Positioner for a valve that can be actuated by a drive

Details Positioner for a valve that can be actuated by a drive Positioner for a valve that can be actuated by a drive

2002-10-04

2004-02-24

Hirsch, Paul J. (Department: 3754)

Valves and valve actuation

Electrically actuated valve

Remote or follow-up control system for electrical actuator

C251S129010

Reexamination Certificate

active

06695282

ABSTRACT:

FIELD OF AND BACKGROUND OF THE INVENTION
The invention relates to a positioner, especially for a valve that can be actuated by a drive.
European Publication EP 0 637 713 A1 discloses such a positioner for a valve that is actuated by a drive. 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 locator of the positioner. The potentiometer detects the actual position of the actuator. A control unit of the positioner 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 positioner through a normalized signal, e.g., a 4 to 20 mA interface or a digital field bus message. Thus, the role of the positioner 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 positioner 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.
OBJECTS OF THE INVENTION
An object of the invention is to provide a positioner, particularly for a valve that is actuated by a drive, which is distinguished by its improved interference immunity 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 novel positioner, for a valve (
2
) that is actuated by a drive (
6
), includes: a locator (
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 generates an actuating signal. The locator includes a permanent magnet (
18
) and a sensor (
50
), and the magnet and sensor are rotatable or displaceable relative to one another in conjunction with a movement of the actuator (
7
). Further, the sensor (
50
) is arranged in an area of a housing (
90
) such that the sensor (
50
) is positioned to detect a relative rotation between the sensor and the magnet when the magnet rotates about an axis of rotation, and is positioned to detect a relative shift between the sensor and the magnet when the magnet is displaced, wherein the shift occurs in a plane that extends substantially perpendicularly to the axis of rotation.
The invention obviates the drawbacks of conventional potentiometers, since it uses a contactless potentiometer that includes a magnet and a magnetoresistive sensor. The novel locator provides the exact actual position of the actuator in either a dynamic or a static case. A non-linearity of the locator's output signal, which is minor in any case, is readily compensated. Between the magnet and the magnetoresistive sensor, a partition can easily be installed for encapsulation and, thus, protection against environmental influences. Therefore, the locator is very rugged and insensitive to dirt and a harsh environment. The magnet is easily mounted outside the sensor housing on a linear or rotary actuator such that its magnetic field lines act on the magnetoresistive sensor through the housing wall. An evaluation circuit is readily integrated in the sensor housing. This evaluation circuit generates a voltage proportional to the angle of rotation, or the linear path, of the magnet by way of the change in resistance of the magnetoresistive sensor. Thus, the evaluation circuit supplies, to a control unit, a signal that corresponds to the actual position and is immune to interference.
A minimum distance between the magnet and the sensor is easily kept to prevent damage to the magnetically hard layers, especially in a GMR sensor, since in this sensor type the strength of the magnetic field may not exceed 15 kA/m. The contactless principle of the novel locator eliminates the problem of a scratching or sticking slide potentiometer. This contactless principle offers advantages in applications where the potentiometer is exposed to continuous vibrations. It is also advantageous in the quasi-static case where the potentiometer position remains unchanged over a long period of time, and where there is a risk that the slider of a slide potentiometer would dig into the resistance layer and possibly get stuck there due to control instability in the system. If the magnet forms the moving part of the locator, which is coupled with the actuator, it couples the actuating movement into the magnetoresistive sensor through its magnetic field, without requiring any mechanical duct. By corresponding add-on parts, an exact rotary or linear motion of the moving part is ensured in a simple manner.
If the magnet is designed as a permanent magnet, a particularly simple structure results, since the magnet does not require a power supply, and thus does not increase the current consumption of the locator.
An advantageous clear increase in the resistance of the magnetoresistive sensor results if a so-called anisotropic magnetoresistive sensor is used. When the

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