Electricity: motive power systems – Positional servo systems – 'reset' systems
Reexamination Certificate
1999-12-03
2001-01-30
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Positional servo systems
'reset' systems
C318S801000, C318S808000, C318S810000, C318S812000, C340S870210, C327S307000, C341S157000
Reexamination Certificate
active
06181099
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to signal correction and more particularly to correcting DC offset in the output of a frequency to voltage converter such as that used in the protection circuit of a motor drive.
The use of motor drives for controlling the speed and/or torque of motors, such as AC induction motors, is well known. Typically, an AC motor drive includes a rectifier section for converting an AC input voltage into a DC voltage for charging a DC bus and an inverter section comprising switching bridges for generating a three phase AC output signal, or an approximation thereof that cause sinusoidal current in the motor, for driving a three phase AC motor. The switching bridges include switching elements that can be controlled in a known manner to vary the frequency of the output signal to thereby control the motor in a desired manner.
Known motor drives include various protection functions and other control functions. Often these functions are accomplished by a microprocessor based controller programmed in a desired manner or analog circuits. For example, it is known to provide regulation of the DC bus voltage for the purpose of overvoltage protection during regenerative braking and/or operation anomalies. It is desirable to electrically isolate the regulation circuit from the DC bus to prevent damage to the regulation circuit. Accordingly, it is known to use a voltage to frequency converter to convert the DC bus voltage to a signal having a frequency that is related to the DC bus voltage, to convert the frequency signal to light pulses, and to transmit the light pulses over a fiber optic link to the protection circuit. A frequency to voltage converter associated with the protection circuit converts the frequency signal back to a voltage signal to permit voltage regulation in a known manner.
There are two conventional types of protection circuits. The first type of protection circuit is an analog circuit that uses known analog components to convert the frequency signal to a voltage signal. Analog circuits are very responsive, i.e. fast. However, analog circuits have an inherent DC offset. In fact, it is difficult to reduce the analog offset to less than ±2%, even when using high grade, expensive components. In many applications, the error due to the DC offset is unacceptable.
The second type of protection circuit is a digital circuit that uses a programmed microprocessor based device and interface components to convert the frequency signal to a voltage signal. Digital protection circuits can be very accurate and are not subject to DC offset problems associated with analog circuits. However, a digital protection circuit must accumulate or sample the pulses of the frequency signal and thus is inherently unresponsive, i.e. slow.
BRIEF SUMMARY OF THE INVENTION
A first aspect of the invention is an apparatus for correcting a DC offset in a frequency to voltage conversion circuit comprising an analog conversion circuit having a frequency to voltage converter and being adapted to receive a frequency signal and convert the frequency signal to a voltage signal having a voltage that varies with the frequency of the frequency signal, and a digital processor adapted to receive the frequency signal and the voltage signal and generate a correction signal that is fed back to the analog conversion circuit to correct DC offset in the voltage signal.
A second aspect of the invention is a method for correcting a DC offset in a frequency to voltage conversion circuit comprising the steps of converting a frequency signal to a voltage signal with an analog circuit, integrating the difference between the voltage signal and the frequency signal to obtain a correction signal, and summing the correction signal and the voltage signal to correct the voltage signal for DC offset.
A third aspect of the invention is an apparatus for correcting a DC offset in a frequency to voltage conversion circuit comprising analog means for converting a frequency signal to a voltage signal, means for integrating the difference between the voltage signal and the frequency signal to obtain a correction signal, and means for summing the correction signal and the voltage signal to correct the voltage signal for DC offset.
A fourth aspect of the invention is an apparatus for correcting a DC offset in a frequency to voltage conversion circuit comprising an analog conversion circuit having a frequency to voltage converter and a summing junction and being adapted to receive a frequency signal and convert the frequency signal to a voltage signal having a voltage that varies with the frequency of the frequency signal, an analog to digital converter coupled to the analog frequency converter to convert the voltage signal to a digital signal, a digital processor adapted to receive the frequency signal and the digital signal, calculate a reference signal based on the frequency signal, calculate a feedback signal based on the digital signal, and integrate the difference between the reference signal and the feedback signal to generate a digital correction signal, and a digital to analog converter coupled to the digital processor and the analog conversion circuit to convert the digital correction signal to an analog correction signal and input the analog correction signal to the summing junction to thereby correct DC offset in the voltage signal.
A fifth aspect of the invention is a motor drive for controlling a motor comprising a DC bus, plural switching bridges coupled to the DC bus, a controller operative to gate the switching bridges to generate a voltage signal to drive a motor, a voltage to frequency converter coupled to the DC bus and being operative to generate a frequency signal having a frequency indicative of voltage of the DC bus, an analog conversion circuit having a frequency to voltage converter and a summing junction and being coupled to the voltage to frequency converter to receive the frequency signal and convert the frequency signal to an output signal having a voltage that varies with the frequency of the frequency signal, an analog to digital converter coupled to the analog frequency converter to convert the output signal to a digital signal, a digital processor adapted to receive the frequency signal and the digital signal, calculate a reference signal based on the frequency signal, calculate a feedback signal based on the digital signal, and integrate the difference between the reference signal and the feedback signal to generate a digital correction signal, and a digital to analog converter coupled to the digital processor and the analog conversion circuit to convert the digital correction signal to an analog correction signal and input the analog correction signal to the summing junction of the analog conversion circuit to thereby correct DC offset in the output signal.
REFERENCES:
patent: 4249120 (1981-02-01), Earle
patent: 5053688 (1991-10-01), Rees
patent: 5105352 (1992-04-01), Iwasa et al.
patent: 6014065 (2000-01-01), Nishikawa
patent: 6091281 (2000-07-01), Yoshida
patent: 60-242540 (1985-12-01), None
patent: 3-238352 (1991-10-01), None
Bixel Paul S.
Cardinal Mark E.
Villamil Robert
General Electric Company
Martin Edgardo San
Nappi Robert E.
Nixon & Peabody LLP
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