Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1999-09-21
2002-11-19
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S686000, C324S688000, C340S870370
Reexamination Certificate
active
06483321
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention deals with a capacitive angular displacement transducer for the measurement of a rotor angle. It encompasses a first stator plate having a number of electrodes in the form of conductive circular ring sectors with a sector angle respectively predefined. The transmitting electrodes are electrically isolated from each other and parallel to a second stator comprising a receiving electrode. A rotor is perpendicularly mounted on a shaft and located coaxially, between and parallel to both stator plates. The rotor has at least one rotor blade in the form of a circular ring sector, the central angle of each rotor blade being essentially equal to the sum of the central angles of n neighboring transmitting electrodes. Furthermore, the invention deals with a measurement signal processing device which can be used to determine the angular shaft position by means of the capacitive angular displacement transducer.
2. Description of the Prior Art
Such a capacitive angular displacement transducer and measurement signal processing device results from the applicant's U.S. Pat. No. 5,598,153, for example. This well known angular sensor features four or eight sector-shaped transmitting electrodes which fully cover a rotational angle of 2&pgr;. The receiving electrode is in the form of a circular ring electrode. The rotor comprises one or two sector-shaped blades, and the shape of each blade has to conform to the rule that the central angle of each rotor blade is equal to the sum of the central angles of two transmitting electrodes of the first stator. The measurement signal processing device of such a capacitive angular transducer includes a generator with four outputs electrically connected to the said four or eight transmitting electrodes of the first stator, as well as an evaluation unit for determining the angular shaft position, connected to the receiving electrode of the second stator. Due to the special geometric properties of the described sensor and to the design of its measurement signal processing unit, a particularly high measurement accuracy is obtained for a measurement range of 360°, although the design is simple in structure and low in cost. The measurement accuracy is almost unaffected by mechanical tolerances such as rotor offset, tilt and the like.
A tile angle transducer known from DE 44 24 538 A1 is not equipped with a transmitting and a receiving electrode, but with an equally designed stator electrode arrangement, whereby the electrodes of the first stator are electrically connected to the electrodes of the second stator and both stators are electrically conductive. The sensor measures the absolute capacitance value, with the objective to obtain the lowest possible frictional resistance, from a first electrode at the first stator by way of a rotor to a second electrode of said first stator, whereby an identical topology of the second stator with corresponding electrodes is connected in parallel. As the principle of operation is non-radiometric, offset and gain error remain uncompensated as a measuring fault in the measuring chain.
A capacitive linear sensor results from EP 0 538 184 A1, whereby a cursor position is determined relative to e.g. a scale equipped with grooves. Thereby, different electrode arrangements at the cursor are disclosed, particularly an arrangement with side by side transmitting, shielding and receiving electrodes.
One task of this invention lies in the further development of the sensor disclosed in U.S. Pat. No. 5,598,153 and its related measurement signal processing device in order to reduce the amount of space consumed by sensor outlines and to achieve easy and simple mounting of all sensor components. Additionally, easy exchange of specific sensor components should be possible, whereby the simple sensor assembly, the low manufacturing cost and the high measurement accuracy should be maintained.
SUMMARY OF THE INVENTION
The task outlined above is accomplished by a sensor of the aforementioned type when at least one stator with an aperture alongside the outer edge is used and the sum of the central angles of all transmitting electrodes is less than 2&pgr;, preferably in the range of &pgr;/12 to 5&pgr;/6, whereby said aperture is chosen to allow during assembly a radial insertion of the shaft along with the rotor to its correct shaft position. The first stator must have a central angle of less than 360° and the central angle of the electrode of the second stator has to be essentially equal to the sum of the central angles of all electrodes of the first stator and the electrodes of the first stator include two shielding electrodes. These inventive measures make it possible to manufacture a sensor which is less space consuming. Above all, the aperture enables the mounting and dismounting of both stators independently of the rotor and of each other. As the electrode area is reduced at the stator in question, it is no longer necessary to construct it as a circular ring disk. Although less space is required and the production is simple, it is possible to measure an angular range of 2&pgr; with high accuracy using a sensor according to the invention, especially as the shielding electrodes largely prevent field deviations caused by auxiliary influences.
If transmitting segments are excited in pairs, it may happen that neighboring electrode pairs do not directly neighbor but lie apart due to their location at the radial edges of the transmitting segments arrangement facing free space. Consequently, the field distribution will be different in the two cases in question. In response to this, an application variant has been developed where the stator has 2n+1 (n≧4) transmitting electrodes. Thus, at any time, n predetermined transmitting electrodes can be electrically coupled to n neighboring transmitting electrodes, so that at any time one single transmitting electrode is available without an electrical link to a neighboring transmitting electrode. This arrangement allows paired electrodes to be directly neighboring at all times, so that whichever pairs of transmitting segment couples are excited, field distribution of the same kind for all electrode pairs results.
Furthermore, it was found to be advantageous if the receiving electrode is in the form of a circular ring sector with a central angle essentially equal to the sum of the central angles of all transmitting electrodes. If necessary, the receiving electrode can be completely surrounded by a shielding electrode.
To achieve a measurement range of 2&pgr; (360°) for the sensor, one advantageous application uses a rotor with at least two blades in the form of circular ring sectors with an equal central angle. In order to obtain a simple but unequivocal relation between the rotor position and the angular shaft position it may be advantageous to use at least two rotor blades with different radial dimensions, whereby the first stator carries two more electrodes in addition to the transmitting electrodes. While the two electrodes have a distance from the shaft which is greater than the radial dimension of a shorter rotor blade, they are located within the area of the radial dimension of a longer rotor blade. Due to the different dimensions of the rotor blades and the corresponding stator electrodes, additional information is obtained in a simple way which provides a reliable absolute angular rotor position.
It is advantageous for practical use to use identical central angles for all transmitting segments, except for the two boundary electrodes located at both radial edges of the stator plate. Corresponding to the expected field distribution, the central angle of the boundary electrodes can be tuned in such a way that at the receiving electrode, on an average, equally sized induced charges (originating from each single transmitting segment) are obtained. Using the expected field distribution as a guideline for sizing he transmitting electrodes proves to be an effective way to achieve a consistent measurement accuracy over the entire mea
Deb Anjan K.
Le N.
Welsh & Katz, LTD.
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