Abstract:
The instant invention teaches a monostable shutter with magnetic blade return using a bistable actuator with blade motion constrained to provide drive force in a single direction and permits the use of compact bistable coil for continuous use by a circuit that applies a high voltage at the beginning of drive that decays to a lower voltage to prevent coil burn out.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This non-provisional application is based on and claims priority through my provisional application titled “Shutter with Bistable Actuator Having Power-Free Magnetic Blade Return” (Ser. No. 61/589,260) filed Jan. 20, 2012. The benefit under 35 USC §119(e) of this United States provisional application is hereby claimed, and the aforementioned application is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]      1 . Field of the Invention 
         [0003]    This invention pertains generally to optical shutters with bistable rotary drive actuators and shutters that move a blade to a given position when power is removed. More specifically, the invention (1) describes a monostable shutter with magnetic blade return using a bistable actuator and (2) permits the use of a compact bistable coil for continuous use while preventing coil burn out. 
         [0004]    2. Description of the Related Art 
         [0005]    Optical shutters use an actuator to drive a blocking element, “blade”, between a first position that blocks a beam of radiation from passing through a designated open area (called an aperture) and a second position that unblocks the beam of radiation, allowing the beam to pass through the aperture. The radiation can be any form of electromagnetic radiation, such as ultra-violet, visible or infrared radiation. The aperture can be in a frame that is directly or indirectly coupled to the actuator. 
         [0006]    The actuator can be electromagnetically activated (an “electromagnetic actuator”) so that it responds to applied electrical power to move the blade between the open and closed position. An electromagnetic actuator can be a linear solenoid, a rotary solenoid, or a brushed or brushless commutated motor. 
         [0007]    Monostable solenoid shutters comprise a coil of wire that provides a magnetic field when electrical power is applied. The magnetic field applies a force to pulls a soft magnetic core in a given direction. Monostable shutters with soft magnetic cores need a spring to returns the core to an original position when power is removed from the core. Monostable shutters arc useful in applications that require the blade to be in a given position when power is removed. Monostable shutters have the disadvantage that they require continuous power to remain in the electrically driven state. 
         [0008]    Bistable shutters are stable in both the open and closed states when power is removed. Bistable shutters can be created using geared motor drives that lock in a given position when unpowered. An over-center spring can be used to create a locking force in either of the two positions. The soft magnetic core of a monostable solenoid can be replaced with a hard magnet that adheres to soft magnetic material in each of the two positions to create a bistable shutter. For example, the rotary drive solenoids (RDS) produced by CVI Melles-Griot are an example of rotary solenoids with a permanent magnet core that is bistable (ref Proc. of SPIE, Vol. 6542, “Advanced electro-mechanical micro-shutters for thermal infrared night vision imaging and applications; Durfee etal). These bistable shutters can have the advantage that the blade position changes with short pulses of voltage to drive the blade between positions. Because bistable shutters are pulsed intermittently, the coils are smaller and lighter to dissipate low amounts of energy over time. Such smaller coils burn out if subjected to continuous power. 
         [0009]    Prior art examples that are or might be related to the technology and/or purposes of the instant invention include: (1) U.S. Pat. No. 4,868,695 issued to Quatro et al. for a “Head/Arm Lock Mechanism for a Disk Drive” (1989) describing a head/arm lock mechanism including a pawl mounted to the armature of a bistable solenoid; (2) U.S. Pat. No. 5,155,522 issued to Castor et al. for “Self Centering Bi-Directional Electromagnetic Actuator” (1992) describing a system for electromagnetically activating the shutter of a camera to provide different aperture openings; (3) U.S. Pat. No. 5,1.59,382 issued to Lee et al. for a “Device and Method for Electromagnetically Activating the Shutter of a Camera” (1992) describing a device and method for electromagnetically activating the shutter of a camera to provide different aperture openings; (4) U.S. Pat. No. 5,497,093 issued to Sundeen et al. for a “Method and Apparatus for Diagnosing a Twin-Coil, Bi-Stable, Magnetically Latched Solenoid” (1996) describing diagnosing the electrical and mechanical operation of a bi-stable magnetically latchning solenoid by monitoring induced voltage across one of a pair of solenoid pairs not being energized; (5) U.S. Pat. No. 5,883,557 issued to Pawlak et al. for a “Magnetically Latching Solenoid Apparatus” (1999) describing a magnetically latching solenoid apparatus characterized by a non-magnetic armature carrying a permanent magnet having poles aligned with the throw axis of the device; and (6) U.S. Pat. No. 7,701,691 issued to Brundisini et al. for a “Control Device for Driving AC Solenoids and DC Bistable Solenoids, Specially for Electrovalves of Irrigation Systems” (2010) describing a control device to drive both AC solenoids and DC bistable solenoids. However, while the foregoing art examples and/or disclosures reveal a variety of forms and systems, none feature the unique combination of elements and advantages offered by the instant invention. More particularly, none disclose, anticipate or obviate a method to convert bistable shutters in monostable applications with blade return in the unpowered state, nor do any disclose, anticipate or obviate how bistable shutters with compact coils that burn out under continuous power can be used in monostable shutters. 
       SUMMARY OF THE INVENTION 
       [0010]    It is an object of this invention to create a monostable shutter with magnetic blade return using a bistable actuator with blade motion constrained to provide drive force in a single direction. It is a further object of the invention to permit the use of compact bistable coil for continuous use by a circuit that applies a high voltage at the beginning of drive that decays to a lower voltage to prevent coil burn out. These objects are accomplished by providing a shutter with blade return on power down comprising: a) a frame with an aperture; b) a bistable actuator with magnetic drive and retention; c) a rotating magnet in said actuator having a magnetic field operable between two angular positions; d) a soft magnet stator arm positioned so that said magnet is attracted to one of two positions to secure said rotating magnet into one of the two rotational positions; c) a blade coupled to said rotating magnet in said actuator having detail to define a first position to cover said frame aperture and a second position to clear said frame aperture, said detail providing two angular positions for said rotating magnet to drive said blade to one position over said frame aperture. The apparatus further includes an electromagnetic coil drive for forcibly driving said rotating magnet and coupled blade to an position that uncovers said frame aperture; means for lowering drive voltage across said coil over time after an fixed voltage has been applied to said coil, which means can advantageously comprise a resistor and capacitor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  provides a schematic top perspective view of a shutter in accordance with the teachings of the invention. 
           [0012]      FIG. 2  provides a schematic bottom view of the shutter in accordance with the teachings of the invention. 
           [0013]      FIG. 3  provides a schematic cross-sectional view of an actuator in accordance with the teachings of the invention. 
           [0014]      FIG. 4  is a plot of torque on the blade as a function of blade angle in the invention. 
           [0015]      FIG. 5  is a circuit that provides a decay drive voltage in accordance with the teachings of the invention. 
       
    
    
     DESCRIPTION 
       [0016]      FIGS. 1 through 3 , provide a basic schematic introduction to the structure of the invention. As illustrated in FIGS. I and  2 , shutter  10  comprises a frame  20  having an aperture  22  and supporting an actuator  30  that is connected to a blade  40  driven by drive pins  32  from actuator  30  via blade holes  42  in a manner typical in the art. Stop pin  50  projects from frame  20  and operates on stop opening  44  in blade  40 . As better seen in  FIG. 3 , actuator  30  has a stator assembly  34  made of low magnetic coercivity metal that conducts a magnetic field. Rotor magnet  38  is connected to drive pins  32 , is pivotally mounted, and will be magnetically attracted to stator arms  36 . Rotor magnet  38  is a permanent magnet that will move in an opposite direction to a magnetic flux induced in stator assembly  34 . A coil  64  is wrapped around stator assembly  34  in the manner typical of the art such that an applied voltage to coil  64  creates a magnetic field that drives rotor magnet  38  away from one position and into the opposing position. 
         [0017]    Drive pins  32  are connected to rotor magnet  38  and interact with/drive blade  40  via their movement in blade holes  42 . Absent stops or controls, rotor magnet  38  and blade  40  will switch between two stable positions based on the direction of current flow through coil  64 . However, the degree and direction of rotation of rotor magnet  34  as well as coupled blade  40  is controlled by stop opening  44  in blade  40  being driven against stop pin  50  due to the previously described linkage of blade  40  to rotor magnet  34  via drive pins  32  and blade holes  42 . In the preferred embodiments illustrated, as more fully explained below, these limit rotor magnet  38  to motion between a 30 degree position and a 10 degree position (both measured relative, in terms of  FIG. 3 , to a vertical position defined as 0 degrees), with the 30 degree position corresponding to the blade  40  position occluding aperture  22  (as shown in  FIG. 1 ) and the 10 degree position corresponding to a blade  40  position exposing aperture  22 . 
         [0018]    The foregoing can be better explained and understood by reference to  FIG. 4 , which provides a plot  70  of torque on blade  40  as a function of blade  40  angle. The zero degree angle is defined as rotor magnet  38  positioned between stator arms  36  so as to be oriented towards neither. Hold curve  71  plots the magnetic force driving blade  40  as a function of blade  40  angle. When rotor magnet  38  is at zero angle the rotational force from the magnetism in rotor magnet  38  is balanced and there is no force rotating blade  40 . When rotor magnet  32  is rotated close to one of the two stator arms  36 , retention force increases and drive force decreases. (See, hold curve  71 ). 
         [0019]    When an electrical current is applied to coil  64  of stator assembly  34 , it forms a magnetic flux interacting with the field from the permanent magnet  38 . In the invention, the current applied and resultant magnetic flux is oriented so as to work against the flux from rotor magnet  38 , driving it from its first stopped position at 30 degrees towards 0 degrees. Drive curves  72  are plotted as a function of the angle with various voltages applied to coil  64  in  FIG. 4 . As illustrated by drive curves  72  in  FIG. 4 , the force is at its peak at the zero degree angle and decays as rotor magnet moves away from 0 angle. As further illustrated by  FIG. 4 , drive force (shown by drive curve  72 ) must be greater than retention force (shown by hold curve  71 ) in order to drive rotor magnet  34  from its locked/blocked position at 30 degrees. In the example plot, 9 volts provides greater torque than holding torque  71 , and one volt does not. Thus, the voltage used for the invention must be chosen to achieve this purpose. 
         [0020]    As previously noted, in the exemplary embodiment of the invention illustrated, the stop detail/opening  44  in blade  40  is designed (in conjunction with stop pin  50 ) to constrain blade motion to one side of torque curve  70  between the 30 degree (first position) and 10 degree (second position). Blade  40  is continuously forced or biased to the far side of the curve (the 30 degree/first position) in accordance with the static magnetic forces delineated in hold curve  71  in its unpowered state. (In safety applications, this is a critical consideration, as it is important that blade  40  close aperture  22  in an unpowered state). The bistable actuator  30  can be pulsed for short periods of time to move the blade from the first position to the second position. The duty cycle of the pulsing is such that the coil is typically undersized for continuous operation. An undersized coil will burn out if continuous voltage is applied. In the invention, decay means is added to shutter  10  to provide a high initial drive voltage when blade  40  is in an initial 30 degree position and the drive voltage decays over time. Decay curve  74  in the exemplary embodiment provides a high, 9 volt, power to the coil initially, and decays applied voltage to 1 volt after the blade has move to the open 10 degree angle, allowing static magnetic forces to once again move rotor magnet  38  so as to urge shutter  10  back to the first (30 degree) position occluding aperture  22 . 
         [0021]      FIG. 5  is a circuit that provides a decay drive voltage in the application. A capacitor  60  and resistor  62  are configured as shown to permit an initial high voltage to decay to a lower voltage. Coil  64  has a high inductance that appears to be open initially. The full drive voltage is applied against coil  4  initially. Capacitor  60  is at zero voltage and shorts out resistor  62 . As field builds in capacitor  60 , voltage develops across resistor  62  to drop voltage across coil  64 . The components are small enough to be parts of shutter  10 , as shown in  FIG. 2 . The values for the three electrical elements are sized based on the dynamic motion of blade  40 . In an example embodiment, coil  64  is  40  ohms and resistor  62  is  200  ohms and capacitor  60  is 100 micro-farads. When nine volts is applied across the circuit, the voltage across coil  64  starts at 9 volts and decays to 1.5 volts after 10 milliseconds. Blade  40  moves within the 10 milliseconds from a 30 degree to 10 degree angle. The final drive voltage is below the voltage required to start blade  40  motion and is low enough to prevent coil  64  from burning out over long operating times. The components required to create the circuit in  FIG. 5  are small enough to be disposed on a circuit board attached to shutter  10  as shown in  FIG. 3 , creating a compact shutter system with a bistable actuator that has magnetic blade return on power down. 
         [0022]    The invention has been described in detail with particular reference to certain preferred embodiments thereof. However, it should be clear that numerous changes and variations can be made without exceeding the scope of the inventive concept outlined. Accordingly, it is to be understood that the embodiment of the invention herein described is merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiment is not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention. 
       PARTS LIST 
       [0000]    
       
           10  shutter 
           20  frame 
           22  aperture 
           30  actuator 
           32  drive pins 
           34  stator assembly 
           36  stator arm 
           38  rotor magnet 
           40  blade 
           42  blade holes 
           44  stop opening 
           50  stop pin 
           60  capacitor 
           62  resistor 
           64  coil 
           70  torque curve 
           71  hold curve 
           72  drive curve 
           74  decay curve