Magnetic materials deliver movement

Tom Shelley investigates a new class of magnetically driven actuator materials achieve order of magnitude bigger movements

Rods of Magnetic Shape Memory material can be made to change their length by up to 6% in a magnetic field in less than a millisecond.

The effect is 50 times greater than in Terfenol D, the material with the greatest response to magnetic field from the previous generation of commercially available products.

Potential applications include automotive valve lifters, vibrators, transducers and a host of micro and nano scale pumps and other devices.

The materials were jointly invented in the early 1990s by Dr Kari Ullako and Dr Robert O'Handley at the Massachusetts Institute of Technology, where Dr Ullako was at the time a visiting scientist. Dr Ullako subsequently set up the Finnish company AdaptaMat in 1996 with colleague Ilkka Aaltio.

Magnetic field induced strains of 0.2% were obtained by Dr Ullako at MIT that same year. Strains of over 10% are said to have now been achieved, although claims for materials offered commercially are that they achieve strains of more than 6%. Operating temperatures for these are in the range -40 deg C to + 60 deg C although it is expected that new developments will raise this to well over 120 deg C. Typical magnetic field strength to produce maximum strain is 400kA/m.

MSM materials are quite different from magnetrostrictive substances, of which Terfenol D is probably the best known. MSM materials are ferromagnetic and possess a controlled martensite twin structure. When the material is exposed to an external magnetic field, the twins with structures oriented in one direction grow at the expense of twins with structures oriented in other directions. Conversely, if the material is subjected to mechanical load, the twins are forced to change direction, affecting a surrounding magnetic field, which can be detected using magnetic field sensors.

AdaptaMat, based in Helsinki, started production in 2001 making single crystal nickel-manganese-gallium actuator elements and the design and manufacture of actuator and sensor devices based on the material.

In order to function as an actuator, a rod of material is pre-stress loaded so that the short crystallographic 'c' axis is oriented along the axis of the rod. If a magnetic field is applied perpendicular to the rod axis, twins grow whose short 'c' axis is parallel to the magnetic field. This causes the rod to grow in directions perpendicular to the field, so that the rod lengthens.

MSM actuators made by AdaptaMat produce strokes of up to 5mm and forces up to 2 kN using elements up to 100mm. Positioning accuracy can be less than one micron. Actuator response time can be as short as 0.2ms, depending on drive current, and frequency response can be from DC up to the kHz range. In service in actuators, MSM materials have been through as many as 500 million cycles with no deterioration of stroke.

Adaptamat's Dr Emmanouel Pagounis writes, "Compared to conventional actuator solutions, e.g. electromagnetics, pneumatics and hydraulics, MSM materials offer considerable advantages in terms of reduced size, fast frequency response, enhanced controllability, reliability and efficiency. In addition, they demonstrate excellent damping properties. "

It can be assumed that because of the nature of the constituents, the material is never going to be cheap. However, even with its much more limited strain, requiring use of substantial amounts of equally expensive material, electronically controlled valve automotive engine valve lifters based on rods of Terfenol D have seriously been put forward. The attraction is that because of the speed and precision of response, these provide good opportunities for improved engine economy.

Commercially available MSM actuators include models: A5-2, 20mm x 30mm x 120mm which has a 5mm stroke from DC to 300 Hz. The A06-3 is 11mm x 23mm x 21mm and has a stroke of 0.6mm from DC to 1,000 Hz. The A1-2, on the other hand is 260mm in diameter and 90mm high, and delivers a 1 kN, 1mm stroke from DC to 100 Hz.

Other devices made include a linear motor, a proportional valve actuator and a micro pump.

The linear motor works on the inchworm principle, clamping at one end, extending and moving the other which then clamps, upon which the first clamp is released and the body is contracted. Magnetostrictive and piezoelectric inchworm motors are already produced commercial using similar principles, despite the relatively small steps possible with such materials. The prototype motor has a no load speed of 40mm/s and exerts a force of 1N. The control system includes two closed loops for current control and position control. The position controller is programmed using National Instruments LabView software.

An MSM actuator has also been designed for a pneumatic proportional poppet valve, taking advantage of a <4ms response. Because the material shape change is large, the valve requires no amplification, either fluid or mechanical.

Mechanical movements can be made bigger than that provided by the basic strain by taking advantage of geometry. By expanding one side of a bi-material element, as in devices based on differential thermal expansion and other small dimensional change effects, a large amount of bending can be produced. It is also possible to make devices that produce torsion in a similar manner.

But it is probably in micro and nano based devices where MSM materials may find their greatest usefulness, in applications requiring only microscopic amounts of material. One experimental device has been a pump based on a square MSM element, with four small chambers around it, one on each side. The square element changes its shape under the action of applied magnetic field, emptying and filling each chamber in turn.

Other potential fluidic applications are seen in automotive injectors, ink jet printers and drug delivery. MSM actuators might also be used in fasteners, brakes, clamps and in automatic assembly. Potential positioning applications include: robotics, manipulators, linear drives, switches and circuit breakers. The high frequency response shows potential applications in shakers, sonar transducers, loudspeakers, drills and vibration cancellation systems. Used as sensors, the material could be used to measure force, position and vibration.

Overall, world wide annual sales of smart materials exceed $1 billion, with piezoelectrics accounting for 75% of the market and magnetostrictive and other shape memory alloys the remaining 25%. The market for devices driven by smart materials is estimated to be $15 billion and growing.


* MSM materials change shape under the influence of applied magnetic field

* Strains achieved are up to 6% to 10%, 50 times greater than that possible with the most responsive magnetostrictive materials

* Operating temperatures are presently within the range -40 deg C to + 60 deg C, with possible extension to 120 deg C. Frequency response is typically up to 1,000Hz for macroscopic devices based on the material. Fatigue life appears to be greater than 500 million cycles.

Eureka says:

In a world increasingly full of smart materials and smart devices, a new material with a two order of magnitude greater responsiveness can be expected to revolutionise a multitude of industries


Tom Shelley

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