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ShapeMemoryActuatorsforAutomotiveApplications

Stoeckel

EngineeringAspectsofShapeMemoryAlloys

(eds.)T.W.Duerig,K.N.Meltonetal.

1990


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Shape Memory Actuators for AutomotiveApplicationsDieter Stoeckel

AbstractWith {he belter understanding of {he metallurgy an d more open discussion wi thin {he sclenU{1c communiCy. shape memoryactuators are becoming increasingly popular for aulomoliue apPlications. This paper surveys current and potential uses.

Actuators are devices which perform a task, like movingan object, either on demand or in response to certainchanges in their environment (temperature, pressure,etc). In a modem car more than 100 ac tuators are usedto contro l engine, transmission and suspensionperformance, to improve safety and reliability andenhance driver comfort. Most of these actuators todayare electric motors, so lenoids, thermobimeta[s, waxmo tors, vacuum or pressure actuators.Shape memory actuators have no t yet penet rated thisgrowing market significan t ly. Th is is due in part to lowmarket awareness about shape memory technology, aswell as little engineering data for the alloys. However,as the scientific community comes to understand themetallurgy and discuss it more openly, shape memoryactuators are becoming increaSing ly popular forau tomotive applications. Japan, which leads in this area,has an impressive number of patent applications. About100 patents for automotive applications are filedannually in Japan, ou tnumbering American andEuropean applications by an order of magnitude. Japannot only leads in the development of new applications,but also in thei r commercialization (Refs 1-8). Shapememory actuators are also produced commercially inEurope, but no base businesses are known in the UnitedStates. This is even more astonishing, as NiT i alloys havebeen developed in the United States and used for over20 years (mainly in the ae rospace industry).Design principlesThere are only two commercia lly available groups ofshape memory alloys for actuator applications today, theCuZn Al alloys and the NiTi alloys. For automotiveapplications, NiTi is preferred because of a number ofadvan tages like high strength, high electrical resistivity,large recovery strains, easy wo rkabi lity, and excellentcorrosion resistance. Therefore, in the following, we willfocus on NiTi alloys only.

The design of shape memory actuators is generallybased on the different stressstrain curves of the materialin its austenitic and martensitic condition. A lthough twoway shape memory actuators are avai lable, most applications use the oneway effect with an ex ternal resetforce. Figure 1 shows the schematic load I deflectiondiagrams for shape memory springs working against aconstant force (load) and against a bias spring. Workagainst a biasing or reset spring is the most commoncase in automotive actuator applications.

A

8! ..

lk n.< t;on lIh i .

1,,",usttnlt.

D

Fig 1 DeSign prinCiple fo r shape memory actuators(lefl : work against bias spring; right; workagainst constant force)Shape memory actuators can be used in two basicallydifferent ways: as thermal or as electrical actuators.Thennal actuators combine the sensing and theactuating functions, responding to a temperaturechange by changing shape andb r generating a force.The functio n of electrical actuators, on the other hand,is simply to move an object or perform a task ondemand. Usually, a current is passed through the shapememory actuator, internally heating it above AI torecover its shape.Thermal actuatorsAs men tioned earlier, thermal actuators respond tochanges in temperature by changing their shape andbrgenera ting a force. Shape memory actua tors, in thisarea, generally compete with thermob imetals and waxmotors, and sometimes with electri clelectronic devicesor vacuumlpneumatic systems. Thermal actuators havea variety of applications in cars. Figure 2 schematicallyshows areas of both potential and realized applicationsfor shape memory thermal actuators.There are three different temperature ranges, inwhich thermal actuators either are already used or couldprovide significant benefits:

low temperature performance (engine,transmission , suspension, brakes) cold sta rt performance cold sta rt performance climate control

302 0261-3059I90I060302- 06 C> 1991 8utterwoflh.Heinemann Ltd MATERIALS & DESIGN VOl. 11 No.6 DECEMBER 1990


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3 42"

78

Fig 2 Potential applications of shape memory thermal actuators in automobiles: (1) radiator shutler; (2) {andutch; (3) fuel management; (4) climate control; (5) engine control; (6) brake ventilation; (7) transmissioncontrol/rattling noise reduction; (8) suspension adjustment

~ ~ \~ ~-40 C 0" 100 C

S l n ~ Cyde NiTi1I .I'han NiT;

Fig 3 Range of transformation temperatures ofcommercially available Ni-Ti alloys

60 - 120C: coo ling systems (engine, brakes) restart ability cl imate control rattling (engine, transmission)

Most of these temperature ranges can be covered byNiTi shape memory alloys available today. Howevercertain limitations, like transformation temperatureranges versus required number of cycles, hysteresiswidth , and stability, have to be considered. Figure 3shows the transformation temperature ranges of themost common Ni:r i alloys. For multiple cycle awlications,standard binary Ni=I'i alloys with tranformation temperatures from -50C to approximately + 70C performwell. These alloys exhibi t a hysteresis width of about 30to 50 degrees and are reasonably stable during cycling.Binary alloys with higher transformation temperatures(up to +120C) tend to 'walk' and, therefore, can only

Most actuator applications require Ni-Ti alloys withboth a narrow hysteresis and high stability of the shapememory effect. The hysteresis width of ternary andq uaterna ry Ni-Ti-Cu alloys, with transformationtemperatures from - 30C to +70C, is only about15C. They show excellent stability even after 100000thermal cycles. Another impor tant advantage of NiTiCu alloys is the low martensitic strength, which allowslow reset forces and thus improved work output.Unfortunately, presently available NiTi alloys havingtransformation temperatures above SOC are notsufficiently stable for multiple cycle applications.However for single cycle applications (eg overtemperature protectors) binary and ternary NiTi alloyswith transformation temperatures up to 150C areavailable. Besides having the highest transformationtemperatures, ternary alloys can provide high austeniteand martensitic strength.

NiTispring cold sleel spring

be used for single or low cycle applications. Wa lk ing is L________ '+___________usually defined as accumulated amnesia. It causesdrifting of the zeropoint during repeated actuation. Fig 4 Function of a thermal valveMATERIALS &. DeSIGN Vol . 11 No. 6 DECEMBER 1990


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Patent literature proposes many applications forshape memory thermal actuators. One successfulapplication is a temperaturesensitive governor va lve ,which controls the shifting pressure in automatictransmissions. This valve's function is shownschematically in Figure 4. At low temperatures, thespring force of a steel bias spring is higher than that ofthe NiTi shape memory spring in the martensitic state.Consequently, the steel spring can compress the NiTispring, pushing the moveable piston of the valve intothe 'closed' position for this particular application, Whenthe temperature of the transmission and thetransmission fluid increases to operating temperature,the NiTi sha pe memory spring in the martensitic state.expands, overcoming the steel spring force, andeventually, pushing the piston into the 'open' position,This pressure regulating valve improves the cold startperformance of the transmission, allowing smoothershifting at low temperatures (Figure 5). Other shapememory governor val ves control the warmup phase ofautomatic transmissions, reducing smog emission andfuel consumption.Figures 6 and 7 show two different versions oftemperaturesensitive boost compensators, and Figure 8a temperaturecompensated valve lifter. Pressureactuators can acquire thermal response by incorporatingshape memory springs into diaphragmtype pressure orvacuum actuators, The low temperature characteristicswould therefore be different from the high temperaturecharacteristics.Another automotive application of shape memorythermal actuators, which has been described frequentlyin the patent literature, is using shape memory washersto reduce gearbox noise. The thermally reactivewashers, either Belleville or wave,washers, are intendedto compensate for the slackness in sp rocket assembliescaused by the differential in thermal expansion ofdissimilar materials, like aluminium and steet, when

temperature increases. Washers are useful because theyprovide very high forces for little motion during recovery.Figures 9 and 10 show examples.

Fig 6 Temperature dependent boost compensator(Reference 1)

II r: Lf

2r " ,ooe jJt:l I l " f 1000'?-II L:QtFig 7 Temperature dependent boost compensator(Reference 2)

Fig 5 Value body with two thermal ualues (cutaway Fig 8sections) Temperatu re compensated valve lifter(Reference 3)

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Fig 10 Rattling noise reduction with "Belleville type" washer (Reference 5)

Fig 9 Rattling noise reduction with shape memorywave washer (Reference 4)

Since conventional shock absorbers tend to be toohard at very low temperatures, they don't providecomfortable driving. This is caused by the high viscosityof the oil in the shock absorber, which usually isbalanced for the temperature range of OC to 100C.A shape memory washer in the shock absorber's valve(Figure 11), which changes the pressure at lowtemperatures, compensates for the oil viscosity.There are many other areas in a car, where shapememory thermal actuators can and will providesignificant changes over competing technologies. Someof the benefits are listed:

MATERIALS & DESIGN Vol. 11 N o. 6 DECEMBER 1990

, , \ \ ' \ , , \ \ , \ \ , \ , , \ \ \ \ \1 \ \

\ \ \ , , \ \ , , \ \ , \ \ , \S , , \ \ S ,

Fig 11 Temperature compensating valve fo r shockabsorbers (Reference 6)

305


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1

4

10Fig 12 Potential applications for electrical shape memory actuators in automobiles: (1) fog/amp [ouvre; (2) engine

hood lock; (3 ) retractable headlight; (4) fuel management; (5) engine control; (6) transmission control;(7) climate control; (8) wiper pressure control ; (9) rear-view mirror adjustment; (10) seatbelt adjustment;(II) central locking system; (12) shock absorber adjustment; (13) filler inlet lock; (14) trunk lock

size high force large motion high work output few mechanical parts non-linear characteristicOn the other hand, for certain applications the followinglimitations have to be taken into account:

limited range of transformation temperatures hysteresis non-linear characteristicElectrical actuatorsElectrical actuators are devices which perform a task ondemand. The stimulus is any voltage applied to thedevice, which is usually an electric motor or a solenoid.If electrically heated above A ., such as by passingcurrent through a wire or spring, Ni T i shape memoryelements become electrical actuators. They can provideinteresting advantages over motors and solenoids like:

small size noiseless operation few mechanical partsTherefore an almost unlimited number of potentialapplications can be found in the patent literature. Figure12 schematically shows the areas in a car whereelectrical shape memory actuators have been suggested.However, few have actually been implemented or seemtechnically and economically feasible because of thelimited range of transformation temperatures of existingshape memory alloys. As shown in Figure 13, theoperating temperature range of a car ranges from- 40 oe to approximately +100oe, with even highertempe ra tures in under hood locations. In order to workproperly at all temperatures, the shape memory alloyhas to have an Mf temperature well above the maximumoperating temperature. Today no NiTi alloys with3De

,cOperat ing T. mperature RU le

/,;iTi "lIoy. a .. today -f "rRequired Tra ns form ation T. mperlluru Mf ATFig 13 Operating temperature range for automobiles

and transformation temperatures ofNiTi alloystransformation temperatures above approximately sooeare available for cyclic applications. euAI.N i shapememory alloys have transformation tempera tures thishigh, but they are brittle and unstable. TiPdNi alloysremain too expensive for routine applications.

Among those applications that are in production isthe remote foglamp louvre opening device. A shapememory spring is wired in series with the lamp. Turningon the fog-lamp passes the lamp current through theactuator, which heats up, contracts and opens the louvre(Figure 14). A reset sp ring closes the louvre when thelamp is turned off. Agure 15 shows the design of theactuator with an integrated overload spring. The areawhere fog-lamps are located on a car is usually very wellventilated, so that the low transformation temperatureof the actuator (-65C) is not an issue.The same applies for windshield wipers. Shapememory actuators could provide an elegant solution forincreasing the pressure at high speeds. Figure 16 showsdifferent designs incorporating a Ni-Ti spring or tensilewire into the wiper arm.

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----. . . . . \ ' , ,,. . , , ~ , .

_ . U M u l i ~ ~ I M . , l [

Fig 14 Fog lamp with shape memory electricalactuator

,,'

'b'

'0'

stroke

;sleeve

Insulating guideMovable termination

terminationspring

Fig 15 Design example {o r a linear shape memoryactuator

Fig 16 Windshield wiper with shape memory pressureactuator (Reference 7)

307

Fig 17 Door locking mechanism with shape memorysprings (Reference 8)

Because of their small size and noiseless operation,shape memory actuators have also been suggested forcentral locking systems (Figure 17), trunk locks and fueltank cap locks. However, there are serious problemswhen ambient temperature approaches thetransformation temperature of the shape memory alloy,for instance when the car is parked in the sun. In thiscase, the shape memory actuator could self actuate orfa il to reset.ConclusionAlthough shape memory actuators can provide significant advantages over conventional devices in certainareas, they have only slowly been penetrating theautomotive market. This is mainly caused by poorinformation and the lack of engineering data for shapememory alloys. However, with the better understandingof the metallurgy and a more open discussion in thescientific community, shape memory actuators arebecoming increasi ngly popular for automo t iveapplications.

Referenc:es1 Hlno Motor Industry, Kokai No. 63-40543 (1988).2 Nissan Diesel Industry. Kokai No. 62169230 (1987).3 Mitsubishl Motor, Kokai No. 62160706 (1987).4 Toyota Motor, Kokal No. 625036] (1987).5 Hlno Motor Industry.6 Aishln Selki.7 Mazda, Kokal No. 60 J73455 (1985).8 Kokusan Metal Industry, Kokai No. 62173472 (/987).

MATERIALS & DESIGN Vol. 11 No.6 DECEMBER 1990

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