US3374337A - Thermal actuator and control therefor - Google Patents

Description

March 19, 1968 B. BURLEY THERMAL ACTUATOR AND CONTROL THEREFOR I Filed Aug. 28, 1964 2 Sheets-Sheet 1 INVENTOR B I LLY BUR LEY ATTORNEY March 19, 1968 B. BURLEY THERMAL ACTUATOR AND CONTROL THEREFOR 2 Sheets-Sheet :2

Filed Aug. 28, 1964 l i /iy Power INVENTOR. BILLY BURLEY flfforng 5 United States Patent 3,374,337 THERMAL ACTUATOR AND CONTROL THEREFOR Billy Burley, Dallas, Tex., assignor to Johnson Service Company, Milwaukee, Wis., a corporation of Wisconsin Continuation-impart of application Ser. No. 250,677,

Jan. 10, 1963. This application Aug. 28, 1964, Ser.

9 Claims. (Cl. 219-501) ABSTRACT OF THE DISCLOSURE A thermal actuator control system includes a normally closed limit switch connecting the gate of a silicon controlled rectifier (SCR) to a signal supply. The power electrodes of the SCR are connected in series with a heating resistor of the actuator. A wax is disposed within a cylinder having a shaft journaled in one end as a piston. The heating resistor is embedded within the wax and produces a mechanical motion as a result of the volumetric change in the Wax. The resistor is a spiral wound coil located within the chamber with a generally U-shaped configuration having the outer end secured as the connecting electrodes or terminals in the outer end of the cylinder and having a base extending around the inner portion of the movable shaft. The limit switch carries only very low gate current and voltage and is a floating contact type responsive to incremental movements of the order of micro inches of the actuator shaft.

A condition-responsive means produces an alternating current signal voltage the magnitude and phase of which are a function of the deviation of the condition from a predetermined value, and means including an RC network and the limit switch applies the signal voltage to the gate of the SCR to vary the conduction angle thereof and the effective level of the DC. current flowing through the heating resistor. The limit switch includes an insulating mounting base disposed within an end chamber of the thermal actuator cylinder with the actuating shaft passing through a central opening. A leaf spring is resiliently mounted as a cantilevered member to the base plate with a breaker member encircling the shaft and clamped between the spring and plate. The breaker member has a diameter somewhat less than the opening in the base plate. A collar on the shaft will pass through the base plate opening into engagement with the breaker to transmit the final shaft motion to the spring. The free end of the spring is aligned with and is tensioned to engage a fixed contact on the base. The silicon controlled rectifier is secured to the back side of the base to form an integral subassembly with the gate terminal in series circuit with the switch contacts. A two-part seal seals about the shaft at the exit opening of the thermal cylinder and includes an encircling rubber member and a thin plastic wafer encircling the shaft and bearing on a planar surface of the cylinder. The rubber member is of a substantial axial length and includes an inner step face projecting inwardly into the expansion chamber and a thin sealing lip adjacent the shaft. The plastic wafer has a central opening through which the shaft projects and is formed with a central cone-shaped portion projecting outwardly of the expansible filled chamber. A thin disc is disposed between the rubber member and the adjacent planar surface and constitutes a seal backup.

increase in temperature. More particularly, the invention has to do with an actuator control system including a floating type limit switch connected in the gate circuit of a by-stable solid state device which regulates the flow of current through the actuator heating element, the limit switch being arranged to be opened by the actuator shaft when the shaft reaches a predetermined extended position. This application is a continuation-in-part of my application filed on Jan. 10, 1963, with Ser. No. 250,677, now abandoned.

In prior systems for controlling resistance-heated thermal actuators, snap-acting switches have been provided for interrupting the heater current when the actuator shaft reaches a fully extended position. In general, the contacts of such switches are connected in series with the actuator heating resistance and it has been found that such switches are subject to many disadvantages, among these being that they are not sufficiently reliable, either mechanically or electrically and that, because of the necessity that such switches interrupt relatively high heating currents, the life of the contacts is greatly reduced. Another major drawback of switches is that they require an appreciable amount of differential actuating movement for make or break operation. Consequently, when the actuator shaft reaches its fully extended position this differential actuating movement often results in undesirable continuous oscillations, or hunting, of the shaft. For example, in installations in which the actuator shaft is designed to maintain a fluid control valve closed when the shaft is in a fully extended position, these shaft oscillations result in undesirable alternate slight opening and closing of the valve.

The present invention has for its principal object the provision of an actuator control system not having the above and other drawbacks of the prior art control sys tems and devices. In achieving this principal object the invention provides a thermal actuator control system including a limit switch which is opened by the actuator shaft when it reaches a predetermined extended position, this switch having normally closed contracts connected in the gate traggering circuit of a bi-stable solid state device such as a silicon controlled rectifier (SCR), the power electrodes of which are connected in series with the heating resistor of the actuator. Since the limit switch need carry only very low gate current and voltage, which may be of the order of 10-20 milliamperes'and one volt, respectively, the switch may be of the floating contact type responsive to incremental movements of the order of micro inches of the actuator shaft. Consequently, the sensitivity of the system response is increased and the differential switch actuating movement of conventional snap-acting switches is eliminated, whereby hunting of the actuator shaft when it reaches its fully extended position is prevented.

The invention also provides a thermal actuator control system including a bi-stable solid state device such as a silicon controlled rectifier which regulates the current flowing through the actuator heating resistor and which is mounted in the actuator housing, whereby the heat generated by the device is utilized by the thermal actuator to increase the efficiency of the control system. Further, by placing the limit switch in the low voltage and current gate circuit, and by mounting the SCR in the actuator housing, the switching effects produced by the switch and the SCR are effectively isolated from the associated condition sensing and electronic signal amplifying means, thus increasing the sensitivity and stability of the system.

The invention also provides a thermal actuator control system consisting of the combination of means for supplying the actuator heating resistor with alternating current, and condition-responsive means for producing an alternating-current signal voltage the magnitude and phase of which are a function of the deviation of the condition from a predetermined value, and means including an R-C network and a limit switch for applying the signal voltage to the gate electrode of the SCR to vary the conduction angle thereof and the effective level of the DC current flowing through theheating resistor, the limit switch being of the floating type having normally closed contacts that are arranged to be opened by the actuator shaft when it reaches a predetermined extended position.

Thermal devices have been suggested such as shown in United States Patent 3,029,595 wherein a wax or similar expansible material is disposed within a cylinder having a shaft journaled in one end as a piston. A heating element is embedded within the wax to melt the latter and produce a mechanical motionas a result of the volumetric change in the wax. A limit switch is preferably provided to terminate energization of the element upon selected extension of the shaft.

In control systems employing thermal actuators and the like'the control circuit is opened and closed a great number of times within the life of the system. The thermal actuator must have a long life and be designed to permit manufacture without undue complications and expense. The limit switch structure and the seal must in particular be designed to maintain reliable operation over long periods. Thus the wax or other fill may be completely melted to a liquid form and the seal must therefor be liquid tight to prevent loss of the fill and interference with the limit switch structure. Applicant has found an exceptionally long life and reliable unit particularly incorporating a unique floating switch structure which is peculiarly adapted to opening and closing low current circuits such as encountered in the gate circuit of silicon controlled rectifiers and a liquid tight seal which will prevent loss of the expansible fill and interference with the switch assembly. The switch assembly includes an insulating mounting base disposed within a suitable end chamber of the thermal actuators unit with the actuating shaft passing through a central opening. A leaf spring is resiliently mounted as a cantilevered member to the base plate with a breaker member encircling the shaft and clamped between the spring and plate. The breaker member has a diameter somewhat less than the opening in the base plate. A collar on the shaft will pass through the base plate opening into engagement with the breaker to transmit the final shaft motion to the spring. The free end of the spring is aligned with and is tensioned to engage a fixed contact on the base.

The mounting of the breaker provides a floating switch structure to compensate for manufacturing tolerance and wear. A silicon controlled rectifier is preferably secured to the back side of the base to form an integral sub assembly which can be readily manufactured and assembled as a part of an integrated actuator unit. The gate terminal of the silicon controlled rectifier or other similar control elements is directly connected in circuit with the switch contacts. In order to provide a highly reliable actuator unit and protect the switch assembly, a special twopart seal is disposed about the shaft at the exit opening of the thermal cylinder which is provided with an internal planar surface. The seal includes an encircling rubber-like member and a thin wafer-like plastic member encircling the shaft and bearing on the planar surface. Generally, the rubber-like member is of a substantial axial length and includes an inner step face projecting inwardly into the expansion chamber and having a thin sealing lip adjacent the shaft. The wafer-like member is a thin, disc disposed between the rubber-like member and the adjacent planar surface and constitutes a seal backup. The wafer-like plastic member has a central opening through which the shaft projects and is formed with a central shaped portion projecting outwardly of the expansible filled chamber. It is found that this seal provides a very long life, permitting in excess of 100,000 strokes, which in combination with the switch structure provides a very reliable, efiicient and rugged thermal actuator unit.

Additionally, sensitivity of the thermal actuator to changes in heat or temperature has been found to be improved by the use of a low mass, high wall density heating element embedded in the wax. The element is preferably a spiral wound coil located within the chamber with a generally U-shaped configuration having the outer end secured as the connecting electrodes or terminals in the outer end of the cylinder and having a base extending around the inner portion of the moveable shaft.

The drawings furnished herewith illustrate embodiments of the invention in which the above features and advantages appear as well as others which will be clear from the following description.

In the drawings:

FIG. 1 is a schematic circuit diagram of a temperature-responsive control system, including the means provided by the invention for controlling the operation of a thermally operated actuator which is responsive to temperature variations detected by the control system:

FIG. 2 is a partial sectional view of the actuator, generally showing particularly a switch and a mounting of the SCR:

FIG. 3 is a longitudinal, central section showing the internal details of an improved thermal actuator:

FIG. 4 is an end view showing a novel floating switch structure of the present invention: and FIG. 5 is an exploded view of the switch structure FIG. 6 is a partial assembly view showing the preferred method for interconnecting the wafer seal.

A temperature-responsive control system for an air conditioning system is shown in circuit diagram in FIG. 1 of the drawings and comprises an AC voltage source 2 which is connected with the primary winding of a transformer 4 by leads including start switch 5. Transformer 4 includes center tapped secondary winding 6 having upper and lower winding halves 6a and 6b. The free terminal of winding half 6a is connected with one terminal of the primary winding 8 of a transformer 10 by lead 12, which includes switch 14. The other terminal of winding 8 is connected with the center tap of secondary winding 6 by neutral lead 16. The secondary windings 18'and 20 of transformer 10 are connected, respectively, in full-wave rectifier circuit 22 and condition-responsive bridge circuit 24. One arm of bridge circuit 24 includes a conditionresponsive element 26, which may be a thermistor, and resistor 28. The other arm includes variable and fixed resistors 30 and 32, respectively. The center taps of secondary windings 18 and 20 are connected to lead 16 by lead 34. Bridge output junction 36 is capacitively connected to the base electrode of transistor 38, which is connected in the first input stage 40 of the audio amplifier electronic control. Thermistor 41 serves to temperature stabilize the electronic control. The collector electrode of transistor 38 is connected to the base electrode of transistor 42 of driver stage circuit 44. The biasing potential applied to the base electrode of transistor 38 is determined by the setting of adjustable resistor 45 of input stage 40, which resistor determines the operating range of the controller. The driver stage circuit output is connected with unijunction transistor 46 of output stage circuit 4-8 by R-C network 50. Full wave'rectifier circuit 22 supplies DC. power to the various transistor stages of the electronic control. Junction 52 of circuit 48 is connected to the gate electrode of SCR 54 by lead 55 and the normally-closed contacts of limit switch 56, which is shown in closed position in FIG. 1. The cathode electrode of SCR 54 is connected with neutral lead 16 by lead 58 and the anode electrode is connected with the free end of winding 6b by lead 60 which includes the resistance heating coil 62 of the expansible medium thermal actuator 64.

The described electronic control system is operative to energize and de-energize the resistance heating coil 62 of the thermal actuator 64, which is of the well known type in which a thermally expansible material is enclosed in a casing with a heating element and, on energization of the heating element, expands to move a shaft or plunger longitudinally from the casing, against the force of a spring, by an amount proportional to the energization of the heating element. Such a condition-responsive electronic control system is of the same general type as that disclosed in my co-pending US. patent application Ser. No. 206,345 filed June 29, 1962, and entitled Condition- ResponsiveElectronic Control, now abandoned, but the present invention is characterized by the provision and operation of the SCR 54 and the floating type limit switch 56, both of which are mounted on the housing of the thermal actuator.

The generally cylindrical casing of the thermal actuator 64 is provided with an end wall 70 having in it a central opening through which the actuator shaft 66 slidably extends. The silicon controlled rectifier 54 is mounted in a casing which is itself mounted in a recess or cup 72 in end wall 70 in heat exchange relation with the expansible material within the actuator casing.

The switch 56 has two contacts 73, 74 which may be formed of spring wire and both of which are rigidly held between blocks 76, 78 which are attached to the outer surface of the end wall 70. Contact 74 has a shoe 80 on the free end thereof, which is positioned to be engaged by an enlarged part 82 on shaft 66 as the shaft moves outwardly from the actuator casing. When this occurs at a predetermined position in the outward travel of the shaft the two switch contacts are separated, the contact 74 resiliently yielding with respect to other contact so that the two contacts return to engagement with each other when the actuator shaft'withdraws. The switch contact 73 is connected to the gate electrode of SCR 54 by lead 84, the movable switch contact 74 is connected to junction 52 of the output stage circuit 48 of the electronic control network by lead 55, the anode of SCR S4 is connected by lead 60 to the heating resistor 62 of the thermal actumm, and the cathode of SCR 54 is connected to neutral lead 16 by lead 58. Operation Assume that the temperature set point of bridge 24, as determined by the setting of variable resistor 30, is 75 F., and that the temperature initially sensed by thermistor 26 is also 75 F. If start switch 5 and control switch 14 are initially in closed and open positions, respectively, transformer is de-energized, the electronic control is de-activated, and no signal voltage. is applied to the gate electrode of SCR 54; Although an AC potential is applied across the anode to cathode circuit of SCR 54 by winding half 6b, SCR 54 remains non conductive and no current flows through actuator resistor 62 because of the lack of a gate triggering signal voltage. 7

Closing of switch 14 causes the electronic control to be activated by the energization of transformer 10. As the temperature sensed by thermistor 26 equals set temperature as determine d by variable resistor 30, the bridge output'at junction 36 is zero. By appropriate setting of adjustable resistor 45 of first input stage 40, a quiescent level of current is caused to flow through stage 40 which causes the potential of junction '52 to be just below the gate triggering potential of SCR 54. Consequently, SCR 54 remains non-conductive and no current flows through actuator heating resistor 62.

If the sensed temperature now increases by a given increment (1 an AC signal voltage is produced at bridge output junction 36 which has a given phase relationship to the reference'voltage of winding 6 and a magnitude that is a function of the temperature increment t This signal voltage is amplified by stages 40 and 44 and is applied to the emittor electrode of uni-junction transistor 46 by R-C network 50. As described in my aforementioned application for Letters Patent, the time constant of R-C network 50 and the phase relationship of the amplified signal voltage appearing at junction 52 relative to the reference voltage produced by winding half :61) are a function of the temperature increment sensed by thermistor 26. If the amplified signal voltage at junction 52 now exceeds the gate triggering potential of SCR 54, the SCR becomes conductive during portions of the positive half cycles of the reference voltage. The effective level of DC current flowing through actuator heating resistor 62 is a function of the conduction angle of SCR 54, i.e. a function of the instantaneous time constant of R-C network 50 and the temperature increment t sensed by thermistor 26. Because of the heating of the expansible medium of the thermal actuator by the current flowing through resistor 62, shaft 66 is moved outwardly of the actuator casing to a position corresponding to temperature (+t F.

If the sensed temperature increases further by increment t the time constant of the R-C network, the conduction angle of SCR 54, and the effective DC current flowing through resistance 62 will increase and the increase will be a function of the total temperature increase (t -H F. Consequently, shaft 66 is additionally moved outwardly to a position corresponding to temperature (75-H 44 F. If this position is the desired end position of travel of shaft 66, any further outward travel of the shaft causes contacts 73, 74 to be separated by movement of the enlarged part 82 of shaft 66 against shoe 80, whereby switch 56 is opened and SCR 54 becomes nonconductive, interrupting the effective DC current flowing through resistor 62. Because of the interruption of the supply of energy to the heating resistor of the thermal actuator, the spring-biased shaft begins a return movement toward its fully retracted position. However, because of the construction of limit switch 56 a very slight retracting movement of shaft 66, which may be of the order of micro inches, causes switch 56 to close, whereby the gate electrode of SCR 54 is again supplied with triggering current to cause the SCR to become conductive and resistance 62 to be energized. Consequently, the opening and closing of switch 56 in the gate circuit of SCR 54 serves to maintain shaft 66 substantially stationary in its fully extended position. It will be apparent that the position of switch 56 on the actuator housing determines the maximum extended position to which shaft 66 may be moved.

If the sensed temperature decreases to set temperature, switch 56 remains closed while shaft 66 is retracted to its initial position. In the event that sensed temperature decreases below set temperature, the amplified signal voltage appearing at junction 52 and at the gate electrode of SCR 54 will be out of phase with the reference voltage applied across the anode and cathode electrodes of SCR 54. Consequently SCR 54 can never become conductive and heating resistor 62 can never become energized when sensed temperature falls below set temperature. This se quence of operations may be reversed, as shown in my co-pending application which is referred to above.

By placing the limit switch in the gate circuit of the SCR the switch may be designed to handle only extremely low levels of current and voltage, for example, levels of the order of 10-20 milliamperes and 1 volt, respectively. Furthermore, by the use of the extremely sensitive floating type limit switch, only very slight movement of shaft 66 in the retracted direction is required to re-close the gate circuit of the SCR, and consequently shaft 66 is maintained substantially stationary in the desired extended position. Additional advantages result from the mounting of the SCR in the casing of the thermal actuator. For example, it is known that the amount of current that can be handled by an SCR is a function of its casing temperature. The thermal actuator affords an adequate heat sink for the SCR and prevents it from operating at a temperature greater than C. Furthermore, the amount of heat generated by the SCR during its operation is dissipated into the thermal actuator, thereby increasing the operating efficiency of the actuator, in terms of the conversion of electrical energy into mechanical work. Also,

by locating the SCR in the thermal actuator housing rather than in the housing of the electronic control, the sharp chopping effects of the power electrode circuit of the SCR are isolated from the condition sensing circuit, whereby the generation of large spike input voltages, which might otherwise tend to render the control inoperative due to regenerative feedback, is avoided. Moreover, by mounting the SCR at a location remote from the electronic control, the heat generated by the SCR does not impair the response of sensing element 26 to condition deviations.

Referring to the drawings and particularly to FIGS. 3-5, an efiicient, reliable thermal actuator construction is shown including a thermal responsive power unit 85 and a switch unit or assembly 86 disposed within a switch housing 87 which is secured to one end and forms an integrated part of the power unit 85.

The illustrated power unit 85 includes a finned cylindrical body 88 having a closed outer end 89 defining an internal chamber which is filled with a suitable expansible material 90 such as a wax and closed by housing 87. A shaft 91 is slidably supported in the internal end of the switch housing 87 and projects into the expansible wax 90. A heating element 92 is also provided within the chamber for selective heating of the wax 90 in accordance with a controlled load which is coupled to the shaft 91 in any suitable manner, not shown. As the wax increases in temperature and changes to a molten state, its volume expands and causes the shaft 91 to move outwardly from the chamber in the manner of a piston. Conversely when the wax 90 cools, the shaft 91 may be forced back into the chamber through any suitable return spring 93.

In the illustrated embodiment of the invention, the heating element 92 is a low mass high watt density memher in the form of a spiral or wire-wound coil embedded within the wax 90 in the form of a U-shaped member. The outer ends the U-shaped element project outwardly through suitable seal 94 in the closed end 89 of the body 88. Seals 94 prevent loss of the wax 90. The base or inner portion of the U-shaped heating element 92 extends around the shaft 91 within the chamber, as shown in FIG. 3.

This shaft is slidably supported within the innermost end of the housing 87 which includes an inwardly projecting central boss portion threaded into a correspondingly threaded portion of the body 88 as at 95. The innermost end of the housing 87 is a planar surface extending diametrically of the cylindrical body 88. The housing 87 is centrally apertured to accommodate the shaft 91 with a suitable bushing or bearing 96 fixed in the innermost end to provide a sliding support for the shaft 91. A sleeve 97 is press fitted over the outer end of shaft 91 and includes an enlargement at the innermost end forming a switch actuating collar or lever 98. The innermost end of sleeve 97 also forms a stop determining the inward most movement of the shaft 91. Outward movement of the shaft 91 is limited by actuation of the improved floating switch structure or assembly 86 of the present invention which is determined by the position of the enlargement 98 of sleeve 97. i

In order to substantially prevent loss of wax 90 and therefore variations in the response and functioning of the thermal actuator and the switch assembly 86, a twopart backto-back seal construction encircles the shaft 91 immediately adjacent the inner flat or planar end of the housing 87. The seal construction includes a wafer-thin seal 99 of Teflon or the like. and an annular rubber-like disc seal 100.

The seal 99 is formed of a suitable plastic such as Teflon or similar bearing material tape. Teflon is a tough, durable material having self lubrication characteristic and has been found to be particularly suitable. The wafer seal 99 includes a central opening tightly engaging the shaft 91 and further includes a central truncated sealing cone 101 preformed to mate with a corresponding charnfered inner edge of the bearing 96. Seal 99 rests on the flat inner end 8 of the housing '87 and bearing 96 with the truncated sealing cone 101 extending axially outwardly in bearing engagement with the edge of bearing 96. In an actual construction a wafer seal has been constructed of a .010 inch thick Teflon tape with a cone angle of 45.

The annular rubber seal fits closely about the shaft and projects into the wax chamber from seal 99. The inwardly facing portion of the rubber seal 100 includes a stepped peripheral portion102 mating with a correspondingly formed edge in the wax chamber within body 88. The innermost end of the rubber seal 100 includes a sealing lip or cone 103 immediately adjacent and bearing on the shaft 91. A highly satisfactory rubber seal has been formed of a Buna nitrile rubber of an 80 durometer hardness. The wafer seal 99 should be carefully assembled with the housing 87 and shaft 91 and preferably is made as shown diagrammatically in FIG. 6. The seal 99 is placed within the housing 87 and a backup tool 104 is placed on top of the seal. Tool 104 is a tubular member having an inner lip 105 bearing on the preformed cone 101. The internal diameter of tool 104 substantially corresponds to the shaft 91 which is assembled within housing 87 with the assembly tool 104 in position. As diagrammatically shown by the dotted rotation lines, shaft 91 is inserted by twisting back and forth while forcing it upwardly through the seal 99. A thin coat of a silicon grease or the like is then preferably placed over the surface of the seal 99 and the shaft 91 and the rubber seal 10.0 is pressed onto the shaft after which housing 87 is assembled with body 88 and floating switch assembly 86.

The illustrated switch assembly 86 constitutes a limited switch unit formed as a subassembly to be mounted within the recessed outer portion of the housing 87.

Generally the switch assembly 86 includes a base or switch plate 106 shown as a disc-like member of a suitable insulating material and having a central opening accommodating the sleeve 97 including the collar 98.

A horseshoe shaped groove 107 is formed in the outer face of the switch plate 106 in radially spaced relation to the central opening. An annular insulating actuator or breaker 108 rests on plate 106 and includes a projection mating with groove 107. The breaker 108 further includes outwardly projecting hub '109 having an internal diameter greater than sleeve 9'7 but less than collar 98. A pair of actuating arms or braces 110 are integrally secured to the outer face of the breaker extending radially outwardly from opposite sides of hub 109. The breaker 108 is clamped to plate 106 and within the groove 107 by the tension of a leaf spring 111 secured to the plate and overlying the breakers.

The leaf spring 111 includes a centralring-shaped portion encircling the shaft and overlying the breaker 108. A contact tip 112 is integrally secured to one side of the spring 111 and is aligned with a button contact 113. Diagna mmatically opposite to the contact tip 112.is a mounting flange 114 integrally connected to the central ring portion by a spring portion 115 having an inverted U-shaped cross-section. A contact lip 116 is integrally formed to the one edge of the fiange 114. The flange 114 is secured to a raised mounting shoulder 117 on the switch plate 106 by suitably mounting screws 118. The illustrative button contact 113 is secured to the outer periphery of the plate 106 by an integral shaft 119 which projects through acorresponding opening in the plate andis peened over onto the back side. A contact lug 120 is secured between the button contact 113 and the plate 106 to provide for electrical interconnection to the contact. The switch actuating collar 98 will be spaced from the leaf spring breaker 108 such that the contact button 113 is engaged by the contact tip 112 as a result of the tension in the leaf spring.

The contacts are connected in circuit to control the conduction of a silicon controlled rectifier 121 or similar solid state device which is secured to the opposite side of the switch plate 106. Such rectifiers are well known devices and rectifier 121 is shown in full outline with the usual three leads or terminals identified respectively as the gatelead 123, the cathode lead 124, and an anode lead 125. The silicon controlled rectifier 121 includes a case tab 126 for identifying the location of the respective leads. In the illustrated embodiment, the rectifier is secured to the underside of the base plate 106 with three leads 123-125 projecting through three correspondingly located openings in the plate. An insulator 127 is secured to the back side of the silicon control rectifier 121 includes four L-shaped legs 128 extending along the side of the silicon control rectifier 128 to the back side of the switch plate 106. One of the four L-shaped legs 128 is aligned with and covers the case tab 126 to fully project and insulate the unit as hereinafter described.

The housing 87 includes an outer cylindrical portion within which the switch assembly 86 is mounted. The base of the cylindrical portion includes a recess generally corresponding to the outer configuration of the silicon control rectifier 121 and the insulator 127.

-In assembly, the silicon control rectifier 1 21 and other components are secured to the base plate 106. The subassembly is held by the silicon control rectifier leads 123-125 inserted into the housing with the silicon controlled rectifier 121 moving into the corresponding recess. The insulator 127 spaces the silicon controlled rectifier 121 from the metal wall of the housing 87.

After being assembled suitable mounting screws 129 project through openings provided in the base plate 106 and thread into correspondingly tap openings in the base of the cylindrical portion of housing -87.

A lead wire 130 connects the switch contacts 112 and 113 and the silicon controlled rectifier 121 into a control circuit for the load and the heating element 92. Lead wire 130 includes three encased conductors 131, 132 and 133 connected respectively to the cathode terminal 124, the anode terminal 125 and contact lug 120 of button contact 113. A jumper lead 134 connects the gate terminal 124 to the contact lip 116 of leaf spring 111. The lead wire 130 extends through a sealed opening in the base portion of the switch housing 87 and along the side of body 88.

A cylindrical extension 135 is secured to the outer end of body 88 extending from the closed end wall 89. The lead wire 130 extends through an opening in extension 135 and the lead conductors 131-133 are connected in circuit with a main circuit wire 136 and to the outer ends of the heating element 92. A sealant 137, such as an epoxy resin or the like, fills the extension to hermetically seal the connections.

Lead Wire 136 is connected to a power connecting control 138 having input power leads 139. A load responsive control such as a thermostat 140 may be provided to vary the heating level.

In operation, the heating element 92 is energized in accordance with the output of the thermostat 140 or other controller. The wax 90 when heated expands to exert a substantial pressure on the shaft 91 which moves out of the cylinder body 88 and actuates the load. When the energization of element 92 is reduced, the wax 90 cools and spring 93 forces the shaft 91 into the cylinder body.

If the load demand is such that the shaft 91 is forced outwardly a permissible full travel, the shaft collar 98 engages the barrier 108 and carries it outwardly to actuate switch assembly 86. This will open the circuit to the gate terminal 124 of the silicon controlled rectifier and the rectifier 121 becomes non-conductive.

It will be apparent to those skilled in the art that other embodiments, as well as modifications of that disclosed, may be made without departing in any way from the spirit or scope of the invention, for the limits of which reference must be made to the appended claims.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. A thermal actuator having a movable shaft member for actuating a load and reciprocally mounted within a cylinder filled with an expansible material and having an electrical control circuit for a load responsive'heating element for heating the material including a solid state switching element having a gate control element to modulate conduction in response to the signal on a signal source wire and thereby control the movement of said shaft, characterized by a floating limit switch assembly connected in circuit between said control element and said signal source wire to open the circuit connection upon said shaft reaching a predetermined limit of movement, said limit swtich being releasably mounted to the thermal actuator unit and having a base plate with an opening through which the shaft and a protrusion therefrom freely passes, a leaf spring mounted to the base plate and operable to open and close the limit switch, and an actuating memberdisposed between the base plate and leaf spring with an opening engaged by the protrusion to transmit shaft generated motion to the leaf spring.

2. The thermal actuator of claim 1 having the solid state switching element secured to the base plate.

3. A modulating control system including a thermal actuator of the type having a housing containing a thermally expansi ble medium and an electrical heating element in said housing in heat exchange relation to the expansible medium, a shaft slidably mounted in said housing in response to the expansion and contraction of the medium established by the changes in the temperature of said medium, and an alternating current supply means for energizing said element, said control system comprising a condition-responsive means establishing an output signal proportional to the condition being sensed, a bistable solid state switching device connected in a first circuit with said heating element and said supply means and having an input element connected in a second circuit to said conditionresponsive means to control the connection of said heating element to said source, and means including a floating type switch connected in the second circuit and operable when closed to establish proportional conduction of the solid state device in accordance with the output of the condition-responsive means, said switch being opened by outward movement of said shaft to a predetermined limit position to open said second circuit and prevent application of the signal to the input element of said switching device for producing conduction and thereby prevent the energization of said heating element in response to the output of the condition-responsive means.

4. The control system of claim 3 wherein the solid state device is a controlled rectifier having a gate electrode connected to the condition responsive means through said switch and is mounted in the actuator housing in heat exchange relationship with the expansible medium therein, and said condition-responsive means generates an alternating current signal voltage the magnitude and phase of which are a function of the magnitude and sense of deviation of a sensed condition from a predetermined value for corresponding proportional energization of said element.

5. The control system of claim 3 wherein the floating switch assembly is releasably mounted to the housing and includes a base plate with an opening through which the shaft and a protrusion on the shaft freely passes, a leaf spring contact mounted to the base plate and movable with respect to a fixed contact, and an actuating member disposed between the base plate and leaf spring with an opening engaged by the protrusion to transmit shaft generated motion to the leaf spring.

6. The control system of claim 3 wherein the housing includes a cup-shaped chamber at one end with the shaft slidably extended therethrough, a base plate mounted adjacent the base of the chamber and having an opening through which the shaft and a protrusion on the shaft freely passes, a leaf spring contact mounted to the base plate and movable with respect to a fixed contact, and an actuating member disposed between the base plate and leaf spring with an opening engaged by the protrusion to transmit shaft generated motion to the leaf spring, said housing having a recess in the base of the chamber, said solid state switching device being secured within said recess and having connection leads extended upwardly through said base plate and connected to the contacts.

7. The control system of claim 5 wherein said leaf spring includes a central ring portion with a mounting flange and a contact tip on diametrically opposite sides and the actuating member is ring-shaped and disposed between the ring portion and the base plate with a projection on the member loosely fitting within a groove in the base plate.

3. The control system of claim 3 having ashaft seal including a wafer formed of a self-lubricating plastic and with a central sealing cone projecting outwardly of the housing and tightly encircles the shaft immediately at the exit of the shaft from the housing and a rubber seal disposed inwardly from the water seal with a mating stepped periphery and a sealing cone on the innermost face.

9. The control system of claim 3 wherein said heating element is a resistor formed of a low mass, high watt density heating Wire, and said element being wire-wound and arranged in a U-shaped configuration with the base portion extending around an intermediate portion of the shaft and the side portions extending generally parallel to the shaft and outwardly thereof.

References Cited UNITED STATES PATENTS 2,524,886 10/1950 Colander et a1. 219210 3,097,314 7/1963 Harriman 219-501 X 3,109,910 11/1963 Fogleman 219501 X 3,136,877 6/1964 Heller 219-501 3,229,071 1/1966 Wisz et a1 219-501 3,333,086 7/1967 William's 219-501 RICHARD M. WOOD, Primary Examiner.

L. H. BEND'ER, Assistant Examiner.

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