US3445172A - Fail-safe system - Google Patents

Description

United States Patent 3,445,172 FAIL-SAFE SYSTEM Robert J. Zielinski, Rochester, N.Y., assignor to American Gas Association, Inc., New York, N.Y., a corporation of New York Filed Aug. 2, 1967, Ser. No. 657,943 Int. Cl. F23g 23/00; F23n /08; H01h 47/24 U.S. Cl. 431-24 4 Claims ABSTRACT OF THE DISCLOSURE A fail-safe electrical circuit arrangement for holding open a thermal gas valve when a flame supplied by the valve is present and for insuring that the valve will be closed when the flame is absent, despite failure of certain circuit components. Alternating voltage is applied to the series combination of the heating coil of the valve 'and the anode cathode path of a silicon controlled rectifier (SCR). The SCR is normally biased off, but is turned on intermittently by intermittent control pulses from a flame-sensing circuit when, and only when, the flame to be sensed is present. The current pulses thereby produced through the SCR apply sufficient power to the heating coil to hold the gas valve open when the flame is present. When the flame is absent, no control pulses are applied, no current flows through the heating coil and the valve closes. Should the SCR become open-circuited, again no current will flow and the valve closes. Should the SCR become short-circuited, in one or both directions, the resultant higher R.M.S. current through the heating c il will cause it, or a fuse in series therewith, to burn out so that the thermal valve is again deactuated and closed. The flame-sensing circuit itself is preferably of a fail-safe type, so that the entire system has fail-safe characteristics.

Background of the invention This invention relates to safety apparatus, and particularly to a circuit for providing fail-safe operation of electrically-operable control means.

There are many applications in which it is desirable or essential to provide safety apparatus which will fail safe in the event of failure of certain components thereof. For example, in the case of a gas flame it is often desirable to sense the presence or absence of the flame, and to insure that gas is supplied to the flame only so long as the flame persists and is shut off when the flame disappears. While many types of circuits are known for sensing a flame and producing gas flow controlling signals in response thereto, in some cases failure of an element of the system, as by open circuiting or short circuiting of one or more components thereof, may result in a false indication of the presence of flame and may therefore cause continued undesired supply of gas even though the flame is absent.

Accordingly it is an object of my invention to provide new and useful safety apparatus.

Another object is to provide such safety apparatus which has fail-safe characteristics.

A further object is to provide such apparatus which will fail safe despite the failure of components thereof.

A still further object is to provide new and useful safety apparatus which will fail safe despite the failure of components thereof, and which is particularly simple, inexpensive, eifective and reliable.

Summary of the inventioni In accordance with the invention, these and other objects are achieved by the provision of a safety system in which a source of supply voltage is connected across the 3,445,172 Patented May 20, 1969 series combination of an electronic switching device and control means, which control means comprises electrical means responsive to electrical power of more than a first value but less than a second value to operate the control means to its actuated condition and responsive to power of at least said second value to place the control means in its deactuated condition; the electronic switching means normal-1y provides a high resistance between its switch terminals but is capable of being rendered intermittently highly conductive in response to pulses applied to its control terminal. Sensing means responsive to the occurrence of an operating condition to be sensed generate pulses which are applied to said control terminal at a rate suflicient to supply said electrical means with power from said source having a value between said first and second values, thereby to actuate the control means. In addition, said supply voltage is suflicient that if the electronic switching means fails by short circuiting, the resultant increased R.M.S. current through the electrical means exceeds said second power value and causes the control means to return to its normal deactuated condition.

In a preferred form of the invention, the flame sensing circuit is of the type described and claimed in the copending application of Robert J. Zielinski and Frederick W. Westberg, Serial No. 417,960, filed December 14, 1964, now Patent No. 3,348,104, and entitled Bias-Controlled A.'C.-'Operable Voltage Threshold Circuit and Systems Employing Same. Such a circuit typically comprises a capacitor connected in series with a flame across the source of alternating voltage, and voltage breakdown means connected effectively in parallel with the flame and having a breakdown voltage greater than the zero-to-peak but less than the peak-to-peak voltage of the alternating voltage. Such a circuit produces output pulses only when flame is present, despite any failures of components thereof. The control means comprises a thermal gas valve f r supplying gas to the flame. In order to provide control of the relatively large currents normally required to operate the thermal valve, the output pulses of the flame sensor are preferably applied to the control gate of a norm-ally-ofi silicon controlled rectifier, the anode and cathode elements of which constitute electronic switch terminals which are in series with the heating element of the thermal valve. In the absence of flame then, no pulses are produced by the flame-sensing circuit and the silicon controlled rectifier remains in its high resistance state so that substantially no current flows through the heating element of the thermal valve and the valve remains closed, as desired. When a flame is present and the silicon controlled rectifier is operating normally, 'it conducts briefly in response to each pulse supplied to its gate electrode and hence applies current pulses of substantial magnitude t the heater coil. These pulses produce power in the heating element suflicient to operate the thermal valve. Now in some cases it is possible for the silicon controlled rectifier to fail by exhibiting an open-circuit or a short-circuit in atleast one direction between its anode and its cathode. If the failure is by open circuiting, then no current is supplied to the thermal valve and it is deactuated as desired for fail-safe operation. If the silicon controlled rectifier fails by shorting between its anode and cathode in one or both directions of conduction, it will apply power to the coil of the thermal valve sufliciently great to burn out the coil and open the circuit, thereby deactu'ating the valve again as desired for fail-safe operation. If desired, a separate fusible element can be utilized in series with the thermal valve coil, which will burn out in response to short-circuiting of the silicon controlled rectifier before the coil does, thereby saving the thermal valve from damage. The entire arrangement therefore provides a high degree of safety in flame sensing and control systems, and at the same time is quite simple, compact and relatively inexpensive.

Description of figures The invention will be more readily understood from a consideration of the following detailed description, taken in connection with the appended drawings, in which:

FIGURE 1 is a schematic block diagram of one form of system in accordance with the invention;

FIGURE 2 is an electrical schematic of a preferred embodiment of the invention; and

FIGURES 3A and 3B are graphical representations to which reference will be made in explaining the mode of operation of the invention in its preferred form.

Description of preferred embodiments Referring now to FIGURE 1, there is shown a gas supply pipe for supplying fuel gas to a burner 12 which when ignited produces a flame 14. The burner 12 is shown only schematically, since the exact form of burner utilized is not of primary significance in connection with the invention, and it will be understood that an igniter for the burner may be employed. In series in the gas supply pipe 10 is a normally-closed thermal valve 18, which may be of the known type which is closed by positive spring pressure unless opened in response to heating by a coil 20, which coil may for example operate through heating of a bimetallic strip to cause opening of the valve. When current is removed from the coil, the heating is discontinued and the valve becomes deactuated and recloses. In this embodiment of the invention a separate fusible element 22 is also connected in series with coil Valve 18, coil 20 and fusible element 22 constitute control means for controlling the flow of gas to the burner, and are characterized by a normally deactuated condition in the absence of heating of coil 20 in which gas flow is cut off by closing of valve 18, and by an actuated condition produced in response to heating of coil 20 in which the valve is open and fuel is supplied to the burner. It will also be appreciated that this control means comprises electrical means, in this example made up of coil 20' and fusible element 22, which will respond to electrical power in excess of a first value to actuate and open valve 18 and which will return valve 18 to its decatuated, closed condition when the power supplied is in excess of a second valve sufiicient to burn out the fusible element 22.

Coil 20 and fusible element 22 are connected in series with a supply voltage source 26 and a normally highresistance electronic switching device 28. Electronic switching device 28 has switch terminals 30 and 32 between which it normally exhibits a high resistance, but is responsive to electrical pulses applied to control terminal 34 thereof to be rendered highly conductive in response to each such pulse. Accordingly, each time a control pulse is applied to control terminal 34, voltage source 26 produces a pulse of current through coil 20 and fusible element 22, no such current being produced however in the absence of control pulses so long as the components of the system are functioning normally.

A flame-sensing electrode contacts flame 14 and is electrically connected to one input terminal 41 of an intermittent-output flame-sensing circuit 42, the other input terminal 43 of which may be grounded and connected to the gas supply line 10. Flame-sensing circuit 42 is operative to produce, at its output terminal 44, an intermittent output which is supplied to control terminal 34 of elecronic switching device 28 to render it intermittently highly conductive when flame 14 is present. In the absence of flame 14, no such output pulses are produced at terminal 44.

Assuming merely by way of example that flame 14 is a pilot flame, when the flame 14 is present flame sensing circuit 42 applies intermittent electrical pulses to control terminal 34 of electronic switching device 28, these pulses being of suflicient frequency and duration to deliver to heater coil 20 an amount of electrical power sufiicient to hold thermal valve 18 open and supply gas to flame 14, without burning out fusible element 22 or the heater coil 20'.

If flame 14 should for any reason disappear, due to a strong draft for example, output pulses from flamesensing circuit 42 will disappear, electronic switching device 28 will remain in its high-resistance condition, substantially no current will flow through heater coil 20, and thermal valve 18 will return to its normal closed condition, cutting off the supply of gas to burner 12 as desired.

If electronic switching device 28 should fail by becoming continuously open-circuited, again no current will flow through coil 20 and thermal valve 18 will close and remain closed, so that although the control system will have failed, it will have failed safe in that gas is no longer supplied to burner 12.

On the other hand, should electronic switching device 28 fail by exhibiting a continuous short circuit between its switching terminals 30 and 32 for both directions of voltage across the switching device, supply voltage source 26 will produce a continuous current through fusible element, 22, which will deliver much more power to element 22 than during normal operation and burn out fusible element 22, terminate current through heater coil 20 and again cause thermal valve 18 to close as desired for fail-safe operation. If the electronic switching device becomes short-circuited for applied voltages of only one polarity, current will be delivered to fusible element 22 only half the time, but still with suflicient power to burn out the fusible element. Accordingly the system fails safe whether the failure is due to an open circuiting or shortcircuiting of the electronic switching device 28.

As will be apparent from the preferred embodiment of the invention now to be described with particular reference to FIGURE 2, the heater coil 20 may itself constitute the fusible element which burns out in response to increased average current from supply voltage source 26, as will now be described.

Referring now to the example shown in FIGURE 2, the supply voltage source comprises the secondary 50 of a transformer 52, the primary 54 of which is supplied with alternating line voltage from any suitable source 56. 52 may constitute a conventional isolating transformer of 1:1 ration such as is commonly utilized in safety equipment. The heater coil of the thermal valve is represented at and also serves as the fusible element which burns out in response to increased power. The normally highresistance electronic switching device comprises the silicon controlled rectifier 62 (SCR) having its gate electrode 64 connected through a resistor 66 to the cathode 68. As shown, the transformer secondary 50, the fusible heating element 60, and the anode and cathode elements of SCR 62 are connected in a common series circuit with each other. The remaining elements of the circuit of FIGURE 2 comprise a flame-sensing circuit of the general type described and claimed in the above-identified copending application.

More particularly, the encircled resistor 70 and diode 72 represent the equivalent resistance and rectifying characteristics of a flame such as flame 14 in FIGURE 1, and will be referred to herein as a flame diode F. Connections are provided to this flame, as by the burner base and a suitable flame electrode, and the flame diode F is connected in series with a capacitor 74 and a resistor 75 across the transformer secondary winding 50. Since the burner base is usually the cathode of the flame, the point 76 between capacitor 74 and the flame diode F is conveniently grounded, as shown. A pair of glow lamps 80 and 82 are connected in series with resistor 66 between point 76 and the lower end of transformer secondary 50.

Details of the operation and adjustment of flame-sensing circuits of the type just described are fully set forth in the above-identified copending application. In brief, the combination of glow lamps 80 and 82 constitutes a voltage breakdown device which is substantially non-conductive until the voltage between point 76 and the lower end 83 of transformer secondary 50 rises above a predetermined level, at which time the glow lamps conduct heavily until said voltage falls below the extinction level for the glow lamps, at which time they resume their non-conductive state. The flame diode F and capacitor 74 respond to the alternating voltage from transformer secondary 50 to produce an increasingly more positive DC voltage component at point 76 relative to the lower end 83 of the transformer secondary 50. Each cycle of the alternating voltage increases this DC component somewhat, until the zero-to-peak value of one positive half-cycle of the applied alternating voltage plus the DC component exceeds the breakdown voltage of the two glow lamps in series. The number of cycles of alternating current required for this to occur can be adjusted by choice of the value of capacitor 74 and resistor 75. The breakdown voltage for the two neon lamps in series is chosen to be greater than the zero-to-peak voltage of the alternating input voltage but less than the peak-to-peak voltage thereof. Firing of the glow lamps can therefore only occur if the rectifying characteristic provided by the flame is present, and cannot be caused by short circuits or open circuits on the flame diode or in capacitor 74, resistor 75 or transformer secondary 50. Accordingly the flame sensing circuit itself embodies important fail-safe characteristics.

Whenever the glow lamps fire, the resultant pulse of current through resistor 66 produces a momentary positive voltage pulse at gate electrode 64 of SCR 62 to render it conductive. Capacitor 74 is thereby rapidly discharged in a small fraction of the time required for a half-wave of the input alternating voltage, but SCR 62 continues to conduct, because of its inherent characteristics, until the positive half-cycle of alternating voltage applied to its anode 85 from the secondary 50 of transformer 52 has fallen to about zero value.

The voltage and time relationships in FIGURE 2 will be more fully appreciated from a consideration of FIG- URES 3A and 3B, which are plots of voltage as ordinates and phase angle of the input-alternating wave as abscissae. In FIGURE 3A the graph A represents the sinusoidal input voltage produced across transformer secondary 50. In FIGURE 3B the graph B represents the voltage produced at gate electrode 64 of SCR 62. As shown, positive voltage pulses such as C are produced in synchronism with, and at a submultiple of the frequency of, the alternating sinewave of input voltage, in this case one pulse for each five cycles of the sinewave. Since these pulses are produced in response to positive peaks of the input sinewave voltage, they occur at or slightly before a phase angle of 1r/2. Each of these pulses triggers the SCR 62 into conduction, and this conduction persists until the alternating voltage has fallen to about zero value, and hence the shaded areas in FIGURE 3A represent the times of conduction through SCR 62 and the coil 60 under normal operating conditions with the flame present. On the other hand, if the SCR 62 fails by exhibiting a short-circuit between its anode and cathode elements for both directions of applied voltage, alternating current will be applied at all times to the coil 60, and hence power will be dissipated in the coil at a rate equal to the RMS value of the sinewave, which is many times greater than the average power delivered to the coil due to conduction during the shaded intervals of FIGURE 3A. Accordingly, by appropriate selection of the resistance of coil 60, in conjunction with the magnitude of the alternating input voltage, the coil can be caused to produce sutficient heating in response to current flowing therein during the shaded intervals to actuate the thermal valve to its open state, and to respond to the RMS value of the sinewave during intervals of short-circuiting of the SCR 62 to burn itself out and hence produce an open circuit and deactuation,

or closing, of the thermal valve. If the SCR fails only for one direction of applied voltage, current will be applied to coil 60 only half the time but the power delivered to the coil will still be many times greater than during normal operation and suflicient to burn out the coil as desired.

It will be understood that the wave forms of FIGURES 3A and 3B are not necessarily to scale and do not necessarily depict the actual wave shapes, but instead are designed merely to illustrate schematically the principle of the invention.

The following more quantitative analysis of the characteristics and operation of the embodiment of FIG URE 2 is presented as an aid to one skilled in the art in designing a circuit suitable for any particular application.

The power P required to be delivered to the heater coil 60 of the thermal valve in order to actuate the valve without burning out the coil is approximately PR=V2/R 1 where:

P =required power in watts,

R=electrical resistance of heater coil 60, and

V=design-center RMS value of AC sinusoidal voltage across coil 60 for operating the valve.

21rRy where y number of input AC cycles per control pulse, E=zero-to-peak voltage from secondary 50, in volts, =phase angle in radians,

0=firing angle, at which SCR becomes conductive (see FIGURE 3).

Intergation of Equation 2 gives:

: E 1r0+SiI1 20 2m 2 4 To operate the thermal valve with pulse power, P must be approximately equal to P or T 3 E 1r0 sin 20) R 21rRy 2 4 Solving Equation 4 for y gives:

1 1r0 sin 20 21r V 2 4 (5) Accordingly, to insure normal operation of the thermal valve when flame is present, the parameters y, E, V and 0 should be selected at least approximately as required by Equation 5. In practice y is determined by the selection of values of components in the flame-sensing circuit and by the quality of rectification provided by the flame. E is determined by the generator voltage and the ratio of transformer 52, and is conveniently made equal to the zero-to-peak value of the power-line voltage, using an isolating transformer having a 1:1 ratio. V is determined by the characteristics of the thermal valve and its heater coil, and 0 is generally slightly less than 1r/2 due to the inherent operation of the flame-sensing circuit.

The valve and circuit values should of course be selected so that the heater coil will burn out when power substantially above its rated level is applied to it, and, particularly, when the SCR is shorted for one direction of conduction therein.

The following specific example of one application and set of values for an embodiment of the invention is provided in the interest of definiteness only, without thereby in any way limiting the scope of the invention. In one embodiment the generator 50 was a standard 115-volt AC RMS line-power generator (so that E=2 /2 115), the thermal valve was designed to operate on V=24 volts RMS, and was substantially 'rr/Z. From Equation 5, y is 5.74 for the above values of E, V and 0. In practice it was found that y could be anywhere in the range 4 to 8 without burning out the heater coil during normal operation. A value of y=6 was chosen, and the following components utilized wtih a flame produced by a Meker burner:

Capacitor 74:0.047 microfarad.

Glow lamps 80 and 82=neon glow lamps, Signalite Type A083.

Resistor 75=0.l megohm.

Resistor 66:1,000 ohms,

With these values the power delivered to the heater coil will be about 12 times normal if the SCR becomes shorted in one direction, and about 24 times normal if the SCR becomes shorted in both directions, either of which conditions is sufficient to burn out the heater coil. As a convenient test, connecting an ordinary diode between anode and cathode of the SCR to stimulate a shortcircuit in one direction will result in burning out the heater coil in about 90 seconds with consequent closing of the thermal valve.

If a separate fuse or other current-responsive circuitopening element is used, it should of course be designed or selected to burn out or operate before the heater coil burns out.

While the invention has been described with particular reference to specific embodiments thereof in the interest of complete definiteness, it will be understood that it may be embodied in a variety of forms diverse from those specifically shown without departing from the spirit and scope of the invention as defined by the appended claims.

I claim:

1. A fail-safe flame-sensing and control circuit for maintaining actuated a normally deactuated thermal gas supply valve for a burner when a flame is present at said burner, comprising:

source of alternating voltage;

a flame sensing circuit supplied with said alternating voltage for producing output pulses at a sub-multiple of frequency of said alternating voltage when said flame is present but not when said flame is absent;

electrical means responsive to electrical power supplied thereto of greater than a first value but less than a second value to maintain said gas supply valve in its actuated state, and responsive to electrical power of at least said second value to place said gas supply valve in its deactuated state;

semiconductor controlled rectifier means having a pair of current discharge elements normally presenting a high resistance between them and having a currentcontrol element;

means connecting said source, said electrical means and said current discharge elements in common series circuit with each other; and

means for applying said output pulses to said currentcontrol element normally to render said semiconductor controlled rectifier intermittently highly conductive during intervals each beginning with the occurrence of one of said output pulses and each ending during the corresponding contemporaneous cycle of said alternating voltage, thereby to increase the difference between the power supplied to said electrical means during normal operation and that supplied to said electrical means when said rectifier is shortcircuited.

2. A circuit in accordance with claim 1, in which said electrical means comprises a fusible element which is responsive to power in excess of said second value to become open-circuited.

3. The circuit of claim 2, in which said fusible element comprises a thermal-valve actuating coil.

4. The circuit of claim 1, in which said flame-sensing circuit comprises capacitive means, means connecting said capacitive means and a flame in series across said source, and voltage-breakdown means in parallel with said flame and having a voltage breakdown level greater than the zero-to-peak value of said alternating voltage but less than the peak-to-peak value thereof.

References Cited UNITED STATES PATENTS 2,248,737 7/1941 Beam 431-46 2,270,722 1/1942 Beam 43146 3,174,528 3/1965 Staring 431- X 3,238,992 3/1966 Forbes 431-69 X 3,273,019 9/1966 Matthews 431-69 X 3,364,971 1/1968 Hayes 431-24 3,348,104 10/1967 Zielinski et al.

FREDERICK L. MATTESON, JR., Primary Examiner.

ROBERT A. DUA, Assistant Examiner.

US. Cl. X.R. 317; 43178

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