| 1. |
- Hillerström, Gunnar, et al.
(author)
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Adaptive rejection of periodic disturbances with unknown period
- 1996
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In: Journal of Dynamic Systems Measurement, and Control. - : ASME International. - 0022-0434 .- 1528-9028. ; 118:3, s. 606-610
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Journal article (peer-reviewed)abstract
- A disturbance cancellation extension by means of Youla parametrization to a stabilizing nominal controller is investigated. In the case of known disturbance model, disturbance frequencies or period time, it may be implemented directly as an add-on device to the existing control system. The nominal control system may be designed without consideration of the deterministic disturbances. This way it may suffice with a PID controller. It may also be a more complex one e.g., designed to obtain certain robustness properties. For known relation between the disturbance frequencies (one or more periodic signals) but unknown fundamental frequency, a frequency shaped disturbance model estimarion scheme is utilized. This makes it possible to e.g., adapt the disturbance model to minimize the error directly with respect to the output. For periodic disturbances the scheme implements a self-tuning discrete time repetitive controller. To illustrate the feasibility of the approach taken it is used for rejecting a periodic disturbance acting on a nonminimum-phase plant.
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| 2. |
- Hillerström, Gunnar
(author)
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Adaptive suppression of vibrations : a repetitive control approach
- 1995
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In: Proceedings of the 1995 American Control Conference. - Piscataway, NJ : IEEE Communications Society. - 0780324455 - 0780324463 ; , s. 3820-3824
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Conference paper (peer-reviewed)abstract
- The aim of this paper is to present an adaptive solution to suppression of vibrations. Adaptation is appropriate whenever the fundamental frequency is unknown or drifting, e.g. when the vibration is caused by a rotational machine with unknown rotational speed. The approach presented here has its roots in repetitive control based on models of isolated frequencies rather than the commonly used delay model. The relationship between different modeled frequencies is fixed by the model structure, and the fundamental frequency is obtained by gradient descent. To show the feasibility of this approach, it was used to reduce vibrations on a lever that were caused by a motor with imbalance in its rotation. As actuator, a standard loudspeaker was used, and the vibration was sensed by an accelerometer.
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| 3. |
- Hillerström, Gunnar
(author)
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Adaptive suppression of vibrations : a repetitive control approach
- 1996
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In: IEEE Transactions on Control Systems Technology. - : Institute of Electrical and Electronics Engineers (IEEE). - 1063-6536 .- 1558-0865. ; 4:1, s. 72-78
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Journal article (peer-reviewed)abstract
- The aim of this paper is to present an adaptive solution to suppression of vibrations. Adaptation is appropriate whenever the fundamental frequency is unknown or drifting, e.g., when the vibration is caused by a rotational machine with unknown rotational speed. The approach presented here has its roots in repetitive control based on models of isolated frequencies rather than the commonly used delay model. The relationship between different modeled frequencies is fixed by the model structure, and the fundamental frequency is obtained by gradient descent or Newton's method. To show the feasibility of this approach, it was used to reduce the vibrations on a lever that were caused by a motor with imbalance in its rotation. As an actuator, a standard loudspeaker was used, and the vibration was sensed by an accelerometer.
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| 4. |
- Hillerström, Gunnar, et al.
(author)
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Application of repetitive control to a peristaltic pump
- 1993
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In: Proceedings. - : IEEE Communications Society. - 0780308611 ; , s. 136-140
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Conference paper (peer-reviewed)abstract
- In many machinery there are rotating axis, which sometimes give problems with mechanically coupled disturbances from e.g. imperfect roundness or loads. These disturbance can be modeled as a nonlinear feedback, and treated under the framework of repetitive control.
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| 5. |
- Hillerström, Gunnar, et al.
(author)
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Application of repetitive control to a peristaltic pump
- 1994
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In: Journal of Dynamic Systems Measurement, and Control. - : ASME International. - 0022-0434 .- 1528-9028. ; 116:4, s. 786-789
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Journal article (peer-reviewed)abstract
- The aim of this paper is to study angular velocity control of a rotating axis, a peristaltic pump where the disturbance is associated with certain angular positions. The use of constant sampling rate then implies varying disturbance period. A discrete repetitive control scheme based on a reduced order disturbance model tuned to the setpoint of the angular velocity is shown to reject the modeled disturbance with asymptotically zero error. With axis angle synchronization of sampling, the plant model is time-varying but it gives a static discrete disturbance model independent of the angular velocity. If the deviation from the nominal working point is not too large this is to prefer while it gives faster convergence
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| 6. |
- Hillerström, Gunnar
(author)
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On repetitive control
- 1994
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Doctoral thesis (other academic/artistic)abstract
- This thesis will concern control systems ability to enhance performance by repeating, while maintaining a stable and sufficiently fast closed loop performance. Iterative techniques can be divided into repetitive and learning control. In principle these techniques only differ in that the learning systems initial condition is reset at the beginning of each cycle while the repetitive system runs continuously. Repetitive controllers, the main theme of this thesis, can be used for rejecting periodic disturbances as well as tracking periodic references with ideally zero error. To accomplish this an internal signal model is utilized. This model is either a delay with a positive feedback around it modeling a general periodic signal, or a combination of harmonic frequencies adding up to a band-limited signal. The delay internal model has some uncomplicated, easy to grasp interpretation. Robustness properties may however not be the best. But if the plant is well determined over a frequency band this may be an excellent approach. With increased sampling rate the robustness properties of the delay model controller will however deteriorate. To cure this, disturbance rejection/tracking properties may be traded against robustness using lowpass filtering. As an alternative the internal model consisting of a number of isolated frequencies is exploited. This model has better robustness properties while preserving the zero error property for a bandlimited input signal. In addition it is shown how fundamental frequency adaptation can be incorporated for the case of unknown or drifting input signal frequencies. In this thesis repetitive controllers have successfully been implemented with two test plants; a peristaltic blood pump used in dialysis treatments, and a process where vibrations caused by a motor with an imbalance are suppressed.
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| 7. |
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| 8. |
- Hillerström, Gunnar, et al.
(author)
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Rejection of periodic disturbances with unknown period : a frequency domain approach
- 1994
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In: Proceedings of the 1994 American Control Conference. - Piscataway, NJ : IEEE Communications Society. - 0780317831 ; , s. 1626-1631
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Conference paper (peer-reviewed)abstract
- A self-tuning discrete time repetitive controller comprising a gradient descent disturbance model estimation and a pseudo feedforward controller is investigated. To illustrate the feasibility of the approach considered an example is given for rejecting a periodic disturbance acting on a nonminimum-phase plant.
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| 9. |
- Hillerström, Gunnar, et al.
(author)
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Repetitive control theory and applications : a survey
- 1996
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In: Elsevier IFAC Publications / IFAC Proceedings series. - 1474-6670. - 0080429084 ; 29:1, s. 1446-1451
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Journal article (peer-reviewed)abstract
- The aim of this paper is to present repetitive control, controllers for rejecting periodic disturbances or tracking periodic reference signals, This survey covers both continuous and discrete time theory, together with a number of applications
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| 10. |
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