Patent Publication Number: US-7595710-B2

Title: Maglatch mechanism for use in lighting control pod

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 60/830,535 entitled “Maglatch Mechanism for Use in Lighting Control Pod,” filed on Jul. 13, 2006, the contents of which are hereby incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to an improved remote controlled circuit breaker and circuit control assembly, and more specifically to remote controlled contacts having a magnetic latch mechanism providing a bi-stable operation. 
     BACKGROUND OF THE INVENTION 
     There has been an increasing demand for remotely controllable circuit breaker assemblies that can reciprocate between an open circuit and a closed circuit in response to a remotely generated command. One advantageous application for such circuit breaker assemblies is in control panelboards that are used for automated control systems such as building management systems. Building management systems may include automated lighting systems, HVAC control systems, fire control, security, and control of refrigerator/freezer systems. Automated lighting systems have been developed for the control of lighting circuits based upon inputs such as the time-of-day, wall switches, occupancy sensors and/or control from a power distribution system. Lighting control systems offer an opportunity to save energy by automating the process of cutting back on the number of lighting fixtures that are illuminated, automatically turning off lighting fixtures when they are not required, or by cutting out artificial lighting altogether when circumstances warrant. For example, ambient light sensors can be used to control lighting circuits in response to ambient light levels. The sensors can serve both switching and automatic dimming functions that can adjust the output of the lighting system continually in response to the amount of daylight striking the ambient light sensor. Occupancy sensors can be used to activate lighting when someone is in a space and to deactivate the lighting, perhaps after a set time interval, when a person is no longer detected in the space. 
     In general, circuit breaker assemblies that can be remotely controlled may be divided into at least two classes. The first is the remote-operated circuit breaker. In a remote-operated circuit breaker, two pairs of contacts are located within a single package. The first (or primary) pair of contacts is used to interrupt short circuits, to interrupt overloads, and to switch the circuit breaker on and off via a handle. The second pair of contacts in a remote operated circuit breaker may be used, for example, in a lighting control application. In some applications, a single pair of contacts serves both functions. 
     Another class of remotely controlled circuit breaker assemblies is an assembly that includes a circuit control pod, or lighting control pod. In such an assembly, a separate relay device or “pod,” including a mechanism to operate a pair of contacts remotely, is attached to a standard circuit breaker that does not have a means of remote operation. The circuit control pod adds an additional pair of contacts in series with the circuit breaker. 
     Several types of mechanisms have been used to remotely operate the contact pair in a circuit control pod. Those include a bi-directional solenoid with an over-center spring, a worm-gear actuated DC motor system, and a multi-linkage solenoid driven mechanism. 
     In the over center design, a solenoid must be sized to work against a non-linear spring force. The solenoid must furthermore have two coils to operate bi-directionally. Those factors can increase the size of the required mechanism. 
     The worm-gear motor design produces a loud noise due to the operation of the DC motor. The worm-gear design is furthermore prone to slippage and failure of the mechanism. Also, when applied in arrays such as those found in standard panel boards having 42 devices, issues such as motor in-rush and under-voltage conditions in the power line must be overcome by increasing the size and complexity of power supplies or the power management system. 
     The multi-linkage solenoid driven mechanism has the disadvantage of requiring several points of rotation, and numerous moving parts. In typical applications, multiple springs are required. Given that a lighting control device is expected to cycle 50,000-100,000 times during its life, the use of multi-spring assemblies increases the risk that frictional wear will cause the mechanism to fail during its intended life. 
     U.S. Pat. No. 4,816,792 to Belbel et al. describes a main circuit breaker contact that may be remotely operated by an electromagnet. The design incorporates a permanent magnet for holding an armature in position. The permanent magnet mechanism operates directly on the circuit breaker contacts. Such a design increases the mass of the circuit breaker mechanism and thus results in parasitic loading of the breaker mechanism, degrading performance. 
     U.S. Pat. No. 6,531,938 to Smith et al. teaches a remote operated circuit breaker assembly having a remote module for remotely operating the circuit breaker. A motor disposed in the module housing operates the breaker switch remotely. The mechanism requires actual operation of the handle of the breaker. Because the breaker handle requires greater force, the actuating device must be a larger and higher-cost unit. 
     There is presently a need for an improved design and method for opening and closing remote controlled contacts. Such a design should have a low cost and should be of high reliability. Such a design should furthermore be compact for use in a small package area. To the inventors&#39; knowledge, no such design is currently available. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention is a contact assembly for reciprocating between a stable closed position to allow current flow through the contact assembly and a stable open position to prevent current flow through the contact assembly. The assembly includes a base, a fixed contact mounted to the base, a contact arm, and a pivot pin for pivotably mounting the contact arm to the base. A moveable contact is mounted on the contact arm for movement between the stable closed position wherein the moveable contact is in contact with the fixed contact, and the stable open position wherein the moveable contact is spaced apart from the fixed contact. A spring exerts a spring force on the contact arm to bias the moveable contact toward the stable closed position of the contact assembly. 
     The contact assembly also includes a magnetic latch solenoid comprising a magnetic armature and a permanent magnet in proximity to the armature when the armature is in a retracted position. The permanent magnet has a magnetic field exerting a latching force on the armature to maintain the armature in the retracted position. The magnetic latch solenoid also includes a coil in proximity with the armature, the coil being adapted to exert a retracting force on the armature in excess of the spring force in a direction of the retracted position of the armature when electrical energy is applied to the coil in a first polarity, and to disrupt the magnetic field when electrical energy is applied to the coil in a second polarity to release the armature from the retracted position. 
     A wrist pin connects the contact arm and the armature, the wrist pin being disposed in a clearance hole in at least one of the contact arm and the armature, permitting relative movement thereof. The retracted position of the armature results in the stable open position of the contact assembly. 
     The spring may be separate from the magnetic latch solenoid. 
     The contact assembly may also include a line terminal for connection with an electrical source of the current flow, and a braided wire connector electrically connecting the line terminal and the contact arm. 
     The contact arm may further comprise a mechanical spring interface, the spring exerting the spring force between the mechanical spring interface and a base of the contact assembly. The mechanical spring interface may be remote from the moveable contact on the contact arm. 
     The contact assembly may also include a load terminal electrically connected to the fixed contact for connecting a current load to the contact assembly. 
     The assembly may comprise a printed circuit board connected to the coil for applying a pulse of electrical energy to the coil in the first polarity to open the contacts and for applying a pulse of electrical energy to the coil in the second polarity to close the contacts. The pulses of electrical energy may be pulse-width-controlled DC signals. 
     Another embodiment of the invention is a method for remotely operating a contact assembly between a stable closed position to allow current flow from a line to a load through the contact assembly and a stable open position to prevent current flow through the contact assembly. The method includes the steps of providing a fixed contact connected to a load side of a circuit breaker, the breaker being set to open the circuit between the line and the load at or above a predetermined current load; providing a moveable contact adapted for movement between the stable closed position wherein the moveable contact is in contact with the fixed contact and the stable open position wherein the moveable contact is spaced apart from the fixed contact; providing a spring exerting a spring force on the moveable contact toward the stable closed position; providing a magnetic latch solenoid including a magnetic armature connected to the moveable contact for movement therewith; a permanent magnet in proximity to the armature when the moveable contact is in the stable open position, the permanent magnet having a magnetic field exerting a latching force on the armature to maintain the armature and moveable contact in the stable open position; and a coil in proximity with the armature; applying electrical energy to the coil in a first polarity to exert an opening force on the armature in excess of the spring force to move the armature and moveable contact to be held in the stable open position by the latching force of the magnetic field; and applying electrical energy to the coil in a second polarity to disrupt the magnetic field and release the armature and moveable contact from the stable open position to be displaced by the spring to the stable closed position. 
     The steps of applying electrical energy to the coil may further comprise applying electrical pulses to the coil. The pulses of electrical energy may be pulse-width-controlled DC signals. The electrical energy may be approximately 1.7 amps at 24 volts DC. The steps of applying electrical energy to the coil may include applying at least one pulse having a duration of less than 50 milliseconds. The step of applying electrical energy to the coil in the second polarity may comprise applying a pulse having a duration of less than 10 milliseconds. 
     Yet another embodiment of the invention is a circuit breaker assembly positionable in a circuit between a line and a load. The circuit breaker assembly comprises a circuit breaker set to open the circuit between the line and the load at or above a predetermined current load; and a contact assembly adapted for reciprocating between a stable closed position to allow current flow through the contact assembly and a stable open position to prevent current flow through the contact assembly. 
     The contact assembly comprises a fixed contact connected to the load side of the circuit breaker; a moveable contact electrically connected to a load side conductor for connection to a load, the moveable contact being moveable between the stable closed position wherein the moveable contact is in contact with the fixed contact, and the stable open position wherein the moveable contact is spaced apart from the fixed contact; a spring exerting a spring force on the moveable contact and biasing the moveable contact toward the stable closed position; and a magnetic latch solenoid. The magnetic latch solenoid comprises a magnetic armature connected to the moveable contact for movement therewith; a permanent magnet in proximity to the armature when the moveable contact is in the stable open position, the permanent magnet having a magnetic field exerting a latching force on the armature to maintain the armature and moveable contact in the stable open position; and a coil in proximity with the armature, the coil being adapted to exert an opening force on the armature in excess of the spring force in a direction of the stable open position when electrical energy is applied to the coil in a first polarity, and to disrupt the magnetic field when electrical energy is applied to the coil in a second polarity to release the armature and moveable contact from the stable open position. 
     The spring may be separate from the magnetic latch solenoid. The contact assembly may further comprise a contact arm having a first end pivotably mounted to a base of the contact assembly, the moveable contact being mounted on a second end of the contact arm. 
     The contact assembly may also include a wrist pin connecting the armature and the contact arm, the wrist pin being disposed in at least one clearance hole permitting relative movement of the armature and the contact arm. A braided wire connector may electrically connect the load side conductor and the contact arm. The contact arm may also comprise a mechanical spring interface, the spring exerting the spring force between the mechanical spring interface and a base of the contact assembly. The mechanical spring interface may be remote from the moveable contact on the contact arm. 
     The contact assembly may also comprise a printed circuit board connected to the coil for applying a pulse of electrical energy to the coil in the first polarity to open the contacts and for applying a pulse of electrical energy to the coil in the second polarity to close the contacts. The pulses of electrical energy may be pulse-width-controlled DC signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are perspective views of a magnetic latch solenoid in extended and retracted positions, respectively, in accordance with the invention. 
         FIG. 2  is a diagrammatic cross sectional view of a magnetic latch solenoid in accordance with the invention. 
         FIG. 3  is a perspective view of a partial electrical contact assembly in accordance with an embodiment of the invention. 
         FIGS. 4A-4D  are simplified force diagrams showing a contact arm and moveable contact in accordance with the invention. 
         FIG. 5  is a perspective view of an electrical contact assembly including a printed circuit board and several associated components, in accordance with an embodiment of the invention. 
         FIGS. 6A and 6B  are perspective views of an electrical contact assembly in open and closed positions, in accordance with an embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     The present invention is a method and apparatus for opening and closing a pair of contacts in a circuit control pod such as a lighting control pod. A magnetic latch solenoid mechanism, or “maglatch,” is employed with a spring that is not located in close proximity to the contacts or to the maglatch in order to provide bi-stable operation. 
     A maglatch is a variation of a solenoid in which a permanent magnet is added to a solenoid. This component allows for translation of electrical signals to a mechanical motion. A maglatch used in a preferred embodiment of the present invention is shown in  FIGS. 1A and 1B . The maglatch includes a maglatch housing  110  and a plunger  160 . The plunger  160  may have a wrist pin hole  130  for accepting a wrist pin as described in more detail below. 
     A schematic cross sectional view of a maglatch  210  in accordance with the invention is shown in  FIG. 2 . A plunger  260  extends from the maglatch  210  and corresponds to the plunger  160  of  FIG. 1A . Other element numbers incremented by multiples of 100 in other figures represent similar elements. A stationary magnetic core  250  surrounds the plunger. The magnetic core may be made of a soft ferrous material that responds to magnetic fields. The plunger  260  is mounted for reciprocating movement in the maglatch, using bushings or other means (not shown) as known in the art. 
     The maglatch  210  further comprises electromagnetic coil  230  connected to a housing of the maglatch (housing  111  of  FIG. 1A ). The coil  230  induces a magnetic field in the core  250  when current is passed through the coil. The magnetic field exerts a force on the armature  250  in an axial direction; i.e., along an axis  270  of the maglatch  210 . When an electrical potential is placed across the coil in first polarity, magnetic force on the armature urges the armature in an upward direction as oriented in  FIG. 2 , retracting the plunger into the maglatch. 
     The maglatch  210  further comprises a permanent magnet  215  and may include a flux guide  216 . When the plunger  260  is in a retracted position and therefore proximate the permanent magnet  215  and flux plate  216 , a strong magnetic circuit is formed through those members, exerting an attractive force on the plunger  260  and “latching” it in the retracted position. 
     The effect of the permanent magnet  215  depends upon the position of the plunger  260 . When the plunger is extended, the magnet provides no function because the air gap  280  in the magnetic circuit is sufficiently large to greatly weaken the field. When the solenoid is pulsed with current in the first polarity, electromagnetic forces on the plunger  260  pull it inward. Once the plunger is retracted, the permanent magnet  215  of the maglatch holds the plunger in place. That holding force creates one of the two stable positions of the switching mechanism of the invention. The holding force is directly dependant upon the strength of the maglatch permanent magnet. The solenoid portion of the maglatch creates the force that allows the plunger to move from the extended position to the retracted position. 
     In order to provide motion in the other direction, i.e., to extend the plunger, the switching mechanism also requires a spring  390 , shown in  FIG. 3 . The spring  390  is mounted externally to the maglatch mechanism  310 , and acts on an L-shaped contact arm  383 , pivotably mounted to a base by a pivot pin  380 . A moveable contact  382  is mounted on the contact arm  383  by welding or another method. In the closed position shown in  FIG. 3 , the moveable contact  382  is in contact with a fixed contact  381 . The spring  390  is preferably a compression spring that places a continuous force on the contact arm  383  to extend the plunger  360 . When the plunger is retracted, the spring force on the plunger  360  is lower than the force on the plunger of the permanent magnet, maintaining the plunger in the stable retracted position. 
     Returning to  FIG. 2 , when a brief DC pulse is provided to the maglatch  210  by applying a potential to the coil  230  in a second polarity, the electromagnetic field of the coil temporarily disables the permanent magnet  215  by interfering with the magnetic circuit containing the plunger  260 . That allows the spring to move the plunger  260  outward until the plunger is fully extended. In the extended position, the spring holds the plunger in its second stable position, with the contacts  381 ,  382  in contact. 
     The contact assembly of the present invention has two stable equilibrium positions: contacts closed and contacts open. Those positions will now be described with reference to  FIGS. 4A-4D . In the “contacts closed” position shown in  FIG. 4A , the force of the spring (F s ) on the contact arm  483  creates a torque about the pivot pin  480 , biasing the moveable contact  482  against the fixed contact (not shown), which acts as a mechanical stop. The spring force therefore holds the contacts closed. A reaction force (F r ) on the moveable contact  482  creates a corresponding torque on the contact arm  483 , maintaining equilibrium. The maglatch does not affect the mechanism. 
       FIG. 4B  shows the contact arm in the stable “contacts opened” position of the contact assembly. The permanent magnet in the maglatch applies a continuous force F L  to the device. The force F L  is greater than the force F s  exerted by the spring. A mechanical stop (not shown) applies a reaction force and prevents further counterclockwise rotation of the arm. 
     The force diagram of  FIG. 4C  shows the contact arm in a non-equilibrium state, resulting in motion from the “contacts closed” position to the “contacts open” position. To start that motion, a force F sol  is generated by the solenoid acting on the armature of the maglatch, placing a torque on the contact arm that exceeds the torque from the spring force F s . The contacts are thereby moved apart as the plunger retracts into the maglatch, until the permanent magnet in the maglatch latches the plunger in the retracted position. 
     The force diagram of  FIG. 4D  illustrates motion of the contact assembly of the invention from the “contacts open” position to the “contacts closed” position. A DC pulse applied across the maglatch solenoid temporarily disables F L  by interfering with the magnetic field of the permanent magnet. As a result, the spring force F s  rotates the contact arm clockwise until the contacts are closed, providing a mechanical stop. 
     A preferred embodiment of the circuit control pod  500  of the invention, including its major components, is described below with reference to  FIG. 5 . 
     The spring  590  is a compression spring located away from the contacts  581 ,  582  to reduce the spring&#39;s exposure to heat generated by opening and closing the contacts. The spring is captured directly by the base and cover (not shown) of the circuit control pod  500 , and acts on the L-shaped contact arm  583 . 
     The contact arm  583  serves several functions. The arm provides a conductor for current flow to the moveable contact  582 . Line current flows from a line side terminal  570  through a braided wire conductor (not shown) that is welded to the contact arm in the region near the pivot pin  580 . The line current then flows from the braid weld site through the arm to the moveable contact  582 . The moveable contact is also welded to the contact arm. Other connection techniques, such as soldering and brazing, may alternatively be used to attach the braid and the moveable contact to the contact arm. 
     The contact arm  583  pivots about the pivot pin  580  to provide the motion to open and close the electrical contacts  581 ,  582 . The arm  583  further provides a mechanical interface  591  with spring  590 . The arm provides mechanical support for both the pivot pin  580  and the wrist pin  530 . 
     In one embodiment of the invention, the contact arm  583  provides mechanical support for an armature  571  used in a “blow closed” mechanism that also includes a magnetic yoke  572  mounted in proximity to the line side conductor  570  and the contact arm  583 . The “blow closed” mechanism operates when excess current flows through the contact arm  583  and the line side conductor  570 , inducing a magnetic field in the yoke  572 , which exerts an attractive force on the armature  571 . That attractive force holds the contacts closed and resists forces at the contacts that otherwise tend to blow the contacts apart under high current loads. 
     The contact arm  583  serves as one of a pair of parallel conductors that additionally holds the contacts  581 ,  582  together under over-current conditions. Current flowing in parallel paths in opposing surfaces of the contact arm  583  and the line side conductor  570  exert attractive forces between those two components. Those attractive forces, in addition to the force of the spring  590  and the above-described “blow-closed” mechanism, hold the contacts closed during an overcurrent condition. The parallel conductors and the “blow-closed” mechanism are described in more detail in the commonly assigned patent application entitled “Design and Method for Keeping Electrical Contacts Closed During Short Circuits,” filed concurrently with the present application, the contents of which are hereby incorporated by reference herein in their entirety. 
     The contact arm  583  may also serve as part of a visual flag indicator (not shown) and as part of an auxiliary contact mechanism (not shown). Further, if the angle of the spring is changed, and the contact arm  583  is slotted to permit translation relative to the pivot pin  580 , the contact arm may be adapted to allow sliding motion between contacts to break tack welds that may result from arcing. 
     The pivot pin  580  provides for smooth rotation of the contact arm  583 . The pin is captured in the base  675  ( FIG. 6 ) and cover (not shown) of the lighting control pod. The pin may be made of hardened steel for additional endurance of the pivot joint. The pivot pin connection provides long life to the joint as compared to known contact arm joints. 
     The contact pair includes a moveable contact  582  and a fixed contact  581 . The contacts make and break the electrical load. The moveable contact  582  is welded directly to the contact arm  583 . The fixed contact  581  is welded to the load terminal  584 . 
     The load terminal  584  provides an electrical connection from the contact  581  to the outside of the circuit control pod. The other end of the load terminal interfaces with a lug  585  for the securing of an external conductor (wire, electrical bus, etc.) to the circuit control pod. Features of the load terminal allow for a robust mechanical and electrical connection. 
     A wrist pin  530  is provided to allow for differences between the linear motion of the maglatch plunger  560  and the rotational motion of the contact arm  583 . For the limited rotational motion of the preferred design relative to the length of the arm, a small amount of clearance is provided in the hole diameter where the wrist pin  530  engages the contact arm  583 . 
     The printed circuit board  573  provides internal control of the circuit control pod. The printed circuit board receives power through an external connector  574 . The printed circuit board  573  switches the polarity and duration of energy supplied to the maglatch  510  so that no additional devices (diode bridge, etc.) are required to operate the maglatch. 
     In a preferred embodiment, the circuit control pod is part of a larger system called an Integrated Lighting Control System. In the Integrated Lighting Control System, a set of many circuit control pods is connected to a computer via a communications bus. Signals to open or close the circuit control pod contacts are sent by the computer down the communication bus. When the signal reaches a circuit control pod, the circuit control pod electronics identify that the signal is intended for a particular circuit control pod. One technique for identifying a particular circuit control pod on a communications bus is disclosed in U.S. Patent Publication No. 20070064360, published Mar. 22, 2007 and entitled “Selection Line and Serial Control of Remote Operated Devices in an Integrated Power Distribution System,” the contents of which are incorporated by reference herein in their entirety. 
     Once the signal is decoded, the circuit control pod printed circuit board  573  issues a positive DC, pulse-width-controlled signal of 18-50 milliseconds in duration to the maglatch  510 . The printed circuit board  573  must properly regulate the pulse width and polarity in order to retract the maglatch plunger  560 . When the opposite motion is desired, the circuit control pod electronics board  573  delivers a negative DC pulse for 2-6 milliseconds. That second pulse temporarily disrupts the field of the permanent magnet within the maglatch  510 , allowing the plunger  560  to extend. 
     A maglatch circuit control pod  600  of the present invention is shown in  FIGS. 6A and 6B  as mounted in a base  675 . The base may be made from a heat-tolerant insulating material such as a high-temperature thermoplastic or a thermoset resin. The pod  600  is shown in an open position in  FIG. 6A , with the maglatch  610  retracted. The pod  600  is shown in a closed position in  FIG. 6B  with the maglatch  610  extended and the contacts  682 ,  681  closed. 
     The maglatch circuit control pod of the present invention has numerous advantages over existing switching devices. As compared to a worm-gear motor design, the device is quiet; the only noise produced being the sound of contacts striking. The device furthermore runs on very low power. For example, a preferred embodiment of the invention requires only about 1.7 A at 24 VDC for 2-25 milliseconds. 
     Operation of maglatch circuit control pod of the present invention is rapid. The inventors have measured response times for a device according to the invention at less than 4.5 milliseconds to break continuity. 
     The device of the invention is compact in part because it does not require a large armature for mechanical advantage. Because the device does not also manage or conflict with circuit breaker functions, it is simplified electrically and mechanically, and does not require compromises on contact design. 
     Due in part to the pivot pin and wrist pin designs, the system has a longer mechanical life. The expected life of a contact assembly according to one embodiment of the invention is in excess of 450,000 cycles. 
     The foregoing detailed description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the description of the invention, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. For example, while the contact arm is described herein as having a particular L-shaped configuration, other contact arm designs may be substituted. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.