Abstract:
A hydraulic actuator includes a hollow tube that has a first opening at a first end of the hollow tube and that has a second opening at a second end of the hollow tube. The hollow tube contains hydraulic fluid. A moveable magnet moves within hollow tube as a result of a magnetic field within the hollow tube. A magnetic field source located outside the hollow tube creates the magnetic field within the hollow tube. When the moveable magnet moves to the first end of the hollow tube, a first piston pushes hydraulic fluid out of the first opening. When the moveable magnet moves to the second end of the hollow tube a second piston pushes hydraulic fluid out of the second opening.

Description:
BACKGROUND 
       [0001]    Hydraulic cylinders are mechanical actuators that get power from pressurized hydraulic fluid. A hydraulic cylinder typically includes a cylinder barrel in which a piston connected to a piston rod moves back and forth. The piston divides the hydraulic cylinder into a first chamber and a second chamber. When the hydraulic pump pushes hydraulic fluid into the first chamber, a valve in the second chamber is open allowing hydraulic fluid to drain from the second chamber into a reservoir as movement of the piston within the hydraulic cylinder increases the volume of the first chamber and correspondingly reduces the volume of the second chamber. Likewise, when the hydraulic pump pushes hydraulic fluid into the second chamber, a valve in the first chamber is open allowing hydraulic fluid to drain from the first chamber into the reservoir as movement of the piston within the hydraulic cylinder increases the volume of the second chamber and correspondingly reduces the volume of the first chamber. 
         [0002]    Typically, the hydraulic pump runs at a constant speed to produce hydraulic pressure. If motion is not imminent, the unused pressured hydraulic fluid is returned to the reservoir or stored in an accumulator. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0003]      FIG. 1  shows a hydraulic actuator system in accordance with an embodiment. 
           [0004]      FIG. 2  is a simplified flow chart illustrating operation of an electronic control circuit within the hydraulic actuator system shown in  FIG. 1  in accordance with an embodiment. 
           [0005]      FIG. 3  shows the hydraulic actuator system shown in  FIG. 1  integrated as part of a robotic joint in accordance with an embodiment. 
           [0006]      FIG. 4  shows a hydraulic actuator used with a hydraulic cylinder in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0007]      FIG. 1  shows a hydraulic actuator system. The hydraulic actuator includes a hydraulic actuator  10 . A cross section of hydraulic actuator  10  is shown in  FIG. 1 . 
         [0008]    A hollow tube  13  is encased by wire windings, represented in  FIG. 1  by wire windings  1 , wire windings  2 , wire windings  3 , wire windings  4 , wire windings  5  and wire windings  6 . Wire windings  1  are separated from wire winding  2  by a space  7 . Wire windings  2  are separated from wire winding  3  by a space  8 . Wire windings  3  are separated from wire winding  4  by a space  9 . Wire windings  4  are separated from wire winding  5  by a space  20 . Wire windings  5  are separated from wire winding  6  by a space  30 . While  FIG. 1  shows six separate windings—wire windings  1 , wire windings  2 , wire windings  3 , wire windings  4 , wire windings  5  and wire windings  6 —the number of wire windings is exemplary and can be varied from one to twenty or even more depending on application and implementer preferences. 
         [0009]    Activating current through a subset of the wire windings produce a magnetic field within hollow tube  13 . A magnet  15  within hollow tube  13  moves as a result of and in response to the magnetic field produced by current through the subset of wire windings. For example, magnet  15  is a rare earth cylindrical magnet. For example, the wire windings are placed over a copper tube  13  and within a ferrous metal tube  11 . Ferrous metal tube  11  contains and intensifies the magnetic field produced by placing current through the subset of wire windings. 
         [0010]    Current through the wire windings produces a Lorentz force that will result in a motional electromotive force on magnet  15  that moves magnet  15  within hollow tube  13 . A piston  17  and a piston  16  isolate magnet  15  from hydraulic fluid  14  within tube  13 . An electronic control circuit  19  provides current to the selected subsets of the wire windings to control movement of magnet  15 . By controlling amplitude of the current and direction of the current through the windings, electronic control circuit can precisely control position of moving magnet  15  within hollow tube  13 . The motional electromotive force placed on magnet  15  varies based on a number of factors such the number of windings in each of the wire windings, the density of windings, the amount of current placed through the selected wire windings, the direction of the current placed through the selected wire windings, the size and shape of magnet  15 , the magnetic properties of magnet  15 , the proximity of the magnet  15  to the wire windings and so on. 
         [0011]    For example, each of wire windings  1 , wire windings  2 , wire windings  3 , wire windings  4 , wire windings  5  and wire windings  6  are separately connected to electronic control circuit  19  allowing electronic control circuit  19  to separately control current through each of the wire windings. For example, electronic control circuit  19  can place pulse width current signals with current flowing in opposite directions on each of two adjacent wire windings. The resulting magnetic field will place and hold magnet  15  in a particular location within hollow tube  13  in proximity of the two adjacent wire windings. By independently varying the pulse width duration in each of the two adjacent wire windings electronic control circuit  19  can move magnet  15  in either direction along hollow tube  13 . 
         [0012]    For example, when magnet  15  is in the proximity of wire windings  3  and wire windings  4 , electronic control circuit  19  can control pulse width signals in wire windings  3  and wire windings  4  to move magnet  15  towards wire windings  5 . Then electronic control circuit  19  can stop the current in wire windings  3  and can control pulse width signals in wire windings  4  and wire windings  5  to move magnet  15  towards wire windings  6 . And so on. For more information on using pulse width current signals through wire windings to create a Lorentz force to precisely move a magnet through magnetic fields, see for example, Bryan Craig Murphy, “Design and Construction of a Precision Tubular Linear Motor and Controller”, Submitted to Texas A&amp;M University, May 2003; Tony Morcos, “The Straight Attraction Part 1” Motion Control, June 2000, pp. 29-33; and Tony Morcos, “The Straight Attraction Part 2” Motion Control, July/August 2000, pp. 24-28. 
         [0013]    When electronic control circuit  19  applies current through various subsets of the windings to move magnet  15  towards a sealing piston seat  18  at an end of tube  13 , hydraulic fluid is forced by piston  17  through a flexible hydraulic fluid transport hose  31  and into a hydraulic muscle  32 . Hydraulic muscle  32  contracts as it receives hydraulic fluid. Attachment structure  33  is pulled and can be used to pull a load, such as is necessary when flexing a robot arm. Also, as electronic control circuit  19  moves magnet  15  towards sealing piston seat  18  of tube  13 , hydraulic fluid is drawn by piston  16  into tube  13  from a flexible hydraulic fluid transport hose  34  and out of a hydraulic muscle  35 . This allows hydraulic muscle  35  to relax and be extended. As can be seen by the above discussion, hollow tube  13  needs to be sufficiently large to provide a volume of hydraulic fluid to hydraulic muscle  32  so that hydraulic muscle  32  can sufficiently contract a desired amount and to provide a volume of hydraulic fluid to hydraulic muscle  35  so that hydraulic muscle  35  can sufficiently contract a desired amount. 
         [0014]    A feedback sensor  38 , electrically connected to electronic control circuit  19 , can be used to monitor extension of attachment structure  36 . This can allow electronic control circuit  19  to precisely control movement. While in  FIG. 1 , feedback sensor  38  is shown positioned to monitor extension of attachment structure  36 , feedback sensor  38  can be located at other locations to monitor other phenomena, such as location of attachment structure  33 , to provide feedback information to electronic control circuit  19 . Also, more than one feedback sensor can be used. 
         [0015]    When electronic control circuit  19  applies current through various subsets of the windings to move magnet  15  towards a sealing piston seat  39  at another end of tube  13 , hydraulic fluid is forced by piston  16  through a flexible hydraulic fluid transport hose  34  and into a hydraulic muscle  35 . Hydraulic muscle  35  contracts as it receives hydraulic fluid. Attachment structure  36  is pulled and can be used to pull a load, such as is necessary when flexing a robot arm. Also, as electronic control circuit  19  moves magnet  15  towards sealing piston seat  39  of tube  13 , hydraulic fluid is drawn by piston  17  into tube  13  from a flexible hydraulic fluid transport hose  31  and out of a hydraulic muscle  32 . This allows hydraulic muscle  32  to relax and be extended. 
         [0016]    The use of motional electromotive force on magnet  15  to pressurize hydraulic fluid makes it easy to allow for compliance to obstructions. That is, when an unexpected obstruction is met during movement, the increased resistance to movement can be detected by the jump in current required to continue the motion. Electronic control circuit  19  can limit the current resulting in stopping the motion of magnet  15  within the magnetic field produced by wire windings  1 , wire windings  2 , wire windings  3 , wire windings  4 , wire windings  5  and wire windings  6 . 
         [0017]      FIG. 2  is a simplified flow chart illustrating operation of electronic control circuit  19 . When operation is started, as illustrated by a block  70 , electronic control circuit  19 , in a block  71  will wait until a position command is received. For example, a position command is sent by a computer, or some other user device in communication with electronic control circuit  19  and configured to send position commands to electronic control signal  19 . 
         [0018]    When a position command is received, in a block  72 , electronic control circuit  19  will compare a requested position in a position command to a current position reported by feedback sensor  38  to calculate a position error. The position error tells how far and what direction attachment structure  36  needs to move in order to be in the requested position. In a block  73  electronic control circuit  19  will generate current through wire windings  1 , wire windings  2 , wire windings  3 , wire windings  4 , wire windings  5  and wire windings  6 . that will move magnet  15  in a direction that will cause attachment structure  36  to move closer to the requested position. In a block  74 , information from feedback sensor  38  will be monitored until attachment structure  36  is in the requested position. 
         [0019]    If it is desired to control speed of motion, commands to electronic control circuit can specify a requested speed of motion (e.g., slow, medium, fast) and electronic control circuit can control current placed through the wire windings to accommodate the requested motion speed. 
         [0020]    The hydraulic actuator system shown in  FIG. 1  can be attached to a lever on a pivot or rack and pinion gear to produce various movements, such as a limited circular movement. Multiple hydraulic actuator systems can be connected together to produce multiple degrees of freedom, such as in the joints of robot arms or legs. 
         [0021]    For example,  FIG. 3  shows the actuator system of  FIG. 1  used as part of a movable joint in a robotics system. Hydraulic actuator  10  is connected to a lever  50  at a pivot  52 . Hydraulic muscle  32  is anchored to hydraulic actuator  10  by a bracket  42 . Hydraulic muscle  35  is anchored to hydraulic actuator  10  by a bracket  44 . Attachment structure  33  is anchored at pivot  54  to an arm  51  of lever  50 . Attachment structure  36  is anchored at pivot  55  to an extended portion  53  of lever  50 . When hydraulic muscle  32  pulls attachment structure  33 , robotic arm  51  pulls toward hydraulic muscle  32  and hydraulic actuator  10 . When hydraulic muscle  35  pulls attachment structure  36 , robotic arm  51  extends away from hydraulic muscle  32  and hydraulic actuator  10 . Robotic arm  51  and hydraulic actuator  10  thus together act as a joint in a robotics system. 
         [0022]      FIG. 4  shows another embodiment where a hydraulic actuator  60  is connected to a hydraulic cylinder  65 . When a magnet within hydraulic actuator  60  is moved towards an end  62  of hydraulic actuator  60 , hydraulic fluid is pushed through a flexible hydraulic fluid transport hose  64  into hydraulic cylinder  65  to correspondingly extend a piston  66  out of hydraulic cylinder  65 . When the magnet within hydraulic actuator  60  is moved towards an end  61  of hydraulic actuator  60 , hydraulic fluid is pushed through a flexible hydraulic fluid transport hose  63  into hydraulic cylinder  65  to correspondingly retract piston  66  into hydraulic cylinder  65 . A feedback sensor  67  monitors position of piston  66  and communicates position information to an electronic control system of hydraulic actuator  60 . 
         [0023]    In the above-discussed embodiments, piston  16 , piston  17 , sealing piston seat  18  and sealing piston seat  39  are constructed for complete seal with no slippage of hydraulic fluid. Alternatively, any or all of piston  16 , piston  17 , sealing piston seat  18  and sealing piston seat  39  can be constructed to allow some pressurized hydraulic fluid to slip past at a certain predetermined pressure to allow for compliance when obstructions in movement are encountered. If this results in loss of calibration of hydraulic actuator  10  or air in fluid chambers, this can be alleviated by appropriately bleeding the hydraulic system of hydraulic actuator  10 . 
         [0024]    Also in the above-described embodiments, electronic control system  19  controls movement of magnet  15  in two directions. In an alternative embodiment, the magnet can be spring loaded on one end to so that motion in one direction is achieved by motional electromotive force and motion in the other direction is achieved by force from the spring. 
         [0025]    Also in the above-described embodiments, magnet  15  moves while the wire windings are stationary with respect to hollow tube  13 . In an alternative embodiment, magnets may be fixed to a hollow tube and be used as a magnetic field source. Within the hollow tube a moveable magnet is an electromagnet that includes wire windings. The electromagnet moves within the hollow tube and as a result of and in response to the magnetic field created by the magnetic field source interacting with the magnet qualities of the moveable magnet produced by the amplitude and current placed through the wire windings. 
         [0026]    Also in the above-described embodiments, a hydraulic actuator is shown connected to hydraulic muscles and a hydraulic cylinder. In alternate embodiments, a hydraulic actuator can be connected to other hydraulic devices. For example, hydraulic actuator  10  can be connected to a hydraulic bladder and used to inflate and deflate the hydraulic bladder to alternate a state of the hydraulic bladder between a limp flexible condition and a stiff or rigid condition. 
         [0027]    The size of hydraulic actuator  10  can be scaled to be larger or smaller to fit requirements of a particular implementation. Hydraulic actuator  10  can be used in products that need circular hydraulic muscle effects that tighten or loosen around an object, producing a squeezing force. The double action valve function of hydraulic actuator  10  both pressurizes fluid depressurizes fluid depending on a configuration of the hydraulic actuator system. Hydraulic actuator  10  can be used with any product that needs to efficiently and fluidly move a load in a straight line in either direction over a limited distance. 
         [0028]    The foregoing discussion discloses and describes merely exemplary methods and implementations. As will be understood by those familiar with the art, the disclosed subject matter may be embodied in other specific forms without departing from the spirit or characteristics thereof. Accordingly, the present disclosure is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.