Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1999-11-30
2001-09-11
Nguyen, Tran (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S233000, C310S248000
Reexamination Certificate
active
06288468
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention originates with a commutation device for a direct-current motor, comprising among other things collector bars and brushes according to the preamble to the main claim, for which the current ripple can be detected with the aid of an electronic circuit and can be evaluated as measure for the speed or the number of motor rotations. The detection, evaluation and use of the alternating component of the current (“ripple”) for direct-current motors as measure for the motor speed is generally known. One method used in this connection is the so-called ripple-count method. With this method, the analog signal of the commutation current proceeds in addition to the higher-frequency ripple with a low-frequent modulation due to production tolerances, caused by differences in the coil sections of the motor armature. In the process, local minima and maxima occur, for which the minima can have absolute higher values than the maxima. For that reason, a local detection by means of an involved, differentiating method is necessary.
The problem of evaluating the analog signal of the commutation current at low cost and with low expenditure for the circuit technology is partially solved with the aid of a known method and arrangement in the DE-A-44 22 083 (German Published Examined Application).
However, a slight geometric displacement of the jointly operating brushes in particular will lead to additional current ripples in the signal course of the commutation current and thus to undesirable false signals since the shape of the brush running surface considerably influences the analog signal of the commutation current.
A known device for improving the analog signal of the commutation current (reference DE-A-31 48 966) represents one such attempt to solve this problem and avoid false signals. In this reference, it is explained that the improved analog signal is the result of a change in the shape of the brush running surface, which has two essentially point-shaped or line-shaped contact faces to the commutator, thus leading to a better coverage with respect to time and geometry, as well as a longer commutation time.
One disadvantage of these arrangements is the loud brush noise caused by the collector bars being run abruptly under the brush. The brush noise is defined, in a manner known per se, as the airborne sound generated in the process, which can be measured with a microphone. With the modified shape of the brush running surface according to the DE-A-44 38 868, the collector bars in each case are run twice per brush under the brush running surface, thereby causing and even stronger noise development.
SUMMARY AND ADVANTAGES OF THE INVENTION
Designing the commutation device according to the invention, represent a better approach to the solution.
The above object generally is achieved according to the present invention by a communication device for a direct current motor, in particular for a mechanically commutated direct current motor, comprising: a commutator with collector bars and brushes, wherein the brushes respectively share a contact face with the commutator, and wherein the ripples in the current, which develop during the commutation of the motor current, are detected with the aid of an electronic circuit and the ripple frequency is evaluated as a measure for the speed and in roder to obtain unambiguous and error-free speed information, the logitudinal edge of the collector bars and the front longitudinal edge of the contact face respectively one of the brushes together enclose an angle (&agr;) that exceeds zero degrees, so that at least two neighboring collector bars are constantly short-circuited via the brush.
The invention makes it possible to obtain unambiguous and error-free speed information from ripples in the commutation current for the mechanically commutated direct-current motor, which can be used for the speed, angle and path detection. Traditionally designed brush running surfaces with a contact face toward the commutator are preferably used for this.
It must be viewed as essential advantage of the commutation device design according to the invention, as defined in the main claim, that the commutation operation requires more time since respectively two neighboring collector bars are short-circuited as a result of the brush surface making contact with the commutator over a larger range of the rotational angle for the direct current motor armature.
Thus, given a geometric displacement of the brushes, it is possible to avoid the occurrence of additional current ripples in the course of the commutation current signal. These ripples are erroneously detected and counted as separate pulses by the evaluation electronics.
Furthermore, the noise development of the motor is reduced owing to the longer commutation time because the momentary ripple within one armature rotation is smaller and the electromagnetic excitation of the motor housing is lower. This noise development corresponds to the so-called structure-born noise, which is measured in the form of vibrations.
An additional advantage of this solution lies in the clearly reduced brush noise. Owing to the design of the commutation device according to the invention, the collector bars no longer run abruptly (at one point in time) under the brush, but enter gradually or continuously.
Given a brush running surface design according to the reference DE-A-31 48 966, for example, for which the brush has two contact faces to the commutator and each collector bar is therefore run twice per brush under a contact face, the brush noise can be improved noticeably and the noise pollution reduced if the commutation device is designed according to the invention.
The measures listed in the dependent claims result in advantageous modifications and improvements of the features listed in the main claim. The option of using standard, commercially available brushes is particularly advantageous, since the brushes are subject to wear and thus must be exchanged more frequently. This results in a reduction in costs.
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Bertsch Hans
Eckstein Dietrich
Kunitz Norman N.
Nguyen Tran
Robert & Bosch GmbH
Venable
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