Abstract:
A safety interlock circuit wherein the safety interlock switches can report their status. Signal generators associated with the safety interlock switches produce status signals that are transmitted on the wiring of the safety interlock circuit. The present invention is ideal for retrofitting current installations because no additional wiring is required for carrying the status signals.

Description:
TECHNICAL FIELD 
   Embodiments relate to safety interlock circuits and safety interlock switches. Embodiments also relate to safety interlock switches that convey their status using the safety interlock circuit itself as a transmission medium. Embodiments are also related to the transmission, reception, and reporting of safety interlock switch status. 
   BACKGROUND OF THE INVENTION 
   Equipment and machinery is often capable of injuring or killing a person when it is operated unsafely. For example, a microwave oven can cause death or injury if it is operated with the door open. The open microwave oven door is an unsafe condition. Safety interlock circuits are electric circuits designed to prevent equipment and machinery from operating when an unsafe condition exists.  FIG. 1 , labeled as “prior art”, illustrates a basic safety interlock circuit. In order to function, the circuit must have a signal to carry. The input signal is introduced into the circuit through the circuit input terminal  101 . The signal then passes through three safety interlock switches  102  and into a machine input terminal  104 . Finally, the signal exits the machine  105  at the machine output terminal  106  and exits the safety interlock circuit at the circuit output terminal  107 . The machine  105  only operates if the signal can pass through it. Any opening in the circuit will prevent the machine  105  from operating. A safety interlock switch  102  must be in the closed state for the signal to pass through it. In the safety interlock circuit of  FIG. 1 , all the safety interlock switches  102  must be closed or the machine  105  will not operate. 
   Returning to the microwave oven example, the circuit input terminal  101  and circuit output terminal  107  could be the prongs on the power cord that is plugged into the wall. In that case, the signal is the AC line power used to power the microwave oven. One safety interlock switch  102  is the door safety switch that opens whenever the door opens. Another safety interlock switch  102  can be set to open whenever the top cover of the microwave oven is removed. The machine  105  is all the parts that rotate food or generate microwave radiation. In this example, the microwave oven cannot operate with either the door open or top cover removed because a safety interlock switch cuts the AC line power. 
   Some equipment requires more power than can be safely carried in a safety interlock circuit. In this case, a control module  202  is required as shown in  FIG. 2 , which is labeled as “prior art”. In  FIG. 2 , the signal passes into the control input terminal  201 , through the control module  202 , and out the control output terminal  203 . Electric power for the machine  105  passes into the power input terminal  204 , through the control module  202 , through the machine  105 , and out the power output terminal  106 . The control module  202  switches machine power on and off based on the presence or absence of the signal. 
   Electric relays are often used for control modules. An electric relay is a common electrical component that uses an electric current as a control signal for opening and closing a switch. Those skilled in the art of electric circuitry know the properties of relays and many functional equivalents of relays wherein a control signal switches power on and off. Some of the similar devices are transistor, vacuum tubes, silicon controlled rectifiers and field effect transistors. 
   A significant problem with safety interlock circuits is that it is often impossible to know which particular safety interlock switch is disabling the machinery. In the microwave oven example, it is easy to see if the door is open. However, if the machinery is an elevator in a skyscraper, the open switch could be on any floor of the building. If a safety interlock switch has disabled an elevator, then the time spent by the maintenance crew just to isolate the problem can be considerable. There are safety interlock switches that can report their state, but they also require a dedicated signaling circuit. A safety interlock circuit with such switches is shown in  FIG. 3 , labeled as “prior art”, wherein each switch uses a dedicated signal wire  302  operable through a monitoring module  301 . Another safety interlock circuit is shown in  FIG. 4 , labeled as “prior art”, wherein the switches share a common signal bus  401 . 
   Examples of a monitoring module  301  are devices that actively monitor interlock switch position, interlock switch contacts, voltage across the interlock switch contacts, or current through the interlock switch. Certain types of switches can monitor their own switch position because they independently open or close multiple independent circuits. 
   The solutions of  FIG. 3  and  FIG. 4  are both used, but they both require signal wires to be installed. There are many installations that already have a safety interlock circuit installed that would benefit from a reporting mechanism for safety interlock switches. However, the wires are already installed and it is often prohibitive to install new wires because some machines are literally miles long. 
   Current diagnostic solutions for safety interlock circuits require additional wiring and additional switches or switch contacts as well as all the extra time, support circuitry, and expense involved in installing, using and maintaining them. 
   The present invention directly addresses the shortcomings of the prior art by supplying a signaling mechanism that does not require more wires, switches, or switch contacts than any presently installed safety interlock circuit. 
   BRIEF SUMMARY 
   The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole. 
   It is therefore, an aspect of the embodiments to provide safety interlock switches and safety interlock circuits that transmit status signals carrying switch status over the wiring of the safety interlock circuit without requiring additional wiring for status signals. 
   Another aspect of the embodiments is that the status signals are received by a receiver and interpreted to yield status information about the safety interlock switches. The status information is then reported. 
   It is a further aspect of certain embodiments that the status signals of many safety interlock switches can be transmitted over a safety interlock circuit without causing interference to one another. 
   A yet further aspect of the embodiments is that safety interlock switch status can be discerned regardless of whether a switch is open or closed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1 , labeled as “prior art”, illustrates a safety interlock circuit; 
       FIG. 2 , labeled as “prior art”, illustrates another safety interlock circuit; 
       FIG. 3 , labeled as “prior art”, illustrates a safety interlock circuit with switch monitoring; 
       FIG. 4 , labeled as “prior art”, illustrates another safety interlock circuit with switch monitoring; 
       FIG. 5  illustrates a functional block diagram of a safety interlock circuit with switch monitoring in accordance with a feature of an embodiment; 
       FIG. 6  illustrates a functional block diagram of a safety interlock switch combined with other components for monitoring and status reporting accordance with another feature of an embodiment; 
       FIG. 7 , labeled as “prior art”, illustrates a single pole single throw switch; 
       FIG. 8 , labeled as “prior art”, illustrates a single pole double throw switch; 
       FIG. 9 , labeled as “prior art”, illustrates a double pole double throw switch; 
       FIG. 10  illustrates a single pole double throw switch combined with other components for monitoring and status reporting accordance with a further feature of an embodiment; and 
       FIG. 11  illustrates a double pole double throw switch combined with other components for monitoring and status reporting accordance with an additional feature of an embodiment. 
   

   DETAILED DESCRIPTION 
   Aspects of the embodiments overcome the limitations of the prior art by using the wires that carry the interlock signal to also transmit signals that carry the switch status instead of using separate wires or a bus to carry the switch status. 
     FIG. 5  illustrates one aspect of the embodiments. The interlock signal is introduced into the interlock circuit  100  at the circuit input terminal  101 . The interlock signal then passes into a safety interlock switch input terminal  108 . The interlock signal then passes through the safety interlock switch  102  if it is closed and then passes out the safety interlock output terminal  109 . If the safety interlock switch  102  is open, the interlock signal can&#39;t pass. The monitor  301  detects the open safety interlock switch  102  and causes the signal generator  501  to generate a status signal. The status signal passes from the signal generator  501  through the status signal coupler  505  and into the safety interlock circuit at the safety interlock switch output terminal  109 . From that point, the status signal can pass through other safety interlock switches until it eventually reaches the machine  105 . 
   Those skilled in the arts of electrical circuitry or electrical signaling are familiar with a vast array of electrical signals, devices for generating those signals, and techniques for coupling those signals into and out of electrical circuits. On contemplation of the embodiments, they could use their skill to produce aspects of the embodiments. 
   Another aspect of the embodiments is that the status signal cannot cause the machine  105  to operate; only the interlock signal can cause the machine  105  to operate. From the machine  105 , the signal passes to the interlock circuit output terminal  107 . However, before the status signal passes out of the safety interlock circuit  100 , a receiver  502  can receive it. The receiver  502  then causes the reporter  503  to report some property or properties of the status signal. Some properties of status signals are the presence of the status signal, information that can be used to identify the signal generator that produced the status signal or status information carried by the status signal. The reporter can report by directly displaying information to a person, sounding an alarm, sending a message to a web site for remote display, or otherwise generating an audible, visual, or electrical signal. 
   A signal is something that may be used to carry information. An aspect of the embodiments is transmitting electrical signals over the wires of the safety interlock circuit. The art of communications systems has found many different types of electrical signals. The embodiments do not require any particular type of electrical signal, only that there be an electrical signal. When two or more signals are present, there is a possibility they will interfere. Interference is when one signal obscures or degrades another. The art of communications systems has found many ways to avoid interference between signals. For purposes of the embodiments, all types of electrical signal are considered equivalent and techniques for avoiding interference between signals are considered equivalent. Techniques for avoiding interference between status signals include, but are not limited to, status signal modulation, time division, collision detection, or collision avoidance 
   In the art of communications systems, modulation is the technique by which signals are caused to carry information. One of the simplest examples is the famed “one if by land, two if by sea” leading to the midnight ride of Paul Revere. A very complicated example is the IEEE 802.11g standard that governs certain wireless Ethernet transmissions. Aspects of the embodiments do require modulation of a signal. More specifically, status information is carried by the status signal. All the modulation techniques by which status information, which includes a switch&#39;s open/close position and identity, can be carried by a status signal are considered equivalent for purposes of the embodiments. 
   Status information is the information that a status signal carries. An example is a status signal that is present only when a particular safety interlock switch is open. When that status signal is not detected at the receiver  502 , then the status information is that the safety interlock switch is closed. When that status signal is detected at the receiver  502 , then the status information is that the switch is open. Another possibility is that a signal generator  501  can generate one status signal when the safety interlock switch  102  is open and a different signal when it is closed. In this manner the status information is that the presence of one signal indicates that a particular switch is open, the presence of the other signal indicates closed, and the absence or presence of both signals indicates an abnormal condition. 
   Another aspect of the embodiments is that the status signal must be incapable of causing the machine  105  to operate. Only the interlock signal can cause the machine  105  to operate. As previously described, the interlock signal is often also the electric power for the machine, such as AC line current for home appliances or 12 volt DC power from a car battery. Historically, there are many instances of signaling via power lines. The methods used to signal via power lines can also be used to for sending and receiving status signals in interlock circuits. However, the embodiments are not limited to any particular signaling method or group of signaling methods. All signaling methods by which an interlock circuit carries both an interlock signal and a status signal are considered equivalent for purposes of the embodiments. 
   A further aspect of the embodiments is coupling the status signal into the wiring of the safety interlock circuit. There are many techniques known in the art of electric circuitry for coupling a signal into a circuit. Capacitive coupling, inductive coupling, and direct wiring are examples of coupling techniques. The embodiments do not depend on the application of any one coupling technique or group of techniques. All techniques that couple a status signal from a signal generator  501  into a safety interlock circuit are considered equivalent. 
   A signal generator  501  is a device that produces a status signal. Aspects of certain embodiments require that every signal generator  501  produce a unique signal. A unique signal is a signal that is unlike any other signal that is intentionally present in the interlock circuit. The reason unique signals are required is so that signal generators can be identified by the signals they produce. Every signal generator in the embodiments is associated with a safety interlock switch. Therefore, a unique signal can be used to identify a safety interlock switch. Additionally, aspects of certain embodiments require a signal generator to produce 2 different signals. If both signals are unique, they can be used to identify the signal generator and thereby the safety interlock switch. Any signal that is not unique can not be used to identify a specific source. 
   In accordance with aspects of certain embodiments,  FIG. 6  illustrates an apparatus  600  that associates an identification module  601  with each safety interlock switch  102  in the system. The reason is that every signal generator  501  must produce a unique signal. The identification module  601  is a device such as a block of jumpers, a DIP switch or electronically programmable memory by which every signal generator  501  in the system can be adjusted to emit a different signal. The safety interlock circuit of  FIG. 5  does not show use of a switch identification, in which case the signal generators  501  must be distinguishable by some other mechanism. In  FIG. 6 , the interlock signal passes through the safety interlock switch  102  when it is closed. However, when it is open the monitor  301  detects it and causes the signal generator  501  to produce a signal that is coupled into the interlock circuit at the safety interlock circuit output terminal  109 . The signal generator  501  generates a status signal that is dependent on the identification module  601 . An example is a signal generator that produces a sinusoidal signal wherein the frequency is set based on the signal identification. In this example, the sinusoidal frequency is the status information. The receiver can use the frequency to identify the signal generator  501  and thereby also identify a specific safety interlock switch  102 . A reporter, such as reporter  503  in  FIG. 5 , can then be used to report the status of the safety interlock switch  102 . 
     FIG. 6  also illustrates another aspect of certain embodiments, the status signal bypass  602 . The status signal bypass  602  is used to supply a signaling path for status signals but not for interlock signals. An interlock signal cannot pass from the input of the safety signal bypass  602  to the output. A status signal can pass from the input of the status signal bypass  602  to the output. It is possible for a status signal to be present at the safety interlock switch input terminal  108 . An open safety interlock switch  102  will not pass any signal, including a status signal. An example of when this can occur is when more than one safety interlock switch is open. A status signal bypass  602  carries status signals past the safety interlock switch  102 . In this manner, the safety interlock circuit can carry many status signals at once. A receiver  502  can receive all the signals and a reporter  503  can report the status information. An implication of this aspect of the embodiments is that many status signals must be able to coexist without interfering with one another. Signaling techniques whereby many signals share the same transmission medium, whether that medium is a wire, the air, or an optical fiber, are common. All the signaling techniques whereby many status signals can share the wires of the safety interlock circuit are considered equivalent for purposes of the embodiments. 
     FIG. 7 , labeled as “prior art”, shows a common circuit symbol for a single pole single throw (SPST) switch  700 . Most switches do not have an input terminal or output terminal because they conduct electricity equally in either direction. When the switch  701  is closed, it conducts electricity, in either direction, between terminal1  702  and terminal2  703 . The circuit symbol appears to show the switch in the open position, however that is not the case because the symbol does not indicate open or closed, it only indicates that there is a switch. 
     FIG. 8 , labeled as “prior art”, shows a common circuit symbol for a single pole double throw (SPDT) switch  800 . When the switch  701  is in one position, it conducts electricity, in either direction, between terminal1  801  and terminal2  802 . When the switch  701  is in the other position, it conducts electricity, in either direction, between terminal1  801  and terminal2  803 . The circuit symbol appears to show the switch in one position, however that is not the case because the symbol does not indicate switch position; it only indicates that there is a switch. 
     FIG. 9 , labeled as “prior art”, shows a common circuit symbol for a double pole double throw (DPDT) switch  900 . It has a switching mechanism that moves two switches at the same time. When the switching mechanism is in one position, one switch  901  conducts electricity, in either direction, between terminalA1  903  and terminalA2  905  and the other switch  902  conducts electricity, in either direction, between terminalB1  904  and terminalB2  906 . When the switching mechanism is in the other position, one switch  901  conducts electricity, in either direction, between terminalA1  903  and terminalA3  907  and the other switch  902  conducts electricity, in either direction, between terminalB1  904  and terminalB2  908 . The circuit symbol appears to show the switching mechanism in one position, however that is not the case because the symbol does not indicate switch position; it only indicates that there is a switch. 
     FIG. 10  shows the apparatus of  FIG. 5  adapted to use a SPDT switch. The SPDT switch  800  is used as both the safety interlock switch  102  and the monitor  301 . Terminal2  802  is connected to the safety interlock switch input terminal  108  and terminal1  801  is connected to the safety interlock switch output terminal  109 . The closed position of the safety interlock switch  102  corresponds to the SPDT switch  800  conducting electricity between terminal1  801  and terminal2  802 . The signal generator  501  is connected to SPDT switch  800  terminal3  803 . The open position of the safety interlock safety switch  102  corresponds to the SPDT switch  800  conducting electricity between terminal1  801  and terminal3  803 . When the safety interlock switch  102  is open, the signal generator  501  is electrically connected to the safety interlock circuit by the SPDT switch. 
     FIG. 11  shows the apparatus of  FIG. 5  adapted to use a DPDT switch. The DPDT switch  900  is used as both the safety interlock switch  102  and the monitor  301 . TerminalA2  905  is connected to the safety interlock switch input terminal  108  and terminalA1  903  is connected to the safety interlock switch output terminal  109 . The closed position of the safety interlock switch  102  corresponds to the DPDT switch  900  conducting electricity between terminalA1  903  and terminalA2  905  and between terminalB1  904  and terminalB2  906 . The signal generator  1101  shown here generates two different status signals and sends status signal 1  to the status signal 1  line  1102  that is connected to DPDT switch  900  terminalB2  906 . It sends status signal 2  to the status signal 2  line  1103  that is connected to DPDT switch  900  terminalB3  908 . When the safety interlock switch  901  is closed, the interlock signal passes through the DPDT switch  900  and status signal 1  also passes through the switch and it is coupled into the safety interlock circuit via the status signal coupler  505 . The open position of the safety interlock switch  102  corresponds to the DPDT switch  900  conducting electricity between terminalA1  903  and terminalA3  907  and between terminalB1  904  and terminalB3  908 . When the safety interlock switch  901  is open, the interlock signal cannot pass through the DPDT switch  900  but status signal 2  does pass through the switch and it is coupled into the safety interlock circuit via the status signal coupler. 
   It will be appreciated that variations of the above-disclosed and other features, aspects and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.