US2964051A

US2964051A - Electropneumatic relay with high power output

Info

Publication number: US2964051A
Authority: US
Inventors: Lawrence T Garnett
Original assignee: Robertshaw Fulton Controls Co
Current assignee: Robertshaw Controls Co
Priority date: 1959-09-15
Filing date: 1959-09-15
Publication date: 1960-12-13
Anticipated expiration: 1977-12-13
Also published as: GB936498A

Description

Dec. 13, 1960 L. T. GARNETT 2,964,0 5]

ELECTROPNEUMATIC RELAY WITH HIGH POWER OUTPUT Filed Sept. 15, 1959 2 Sheets-Sheet 1 JNVENTOR. LA M/KEA/C'E TGAEA/E-TT BY 777m Myle maioww, faifih'm 4424/ WW Arrow/E45.

Dec. 13, 1960 T. GARNETT 2,964,051

ELECTROPNEUMATIC RELAY WITH HIGH POWER OUTPUT Filed Sept. 15, 1.959 2 Sheets-Sheet 2 IN V EN TOR.

L LAM/EE-A/CE TGAEA/E-TT BY 77/624072, zommmfl, (QM/1mm WW Arron/5f;

.to improve its flow regulation characteristics.

ELECTROPNEUMATIC RELAY WITH HIGH POWER OUTPUT Lawrence T. Garnett, 'Stratford, Conn., assignor, by mesne assignments, to Robertshaw-Fulton Controls Company, Richmond, Va., a corporation of Delaware Filed'Sept. 15, 1959, Ser. No. 840,174

12 Claims. (Cl. 137-85) The present invention relates to electropneumatic relays of the type which are adapted to translate an electrical signal into a corresponding pneumatic signal, and the invention has for an object the provision of an electropneumatic relay unit which is arranged to provide a high power level output, while, at the same time, providing good dynamic response and sensitivity to changes in the electrical input signal.

It is another object of the present invention to provide an electropneumatic relay with facilities for operation in hazardous areas wherein the relay is provided with adequate flame-proofing for industrial specifications in such areas.

It is still another object of the present invention to provide a new and improved electropneumatic relay wherein facilities for explosion-proofing are provided in the lower power level portion of the unit so as not to interfere with the sensitivity and dynamic response of the instrument.

It is still another object of the present invention to provide a new and improved electropneumatic relay where in flame arresters are employed which have a relatively low pressure drop and provide explosion proofing without interfering with the sensitivity and dynamic response of the instrument.

Briefly, the above and further objects of the invention are achieved by providing an electropneumatic relay comprising a body member having an air chamber therein and means including a restricting orifice for supplying air from an air inlet to said air chamber. A nozzle is positioned in communication with this air chamber so that air issues from the nozzle and closure of the nozzle is controlled by a flapper carried on a pivotally mounted beam. This beam also carries a coil which is positioned within a magnetic field so that upon application of an electrical input signal to the coil the beam is deflected about the pivotal axis thereof in proportion to the electrical input signal. As a result, the flapper provides a variable impedance to air issuing from the nozzle and the air issuing from the nozzle also exerts a feedback force on the beam in opposition to the deflection thereof by the electrical input signal so that the back pressure in the nozzle is proportional to the elec trical input signal. Air from the air chamber is also supplied to a power booster unit which is provided integrally with the electropneumatic relay and includes opposed fiexible input and output diaphragms, the air from the air chamber being supplied to the input diaphragm of the booster unit. A control valve is also positioned in the body member and is movable in response to movement of the input diaphragm, this control valve being arranged to supply air from the air inlet of the relay to the output section of the power booster. The power booster also includes a feedback passageway in the body member connecting the output section to the output diaphragm so as to stabilize the pneumatic booster unit and Since atent both the electropneumatic relay and the power booster are provided with feedback connections, these elements follow accurately and faithfully the variations in the electrical input signal. However, since the electrical input signal is translated to a pneumatic signal at a relatively low power level at which level the air issuing from the nozzle is sutficient to provide the necessary feedback force for the beam, the sensitivity and dynamic response of the instrument are preserved while provid ing the advantages to the overall system of a feedback type of operation.

The invention, both as to-its organization and method of operation, together with further objects and advantages thereof will be best understood by reference to the following specification taken in connection with the accompanying drawings, in which:

Fig. 1 is a sectional, side elevational View of the electropneumatic relay and power booster of the present invention;

Fig. 2 is a sectional view taken along the lines 2--2 of Fig. I; and

Fig. 3 is a sectional view taken along the lines 3-3 of Fig. 1.

Referring now to the drawings, the electropneumatic relay of the present invention is therein illustrated as comprising a body member 10 to which a depending cover member 11 is adapted to be secured so as to provide an explosion-proof housing for the chamber 12 within which the electrical components of the electropneumatic relay are positioned. The body member 10 is provided with four depending bosses 15 which are employed to support a permanent magnet structure indicated generally at 16 within the chamber 12, this magnet structure being secured to the bosses 15 by means of the screws 14. More particularly, the permanent magnet structure comprises a top magnet plate 17 and a U- shaped member 18 which are connected together by means of the screws 19 to form, together with a magnet and pole piece 20, a permanent magnet circuit. The top magnet plate 17 is provided with a central aperture therein which cooperates with an enlarged portion 22 on the upper end of the pole piece 74 to provide an annular air gap 23. Within the air gap 23 there is positioned a cylindrical coil form 24 on which is mounted a coil 25 in such manner that the coil 25 is positioned within the air gap 23. The coil form 24 is mounted on the underside .of a beam member 30, the beam 30 being pivotally supported on the magnet structure 16 by means of a pair of thin resilient flexure members 31 and 32 (Fig. 2). Thus, one end of the flexure members 31 and 32 is secured to the topmagnet plate 17 by means of the screws 35 and the other end of the flexure members 31 and 32 is secured to a spacer element 36 which is clamped between the flexure members 31 and 32 and the bottom side of the beam 30 by means of the screws 37. The spacer member 36 together with a depending block 38 acts as a counterweight, the block 38 being secured to the beam 30 by means of the screws 39. The ends of the coil 25 are secured to lead wires 45 and 46 which are in turn connected to the respective terminal screws 48 and .49 of a terminal block 50 which is secured to one side of the U-shaped member 18. The electrical input signal supplied to the electropneumatic relay of Fig. 1 from any suitable source is connected to the terminal block 50 by any suitable encapsulated means (not shown); this connection being preferably made through the body member 10 by any suitable arrangement which will preserve the explosion-proof qualities of the electropneumatic relay. It is intended that the electrical signal which is supplied to the coil 25 be a direct currentsignal which varies in magnitude in accordance with 'a measured variable or other controlling factor. Alternating current ripple and other noise components in the input signal supplied to the

terminals

48 and 49 is preferably removed by means of a bypass capacitor 51 which is supported on the base portion of the U-shaped member 18 and is connected to the

terminals

48 and 49 by the

lead wires

52 and 53.

In order to translate the electrical signal applied to the coil 25 into a corresponding pneumatic signal, a nozzle member 55 is positioned in the body member 10, the nozzle member 55 being adapted to cooperate with a nozzle closure member, or flapper, in the form of a flat sided spherical ball 56, the ball 56 being retained against the upper surface of the beam 30 by means of a spring member 57 in such manner that it can conform to the plane of the lip of thenozzle 55 when the ball 56 is moved into engagement therewith. Air at approximately 20 p.s.i. is supplied through an air inlet 60 and a filter screen 61 to a restricting orifice 62. The air issuing from the orifice 62 is supplied to an air chamber indicated generally at 63, the chamber 63 being formed in a cap member 64 which is threaded into a top opening recess 65 in the body member 10, there being provided passageways 66 and 67 in the members 64 and 10, respectively, which connect the air chamber 63 with the orifice 62. The recess 65 also communicates through a passageway 68 with the nozzle 55 so that air pressure of from 3 to 15 p.s.i. is built up in the air chamber 63 due to the back pressure of the nozzle 55, this pressure depending upon the position of the beam 30 and hence the nozzle closure member 56 with respect to the end of the nozzle 55.

When an electrical signal is applied to the coil 25 the beam 30 is deflected in such a direction that the nozzle closure member 56 is moved toward the lip of the nozzle 55. If the electrical input signal is relatively small, a relatively small force is exerted on the beam 30 and this force is counterbalanced by the force of the air issuing from the nozzle 55 while the ball 56 is spaced at relatively large distance from the end of the nozzle. On the other hand, if a relatively large electrical input signal is applied to the coil 25 a larger force is exerted on the beam 30 so that the beam is rebalanced by the force of the air issuing from the nozzle 55 when the closure member 56 is relatively close to the rim of the nozzle 55. Accordingly, the air pressure built up in the chamber 63 will vary proportionately with the electrical input signal applied to the coil 25.

In order to limit movement of the beam 30 in the direction in which the flapper 56 is moved away from the nozzle 55, there is provided a positive stop in the form of a set screw 94 which is threaded through the body member and is adapted to engage the upper surface of the beam 30. With this arrangement damage to the beam 30 is prevented when the relay is subjected to shock impacts in said direction.

In order to provide a span adjustment so that the electropneumatic relay may be arranged to function properly with any desired range of input signals, there is provided on the pole piece 74 a collar member 75 which is secured to the pole piece 74 and is externally threaded. A brass ring 76 which is threaded both internally and externally is threaded onto the collar 75 and is positioned so that the upper end of the ring 76 engages the bottom surface of the top magnet plate 17. An outer adjustment ring 78, which is made of iron or other magnetic material, is then threaded onto the outside of the brass ring 76 and after the iron ring 78 is adjusted properly it may be locked in place by means of a locking screw 80 which is threaded through the side of the U-shaped member 18 and engages the outer periphery of the ring 78. The iron ring 78 functions to shunt a variable portion of the flux normally developed in the air gap 23 and thereby varies the amount of force exerted on the beam 30' by a given input current flowing through the coil 25. Thus, as the ring 78 is threaded upwardly towards the top magnet plate 17 a larger proportion of the flux is shunted in the air gap 23 is lowered.

In order to provide a zero adjustment for the beam 30 which is adjustable from the exterior of the electropneu-- matic relay, a coil spring is provided, the upper end of which is connected to the counterweight 38 by means of a screw 86 and the bottom end of which is connected to a U-shaped member 87 which may be adjustably positioned on a zero adjustment screw 88. The adjustment screw 88 is rotatably mounted in a sleeve 89 which is secured in the body member 10 in such manner that the slotted head portion 90 of the screw 88 is positioned in a recess 91 formed in the body member 10. The U- shaped member 87 is provided with a slot 92 through which extends a split sleeve 93, the sleeve 93 being secured to one arm of the U-shaped member 18 and preventing the member 87 from rotating as the adjustment screw 88 is rotated during the zero adjustment- In addition, a coil spring 95 is positioned between the bottom end of the sleeve 89 and the upper arm of the member 87 so as to remove backlash in the zero adjust-- ment mechanism and the U-shaped member 87 is also provided with arm portions which are sprung slightly outwardly in the unstressed position of the member 87/ so that these arm portions exert a side thrust on the: threads of the screw 88 and hence further act to remove: backlash from the zero adjustment mechanism. A cover:

member 96 is secured to the body member 10 by means of the screw 97 and is employed normally to cover thehead 90 of the adjustment screw 88 so that this adjust-- ment will not be misaligned inadvertently. Preferably,. a relatively small clearance is provided between the: threads of the adjustment screw 88 and the sleeve 89 so that a relatively long flame path of narrow dimensionsis provided between the chamber 12 within which the electrical components of the electropneumatic relay are positioned and the exterior of the relay itself.

It will be noted that air issuing from the nozzle 55 into the chamber 12 must be vented to the atmosphere in order to permit the above described back pressure to be built up in the chamber 63 which is proportional to the electrical input signal applied to the coil 25. Furthermore, the air in the chamber 12 should be vented to atmosphere through a medium which produces a relatively low pressure drop since the air pressures at which the nozzle 55 operates are at relatively low power levels. On the other hand, if the electropneumatic relay is to be explosion-proof and is to meet industrial standards for operation in hazardous areas suitable flame arrester means must be provided between the chamber 12 and the exterior of the electropneumatic relay. In accordance with an important aspect of the present invention, these conflicting requirements are met by providing a plug member 100 which is threaded into an opening in the body member 10 and is provided with a central vent opening 101 therethrough. The plug 100 is provided with a disc portion 102 which is adapted to seat on the under surface of the body member 10 when the plug 100 is threaded thereinto and a flame arrester element 103 is provided through which all of the air from the chamber 12 must pass in order to be vented through the opening 101 to the atmosphere. The flame arrester element 103 is preferably made of sintered bronze and it has been found that a suitable element 103 may have a mean pore opening of 0.0025 inch and a particle size filter rating of 40 microns, the element 103 having such thickness and area that it has a maximum pressure drop of six millimeters of mercury at a flow of 0.13 standard cubic feet per minute of air.

In order to'secure the flame arrester element 103 to the disc portion 102 of the plug 100, a bronze or copper plated ring 105 is provided. The ring 105 is first secured an arrangement haslie advantage of. solidly securthe ring-105 to. the element 103 without closing the pore spaces of the element 103 which would result. if a conventional soldering operation is performed. The ring 105 may then be secured to the disc portion 102 of'the .plug100 by any suitable soldering operation without in vided with a suitable flame arrester arrangement so that in the event of -a flash-back through the nozzle 55 the flame will .be prevented from escaping through the air chamber 63 to the exterior of the electro-pneumatic re- .lay. To this end, ,a disc type flame arrester element 110 is provided in the cap member 64, the disc element 110 also being of sintered bronze and having the same low pressure drop specifications as the disc element 103. The disc element 110 is likewise sintered onto a .bronze or copper plated ring 111 which is in turn soldered or otherwise secured to the recessed bottom portion of the cap member 64. Accordingly, a flame arrester means of suitable flame path is provided for both the vent 101 to theatmosphere and forair supplied to the air chamber 63 so that a suitably long and tortuous flame path is established, while, at the same time, providing a sufficiently low pressure drop that theelectropneumatic relay can function properly with high sensitivity.

Considering now the manner in which the power level of the electropneumatic relay is increased by means of the booster portion of the relay, it will be understood from theprevious description that the air pressure in the, chamber 63 varies in accordance with the electrical input signalapplied to the coil 25. Furthermore, since the conversion from an electrical signal to a pneumatic signal is achieved at a relatively low power level a relatively fast response to variations in the input signal is achieved and good dynamic response of the electropneumatic relay is provided. Also, the conversion of the electrical signal to a pneumatic signal at a relatively low power level-facilitates the explosion-proofing of the relay by use of the above described flame arrester elements 103 and 110 which may be of relatively small area and have a low pressure drop thereacross. However, in order to provide a useful pneumatic output signal it is necessary to increase the power level thereof and, in accordance with another important aspect of the invention, this is achieved by a booster unit of the relay which is a part of the electropneumatic relay itself and is pro vided with its oun pneumatic feedback loop for stabilization. However, an overall feedback loop between the output of the booster unit and the beam 30 is not employed since such an arrangement would requirea considerably longer time lag and hence would substantially decrease the dynamic response of the instrument.

The power booster section of the electropneumatic relay comprises an annular body member 115 and an upper body member 116, these members being secured to the body member by means of screws 117 (Fig. 3). A flexible input diaphragm 120 is clamped between the body member 10 and the annular ring 115 and a flexible output diaphragm 121 is clamped between the annular member 115 and the upper body member 116 when these members are assembled to the body member 10. An air inlet passageway 122 is provided in the annular member -115 and an air inlet passage 123 is provided in the body member-116, thepassage 123 communicating with the main air inlet 1240f the relayand the

diaphragms

120 and 121 being provided with suitable openings therethrough which register with the passageways 122 and 123 .so thatair supplied to-the the passageways 123,.122.and 6010 the filter-screen 61in .pressure variations occur.

inlet 124 may passflthrough the manner described in detail heretofore.

The upper surface of the body member 10 is provided with a shallow depression so as to form an input diaphragm chamber 131 which communicates .with the chamber 63 through apassageway 132 at the center of the cap member 64. The input and

output diaphragms

120 and 121 are spaced apart by means of a movable member 135 which is provided with .an annular bottom flange 136 which supports the central portion of the input diaphragm 120 and an upper flange 137 which supports the central portion of the upper diaphragm .121. A sleeve 140 is threaded into the top of a central aperture 141 in the member 135, the sleeve .140 being provided with a flange 142 which functions to clamp the central portion of the upper diaphragm 1 21 between a disc washer 143 and the uppersurface of the flange 137 of the member 135. The sleeve 140 is provided with a central aperture 144 therethrough and a double poppet valve 145 is arranged with the bottom poppet 146 thereof adapted to seat in .a valveseat formed in the upper end of the opening.144 in the sleeve 140. A flexible diaphragm 150is positioned in an annular groove 151 formed in the upper end of the sleeve 140, the periphery of the diaphragm 150 being retained against an annular step 152 formed in the upper body. member 116 by means of a retaining ring 153 which is forced into place so as to hold the outer rim of the diaphragm 15.0 against the step 152. The diaphragm 150 is sufliciently flexible to permit the diaphragm members 135 and 140 to be moved in accordance with variations in thepneumatic input signal applied to the input diaphragm 120. A coil spring 155 is positioned between the retaining ring 153 and the washer 143. so as to providev a biasing force against .which the diaphragm 120 can act.

Considering now the operation of the booster portion .of the electropneumatic relay, it will be evident that a force is exerted. upwardly on the. members135 and 14,0 which is proportional to thepressure in the input diaphragm chamber 131 times the area of the input diaphragm 120, it being understood that the pressure of the air in the diaphragm chamber 131 is the same as the pressure in the air chamber 63 by virtue of the passageway 132. Accordingly, the members 135 and '140 are moved upwardly against the force of the spring 155 with the result thatjthe upper poppet 157 is raised from the valve seat 158 and permits air to flow from the input line 124 through the opening 159 in the valve seat;158 to the output line 160 of the booster unit.

In order to provide a feedback connection between the output and input sections of the booster unit, there is provided a passageway 1 65 through which the output air is supplied to the upper side of the output diaphragm 121 so as to exert a force on this diaphragm which tends to move the members 135 and 140 downwardly. A spring 166 which is positioned between a cap member 167 in the upper end of the upper body member 116 and the upper end of the poppet valve 145 acts to hold the lower poppet 146 against the valve seat 140 and as the member 140 moves downwardly the upper poppet 157 closes off the inlet air as the pressure in the output section of the booster unit becomes equal to the pressure in the input diaphragm chamber 131, it being understood that the input and

output diaphragms

120 and 121 have equal areas and hence the output pressure developed by the booster unit is equal to that applied to the input diaphragm 120 although the power level, i.e., flow capacity of the booster section, is substantially greater due to the large area of the poppet 157 and the amount which this poppet is opened when In this connection it will be noted that the small area diaphragm ISO-prevents the velocity head developed when the poppet;157 is opened from changing the output as it Would'if the diaphragm 150 were omitted or were made equal in area to the diaphragm 121. However, the feedback passageway 165 is sufliciently large to provide good following action without introducing changes in the output pressure due to velocity head variations.

In the event that the pressure in the input diaphragm chamber 131 decreases, as for example when the electrical input signal decreases, the force exerted on the upper diaphragm 121 becomes greater than that exerted on the lower diaphragm 120 with the result that the member 140 moves away from the bottom poppet 146 and permits the output air to bleed through the

passageways

144 and 141 to the atmosphere until a pressureforce balance is reestablished, at which point the poppet 146 is again seated on the upper end of the member 140. In this manner, the booster unit is able to follow accurately variations in the input pressure supplied to the input diaphragm 120 while providing a high power level output for the electropneumatic relay. It will be noted, however, that the feedback connection 165 is provided only to the output of the power booster unit and does not comprise an overall feedback connection between the output of the booster unit and the beam 30. Accordingly, the dynamic response of theelectropneumatic relay is substantially improved. It will also be noted that the feedback passageway 165 is located at a reduced area portion of the outlet passage 160 so that during flow the feedback pressure signal to the diaphragm 121 is further reduced which results in even wider opening of the poppet 157 and therefore, provides greater air flow and hence capacity of the booster unit.

While a particular embodiment of the invention has been shown and described it will be recognized that various modifications will readily occur to those skilled in the art and it is therefore contemplated by the appended claims to cover any such modifications as fall within the true spirit and scope of the invention.

What is claimed as new and is desired to be secured by Letters Patent of the United States is:

1. An electropneumatic relay comprising a pivotally mounted beam, a coil mounted on said beam and positioned within a magnetic field, means for applying an electrical input signal to said coil, thereby to deflect said beam about the pivotal axis thereof, a flapper carried by said beam, a nozzle positioned adjacent said flapper, an air supply, means including an orifice for supplying air from said supply to said nozzle, said flapper being positioned to provide a variable impedance to air issuing from said nozzle while at the same time air issuing from said nozzle exerts a feedback force on said beam in opposition to the deflection thereof by said electrical input signal whereby the back pressure in said nozzle is proportional to said electrical input signal, pneumatic power amplifying means having an input section and an output section and having an internal pneumatic feedback connection between said input and output sections thereof, means for supplying said nozzle back pressure to said input section of said pneumatic power amplifying means, and means including a control valve controlled by said input section of said pneumatic power amplifying means for supplying air from said air supply to said output section of said pneumatic power amplifying means, whereby the pressure established in said output section is proportional to said nozzle back pressure.

2. An electropneumatic relay comprising a pivotally mounted beam, a coil mounted on said beam and positioned within a magnetic field, means for applying an electrical input signal to said coil, thereby to deflect said beam about the pivotal axis thereof, a flapper carried by said beam, an air supply, a body member having an air chamber therein, means including a restricting orifice for supplying air from said air supply to said air chamber, a nozzle positioned adjacent said flapper and connected to said air chamber, said flapper being positioned to provide a variable impedance to air issuing from said nozzle while at the same time air issuing from said nozzle exerts a feedback force on said beam in opposition to the deflection thereof by said electrical input signal whereby the back pressure in said nozzle is proportional to said electrical input signal, pneumatic power amplifying means mounted in said body member and having input and output sections, means connecting said air chamber to the input section of said pneumatic power amplifying means internally of said body member, means including a control valve positioned in said body member and controlled by said inuput section of said pneumatic power amplifying means for supplying air from said air supply to said output section of said pneumatic power amplifying means so that the pressure established in said output section is proportional to said nozzle back pressure, and a feedback passageway in said body member interconnecting said input and output sections, thereby to provide linear response of said pneumatic power amplifying means to said nozzle back pressure.

3. An electropneumatic relay comprising a pivotally mounted beam, a coil mounted on said beam and positioned within a magnetic field, means for applying an electrical input signal to said coil, thereby to deflect said beam about the pivotal axis thereof, a flapper carried by said beam, a body member having an air supply inlet and an air chamber therein, means including a restricting orifice for supplying air to said chamber from said inlet, a nozzle positioned in said body member adjacent said flapper and connected to said air chamber, said flapper being positioned to provide a variable impedance to air issuing from said nozzle while at the same time air issuing from said nozzle exerts a feedback force on said beam in opposition to the deflection thereof by said electrical input signal, whereby the back pressure in said nozzle is proportional to said electrical input signal, pneumatic power amplifying means mounted in said body member and having input and output sections including opposed flexible input and output diaphragms, means for supplying air from said air chamber to said input diaphragm, a control valve positioned in said body and movable in response to movement of said input diaphragm, said control valve being arranged to supply air from said air inlet to said output section of said power amplifying means, and means including a feedback passageway in said body connecting said output section to said output diaphragm, thereby to stabilize said pneumatic power amplifying means against variations in pressure of the air supplied to said inlet.

'4. An electropneumatic relay, comprising a base member, a permanent magnet structure supported on said base member and including means defining an annular air gap, a pivotally mounted beam including a coil positioned Within said air gap, a nozzle mounted in said base member, means including a restricting orifice for supplying air to said nozzle, nozzle closure means carried by said beam and adapted to present an impedance to air issuing from said nozzle which varies with movement of said beam, means for supplying an electrical signal to said coil so as to produce a corresponding deflection of said beam, the air issuing from said nozzle exerting a feedback force on said beam in opposition to the deflection thereof by said electrical signal so that the back pressure in said nozzle is proportional to said electrical signal, an explosion-proof housing connected to said base member and enclosing said permanent magnet structure, beam and nozzle, and flame arrester means for venting said housing to the atmosphere.

5. An electropneumatic relay, comprising a base member, a permanent magnet structure supported on said base member and including means defining an annular air gap, a pivotally mounted beam including a coil positioned within said air gap, a nozzle mounted in said base member,

9 means including a restricting orifice for supplying air to said nozzle, nozzle closure means carried by said beam and adapted to present an impedance to air issuingfrom said nozzle which varies withmbVeni'ent of said beam, means for supplying anelectrical signal to said coil so as to produce a corresponding deflection of said beam, the 'air issuing from said nozzle exerting a feedback force on said beam in opposition to the deflection thereof by said electrical signal so that the back pressure in said nozzle is proportional to said electrical signal, an explosion-proof housing connected to said base member and enclosing said permanent magnet structure, beam and nozzle, flame arrester means positioned in said base member between said nozzle and said restricting orifice for preventing flashbacks through said nozzle from reaching the exterior of said relay.

6. An electropneumatic relay, comprising an explosion-proof housing, a pivotally mounted beam positioned within said housing, electromagnetic current responsive means for deflecting said beam, a nozzle positioned within said housing, means including a restricting orifice for supplying air from the exterior of said relay to said nozzle, nozzle closure means carried by said beam and adapted to present an impedance to air issuing from said nozzle which varies with deflection of said beam, the air issuing from said nozzle into said housing exerting a feedback force on said beam in opposition to the deflection thereof by said current responsive means so that a back pressure is built up in said nozzle which corresponds to the position of said beam, and flame arrester means for venting said housing to the atmosphere.

7. An electropneumatic relay, comprising an explosionproof housing, a pivotally mounted beam positioned within said housing, electromagnetic current responsive means for deflecting said beam, a nozzle positioned within said housing, means including a restricting orifice for supplying air from the exterior of said relay to said nozzle, nozzle closure means carried by said beam and adapted to present an impedance to air issuing from said nozzle which varies with deflection of said beam, the air issuing from said nozzle into said housing exerting a feedback force on said beam in opposition to the deflection thereof by said current responsive means so that a back pressure is built up in said nozzle which corresponds to the position of said beam, and flame arrester means for venting said housing to the atmosphere, said flame arrester means including a sintered metal element through which air within said housing passes to escape from said housing.

8. An electropneumatic relay, comprising an explosionproof housing, a pivotally mounted beam positioned within said housing, electromagnetic current responsive means for deflecting said beam, a nozzle positioned within said housing, means including a restricting orifice for supplying air from the exterior of said relay to said nozzle, nozzle closure means carried by said beam and adapted to present an impedance to air issuing from said nozzle which varies with deflection of said beam, the air issuing from said nozzle into said housing exerting a feedback force on said beam in opposition to the deflection thereof by said current responsive means so that a back pressure is built up in said nozzle which corresponds to the position of said beam, and flame arrester means including a sintered metal element having a relatively low pressure drop thereacross for venting said housing to the atmosphere.

9. An electropneumatic relay, comprising an explosionproof housing, a pivotally mounted beam positioned within said housing, electromagnetic current responsive means for deflecting said beam, a nozzle positioned within said housing, means including a restricting orifice for supplying air from the exterior of said relay to said nozzle, nozzle closure means carried by said beam and adapted to present an impedance to air issuing from said nozzle which varies with deflection of said beam, the air issuing from said nozzle into said housing exerting a feedback force on said beam in opposition to the.'d'eflectionfthereof by said current'resp'onsive means so that aback pressureiis passing air within said'housing therethrough with a relatively small pressure drop thereacross while providing a relatively long and tortuous flame path to the exterior of said housing.

10. An electropneumatic relay, comprising an explosion-proof housing, a pivotally mounted beam positioned within said housing, electromagnetic current responsive means for deflecting said beam, a nozzle positioned within said housing, means including a restricting orifice for supplying air from the exterior of said relay to said nozzle, nozzle closure means carried by said beam and adapted to present an impedance to air issuing from said nozzle which varies with deflection of said beam, the air issuing from said nozzle into said housing exerting a feedback force on said beam in opposition to the deflection thereof by said current responsive means so that a back pressure is built up in said nozzle which corresponds to the position of said beam, means defining a vent opening through said housing to the atmosphere, a removable plug member positioned in said opening and having a central aperture extending therethrough, a disc element comprised of sintered metal, and means including a metallic ring which is sintered to said disc element and is soldered to said plug member so as to block the central aperture therethrough, said sintered metal disc element passing air within said housing therethrough while providing a relatively long and tortuous flame path to the exterior of said housing.

11. An electropneumatic relay, comprising a base member, a removable cover member adapted to be connected to said base member to form an explosion-proof housing, a pivotally mounted beam positioned within said housing, electromagnetic current responsive means for deflecting said beam, an air chamber within said base member, means including a restricting orifice for supplying air from the exterior of said relay to said air chamber, a nozzle positioned on said base member in communication with said air chamber and adapted to exhaust air into said housing, nozzle closure means carried by said beam and adapted to present an impedance to air issuing from said nozzle which varies with deflection of said beam, the air issuing from said nozzle into said housing exerting a feedback force on said beam in opposition to the deflection thereof by said current responsive means so that a back pressure is built up in said nozzle and air chamber which corresponds to the position of said beam, and flame arrester means positioned in said base member between said nozzle and said restricting orifice for preventing flashbacks through said nozzle from reaching the exterior of said relay.

12. An electropneumatic relay, comprising a base member, a removable cover member adapted to be connected to said base member to form an explosion-proof housing, a pivotally mounted beam positioned within said housing, electromagnetic current responsive means for deflecting said beam, an air chamber within said base member, means including a restricting orifice for supplying air from the exterior of said relay to said air chamber, a nozzle positioned on said base member in communication with said air chamber and adapted to exhaust air into said housing, nozzle closure means carried by said beam and adapted to present an impedance to air issuing from said nozzle which.varies with deflection of said beam, the air issuing from said nozzle into said housing exerting a feedback force on said beam in opposition to the deflection thereof by said current responsive means so that a back pressure is built up in said nozzle which corresponds to 1 1 the position of said beam, and flame arrester means posi- References Cited in the file of this patent tioned in said base member between said nozzle and said UNITED STATES PATENTS restnctmg orifice for preventing flashbacks through sald nozzle from reaching the exterior of said relay, said flame 2,915,045 Mackenzle 1959 arrester means comprising a sintered metal element which 5 is positioned so that all of the air transmitted between said FOREIGN PATENTS nozzle and said air chamber passes through said element. 795,237 Great Britain May 21, 1958