Electricity: motive power systems – Positional servo systems – With compensating features
Reexamination Certificate
2002-02-08
2003-07-22
Masih, Karen (Department: 2837)
Electricity: motive power systems
Positional servo systems
With compensating features
C318S649000, C318S611000, C318S606000
Reexamination Certificate
active
06597146
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The field of the invention is motor controllers and more specifically filtering systems for removing cyclic load disturbances from motor control systems.
An exemplary motor control system includes a controller and a motor drive. The drive is linked to a motor and provides currents to motor windings thereby causing a motor rotor to rotate within a stator construct. To this end, a controller typically receives one or more command signals that indicate intended motor operating characteristics. For instance, one exemplary intended characteristic may be rotor velocity which is received by the controller in the form of a command velocity signal. The controller is programmed to provide control signals to the drive to cause the motor to operate in accordance with the command signals. The drive receives the control signals and attempts to drive the motor in accordance therewith.
Unfortunately, many motor drive applications are characterized by mechanical and electrical disturbances that hamper the control process and cause the motor to operate in other than the commanded fashion. Some of the disturbances are non-cyclic while others are cyclic. For instance, an increased load is a change that occurs over an operating period and therefore is not cyclic. In contrast, mechanical resonances (e.g., a dual inertia motor-load system with a spring-like coupling that exists between the inertias) and electrical disturbances (e.g., system force induced by the mutual torques that exist in the harmonics associated with back electromotive forces (EMFs)) are typically cyclic and hence occur at system specific and recurring frequencies.
To minimize the affects of system disturbances, many control systems include one or more sensors and corresponding feedback loops. The sensors are configured and positioned so as to measure motor operating parameters and provide feedback signals to the controller. For instance, one feedback signal may include a feedback velocity signal. In the case of a velocity feedback system, the controller compares the command velocity signal to the feedback velocity signal and generates an error command signal for controlling the drive. The controller may simply subtract the feedback signal from the command velocity signal and use the difference as a velocity error signal for controlling the drive. Typically, to expedite control, the velocity error signal is manipulated (e.g., via a PI controller) prior to being fed to the drive.
Conventional feedback systems achieve their end results (i.e., reduce the affects of system disturbances) in a less than optimal fashion by overshooting and undershooting command signals and thereby cause system errors. For instance, where motor velocity is below a commanded velocity, conventional feedback control systems simply increase the velocity error signal until the feedback velocity is above the commanded velocity. Thereafter the feedback signal indicates that the motor velocity is higher than the commanded velocity and the controller reduces the velocity error signal until the motor velocity, as represented by the feedback signal, is again below the commanded velocity. This overshooting and undershooting process is repeated without end and never reaching constant steady state.
With respect to non-cyclic disturbances (e.g., a slowly changing load), conventional feedback controllers work relatively well as, after several cycles, a steady state condition should result. Unfortunately, in the case of cyclic disturbances, the target compensated signal must inversely mirror the changing disturbance and therefore the overshoot and undershoot problem persists.
For instance, assume a command velocity of 2 Hz and a cyclic disturbance including an 8 Hz component and a 16 Hz component. In this case, the combined feedback signal includes 2, 8 and 16 Hz components and therefore the velocity error signal is dynamic and includes 8 and 16 Hz components. Thus, despite the recurring cyclic nature of the disturbances, the conventional feedback system would be unable to eliminate the disturbances without generating persistent overshoot and undershoot related noise.
BRIEF SUMMARY OF THE INVENTION
It has been recognized that, in the case of cyclic-type disturbances, the frequencies of the most prominent instantaneous cyclic disturbances can be determined and used to, in effect, anticipate the cyclic characteristics of subsequent disturbances, the anticipated characteristics thereafter being used to modify the velocity error signal that represents the difference between the commanded signal and the feedback signal. To this end, to anticipate subsequent characteristics of a cyclic disturbance, three characteristics of the disturbance have to be identified. First, the frequencies corresponding to the separate components of the disturbance have to be identified. Second, the phases of the separate components of the disturbance have to be identified. Third, the amplitudes of the disturbance components have to be identified. After frequencies, phases and amplitudes of the disturbance components are identified, a compensation signal mirroring the disturbance can be generated and subtracted from the manipulated velocity error signal to generate a corrected error signal that essentially anticipates the disturbance and compensates therefore.
To this end, an exemplary embodiment of the invention includes an apparatus for reducing cyclic noise in a motor drive system where the drive system subtracts a velocity feedback signal from a velocity command to generate a velocity error signal and uses the velocity error signal to control other drive system components, the apparatus comprising a frequency identifier that receives a velocity feedback signal and identifies unintended frequencies of unintended components of the feedback signal where each unintended component is characterized by a component specific phase and amplitude, a sine generator that generates a combined signal characterized by a separate sinusoidal component corresponding to each of the undesirable frequencies, a tuning signal identifier linked to at least one system component output and providing a tuning signal corresponding to the at least one component output, an LMS module that mathematically combines the combined signal and the tuning signal to generate a compensation signal characterized by a separate component corresponding to each identified unintended frequency where each separate component of the compensation signal is essentially in phase with and has an amplitude similar to a corresponding unintended component; and a regulator module that mathematically combines the velocity error signal and the compensation signal to generate a corrected signal used to control other drive components.
In at least one embodiment the sine generator includes a plurality of separate sine modules and a second summer, the transform module providing a separate one of the unintended frequencies to a different one of the sine modules, each sine module receiving an unintended frequency generating a sine wave signal at the unintended frequency, the sine wave signals provided to the second summer and the second summer mathematically combining the sine wave signals to generate the combined signal. The second summer may mathematically combine by adding the sine wave signals.
In at least one embodiment the frequency identifier includes a Fast Fourier Transformer (FFT) module that generates frequency signals corresponding to each frequency present in the feedback signal. The frequency identifier may further include a frequency discriminator that receives the frequencies generated by the FFT module and identifies a sub-set of the received frequencies as unintended frequencies.
The discriminator may also receive the command velocity signal and, when the command velocity signal is a square wave having a fundamental frequency, identifies odd multiple harmonics of the command velocit
Rehm Thomas J.
Schmidt Peter B.
Gerasimow Alexander M.
Masih Karen
Quarles & Brady
Rockwell Automation Technologies Inc.
Walbrun William R.
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