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08/06/2009
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Tom Shelley reports on two new actuator-sensor technologies unveiled at the Hannover Fair
 Magnetic shape memory (MSM) actuators and patch transducer actuators were on show at the Hannover Fair held in April. The technologies, dubbed DuraAct, are able to produce precise small motions and can also function as sensors.
The MSM actuators are capable of producing large forces in small spaces and produce strokes of 3 to 5%. Although this may not sound like much, this is 50 times greater than the best conventional magnetostrictive device available. The patch transducer actuators are neatly packaged flat pack piezoelectric devices, with the active parts sandwiched between two layers of glass fibre reinforced polymer.
The example shown at the Hannover Fair was an extending push rod in a solenoid coil. In the presence of a magnetic field, ranging from 0.1 to 0.7 Tesla, the amount of crystal in the alternative twinned orientation increases progressively as the field increases. This in turn increases the length of a rod actuator in proportion to the field strength.
The material tolerates humidity, which allows the material to be used for fuel injectors, valve lifters, and to operate pneumatic and hydraulic valves. In fact, both devices are beginning to find uses in the automotive, aerospace and renewable sector.
MSM material is the invention of Dr Kari Ullakko. He started Adaptamat, a Helsinki based company started in 1996. The big difference between MSM and conventional magnetostrictive materials is that it consists of two possible twinned variants, with different crystallographic orientations.
The actuators are made from ferromagnetic single crystals of 50% nickel, 30% manganese and 20% gallium. This may sound expensive, but for most intended applications, the amount required is very little, and Dr Ullakko says that small device typically costs no more than $0.50.
Originally Dr Ullakko was only able to produce magnetic field induced strains of 0.2% in an MSM material when he was at the Massachusetts Institute of Technology (MIT) in the early to mid-nineties. However, he has since been able to increase this to more than 6%.
An interesting phenomenon is that the patch actuator transducers produce small movements when a voltage is applied to the piezoelectric material and conversely produce a small amount of electrical charge from a change displacement or force. This means the devices can also function as sensors by measuring the difference in magnetic permeability arising from strain, stress, or magnetic field.
And both devices can also combine both these functions. This opens up a number of potential applications in vibration damping and composite structure health diagnosis. By building separate layers into the same device, so that one layer acts as a driving transducer while another measures movement. Or, they can be mounted separately with some patches producing vibrations while others detecting the waves that they produce.
With composite materials this can be used as a means of detecting micro cracks by comparing the detected vibrations with those produced when the composite is undamaged. Methods of detecting damage in composites in service, particularly in commercial airliner wings, have been under investigation for years. Presently, such structures are subjected to initial and subsequent surveys, using a combination of visual observation and ultrasonic examination.
The other potential application is in vibration damping, using patch transducer actuators as sensors and either using other patch transducer actuators to produce damping counter vibrations in a servo loop or by linking them to other high-speed devices, such as the MSM actuators.
Such systems could also be used as a means of changing material shapes in response to changing requirement. This is a field of considerable interest to the makers of unmanned aerial vehicles who want to be able to change wing shapes in order to optimise them for landing, flying slowly for observation purposes or for high speed cruise speed.
The other application cited, particularly for the patch transducer actuators, but also claimed as a possibility for the MSM actuators, is harvesting energy from vibrations. Power output from the patch transducers is mW rather than Watts, but this is more than sufficient to power applications such as Bluetooth low power radio data links.
The demonstration unit on the stand at Hannover had a patch transducer actuator on a thin beam. Flicking the beam produced quite sufficient power to light up a couple of LEDs, and nobody managed to break the device, despite all the students making visits. Applications are seen in structural and human medical health monitoring.
Mario Rauer, sales and marketing manager said: "The breakthrough is the way that the thin piezoceramic foil is mounted between two conductive films, and embedded within a composite polymer structure. The brittle piezoceramic is mechanically pre-stressed and electrically insulated. And because of the pre-stress, you can bow it. And that means the patches can be applied to curved surfaces with a radius of as little as 20mm."
The actuators function in two ways, either they can be made to contract when energised using the inverse piezo effect or attached to a substrate and be used to make it bend. Although typical linear contraction is only around 1µm/m/V, with a maximum applied voltage of up to 1,000V, the maximum bending deflection when bonded to a thin piece of metal is about 0.8mm.
Rauer added: "The actuators can be positioned very precisely for micro positioning, particularly in the micro electronics industry and where electronics compensate for temperature effects."
Pointers
Magnetic Shape Memory actuators have been developed that can deliver high force strokes of up to 3% to 5%, 50 times greater than is possible than conventional magnetostrictive materials
Piezoelectric patch transducers can also be used to make precision actuators
Both types of device can also be used as sensors and energy harvesters, although such applications are more advanced for the latter than the former
Combinations of fast actuators and sensors, either of the same or both types can be used for composite structural health monitoring, active vibration damping and the fabrication of "Smart" adaptable structures
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Author
Tom Shelley
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