Electricity: motive power systems – Positional servo systems – With particular 'error-detecting' means
Reexamination Certificate
2000-02-11
2001-05-15
Dang, Khanh (Department: 2837)
Electricity: motive power systems
Positional servo systems
With particular 'error-detecting' means
C318S603000, C318S715000, C324S207250
Reexamination Certificate
active
06232739
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to an apparatus for determining a position of a rotating member of an electric machine, and, more particularly, to an apparatus for accurately determining the position having a pulse switching strategy.
2. Disclosure of Related Art
For conventional variable frequency induction machines (i.e., electric induction motors), it is desirable to determine incremental angular position of the rotor. The incremental position is used to control the stator electrical excitation frequency so as to maintain a desired slip frequency (i.e., the difference between the rotor speed or frequency and the applied stator excitation frequency). As known, failure to maintain the proper slip frequency will result in a loss of torque production as well as a loss in efficiency.
A number of position determining approaches are known. For example, it is known to employ a rotating, ferromagnetic target wheel and a sensor to determine position. Exemplary of this approach is seen by reference to U.S. Pat. No. 5,754,042 entitled “MAGNETORESISTIVE ENCODER FOR TRACKING THE ANGULAR POSITION OF A ROTATING FERROMAGNETIC TARGET WHEEL” issued to Schroeder et al. Schroeder et al. disclose a target wheel having a plurality of teeth, separated by slots, angularly spaced around the periphery thereof. Schroeder et al. further disclose two magnetoresistive (MR) sensors positioned adjacent the target wheel, each generating a signal with transitions between two voltage levels at the passage of the leading and trailing edges of a tooth. As applied to induction machines, it is further known to use two sensors (fixed MR or Hall effect) in quadrature (i.e., the sensors are spaced apart a distance equal to one-half tooth). The state transitions or “edges” of the sensor output signal, which correspond to the leading and trailing edges of the tooth as it passes the sensor, are counted by a controller to calculate incremental position.
As further background, it is a characteristic of induction machines to require the highest incremental position resolution at relatively low speeds, when the time between successive pulses is longest, and the acceleration rate of the machine is typically the greatest. At high rotational speeds, the time between successive incremental position pulses is short, relative to the rotor acceleration rate, and thus the resultant speed error given acceleration over time is relatively low. Hence a lower resolution encoder is allowable. However, during startup or at low speeds, a resultant speed error given acceleration over time becomes unacceptably large, which limits the torque production of the induction machine and the response time of the system including the machine.
One approach therefore, taken in the art, is to simply provide a single, high resolution sensing system to accommodate the high-resolution requirements of relatively low speed operation. However, there are shortcomings to this approach. For example, optical encoders, if used, provide high resolution, but are costly. Alternatively, the target wheel in a sensor/target wheel approach can be made to have more teeth, thereby increasing resolution. There are, however, several factors limiting the number of teeth that can be produced on the target wheel (i.e., the greater number of teeth, the higher the resolution). First, as to manufacturing costs, tolerance requirements for a very high tooth-count target wheel and the increased time to form such a wheel add to the overall cost of the system. Second, magnetic design constraints limit the number of teeth that can be sensed on a target wheel of a given diameter. That is, proper operation of the sensors requires a minimum spacing between teeth. Finally, there are electrical concerns. The sensor devices conventionally used have predetermined rise and fall times (i.e., for the sensor output signal to transition between low and high states when passing leading and trailing edges of a tooth). At low speeds, such rise and fall times are generally inconsequential. However, at higher speeds, the rise and fall times interfere in applications where two sensors are used in quadrature. That is, at high speeds, the rise and fall times may causes the fall of the first sensor output signal to substantially coincide in time with the rise of a second, adjacent sensor output signal, rather than be spaced apart by one-half tooth (e.g., in quadrature). This can confuse edge detection circuitry into only recognizing one “edge” when two “edges” should be counted. This situation leads to errors in determining incremental position. Thus, rise and fall times, tolerances, and duty cycle also limit the number of teeth on the target wheel.
There is therefore a need to provide an apparatus for position determination that minimizes or eliminates one or more of the shortcomings as set forth above.
SUMMARY OF THE INVENTION
An apparatus for determining a position of a rotating member of an electric machine in accordance with the present invention is characterized by the features specified in claim
1
.
An apparatus for determining incremental position of a rotating member according to the invention has as one advantage a reduced cost relative to comparable resolution conventional sensing systems. Another advantage of the present invention is that a higher resolution apparatus can be provided as compared to a conventional sensing system having a similar cost. The cost can be reduced and/or resolution can be increased, in accordance with the invention, by providing a plurality of sensors in a sensor assembly to be utilized with a target wheel having a reduced number of teeth. A lower tooth-count target wheel is less expensive to manufacture than a higher tooth-count target wheel, all other factors being the same.
These and other advantages are realized by an apparatus according to the invention for determining a position of a rotating member of an electric machine. The apparatus includes a target wheel, a sensor assembly and a processing circuit. The target wheel is configured to have a plurality of teeth, separated by slots, angularly spaced around the periphery thereof. The sensor assembly is located proximate the periphery and has a plurality of sensors each configured to generate a respective output signal that transitions between first and second states at the passage thereby of a respective leading and trailing edge of each tooth. The processing circuit is provided for generating a position signal in response to the sensor output signals, characterized in that: each sensor is spaced from an adjacent sensor by a predetermined distance; and, the processing circuit has (i) a first mode of operation wherein a first number of the plurality of sensor output signals are utilized to generate the position signal, and (ii) a second mode of operation wherein a second number of sensor output signals different from the first number are utilized to generate the position signal, the processing circuit being configured to switch from the first mode to the second mode when the rotating member reaches a first predetermined speed. Preferably, the second number is less than the first number. By increasing the number of sensors, more sensor output signal transitions or “edges” can be generated per tooth, providing increased resolution, particularly useful for low speed operation. Therefore, a lower tooth-count target wheel can be used to obtain comparable resolution as conventional systems. At higher speeds, where rise/fall times of the sensors may interfere, as described above, and where high resolution is not needed, the number of sensor output signals used is reduced (i.e., ignoring pulses from several sensors), thereby eliminating the electrical problem described above.
REFERENCES:
patent: 4766359 (1988-08-01), Smith et al.
patent: 4883973 (1989-11-01), Lakey et al.
patent: 5185918 (1993-02-01), Shafer, Jr.
patent: 5731702 (1998-03-01), Schroeder et al.
patent: 5754042 (1998-05-01), Schroeder et al.
patent: 5811957 (1998-09-01), Bose et al.
patent: 5864217
Krefta Ronald John
Walters James E.
Dang Khanh
Delphi Technologies Inc.
Dobrowitsky Margaret A.
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