Patent Application
20060237637
This invention relates to a motor system having a motor, an encoder and a flexible coupling. More particularly, the present invention relates to the flexible coupling having a provision by which the stators of the motor and the encoder are connected.
The basic components of a motor generally include a rotor that spins inside a housing (i.e., a stator) that does not move. The rotor spins in the electromagnetic field contained in the stator. A shaft is generally connected to the spinning rotor thereby transferring the rotational movement to a load connected to the shaft. A motor system usually includes an encoder (or resolver) to control the operation of the motor system. The encoder is connected to the motor system to provide the position and speed information of the rotor of the motor system. This information may be used by a user to control the operation of the motor system using, for example, an external motor controller with associated electronics.
Housed rotary optical encoders are the most common type of encoders used in a motor system to provide the rotary position of the motor. A housed rotary optical encoder typically includes a housing (i.e., a stator) to support precision bearings and a shaft with an optical disk attached thereto. The shaft of the rotary optical encoder is usually rigidly coupled to the shaft of the motor to detect the rotational position of the motor.
A flexible stator coupling is used in the housed rotary optical encoder to prevent rotation of the encoder housing with respect to the motor housing while allowing radial and axial misalignment, both static and dynamic. Despite its flexibility in the radial and axial directions, the coupling must have high torsional stiffness in order to prevent undesirable dynamic positioning errors from the encoder.
Couplings for the purpose of joining housed encoders and motors are commercially available which are stiff torsionally. While these couplings have been quite successful in a majority of applications, they experience fatigue failures in certain applications that require a large amount of radial and axial misalignment.
Bellows couplings have been used for connection of encoder and motor stators which are formed from elastomeric materials, however these are not suitable for certain applications which require high positional accuracy, both static and dynamic. Metal bellows couplings have also been used to couple encoder shaft to motor shaft, whereby the motor and encoder stators are rigidly coupled. In this arrangement, the coupling diameter is small since it is mounted to the shaft, therefore the torsional stiffness is lower and the positional errors in operation are higher. The smaller diameter of the shaft-coupling also allows a lesser amount of radial and axial misalignment.
The above-identified problems are solved and a technical advance is achieved in the art by providing a method and system that connects the motor and the encoder with a flexible coupling thereby achieving a high torsional stiffness in the motor system, along with obtaining capacity for handling larger amounts of radial and axial misalignment without experiencing fatigue damage.
In accordance with an aspect of the invention, there is provided a bellows coupling that connects the housing of a motor (i.e., motor stator) and the housing of an encoder (i.e., encoder stator).
In accordance with another aspect of the invention, there is provided a motor system comprising a motor, having a shaft and a housing, capable of driving a load connected to the shaft of the motor; an encoder, having a shaft and a housing, capable of detecting the rotational position of the shaft of the motor; a flexible coupling capable of connecting the housing of the motor to the housing of the encoder, wherein the shaft of the motor and the shaft of the encoder are connected with a rigid connection.
Other and further aspects of the present invention will become apparent during the course of the following detailed description and by reference to the attached drawings.
One aspect of the present invention is directed to the connection between a motor and encoder. In particular, a flexible bellows coupling is used to connect the encoder housing (i.e., encoder stator) with the motor housing (i.e., motor stator) in a motor system. It is assumed that the shaft of the motor and the encoder are rigidly connected.
In the present invention, the convolutions of the bellows coupling can be oriented in two ways, i.e., stacked and concentric. In the stacked embodiment, the convolutions are stacked one on top of another. In the concentric embodiment, the convolutions are in concentric layers outward from a central axis.
Referring to
Table I shows working conditions during the movement of the flexible coupling.
An experiment has been performed to test the flexible coupling built according to the embodiment as described above. The motor system embodying the present invention shows nearly the same frequency of torsional resonance as the flexible coupling previously used.
A second experiment was performed to verify the life of the bellows coupling when operated under combined radial and axial misalignment. The bellows coupling survived 113 million cycles under a 0.009 inches radial misalignment in combination with 0.012 inches radial run-out with no degradation or damage. The original non-bellows coupling was tested in a similar manner under lower levels of offset (0.004 inches radial runout) and failed due to fatigue crack propagation after as little as 4 million cycles.
Although illustrative embodiments of the present invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to these embodiments, and that various changes and further modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention, which is defined in the claims, below. For an example, while the flexible coupling of the present invention connects the motor stator and the encoder stator having a similar diameter, this invention may also be applied easily to the motor stator and encoder stator having different diameter without significant modification. Additionally, while the flexible coupling of the present invention uses a flange type connection and a bolt/nut type connection, other types of connections may be used as well within the scope of the invention.
