooling during the stretching process. However, Zeininger adds, more work is needed to fine-tune the physical interplay of the wires. He envisions future SMA actuators and drives capable of controlling a car’s rear-view mirror or air vents—or to pump geothermally heated water from the depths of the earth. The required heat, in that case, would be available for free.

Plastic Hearts. A similar expansion in length is a feature of plastics known as electroactive polymers (EAPs). In a liquid, these long molecular chains stretch or contract when a current passes through them. This is due to a chemical reaction in which the polymers are oxidized or reduced. During these transitions, their molecular bond angles change. Such polymers could, for instance, be used in artificial muscles. Dr. Frank Arndt heads the elastomer actuator project at Siemens in Berlin. "We apply a voltage to two thin metal layers, between which a layer of resilient silicone is sandwiched," he says. If these metal plates are oppositely charged, they attract each other electrostatically and squeeze the silicone. When the voltage is removed, the silicone expands and pushes the plates apart. Such a "plastic heart" would be inexpensive. The design has achieved expansions of 30 %. "We’ll demonstrate that this plastic heart really beats before the end of the year," says Arndt.
                                                                                                        Andrea Hoferichter