Abstract:
A frangible actuator and switch isolates a defective cell in a battery by switching an electrical circuit when the current through a fusible link exceeds a predetermined value. The high impedance of the defective cell causes most of the battery&#39;s current to flow through the fusible link. The actuator releases a spring-loaded plunger when the high current causes tensile failure of a fusible link. Electrical contacts coupled to the pre-loaded plunger are displaced by a predetermined distance, causing the contacts to move into or out of contact with electrical terminals. The actuator includes two mating parts held together by a restrainig wire, which is in turn held in place by the fusible link. When the fuse melts, fails in tension or otherwise triggers due to excessive current, the restraining wire loosens and allows the two actuator parts to separate. This separation in turn permits the spring loaded plunger to advance, triggering the switching action. The actuator contains the fusible link on an insulator portion rather than on one of the mating parts, so that the connecting wires will not mechanically interfere with the separation of the mating parts. Pins are used between the mating parts to prevent unwanted rotation of the parts and to prevent the resulting false activation of that could thereby occur. The switching contacts use toroidal springs as contact elements to maximize contact area and thereby increase current capacity of the switch.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is derived from provisional application 60/078,312, filed Mar. 17, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrical switching device utilizing a frangible actuator, specifically to a device for bypassing (i.e., isolating) a failed battery cell utilizing an improved frangible actuator. 
     2. Description of the Related Technology 
     A multi-cell battery typically has the cells connected in series so that their voltages will be summed to produce a battery with a higher voltage than could be obtained with a single cell. Unfortunately, when a battery cells fails, it generally develops a high resistance. Since this resistance is in series with the other cells, it effectively disables the entire battery, even though the remaining good cells would be sufficient to keep the battery operating in a slightly degraded mode. For large batteries, where battery cost is high and replacement is difficult, it makes sense to use actuators to detect and isolate failed cells so that the battery can keep operating. Since a defective cell generally cannot be repaired, such actuators are generally one-way single-use actuators, and can be frangible (i.e., they activate by separating). 
     Conventional actuators (used for a variety of purposes) have a number of deficiencies. For the switch portion, a commonly-used structure provides a conductive tube with the end slightly bent in. A slightly smaller conductive cylinder fits within the tube so that it just touches the inwardly indented portion of the tube&#39;s rim. A second tube with a similarly indented end faces the first. Upon activation, the cylinder passes into the second tube, contacting its rim and providing an electrical connection between the two tubes. Unfortunately, such devices can only be used as simple on-off switches. In addition, they provide a minimal contact area, which limits the amount of occurrent that can be conducted. Also, the degradation of contact force due to heating is reduced. 
     Another problem with conventional actuators is that they employ frangible cylinder-type actuators, which are prone to mechanical failure, due to the manner in which certain portions can interfere with other portions during actuation. An actuator is needed that maintains the simplicity of conventional actuators, but with improved reliability and higher current capacity. 
     SUMMARY OF THE INVENTION 
     A frangible actuator may contain a plurality of separable parts, preferably in the form of two cylinder halves pressed together to form an overall cylinder shape. The cylinder halves may be held together by wrapping a restraining wire around them multiple times, and securing the ends of the restraining wire so that it stays in this position. One end of the restraining wire may be secured to one of the separable halves, while the other end may be secured by a sensor that detects when an electrical current exceeds a predetermined threshold. The sensor is preferably a fusible link which melts, separates, deforms or otherwise fails in tension when the current through it exceeds the threshold, thereby releasing the end of the restraining wire. Once the restraining wire is released, the cylinder halves may be free to separate. 
     A spring-loaded plunger may be held in place by the cylinder halves in their restrained position, with the end of the plunger pressed against a conical surface formed between the two halves. When the cylinder halves are allowed to separate, the force of the plunger against this conical surface may force the cylinder halves apart, allowing the plunger to continue moving forward between the cylinder halves until stopped by a physical obstacle. This motion of the plunger may activate a switch. 
     The sensor may be attached to an insulator at one end of the cylinder, so that the electrical connections to the sensor are held away from the cylinder halves to avoid physically interfering with them during separation. The insulator may also include two pins disposed between the cylinder halves to prevent them from rotating under the urging of the restraining wire, which can be made of a spring-like material and be spring loaded in its restraining position. 
     The actuator can activate an electrical switch. The switch may include a contact base formed as a conductive cylinder which slides axially within the bore of multiple electrical terminals. By attaching the contact base to the end of a non-conductive cylinder of the same diameter, the total conductive/non-conductive cylinder may slide within the terminal bores, making or breaking contact with each terminal according to which portion of the cylinder is within that terminal. Reliable electrical contact may be achieved by placing toroidal contact elements within annular grooves in the conductive cylinder. Each contact element may be in the form of a coiled spring with its two ends attached to each other, thus forming a toroid having a spiral spring traversing the circle of the toroid. By sizing the various elements so that outermost portions of the toroid are slightly larger than the diameter of the terminal bore, the contact element may be slightly compressibly deformed when within the terminal bore, thus creating a spring-loaded force at each contact point. Switches formed in this manner may be configured with one or more poles, single- or double-throw, make-before-break or break-before-make, or any combination of these, simply by changing the number and spacing of contact elements and the spacing between terminals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a-c  show a section of the invention in various positions respectively in a pre-activated state, a transitional state and a post-activated state. FIG. 1 a ′ is a cross-sectional view of the invention taken along line  1   a ′— 1   a ′ of FIG. 1 a . FIG. 1 a ″ is an end view taken from the end opposite to that shown in FIG. 1 a ′ of the invention depicted in FIG. 1 a.    
     FIGS. 2 a-b  show the circuitry of the invention respectively before and after actuation. 
     FIGS. 3 a-c  show details of the actuator illustrated in FIG. 1 a , in which FIGS. 3 a  and  3   b  are taken orthogonally to one another. 
     FIGS. 4 a-f  shows various alternative configurations of the switch depicted in FIGS. 2 a  and  2   b , in which FIG. 4 a  switch is a double-pole single-throw switch, the FIG. 4 b  switch is a single-pole double-throw switch, the FIG. 4 c  switch is a single-pole double-throw switch, the FIG. 4 d  switch comprises two separate single-pole single-throw switches, the FIG. 4 e  switch is a single-pole single-throw switch, and the FIG. 4 f  switch is a double-pole switch with one pole being single-throw and the other being double-throw. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention can be a one-shot or single-use device which, upon application of a predetermined electrical stimulus, provides at least one closed and/or open switch connection. Depending on the arrangement of the switching elements, a preferred embodiment of the switch may be configured as a single-pole single-throw or a single-pole double-throw switch. Contacts may be configured in a make-before-break or break-before-make arrangement. Ganged and multiple switch designs are also possible. 
     Switch Assembly 
     FIG. 1 a ′ and  1   a ″ show the battery cell bypass with frangible actuator, in an embodiment adapted for a single-pole double-throw switch. As can be seen, in the “before activation” position, the circuit between terminal T- 1  and terminal T- 2  is closed by conductive contact base  30 , whereas the circuit between terminal T- 2  and terminal T- 3  is opened by non-conductive base  32 . FIG. 1 b  shows the frangible actuator opening, allowing the plunger  14  (generally a dielectric plunger with a conductive tip) to be urged forward by compression spring  18 . Terminals T- 1 , T- 2  and T- 3  are all shown connected in a make-before-break mode. As the compression spring urges the plunger after forward to its final position, FIG. 1 c  shows the circuit between terminals T- 1  and T- 2  has been opened and the circuit between terminals T- 2  and T- 3  has been closed, completing the operation. Improved electrical contact between contact base  30  and the terminals can be achieved by the use of coiled springs  25 ,  27 ,  29  and  29   a  formed in a toroidal shape. Although FIG. 1 a  shows a preferred embodiment employing two pairs of coiled springs  25 , 27  and  29   29   a,  other combinations may also be employed for specific applications, such as two single coiled springs (not shown) replacing the two pairs shown ( 25 ,  27  and  29 ,  29   a ) or one pair of coiled springs  25 ,  27  and a single coiled spring (not shown) replacing paired springs  29 ,  29   a.  Other configurations are also useful, again depending on the specific application. 
     FIG. 1 c  also shows a cross section of the actuator/switch assembly  1  in its pre-actuated condition. In a preferred embodiment, cylindrically-shaped housing  12  can provide physical support for plunger  14 , non-conductive base  32 , and contact base  30 . Plunger  14 , base  32  and contact base  30  may be effectively attached to each other so that they move as a single unit in a longitudinal direction within housing  12 , and this movement may provide the switching action. Contact base  30  may be made of electrically conductive material, with contacts elements  25 , 27  and  29 ,  29   a  providing dependable electrical contact between contact base  30  and electrical terminals T- 1 , T- 2 , and T- 3 . In a preferred embodiment, contact elements  25 ,  27  and  29 ,  29   a  encircle contact base  30  in recessed annular grooves, and make contact with an inside surface of a circular bore within terminals T- 1 , T- 2 , T- 3 . This “full circle” contact area provides for a large contact surface, permitting the switch to carry more current than it could with a single-point contact area. In a preferred embodiment, contact elements  25 ,  27  and  29 ,  29   a  toroidal springs, which can be formed by connecting the two ends of a standard spiral-coiled spring together so the spring assumes the overall shape of a toroid. The various elements of the switch may be sized so that the outer diameter of the toroid is slightly larger that an annular contact surface of a terminal, thereby compressing or deforming the contact element when it is moved into contact with the terminal. The spring-like resistance of the contact element may thus be used to assure good contact at each point. This shape can provide a separate contact point with the terminal for each turn of the spiral in the contact element spring, thereby creating many contact points. With the current flow thereby distributed over a larger area, current density at any given point can be maintained at a lower level, with a corresponding reduction in heat generation and an increase in the surface area for dissipating the heat. This configuration also improves reliability, since poor contact at any given point (due to corrosion, physical defect, etc.) is essentially in parallel with many other good contact points, and thus has little effect on overall current flow. 
     FIGS. 1 a,    1   b,  and  1   c  show the sequence of movement during an activation cycle. In the pre-activated state of FIG. 1 a,  terminal T- 1  is electrically connected to terminal T- 2  through contact elements  29  and  29   a,  contact base  30 , and contact elements  25 , 27 . As contact base  30  moves to the left (as left is depicted in the drawing), terminals T- 1 , T- 2 , and T- 3  are all connected together in the transitional state of FIG. 1 b.  This is a make-before-break configuration, since a new connection is made with terminal T- 3  before the old connection with terminal T- 1  is broken. A break-before-make switch could be configured by spacing T- 1  and T- 3  farther apart, so that they are never connected to T- 2  at the same time. FIG. 1 c  shows the post-activated state, in which terminal T- 2  is connected to T- 3  through contact elements  29  and  29   a,  contact base  30 , and contacts  25 ,  27 . 
     Another advantage of the toroid-spring contact elements is that all the forces required to assure electrical continuity are contained within the contacts themselves, and therefore are not reliant upon any external members or features to react upon. 
     Although FIGS. 1 a-c  show a single-pole, double throw, make-before-break switch, other configurations can be easily incorporated. Additional poles can be implemented by adding more contact bases  30 , electrically isolated from each other, if separate electrical circuits are to be switched. Single/double throw operation can be implemented simply by changing the quantity of the contact elements and associated terminals. Break-before-make or make-before-break can be implemented by simply changing the spacing between contact elements. FIGS. 4 a-f  show several different switch types which might be implemented. In each figure, the two terminals at the bottom of the figure represent the fusible link (which may take the form or a fusible link wire), while the remaining terminals represent the switch terminals. FIG. 4 a  shows a double-pole single-throw switch, FIG. 4 b  a single-pole double-throw switch, FIG. 4 c  a single-pole single-throw combined with a separate single-pole double-throw, FIG. 4 d  shows two separate single-pole single-throw switches, FIG. 4 e  has one single-pole single-throw switch, and FIG. 4 f  double-pole switch with one pole being single-throw and the other double-throw. 
     Regardless of the switching configuration, the necessary force for the switching action may be provided by spring  18 , which is normally constrained from motion because plunger  14  is prevented from moving by frangible actuator  10 . When actuator  10  is split into two halves as shown in FIG. 1 b,  plunger  14  is free to move between the two halves, and the force of spring  18  can urge plunger  14 , non-conductive base  32 , and contact base  30  toward insulator  65  as shown in FIG. 1 c.    
     Actuator 
     The actuator may be an enabling device that initially restrains a coaxially located shaft from axial movement, but releases the restraint upon application of a predetermined minimum amount of electrical current. The actuator includes a fusible link, which may include any resistive material that decreases its tensile strength in response to an increase in temperature. In a preferred embodiment, the fusible link is a length of 18-8 stainless steel wire. The actuator also includes a restraining wire, an insulator assembly, and two cylinder halves. A preferred embodiment may use several mechanical advantages to multiply the holding capability of the fusible link, such as inclined planes or cones, and multiple wraps of a restraining wire. Reliability may be improved by using anti-rotation pins, and by connecting the fusible link to a non-moving part. A current sensor, such as the fusible link, may be used to hold the restraining wire in place during normal operation, but release the restraining wire when an overcurrent condition is detected. The fusible link may be made of high-strength, corrosion-resistant, heat-resistant material with a length and diameter sufficient to create the necessary thermal and electrical effects. In a preferred embodiment, the fusible link may have an electrical resistance of about one ohm, and a diameter large enough to continuously dissipate the heat generated by a predetermined-maximum current (in one embodiment, one amp) but small enough to heat past its stress-failure point if the current exceeds that predetermined maximum. When the fusible link temperature exceeds its stress-failure point, it can release the restraining wire, which in turn may release the mechanical components of the actuator. In a preferred embodiment, stress failure is characterized by melting, separation or other tensile failure of the fusible link. 
     FIGS. 3 a-   3   c  show a preferred configuration of actuator  10  in greater detail. As shown, the two halves  62 ,  64  of a cylinder-shaped device may be bound together by multiple wraps of a restraining wire  50 . Restraining wire  50  may be secured at one end to one of the cylinder halves, and restrained at the other end by fusible link  46 . Fusible link  46  may be terminated at either end by electrical terminals T- 4  and T- 5 . These terminals may be attached to insulator  65  through pre-formed holes. Access to the terminals by fusible link  46  may be acquired through access holes  67 . Conventional devices typically attach the fusible link to one of the cylinder halves, where the fuse terminals or connecting wires could get caught in the uncoiling restraining wire and jam it, preventing actuation. By placing the fusible link on the non-moving, non-frangible insulator as shown, the present invention prevents this problem by keeping all such components away from the uncoiling restraining wire  50  and the moving cylinder halves  62 ,  64 . 
     When bound together as described, cylinder halves  62 ,  64  may form a pyramid-shaped or cone-shaped recess  68  at one end. In the non-actuated position shown in FIG. 1 a,  plunger  14  may be pressed into this recess, where it tries to force cylinder halves  62 ,  64  apart with the insertion force provided by spring  18 . But since the two cylinder halves are tightly bound together by restraining wire  50 , this force may be unable to cause separation. 
     When a stress failure of fusible link  46  occurs, it can release the end of restraining wire  50 , which in turn releases cylinder halves  62 , 64 , allowing them to separate. The force of plunger  14  against recess  68  may force cylinder halves  62 ,  64  apart, allowing plunger  14  to penetrate between the cylinder halves until it is stopped by recess  66  in insulator  65 . In a preferred embodiment, restraining wire  50  may be made of spring-like material, which in its unrestrained state is either straight or has a curvature larger than in its restrained state. When such a wire is released, it may “uncoil” from the cylinder, thus releasing the two cylinder halves. The interior walls of housing  12  can prevent the unrestrained wire from flying out too far and possibly interfering from with other parts of the device. In an alternate embodiment, restraining wire  50  may simply be flexible wire without the “memory shape” characteristics of a spring, and may be forced to uncoil simply by the force of plunger  14  separating the two cylinder halves. This configuration may require greater force from plunger spring  18 , since it must overcome the friction of restraining wire  50  against the cylinder halves. 
     Since a spring-loaded restraining wire  50  can impart a twisting force on the cylinder, cylinder halves  62 ,  64  must be prevented from rotating and thereby unwinding wire  50 , causing the actuator to inadvertently actuate. This prevention may be accomplished with pins  70 ,  72  inserted between the cylinder halves and attached to insulator  65 . As shown in FIG. 3 c,  these pins can prevent cylinder halves  62 ,  64  from rotating but do not impede separation. Since plunger shaft  14  fits between the two pins, the pins also prevent the cylinder halves from interfering with the plunger during actuation. Conventional devices typically place the cylinders in a recess in the insulator, where frictional forces between the cylinder half and the insulator can impede the separation motion. 
     Fuse terminals T- 4  and T- 5  are shown as conductive posts, with fusible link  46  shown as a short piece of wire connected between terminals T- 4  and T- 5 . Referring to both FIGS. 2 a  and  3   a,  as the cell fails and current flowing through diode  44  or  45  (or as noted below, other voltage sensitive electrical component) exceeds the diode threshold limit, such current is sufficient to heat and cause tensile failure of the fusible link  46 . Restraining wire  50  is normally held in place by having its end  51  hooked over fusible link  46 . When fusible link  46  fails in tension, hook end  51  is released, and restraining wire  50  is allowed to uncoil, thus allowing the two cylinder halves  62  and  64  to separate. Referring back to FIG. 1 a,  prior to separation, initiator segments  62 ,  64  in their closed position restrain the movement of plunger  14 . As shown in FIG. 1 b,  when the restraining effect of restraining wire  50  is removed, plunger  14  is urged forward by spring  18 , causing cylinder halves  62 ,  64  to be spread apart by the force of plunger  14  against angled recess  68  (see FIG. 3 b ). Once cylinder halves  62 ,  64  are open sufficiently wide, plunger  14  may continue moving forward essentially without resistance, until plunger  14  encounters end  66  of the bore, as shown in FIG. 1 c.    
     The time it takes for the actuator to actuate is the sum of the time it takes fusible link  46  to melt or otherwise fail in tension, and the time for the mechanical parts to complete their motion. In a preferred embodiment, this total time is a few milliseconds. Variation in this time may be primarily due to the actuating current, which dictates how long it takes fusible link  46  to heat up and fail in tension. The time should be consistent for any given actuating current. 
     Although the cylinder halves are so named because of their shape in a preferred embodiment, they might assume various other geometric shapes as well, and there might be more than two such parts. An important consideration is that their shape and quantity permit the uncoiling of the restraining wire during actuation. 
     Circuit 
     FIGS. 2 a  and  2   b  show how the device is used in the context of a battery cell bypass. FIG. 2 a  schematically shows bypass circuit  42  attached to a battery cell # 2  in which all cells are functioning. As can be seen, the cells are connected in series, so that if one cell fails by developing high resistance (the normal failure mode for a cell), the entire battery fails, even though all other cells may be functional. In a preferred embodiment, a bypass switch and sensor mechanism includes a voltage detector, such as diodes  44 , 45 , for detecting a voltage drop across a battery cell. Diodes  44 ,  45  may be connected in parallel and together connected in series with a fuse, actuator, or other current-activated cutoff device, such as fusible link  46  (FIG. 3 a ) between terminals T- 4  and T- 5 . Fusible link  46  is adapted for triggering switch  47 , which has terminals T- 1 , T- 2  and T- 3 , and is connected between cell # 1  and cell # 3 . In normal operation as shown in FIG. 2 a,  diodes  44 ,  45  block current flowing in either direction unless the voltage drop across the diodes exceeds the small threshold value of the forward-biased diode. As long as the impedance of the diodes is much greater than that of the battery cell, most of the current will flow through the cell rather than the diode. Using two diodes with opposite polarity allows the sensor to operate with either battery polarity. 
     Each diode therefore effectively functions as a conductor in one direction and a high resistance insulator in the other direction, causing most of the current from cell # 1  to travel through cell # 2  to cell # 3 . FIG. 2 b  shows that in the event of a failure of cell # 2 , most of the current from cell # 1  (which is greater than the threshold limits of the diodes) cannot pass through the high resistance of cell # 2  and therefore passes through the diodes and through fusible link  46 . When fusible link  46  melts, fails in tension or otherwise triggers, this actuates switch  47 , causing the circuit between terminals T- 1  and T- 2  to be broken and the circuit between T- 2  and T- 3  to be completed. As can also be seen, terminal T- 3  is connected to a bypass circuit beginning at the end of cell # 1 , such that with the bypass switch activated, a closed circuit exists between cell # 1  and cell # 3 , bypassing cell # 2  and allowing the battery to continue functioning despite the loss of that cell. In a typical application, a good battery cell may have an internal impedance of a few milli-ohms or less, a defective cell may have an impedance of hundreds of ohms or higher, and fusible link  46  in series with diodes  44 ,  45  may have a resistance of about one ohm. 
     Cell # 3  is shown with a similar bypass circuit  43 . In a preferred embodiment, every cell in the battery will be protected by a bypass switch of the type described. Although the switch shown is a single-pole double-throw switch, other possible combinations may be used depending on the specific application. FIGS. 2 a  and  2   b  show one possible embodiment of the invention. Alternately, transistors rather than diodes can be used to activate a bypass circuit as soon as a predetermined power level is detected. The circuit may also be activated by sensors that sense gaseous pressure or temperature within a given cell. 
     The embodiments of the invention described herein are illustrative and not restrictive. Numerous variations may occur to those of skill in the art that fall within the spirit of the invention. The scope of the invention is therefore limited not by the particular examples described herein, but only by the scope of the attached claims.