Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-10-05
2003-05-20
Nguyen, Tran (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S191000, C310S261100
Reexamination Certificate
active
06566777
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a class of actuators capable of configuration to provide high-speed low-torque or to provide low speed high-torque output. The invention includes a method to convert electrical energy directly into mechanical energy utilizing an elastically deformable flexible rotor.
2. Description of Related Art
Conventional actuators of a given power rating, such as electrically or hydraulically powered motors, normally rotate at high speed with low torque. The speed is determined by the electrical excitation frequency and the number of motor poles for an electric motor; or by the flow rate for a hydraulic or pneumatic motor. To obtain low speed and high torque output at rated power, actuators are often coupled with any of several mechanical speed reducers, well known in the mechanical arts, such as chain driven gear sets, belt driven pulley sets and direct gear reduction. Gear reducers may include several stages of speed reduction as in stepped gear reduction or planetary gearing systems. In many industrial applications, high ratio reducers are commonly used with conventional motors running at a relatively high speed, typically 1000 to 5000 rpm or more, to obtain low-speed output rotation with high torque. Otherwise, high power high-torque motors would be used at great penalty of cost, size, and weight.
Some electrically powered motors are commercially available that generate high-torque and operate at low speed by employing a large number of electromagnetic stator poles. However, such motors are usually bulky and expensive. Similarly, low-speed and high-torque hydraulic motors are bulky and heavy with the additional requirement of a separate hydraulic power supply. Applications that are cost sensitive often utilize a high-speed motor coupled to a commercially available gear reduction system such as a multi-ratio gear reducer, a worm gear reducer or a planocentric motion reducer. For many applications it's desirable that the actuator and the speed reducer are provided with a central hole to pass process cables and lines through motorized joints.
The Harmonic Drive U.S. Pat. No. 3,196,713, is a well known commercial speed reducer which includes a flexible internal shell having externally cut gear teeth that engage a rigid outer shell having internally cut gear teeth. The flexible shell is deformed elliptically by a rotating elliptical cam to engage the outer shell at two diametrically opposed locations. The rotating cam imparts a rotating elastic wave into the flexible shell and causes the shell to rotate about its central axis. The flexible shell is usually coupled to an output shaft that rotates rigidly with it. The difference in the number of teeth between the flexible shell and the outer rigid shell defines the ratio of rotation between the speed of the motor that rotates the cam and the speed of the output shaft. Conventionally, the motor is external to the speed reducer and is coupled mechanically to the elliptical cam. Since the cam is rotated at the high speed of the motor, its inertia negatively impacts the servo controllability of an output load. Alternately, in Ohm, U.S. Pat. No. 4,044,274, the flexible shell is placed within tile air-gap between the motor's rotor and stator to provide a closely integrated actuator. However, such arrangement increases the width of the air-gap and reduces the power conversion efficiency of the motor.
Another type of motor achieves gear reduction to high torque using a rigid gear shell that progresses within a rigid fixed outer gear having a larger number of gear teeth. A rotating magnetic field, generated by stator poles mounted along the circumference of the outer gear, can induce a rigid style gear shell to roll along the outer gear in an orbital fashion. The low gear shell inertia and the absence of a bulky mechanical element rotating at high speed are features conducive of a desirable low-speed motion control. For example, in Pitchford et al, in U.S. Pat. No. 4,379,976, a rigid orbiting shell progressively engages stator gear teeth and rotor gear teeth along one common line of contact in a planocentric motion. This type of single point loading reduces possible torque output of the motor, promotes vibration, and generates excessive loads on the rotor bearings compared to the present invention. In addition, the motor poles are energized in steps and are not controllable for smooth motion.
Humphreys, U.S. Pat. No. 3,561,006, discloses an electromagnetic actuator having an electromagnetic stator that elliptically deforms a coaxial spline having internal as well as external gear teeth. The coaxial spline progressively engages matching external teeth on an output spline and stator internal teeth, the progressive rotation of the output spline being transmitted to a power output shaft at a rate reduced from the electromagnetic rotation rate. Humphreys employs magnetic shim stock to reduce magnetic reluctance and suggests roughened surfaces instead of gearing for surface engagement. This prior art suffers from the multiplicity of gear engagement surfaces, which is subject to wear, frictional losses and slip with frictional engagement. The, stator, coaxial spline, and output spline elements all serve both torque transmission and magnetic circuit functions. These functions require conflicting material properties of hardness and magnetic reluctance with one usually attained at the detriment of the other. Hence, power conversion efficiency and durability are compromised. The stepping motion of the device is also a serious limitation.
Kondoh et al, in U.S. Pat. No. 5,497,041 (1996) discloses a low-speed motor wherein a rotating magnetic field is formed in a geared outer stator to induce progressive deformation in a geared inner flexible shell containing a series of permanent magnets with alternating polarity. The progressive rotation of the inner flexible shell is transmitted to a power output shaft. In this prior art, the flexible shell is naturally circular and assumed to deform elliptically when the magnetic field is applied. However there is no mechanism to assure such desired elliptical form; the flexible ring could assume the least energy position of single-point contact with the stator and remain circular rather than the desired two-point contact of elliptical deformation which has a higher elastic energy level. The rotor naturally assumes the least energy circular configuration and may jam into a non-rotating vibratory state. In addition, the position of the Kondoh internal gear may become indefinite relative to the position of the rotating magnetic field resulting in compromised precision with this actuator configuration.
The prior art addresses electromagnetic actuators that combine electric motor principles with high gear ratio flexible speed reducers. However, these actuators are impractical for many applications due to the incompatible design considerations involved in combining the functions of electromagnetic permeability and gear engagement in the stator and rotor parts of the motor. Prior art also requires gearing between the rotor and stator elements to avoid slippage in high torque applications. Optimal rotor geometry is not inherent in much of the prior art. These shortcomings are addressed in the present invention.
The unique construction of the present invention overcomes these serious shortcomings and provides other advantages in several ways. In one embodiment, the actuator utilizes the large magnetic attractive forces and friction between the stator and a ferromagnetic rotor flexible shell for the transmission of high torque at low speed, thus avoiding the mechanical complexity and financial cost associated with gearing. Another embodiment includes a series of uniformly polarized permanent magnet segments radially mounted circumferentially to the rotor flexible shell to generate an elliptical rotor shape during electromagnetic interaction with the stator and propagates an elastic wave into the flexible shell. Optimal rotor shape can also be pro
Baker Gary
Nguyen Tran
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