Thermal actuators for operating the contacts of a thermal switch are known. Typically, such actua¬ tors employ a bimetallic member, such as, for example, a disc, that has a high temperature stable state and a low temperature stable state that snaps with a snap action from the low temperature stable state to the high temperature stable state upon heating and returns to the low temperature stable state upon cooling. In such devices, the bimetallic member snaps to its high temperature stable state at a predetermined tempera¬ ture known as the upper set point and returns to its low temperature stable state at a lower temperature known as the lower set point. The temperature differ¬ ence between the upper and lower set points is known as the temperature differential. The temperature differential of an actuator may be adjusted to suit the desired application by adjusting the geometry of the bimetallic disc.

hen operating the contacts of a thermal switch it is desirable to have a strong snap action to insure a rapid opening and closing of the contacts in order to reduce arcing and contact resistance to thereby prolong the life of the contact. A strong snap action is not difficult to achieve with an actua¬ tor employing a bimetallic member as long as the temperature differential of the member is relatively high. However, when the temperature differential is reduced, the quality of the snap action is also reduced and the contacts will be opened and closed more slowly.

One attempt to provide a quick acting tem¬ perature controlled electric switch is illustrated in United States Patent No. 1,918,491. The device dis- closed in the '491 patent utilizes two bimetallic discs carrying opposed contacts. The two discs have different thermal characteristics and cooperate to maintain pressure on the contacts as the opening tem¬ perature is approached. However, the device illus- trated in the '491 patent will still have a relatively sluggish snap action when discs having a low tempera¬ ture differential are used. Also the design is rela¬ tively complex, and cannot readily control externally mounted contacts. Consequently, the design cannot easily be integrated into existing switch designs.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a thermal actuator that overcomes many of the disadvantages of the prior art thermal actuators.

It is another object of the present inven¬ tion to provide a thermal actuator particularly usable for controlling the operation of a thermally controlled switch. It is another object of the present inven¬ tion to provide a thermal actuator that provides rapid snap action even in low temperature differential applications.

It is another object of the present inven¬ tion to provide a simple positive snap action actuator that can be retrofitted into existing designs.

Thus, in accordance with a preferred embodi- ment of the invention, two bistable bimetallic members are mechanically coupled together to move in unison. The temperature differential of each individual member is relatively high to assure good snap action, but the upper and lower set points of the two members are different and selected so that one of the members controls the separation of the contacts and the other member controls the closing of the contacts. Thus, by appropriately selecting the set points of the two members, a temperature differential for the combina- tion of the two members that is lower than the tempera¬ ture differential of either individual member may be obtained.

DESCRIPTION OF THE DRAWING These and other objects and advantages of the present invention will become readily apparent upon consideration of the following detailed descrip¬ tion and attached drawing, wherein:

FIG. 1 is a side sectional view of the actuator according to the invention used to control the operation of a switch showing the actuator in its low temperature stable state position;

FIG. 2 is a side sectional view similar to FIG. 1 showing the actuator in its high temperature stable state; and FIG. 3 is a temperature bar graph illustrat¬ ing the operating principle of the actuator according to the invention.

DETAILED DESCRIPTION Referring now to the drawing, with particular attention to FIG. 1, there is shown a thermal actuator according to the invention generally designated by the reference numeral 10. The actuator 10 is shown

in the environment of a thermal switch for purposes of illustration, because it is particularly suitable to such applications, but it should be understood that the actuator 10 could be used to actuate other devices. The actuator 10 comprises, in the illustrat¬ ed embodiment, a pair of bistable bimetallic members, which in the present embodiment, comprise a pair of discs 12 and 14. While the discs 12 and 14 are refer¬ red to as bimetallic because they are fabricated from two layers of metal such as Invar (an alloy of iron, nickel, carbon, manganese and silicon that has a low coefficient of expansion) and steel having different coefficients of expansion, they may be fabricated from other materials having different temperature coefficients, whether metallic or non-metallic, and the term bimetallic is intended to encompass any struc¬ ture utilizing material of different temperature co¬ efficients as a thermal actuator. A third disc 16, fabricated from a flexible, high temperature material is interposed between the discs 12 and 14. One mater¬ ial suitable for the disc 16 is a polyimide film manu¬ factured under the trade name Kapton by DuPont. The thickness of the disc 16 is selected to provide an optimum spacing between the discs 12 and 14 that maxi- mizes the snap action of the actuator. Clearance is provided between the three discs to enhance operation as will be explained below. The discs 12, 14 and 16 are captured within a housing 18 by an inner housing 19 containing a thermal switch having moveable contact 20 that is movable into and out of engagement with fixed contact 22 by the discs 12 and 14. Shoulders 24 and 26 extending from the respective housings 18 and 19 engage the periphery of the discs 12, 14 and 16 and serve to retain the discs 12, 14 and 16 in position. The spacing between the shoulders 24 and 26 is adjusted so that the distance between the shoulders 24 and 26 is approximately 5 mils greater

than the combined thickness of the discs 12, 14 and 16 so that the discs 12, 14 and 16 are loosely held in place and the operation enhancing clearance is provided. A resilient member 28 maintains the con- tacts 20 and 22 closed when the actuator 10 is in its high temperature position (FIG. 2) , and the contacts are opened by pressure exerted on the member 28 by the actuator 10 via a striker pin 30 when the actuator 10 reaches its low temperature position (FIG. 1) to open and close a circuit between a pair of terminals 32 and 34.

While the actuator 10 is described in con- junction with a simple mechanical switch, it should be understood that the actuator may be used to operate any suitable device including other switches including solid state switches, devices that modulate the flow of current or otherwise control the operation of an electrical circuit, or even to mechanical controllers such as valves that control a thermally responsive hydraulic or pneumatic control system.

Each of the discs 12 and 14 has a high tem¬ perature stable state and a low temperature stable state. For example, the low temperature stable state for both discs may be such that each disc has its concave side down at room temperature (FIG. 1) , and upon heating to a higher temperature, the disc snaps to its high temperature stable state for example, to a state wherein the concave side of the disc is up (FIG. 2). Upon cooling, the concave side remains up until the disc is cooled sufficiently to reverse to its low temperature stable state.

The thermal characteristics of the discs 12 and 14 are illustrated in FIG. 3. The thermal charac¬ teristics of the disc 12 are illustrated by a portion so of a bar graph 49 which illustrates an upper set point 52 and a lower set point 54 of the disc 12. Similarly, a portion 56 of the bar graph 49 illus-

trates the thermal characteristics of the disc 14 and shows an upper set point 58 and a lower set point 60 of the disc 14. The upper and lower set points 52, 54 and 58, 60 of the respective bar graph portions 50 and 56 define the temperatures at which the respective discs 12 and 14 transfer from their low temperature stable state to their high temperature stable state upon heating and to their low temperature stable states upon cooling. For example, at room temperature the disc 12, operating separately, is in its low tempera¬ ture stable state with its concave side down. Upon heating, the lower side of the disc 12 expands more rapidly than the upper side, thus tending to cause the disc 12 to snap so that its concave side faces upwardly. This snapping action occurs at the upper set point 52. Thus, above the upper set point 52, which corresponds to 145° Fahrenheit for the disc 12, the disc will be in its high temperature stable state with its concave side facing upwardly. Upon cooling, the disc will not revert to its low temperature stable state when the temperature drops below the upper set point 52 (145° Fahrenheit) but will remain in a posi¬ tion corresponding to the high temperature stable position until the lower set point 54, which corre- sponds to 110° Fahrenheit in the illustrated embodi¬ ment, is reached. Below the lower set point 54, the disc 12 will revert to its low temperature stable state.

Below the lower set point 54, the disc 12 will remain in its low temperature stable state, and will resist any mechanical pressure to cause it to change to the high temperature stable state, and upon removal of any such physical pressure, will revert to its low temperature stable state. Similarly, above the upper set point 52, the disc 12 will resist any pressure to cause it to assume the low temperature stable state and will return to the high temperature

stable state upon removal of such pressure. However, in the range of temperatures illustrated by the bar graph portion 50 between the upper set point 52 and the lower set point 54, the disc 12 is in a vacillation range wherein it may assume either the high temperature stable state position or the low temperature stable state position. Absent any mechanical pressure, the disc will remain in whatever state it was in when it entered the vacillation range, but the disc can be mechanically moved between the high temperature and lower temperature stable state positions by mechanical pressure and retain the position to which it has been moved. The above description also applies to the operation of the disc 14 except that the temperatures corresponding to the upper and lower set points of the disc 14 are lower than those of the disc 12, as shown by the upper and lower set points 58 and 60 at opposite ends of the bar graph portion 56 that illus¬ trates the vacillation range of the disc 14 (FIG. 3) . As can be seen from the graph of FIG. 3, the temperature difference between the upper and lower set points of each of the discs 12 and 14, is approxi¬ mately 35° Fahrenheit, i.e. between 110° Fahrenheit and 145° Fahrenheit for the disc 12 and between 80° Fahrenheit and 115° Fahrenheit for the disc 14. The difference in temperature between the upper and lower set points is known as the temperature differential of the disc, and in general, a relatively high temper¬ ature differential as the one illustrated in FIG. 3 provides for a strong snap action at the transition between the high and low temperature stable states.

However, in many applications, a- temperature differential large enough to assure vigorous snap action is undesirable for other reasons, for example, in applications wherein it is necessary to maintain the temperature within a very narrow range of tempera¬ tures. In such an application, a thermal switch having

a small temperature differential is required. This is accomplished by the actuator according to the inven¬ tion by utilizing two bimetallic members having rela¬ tively high temperature differentials, but different upper and lower set points. The two bimetallic members have overlapping vacillation ranges and are mechanical¬ ly coupled together so that when one member changes state it will exert a mechanical pressure on the other member. Because the two members have overlapping vacillation ranges, one will be in its vacillation range when the other changes state. Thus, when one member changes state, it will cause the other member also to change state by applying pressure to it. The previously mentioned spacing between the members per- mits the member changing state to gain momentum prior to actuating the other member to thereby provide a more positive snap action. For example, in the illus¬ trated embodiment, the thickness of each of the bi¬ metallic discs is 8 mils, the thickness of the Kapton disc is 5 mils and the spacing between the shoulders 24 and 26 is 26 mils to provide the desired spacing. The value of the spacing is determined empirically and may vary as a function of the thermal and mechani¬ cal characteristics of the particular discs that are used. Also, as previously mentioned, the thickness of the disc 16 is important in optimizing snap action, and a thickness of 5 mils has been empirically deter¬ mined to be optimum for use with the aforementioned 8 mil bimetallic discs. As illustrated, the disc 12 having the higher temperature set points is disposed with its convex side adjacent to the concave side of the disc 14 hav¬ ing the lower temperature set points. A third disc, such as the disc 16, serves to transfer energy between the two discs 12 and 14. Wi h the arrangement shown in FIG. 1, both discs 12 and 14 are in their lower stable state positions with their concave sides facing

down. Upon heating, the lower set point 60 of the disc 14 is first encountered. At this point, the disc 14 enters its vacillation range, but the disc 12 is still in its low temperature stable state, and thus no change occurs. Upon further heating, the lower set point 54 of the disc 12 is reached, and at this point, both the discs 12 and 14 are in their vacillation ranges. However, absent any mechanical pressure, no change will occur.

Upon further heating, the upper set point 58 of the disc 14 will be reached. At this point, the disc 14 will change from its low temperature stable state to its high temperature stable state and apply pressure to the disc 12. Because the disc 12 is in its vacillation range, the pressure from the disc 14 will cause it to change position from its low tempera¬ ture stable state position to its high temperature stable state position, and the assembly will snap to a position corresponding to the high temperature stable state position and actuate the switch. The assembly will remain in the high temperature stable state posi¬ tion until the temperature reaches the lower set point 54 of the disc 12. At this point, the disc 12 will revert to its low temperature stable state position and exert pressure on the disc 14 which is in its vacillation range, and the assembly will return to the low temperature stable state position.

Thus, in the illustrated embodiment, the transition from the low temperature stable state to the high temperature stable state position is deter¬ mined by the disc 14 and the transition from the high temperature stable state position to the low tempera¬ ture stable state position is controlled by the disc 12. Consequently, by appropriately selecting the upper and lower set points of the two discs, a small temperature differential (5° in the illustrated embodi¬ ment) may be obtained without sacrificing the effec-

tiveness of the snap action switching, because the snap action switching is controlled by the temperature differentials of the individual discs which may be maintained at a high value. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically de- scribed above.

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