Abstract:
An actuator for a device such as a tappet of a valve. The actuator having a thermostatic operating element electrically heated by an electrical heating element, and including an operating piston. A proportional piston stroke regulator detects and controls the position of the operating piston. The proportional piston stroke regulator includes a piston travel detection device, that detects the actual position of the operating piston, connected to a conventional closed-loop controller which compares the measured piston position to a predetermined piston control position entered into the closed-loop controller. Based on the comparison, the closed-loop controller regulates the electrical supply to the electrical heating element.

Description:
FIELD OF THE INVENTION 
     The invention relates to an actuator with an electrically heatable thermostatic operating element heated by an electrical heating element and having a housing containing an expanding material and an operating piston that is extensible from the housing, the piston position being controlled by a proportional piston stroke regulator. 
     BACKGROUND OF THE INVENTION 
     An actuator of this type is known, for example, in German Patent Disclosure DE 41 38 523 A1. In that construction, the regulation of the position of the operating piston is based on the electrical resistance of the thermostatic operating element, which is assumed to vary as a function of the change in volume of the expanding material, so that the electrical resistance of the operating element corresponds to the position of the operating piston. This design is not totally reliable as changes in ambient temperatures, ambient pressures or combinations thereof potentially change the expansion for a given temperature as measured by the operating piston position since the electrical resistance measured is not truly proportional to the actual temperature. Furthermore, the starting ambient temperature surrounding the operating element would either have to be a constant, so the piston always started at the same position with the same corresponding measured electrical resistance, or the piston position would have to be re-calibrated before every use of the operating element. The operability of the exemplary embodiments with which an electrical resistance varying with the change in volume of the expanding material is to be detected is not always consistent with precise and accurate movement and regulation of the operating piston. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide an actuator with a way of precisely measuring and regulating the position of the operating piston. 
     This object is attained by controlling the operating piston position with a proportional piston stroke regulator, which includes a piston travel detection device connected to a conventional closed-loop control circuit. The closed-loop control circuit compares the detected position of the operating piston with an entered pre-determined piston control position and regulates the electrical current supply to the electrical heating element accordingly. 
     By the present invention, it is possible to provide very precise measurement and regulation of the operating piston position. In particular, very precise movement of the operating piston to pre-determined positions is possible. 
     In a specific embodiment of the invention, the conventional closed-loop control circuit controls the supply of pulses of alternating current to the electrical heating element by the use of a relay, in particular a triac. By supplying the electrical heating element with an adjustable intermittent duration of pulses of electrical current, the electrically heatable thermostatic operating element behaves in the manner consistent with a proportional, integral, derivative (PID) controller, enabling very precise piston position regulation. Precision is enhanced if the operating temperature of the expanding material, that is, the temperature at which the expanding material begins to change from its original unheated state, is markedly above the ambient temperature, so that relatively rapid cooling and retraction of the operating piston occurs when the electrical heating element is not supplied with electrical current. 
     In another specific embodiment of the invention, the maximum travel of an actuator element of a device to be actuated is determined and stored in the memory of the conventional closed-loop control circuit. It is thus possible within the closed-loop control circuit to detect the maximum travel of the device to be actuated, for example a tappet of a valve, and to evenly distribute the pre-determined piston control positions along the entire length of travel, for example between the opening position and the closing position of a tappet of a valve, thereby providing a number of adjustable positions available to the actuating element of the device to be actuated. 
     In a specific structural embodiment of the invention, an outer housing encloses a stationary base body on which the electrically heatable thermostatic operating element, a motion transfer member moveable with the operating piston and the piston travel detection device are supported, the piston travel detection device being disposed between the motion transfer member and the outer housing or the base body. A preferred construction of the piston travel detection device includes a stationary element, mounted on the outer housing or the stationary base body, and a relative position element associated with the stationary element and moveable with the motion transfer device and the operating piston. 
     In a specific feature of the structural embodiment of the invention, the relative position element moveable with the motion transfer device and the operating piston is disposed on a circuit board which is mounted on the motion transfer member. It is advantageous if the conventional closed-loop control circuit and the relay are disposed on the circuit board as well. 
     Further characteristics and advantages of the invention will become apparent from the following description of the exemplary embodiment shown in the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of an actuator of the present invention; and 
     FIG. 2 is a vertical sectional view of an actuator of the preferred structural embodiment of the present invention mounted on a flow valve, with the components in the position where the thermostatic operating element is unheated. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The actuator shown in FIG. 1 includes an electrically heatable thermostatic operating element  10  having a metal housing  11  containing an expanding material, in particular a wax mixture, and a guide insert  12  secured to one end of the housing  11  through which an operating piston  13  extends outwardly from the housing. A flexible diaphragm internal to the housing  11  and sealingly attached to the upper edge of the housing  11  by the guide insert  12 , surrounds the operating piston  13 , isolating the operating piston  13  from the expanding material contained in the housing  11 . 
     An electrical heating element  14 , such as a positive temperature coefficient (PTC) resistor, is located under the housing  11  of thermostatic operating element  10 . The electrical heating element  14  is connected to an electrical current source in the manner disclosed in U.S. Pat. No. 5,897,055 (German Patent Disclosure DE 197 05 721 A1). The electrical heating element  14  heats the thermostatic operating element  10  such that the expanding material increases its volume, thus in accordance with its increase in volume driving the operating piston  13  out of the housing  11  through the guide insert  12 . The operating temperature of the expanding material, in particular the temperature at which it changes from its unheated state, is defined as sufficiently high compared with the ambient temperature that rapid cooling occurs after the electrical current is removed from the electrical heating element  14 . For example, if the thermostatic operating element  10  is used in an area with the ambient temperature being room temperature (approximately 22° C.), then an operating temperature of 70.4° C. may be desired for the expanding material. 
     The electrical heating element  14  is connected to a 24 V alternating current electrical source by an electrical supply line. A relay  15 , particularly a conventional triac, is integrated into the electrical supply line, enabling the electrical heating element  14  to be supplied with electrical power in pulses of adjustable intermittent duration. 
     The relay  15  is operated by a conventional closed-loop controller  16 , which forms a closed-loop control circuit. Also connected to the closed-loop controller  16  is a piston travel detection device  17  moveable with the operating piston  13 . The piston travel detection device  17  transmits a signal to the closed-loop controller  16  corresponding to the detected position of the operating piston  13 . The detected position of the operating piston  13  is compared by the closed-loop controller  16  to a pre-determined piston control position entered into the closed-loop controller  16 . Any deviation between the detected position and the predetermined piston control position causes the closed-loop controller  16  to operate the relay  15  in such a way that pulses of electrical current are intermittently sent to or blocked from the electrical heating element  14 , sequentially energizing and de-energizing the electrical heating element  14 . 
     The pre-determined piston control positions for the operating piston  13  are entered into the conventional closed-loop controller  16  as direct voltage values in a range between 0 V and 10 V. These direct voltage values are coordinated within the closed-loop controller  16  with the detected travel distance of the operating piston  13 , with the 10 V direct voltage value corresponding to the maximum travel of the operating piston  13  and the remaining predetermined piston control positions evenly distributed along the entire travel length of the operating piston  13 . This coordination of the direct voltage values and the travel of the operating piston  13  within the closed-loop controller  16  is achieved in the following manner. 
     The conventional closed-loop controller  16  causes the electrical heating element  14  to be energized for a sufficient amount of time such that the expanding material of the thermostatic operating element  10  increases in volume, causing the operating piston  13  to engage and move the actuating element of the device to be actuated to its maximum point. For example, if the device to be actuated is a valve and the outward motion of the operating piston  13  acts to close the valve, then the travel of the valve and the operating piston  13  would be from the valve&#39;s open position to its closed position. On the other hand, if the device to be actuated is a valve tappet such that the outward motion of the operating piston  13  acts to open the valve tappet, then the travel of the device and the operating piston would be from the device&#39;s closed position to its open position. The maximum travel distance required of the operating piston  13  is registered when the actuating element of the device to be actuated reaches its maximum travel, and the corresponding maximum pre-determined piston control position is entered in the closed-loop controller  16 . The closed-loop controller  16  then causes the electrical heating element  14  to be de-energized, allowing the expanding material of the thermostatic operating element  10  to cool to ambient temperature, thereby decreasing in volume. The operating piston  13  is then forced back into the housing  11  by a compression spring, not shown in FIG.  1 . As the operating piston  13  withdraws into the housing  11 , the actuating element of the device to be actuated follows the operating piston  13  until the minimum position for the actuating element (open or closed) is reached. Thus the minimum distance required of the operating piston  13  is determined and the corresponding minimum pre-determined piston control position is entered in the closed-loop controller  16 . The remaining pre-determined piston control position direct voltages are distributed evenly along the travel length of the operating piston  13 . For example, if the maximum travel distance of the operating piston  13  was determined to be 2.5 mm and the minimum travel distance was 0 mm, then the maximum pre-determined control position entered would correspond to 2.5 mm, the minimum pre-determined control position entered would correspond to 0 mm, and the remaining pre-determined control positions would be entered to correspond to a travel distance of 0.25 mm for each voltage step (i.e., each 1 V is 0.25 mm). The maximum travel of the actuating element of the device to be actuated can be readjusted with little difficulty, making the actuator adaptable to any variations in the maximum travel distance of the actuating element. The pre-determined control positions for the operating piston  13  can be determined the first time electrical power is supplied to the electrical heating element  14  and readjusted after every power supply disruption. 
     The piston travel detection device  17  can operate by a variety of conventional methods, including, but not limited to, the following: utilizing the Hall effect by using a magnetic field sensor; using the change in capacitance of a capacitor; by magnetoresistivity; as a system employing the Wiegand effect; and by an optical method using applied, detectable markings. The preferred method of the present invention employs a two-piece apparatus—a ferrite bar that penetrates into a electromagnetic coil to which an electrical voltage has been supplied. The distance in which the bar penetrates the coil creates a detectable change in inductance, corresponding to the change in the position of the operating piston  13 . 
     The actuator shown in FIG. 2 corresponds generally to the actuator disclosed in U.S. Pat. No. 5,897,055 (German Patent Disclosure DE 197 05 721 A1), which is incorporated herein by reference. The actuator includes a base body member  24 , which can be secured by a union nut  23  to threads of a device to be actuated, such as a valve. As shown in FIG. 2, the actuator is mounted to a connection stub  22  integral to valve  19  by the union nut  23 , the actuator situated in a manner to engage and operate a valve tappet  20  which carries a valve plate  21  that is associated with a valve seat, not shown in detail, where the valve tappet  20  is biased by a compression spring  30  which opens the valve  19  when no external load is acting on the valve tappet  20 . A motion transfer member, assembled from two parts  26 ,  27  is disposed on the base body member  24  for movement relative thereto, and the operating piston  13  of a thermostatic operating element  10  is in operable engagement therewith. The housing  11  of the thermostatic operating element  10  is supported in stationary fashion on the base body member  24  and is seated on an electrical heating element  14 , such as a positive temperature coefficient resistor (PTC), which is supported on the base body member  24  This PTC resistor is connected to an electric current source in the manner disclosed in U.S. Pat. No. 5,897,055 (German Patent Disclosure DE 197 05 721 A1). The thermostatic operating element  10  is oriented such that the operating piston  13  moves in a direction away from the union nut  23  when the operating element  10  is actuated. 
     With this arrangement of the components, when the electrical heating element  14  is energized, it heats the operating element  10  to cause expansion of the wax mixture in the operating element  10 , thereby forcing the operating piston  13  outward, which in turn moves the motion transfer member,  26 ,  27  against the bias of the compression spring  30 , which movement results in the motion transfer member  26 ,  27  moving an actuating element of the device being actuated. 
     The compression spring  30  is supported by an annular collar  35  disposed on one part  26  of the motion transfer member  26 ,  27  and prestressed against an outer housing  31 . The outer housing  31  encloses the thermostatic operating element  10  including the operating piston  13 , heating element  14 , compression spring  30 , motion transfer member  26 ,  27  and the base body member  24 . The outer housing  31  is secured to the base body member  24 . 
     On its side toward the union nut  23 , the part  27  of the motion transfer member  26 ,  27  penetrates the base body member  24  and includes a pressure plate  28  which forms a bearing face for the element to be actuated, in FIG. 2 a valve tappet  20 . A circuit board  29  is mounted on the face end of the part  26  of the motion transfer member  26 ,  27  that is opposite the operating piston  13 . The piston travel detection device shown in FIG. 1 includes an electromagnetic coil  32  mounted on the surface of the circuit board  29  and a ferrite bar  33  secured to the inside surface of the top of the outer housing  31 . The ferrite bar  33  is positioned such that the ferrite bar  33  penetrates the electromagnetic coil  32  when the operating piston  13  moves outward, pushing the part  26  of the motion transfer member  26 ,  27  toward the top of the outer housing  31 . The closed-loop controller and the relay are also disposed upon the circuit board  29 . 
     A window  34  of transparent material is inserted into the outer housing  31  facing the annular collar  35  on the part  26  or the motion transfer member  26 ,  27  such that it is possible to view the position and functioning of the actuator external of the outer housing  31 . 
     It will therefore be readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application. Many embodiments and adaptations of the present invention other than those herein described, as well as many variations, modifications and equivalent arrangements, will be apparent from or reasonably suggested by the present invention and the foregoing description thereof, without departing from the substance or scope of the present invention. Accordingly, while the present invention has been described herein in detail in relation to its preferred embodiment, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for purposes of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended or to be construed to limit the present invention or otherwise to exclude any such other embodiments, adaptations, variations, modifications and equivalent arrangements, the present invention being limited only by the claims appended hereto and the equivalents thereof