Abstract:
A limit switch includes a housing, a plunger, an actuator, a prime contactor, a redundant prime contactor, an adjustment contactor, and a redundant adjustment contactor disposed within the housing. The prime contactor and adjustment contactor are electrically coupled in series. The prime and redundant prime contactors are openable by the plunger, and the adjustment and redundant adjustment contactors are closable by the plunger. The redundant adjustment contact is electrically coupled in series with the redundant prime contactor. The limit switch controls a circuit by aligning the limit switch in a mid-position alignment. Opening the prime contactor or redundant prime contactor de-energizes the electrical circuit. Closing the prime contactor and the redundant prime contactor while the adjustment contactor and the redundant adjustment contactor are maintained closed energizes the electrical circuit. Opening the adjustment contactor or redundant adjustment contactor when the actuator is misaligned from the plunger de-energizes the electrical circuit.

Description:
BACKGROUND 
     In electromechanical systems, switches provide the function of making and breaking electrical contacts and consequently electrical circuits. In certain system applications, there is a need to employ a switch that is capable of breaking and making electrical contact in circuits that control and/or power equipment. The equipment is desired to have an automatic electrical disconnect installed into the operating circuitry so that upon the occurrence of a selected condition, the circuit can be de-energized. 
     One type of switch is a limit switch which is a device that serves the function of connecting and disconnecting circuits in a fashion that is related to a given set of inputs. The inputs typically are mechanically derived positions that are linked to the device that is being protected by the limit switch. For example, when a device has reached a point of travel in a particular direction, the limit switch is activated to disconnect the electrical circuit usually stopping the mechanical motion of the device. In this fashion the limit switch limits the degree of travel or motion of the device and so can protect the device. 
     If a spring is broken in such a switch and no longer provides a bias, then contacts will not be opened upon actuation. Additionally if the contacts become welded perhaps from arcing across the contacts in the circuit, the spring bias may not be capable of opening the contacts. The circuit will remain energized even though the limit switch has been positioned to release the contacts. A spring failure or a stuck contact will prevent the contacts from opening. Damage to the equipment may result. 
     SUMMARY 
     Electrical circuits are controllable in electromechanical systems through limit switches. Limit switches can open and close electrical circuits to protect equipment. A limit switch is disclosed comprising a housing, a prime contactor and an adjustment contactor disposed within the housing. The prime contactor and the adjustment contactor are electrically coupled in series. A plunger is disposed in the housing. The prime contactor is openable by the plunger, and the adjustment contactor is closable by the plunger. A redundant prime contactor and a redundant adjustment contactor are also disposed within the housing. The redundant adjustment contactor is electrically coupled in series with the redundant prime contactor. The redundant prime contactor is openable by the plunger, and the redundant adjustment contactor is closable by the plunger. An actuator is in operable communication with the plunger. 
     A method of controlling a circuit with a limit switch is disclosed comprising aligning the limit switch in a mid-position alignment. The circuit is controlled with the limit switch by actuating the limit switch by direct acting contact by a plunger against a prime contactor. Opening the prime contactor, or opening the redundant prime contactor, de-energizes the electrical circuit of the limit switch. Closing the prime contactor along with the redundant prime contactor while the adjustment contactor and the redundant adjustment contactor is maintained closed, can be done to energize the electrical circuit. Opening the adjustment contactor or the redundant adjustment contactor when an actuator is misaligned from the plunger can be completed to de-energize the electrical circuit. 
     The above described and other features and advantages of the invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the figures, wherein like elements are numbered alike: 
     FIG. 1 is a schematic diagram of an. exemplary embodiment of a limit switch with all contacts closed; 
     FIG. 2 is schematic diagram of an exemplary embodiment of a limit switch with a prime contactor open; 
     FIG. 3 is a schematic diagram of an exemplary embodiment of a limit switch with a adjustment contactor open; and 
     FIG. 4 is a schematic diagram of an exemplary embodiment of a limit switch. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an exemplary embodiment of a limit switch  10  is shown. The limit switch  10  generally includes a switch  12  and an actuator  14  mechanically coupled. in order that upon movement of the actuator  14 , the switch  12  moves and changes the state of an electrical circuit  16 . In other words, through movement of actuator  14  the switch  12  can make or break the electrical circuit  16  allowing an electrical current  18  to flow or shutting off the electrical current  18 . 
     The actuator  14  can be a cam mechanism or similar lobe that translates rotary motion to linear translation or movement. It will be appreciated that the actuator is not limited to transferring rotary motion to linear motion but the actuator can also transfer linear motion to linear motion. In one embodiment, the actuator  14  is a cam that acts with the switch  12 . The cam rotates about a shaft  20 . A handle or a guard (not shown) can be attached to the shaft  20  so that an operator can rotate the handle or actuate the guard and subsequently rotate the actuator  14  about the shaft  20 . 
     The switch  12  includes a housing  22 , which preferably contains and protects the subcomponents of the switch  12 . Contained within the housing  22  is a part of the electrical circuit  16  that performs the switching function of making and breaking the electrical circuit  16 . As part of the electrical circuit  16  two contacts (or contactors) are employed to perform the switching function; these are a prime contactor  24  and an adjustment contactor  26 . In another embodiment, an additional set of redundant contactors in an additional circuit is employed in parallel with the prime contactor  24  and adjustment contactor  26  of electrical circuit  16 . The electrical circuit  16  is equipped with two contactors having the functions of direct-action breaking and misadjustment detection. Further detail is described with reference to FIG. 4 below. 
     As shown in FIG. 1, the prime contactor  24  is the primary component in the switch  12  for making and breaking the electrical circuit  16 . It is the primary component because the prime contactor is physically moved in normal operation to make contact and break contact. The adjustment contactor  26  acts to break the electrical circuit when the limit switch  10  is misadjusted or misaligned. The adjustment contactor  26  remains closed or in contact to complete the electrical circuit  16  during normal operation. 
     A plunger  28  is disposed in the housing  22  to facilitate the mechanical aspect of the switching function. In one embodiment, the plunger  28  comprises a shaft having two ends, a first end  27  and a second end  29  with the first end  27  being proximate to the actuator  14  and the second end  29  being distal from the actuator  14 . The plunger  28  can be coupled to the housing  22  such that motion of the actuator  14  is transferred to the plunger  28  to facilitate the switching function of the switch  12 . The plunger  28  extends outside of the housing  22  where the plunger  28  is in operable communication with the actuator  14 . The plunger  28  supports the prime contactor  24 , as well as moves it in and out of electrical connection with the electrical circuit  16 . 
     In normal operation of the switch  12 , the prime contactor  24  opens and closes (breaks electrical connection, makes electrical connection) upon actuation of the plunger  28  in the switch  12 . In one embodiment, the prime contactor  24  has two prime contact points  30  and one prime contact plate  32 . It is appreciated that more than two prime contact points  30  and more than one prime contact plate  32  can be utilized to conduct the electrical current  18  through the electrical circuit  16 . The prime contact plate  38  is a platen shape but any configuration can be employed to conduct the electrical current  18  through the electrical circuit  16 . The prime contact plate  32  is mounted on the plunger  28  such that movement of the plunger  28  translates into movement of the prime contact plate  32 . The plunger  28  and the prime contact plate  32  move in a linear fashion in one embodiment. 
     A prime spring  34  is coupled to the plunger  28  and the prime contact plate  32 . The prime spring  34  provides a spring bias (or spring force) that acts on the plunger  28  and the prime contact plate  32 . The prime spring  34  closes the prime contact plate  32  against the prime contact points  30 , upon some condition when the plunger  28  is no longer driven against the prime spring  34 . Thereby the prime contactor  24  makes or completes the electrical circuit  16 . The prime contactor  24  breaks the electrical circuit  16  upon some condition when the actuator  14  drives the plunger  28  against the force of the prime spring  34  to move the prime contact plate  32  disconnecting the prime contact plate  32  from the prime contact points  30 . 
     FIG. 1 shows adjustment contactor  26 . Adjustment contactor includes a number of adjustment contact points  36  and an adjustment contact plate  38 . In one embodiment, the adjustment contactor  26  has two adjustment contact points  36  and one adjustment contact plate  38 . It is appreciated that more than two adjustment contact points  36  and more than one adjustment contact plate  38  can be utilized to conduct the electrical current  18  through the electrical circuit  16 . The adjustment contact plate  38  is a platen shape but any configuration can be employed to conduct the electrical current  18  through the electrical circuit  16 . The adjustment contact plate  38  is coupled to an adjustment spring  40 . The adjustment spring  40  is coupled to the housing  22 . It is contemplated that the adjustment spring  40  can be coupled to other elements in order to provide a spring bias means to the adjustment contact plate  38 . The adjustment contact plate  38  is normally in contact with the adjustment contact points  36  in normal operation of the switch  12 . The plunger  28  forces the adjustment contact plate  38  against the adjustment contact points  36  in opposition to the bias of the adjustment spring  40 . When the plunger  28  is disconnected from the actuator  14  or is overcome by the adjustment spring bias, allowing the adjustment spring bias to move the plunger  28  and the adjustment contact plate  38 , the adjustment contact plate  38  disconnects from the adjustment contact points  36 . Thereby the adjustment contactor  26  opens or breaks the electrical circuit  16 . 
     Referring to FIG. 1, another aspect of the limit switch  10  is the mid-travel position window. The mid-travel position window can be understood as the configuration of the limit switch  10  when the actuator  14  holds or positions the plunger  28  in a mid-stroke. The plunger in a full stroke can be seen in FIG.  2 . The mid-stroke position has the prime contactor  24  in a closed position and the adjustment contactor  26  in a closed position thereby energizing the electrical circuit  16 . In the mid-stroke position, the limit switch  10  conducts current  18 . The degree of travel or stroke length of the plunger  28  in relationship to the contacts is known as a mid-travel position window. In one embodiment, the mid-travel position window of the limit switch  10  is about three millimeters. The mid-travel position window can also be characterized as the amount of contact overlap between the prime contactor  24  and the adjustment contactor  26 . The contact overlap allows the switch  12  to be adjusted so that normally open contacts close while keeping the normally closed contacts closed. By keeping the window relatively large and maintaining the switch  12  adjusted in a mid-stroke or partial actuation position the limit switch  10  can be difficult to render inoperable when tampered with (e.g. attempting to bypass the switch). In embodiments that employ a guard (not shown) with the actuator  14 , the mid-travel position allows for detection of the guard being actuated, as well as the removal of the guard or misalignment of the switch  12 . Fully actuating the switch  12  by tripping the guard or misadjusting the switch causes forced, direct-action opening of the prime contactor  24 . Releasing the switch  12  by removing the guard or misadjusting the switch  12  causes the adjustment contactor  26  to open. 
     The limit switch  10  can be understood by further describing the functions of the subcomponents with reference to FIGS. 2 and 3. FIG. 2 shows the limit switch  10  in a normal de-energized configuration with the prime contactor  24  open. The actuator  14  is shown adjusted to a position that drives or pushes the plunger  28 . The plunger  28  has an actuator interface  44  disposed at the first end  27 . The actuator  14  has a plunger interface  42  proximate to the plunger  28 . In one embodiment, the plunger interface  42  is arranged to receive the actuator interface  44  in a nestable fashion as shown in FIG.  1 . The actuator  14 , in one embodiment, actuates to drive the plunger  28  upon a cam motion or rotation as shown in FIG.  2 . The actuator interface is a roller rotatably mounted on the plunger  28  or any means for promoting a transfer of rotary or linear motion of the actuator  14  into a linear motion of the plunger  28 . In the embodiment shown, the actuator  14  rotates and drives the plunger  28  against the spring force or bias of the prime spring  34  thereby compressing the prime spring  34 . It can be appreciated that the prime spring  34  can be configured in another embodiment, so that the plunger  28  forced against the prime spring  34  extends the prime spring  34  instead of compressing it. 
     In any configuration, the bias of the prime spring  34  acts to push or drive the prime contactor  24  closed. The actuator  14  drives the plunger  28  to open the prime contactor  24 . This configuration of having the actuator positively drive the prime contactor  24  open is also known as direct-acting contacts. Direct-acting contacts are mechanically forced open as the switch  14  is actuated. Mechanical failure of the prime contactor  24  subcomponents, (e.g., through contact weld), can be protected against by the mechanical opening of the prime contactor  24 . 
     In the normal open configuration of limit switch  10 , the adjustment contactor  26  is normally closed. To maintain the adjustment contactor  26  closed, the plunger  28  drives against the force or bias of the adjustment spring  40 . The adjustment contact plate  38  is pushed into electrical connection with the adjustment contact points  36  (also shown in FIG.  1 ). The electrical circuit  16  is in a de-energized state even though the adjustment contactor  26  is closed because the prime contactor  24  is open. 
     Referring now to FIG. 3, a limit switch  10  is shown in an off-normal state or abnormal configuration with the adjustment contactor  26  open. The actuator  14  is shown dislodged from a normal position such as shown in FIG.  1  and FIG.  2 . The actuator  14  is out of contact with the plunger  28 . In this configuration, the plunger  28  overcomes the force that is applied against the adjustment spring  40  so that the adjustment contactor  26  opens, de-energizing the electrical circuit  16 . This configuration is an example of the misalignment detection that is embodied in the limit switch  10 . In the event of a failure of the actuator  14  and resultant release of the plunger  28  from the adjustment contactor  26  the limit switch  10  defaults into a de-energized condition. FIG. 3 illustrates a condition where the actuator  14  may be broken or may be removed or any other off-normal condition that creates a potential event for the device that the limit switch  10  is installed to protect. Even though the plunger  28  has allowed the primary contact  24  to close, the adjustment contactor  26  opens and maintains the electrical circuit  16  de-energized. 
     Referring to FIG. 4 an exemplary embodiment of the limit switch  110  is shown equipped with redundant circuits and redundant sets of contactors. The limit switch  110  has redundancy in order to allow the limit switch  110  to function in the event of a failure of the any one of the contactors. The redundant components function similarly to their counter parts of the limit switch  10 . 
     Limit switch  110  is equipped with a redundant plunger  46  coupled to the plunger  128 . It is contemplated that a single plunger can be employed instead of two plungers. The redundant plunger  46  is mounted in the housing  122 . Plunger  128  drives redundant plunger  46 . Redundant plunger  46  comprises a shaft having two ends, a first end  48  and a second end  50 . Redundant plunger  46  is connected to plunger  128  near the first end  48 . Redundant plunger  46  is mounted such that motion of the plunger  128  is transferred to the redundant plunger  46  to facilitate the switching function. Redundant plunger  46  supports a redundant prime contactor  52 . Redundant plunger moves redundant prime contactor in and out of electrical connection with the redundant electrical circuit  117 . 
     In addition to the electrical circuit  116 , as well as to the prime contactor  124  and the adjustment contactor  126 , a redundant electrical circuit  117  having a redundant prime contactor  52  and a redundant adjustment contactor  54  are employed to perform the redundant switching function. The redundant prime contactor  52  duplicates the action of the prime contactor  124 . The redundant adjustment contactor  54  duplicates the action of the adjustment contactor  126 . In normal operation of the switch  112 , the prime contactor  124  and redundant contactor  52  opens and closes upon actuation of the plunger  128  and redundant plunger  46 . 
     The redundant prime contactor  52  similarly to the prime contactor  124  is provided with a redundant prime contact plate  56  and redundant prime contact points  58 . The embodiment shown in FIG. 4 has one redundant prime contact plate  56  and two redundant prime contact points  58 . It is contemplated that any number of redundant prime contact plates  56  and redundant prime contact points  58  can be employed. The redundant prime contact plate  56  makes electrical connection with the redundant prime contact points  58  to provide electrical conductivity for redundant electrical circuit  117 . Redundant prime contact plate  56  is coupled to the redundant plunger  46  such that movement of the redundant plunger  46  translates into movement of the redundant prime contact plate  56  relative the redundant prime contact points  58 . 
     A redundant prime spring  60  is coupled to the redundant plunger  46  and the redundant prime contact plate  56 . The redundant prime spring  60  provides a spring bias that acts on the redundant plunger  46  and the redundant prime contact plate  56 . The redundant prime spring  60  closes the redundant prime contact plate  56  against the redundant prime contact points  58  upon some condition when the redundant plunger  46  is no longer driven against the redundant prime spring  60 . Thereby the redundant prime contactor  52  completes the redundant electrical circuit  117 . It is also contemplated that the redundant prime contact plate  56  and the prime contact plate  124  are the same plate with electrical insulation separating the electrical circuits  116  and  117 . The contact plates  124  and  56  can be in operable communication with each other, such that plunger  128  can be driven to open the electrical circuits  116  and  117 . 
     FIG. 4 shows redundant adjustment contactor  54 . In one embodiment, the redundant adjustment contactor  54  has two redundant adjustment contact points  64  and one redundant adjustment contact plate  66 . It is appreciated that more than two redundant adjustment contact points  64  and more than one redundant adjustment contact plate  66  can be utilized to conduct the electrical current  119  through the redundant electrical circuit  117 . The redundant adjustment contact plate  66  is a platen shape but any configuration can be employed to conduct the electrical current  119  through the redundant electrical circuit  117 . The redundant adjustment contact plate  66  is coupled to a redundant adjustment spring  68 . The redundant adjustment spring  68  is coupled to the housing  122 . It is contemplated that the redundant adjustment spring  68  can be coupled to other elements in order to provide a spring bias means to the redundant adjustment contact plate  66 . The redundant adjustment contact plate  66  is normally in contact with the redundant adjustment contact points  64  in normal operation of the switch  112 . The redundant plunger  46  forces the redundant adjustment contact plate  66  against the redundant adjustment contact points  64  in opposition to the bias of the redundant adjustment spring  68 . When the plunger  128  is disconnected from the actuator  114  or is overcome by spring bias, allowing spring bias to move the plunger  128  and the redundant plunger  46 , the redundant adjustment contact plate  66  disconnects from the redundant adjustment contact points  64 . Thereby the redundant adjustment contactor  54  opens or breaks the redundant electrical circuit  117 . 
     As shown in FIG. 4, the limit switch  110  is in an off-normal condition with both the prime contactor  124  and the adjustment contactor  126  closed. The actuator  114  is shown dislodged from a normal position such that the plunger  128  is dislodged from applying a force on the adjustment contactor  126 . The prime contactor  124  is closed and the adjustment contactor  126  is closed thereby potentially energizing the electrical circuit  116 . In this configuration a condition can exist in which the actuator  114  is disabled and the electrical circuit  116  is energized. As seen in FIG. 3, even though the actuator  14  is disabled, the limit switch  10  would maintain the electrical circuit  16  de-energized due to the adjustment contactor  26  being open and thereby de-energizing the electrical circuit  16 . However, FIG. 4 illustrates a condition where the adjustment contactor  126  has become stuck or failed closed. The adjustment contact plate  138  can weld to the adjustment contact points  136  due to arcing. The adjustment spring  140  can fail to provide a spring bias. The result of the failure of the adjustment contactor  126  to open is that the electrical circuit  116  can potentially remain energized when operators may expect it to be de-energized. The redundant set of contactors,  52  and  54  provides the additional set of contactors to maintain the redundant electrical circuit  117  de-energized. In this condition, the off-normal condition is detectable by monitoring the redundant set of contacts, redundant prime contactor  52  and redundant adjustment contactor  54 . The redundant adjustment contactor  54  will open upon the failed actuator  114  and de-energize the redundant electrical circuit  117 . The electrical circuit  116  and the redundant electrical circuit  117  can be wired to a safety circuit not shown such that upon failure of the contactors in electrical circuit  116 , redundant electrical circuit  117  will maintain the device in a de-energized condition. The failed adjustment contactor  126  can then be detected upon inspection of the limit switch  110 . Because the limit switch  110  is maintained in a mid-travel position, damaged, misadjusted, or removed actuators  114  can be detected. 
     An additional circuit can have detection logic and components that detect the position of the contactors and provide indication based on the contactors position. Control circuitry can also be wired to include controlling the device that the limit switch  110  is equipped to protect. 
     The limit switch  110  is more usable due to the larger range of the mid-travel position window. The limit switch  110  has redundant contacts in electrical series that allow for use of the limit switch  110  to be used in safety circuits requiring monitored redundancy, as well as circuits where monitoring and/or redundancy is not required. The prime contactor  124  can be opened by direct action and the adjustment contactor  126  can be opened by spring action. On a failure of the limit switch  110  the failure mode is open, thus maintaining the circuit de-energized. 
     While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.