Abstract:
Methods and apparatus are provided for handling a STUCK-ON microphone push-to-talk (PTT) switch. The apparatus comprises an input device receiving the PTT output, switches for selecting a radio, and a controller that receives this information and determines whether the PTT switch is STUCK-ON. Until STUCK-ON occurs, the controller passes the microphone signal and a PTT initiated TRANSCVER-ON command to the radio causing it to transmit. When STUCK-ON occurs, the state of the PTT switch is ignored and the transmitter shut off. The status of other PTT switches and radios are not affected. In the method, the active PTT signal results in a TRASNCVR-ON command to the transmitter until a PTT-ON timer reaches a predetermined critical value Tc, whereupon the PTT-ON signal is blocked, the transmitter returned to stand-by and an alarm turned on to warn the pilot of the stuck PTT switch.

Description:
TECHNICAL FIELD 
   The present invention generally relates to a system and method for communication from vehicles, and more particularly to a system and method for handling a stuck microphone condition in aircraft. 
   BACKGROUND 
   Modern aircraft are generally equipped with several means of radio communication between the pilot or other aircraft personnel and ground stations or other aircraft. For convenience of explanation the present invention is described in the context of pilot communication but persons of skill in the art will understand that it applies to any other flight crewmembers and even to passengers who may have access to communication facilities on the aircraft. Typically the pilot has several radios and several microphones that he or she may use. The pilot selects a particular microphone and radio by means of one or more switches on a control panel. The selected microphone itself is typically activated by depressing a ‘Push-To-Talk’ (PTT) switch whereupon the radio to which the microphone is coupled will transmit whatever message the pilot speaks into the microphone. The PTT may be a part of the microphone itself, as for example with a hand held mike or it may be remotely located. Pilots often wear a headset with a boom mike whose PTT switch is located on the aircraft control yoke. These are non-limiting examples of typical arrangements. The particular microphone and PTT switch arrangement is not critical. 
   Because of safety considerations it is important that a stuck PTT switch in a particular aircraft not preempt the particular radio channel being used, e.g., cause that aircraft&#39;s transmitter to stay on for an extended period of time, thereby preventing others sharing the same channel from communicating. A stuck PTT switch condition occurs when a PTT switch has been depressed or otherwise activated for a time exceeding a preset threshold Tc. A stuck PTT switch condition can result from several causes as, for example: (i) the pilot has held the PTT switch closed for a time exceeding Tc, or (ii) mechanical or electrical failure has occurred which prevents the PTT switch from returning to its OFF state. Typical present day avionics systems do not distinguish between these conditions. In either case, once Tc is exceeded the avionics system deactivates all of the pilot&#39;s PTT switches so that further transmission is precluded and the radio channel is cleared for use by others. The pilot&#39;s PTT switches remain disabled until the stuck-switch condition is cleared. Even when only one of the pilot&#39;s available PTT switches is ‘stuck’, for example on a hand mike, his other PTT switch, e.g. for a boom mike, is also disabled. Thus, when a ‘stuck PTT switch’ condition occurs in the prior art, all of the pilot&#39;s normal means of communication are disabled. This usually does not affect the copilot&#39;s communication capability in a dual control aircraft with duplicate microphones, PTT switches, and so forth. In addition, per RTCA/DO-207 the pilot usually has a safety over-ride switch that permits essential communication by the pilot in emergencies. This over-ride switch must be a protected switch so that it cannot be accidentally left activated. This arrangement while workable is inconvenient and more flexible means of dealing with a stuck PTT switch condition are needed. 
   Accordingly, it is desirable to provide a ‘stuck PTT switch’ response that allows non-stuck PTT switches and corresponding mikes to be used so that normal communications can continue on the alternative systems, despite the ‘stuck switch’ condition on one of the available mikes. In addition, it is desirable to provide this improved capability with minimum cost and little or no added complexity. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
   BRIEF SUMMARY 
   Method and apparatus are provided for handling a STUCK-ON microphone push-to-talk (PTT) switch. The apparatus comprises a radio for transmitting a signal derived from a microphone, a PTT switch having an ON/OFF output and an associated microphone for providing a communication signal to the radio, switches for selecting the radio from one or more available, and a controller coupled to the selected microphone and PTT switch, that determines whether the PTT switch is STUCK-ON. Until STUCK-ON occurs, the controller passes the microphone signal and a PTT-ON initiated TRANSCVER-ON command to the radio causing it to transmit When STUCK-ON occurs, the state of the PTT switch is ignored (e.g., it is disabled or deselected) and the transmitter shut off. The status of other PTT switches and radios are not affected. 
   A method is provided wherein a PTT-ON signal from the selected PTT results in a TRASNCVR-ON command sent to the transmitter so that it transmits a communication signal based on the microphone output, and a timer started. When the PTT-ON timer reaches a predetermined critical value Tc, the PTT-ON signal is rendered ineffective, that is, blocked, disabled or deselected. The transmitter is returned to stand-by, the timer reset and an alarm turned on to warn the pilot of the stuck PTT switch, without affecting the response of other PTT switches. In the preferred embodiment, the PTT output status is checked and if it returns to PTT-OFF, then the alarm is cancelled and the PTT output is re-enabled, i.e., no longer blocked. If the PTT output changes to PTT-OFF before the timer reaches Tc, then the transmitter is turned off and the timer reset. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
       FIG. 1  is a simplified schematic diagram of an aircraft communication system according to the present invention; 
       FIG. 2  is a simplified schematic diagram of a portion of the system of  FIG. 1  showing further details; 
       FIG. 3  is a simplified flow chart illustrating the method of the present invention; and 
       FIG. 4  is a simplified flow chart illustrating a further embodiment of the method of the present invention. 
   

   DETAILED DESCRIPTION 
   The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. As used herein the words “radio” and “transceiver” (abbreviated “TRANSCVR”), whether upper or lower case, are used interchangeably to refer to electronic equipment capable of transmitting a radio communication signal. The words “receive” and “standby” in reference to such radios are also used interchangeably to describe the condition when the radio is not transmitting. With respect to a PTT switch, the term “active” is intended to mean that the switch output is ON, that is, commanding a transmit action, and the term “enabled” is intended to mean that the switch output is not being blocked from the TRANSCVR, that is, it is not disabled or deselected. 
     FIG. 1  is a simplified schematic diagram of aircraft communication system  8  according to the present invention. System  8  comprises one or more radios or transceivers (TRANSCVR)  10 ,  12 , avionics display unit (ADU)  14  and network interface module (NIM)  16 , respectively coupled by communication bus  15  which includes at least buses  15 A,  15 B, and  15 C. Network interface module (NIM)  16  is coupled to audio control panel (ACP)  18  by another communication bus  17 . Audio control panel (ACP)  18  has a number of switches, for example switches  30 - 35 , used by the pilot to select the radio transceivers (TRANSCVR) for communication. Coupled to ACP  18  are, for example, hand held mike  20  with integral PTT switch  22  and boom mike  24  and its corresponding PTT switch  26 . Switch  26  is conveniently located on aircraft control yoke  28 . Cable  21  couples the audio signal of mike  20 , and cable  23  couples the PTT signal for mike  20 , to ACP  18 . Similarly, cable  25  couples the audio signal from boom mike  24 , and cable  27  couples the corresponding PTT signal from PTT switch  26  on control yoke  28 , to ACP  18 . 
   Audio control panel (ACP)  18  receives the PTT and audio signals from the respective microphone selected by the pilot. The pilot uses panel switches  30 - 35  to select the radio (TRANSCVR) through which he or she desires to transmit. With the PTT signal, and the selected panel switch, ACP  18  has sufficient information to send over communication bus  17  to NIM unit  16  a PTT activation signal, an address for the desired radio and, for example, a digitized audio signal corresponding to the audio input received from the selected microphone. NIM  16  conveniently but not essentially includes an audio processor card (APC) coupled to bus  17 . The APC conveniently receives from bus  17  the PTT signal, the digitized audio signal, and the radio address and issues a corresponding PTT signal and sends an audio signal directly to the selected radio  10  or  12  depending on the address. The PTT signal issued by NIM  16  (e.g., TRANSCVR-ON) over buses  15 B or  15 C, corresponds to the signal required by radio  10  or radio  12  to switch it from the receive (or standby mode) to the transmit mode. 
   When ACP  18  detects that a ‘stuck PTT switch’ condition has occurred, it deactivates or deselects the PTT signal going to NIM  16  according to which of the available PTT switches is in the ‘stuck’ condition. NIM  16  in turn deactivates the PTT signal going to the selected radio. At substantially the same time, ACP  18  sends an error message over bus  17  to NIM  16  which relays the message to avionics display unit (ADU)  14  via bus  15 A to cause ADU  14  to illuminate a tell-tale warning light or other alarm to warn the pilot that a stuck PTT switch condition has occurred and, preferably, which of the available PTT switches is ‘stuck’. This allows the pilot to recycle the switch if he or she has held it down too long or to change to another mike and PTT switch if the ‘stuck switch’ condition is caused by a mechanical or electrical failure. Only the microphone experiencing the ‘stuck switch’ condition is disabled. The other microphones and PTT switch(es) are left unaffected and thus, may continue to be used by the pilot in a normal manner. 
     FIG. 2  is a simplified schematic diagram of electronic subsystem  50  showing further details of the part of system  8  within outline  19  of  FIG. 1 . For conveniance of explanation handheld mike PTT switch  22  is identified as corresponding to MIC-1 and boom mike PTT switch  26  is identified as corresponding to MIC-2, but this is not intended to be limiting. While subsystem  50  within outline  19  is shown in  FIG. 1  as being partitioned into ACP  18  and NIM  16 , this is merely for convenience of explanation and is not essential. As far as dealing with a ‘stuck PTT’ condition is concerned, it does not matter if a particular function is performed within ACP  18  or NIM  16 . Accordingly,  FIG. 2  provides a simplified schematic diagram of the combined function of ACP  18  and NIM  16  with respect to dealing with a ‘stuck PTT’ condition. 
   Subsystem  50  receives PTT signals from MIC-1 PTT switch  22  and from MIC-2 PTT switch  26  over leads  23 ,  27  respectively. The nature of the PTT signal can be +/−Vcc, Ground, Vbb, or whatever voltage, current or impedance level is convenient to indicate “switch depressed” or “switch released”, i.e., PTT-ON or PTT-OFF. Persons of skill in the art will understand how to choose the respective ON and OFF levels to suit their particular application. As used herein with respect to a PTT switch, “ON” is intended to refer to the condition where signal transmission is desired (e.g., PTT switch depressed), and “OFF” to refer to the condition where signal transmission is not intended (e.g., PTT switch released). 
   The PTT signals from MIC-1 and MIC-2 are conveniently received by INPUT BUFFER  52  over leads or wires  23 ,  27 . INPUT BUFFER  52  conveniently adjusts the ON/OFF signals from the PTT switches to suit whatever voltage or current levels are appropriate for processing by subsequent elements of subsystem  50 . INPUT BUFFER  52  communicates the state of the PTT switches (i.e., ON or OFF) over bus  53  to CONTROLLER  54 . CONTROLLER  54  also receives inputs from TRANSCVR SELECT SWITCHES  58  (e.g., switches  30 - 35  of  FIG. 1 ) over bus  59  so that it knows which of RADIO- 1  and RADIO- 2  have been selected by the pilot. The arrangement shown in  FIG. 2  whereby the inputs from BUFFER  52  are received by CONTROLLER  54  has the advantage that it allows CONTROLLER  54  to continually determine the state of the PTT switches, but this is not essential. 
   CONTROLLER  54  is coupled to MEMORY  60  via bus  55  and to OUTPUT BUFFER  64  by bus  63 . MEMORY  60  conveniently stores program instructions for CONTROLLER  54  as well as intermediate data variables and predetermined constants such as Tc. OUTPUT BUFFER  64  is coupled to RADIOS  10 ,  12  over buses  15 B,  15 C, respectively. OUTPUT BUFFER  64  conveniently provides level translation or whatever other remaining signal manipulation is needed to correctly interface a PTT signal or equivalent transmitter command (e.g., TRANSCVR-ON or TRANSCVR-OFF), audio signal and RADIO address signal to the destination RADIO. 
   CONTROLLER  54  conveniently contains one or more TIMERS  62 , or equivalent timing functions (e.g., software timers). CONTROLLER  54  receives the PTT switch signal (e.g., PTT-ON) from the selected MIC and the identification of the selected RADIO. CONTROLLER  54  then starts and monitors a TIMER, performs whatever digitization, compression or other audio signal manipulation is needed for the selected RADIO and sends a TRANSCVR-ON command, processed audio information and RADIO address to OUTPUT BUFFER  64 , as long as the lapsed time t since receiving the PTT-ON signal is less than the predetermined value Tc stored, for example, in MEMORY  60 . When t≧Tc, then the PTT-ON input is deselected, disabled or ignored, CONTROLLER  54  no longer sends the TRANSCVR-ON command to OUTPUT BUFFER  64  and the selected RADIO  10 ,  12  stops transmitting and returns to standby mode. When CONTROLLER  54  determines that t ≧Tc, it sends an ALARM signal via OUTPUT BUFFER  64  to ADU  14  over bus  15 A to alert the pilot that a ‘stuck PTT switch’ condition exists, as discussed previously in connection with  FIG. 1 . An error flag may also be sent to MEMORY  60  to indicate that the particular PTT switch concerned has reached a ‘stuck switch’ condition so that its signals are ignored until the ‘stuck switch’ condition is cleared. This is convenient but not essential. The ‘stuck switch’ alarm and flag are maintained until the ‘stuck switch’ status is cleared, as for example, by the pilot recycling the PTT switch if it has not failed. If the PTT switch has a mechanical or electrical failure, then the alarm and flag continue until the system is powered down and/or the defective PTT switch or other defective part is replaced. 
     FIG. 3  is a simplified flow chart illustrating method  100  of the present invention. As used herein, “PTT” refers to any push-to-talk switch, e.g., PTT- 1 , PTT- 2  or others that may be present in the system. Commencing with START  102 , which usefully occurs when system  8  is powered up, subsystem  50  executes PTT ACTIVE ? query  104  wherein it is determined whether or not the PTT switch is ON, that is, has the pilot depressed PTT switch  22  on hand-mike  20  or equivalent. This is conveniently accomplished by CONTROLLER  54  detecting whether a signal on input line  23 ,  27  corresponds to the PTT-ON state. If the outcome of query  104  is YES (TRUE) then step  106  is executed. Step  106  comprises TRANSCVR-ON sub-step  106 - 1  and TIMER-ON sub-step  106 - 2 . Steps  106 - 1  and  106 - 2  may be executed in either order. In step  106 - 1  the TRANSCVR-ON command is coupled from subsystem  50  to the appropriate RADIO transceiver (TRANSCVR) e.g., radio  10  or  12  selected by the pilot. This causes the RADIO to switch from the receive or standby to the transmit mode. Sub-step  106 - 2  turns on timer  62  (see  FIG. 2 ), which begins to court-up to Tc or to count down from Tc to zero. Either approach is useful. While the timing function is described herein as a counter, this is merely for convenience of explanation and persons of skill in the art will understand that any type of timing function may be employed. As used herein, the words “counter” and “timer” are intended to include these other alternatives. 
   TIMER≧TC ? query  108  is then executed wherein it is determined whether or not the running time from step  106 - 2  has reached the predetermined critical time value Tc. If the outcome of query  108  is NO (FALSE) then method  100  loops back to START  102  and query  104  via path  109 . As long as the PTT switch is active (query  104 =YES (TRUE)) and the counter running time is less than Tc (query  108 =NO (FALSE)), then method  100  will repeat steps  104 ,  106 ,  108  and TRANSCVR-ON will remain coupled to the appropriate radio  10 ,  12  thereby holding the radio in the transmit state. If the pilot releases the PTT switch before time reaches Tc, then on the next loop-back via pathway  109 , the outcome of query step  104  becomes NO (FALSE) and step  105  is executed. In sub-step  105 - 1  the TRANSCVR is turned off and in sub-step  105 - 2  the timer is reset to the start count, as the method loops back to the beginning, Sub-steps  105 - 1  and  105 - 2  may be performed in either order. 
   If the outcome of query  108  is YES (TRUE) indicating that the time count has exceeded the predetermined critical value Tc, then step  110  is executed. In sub-step  110 - 1 , the PTT switch is disabled, that is, TRANSCVR-ON is no longer coupled to radio  10 ,  12 , thereby causing radio  10 ,  12  to cease transmitting and revert to the receive or standby state. In sub-step  110 - 2 , timer  62  is reset to its starting value and in sub-step  110 - 3  an alarm message is sent along bus  15 A to ADU  14  causing the appropriate warning or caution light to illuminate or other alarm to turn on, alerting the pilot to the ‘PTT-Stuck’ condition and, preferably which PTT switch is in the ‘stuck’ state. Steps  110 - 1 ,  110 - 2 ,  110 - 3  may be performed in any order. Following step  110 , PTT ACTIVE ? query  112  is executed to determine whether the PTT switch is still activated (e.g., the PTT switch is depressed by the pilot or there is a continuing mechanical or electrical failure in the ON-STATE). If the outcome of query  112  is YES (TRUE) meaning that the PTT switch is still stuck in the ON-STATE, method  100  loops back via path  113  and query  112  is repeated. 
   If the outcome of query  112  is NO (FALSE) then step  114  is executed. In step  114 - 1 , the PTT switch is enabled again, that is, the PTT-ON state is no longer prevented from reaching radio  10 ,  12  and if the PTT switch is subsequently activated, a TRANSCVR-ON command will be coupled to radio  10  or  12  depending on which has been selected by the pilot. In step  114 - 2  the alarm set in step  110 - 3  is disabled, e.g., turned OFF, and operation of the PTT switch is once again fully normal. Steps  114 -l,and  14 - 2  may be performed in either order. Following step  114 , method  100  loops back via path  115  to START  102  and query  104  wherein system  8  once again monitors the status of the PTT switch. While method  100  has been described for a single PTT switch, this is merely for convenience of explanation and persons of skill in the art will understand that it applies to any of the PTT switches coupled to system  8 . 
     FIG. 4  is a simplified flow chart illustrating method  200  according to a further embodiment of the present invention, wherein two PTT switches are being employed, e.g., PTT- 1  and PTT- 2 . For convenience of explanation these are abbreviated here and in  FIG. 4  as PTT 1  and PTT 2 . Commencing with START  202 , query  204  is executed wherein it is determined whether or not both PTT 1  and PTT 2  are in the “ON” state (e.g., PTT 1  &amp; PTT 2  ACTIVE?). If the answer to query  204  is YES (TRUE) then query  206  is executed wherein it is determined whether or not either of PTT 1  or PTT 2  is in the enabled state (e.g., PTT 1  OR PTT 2  ENABLED ?), that is, not disabled because of a previous, uncleared “STUCK-ON” condition (see the discussion of  FIG. 3 ). If the outcome of query  206  is NO (FALSE) then method  200  returns to start  202  via path  207 . 
   If the outcome of query  206  is YES (TRUE) indicating that one or the other of PTT 1  or PTT 2  is enabled (capable of working), then step  208  is executed wherein the timer is started (TIMER ON) and the corresponding transceiver is turned on (TRANSCVR ON). These sub-steps may be performed in either order. Following step  208 , query  210  is executed wherein it is determined whether or not the timer has counted to or exceeded its critical value (e.g., TIMER≧Tc ?). If the outcome of query  210  is NO (FALSE) then method  200  returns to start  202  and initial query  204  via path  211 . The transmitter remains in the ON-STATE as long as the PTT switch continues to be depressed and t&lt;Tc. If the outcome of query  210  is YES (TRUE) then step  212  is executed wherein the alarm is turned on (e.g., ALARM ON), the timer is reset (e.g., RESET TIMER), both PTT 1  and PTT 2  are disabled (e.g., DISABLE PTT 1 &amp; 2 ), and the active transceiver is turned off (TRANSCVR OFF), that is switched back to the receive or standby state. These sub-steps may be executed in any order. The method then returns to start  202  and initial query  204  via path  213 . 
   Returning now to query  204 , if the outcome of query  204  is NO (FALSE), then query  214  is executed wherein it is determined whether PTT 1  is active (e.g., PTT 1  ACTIVE ?), that is, is PTT 1  depressed. If the outcome of query  214  is YES (TRUE) then step  216  is executed wherein the alarm is turned off (e.g., ALARM OFF) and PTT 2  is enabled (e.g., ENABLE PTT 2 ), that is, placed in a state where it is capable of activating a transmitter if pressed. Query  218  is then executed wherein it is determined whether or not PTT 1  is enabled (e.g., PTT 1  ENABLED ?), that is capable of functioning to activate a transmitter. If the outcome of query  218  is NO (FALSE) then the method returns to start  202  and initial query  204  via path  219 . If the outcome of query  218  is YES (TRUE) then step  220  is executed wherein the timer is started (e.g., TIMER ON) and the radio is put into the transmit mode (e.g., TRANSCVR ON). Following step  220 , “TIMER≧Tc ?” query  222  is executed in the same manner as for query  210 . If the outcome of query  222  is NO (FALSE) then the method returns to start  202  and initial query  204  via path  223 . If the outcome of query  222  is YES (TRUE) then step  224  is performed wherein the alarm is turned ON (e.g., ALARM ON), the timer is reset (e.g., RESET TIMER), PTT 1  is disabled (e.g., DISABLE PTT 1 ), that is, rendered ineffective, and the corresponding transceiver is switched from the transmit state back to the receive or standby state (e.g., TRANSCVR OFF). These sub-steps may be performed in any order. Following step  224 , the method returns to start  202  and initial query  204  via path  225 . 
   Returning now to query  214 , if the outcome of query  214  is NO (FALSE) then step  226  is executed wherein the alarm is turned off (e.g., ALARM OFF) and PTT 1  is enabled (e.g., ENABLE PTT 1 ). Query  228  is then executed wherein it is determined whether or not PTT 2  is active (e.g., PTT 2  ACTIVE ?). If the outcome of query  228  is NO (FALSE) then step  238  is executed where the alarm is turned off (e.g., ALARM OFF), the timer is reset (e.g., RESET TIMER), and PTT 2  is enabled (e.g., ENABLE PTT 2 ). Thereafter, the method returns to start  202  and initial query  204  via path  239 . If the outcome of query  228  is YES (TRUE) then query  230  is executed wherein it is determined whether or not PTT 2  is capable of working (e.g., PTT 2  ENABLED ?). If the outcome of query  230  is NO (FALSE) then method  200  returns to start  202  and initial query  204  via path  231 . If the outcome of query  230  is YES (TRUE) then step  232  is executed wherein the timer is started (e.g., TIMER ON) and the selected radio is put into the transmit mode (e.g., TRANSCVR ON). Following step  232 , “TIMER≧Tc ?” query  234  is executed in the same manner as for queries  210  and  222 , that is, has the timer counted to or past the predetermined value Tc. If the outcome of query  234  is NO (FALSE) then the method returns to start  202  and initial query  204  via path  235 . If the outcome of query  234  is YES (TRUE) then step  236  is performed wherein the alarm is turned ON (e.g., ALARM ON), the timer is reset (e.g., RESET TIMER), PTT 2  is disabled (e.g., DISABLE PTT 2 ), that is, rendered ineffective, and the corresponding transceiver is switched from the transmit state back to the receive or standby state (e.g., TRANSCVR OFF). These sub-steps may be performed in any order. Following step  236 , the method returns to start  202  and initial query  204  via path  237 . 
   Among other things, the above-described method has the advantage that when a particular PTT switch enters a STUCK-ON state, that only the communication path corresponding to that particular PTT switch is disabled. The remaining PTT switches remain active and able to provide communications in the normal manner. This is a significant improvement over the prior art wherein a single PTT failure disabled all PTT switches coupled to the same ACP. Further advantages of the present invention are that: (1) it can automatically monitor and flag (and alarm) a stuck PTT switch even if it has not been selected by the pilot, and (2) it can continually poll stuck PTT switch status and reset it to an active state once the STUCK-ON condition is no longer present. 
   While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof