Last set

Question 1

Compare the thermal and the magnetic shape memory effect.

Answer 1

Thermal Shape Memory Effect: -almost stepwise deformation possible in the temperature range of 10-50 K.

• a varierty of deformations is possible (elongation, contraction, deflection, torsion)
• high energy density
• effect can be limited to certain regions of the element
• simple mechanism
• silent, clean and spark free working conditions.
  dis: -stability of the effect depends highly on the quality of the alloy
• low energy efficiency
• limited bandwidth due to heating and cooling restricitons
• degradation and fatigue

Magnetic Shape Memory effect:

• similar to the thermal shape memory effect but no heating/cooling restrictions
• faster switching speeds can be reached.

Question 2

What limits the maximum useful frequency in a magnetic shape memory actuator?

Answer 2

the magnetic core type, eddy currents in the core and the inertia of the moving objects. (restoring and drive electronics also)

Question 3

Compare the energy densities of thermal and magnetic shape memory alloys.

Answer 3

Thermal Shape memory alloys exhibit a higher density (in the order of 800-2000 J/kg) than magnetic shape memory alloys (15-30 J/kg)

Question 4

Discuss the most important challenges in practical applications of ERF.

Answer 4

• The measurement of the properties of ERF is not standarized, which makes it very difficult to compare between different manufacturers and different material.
• long term behavior is not well defined.
• insufficient thermal and chemical stability
• inadequate yield stress and response time
• isolation problems due to high energy
• high temperature dependence.

Question 5

What limits the useful temperature range for ERF based actuators?

Answer 5

The conductivity increases sharply with the temperature increase. This limits the useful temperature range of ERF based actuators.

Question 6

Discuss the most important challenges in practical use of MRF.

Answer 6

Due to the large difference in density between the particles and the carrier fluid, MRF suffer from gravitational settling.
Iron particles can also be very abrasive.
A solution would be to minituarize the particles, which is possible, but that would lead to expensive fabrication costs.

Question 7

What limits the maximum frequency achievable by MRF based actuators?

Answer 7

• magnetic field saturation

- magnetization of the particles.

Question 8

Compare ERF and MRF actuators with respect to their minituriazation potential.

Answer 8

The higher yield strength of MRF means that the same mechanical power transmission can be achieved by a smaller active fluid volume of MRF than ERF.

Question 9

Discuss the difference between ERF and MRF based actuators.

Answer 9

1 MRF: - magnetic polarized particles in a non magnetic carrier fluid.

• magnetic field generation is easier.
• better operable temperature range (-140 C up to 150 C)
• no isolation problems, no conductivity problems
• higher yield strength
• less stable against gravitational settling
• more freedom with additives
• safer than ERF
• surpassed ERF by far in industrial applications

2 ERF: -charged particles with a dipole in a non conducting carrier fluid.

• lower base viscosity
• operable temperature range only up to 70 C
• simpler actuator design
• limited due to breakthrough voltage.
• substantial joules heating
• additives have a significant influence on the dielectric properties.

Question 10

In which way can DEA be compared to human muscles?

Answer 10

Due to their soft characteristic and to the comparable stress/strain range, DEA are often called artificial muscles.

Question 11

Why do most commercial DEA use silicones and polyurethanes and not acrylic elastomers?

Answer 11

Acrylic elastomers have in general a quite good DEA behavior, however, they exhibit a significant creep behavior, which makes them in the long term view not useful for industrial applications.

Question 12

Why is the development of suitable electrodes for DEA more difficult than the design of electrodes for piezoelectric actuators?

Answer 12

The electrode design for DEA is very critical, they must allow a high conductivity without blocking the motion of the elastomer foil. They must be softer than the already soft elastomeric material and last for millions of cycles.

Question 13

Describe the two major processes for the fabrication of multilayer DEA.

Answer 13

cyclic spin coating of two component addition curing silicon, accelerated by thermal radiation. The electrodes are deposited by a graphite based spraying process using shadow masks structured by lithography.
the second method uses commercially available DEA films and follows the idea of stacking and folding of the DEA film.

Question 14

What limits the dynamic properties of DEA?

Answer 14

Due to their very soft character, the dynamic properties of DEA are strongly influenced by the environment in which they are applied.

Question 15

Compare DEA to piezoelectric actuators.

Answer 15

Both are manufactured in thin plates due to the high electric field requirements, and both get stacked in order to increase the stroke.

DEA: -operating voltages in the order of a few kVs.

• higher strokes
• larger strain
• less stiffness
• less energy density
• critical design of the electrodes

Piezoelectric: -driving voltages of 200 to 1000 V

• lower strokes (between 20 and 20 um)
• higher forces generated up to 30000N

Question 16

Discuss the advantages and disadvantages of using DEA for pneumatic valves.

Answer 16

Advantages: - less power consumption, since they don’t require any power to hold the valve in a given state.

• easy to minituriaze.
  disadvantages: - DC-DC converter is needed, to drive the valves with low voltages and actuate them with high voltages.
• more complex actuator design.

Question 17

Discuss the advantages and disadvantages of using DEA for biomedical applications.

Answer 17

Advantages: -fit well to our meaning of appearance. (humanoid robots)

• due to the very soft material, they are well suited for prosthesis.
  disadvantages: - they need a high operating voltage.

Question 18

Why does the magnetostrictive coefficient of a polycrystalline ion sample depend on the applied magnetic field?

Answer 18

Iron is a special case because of the declared anisotropy of its magnetostrictive properties. Along the (100) direction, it has a positive magnetostriction. Therefore a singlecrystalline piece of iron expands if a magnetic fied is applied along the 100 direction.
However, along the (111) direction iron exhibits a negative magnetostriction.
Since the magnetostrictive constants are field-dependent, a polycrystalline Iron sample shows a field -dependent change in sign of the magnetostrictive effect.

Question 19

Why does the magnetostrictive effect in TERFENOL-D depend on the mechanical prestress of the sample?

Answer 19

3 reasons:

• TERFENOL-D deforms in a magnetic field in order to minimize its internal energy. It has no stable zero position for the magnetic field or the prestress. A mechanically pre-stressed TERFENOL-D rod will return to its initial shape after switching off the magnetic field.
• the maximum value of the magnetostrictive strain depends strongly on the mechanical prestress. By choosing the optimum prestress, strokes of up to 50 % higher can be reached.
• the maximum allowable tensile stress of TERFENOL-D is much smaller than the maximum allowable compressive stress. Thus, the rod should be kept under compressive stress by a suitable mechanical device.

Question 20

Discuss the advantages and disadvantages of magnetostrictive actuators.

Answer 20

Advantages: - considerably higher operating temperatures.
- large force generating capabilities.
- no stacking needed
Disadvantages: -low voltage, high current operating demands (high energy)
-very high fabrication costs (TERFENOL-D is expensive).
-frequency dependent displacement.

Question 21

Why is the two way shape memory effect less effective than the one-way shape memory effect?

Answer 21

In the two way shape memory effect, no external restoring force is needed. The effect is much smaller and the material have significantly shorter lifetime cycles.

Question 22

Are thermal shape memory alloy based actuators useful for fast periodic movements?

Answer 22

Thermal shape memory alloy based actuators require a substantial heat transfer from and into the shape memory structure, which naturally limits the actuation speed. However, using a very thin wire of about 25 um and current pulses, fast switching speeds of about 11 ms can be reached.

Question 23

What determines the effective switching speed of a thermal shape memory alloy?

Answer 23

The cooling/heating rate of the SMA structure.

Question 24

How much deformation can a shape memory alloy tolerate without degradation of its performance?

Answer 24

8 % of its total length in cooled state.

Question 25

Discuss at least 3 applications of SMA, e.g. in the fields of medical and aerospace engineering.

Answer 25

Medical engineering: - medical endoscopes, or catheters.(as memory springs, NiTi is non toxic to the human body, so very well suited)
Aerospace engineering: very compact robot for the inspection of compressor turbine blades by eddy current sensors. The robot needs to have high mobility, which can be achieved by multiple joints. The movement of the joints can be generated by SMA springs acting against conventional springs.

Ventilation flaps which close in case of fire (SMA used as grippers).

Question 26

Find out about the manufacturing procedures for SMA.

Answer 26

SMA can be manufactured either by laser cutting, or by etching a thin layer of SMA deposited by sputtering.