Abstract:
A high-isolation call cord switch includes connectivity for a standard hospital-style push-button call cord while using low voltage alternating current to isolate detection circuitry from static electricity. An alternating current signal is used to permit transformer coupling, with its intrinsic isolation capability, between the call cord switch and the detection circuit.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to sensing electronics. More particularly, the present invention relates to voltage-isolated detection circuits for communication in controlled environments.  
       BACKGROUND OF THE INVENTION  
       [0002]     Circuit protection and bidirectional electrical isolation are strongly desirable attributes of electrical circuits likely to be exposed to outside stresses as well as to electrical circuits with the potential to introduce hazards. Examples of outside stresses include electrostatic discharge (ESD) events from such sources as lightning, other natural events, and human contact. Examples of hazards potentially able to be introduced by electronic circuits include interference with medical test results and causation of sparks that can ignite flammable mixtures such as medical gases mixed with air or oxygen.  
         [0003]     One area of particular interest is ESD events that occur in nominally sheltered areas such as buildings, and particularly those occurring in locations such as hospitals and other care facilities. ESD is caused by separation of similar materials, such as peeling a strongly insulating material like polyethylene film from a roll; by repeated rubbing between differing materials, such as the classic rabbit fur and amber rod demonstration; and by many everyday events such as walking across a linoleum floor in leather-soled shoes. ESD is gauged by voltage and available energy storage, and by such factors as the humidity-dependent conductivity of air, which factors contribute to a determination of the distance a spark can leap.  
         [0004]     ESD events are diminished by bleeding off static charge through ameliorative strategies, such as replacing static-promoting furniture, clothing, bedding, and carpets with dissipative types, applying carefully formulated waxes to linoleum floors, adding extra water to dry air, and the like. However, the complete elimination of ESD is seldom practical. A balanced approach to ESD control combines amelioration strategies with electronic circuit designs that increase immunity of circuits to damage due to those ESD events that are not successfully suppressed.  
         [0005]     Several ESD models exist to permit and verify successful apparatus design for the prevention of damage. Typical ESD models include combinations of voltage levels, energy storage levels (capacitance), and pulse rise time to emulate one or more modes analogous to such events as a human walking across a static-promoting floor in dry weather and touching a test subject device with a finger. Since established ESD models generally use voltage levels well below 100 kilovolts (KV), that level is a useful threshold for developing an apparatus that is substantially ESD resistant.  
         [0006]     In general, circuit designs that have heightened intrinsic ESD immunity are desirable in comparison to lower ESD immunity circuits. Tradeoffs between such devices in order to enhance survivability in normal use involve such issues as higher power consumption, slower response time, diminished accuracy, and higher cost. Thus, any circuit improvement that can potentially improve ESD immunity with minimal penalties may be viewed with great interest.  
         [0007]     In particular, in a hospital setting, a human interface device such as a call cord apparatus may be viewed as a candidate for improved ESD protection. A call cord is a device used at a hospital bed and elsewhere, typically electrically connected to a wall-mounted electrical enclosure. The call cord, itself, is, in some embodiments, a wire pair ending at the wall end in a plug and at the opposite end in a handle with a pushbutton switch. A call cord is typically used by a patient to request the attention of an attendant. Because the call cord contains conductive components and is covered in insulating material, adding a requirement for ESD dissipative qualities is an undesirable imposition on the design of the call cord itself.  
         [0008]     A further call cord restriction that requires consideration is the possibility that the call cord itself could be a source of physical hazard. A spark of any size occurring exterior to or within a call cord, such as a spark associated with a switch closure, is potentially capable of causing electrical interference with devices in its immediate vicinity. In addition, an improperly configured call cord could be a source of undesirable arcing.  
         [0009]     Accordingly, it is desirable to provide a method and apparatus that allow a call cord-type switch for use in a hospital grade wall-mount signal annunciator to operate in an essentially normal mode, while establishing an ESD immunity threshold on the order of 100 Kilovolts and providing substantial immunity from spark generation.  
       SUMMARY OF THE INVENTION  
       [0010]     The foregoing needs are met, to a great extent, by the present invention, wherein in one aspect an apparatus is provided that in some embodiments provides a method and apparatus that allow a call cord-type switch in a hospital grade wall-mount signal annunciator to operate in an essentially normal mode, while establishing an ESD immunity threshold on the order of 100 Kilovolts and providing substantial immunity from spark generation.  
         [0011]     In accordance with one embodiment of the present invention, a detection circuit is presented. The detection circuit comprises a transformer primary circuit and a transformer secondary circuit. The transformer primary circuit further comprises an alternating-current signal (AC) source, an impedance in series with the AC source, and a transformer primary winding in series with the AC source and the impedance. The transformer primary circuit establishes an electrical circuit by completing a signal path to the AC source. A detector is positioned across one of the impedance and the primary winding. The transformer secondary circuit further comprises a transformer secondary winding magnetically coupled to the primary winding, and a plurality of load devices switchably connected across the secondary winding one of not at all, individually, and in combination.  
         [0012]     In accordance with another embodiment of the present invention, a detection circuit is provided. The detection circuit comprises means for generating a generally sinusoidal signal applied to a series string comprising a impedance and a transformer primary winding, means for detecting positioned across one of the impedance and the transformer primary winding, means for detachably coupling a first load across a transformer secondary winding magnetically coupled to the transformer primary winding, and means for detachably coupling a second load across the transformer secondary winding.  
         [0013]     In accordance with yet another embodiment of the present invention, a method for monitoring a call cord is provided. The method comprises the steps of generating a generally sinusoidal signal across a series string comprising a impedance and a primary winding of a transformer, detecting a voltage across one of the impedance and the primary winding of the transformer, isolating a secondary winding of the transformer from the primary winding of the transformer, detachably switching a first load across the secondary winding of the transformer, wherein the secondary winding is magnetically coupled to the primary winding of the transformer, and detachably switching a second load across the secondary winding of the transformer.  
         [0014]     There have thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto.  
         [0015]     In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as in the abstract, are for the purpose of description and should not be regarded as limiting.  
         [0016]     As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be used as a basis for the designing of other structures, methods, and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a block diagram of the detector according to a preferred embodiment of the invention.  
         [0018]      FIG. 2  is an overall schematic diagram of a preferred embodiment of the invention.  
         [0019]      FIG. 3  is a detailed schematic diagram of a specific section of the invention.  
         [0020]      FIG. 4  is a schematic diagram of an alternate embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]     The invention will now be described with reference to the drawing figures, in which like reference numerals refer to like parts throughout. An embodiment in accordance with the present invention provides an impedance element, such as a fixed resistor, and a transformer primary winding. The pair functions as a load, wired in series, and is driven by an alternating current (AC) source, preferably characterized by low power output and low output impedance. The output of the AC source is preferably a sinusoidal steady state signal. In general, a signal source loaded by a series combination of load devices, such as an impedance element and a transformer primary winding, where the transformer primary winding has transformer coupling to at least one secondary winding, exhibits voltage division between the load devices. A detector placed across either the impedance element or the transformer primary winding detects a waveform similar to that of the AC source, scaled according to the division ratio between the two load devices and altered in phase by the reactive component of the impedance of each load device.  
         [0022]     The circuit into which the secondary winding of the transformer is wired couples to the primary winding. For example, if the secondary is an open circuit, the waveform in the primary circuit will cause the detector to sense a particular signal, from which can be extracted a DC voltage, the magnitude of which is proportional to the impedance ratio of the impedance element and the coupled transformer components. Placing a resistive or reactive load or a short circuit across the transformer secondary winding will change the primary-circuit AC output level. This primary-circuit AC output level is detected, conditioned, and used as an input to other electronic circuits.  
         [0023]      FIG. 1  illustrates an embodiment of the present inventive apparatus. For a detector primary circuit  10 , a sine wave generator  12  drives an impedance  14  and the primary of a transformer  16 . In the mode shown, both a first switch  18  and a second switch  20  are open, corresponding to an unplugged call cord  22 . A voltmeter  24 , which serves here as a generic detector, detects a baseline AC voltage V PRI . When the call cord  22  is plugged in, the first switch  18  is closed, which, by transformer coupling, inserts a second load device  26  into the primary circuit, with an impedance scaled by the square of the turns ratio of the transformer  16 . The effect of this coupling, for any real (i.e., resistive) impedance, is to draw down the AC voltage V PRI  at the voltmeter  24 . When a call cord  22  pushbutton is pressed, the second switch  20  is closed, causing the voltage across the transformer  16  primary winding to drop nearly to zero. The voltmeter  24  in  FIG. 1  converts each of these conditions to a logic signal.  
         [0024]     Transformers of various construction styles may be advantageous for different embodiments of the inventive apparatus. Ferrite core, iron core, and nonferromagnetic core such as air core are some of the potentially useful constructions. Windings in the form of printed circuit tracks on circuit boards constitute one of the types that may be useful in some embodiments.  
         [0025]     The configuration shown provides isolation to the extent that the transformer  16  has isolated primary and secondary windings. Isolation of primary from secondary windings can be realized in a variety of ways. For example, ordinary enameled magnet wire wound with bifilar windings provides isolation to the extent of the breakdown voltage of two layers of wire insulation. If the two windings are wound onto opposite sides of a toroidal core, then the breakdown voltage may be a function of the air gap between the windings as well as the breakdown voltage characteristic of the wire insulation. If the two coils are wound on a multiaperture core or on two separate bobbins fitted onto a split core, then for some core designs the level of isolation can be increased further still.  
         [0026]     A particularly useful core style in a preferred embodiment is a ferrite (powdered iron and other metal oxides and ceramic binders) core in an “E” shape with a ferrite plate assembled onto and closing the open legs of the “E”. This transformer core style can be made in various sizes, and can provide acceptable coupling even for small signals using large core sizes. The wiring for the transformer in the preferred embodiment is wrapped on bobbins that fit closely on the outer legs of the “E”. The potentially large size of the core affords large gaps for ESD isolation—that is, where there is a relatively large physical distance between materials at different static potentials, a higher voltage is required to cause an arc, so the arc is less likely to occur, while slow, usually harmless bleedoff of the charge becomes more likely.  
         [0027]     It may be observed that the effect of a voltage spike due to ESD, if applied to one terminal of the transformer  16  secondary, may cause a differential-mode voltage pulse or glitch to be coupled back into the detection circuit. However, if the transformer  16  is configured for operation in the vicinity of the audio band (that is, at frequencies generally above 120 Hz and generally below 100 KHz), the largest proportion of an ESD pulse will fail to couple through the windings of the transformer  16 . Similarly, isolation afforded between the windings can largely prevent an ESD pulse from coupling from secondary to primary as a common-mode static pulse.  
         [0028]     Further enhancement can be provided by adding transient suppressive devices such as spark gaps or transient suppression diodes to the primary and/or secondary circuits, as well as rise time-limiting devices such as series resistor-capacitor networks.  
         [0029]     ESD pulse transmission from primary to secondary is also generally inhibited by the isolation afforded by the transformer, which isolation can be further enhanced by the turns ratio of the transformer being made comparatively large, such as 10:1 or 100:1, with the primary on the high voltage side and the secondary on the high current, low voltage side. Such a configuration permits the call cord  22  to be operated at a reduced voltage level and thereby generally reduces the possibility for appreciable signal radiation from the call cord  22  acting as an antenna.  
         [0030]      FIG. 2  is a circuit diagram  34  showing the features of an embodiment of the invention in analog form. Here, the AC source  12  includes an oscillator  36  followed by a buffer/driver  38 . The generic impedance device  14  of  FIG. 1  is seen to be a resistor  40  chosen to have a value in a desired range.  
         [0031]     In other embodiments, a device other than a resistor  40  may be useful for the impedance device  14 . Such devices may include capacitors, semiconductor devices such as constant-current diodes or zener diodes, networks of passive (R/L/C) components, and other circuit configurations as dictated by system requirements or user preference. The fixed resistor  40  is effective in the AC circuit of the embodiment shown.  
         [0032]     The transformer  16  and call cord-side circuit components in  FIG. 2  are arranged in the same general fashion as those shown previously in  FIG. 1 . The detector  42  is shown to be a circuit device known variously by such names as a high-speed rectifier, an ideal diode, and an AM demodulator. The output of this circuit is shown to drive a filter  44 , the combination of which with the high-speed rectifier is known as an averaging detector  46 . To distinguish more clearly between the overall circuit and the subcircuit, the term ideal diode  42  will be used hereinafter.  
         [0033]     The averaging detector  46  output is a voltage corresponding to the envelope of the AC source  12  as affected by load variations. If the AC source  12  has an output that is substantially constant, then the switching of the cord detect switch  18  by inserting the call cord  22  divides the input signal, lowering the ideal diode  42  input V PRI  and thereby the averaging detector  46  output V OUT . Activating the call cord switch  20  will further load the circuit, lowering the averaging detector  46  output further still.  
         [0034]     The output V OUT  of the averaging detector  46 , shown in  FIG. 2 , is fed to a first comparator  48  and a second comparator  50 , the reference voltages for which are chosen to allow the first comparator  48  to trip at a first voltage threshold V REF1  and the second comparator  50  to trip at a second voltage threshold V REF2 . When the call cord  22  is unplugged, the voltage is high enough that neither comparator  48  nor  50  is tripped, producing a logic output for the first comparator  48  and second comparator  50  of [0,0].  
         [0035]     When the call cord  22  is plugged in, but its button is not pressed, closure of the first switch  18  divides the AC signal between the first resistor  40  and the second resistor  52  as scaled by the square of the transformer  16  turns ratio. Activation of the first comparator  48  but not the second  50  produces a logic output of [1,0].  
         [0036]     When the switch  20  on the call cord  22  is activated, a substantially short circuit is applied to the secondary of the transformer  16 . Scaled by the square of the transformer  16  turns ratio, this short appears effectively as a short in the primary-side AC voltage divider, reversing the state of the second comparator  50 , and producing an output signal of [1,1].  
         [0037]     Each of the logic states can be used to control system functions, such as by generating a “system operational” signal and a “system operational and activated” signal.  
         [0038]     It may be observed that considerations such as loop gain and diode polarity in the ideal diode  42  may be changed, which can in some configurations reverse the operation of the comparators. Similarly, configuring the ideal diode  42  across the resistor  40  instead of from the transformer  16  primary winding to ground reverses the polarity of the changes that occur due to circuit operation. System design considerations such as a requirement for additional DC power supplies in some configurations when compared to others may be factors in selecting an embodiment.  
         [0039]      FIG. 3  is a schematic of the detector  46 . A voltage V PRI  present at the input  60  to the detector  46  circuit causes a swing of the same polarity at the detector output  62 . With the diodes  64  and  66  in the feedback loop of the ideal diode  42  in the polarity shown, positive input swings with respect to a DC reference voltage level at the noninverting input  68  of the operational amplifier  70  appear at the output  62 , while negative input swings reverse-bias the diode  64  and forward bias the diode  66  and produce zero output. Output signal magnitude is a function of gain and is controlled (for positive signals) by the values of the resistors R B  and R C , according to the formula V OUT =V PRI *(1+(R B /R C )), neglecting any effects of the R D /C 1  filter and any loading by the inputs to comparators U 2 and U   3 . With the noninverting detector  46  design as shown, the detector output  62  is most positive when the call cord  22  is unplugged, and least positive when the call cord  22  is plugged in and the call cord switch  20  is activated.  
         [0040]     Alternative circuit embodiments, such as inverting amplifier configurations, are also realizable, but require different computations. Similarly, a single semiconductor diode can perform substantially the function described, with an offset approximately equal to the rated forward drop of the diode. If a Schottky diode is used, then the forward drop is typically on the order of 0.2 volts for positive signals.  
         [0041]      FIG. 4  shows that another configuration  78  can generate more logic states than the call cord configuration shown in  FIGS. 1, 2 , and  3 . The configuration of  FIGS. 1, 2 , and  3  allows three logic states to be produced, namely, unplugged, plugged, and pressed. For convenience, in  FIG. 4  the turns ratio of the transformer  80  is assumed to be 1:1. A first resistor  82  is assigned a resistance value  8 R. If a second resistor  84  is also assigned a value  8 R, a third resistor  86  a value  4 R, a fourth resistor  88  a value  2 R, and a fifth resistor  90  a value R, then leaving the call cord  92  unplugged still activates none of the switches  94 ,  96 ,  98 , and  100 . With the secondary open, the reference voltage V REF  is present across a series string of the first resistor  82  and the primary of the transformer  80 . This represents the nominal condition for the detection circuit.  
         [0042]     Plugging in the call cord  92 , which closes the cord sense switch  94 , and omitting the contribution from the resistance of the transformer  80 , an output of 0.5 V REF  is obtained. Activating the first call cord switch  96  then produces an output of 0.25 V REF , while activating the second switch  98  alone produces an output of 0.167 V REF , and activating the first and second switches  96  and  98  together produces an output of 0.125 V REF . Activating the last call cord switch  100 , which has, in the embodiment shown, a resistor lower in value than the combination of all of the other resistors in parallel, produces an output closer to zero than any combination of other switch closures, and can thus serve as an override or emergency switch. If the output generated by activating one or more switches is rectified, filtered, and fed to a multiplicity of comparators with appropriate settings for their voltage thresholds, then a multiplicity of logic states can be extracted from the button-press status of the call cord switches. Thus, a multiplicity of information items can be extracted from a multifunction call cord, while the desirable degree of voltage isolation is preserved.  
         [0043]     The final resistor  90  may have a resistance of zero ohms—that is, no resistance or no resistor at all—in which case pressing the last switch  100  results in largely the same behavior exhibited by the circuit in  FIG. 2 . This phenomenon may prevent the other switches from being detected while the last switch  100  remains pressed.  
         [0044]     An additional resistance may be permanently positioned across the transformer  16  secondary winding in some embodiments.  
         [0045]     Response speed is a function of oscillator frequency and component values. For many low-frequency oscillators, such as those that run at 32,768 Hz, response time with typical sense components may be a few thousandths of a second, and thus indistinguishable from instantaneous to a user. Other oscillator frequency regimes may be suitable for some embodiments.  
         [0046]     The use of a sine wave signal has been noted herein as preferable. Alternatives to sine wave signals, such as ramp, square, and rectangular wave signals, and the like, have greater harmonic content than a sine wave. These harmonics, even at low amplitudes, can increase spurious radiation by the call cord, for example in event of a shield failure. At the opposite extreme, an approximate sine wave that has deliberately introduced, fairly low-rate frequency modulation to the extent of a few percent may show a lower effective level of detectable emission than a strictly steady-state signal. Other strategies, such as pseudo-random bursts of waveforms and/or cos 2 x amplitude modulation envelopes synchronized to the detectors, may also be effective at reducing effective emission levels.  
         [0047]     A microprocessor/microcontroller/digital signal processor with a built-in or added-on analog-to-digital converter can accept either an unrectified AC input or a rectified DC version of the AC input for conversion. Such a microcontroller can then either mathematically convert the detected signal by discarding negative values and averaging positive values, for example, or can recognize positive peaks within a time window as indicative of switch status. A sample rate not less than the Nyquist rate is advisable for detecting positive peaks in a raw AC signal, while a peak detection provision such as an external rectifier/filter assembly can in some embodiments greatly reduce the required sample rate, depending on the time constant of the filter. An upper limit on the filter time constant may be established by determining how slow the detection of a switching event can be without appreciably degrading system functionality for a particular application.  
         [0048]     Although an example of a voltage-isolated call cord detector is shown that uses multiple operational amplifiers and comparators, as well as a variety of discrete passive components, it will be appreciated that alternative devices, such as analog-to-digital converters, can produce substantially the same degree of ESD protection, provided the isolation transformer is employed largely as shown. Also, although the voltage-isolated call cord detector is useful in support of medical and related arts, it can also be used in other environments such as manufacturing, warehousing, and office environments where risk of damage to electronic devices due to ESD may be a principal consideration.  
         [0049]     The many features and advantages of the invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and, accordingly, all suitable modifications and equivalents may be resorted to that fall within the scope of the invention.