Patent Publication Number: US-2005115235-A1

Title: Shape memory alloy actuator

Description:
The present invention relates to a shape memory alloy actuator. Shape memory alloy actuators utilizing the property of a wire made of shape memory alloys such as a nickel titanium alloy (commonly called nitinol) of contracting when heated past the transformation temperature are well known. The known actuators are all based on the contraction of the wire to directly provide the activating effect of the actuator. Hereby, the contraction speed and load capacity of the wire during transformation directly determines the speed and exerted force of the activating effect of the actuator.  
      For many uses it is desirable to attain an activating speed and force characteristic different from what can be achieved by directly utilizing the contraction of the shape memory alloy wire.  
      The main object of the invention is to provide a shape memory alloy actuator where the activating effect and force thereof is not directly dependent on the shape memory alloy wire incorporated in the actuator.  
      According to the invention, this object is obtained by the actuator comprising a body constituting or connected to an activating member such that said activating member is moved between a first and a second position when said body is moved between a third and a fourth position, releasable holding means for holding said body in said third position, at least one first and at least one second wire made of a shape memory alloy such as nitinol, said first wire being connected to said body such that shortening of the length of said first wire exerts a force on said body for moving same from said fourth to said third position, and a biasing means, such as a tension or compression spring or a piston and cylinder mechanism, attached to said body for biasing said body for moving same from said third to said fourth position, said second wire being arranged such relative to said holding means that shortening of the length of said second wire or wires releases said holding means allowing said biasing means to move said body from said third position to said fourth position.  
      Hereby the activating effect is determined directly by the characteristics and arrangement of the biasing means and not directly by the characteristics and arrangement of the shape memory wire incorporated.  
      Preferably, the actuator according to the invention further comprises means for intermittently directing an electric current through said first and second wires for heating same to at least the shape memory alloy transformation temperature. Hereby a particularly effective way of heating the wire is provided.  
      Advantageously, said holding means may comprise a brake mechanism and/or a pawl mechanism.  
      In the currently preferred embodiment of an actuator according the invention, said body is pivotably attached to a frame, said first and second wires are attached at one end thereof to said frame and connected at the other end thereof with said body such that shortening of the length of said first wire exerts a pivoting force on said body in one pivoting direction and shortening of the length of said second wire exerts a pivoting force on said body in the opposite pivoting direction, and said biasing means is attached to said frame and arranged for exerting a pivoting force on said body in at least one of said pivoting directions.  
      Hereby a particularly efficient way of establishing different types of holding means is obtained, one of said holding means being obtained by said biasing means being arranged for exerting a pivoting force on said body in both said pivoting directions with an intermediate balance point wherein said biasing means does not exert a pivoting force on said body.  
      The invention furthermore relates to a shape memory alloy actuator comprising: 
          a frame with a body pivotably attached thereto,     at least one first wire and at least one second wire made of a shape memory alloy such as nitinol attached at one end thereof to said frame and connected at the other end thereof with said body such that shortening of the length of said first wire exerts a pivoting force on said body in one pivoting direction and shortening of the length of said second wire exerts a pivoting force on said body in the opposite pivoting direction, and     a biasing means attached to said frame and arranged for exerting a pivoting force on said body in at least one of said pivoting directions.        

      Advantageously, said biasing means may comprise a tension spring, a compression spring or a piston and cylinder mechanism, and the actuator may advantageously further comprise means for intermittently directing an electric current through said first and second wires for heating same to at least the shape memory alloy transformation temperature.  
      Furthermore, the invention relates to a shape memory alloy actuator comprising: 
          a body with an activating member connected thereto such that said activating member is moved between a first and a second position when said body is moved between a third and a fourth position,     holding means for holding said body in said third position,     at least one first wire and at least one second wire made of a shape memory alloy such as nitinol, said first wire being connected to said body such that shortening of the length of said first wire exerts a force on said body for moving same from said fourth to said third position, and     a biasing means, such as a tension or compression spring or a piston and cylinder mechanism attached to said body for biasing said body for moving same from said third to said fourth position,     said second wire being arranged such relative to said holding means that shortening of the length of said second wire releases said body from said holding means such that said biasing means may move said body from said third position to said fourth position.        

      In the currently preferred embodiment of the actuator according to the invention, the body is pivotably attached to a frame, said first and second wires are attached at one end thereof to said frame and connected at the other end thereof with said body such that shortening of the length of said first wire exerts a pivoting force on said body in one pivoting direction and shortening of the length of said second wire exerts a pivoting force on said body in the opposite pivoting direction, and said biasing means is attached to said frame and arranged for exerting a pivoting force on said body in at least one of said pivoting directions.  
      Preferably, said biasing means is arranged for exerting a pivoting force on said body in both said pivoting directions with an intermediate balance point wherein said biasing means does not exert a pivoting force on said body. 
    
    
      The invention will be described more in detail in the following with reference to various embodiments of a shape memory alloy actuator according to the invention shown, solely by way of example, in the accompanying drawings, where  
       FIGS. 1 and 2  are schematic illustrations of a first embodiment of an actuator according to the invention in two different positions, namely with the activating pin fully retracted in  FIG. 1 , and with the activating pin fully extended in  FIG. 2 ,  
       FIGS. 3 and 4  are schematic illustrations of a second and third embodiment, respectively, of an actuator according to the invention,  
       FIGS. 5 and 6  are perspective elevational views of the currently preferred embodiment (similar to the first embodiment shown in  FIGS. 1 and 2 ) of an actuator according to the invention with the housing cover removed and the activating pin fully retracted in  FIG. 5 , and with the housing cover in place and the activating pin fully extended in  FIG. 6 , and  
       FIGS. 7-9  are schematic illustrations of three further embodiments of an actuator according to the invention. 
    
    
      Referring now to  FIGS. 1 and 2 , a pivotable body in the form of a circular disc  1  is arranged for pivoting around a central pivot  2  fixedly attached to a not shown frame of the actuator, and the disc  1  is provided with a peripheral extension  3  and a yoke-like peripheral extension  5 . A tension coil spring  6  is at one end thereof pivotably attached to a fastening pin  7  fixedly attached to said frame and is at the other end thereof pivotably attached to a fastening pin  8  fixedly attached to the peripheral extension  3 .  
      Two wires or filaments  9  and  10  of a shape memory alloy such as nickel titanium alloy or nitinol, for instance supplied by the company DYNALLOY, INC, of Costa Mesa, Calif., USA, under the trade name FLEXINOL, are attached at one end thereof to electrically conductive terminals  11  and  12 , respectively, fixedly attached to said frame.  
      The other end of each of the wires  9  and  10  is attached to an electrically conductive terminal  13  fixedly attached to the periphery of the disc  1 . The wires  9  and  10  extend along the periphery of the disc  1  such that the wires  9  and  10  when tensioned extend along and are supported by said periphery. In the drawings the wires  9  and  10  are shown spaced from said periphery for the sake of clarity.  
      A sliding body  14  having two arms  15  and  16  is arranged for sliding movement between two stop pins  17  and  18  attached to the frame. A pin  19  attached to the sliding body  14  is received in the fork  5   a  of the yoke-like extension  5  such that the pin  19  may slide and rotate freely in the fork when the disc  1  pivots from the position shown in  FIG. 1  to the position shown in  FIG. 2  thereby slidingly displacing the body  14  from abutment against stop pin  18  to abutment against stop pin  17  with the arm  15 , constituting the activating pin of the actuator, fully extended.  
      A proximity sensor  20  is attached to the frame and connected to not shown electrical conductors for transmitting a signal from the sensor to a not shown receiver. The terminals  11  and  12  are likewise each connected to an electrical conductor, not shown, connected to a not shown power source for supplying electrical power to the wires  9  and  10  for resistance heating thereof, the terminal  13  being likewise connected to the not shown power source through a not shown electrical conductor for closing the resistance heating circuit.  
      In use, the wires  9  and  10  are intermittently heated to the transformation or transition temperature (from martensitic to austenitic state) of the shape memory alloy which temperature for nitinol is approximately 90° C. Thereby the length of the wire is shortened. When the wire cools to below 90° C. the length thereof reverts to normal, i.e. the wire lengthens. The speed at which the shortening takes place, i.e. the contraction time, is directly related to the current input. i.e. the voltage applied over the terminals  11  or  12  and  13 .  
      In the position depicted in  FIG. 1 , the disc  1  is in its outermost counter-clockwise position with the arm  15  fully retracted and with the wire  9  cooled to below 90° C. and the wire  10  heated to above 90° C. by applying an electrical voltage between the terminal  12  and  13  whereby an electrical current wil flow through the wire  10 . The disc  1  has therefore been rotated counter-clockwise to the position shown by the contraction force exerted by the wire  10 .  
      In the next step, the wire  10  is cooled to below 90° C. and thereby lengthens to the shape indicated by the dotted line  10   a  in  FIG. 1 . The actuator is now ready to perform an activating extension of the arm  15  towards the left, the end of the arm  15  being intended to come into contact with a not shown lever or button and depress or activate same during the movement of the arm  15  to the extended leftwards position thereof as depicted in  FIG. 2 .  
      Thereafter or simultaneously, the wire  9  is heated to above 90° C. whereby it contracts and exerts a clock-wise force on the disc  1  pivoting it clock-wise around the pivot  2  past the balance position of the disc  1  and spring  6  in which the attachment pins  7  and  8  of the spring  6  are aligned with the pivot  2 .  
      When the disc  1  has rotated clock-wise past said balance point, the tension force exerted by the spring  7  will continue the clock-wise rotation of the disc  1  to the position shown in  FIG. 2  with the arm  15  fully extended and the wire  9  slack though still above 90° C. This is the actual activating movement of the actuator where the force applied to the sliding body  14  by the extension  5  increases because of the increasing torque arm of the tension force exerted by the spring  6  on the disc  1 .  
      For many applications where the force necessary to perform the function of the actuator, for instance depress a pump piston, increases during the activating stroke, said increase of the spring force torque arm as the disc  1  rotates is an advantageous feature.  
      Finally, the wire  10  is heated above 90° C. so that it contracts and pivots the disc  1  back to the position shown in  FIG. 1  whereby the activating cycle is ready to be repeated.  
      The length of the wire  10  is larger than the length of the wire  9  because the contraction or shortening of the wire  10  must be large enough to pivot the disc  1  from the position shown in  FIG. 2  past the balance point mentioned above while the shortening of the wire  9  only has to be enough the pivot the disc  1  from the position shown in  FIG. 1  past said balance point.  
      Nitinol wires will typically contract about 4%-4.5% when heated past the transition temperature. The uncontracted length of the wire  10  should be enough to ensure that the uncontracted wire is fully extended in the position shown in  FIG. 2  and that the contracted wire  10  is fully extended when the disc  1  is at least slightly past said balance point in the counter-clockwise direction, i.e. the uncontracted length of wire  10  should be about 22-25 times the distance of travel of terminal  13  between the  FIG. 2  position thereof and the balance point position thereof.  
      The necessary contraction force to be exerted by wires  9  and  10  are rather different because the contraction force of wire  9  only has to counteract the torque of the spring force of spring  6  with the relatively small torque arm in  FIG. 1  while the contraction force of wire  10  has to counteract the considerably larger torque of said spring force in  FIG. 2 . The contraction force of a nitinol wire is larger the larger the diameter or cross sectional area of the wire. The cross sectional area of wire  10  is thus considerably larger than the cross sectional area of wire  9  or there may be a number of wires  10  with the same cross sectional area.  
      The latter possibility is chosen if it is necessary that the cooling-off time for the wires  10  is as short of possible so that the interval between the activating cycles may be as short as possible. Several small diameter wires with a certain total cross sectional area will cool more rapidly than a single larger diameter wire with the same cross sectional area.  
      The signal emitted by the proximity sensor  20  each time the extension  3  is in the position shown in  FIG. 2  may be utilized for many different purposes such as for instance a mere monitoring of the correct function of the actuator or for controlling the timing of the heating of the wires  9  and  10  and thereby the timing of the activating stroke of the sliding body  14 . Naturally, the location of the proximity sensor or of any other type of sensor for sensing the position of the disc  1  may be varied according to the purpose thereof, and several such sensors may be provided in different locations for instance for achieving a more complex control of the timing of the activating effect of the actuator.  
      Referring now to  FIG. 3 , this embodiment differs from the embodiment of  FIGS. 1-2  in that a double activating effect may be achieved for each cycle of heating and cooling the shape memory wires  21  and  22  that in this case are of equal length and cross sectional area. The rotation of the disc  1  counter-clockwise and clockwise is limited by stop pins  23  and  24 , respectively.  
      The activating member may be a sliding body similar to body  14  in  FIG. 1-2  where both the arm  15  and the arm  16  perform an activating function, or the activating function may be a pull/push activation by for instance arm  15 .  
      The disc  1  may alternatively be provided with a central torsion shaft projecting at right angles to the plane of the disc  1  as a prolongation of the pivot  2  such that the torsion shaft functions as the activating member by for instance rotating a lever to and fro. Many different types of activating members connected to the disc  1  will be obvious to those skilled in the art.  
      In the position shown in  FIG. 3 , the disc  1  has just performed an activating rotation counter-clockwise under the influence of the counter-clockwise torque of the force of the spring  6  and is ready for the initiation of a rotation clockwise by heating the wire  21  so that the disc  1  is rotated against the counter-clockwise torque of the spring force until the balance point is passed. Then the activating rotation clockwise is performed by the clockwise torque of the spring force.  
      Referring now to  FIG. 4 , the terminal  13  of the embodiments of  FIGS. 1-3  has been substituted by a combined terminal and abutment member  28  for abutting the stop pins  24  and  25 . Furthermore, another type of biasing means is utilized, namely a piston and cylinder mechanism comprising a pressurized cylinder  24  pivotably attached to pin  7 , a piston  26  and a piston rod  27  pivotably attached to the disc  1  by means of a pin  27 .  
      The piston and cylinder mechanism  24 - 25  functions like a compression spring and could in fact be substituted by a compression spring. In  FIG. 4  the disc  1  is in the balance point position where the pin  7 , the pin  27  and the pivot  2  are aligned such that the pressure exerted on the disc  1  by the piston rod  25  does not produce any torque on the disc  1 . In the situation shown in  FIG. 4 , the wire  22  is contracting and rotating the disc counter-clockwise past the balance point. As soon as the balance point has been passed, the torque from the piston rod  25  will cause the activating counter-clockwise rotation of the disc  1  until the member  28  abuts the stop pin  23  whereupon a clockwise rotation may be initiated in a manner very similar to that described above in relation to  FIG. 3 .  
      Obviously, the tension spring  6  in  FIGS. 1-2  could also be substituted by a piston and cylinder mechanism or a compression spring in an arrangement similar to  FIG. 4 .  
      Referring now to  FIGS. 5-6 , the currently preferred embodiment of an actuator according to the invention is very similar to the embodiment schematically shown in  FIGS. 1-2 . All elements common to the embodiments of  FIGS. 1-2  and  FIGS. 5-6  are referenced by the same numerals.  
      The disc  1  and sliding body  14  are enclosed in a housing  30  having a cover  31  in which a slit  32  is provided for allowing free movement of the pin  8  extending through the slit  32 .  
      Electrical conductors  33 ,  34  and  35  are connected to terminals  13 ,  11  and  12 , respectively, for supplying electrical current from a not shown battery for resistance heating of the nitinol wires  9  and  10  to the transformation temperature of about 90° C. Electrical conductors  36  connect the proximity sensor  20  to a not shown receiver for transmitting signals thereto.  
      The position shown in  FIG. 5  corresponds to the position shown in  FIG. 1 , while  FIG. 6  correspond to  FIG. 2 . As regards the operation of the actuator, reference is made to the description thereof above in connection with  FIGS. 1-2 .  
      The actuator of  FIGS. 5-6  is well suited for depressing the piston of a pump, for instance a medical infusion pump, by means of the arm  15 . In connection with such use with an infusion pump, eight FLEXINOL wires with a diameter of 0.002 Inches are used as the wires  9  while two of the same wires are used as wires  10 . The cooling of the wires  9  and  10  takes place by natural radiation and convection, and the current input for heating the wires is chosen such that the actuator may activate the pump several times per second. An additional coil tension spring  6  may be arranged on the other side of the housing attached to a prolongation of the same pins  8  and  7  as used for the visible spring  6 . As regards the characteristics of the FLEXINOL wires, reference is made to the relevant publications from DYNALLOY, INC, which are readily available.  
      Referring now to  FIG. 7 , a body  40  is arranged for movement in a track  41 , the body  40  being connected with a not shown activating member. Shape memory alloy wires  42  and  43  are attached to a not shown frame at  44  and  45 , respectively, and to the body  40 , the wire  42  extending around a pin or pulley  46  attached to said frame. A tension spring  47  is attached to the frame at  48  and to the body  40 . The wires  42  and  43  are connected to electrical terminals at  44 ,  45  and  40  for being supplied with electrical resistance heating current as described above with respect to  FIGS. 1-6 .  
      When the body  40  is in the extreme left position in the track  41 , the geometry of the track  41  relative to the corresponding angle of the spring  47  will entail that the spring  47  cannot move the body  40  in the track. When the wire  43  is heated, the contraction thereof will, together with the force from the spring  47  move the body to the position shown in  FIG. 7  which is past the balance point whereby the force of the spring  47  is enough to continue moving the body towards the right in the track  41 . The shape and orientation of the leftmost portion of the track  41  thus functions as a holding means for holding the body in the position in the said leftmost portion of the track  41 . Thus, the contraction of the wire  45  releases the body  41  from the thus defined holding means such that the spring  47  can move the body to the rightmost extremity of the track  41  indicated by dotted lines  40 a with the corresponding position of the two wires and the spring being indicated by the dotted lines  42   a,    43   a  and  47   a.    
      Subsequent contraction of the wire  42   a  will move the body from the position  40   a  towards the left, thereby “cocking” the spring  47  when the body has passed said balance point. The activating stroke of the actuator takes place when the spring is released from the “cocked” condition and moves the body  41  to the right in the track  41 .  
      Referring now to  FIG. 8 , basically the same arrangement as in  FIG. 7  has been utilized except that the leftmost portion of the track  41  for holding the body  40  with the spring  47  “cocked” has been eliminated and substituted by a depression  49  in the track with a depth sufficient to prevent the spring force from moving the body to the right until being assisted by contraction of the wire  43 .  
      Instead of the depression  49  in  FIG. 8 , a friction brake could be installed in the track  41  for instance by slightly reducing the width of the track. Hereby the holding means for holding the body  40  and “cocking” the spring  47  would be the friction brake.  
      In the embodiment shown in  FIG. 9 , a body  50  is arranged for displacement in a track  51  and shape memory alloy wires  52  and  53  are attached to pins  54  and  55 , respectively, the wire  52  being also attached to the body  50 . A tension spring  56  is attached to pin  57  and the body  50 .  
      A pawl  58  pivotably attached at  59  to the inner surface of the track  51  is biased towards the inside of the track by a compression spring  60  and the pawl  58  is attached to the wire  53  such that shortening of the wire  53  will pull the pawl  58  toward the inner surface of the track against the bias of the compression spring  60 .  
      In the position shown in  FIG. 9 , the wire  52  has been heated so as to contract and pull the body past the pawl  58  by compressing the spring  60 . In this position the spring  56  is “cocked” and the body  50  is held in this position by the holding means constituted by the pawl  58 .  
      Subsequently, the wire  52  is cooled for lengthening thereof and the wire  53  is heated for shortening thereof such that the pawl  58  is pivoted to a position flush with the inner surface of the track  51 . Hereby the body  50  is released and may be pulled by the tension force of the spring  56  to achieve an activating motion of the body downwards as seen in  FIG. 9   
      It will be obvious to those skilled in the art that a pawl mechanism similar to the one described in connection with  FIG. 9  may be employed with the other embodiments shown in  FIGS. 1-2  and  FIG. 8  by utilizing the wires  9  and  43 , respectively, to release a corresponding pawl mechanism instead of as described and shown.  
      In broad terms, the basic idea of the invention could be said to be to combine influencing a body (that constitutes or is connected to an activating member) with a biasing means and two shape memory alloy wires, contraction of one wire causing the body to move against the influence of the biasing means, thereby, so to say, “cocking” (tensioning or compressing) said biasing means, and contraction of the other wire causing the body to move under the influence of the biasing means by releasing at least part of said tension or compression of the biasing means achieved by said “cocking” thereof.  
      It will be obvious to those skilled in the art that many variations of the shown embodiments are conceivable for the application of the above basic inventive idea.  
      An actuator according to the invention may be used inter alia for a great variety of pushing and/or pulling actions, rotating actions, for locking bolts in car doors, hospital beds etc, for release trigger mechanisms for instance for cash registers, for signal arms for toy railroads, for robots for instance for picking up or sorting objects, for opening and closing valves and so on.