Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
2002-11-07
2004-06-29
Strecker, Gerard R. (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207120, C324S207240, C324S207250
Reexamination Certificate
active
06756779
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an inductive measuring transducer for determining the position of a body which is movable relative to another body, one of such bodies being equipped with at least one inductive sensor component that produces a local magnetic field and the other one of such bodies comprising an inductive pickup into which the local magnetic field is injected, and to an electronic circuit for supplying the inductive sensor component with an alternating voltage and a circuit for evaluating the output voltages of the sensor. It is the object of the invention to be described hereafter, to provide a measuring transducer which achieves a sufficiently high degree of precision at low expense.
Measuring transducers of that kind are used as measuring transducers preferably for measuring angles and displacements.
For converting an angular position of a rotor relative to a stator to an electric signal a large number of measuring methods have been known that can be implemented in the most different ways, depending on the particular application. With respect to insensitivity to environmental influences, the inductive measuring principle has been found to be especially well suited. Contrary to optical or capacitive methods, it is particularly insensitive to contamination and moisture condensation. Magnetic sensors may be influenced by external fields, or by aging or demagnetization of the permanent magnets employed. In addition, they are not very much suited for hollow-shaft sensors because normally the sensor has to be arranged at the center of rotation. Potentiometric angle sensors are connected with the disadvantage that the wipers and resistance layer are subject to wear.
Inductive angle sensors, such as resolvers or synchros, are known and have proven their value in practice. Because of their symmetric structure, the latter are relatively insensitive to eccentricity. On the other hand, however, only small gaps are possible between rotor and stator. Due to their structure, they are relatively expensive because precision-made and precision-wound stators and rotors lead to high production costs and material input.
DE 41 13 745 A1 describes a structure where a centrally arranged excitation coil magnetizes a pot-type ferrite core, fitted on the rotor shaft and comprising outer pot halves of different diameters, a circuit board with an induction loop being arranged between the outer pot halves, from which a voltage can be picked off by suitable pickoffs for obtaining the angle information. The disadvantage of that method mainly lies in the asymmetric structure of the rotor which has the result that any displacement of the center of rotation of the rotor results in a relatively important measuring error. Added to this, the method requires a core that covers the entire angular range. Especially when employed as a hollow-shaft sensor, a very large and expensive core is required. Contrary to optical systems, determining the effective center of such a system is quite difficult with such a system because of the nature and propagation of the magnetic field lines in such systems. In addition to the requirement of having the circuit board exactly positioned, a precise bearing system is also necessary.
In many applications it is possible either not at all, or only at high expense, for example by the use of additional bearings, to align the rotor exactly with the stator. In practice, eccentricities and axial displacements of more than ±1 mm may be encountered. In the case of asymmetric systems it is then necessary to work with large diameters, which frequently leads to considerable difficulties in use.
DE 197 57 689 A1 describes an inductive measuring transducer which uses a conductor loop and a resistor network to scan a movable alternating magnetic field and derive a position-dependent output voltage. While that system is well suited for measuring limited displacement and angular ranges, it can be adapted to a continuous measuring range of 360° only with considerable additional input.
SUMMARY OF THE INVENTION
Contrary to the known solution, the invention proposes to determine the measuring voltage by the steps of inducing a voltage, by one or more inductive sensor components mounted on a rotor, which cover only a small portion of a rotating conductor path, in a closed circular conductor loop extending along the path described by an inductive pickup element, and forming a function, defined by the position and value of different resistors, of the voltages encountered at the connection points of the different resistors and the conductor loop, by resistors distributed and connected along the periphery of the conductor path, the other ends of such resistors being connected one with the other in different groups. For example, a mean value of the voltage over an angular range can be obtained at a connection point if resistors with equal resistance values, arranged at equal spacings over that angular range, have their one ends connected to the conductor loop and their other ends connected to the connection point of the resistor group from which the voltage can be picked off. In order to permit the angle to be clearly determined over 360°, it is necessary to obtain at least two different voltage curve shapes, related to the angular position of the rotor. This is achieved by forming a plurality of resistor groups connected to different portions of the conductor loop.
When a single inductive sensor component is used, an alternating voltage is induced in the conductor loop over the angle filled by the air gap of the relatively small sensor component. That voltage produces in the conductor loop a current which, due to the inductive and ohmic resistance of the conductor loop, leads to a uniform voltage drop over the circumference. It can be shown here that by using resistor groups, each defining the mean value of a quadrant of the conductor path, a voltage curve shape as necessary for determining a clear angular value over a range of 360° is obtained.
If a single sensor component is used only, any displacement of the center of rotation of the rotor relative to the center of the stator in the direction of movement of the inductive sensor may, however, lead to measuring errors. Similarly, a not perfectly uniform behavior of the impedance of the conductor path over the circumference may cause additional measuring errors.
According to a further embodiment of the invention, this is avoided by using two inductive sensor components, offset by 180°, to induce voltages of opposed phase and equal amplitude in the conductor loop. The conductor loop then constitutes an electric circuit with two opposed voltage sources, where no differential voltage is present and, thus, no current flows between the voltage sources. Between the different sensor components, a conductor loop carries a voltage of constant amplitude, while the oppositely arranged portion of the conductor loop carries a voltage of opposed phase and equal amplitude. Due to the fact that there is no current flow in the conductor loop, no current drop produced by the impedance of the conductor loop will be encountered, either. Thus, the impedance of the conductor loop does not enter into the measuring result.
Any displacement of the center of rotation of the rotor relative to the stator in or against the direction of movement of the two oppositely arranged inductive sensors will result in opposed errors which largely balance each other out. It is, thus, possible to build up angle sensors especially for applications that do not have a bearing system of their own and/or where assembly tolerances are high.
A corresponding magnetic potential and, thus, a corresponding excitation current is required for generating the field in the air gap of the sensor component.
In principle, excitation by a winding connected directly to an oscillator would seem possible. In practice, this is however not practicable in most of the cases because of the movable lines required in this case and the limited angle of rotation.
In the case of a
Gleixner Franz
Sester Stefan
Horst Siedle GmbH & Co. KG
Ostrolenk Faber Gerb & Soffen, LLP
Strecker Gerard R.
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