Power plants – Motor operated by expansion and/or contraction of a unit of... – Mass is a solid
Reexamination Certificate
2000-08-11
2003-06-10
Nguyen, Hoang (Department: 3748)
Power plants
Motor operated by expansion and/or contraction of a unit of...
Mass is a solid
C060S528000
Reexamination Certificate
active
06574958
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
This invention relates to shape-memory alloy (SMA) actuators and other actuators using electromechanically active materials [collectively referred to in this application as SMA actuators] and to methods for their control. In particular, this invention relates to SMA actuators that are capable of miniaturization to achieve fast (sub-second) response, and to control methods for SMA actuators in general, and also in particular for the miniaturizable SMA actuators of this invention for low power consumption, resistance/obstacle sensing, and positional control.
(b) Description of Related Art
A class of materials was discovered in the 1950s that exhibit what is known as the shape memory effect. See, for example, K. Otsuka, C. M. Wayman, “
Shape Memory Materials
”, Cambridge University Press, Cambridge, England, 1998, ISBN 0-521-44487X. These materials exhibit a thermoelastic martensite transformation; i.e. they are pliable below a certain transition temperature because the material is in its martensite phase and can be easily deformed. When their temperature is raised above the transition temperature the material reverts to its austenite phase and its previous shape, generating a large force as it does so. Example of such materials are approximately 50:50 atom percent titanium-nickel (TiNi) alloys, optionally containing small quantities of other metals to provide enhanced stability or to alter the martensite-austenite transition temperatures; and these can be formulated and treated to exhibit the shape memory effect. Other such alloys include Cu/Al/Ni and Cu/Al/Zn alloys, sometimes known as &bgr;-brasses. Such alloys are generically referred to as shape memory alloys (SMA) and are commercially available from a number of sources in wire form, with diameters from as low as 37 &mgr;m to 1 mm or greater. See, for example, Dynalloy Corp., “Technical Characteristics of Flexinol Actuator Wires”, Technical Information Pamphlet Dynalloy Corp., 18662 MacArthur Boulevard, Suite 103, Irvine Calif. 92715, USA.
SMA wires are wires of shape memory alloy that are treated such that they can be easily stretched along their longitudinal axis while in the martensite phase, thus re-arranging their atomic crystalline structure. Once stretched they remain that way until they are heated above their austenite transition temperature, at which point the crystalline structure is restored to its original (remembered) austenite configuration. This reversion not only returns the wire to its original length, but also generates a large force, typically on the order of 50 Kgf/mm
2
cross-sectional area, depending on the alloy and its treatment. Because of the large available force per cross-sectional area, SMA wires are normally produced. in small diameters. For example, a 100 &mgr;m diameter wire can deliver about 250 g of force. To obtain more force, thicker wires or multiple wires are required.
Although SMAs have been known since 1951, they has found limited commercial actuator applications due to some inherent limitations in the physical processes which create the shape memory properties. This lack of commercial applications is due to a combination of the following factors:
(1) Limited Displacement
A TiNi SMA wire can contract by at most 8% of its length during the thermoelastic martensite to austenite transition. However, it can only sustain a few cycles at this strain level before it fails. For a reasonable cycle life, the maximum strain is in the 3-5% range. As an example, for an actuator with reasonable cycle life, it requires over 25 cm of SMA wire to produce 1 cm of movement.
(2) Minimum Bend Radius
An obvious solution to packaging long lengths of SMA into small spaces is to use some kind of pulley system. Unfortunately SMA wires can be damaged if they are routed around sharp bends. Typically an SMA wire should not be bent around a radius less than fifty times the wire diameter. As an example, a 250 &mgr;m diameter wire has a minimum bending radius of 1.25 cm. It should be noted that the term “minimum bending radius” as used here means the minimum radius within which an SMA wire can be bent and still be capable of repeated austenite-martensite cycling without damage. The addition of a large number of small pulleys makes the system mechanically complex, eliminating one of the attractions of using SMA in the first place. Also the minimum bend radius requirement places a lower limit on actuator size.
(3) Cycle Time
An SMA wire is normally resistively heated by passing an electric current through it. The wire then has to cool below its transition temperature before it can be stretched back to its starting position. If this cooling is achieved by convection in still air, then it can take many seconds before the actuator can be used again. The 250 &mgr;m wire discussed above has a best cycle time of about 5 seconds or more. Thus, as an example, Stiquito, an SMA powered walking insect [J. M. Conrad, J. W. Mills, “Stiquito: Advanced Experiments with a Simple and Inexpensive Robot”, IEEE Computer Society Press, Los Alamitos Calif., USA, ISBN 0-8186-7408-3] achieves a walking speed of only 3-10 cm/min. Since the rate of cooling depends on the ratio of the surface area of the wire to its volume, changes in wire diameter dramatically affect the cycle time.
To overcome these limitations designers of SMA based actuators have typically used long straight wires or coils. See, for example, M. Hashimoto, M. Takeda, H. Sagawa, I. Chiba, K. Sato, “Application of Shape Memory Alloy to Robotic Actuators”,
J. Robotic Systems
, 2(1), 3-25 (1985); K. Kuribayashi, “A New Actuator of a Joint Mechanism using TiNi Alloy Wire”,
Int. J. Robotics
, 4(4), 47-58 (1986); K. Ikuta, “Micro/Miniature Shape Memory Alloy Actuator”,
IEEE Robotics and Automation
, 3, 2151-2161 (1990); and K. Ikuta, M. Tsukamoto, S. Hirose, “Shape Memory Alloy Servo Actuator with Electrical Resistance Feedback and Application for Active Endoscope”,
Proc. IEEE Int. Conf. on Robotics and Information
, 427-430 (1988). Clearly, in many applications, especially where miniaturization is desired, it is impractical to use long straight wires. Coils, although greatly increasing the stroke delivered, significantly decrease the available force; and, to compensate for the drop in force, thicker wires are used which reduce the responsiveness of the resulting actuator, making it unsuitable for many applications.
Other mechanisms commonly used to mechanically amplify the available displacement, such as those disclosed in D. Grant, V. Hayward, “Variable Control Structure of Shape Memory Alloy Actuators”,
IEEE Control Systems
, 17(3), 80-88 (1997) and in U.S. Pat. No. 4,806,815, suffer from the same limitation on available force, again leading to the requirement for thicker wires and the attendant problems with cycle time.
As discussed above, SMA materials can be used as the motive force for an actuator [See, for example, T. Waram, “
Actuator Design Using Shape Memory Alloys
”, 1993, ISBN 0-9699428-0-X], whose position can be controlled by monitoring the electrical resistance of the alloy. See, for example, K. Ikuta, M. Tsukamoto, S. Hirose, “Shape Memory Alloy Servo Actuator with Electrical Resistance Feedback and Application for Active Endoscope”, discussed above.
A common method of heating SMA actuators to their transition temperature is pulse width modulation (PWM). In this scheme, a fixed voltage is applied for a percentage of a pre-set period. As the percentage on-time to off-time in a single period (referred to as the duty cycle) is changed, the aggregate amount of power delivered to the SMA can be controlled. This scheme is popular because of the ease with which it can be implemented in digital systems, where a single transistor is all that is required to drive an actuator, obviating the need for digital-to-analog conversion and the associated amplifiers.
In a simple example, a PWM generator supplies PWM pulses to the SMA element at a duty cycle and period specified by a digital contr
Cooley & Godward LLP
NanoMuscle, Inc.
Nguyen Hoang
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