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    ACTUATORS - SMA MUSCLE WIRE

    SMA muscle wire

    note: this page is a place holder until a better tutorial is written

    SMA's have been used to mimic muscle, as well as act as levers, and even camera lens wipers! They have been used on Pathfinder on Mars and arterial stents for restoring blood flow to clogged arteries too. Cell phone antennas, orthodontic braces, and even eyeglasses use SMA wires for their extreme flexibility.

    SMA's, or Shape Memory Alloys, are basically materials that change shape when energy is applied to, or removed from, them. These unique metals, made from combinations of two or more elements, exhibit hardness and elasticity properties that change radically at distinct temperatures. There is a large variety currently available, each activated by different forms of energy, often operating under entirely different physical properties. Memory metal, shape memory alloy, Nitinol, Muscle Wires, Flexinol, BioMetal and NanoMuscles are not just marketing hype names, but each also has very differrent properties and uses.

    After alloying and basic processing, SMAs can be formed into any shape (for example, a coil spring), and then set to that shape by a high heat treatment. When cooled, they may be bent, stretched or deformed (within limits) and then with subsequent moderate heating (well below the heat setting temperature), they can recover some or all of the deformation - then move when heated! However, current problems with SMA technology are low force output and extremely low power efficiency (often below 10%). This may be the killer reason NOT to use SMA's. Obviously the inefficiency results as a loss of heat to the environment. Cooling methods will also need to be applied to increase frequency.

    Notes on Muscle Wire and other SMA's

  • SMA's actuate when an energy is applied or removed, such as heat or electrical
  • The material changes shape, or extends/contracts some percentage of its total length
  • Hysteresis is when an SMA has a reduced ability due to wear, such as a decrease in maximum shape change
  • Voltage

  • Not Polarized (current can be reversed)
  • Typically any voltage, DC or AC
  • Higher voltages generally mean more torque and actuation speed, but also require more power and can lead to hysteresis
  • Power

  • Purely dependent on applied voltage and electrical resistance of the material

    Torque

  • Very weak, dependent on material
  • Place SMA's in parallel for increased strength
  • Velocity

  • Fairly slow, dependent on material
  • Often a function of cooling and heating rates
  • Efficiency

  • Extremely innefficient
  • Thermal energy is lost to the environment
  • Cooling methods need to be employed
  • Control Methods

  • Linear Encoder - device which measures distance
  • Voltmeter - electrical resistance changes non-linearly when the material changes shape
  • Half-Bridge - circuit which allows digital mono-directional control of an SMA through a computer
  • Vision Feedback - use of a video camera to measure length changes for a feedback loop




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