Valves and valve actuation – Permanent or constantly energized magnet actuator
Reexamination Certificate
2002-11-18
2003-12-02
Bastranelli, John (Department: 3754)
Valves and valve actuation
Permanent or constantly energized magnet actuator
C251S129040, C324S207210
Reexamination Certificate
active
06655652
ABSTRACT:
FIELD OF AND BACKGROUND OF THE INVENTION
The invention relates to a position controller, particularly for a drive-actuated valve, with an inherent safety design.
European Publication EP 0 637 713 A1 discloses such a position controller for a drive-actuated valve. The valve is installed in a pipe and controls the passage of a medium by way of a corresponding stroke of a closing element that interacts with a valve seat. A pneumatic drive is connected, by a push rod, with the closing element. A lever engages with the push rod and acts on a potentiometer, which functions as a position transmitter of the position controller. The potentiometer detects the actual position of the actuator. A control unit of the position controller compares this actual position with a predefined desired position. As a function of the determined deviation, the control unit generates an actuating signal to control the pneumatic drive. The desired value is predefined for the position controller through a normalized signal, e.g., a 4 to 20 mA interface or a digital field bus message. Thus, the role of the position controller is to convert the predefined desired value of the actuator position into a pneumatic pressure signal that is supplied to the pneumatic drive and results in a corresponding position of the push rod.
In addition, flap valves are known in the art in which the opening angle of a rotary valve is detected by means of a rotary potentiometer. In this case, a position controller generates an actuating signal for a rotary actuator that controls the rotary valve.
Slide potentiometers, because of their simple and inexpensive construction, are frequently used for position detection. Their advantage is that they produce a usable electrical actuating signal in a relatively simple manner with low power consumption. For instance, a 10 k&OHgr; potentiometer operated at 3 V consumes a maximum of 300 &mgr;A. The stroke or rotary movement of the actuator is applied to the potentiometer's axis of movement via corresponding add-on parts, e.g., a rotary lever with a switchable gear drive, and the component voltage detected by the potentiometer is transmitted to the analog input of an analog or digital control unit. The detection range of the angle of rotation for rotary actuators is typically 120° maximum. For linear actuators, typically the detection range is 15 mm maximum. The linear motion can also be converted into an angle of rotation of 120° maximum by means of a conversion mechanism.
In many areas of process and power technology, the fault-free operation of a plant depends on the flawless functioning of the control valves used. Downtimes of plants or plant parts caused by component failures significantly reduce the production capacity and the possible utilization of the plant. Thus, reducing downtimes and increasing system reliability are essential goals for efficient plant operation.
Due to their construction, the electromechanical slide potentiometers, which are frequently used for rotary or linear position detection, have drawbacks regarding their long-term stability because of wear and oxidation of the contact paths as well as because of their vibration fatigue limit. After prolonged quasi-static operation, their sliders tend to stick. Due to mechanical wear, the sliders and the resistive coatings eventually wear or their quality changes as a result of aging and oxidation. In electromechanical slide potentiometers, the rotary or linear motion is transmitted by means of a continuous shaft. Suitable encapsulation against environmental influences is therefore very costly and in itself is susceptible to aging and wear.
European Patent EP 0 680 614 B1 discloses a device for detecting an angular position of an object. The sensors described in this patent specification are based on the giant magnetoresistive (GMR) effect and consist of alternating magnetically hard and magnetically soft metal layers. These layers are each only a few atoms thick and are sputtered onto a silicon substrate. The resistance of the sensors greatly depends on the direction of a magnetic field acting on them. A GMR sensor is thus very well suited to detect a change in the angular position of a magnet.
Electropneumatic position controllers are frequently used in areas where the atmosphere can change into an explosive gas mixture. Here, it must be ensured that no risk arises from any of the possible operating states of the position controller. For this purpose, the occurrence of an ignition spark must be either excluded by suitable means or ignition of the atmosphere in the environment of the position controller must be reliably prevented.
OBJECTS OF THE INVENTION
One object of the invention is to provide a position controller, particularly for a drive-actuated valve, which is distinguished by its improved interference immunity and its inherently safe design, while being inexpensive to produce.
SUMMARY OF THE INVENTION
To attain this and other objects, according to the principles of the present invention, and according to one formulation, the a novel position controller, for a valve (
2
) actuated by a drive (
6
), having an inherent safety design, includes: a position transmitter (
9
) that detects the actual position of an actuator (
7
), and a control unit (
13
) that compares the actual position with a predefined desired position and that generating an actuating signal, wherein the position transmitter includes a magnet (
10
) and a magnetoresistive sensor (
50
), and wherein the magnet and sensor are rotatable or displaceable relative to one another in conjunction with a movement of the actuator (
7
), wherein the control unit (
13
) is arranged in a flame-proof enclosure that is separate from a housing of the position transmitter (
9
), and further wherein an interface (
18
) of the control unit (
13
) to the position transmitter (
9
) as well as the position transmitter (
9
) itself have an inherent electrical safety design.
The invention has the advantage that an inherently safe design of the position controller is realized at relatively low production costs. High investments for protecting the operating voltage, which would be incurred in a position controller with an inherent electrical safety design not having an ignition-proof enclosure, are avoided due to the flame-proof housing of the control unit. For the position controller, it is sufficient to use an inexpensive plastic housing. There are no increased ignition protection requirements regarding the electrical connection between the control unit and the position controller. The control unit is mechanically and thermally decoupled from the actuator. Mechanical vibrations and shocks, as well as high or low temperatures, of the plant are thus not transmitted to the control unit. With an appropriate encapsulation of the position controller, it is possible to use the unit in a chemically loaded atmosphere or in an explosive environment because of the dual protection achieved. The position controller, which is located outside the housing of the control unit, is advantageously mounted to the actuator even on small drives or under extreme loads that are due, for example, to mechanical vibrations, shocks or extreme temperatures. The position controller then is electrically connected to the control unit at low cost. The control unit is mounted at some distance, e.g., on a mounting tube or a similar mounting aid, from the position controller and then connected thereto by an electrical cable connection, Further the control unit is connected with the pneumatic drive via one or two pneumatic lines.
The invention further obviates the drawbacks of conventional potentiometers, since it uses a contactless potentiometer that comprises a magnet and a magnetoresistive sensor. The novel position transmitter provides the exact actual position of the actuator in either a dynamic or a static case. A non-linearity of the position transmitter's output signal, which is minor in any case, is readily compensated. Between the magnet and the magnetoresistive sensor, a partition can easil
Bastranelli John
Siemens Aktiengesellschaft
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